Drug-resistant epilepsy

Epilepsia, 46(6):858–877, 2005

Blackwell Publishing, Inc.

⃝C 2005 International League Against Epilepsy

Summary: Drug-resistant epilepsy with uncontrolled severe seizures despite state-of-the-art medical treatment continues to be a major clinical problem for up to one in three patients with epilepsy. Although drug resistance may emerge or remit in the course of epilepsy or its treatment, in most patients, drug re- sistance seems to be continuous and to occur de novo. Unfortu- nately, current antiepileptic drugs (AEDs) do not seem to prevent or to reverse drug resistance in most patients, but add-on therapy with novel AEDs is able to exert a modest seizure reduction in as many as 50% of patients in short-term clinical trials, and a few become seizure free during the trial. It is not known why and how epilepsy becomes drug resistant, while other patients with seemingly identical seizure types can achieve seizure con- trol with medication. Several putative mechanisms underlying drug resistance in epilepsy have been identified in recent years.

During treatment with a variety of different antiepilep- tic drugs (AEDs), as many as 20–40% of newly treated patients with epilepsy will not enter long-term remission for several years (1–4). Despite state-of-the-art medical management with modern AEDs, a number of these pa- tients continue to have drug-resistant epilepsy with fre- quent debilitating seizures. Considering that epilepsy is one of the most common chronic neurologic disorders, drug-resistant epilepsy is a major public health problem. The consequences of drug-resistant epilepsy can be quite severe, including mortality rates that are 4 to 7 times higher in people with drug-resistant seizures (5). It is not known why and how epilepsy becomes drug resistant in some pa- tients while others with seemingly identical seizure types and epilepsy syndromes can achieve seizure control with medication (6–9). Thus a pressing need exists to under- stand better the neurobiologic and clinical mechanisms underlying intractability of epilepsy to form a basis for de-

Accepted January 25, 2005.

Address correspondence and reprint requests to Prof. Dr. Dieter Schmidt at Epilepsy Research Group, Goethestr.5, D-14163 Berlin, Ger- many. E-mail: dbschmidt@t-online.de

Based on experimental and clinical studies, two major neuro- biologic theories have been put forward: (a) removal of AEDs from the epileptogenic tissue through excessive expression of multidrug transporters, and (b) reduced drug-target sensitivity in epileptogenic brain tissue. On the clinical side, genetic and clinical features and structural brain lesions have been associ- ated with drug resistance in epilepsy. In this article, we review the laboratory and clinical evidence to date supporting the drug- transport and the drug-target hypotheses and provide directions for future research, to define more clearly the role of these hy- potheses in the clinical spectrum of drug-resistant epilepsy. Key Words: Pharmacoresistance—Antiepileptic drugs—Multidrug transporters—Sodium channels—GABA receptors—Target hy- pothesis.

velopment of new drugs or better treatment approaches for intractable epilepsy. Several putative neurobiologic mech- anisms underlying drug resistance in epilepsy have been identified in recent years. Two major theories have been put forward: (a) removal of AEDs from the epileptogenic tissue through excessive expression of multidrug trans- porters, and (b) reduced drug-target sensitivity in epilep- togenic brain studied in animal models and in humans. On the clinical side, the complex natural history of drug resis- tance in epilepsy is discussed. The goals of our review are (a) to discuss critically the diverse clinical characteristics of drug resistance; (b) to evaluate critically the current neurobiologic hypotheses, particularly the transport and the target theories, and point out their strengths and weak- nesses; (c) to make an attempt to correlate clinical char- acteristics of drug resistance with hypothesized biologic mechanisms; (d) to assess critically the ability of hypoth- esized biologic mechanisms, to better understand the clin- ical challenges and issues of drug resistance in epilepsy; and (e) to provide suggestions for future research to bet- ter define their role in the pathogenesis of drug-resistant epilepsy.

Critical Review

Drug Resistance in Epilepsy: Putative Neurobiologic and Clinical Mechanisms

∗Dieter Schmidt and †Wolfgang Lo ̈scher

∗Epilepsy Research Group, Berlin; and †Department of Pharmacology, Toxicology, and Pharmacy, University of Veterinary Medicine, Hannover, Germany

858

DEFINITION AND EPIDEMIOLOGY OF DRUG-RESISTANT EPILEPSY

Definition

Although no single accepted definition exists of drug- resistant epilepsy, different definitions may be appropri- ate, depending on the type of seizure and epilepsy syn- drome and the purpose for which the definition is used. Definitions usually include the number of AED failures and the minimal remission or seizure frequency during a specified duration of therapy. For example, inclusion cri- teria for add-on AED trials often require four complex par- tial seizures with or without secondary generalization per 28 days during a 2-month baseline period despite adequate treatment with one to three AEDs to qualify as pharma- coresistant or medically refractory (10). The National As- sociation of Epilepsy Centers has considered epilepsy as pharmacoresistant when seizures do not come under con- trol after 9 months of treatment under the care of a neurolo- gist (11). Recently, Berg et al. (12) defined pharmacoresis- tant epilepsy in children as failure of two or more drugs and one or more seizures per month over an 18-month period. Pragmatically, the number of AEDs tested depends on the chances of alternative therapies such as epilepsy surgery, vagus nerve stimulation or ketogenic diet, the patient’s interest, and available procedures (13). Based on partly prospective clinical observations in a series of patients with newly diagnosed epilepsy, those who did not achieve complete seizure control for 12 consecutive months with the first two or three AEDs were given the predictive diag- nosis of refractory or drug-resistant epilepsy (4). These in- dividuals should be referred to an epilepsy service for fur- ther diagnostic evaluation including videotelemetry, opti- mization of pharmacotherapy, and consideration of other therapies, particularly epilepsy surgery. In general, many experts would agree that whenever a patient does not be- come seizure free for 12 months during long-term state-of- the-art treatment with several suitable AEDs at maximal tolerated doses, the epilepsy can be broadly classified as drug-resistant, pharmacoresistant, or medically refractory (14–16). In addition, conceptual and practical differences exist between “not seizure free” and “drug resistant.” Cer- tainly for the purpose of epilepsy surgery, from which the pathology data concerning target sensitivity and trans- porter overexpression are derived, the two scenarios are not equivalent.

Epidemiology of drug-resistant epilepsy

Drug resistance (by any definition) differs widely among patients with different types of seizures and epilepsy syndromes and the population studied (2,17,18). In a population-based study of 176 Finnish children with epilepsy treated with AEDs, seen first between 1961 and 1964 and followed up until 1992, partial epilepsies ac- counted for 68% of cases (17). Of the 64 surviving pa- tients who were not in terminal 5-year remission, the vast

Epidemiology of drug-resistant epilepsies. The figure from a long-term prospective study shows that, in many cases, drug resistance may be largely determined by the underlying epilepsy syndrome. The majority of surviving patients (49%, 46 of 93) with symptomatic partial or symptomatic generalized epilepsies (78%, seven of nine) are drug resistant, whereas only a minority of pa- tients with idiopathic generalized epilepsies (13%, four of 30) do not enter 5-year terminal remission, 92% of patients not in remis- sion were taking continued AEDs (adapted from 17). See text for definition of drug resistance. IGE, Idiopathic generalized epilepsy; IPE, idiopathic partial epilepsy; SGE, symptomatic generalized epilepsy; SPE, symptomatic partial epilepsy.

majority of 59 patients were receiving continued AED treatment and can be considered drug resistant, whereas five (8%) patients were seizure free for a longer period or periods, and the medication was discontinued. In this sample, only 13% of all patients with idiopathic general- ized epilepsies, and no case with idiopathic partial epilep- sies, were not in remission. In contrast, 78% of patients with altogether rare symptomatic generalized epilepsies and 49% of patients with the much more common symp- tomatic partial epilepsies were not in remission (Fig. 1). Although not all patients can be considered drug resistant, as discussed earlier, the study by Sillanpa ̈a ̈ (17) was cho- sen because it has the advantage of being prospective and having an extended follow-up of patients long enough to register the majority of typical drug-resistant cases seen in surgical series. In contrast, prospective population-based studies starting in adults and covering the initial several years reported a lower percentage of patients (20%) who were considered to be drug resistant according to the au- thors’ definition. However, the latter studies may not have been long enough, as pointed out by Anne Berg (15). In hospital-based and only partly prospective studies such as one from Scotland, 37% of this cohort was considered drug resistant (4). The definition of drug resistance used by the Scottish authors was having no seizures for 12 months at the last follow-up, which ranged widely from 2 to 16 years (median, 5 years).

In this review, we focus on the mechanisms of in- tractability of the symptomatic partial epilepsies and, in particular, the most common type of epilepsy in adults, which is temporal lobe epilepsy (TLE). Because as many as 75% of patients with mesial TLE are considered to

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FIG. 1.

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860 D.SCHMIDTANDW.LO ̈SCHER

have drug-resistant epilepsy, most of the data we discuss in this review will be related to this most common type of drug-resistant epilepsy in patients undergoing surgery (18,19).

Clinical features associated with drug resistance

Once diagnostic and treatment errors are excluded that may lead to pseudo-pharmacoresistance, which has been covered elsewhere (16,19), a core population of drug-resistant epilepsy remains, in which a host of well- described clinical factors exist and have been associated with intractability (7,20). In particular, an as yet unre- solved debate concerns whether hippocampal sclerosis— as seen in magnetic resonance imaging (MRI)—is or is not a cause of pharmacoresistance in patients with mesial TLE (21–24). The difference in outcome among patients with mesial TLE and hippocampal sclerosis suggests that prognosis varies among patients with hippocampal scle- rosis, and preferably those with a poorer prognosis (for any reason) were seen in a tertiary referral center. Alter- natively, as is discussed later, it is possible that mesial TLE with hippocampal sclerosis is a progressive disorder in which the proportion of patients with poor prognosis increases in the course of the disorder, at least in some patients.

None of the clinical factors associated with intractabil- ity in the literature constitutes a possible target for new drug-treatment strategies, for two main reasons. One, AEDs are not able to prevent the development of symp- tomatic acquired epilepsy (25,26), and two, although AEDs are impressively blocking seizures in many patients, currently no evidence exists that they influence the course of epilepsy and prevent pharmacoresistant epilepsy. A re- cent study confirmed that early AED treatment undoubt- edly reduces the number of seizures but is not able to im- prove the course of epilepsy, as indicated by very similar 2-year remission rates for early and delayed AED regi- mens 5 years after starting treatment (27). Therefore as pointed out by Hauser (28), epilepsy in all patients could currently be labelled pharmacoresistant.

A further challenge for our understanding of mech- anisms of pharmacoresistance is posed by the different patterns of drug-resistant epilepsy. Although in most pa- tients, drug resistance seems to have been present de novo, even before the first AED was started (15,29–32), this is not always the case. In other patients with easily treatable epilepsy, drug resistance seems to develop later (32), and in a third group of patients, drug resistance appears to re- mit in the course of epilepsy or its treatment (10). These clinical patterns of drug resistance are discussed in the next section.

Patterns of drug resistance in epilepsy

Three seminal observations coexist as to the evolution of drug resistance as measured in the time to onset of in- tractability (15,29–32). The de novo theory claims that in

Seizure control

No seizure control

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1 2 3 4 5 6 7 8 9 10 11 12 Time (years)

FIG.2. Differentclinicalpatternsofdrugresistance.Denovocon- tinous drug resistance (—), reversal of drug resistance, possibly in a intermittent pattern in which periods of remission are followed by periods of uncontrolled seizures (- – -), and progression to drug resistance of delayed onset with persistent loss of efficacy after initial control (–––). See text for description of clinical patterns and discussion of putative mechanisms.

most cases, pharmacoresistance has been fully developed before the first seizure or at least before the start of AED treatment. Alternatively, emerging evidence from one ret- rospective study of a highly selected group of largely TLE patients undergoing temporal lobe surgery indicates that, at least in some patients with easily treatable epilepsy, pharmacoresistance requiring surgery develops years later in the course of their epilepsy (32). In addition, transient reversal of drug resistance, possibly in a wax and wane pattern with a remitting-relapsing course is seen in pa- tients with previously drug-resistant partial epilepsy will respond to become seizure free during trials of new AEDs (10). These different theoretical patterns of intractability, which are illustrated in Fig. 2, are discussed in more detail in the following section.

De novo drug resistance

One line of support for de novo existence of pharma- coresistance is that most patients who fail to respond do so during the first ever exposure to AEDs (Fig. 2). Only 11% of the 113 patients studied in a hospital-based cohort from Scotland in whom the first drug failed primarily because of lack of efficacy (as opposed to adverse events or other causes) later reported a 12-month remission with other AEDs (4). In a detailed updated analysis of a larger sam- ple, only 7% of patients for whom two well-tolerated treat- ment schedules failed achieved remission and for those for whom three regimens failed, this figure decreased further to just 3% (20). Thus patients not responding to two well- tolerated AED regimens were likely to have refractory epilepsy; the majority of these were refractory de novo (20). Camfield et al. (31) also found that response to the first AED was highly predictive of outcome in 417 chil- dren treated for epilepsy. In 345 children responding to the first AED, 61% eventually went into remission ver- sus only 30 (42%) of 72 who failed to respond to the first AED. It seems clear from these large observational studies

that many patients are destined to become pharmacoresis- tant even before their first exposure to an AED. Many patients therefore do not evolve into a pharmacoresistant state, rather their disorder is one that at onset does not re- spond fully to currently available AEDs. A further impor- tant characteristic of drug-refractory epilepsy is that most patients with intractable epilepsy are resistant to most, and often all, AEDs when seizure remission is the end point (7). As a consequence, patients not controlled with monotherapy with the first AED have a chance of <20% to be controlled with other AEDs, even when using AEDs (alone or in combination) that act by diverse mechanisms (4). This multidrug resistance of most patients with refrac- tory epilepsy is important when discussing mechanisms of pharmacoresistance in epilepsy. However, contrary to the prevalent assumption that refractory epilepsy always announces itself as refractory from the onset, mounting evidence indicates that this may not always be the case, and in a number of patients, pharmacoresistant epilepsy develops after they responded well to the first AED.

Is there progression from remission to drug resistance?

In support of evolution from remission to drug resis- tance, several lines of suggestive evidence exist.

1. Brodie (20) reported that among 780 patients with newly diagnosed epilepsy, in 9% of patients start- ing on AED therapy, pharmacoresistant epilepsy subsequently developed after an initial good re- sponse. In a Canadian study of children, in 4% of patients, pharmacoresistant epilepsy developed af- ter they responded well to the first AED (31). Ac- cording to one, albeit retrospective observation, a substantial proportion of partial epilepsy may not become clearly pharmacoresistant for many years after onset (32).

2. In the analysis of Berg et al. (32), a prior remission of ≥5 years was reported by 8.5% of surgical can- didates with pharmacoresistant epilepsy. The time to pharmacoresistance may be several years and extend to >10 years, especially if the onset is dur- ing childhood. Consequently, if a study considered outcome at 2 years after onset of treatment, many individuals with mesial TLE might be considered well controlled.

3. Engel has referred to the “stuttering” course of mesial TLE (33). In his experience, “seizures typi- cally respond to medication initially. . . .there may be several years of remission. . . .disabling complex partial seizures return during adolescence” (33). This suggests a complex natural history that we have not fully appreciated as well as a potential window of opportunity for early secondary inter- vention (15). However, we currently cannot be cer- tain that progression of mesial TLE into drug re-

sistance exists, mainly because we have no large- scale, long-term, prospective clinical studies. Nev- ertheless, several lines of additional indirect evi- dence support a progressive nature of mesial TLE, at least in some patients.

4. Recent animal experimental data suggest that mor- phologic and functional consequences follow re- peated, brief seizures (34).

5. A prospective MRI volumetric study of 24 pa- tients with newly diagnosed TLE demonstrated a hippocampal volume reduction of 9% on the side of seizure onset (over a 3.5-year period of obser- vation) and a correlation of hippocampal volume loss with the number of secondarily generalized seizures, suggesting seizure-induced damage (35). A second prospective study of 12 patients with TLE also showed a small (1%) hippocampal vol- ume loss on MRI over a 3.4-year observation pe- riod ipsilateral to the seizure focus that was as- sociated with recurring complex partial seizures; seizure-free patients (n = 3) did not show volume loss (36). Other studies by John Duncan’s group in London (37) suggested a progressive more non- specific neocortical atrophy associated with uncon- trolled epilepsy. However, the cause-and-effect re- lation between volume loss and drug responsive- ness remains unclear (i.e., no evidence suggests that the smaller the hippocampus, the more drug re- sistant the patient’s epilepsy becomes). As briefly discussed earlier, no clear evidence suggests that hippocampal sclerosis is associated with drug re- sistance.

6. Helmstaedter’s studies on memory function in TLE suggest that long-term memory outcome is a func- tion of seizure control (38).

7. The “silent period” between the occurrence of a risk factor and the onset of habitual seizures and apparently increasing resistance to medical therapy suggests an ongoing process (39). However, none of these seven lines of indirect evidence for pro- gression of epilepsy provides proof of progressive pharmacoresistance in patients with mesial TLE. Additional longitudinal clinical studies of patients with mesial TLE with hippocampal sclerosis are needed.

Further clinical observations possibly suggest the de- velopment of pharmacoresistance in the course of the dis- order. One is the clinical observation, summarized in detail in a recent review that an estimated 7–9% of seizure-free patients who undergo preplanned AED discontinuation do not achieve remission again but turn out to have pharma- coresistant epilepsy, and second, that it may take many years for as many as 50% of patients to achieve remis- sion after AED discontinuation (40). Two, in a study in

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862 D.SCHMIDTANDW.LO ̈SCHER

TABLE 1. Seizure outcome of randomized controlled add-on trials of antiepileptic drugs in patients with previously pharmacoresistant partial epilepsy

their description of patterns of remission in a book chap- ter, Shorvon and Sander (29) described an intermittent pattern in which active epilepsy is interrupted by periods of remission. They defined remission as a period of free- dom from seizures of ≥2 years. In a group of 181 patients with chronic uncontrolled seizures attending a special- ized hospital outpatient service, the intermittent pattern was found in 39 (22%) patients. Unfortunately, important information is missing, such as whether seizure relapse was due to drug discontinuation. Relapse is not equivalent to drug resistance, and no information was made avail- able on how often relapse seizures could be controlled again. Despite these shortcomings, these data suggest that in some patients, pharmacoresistance may be reversible, at least for a period of several years. Although no claim can be made or is intended that AEDs are involved in reversing pharmacoresistance, these data show that any theory for pharmacoresistance must take into account that pharmacoresistance may be progressive or reversible in some patients with partial epilepsy. For our discussion, it is important to consider that neurobiologic mechanisms of pharmacoresistance may be different in patients who have never responded to an AED versus those who progressed to pharmacoresistance after they responded initially to therapy. In addition, the mechanisms of reversing phar- macoresistance may differ from those generating phar- macoresistance. More specifically, because, as reviewed earlier, combination treatment with novel AEDs is unde- niably improving seizure frequency by ≥50% in as many as 50% of patients with chronic partial epilepsy that was resistant to previous AEDs often for decades (see Table 1), a number of questions arise. Although seizure reduc- tion cannot be regarded as equivalent to remission, which is seen in only 5–8% of previously drug-resistant patients with the introduction of a drug (42), the question remains, how are novel AEDs achieving beneficial seizure reduc- tion in individuals with chronic refractory epilepsy, and why does treatment with diverse AEDs fail initially or later in the course of treatment in so many patients?

After having reviewed the clinical complexity includ- ing different patterns of drug resistance in epilepsy, we critically discuss two major hypotheses of drug resistance that are plausible and for which a reasonable body of sug- gestive evidence exists.

PUTATIVE EXPERIMENTAL AND CLINICAL MECHANISMS OF DRUG RESISTANCE IN EPILEPSY

Currently, two major hypotheses may explain med- ical refractoriness of epilepsy, the target hypothesis and the multidrug-transporter hypothesis (26). Based on the target hypothesis, intrinsic or acquired changes in AED targets in the brain underlie the development of drug-resistant epilepsy, whereas the multidrug-transporter

AED

Gabapentin Lamotrigine Levetiracetam Oxcarbazepine Pregabalin Tiagabine Topiramate Zonisamide

Highest responder rate

Percentage of patients seizure free

26%

34%

40% 8% 50%

51% 5% 36%

51%

43%

Adapted from refs. 10 and 42. Responder rate: percentage of patients with a minimum of 50% seizure reduction over baseline. The percentage of patients (completers) who are seizure free during the last 28 days of the trial (no published data found for other AEDs). This figure, however, does not represent the likelihood of patients remaining seizure free over a long-term period.

which patients after a 2-year remission were randomized to continued AED treatment or slow withdrawal, 22% with continued treatment relapsed during the following 2 years. Although occasional poor drug compliance cannot be ex- cluded, according to the authors, it accounted for only a small proportion of the risk to the group continuing with treatment (41). The evidence is, however, weakened by the fact that a number of patients randomized to the contin- ued therapy group nonetheless reduced or withdrew from treatment. It also is unclear how many of the relapsed pa- tients later attained seizure control again. If prospective studies confirm these preliminary findings and the under- lying mechanisms generating these associations are better understood, it may be possible to consider interventions that might interrupt these processes and in the future pre- vent some forms of epilepsy from becoming pharmacore- sistant. Fortunately, other patients who start out to have pharmacoresistant epilepsy seem to benefit from clinically relevant seizure reduction when treated with add-on novel AEDs.

Is drug resistance reversible?

The reverse process, namely that previously pharma- coresistant epilepsy can be attenuated, at least in part and transiently, is a well-known fact in a number of placebo- controlled randomized trials in which add-on treatment with a modern AED achieves significant seizure reduc- tion in as many as 50% of patients with chronic pre- viously refractory epilepsy with often weekly seizures before the introduction of the new drug (Table 1). Fur- thermore and most welcome, some patients—in the 5–8% range—will be seizure free during the trial. This figure, however, does not represent the likelihood of patients re- maining seizure free over a long-term period. In contrast, significantly lower responder rates and no seizure-free pa- tients are seen during the administration of placebo (10). In

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Carbamazepine Ethosuximide Lamotrigine Oxcarbazepine Phenytoin Pregabalin Zonisamide

INaf INaP? INaf

INaf

INaf INaP

+ − − − − + + + + + − − + − − + − − + − +

+ + + + − − + − −

+ − + + − − + ? ? + + + + + − + + +

INaf

Predominant effect on GABA-mediated mechanisms

Benzodiazepines Tiagabine Vigabatrin

Mixed, complex actions

Felbamate Gabapentin Levetiracetamc Topiramate Phenobarbital Valproate

INaf

INaf INaP

INaf? INaP

HVA

HVA (α2δ) HVA

HVA

HVA T-type?

NMDA

KA/AMPA AMPA NMDA

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TABLE 2. Current antiepileptic drugs and the molecular targets through which they are thought to exert their therapeutic activity in epilepsy

Molecular targets of antiepileptic drugs

Clinical efficacy of antiepileptic drugs

Drugs Na+ channels

Predominant effect on voltage-dependent

Ca2+ channels

ion channels HVA

T-type

HVA

HVA

HVA

HVA (α2δ) T-type

GABA system

GABAAR GABA-transporter↓ GABA-T↓

GABAAR

GABA turnover↑ Reverses zinc and DMCM GABAAR

GABAAR

GABA turnover↑

Glutamate receptors

Partial Myoclonic seizuresa seizuresb

Absence seizures

For references, see text and Rogawski and Lo ̈scher (44). Note that not all molecular targets of AEDs are shown in this table; some additional targets are discussed in the text. Increase of a process is indicated by “↑” and decrease by “↓”.

AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid; GABA, γ-aminobutyric acid; GABAAR, GABAA receptors; GABA-T, GABA aminotransferase; GTC, generalized tonic–clonic; HVA, high-voltage activated; INaf, fast sodium current; INaP, persistent sodium current; KA, kainate; NMDA, N-methyl-D-aspartate. Zinc and DMCM (methyl-6,7-dimethoxy-4-ethyl-ß-carboline-3-carboxylate) are negative allosteric modulators of GABAA receptors.

aPartial seizures with or without secondary generalization, and all tonic–clonic seizures except those seen in patients with idiopathic generalized epilepsies (IGEs).

bIncludes patients with primary generalized tonic–clonic seizures as seen in IGE.

cLevetiracetam binds with high affinity to synaptic vesicle protein 2A (SV2A), a ubiquitous protein that is associated with synaptic vesicles and is believed to participate in the regulation of Ca2+-dependent neurotransmitter release (45).

hypothesis claims that the target is never reached because intrinsic or acquired overexpression of multidrug trans- porters at the blood–brain barrier (BBB) restricts brain uptake of AEDs (26). Of course, these hypotheses are not exclusive but may coexist in the same patient. Further- more, intrinsic or acquired resistance to AEDs is certainly a multifactorial phenomenon, and it would be naive to expect that two hypotheses are sufficient to explain in- tractability. However, because these two hypotheses are biologically reasonable and have attracted a great deal of interest, they are described and discussed in more detail in this review.

THE TARGET HYPOTHESIS

Major targets and mechanisms of antiepileptic drugs

In the absence of a specific etiologic understanding of most forms of epilepsy, approaches to drug therapy must necessarily be directed at the control of symptoms (i.e., the suppression of seizures) (9,43). Prolonged administration of AEDs is the treatment modality of first choice. The se- lection of an AED is based primarily on its efficacy for the specific types of seizures exhibited by the patient. Table 2 illustrates the clinical efficacies of many of the commonly

used AEDs against major types of epileptic seizures to- gether with major AED targets. The goal of therapy is to keep the patient free of seizures without interfering with normal brain function or producing other untoward effects and adversely affecting the patient’s quality of life.

To exhibit antiepileptic activity, a drug must act on one or more target molecules in the brain. These targets include ion channels, neurotransmitter receptors, and transporters or metabolic enzymes involved in the release, uptake, and metabolism of neurotransmitters (44). Based on the spe- cific targets involved in AED mechanisms, AEDs can be divided mechanistically into drugs acting by (a) modu- lation of voltage-gated ion channels (including sodium, calcium, and potassium channels); (b) enhancement of synaptic inhibition [e.g., by potentiating inhibition medi- ated by γ -aminobutyric acid (GABA)]; and (c) inhibition of synaptic excitation (e.g., by blockade of glutamate re- ceptors) (44). The fact that several AEDs act by more than one of these mechanisms is thought to explain their broad spectrum of clinical efficacy. However, this con- cept ignores important pharmacologic and clinical differ- ences between AEDs that cannot be explained solely on the basis of the mechanisms illustrated in Table 2. For in- stance, lamotrigine (LTG) shares its effects on sodium and

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864 D.SCHMIDTANDW.LO ̈SCHER

TABLE 3. Loss of drug effects on the use-dependent block of Na+ channels in epileptogenic brain tissue

Antiepileptic drug

Carbamazepine Carbamazepine Carbamazepine Carbamazepine Lamotrigine Phenytoin Valproate Valproate Valproate

Epileptogenic hippocampal tissue from

Patients

Patients

Kindled rats

Epileptic rats (pilocarpine model) Epileptic rats (pilocarpine model) Epileptic rats (pilocarpine model) Patients

Kindled rats

Epileptic rats (pilocarpine model)

Type of hippocampal neuron examined

CA1 neurons Dentate granule cells CA1 neurons Dentate granule cells Dentate granule cells Dentate granule cells CA1 neurons

CA1 neurons Dentate granule cells

Loss of drug effect on Na+ channels

++ (50% loss)

+++ (complete loss)

++ (50% loss but transient) +++ (complete loss)

No loss

+ (partial loss)

No loss

No loss

No loss

Reference

Vreugdenhil et al. (46)

Remy et al. (47)

Vreugdenhil and Wadman (48) Remy et al. (49)

Remy et al. (49)

Remy et al. (49)

Vreugdenhil et al. (46) Vreugdenhil and Wadman (48) Remy et al. (49)

calcium channels with carbamazepine (CBZ), but these drugs clearly differ in their clinical efficacies (Table 2). Thus certainly other cellular and molecular targets (not covered in Table 2) explain the differences in pharma- cology between AEDs in the same mechanistic category (43). Knowledge on these additional mechanisms rapidly increases, as illustrated by the novel AED levetiracetam (LEV), the putative mechanism of which was identified only recently and is not shared by any other AED (45).

Voltage-gated sodium channels

The target hypothesis is based primarily on studies with CBZ on voltage-gated sodium channels in hippocampal neurons (Table 3). The primary mechanism of this major AED is well established and thought to be related to its action on voltage-gated Na+ channels, which are integral to the generation of seizure discharges (44). CBZ [and other AEDs, such as phenytoin (PHT) or LTG, which act on voltage-gated Na+ channels] has several concomitant effects on Na+ channels. First, it reduces the maximal amplitude of Na+ current; second, it shifts the voltage dependence of inactivation into a hyperpolarizing direc- tion, thus reducing the number of available Na+ chan- nels; and third, it slows the recovery from inactivation of these channels. This latter mechanism may be particularly relevant to the treatment of epilepsy because it causes a preferential reduction of Na+ -channel availability during high-frequency discharges (44).

Wytse Wadman’s group (46) was the first to report that the modulation of sodium current inactivation by CBZ in hippocampal CA1 neurons from patients with TLE and mesial temporal lobe sclerosis was only half of that en- countered in neocortical neurons from the same patients, and only half of that encountered in CA1 neurons from patients without mesial temporal lobe sclerosis (Table 3). Similar observations were obtained in the kindling model of TLE, in that the CBZ response of sodium channels of CA1 neurons isolated from the epileptic focus of fully kin- dled rats was only half of that in control rats (Table 3; 48). Vreugdenhil et al. (46) suggested that the reduced CBZ sensitivity at the cellular level is involved in the inability of CBZ to control seizures with intractable TLE and may be

a function of seizure activity rather than of the underlying epileptogenicity of the tissue.

More recently, Heinz Beck’s group (47) substantiated and extended these data by showing that the use-dependent block of voltage-dependent Na+ channels of dentate gran- ule cells by CBZ is completely lost in patients with CBZ-resistant TLE in comparison to patients clinically responsive to this AED (Table 3). In addition to the loss of use-dependent inhibition of Na+ channels by CBZ, the fast recovery from inactivation of the fast Na+ cur- rent was CBZ insensitive in pharmacoresistant patients, whereas recovery was markedly slowed in cells from CBZ-responsive patients (47). Consistent with these data from patients with intractable TLE, Remy et al. (47) also showed that use-dependent block of Na+ channels by CBZ is absent in the pilocarpine rat model of TLE (Table 3). Based on these data, the authors suggested that a loss of Na+-channel drug sensitivity may explain the develop- ment of drug-resistant epilepsy. In a subsequent study in the rat pilocarpine model in TLE, Remy et al. (49) demon- strated that the effects of PHT on fast recovery from inac- tivation of Na+ channels of hippocampal granule neurons were significantly reduced, although not as pronounced as observed with CBZ (Table 3), substantiating the notion that reduced pharmacosensitivity of Na+ channels may contribute to the development of drug resistance. In con- trast to CBZ and PHT, LTG slowed the time course of recovery from fast inactivation both in epileptic and con- trol rats without significant intergroup differences (Table 3; 49). Valproate (VPA) did not appear to alter the fast recovery from inactivation of Na+ channels in either ex- perimental group (49). In contrast to these findings from dentate granule cells, slowing of fast recovery from inac- tivation of Na+ channels by VPA has been described for CA1 neurons from both patients and rats (46,48). In the latter studies, this effect of VPA was not different between patients with or without mesial temporal lobe sclerosis or between kindled rats and controls.

To evaluate which molecular and functional changes in voltage-dependent Na+ channels may underlie the lost or reduced pharmacosensitivity of these channels in the pilocarpine model of TLE, Ellerkman et al. (50)

Epilepsia, Vol. 46, No. 6, 2005

studied the expression of Na+-channel subunits in this model. Voltage-dependent Na+ channels are composed of a pore-forming α subunit associated with two auxiliary β subunits (44,51). Up to now, nine different α subunits have been identified that have been termed Nav 1.1–1.9. Of these, Nav1.1–1.3, 1.5, and 1.6 have been shown to be expressed in the brain. α Subunits have four homologous domains (I to IV) containing six transmembrane α helices (S1–S6). AEDs affect Na+ channels by binding to a recep- tor site in segment S6 in domains III and IV in the pore of the channel (44,51). In epileptic rats, the changes in Na+- channel pharmacosensitivity were associated with down- regulation of the accessory β 1 and β 2 Na+ -channel sub- unit messenger RNA (mRNA) as well as the pore-forming Nav 1.2 and Nav 1.6 subunit mRNA in dentate granule cells (50). Na+-channel β subunits are multifunctional (52). They modulate channel gating and regulate the level of channel expression at the plasma membrane. Downregu- lation of β subunits in the dentate gyrus has been found in a number of epilepsy models, and a mutation in the β1 subunit gene (SCN1B) has been linked to generalized epilepsy with febrile seizures plus type 1 (GEFS+1) in a human family with this disease (52). The downregula- tion of β subunits observed by Ellerkmann et al. (50) in epileptic rats may constitute a candidate mechanism on the molecular level to explain the loss of efficacy of CBZ and PHT on voltage-gated Na+ channels observed in these rats (Table 3). However, so far, these findings are purely correlative, and a prospective experiment linking altered expression of a particular subunit to drug insensitivity of the channel complex is still lacking.

In general, several open questions and inconsistencies exist with regard to the target hypothesis. First, the loss of the CBZ effect on Na+ channels in kindled rats reported by Vreugdenhil and Wadman (48) was only transient. Five weeks after kindling, the inhibitory effect of CBZ on Na+ channels had recovered to a level not different from that of controls. Second, in contrast to studies on Na+ chan- nels from patients with drug-refractory epilepsy, rats in the studies of Wadman’s and Beck’s groups were not pre- selected with respect to their in vivo pharmacosensitivity to AEDs. Seizures in kindled rats are highly responsive to CBZ in vivo (53), so that the transient loss of the CBZ in vitro effect on Na+ channels of CA1 neurons reported by Vreugdenhil and Wadman (48) is not associated with a poor or absent in vivo response to the CBZ anticonvul- sant efficacy. Similarly, despite the loss or reduction of the CBZ and PHT modulatory effects on dentate granule cells reported for epileptic rats of the pilocarpine model of TLE (47,49), spontaneous recurrent seizures in these rats can be successfully suppressed by CBZ and PHT in vivo (54).

However,asshownbyLo ̈scher’sgroup,ratsfromboth the kindling and pilocapine models of TLE exhibit marked interindividual differences in AED responsiveness. Thus by repeated testing of amygdala-kindled rats with PHT,

three subgroups can be selected, responders, nonrespon- ders, and variable responders (55). Nonresponders do not show any anticonvulsant response after treatment with maximal tolerable doses of PHT, although plasma drug levels are the same as those in responders (55). The PHT resistance of kindled nonresponders extends to other ma- jor AEDs, including CBZ, VPA, and LTG (55). Similarly, AED responders and nonresponders can be selected from the pilocarpine model of TLE (56). For proof of principle of the target hypothesis, it would therefore be important to compare the pharmacosensitivity of Na+ channels of re- sponders and nonresponders selected from TLE models.

Such a comparison was recently published for the rat kindling model (57). Responders and nonresponders were selected by repeated testing with PHT in vivo, followed by evaluation of the PHT in vitro effects on Na+ and Ca2+ channels of hippocampal CA1 neurons (57). PHT resis- tance was not associated with altered tonic block of Na+ channels by PHT, but recovery from Na+-channel inacti- vation and use-dependent blocking effects were not stud- ied.

As a proof of principle for the target hypothesis, it will be important to demonstrate that AED-resistant subgroups of epileptic rats differ from AED-responsive subgroups in their AED-target sensitivity. Such a proof of princi- ple is difficult to obtain in patients, because, in contrast to patients with intractable epilepsy, patients responding to AEDs in general do not undergo surgical treatment for their epilepsy. Although Remy et al. (47) obtained surgical “reference” specimens from two patients who responded well to treatment with CBZ for comparison with 10 pa- tients with CBZ-resistant TLE, differences in age, gender, history of epilepsy, AED treatment, and various other vari- ables may form a bias for such a comparison.

Another problem with the target hypothesis is the appar- ent paradox that although the CBZ and PHT modulatory effects on voltage-dependent Na+ channels were lost or partially lost in the pilocarpine model of TLE, LTG signif- icantly retarded recovery from inactivation of Na+ chan- nels in this model without a difference from control (Table 3; 47,49), although all three AEDs are thought to act by the same mechanism(s) (Table 2). Furthermore, these ob- servations from the pilocarpine model are not consistent with the clinical situation, because many patients who are resistant to CBZ or PHT are also resistant to LTG (4). The observations of Vreugdenhil et al. (46) on the VPA effects on sodium currents in neurons isolated from patients with pharmacoresistant TLE also argue against alterations in the Na+ channel as the only explanation for intractability in these patients.

In addition to changes in AED efficacy as a result of epilepsy-associatedalterationsinthedensity,distribution, or molecular structure of voltage-operated ion channels, drug efficacy may be altered as a result of ion-channel mutations underlying some familial forms of epilepsy (8).

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866 D.SCHMIDTANDW.LO ̈SCHER

An increasing number of channelopathies, including mu- tations in voltage-gated and neurotransmitter-gated ion channels, are being identified in human genetic epilep- sies (58,59). However, if and how these mutations may alter drug response has not been clearly defined as yet. Apart from mutations of ion channels, polymorphisms in ion-channel genes may play a role in altered AED respon- siveness, as recently indicated for the SCN1A neuronal sodium channel gene (60), although the latter data are as yet available only in abstract form.

GABAA receptors

Apart from voltage-dependent ion channels, other drug targets may be altered in patients with intractable epilepsy. As shown in Table 2, GABA-mediated inhibition repre- sents an important AED target (44). The type A (GABAA) receptors mediate the majority of fast inhibitory neuro- transmission in the brain, and alterations in GABAA- receptor function are believed to be involved in the pathol- ogy of several neurologic and psychiatric illnesses, such as epilepsy, anxiety, Alzheimer disease, and schizophrenia. GABAA receptors can be assembled from seven distinct subunit families defined by sequence similarity: alpha(1- 6), beta(1-3), gamma(1-3), delta, pi, theta, and rho(1-3) (61). Most GABAA-receptor subtypes in the brain are be- lieved to be composed of alpha, beta, and gamma sub- units. The role of the other subunits, which have a very limited expression pattern in the brain, remains to be de- termined, but it is possible that they substitute for the gamma-subunit in alpha-beta-gamma combinations (61). The different GABAA-receptor subtypes can be distin- guished by their channel kinetics, affinity for GABA, rate of desensitization, subcellular positioning, regional dis- tribution, and pharmacology (61). The major GABAA- receptor subtype (60% of all GABAA receptors) is assem- bled from the subunits alpha1beta2gamma2, with only a few brain regions lacking this receptor. This receptor sub- type mediates to a large extent the anticonvulsant action of benzodiazepines (BZDs), whereas, for instance, alpha4- or alpha6-containing subunit assemblies are insensitive to BZDs and other BZD site agonists such as zolpidem (61,62). Thus any change in the subunit composition of GABAA receptors can have dramatic consequences for the anticonvulsant efficacy of BZDs and possibly other AEDs that act via the GABAA receptor (Table 2).

A number of studies have shown that epilepsy (clini- cal or experimental) is associated with large changes in GABAA-receptor subunit expression. In human TLE and post–status epilepticus models of TLE, such as the pilo- carpine model, decreased GABAA -receptor subunit stain- ing, reflecting cell loss, is observed in the hippocampal for- mation, but a reorganization of distinct GABAA receptor subtypes is found in surviving hippocampal neurons, un- derlining the potential for synaptic plasticity in the GABA system (66–74). These changes in GABAA-receptor sub-

unit expression correlate with profound alterations in re- ceptor function and pharmacology (74–76).

In normal dentate granule cells, GABAA receptors are insensitive to zinc, which is released from mossy fibers and functions as a negative allosteric modulator of GABAA receptors. This zinc insensitivity of normal GABAA re- ceptors is a result of high levels of expression of the al- pha1 subunit in these cells (76). In epileptic rats, expres- sion of the alpha1 subunit decreases, and expression of alpha4 and delta subunits increases, leading to an assem- bly of GABAA receptors that are strikingly zinc sensitive. In addition to the enhanced zinc sensitivity, GABAA re- ceptors from the epileptic hippocampus lose their sensi- tivity to augmentation by the BZD-type site I modulator zolpidem (77). Coulter (75,76) proposed that this tem- poral and spatial juxtaposition of these pathophysiologic alterations may compromise normal “gatekeeper” func- tion of the dentate gyrus through dynamic zinc-induced failure of inhibition, predisposing the hippocampal circuit to generate seizures. Of course, assuming that similar al- terations in GABAA -receptor function and pharmacology also take place in the epileptogenic human hippocampus, this could lead to reduced efficacy of AEDs acting via GABA-mediated inhibition (Table 2).

The best evidence that changes in GABAA recep- tors, such as those occurring during epileptogenesis, can lead to drug resistance comes from a series of studies of Bob Macdonald’s group with the pilocarpine model (78–80). The latter group demonstrated that during a pilocarpine-induced status epilepticus, a substantial re- duction of potency occurs for termination of seizures by AEDs that enhance GABAA-mediated inhibition, such as BZDs and phenobarbital (PB). This progressive devel- opment of pharmacoresistance during a sustained status epilepticus is paralleled by alterations in the functional properties of dentate granule cell GABAA receptors. The authors concluded that rapid modulation of GABAA re- ceptors during status epilepticus may result in pharma- coresistance to AEDs that enhance GABAA receptor– mediated inhibition (80).

However, despite the plastic changes in GABAA- receptor subunits demonstrated in rats from the pilo- carpine model of TLE, spontaneous recurrent seizures de- veloping after the status epilepticus in such rats can be suppressed by drugs such as PB or VPA (54), which, at least in part, act via effects on GABA-mediated inhibition (Table 2). Interestingly, the novel AED LEV has been shown to reverse the inhibitory effects of the nega- tive allosteric modulators zinc and β-carbolines (such as DMCM) on GABAA receptor–mediated responses (81). Prolonged administration of LEV in rats with spontaneous recurrent epileptic seizures developing after a pilocarpine- induced status epilepticus has been found to be highly efficacious to block the seizures in part of the animals (56). However, another part of the epileptic rat cohort was

Epilepsia, Vol. 46, No. 6, 2005

PHARMACORESISTANCE 867

The multidrug transporter hypothesis of drug resistance

A. Normal expression

of multidrug transporters

Neuron

mdt

Blood

Endothelial cell

Extracellular space

Glial cell

FIG. 3. A: Schematic representation of brain capillary endothelial cells that form the blood–brain barrier (BBB) and the role of multidrug transporters (mdts) in drug transport through the BBB. Unlike endothelial cells in most tissues, endothelial cells of capillaries in the brain are joined by tight junctions and lack intercellular pores and pinocytotic vesicles. The capillary endothelium is surrounded by a basement membrane (not illustrated) and a sheath of processes from perivascular astrocytes (glial endfeet), which contribute to the function of the BBB. The functional consequence of these features is that brain capillaries act in a passive manner to restrict penetration of hydrophilic, ionized (polar), or large substances, but highly lipophilic drugs (like most antiepileptic drugs) penetrate easily through the BBB by simple diffusion (dashed arrows). As an active defense mechanism of the BBB against lipophilic substances, adenosine triphosphate–dependent mdts, such as P-glycoprotein (Pgp) or MRP2, which are located in the apical (luminal) cell membrane of capillary endothelial cells, act as outwardly directed active efflux pumps, transferring part of the drug, which has entered the endothelial cells by diffusion, back into blood, thus limiting penetration of many lipophilic drugs into brain parenchyma. Furthermore, by reducing drug concentration in the endothelial cells, mdt proteins may indirectly promote flux from the brain extracellular space into endothelial cells, followed by extrusion into the blood (103). B: In epileptogenic brain tissue these multidrug transporters are overexpressed in capillary endothelial cells and astrocytes around blood vessels, so that now the glial endfeet may contribute to the barrier function as a “second line defense” mechanism. Furthermore, as shown for Pgp and MRP1, overexpression of multidrug transporters may occur in parenchymal astrocytes and presumably protects these cells from apoptosis (104). Recently, increased expression of Pgp was demonstrated in neurons of epileptogenic brain tissue, thus affecting AEDs, which act via intraneuronal targets (93).

B. Overexpression of multidrug transporters after seizures

Neuron

Endothelial cell

mdt

Blood

Glial cell

Expression:

In endothelial cells

In perivascular astrocytes In parenchymal astrocytes In neurons

Drug uptake Drug extrusion

Protection from apoptosis?

Extracellular space

Drug penetration

resistant to LEV, which—in view of the reversal of zinc’s effects on GABAA receptors produced by LEV—does not seem to support the idea that altered zinc sensitivity of GABAA receptors may be associated with pharmacoresis- tance. Thus whether the plastic changes in hippocampal GABAA receptors found in rat models of TLE and patients with TLE are predominantly involved in epileptogenesis or also can lead to drug resistance of spontaneous seizures developing as a consequence of epileptogenesis remains to be determined.

In view of the changes in subunit composition of voltage-dependent Na+ channels and GABAA receptors in the epileptogenic hippocampus, it is tempting to spec- ulate that development of new AEDs that act specifically on these altered targets would be an interesting strategy for treatment of intractable epilepsy (26). However, as dis- cussed earlier, various inconsistencies exist in the target hypothesis that must be resolved before this hypothesis can form the basis of a new “rational” strategy for drug development. Furthermore, the fact that most patients re- sistant to AED treatment are resistant to a broad range of

AEDs with different mechanisms of action suggests that other, less specific mechanisms contribute to drug resis- tance (9). The most prominent hypothesis in this respect, the multidrug-transporter hypothesis, was first explored in chemotherapy-resistant cancer but currently attracts grow- ing interest as a putative mechanism to explain drug re- sistance in epilepsy by reduced penetration of AEDs into the brain (9).

MULTIDRUG TRANSPORTER HYPOTHESIS

The BBB (Fig. 3A) and the blood–cerebrospinal fluid (CSF) barrier (BCSB) form very effective barriers to the free diffusion of many hydrophilic, polar drugs into the brain (82). In general, the more lipid soluble a drug, the more readily it penetrates into the brain. However, a sig- nificant number of lipid-soluble molecules, among them many useful therapeutic drugs, have lower brain perme- ability than would be predicted from a determination of their lipid solubility. These molecules are substrates for drug-efflux transporters, which are present in the BBB

Epilepsia, Vol. 46, No. 6, 2005

Glial endfoot

Glial endfoot

868 D.SCHMIDTANDW.LO ̈SCHER

TABLE 4. Overexpression of multidrug transporters in epileptogenic brain tissue

Multidrug transporter overexpressed in

Capillary

endothelial cells Astrocytes Neurons

+ + + + + +

? + + + ? ? − + +

+ + ? − ? ? + ? ? − − −

Multidrug transporter (protein and/or gene)

P-glycoprotein (MDR1)

MRP1

MRP2 MRP3 MRP5 BCRP

Epileptogenic brain tissue from

Patients

Rats (kainate)

Rats (pilocarpine) Rats (kindling) Patients

Patients Patients Patients Patients

Reference

Tishler et al. (85); Sisodiya et al. (86,87); Dombrowski et al. (88); Aronica et al. (89,90)

Zhang et al. (91); Seegers et al (92); Volk et al. (93,94)

Volk et al. (93)

Volk et al. (94); Potschka et al. (95) Sisodiya et al. (87,96); Dombrowski et al.

(88); Aronica et al. (89)

Dombrowski et al. (88); Aronica et al. (89) Dombrowski et al. (88)

Dombrowski et al. (88)

Sisodiya et al. (97)

With respect to the data on epileptogenic brain tissue from patients, it should be noted that the expression of drug transporters may be pathology specific. A detailed summary of data on overexpression of efflux transporters in different types of epileptogenic lesions (hippocampal sclerosis, focal cortical dysplasia, tuberous sclerosis, dysembryoplastic neuroepithelial tumors, ganglioglioma, Rasmussen encephalitis) has been recently presented in a review by Kwan and Brodie (98).

and BCSB, and the activity of these transporters very ef- ficiently removes these drugs from the CNS, thus limiting brain uptake (Fig. 3A). P-glycoprotein (Pgp) was the first of these adenosine triphosphate (ATP)-binding cassette (ABC) transporters to be described, followed by the mul- tidrug resistance–associated proteins (MRPs) and more re- cently breast cancer–resistance protein (BCRP) (82–84). All are expressed in the BBB and blood–CSF barrier and combine to reduce the brain penetration of many drugs. This may create a major hurdle when it comes to the de- livery of therapeutics to the brain, particularly when these efflux transporters are overexpressed, which may lead to the phenomenon of “multidrug resistance” known from cancer chemotherapy (82).

As shown in Table 4, accumulating evidence demon- strates that multidrug transporters such as Pgp and mem- bers of the MRP family are overexpressed in epileptogenic brain tissue surgically resected from patients with medi- cally intractable epilepsy (98–102). In this tissue, overex- pression of Pgp and/or MRPs has been found in capillary endothelial cells that form the BBB and in astrocyte foot

processes that add to BBB function (Fig. 3B; Table 4). Furthermore, increased expression of these efflux trans- porters has been reported in BBB and brain parenchyma of rat models of TLE such as the kindling, pilocarpine, and kainate models (Table 4; 102).

When discussing putative transporter roles in pharma- coresistance, an important aspect is related to the orien- tation (apical vs. basolateral) of drug transporters in cel- lular membranes. A function in decreasing brain uptake of drugs can be ascribed only to those transporters facing the luminal side of the brain microvasculature (83,84). Among the multidrug transporters that have been shown to be overexpressed in epileptogenic brain tissue of pa- tients with intractable epilepsy (Table 4), Pgp and most MRPs (MRP1, MRP2, and MRP5) are expressed at the apical (luminal) side of brain capillary endothelial cells, which is the appropriate position for an efflux transporter to restrict brain uptake of drugs. MRP1 and MRP5 have generally been considered to be expressed in the basolat- eral membrane of polarized epithelial cells (118). How- ever, a recent study, using centrifugal separation of apical

TABLE 5. Active transport of antiepileptic drugs by multidrug transporters in the brain Multidrug transporter in the brain

Antiepileptic drug

Phenytoin Carbamazepine Valproate Phenobarbital Felbamate Lamotrigine Gabapentin Topiramate Levetiracetam

MRPs Other + + ?

? + +

+ − ?

+ − ?

+ − ?

+ ? +

+ ? ?

− − ?

Reference

Potschka and Lo ̈scher (105,106); Rizzi et al. (107); Potschka et al. (108) Potschka et al. (109); Rizzi et al. (107); Sills et al. (110)

Frey and Lo ̈scher (111); Huai-Yun et al. (112); Gibbs et al. (113) Potschka et al. (114); Potschka et al. (108)

Potschka et al. (114,115) Potschka et al. (114,115)

Luer et al. (116); Sills et al. (110) Sills et al. (110)

Potschka et al. (117)

P-glycoprotein

+ + ?

Epilepsia, Vol. 46, No. 6, 2005

and basolateral membranes of brain microvessel endothe- lial cells with subsequent Western Blot analysis as well as confocal laser scanning microscopy, demonstrated a predominantly apical localization of MRP1 and MRP5 in these cells (119). These data gave first evidence that the localization of MRPs in endothelial cells forming the BBB may differ from that observed in polarized epithelial cells.

In addition to overexpression of multidrug transporters in the BBB, many of the available pathology studies have demonstrated overexpression of Pgp or MRPs in nonen- dothelial cells of epileptogenic lesions (e.g., in dysplastic neurons or reactive astrocytes) that are not part of the BBB (Table 4, Fig. 3B). It has been proposed that the overex- pression of multidrug transporters in such nonendothelial cells may serve a drug efflux–independent cytoprotective function by inhibiting apoptosis (93,104). However, over- expression of Pgp in neurons could affect AEDs that act via intraneuronal targets by limiting the penetration of such drugs into neurons (93).

Pgp and MRPs that are apically localized in the BBB are thought to act as an active defense mechanism, restricting the penetration of lipophilic substances into the brain (82). A large variety of compounds, including many lipophilic drugs, are substrates for either Pgp or MRPs or both (102). It is thus not astonishing that various AEDs, which have been made lipophilic to penetrate into the brain, seem to be substrates for multidrug efflux transporters in the BBB (Table 5). Overexpression of such transporters in epilep- togenic tissue is likely to reduce the amount of drug that reaches its target(s) in this tissue, which would be a plausi- ble explanation for pharmacoresistance (98–100). In line with this concept, recent data from the kindling model of TLE show that pharmacoresistant rats selected from this model have significantly higher expression of Pgp in capillary endothelial cells of the epileptic focus (the ipsi- lateral amygdala) than do pharmacosensitive rats (95). No such overexpression was seen in adjacent brain regions, explaining that PHT-resistant kindled rats lack the anti- convulsant but not the adverse effects of PHT (55). Over- expression of Pgp (and MRPs) in focal (epileptogenic) but not parafocal tissue has also been shown for patients with intractable TLE (101) and would explain that such patients exhibit the same central side effects of AEDs as do pharmacosensitive patients, but lack the antiepileptic effect because uptake into epileptogenic brain tissue is reduced by overexpression of multidrug transporters.

We previously evaluated various AEDs with diverse mechanisms in PHT-resistant kindled rats (55). Except for LEV, all AEDs were less efficacious in PHT nonre- sponders than in PHT responders (55). Our recent finding of increased Pgp expression in the kindled amygdala of PHT-resistant vs. PHT-responsive kindled rats (95) thus prompted us to compare whether inefficacy of AEDs in this model is correlated with efflux transport by Pgp in the BBB. All AEDs that are substrates for Pgp (see Table 5) are

inefficacious in PHT-resistant kindled rats with increased expression of Pgp, whereas LEV, which is not a substrate for Pgp in rats (117), is very efficacious to block seizures in these rats. This is, of course, only a correlative finding, which, however, is in line with the multidrug-transporter hypothesis.

Pgp and MRPs can be blocked by specific inhibitors, which raises the option to use such inhibitors as adjunctive treatment for medically refractory epilepsy (99,101). Fur- thermore, bypassing multidrug transporters in the BBB by specifically designed AEDs or preparations of AEDs may be an attractive therapeutic option (101). However, al- though overexpression of multidrug transporters is a novel and reasonable hypothesis to explain multidrug resistance in epilepsy, it is clear that further studies are needed to establish this concept (98,101).

Most experimental and clinical studies on the multidrug-transporter hypothesis of intractable epilepsy have examined the expression and function of Pgp in the brain, whereas only relatively few studies dealt with other multidrug transporters, such as MRP1 or MRP2 (Table 4). In animal models of TLE, seizures have been shown to induce a transient overexpression of Pgp in the hip- pocampus and other brain regions thought to be involved in seizure initiation and propagation (102). Consistent with the multidrug-transporter hypothesis, after kainate- induced seizures in mice, the brain/plasma ratio of PHT was significantly reduced in the hippocampus at time of maximal overexpression of the gene (mdr1) encoding for Pgp (107). As noted earlier, overexpression of Pgp in amygdala-kindled rats was associated with a loss of phar- macosensitivity to PHT and various other AEDs (95). The transient overexpression of Pgp observed after different types of experimentally induced seizures in rodents in- dicates that overexpression of Pgp and presumably also other multidrug transporters can be acquired (i.e., develop as a consequence of uncontrolled seizures). In addition to acquired (seizure-induced) overexpression of multidrug transporters such as Pgp, overexpression may be intrin- sic (constitutive) (e.g., as a result of polymorphisms in the gene encoding Pgp) (120). Thus genetically increased expression of multidrug transporters could be associated with the de novo pattern of drug resistance in epilepsy.

Hitherto >50 single nucleotide polymorphisms (SNPs) and insertion/deletion polymorphisms in the large MDR1 (ABCB1) gene (209 kb) that encodes Pgp have been re- ported, and mutations at positions 2677 and 3435 have been associated with alteration of Pgp expression and/or function (121,122). A silent C to T transition in exon 26 of ABCB1 (3435C>T) has been associated with differences in Pgp levels and activity (123). This polymorphism has re- cently been associated with drug-resistant epilepsy (120). In the latter British study in 315 patients with epilepsy, classified as drug resistant in 200 and drug responsive in 115, patients with drug-resistant epilepsy were more

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870 D.SCHMIDTANDW.LO ̈SCHER

likely to have the CC genotype at the MDR1 C3435T polymorphism (120). Siddiqui et al. (120) concluded that their pharmacogenomic results identified a genetic fac- tor associated with resistance to AEDs. However, a recent study from Australia in 401 drug-resistant and 208 drug- responsive subjects with epilepsy failed to replicate an as- sociation of the CC phenotype at the ABCB1 C3435T poly- morphism with multidrug-resistant epilepsy (124). Tan et al. (124) concluded that the initial association published by Siddiqui et al. (120) may have arisen by chance. In a study from Austria (125), in which functional variants in ABCB1 were genotyped in 210 patients with TLE, a com- mon ABCB1 gene haplotype was identified that was signif- icantly associated with the risk for pharmacoresistance. It is not clear yet whether differences in the examined popu- lations may explain the apparent discrepancy between the two European studies and the Australian study.

Similar to the target hypothesis of intractability, a num- ber of open questions remain with regard to the multidrug- transporter hypothesis. First, similar to the target hypothe- sis, many of the reports relating to the transporter hypoth- esis are purely correlative, and any compelling proof of principle of this hypothesis is still lacking.

Second, not all AEDs are substrates for multidrug trans- porters such as Pgp. For instance, as mentioned earlier, LEV seems not be transported by Pgp or MRPs at the BBB (117). Interestingly, LEV was also the only AED that was equally efficacious in kindled PHT responders and non- responders (55). However, patients with epilepsy not re- sponding to major AEDs often are also not responsive to LEV, as shown in Table 1. It is clear from these studies that the multidrug-transporter hypothesis does not explain pharmacoresistance to AEDs in general. Furthermore, for several AEDs such as VPA, oxcarbazepine (OXC), zon- isamide (ZNS), ethosuximide (ESM), BZDs, vigabatrin (VGB), tiagabine (TGB), and pregabalin (PGB), it is still not known whether they are substrates for multidrug trans- porters at all. In addition, for most AEDs shown as sub- strates in Table 5, transport must be confirmed by dif- ferent techniques. For instance, our own work on trans- port of AEDs by multidrug transporters, using a brain microdialysis model in rats (cf. 99,102), can be criticized because of the lack of selectivity of some of the trans- port inhibitors used, so that we cannot exclude the possi- bility of other transporters actually mediating the results claimed to relate to the transporters such as Pgp shown in Table 5. In line with this criticism, existing data on transport of AEDs by multidrug transporters such as Pgp are not unequivocal. The best example is CBZ, for which three groups independently found evidence of transport by Pgp by using different experimental approaches in- cluding Pgp knockout mice (107,109,126), whereas Owen et al. (127) failed to demonstrate any transport of CBZ by Pgp. This example illustrates that studies on a pre- sumably weak Pgp substrate such as CBZ may lead to

controversial data, depending on the experimental tech- nique used for studying drug transport and the dose or concentration of the test drug. Furthermore, transport of a weak substrate in a tissue or cell culture with “normal” expression of the transporter may dramatically differ from transport in a tissue or cell culture in which the transporter is overexpressed, such as occurring in epileptogenic brain tissue.

Third, the molecular mechanisms underlying the over- expression of multidrug transporters in epileptogenic brain tissue are not sufficiently known. As mentioned earlier, experimental and clinical evidence indicates that increased expression of multidrug transporters such as Pgp may be either constitutive (intrinsic) or acquired (in- duced) (e.g., as a result of frequent seizures). Such seizure- induced, regionally restricted overexpression of multidrug transporters may be a second-line defense mechanism of the BBB because of transient BBB opening during seizures and chronic dysregulation of BBB function such as indicated by endothelial cell alterations, abnormal tight junctions, and thickening of the basal membrane in hu- man epileptic tissue (128–130). Thus overexpression of multidrug transporters may be one mechanism in the way seizures beget (drug-resistant) seizures (i.e., by impair- ing AED penetration into the brain). Although proba- bly uncommon and controversial, a progressive risk of seizures with increasing numbers of seizures has been shown in some patients (131,132). In addition, seizure- induced overexpression of multidrug transporters also may offer one possible explanation why a high frequency of seizures before treatment may negatively affect the re- sponse to AEDs (133). As indicated by animal data, such seizure-induced upregulation of multidrug transporters is transient (i.e., expression normalized within days to weeks in the absence of seizures). Conversely, AEDs themselves do not seem to induce increased expression of multidrug transporters in the brain (102).

Fourth, limited proof still exists that multidrug trans- porters are functionally important in human drug-resistant epilepsy (101). For direct proof of principle, it should be demonstrated that drug resistance can be reversed by ad- junctive treatment with a Pgp and/or MRP inhibitor, but only anecdotal data are available in this respect (134). In- hibitors of Pgp and, more recently, of MRPs are currently clinically evaluated for reversal or prevention of intrinsic and acquired multidrug resistance in human cancer (135). Systemic or local administration of such inhibitors also may prove useful in pharmacoresistant epilepsy, as indi- cated by the recent report of Summers et al. (134). Animal studies demonstrate that Pgp or MRP inhibition results in enhanced anticonvulsant efficacy of AEDs (102). Thus pharmacologic inhibition of Pgp or MRPs could form a novel clinical strategy to prevent and overcome drug re- sistance in patients with altered expression of multidrug transporters, but this requires validation.

Epilepsia, Vol. 46, No. 6, 2005

TABLE 6. Experimental evidence for the target and multidrug transporters hypotheses of intractable epilepsy

Substrate for multidrug transporters at the blood–brain barrier

multidrug-transporter hypotheses according to these cri- teria are summarized. Much less experimental and/or clin- ical evidence is available for other potential hypotheses.

OTHER HYPOTHESES

From a conceptual view, three general categories of pathomechanisms may be involved in intrinsic or acquired drug-refractoriness of epilepsy (for details, see 136):

• Genetics

– e.g., gene polymorphisms or inherited mutations

AED

Benzodiazepines +?

PHARMACORESISTANCE 871

Loss of target sensitivity

Carbamazepine Felbamate Gabapentin Lamotrigine Levetiracetam Phenobarbital Phenytoin Topiramate Valproate

+ + ? + ? + − + ? − ? + + + ? + − +

Disease-related mechanisms

– etiology of the seizures

– progression of disease

– structural brain alterations and/or network

changes

– alterations in drug target(s)

– alterations in drug uptake into the brain

Drug-related mechanisms

– loss of efficacy (tolerance)

– ineffective mechanism of drug action

As shown in Table 6, several major AEDs are affected by both loss of target sensitivity and decreased brain pen- etration as a result of increased expression of multidrug transporters in epileptogenic brain tissue. In view of the likely possibility that these two mechanisms of drug re- fractoriness can coexist in the same epileptogenic brain tissue, pharmacologic strategies for improved treatment of drug-refractory epilepsy may be complex. Furthermore, certainly other mechanisms contribute to the development of pharmacoresistance and have to be dealt with when thinking about effective therapeutic agents for hitherto in- tractable types of epilepsy. However, as recently pointed out by Sisodiya (101), for any postulated AED-resistance mechanism, a set of criteria have to be satisfied before its role in drug-resistant epilepsy can be accepted: (a) the mechanism must be detectable in epileptogenic brain tis- sue; (b) the mechanism must have appropriate functional- ity; (c) the mechanism must be active in drug resistance; and (d) overcoming the mechanism should affect drug resistance. In Table 7, data available for the target and

As discussed earlier, genetic factors such as polymor- phisms in genes encoding for drug targets or multidrug transporters may be important and explain why two pa- tients with the same type of epilepsy or seizures may dif- fer in their initial responses to AEDs. Apart from alter- ations in drug targets and drug uptake into the brain, other disease-related factors are certainly important to under- stand putative mechanisms contributing to the different clinical patterns of drug resistance (see Fig. 2), including the etiology of the seizures, progression of epilepsy un- der treatment with AEDs, and seizure-induced synaptic reorganization.

TABLE 7. Criteria for drug resistance mechanisms in epilepsy Criteria for drug-resistance mechanisms in epilepsy

Experimental Clinical Hypothesis evidence evidence

1. Mechanism is detectable in epileptogenic brain tissue Target hypothesis + +

Transporter hypothesis + +

2. Mechanism has appropriate functionality to mediate drug resistance Target hypothesis + + Transporter hypothesis + ?

3. Mechanism is active in drug resistance in vivo

Target hypothesis ? ? Transporter hypothesis + ?

References (examples)

Vreugdenhil et al. (46); Vreugdenhil and Wadman (48); Remy et al., (47,49)

Tishler et al. (85); Sisodiya et al. (86,87); Zhang et al. (91); Dombrowski et al. (88); Rizzi et al. (107); Seegers et al. (92); Aronica et al. (89,90); Volk et al. (93,94)

Vreugdenhil et al. (46); Vreugdenhil and Wadman (48); Remy et al. (47,49) Rizzi et al. (107); Potschka et al. (117)

Sills et al. (110)

4. Overcoming the mechanism counteracts drug resistance

Target hypothesis ? ?

Transporter hypothesis ? (+) Summers et al. (134)

Adapted from Sisodiya (101).

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872 D.SCHMIDTANDW.LO ̈SCHER

Furthermore, drug-related factors are most likely in- volved in insufficient seizure control, including loss of an- ticonvulsant efficacy during treatment after initial control (i.e., development of tolerance to the anticonvulsant effect) or ineffective mechanisms of action of currently available AEDs in difficult-to-control types of epilepsy, which may contribute to de novo drug resistance. Although reduction in severity of adverse effects during prolonged treatment with AEDs is a well-known phenomenon in the treatment of patients with epilepsy (137), the possibility that the antiepileptic effect of an AED (other than BZDs) may decrease during treatment and thus contribute to delayed- onset drug resistance as a result of development of toler- ance to the anticonvulsant effect is commonly dismissed by most neurologists in epilepsy therapy. A recent anal- ysis of add-on studies in patients with refractory partial epilepsy showed that in almost 30% of the patients, the added AEDs [including CBZ, PHT, LTG, and gabapentin (GBP)] were associated only with a temporary response, indicating development of tolerance to the anticonvulsant effect (138). Such tolerance can be predicted from long- term studies in rodents (53,137) and should be considered a factor in drug-refractory seizures.

Several types of epilepsy, most notably TLE, are known to be associated with complex alterations in the mor- phology and function of neural circuits in the hippocam- pus and other brain regions (13,139,140), leading to an epileptic network that may be or may become insensi- tive for as yet unknown reasons to AED actions. Con- sistent with this view, medical treatment fails in 75% of patients with mesial TLE (18). Brain abnormalities, par- ticularly hippocampal sclerosis, have been assumed to be a major prognostic factor for failure to control seizures (24). However, as discussed earlier in more detail, it is un- clear whether hippocampal sclerosis or progression of hip- pocampal sclerosis is in fact associated with pharmacore- sistance in patients with TLE. In addition to hippocampal sclerosis, malformations of cortical development seem to be responsible for many cases of refractory epilepsy in children and adults (87). Despite its intuitive attractive- ness, the network hypothesis of drug-refractoriness must be confirmed by studies showing that responders and non- responders differ in terms of network functions. An inher- ent problem for investigating altered neuronal network sensitivity to AEDs is that these alterations may be ab- sent when the same neurons are isolated from the network where emergent properties are lost (141). Based on the idea of emergent properties of neuronal networks as tar- gets for AEDs (141), examination of network mechanisms of AEDs in epilepsy models may identify general mecha- nisms and emergent targets for drug action and how these drug actions are altered by changes in the network such as occurring in TLE.

In addition to changes in networks or drug targets, metabolic changes in epileptogenic brain tissue may be

associated with altered drug responses. For instance, in patients with TLE, increased extracellular glutamate lev- els in the epileptogenic hippocampus both during and af- ter clinical seizures have been reported. These increased glutamate levels could be the result of malfunctioning or downregulation or both of glutamate transporters (142). Whether this alteration of glutamate uptake contributes to pharmacoresistance remains to be determined.

After the discussion of the major current hypotheses that try to explain drug resistance, we briefly assess the clinical evidence in support of these theories. How are novel AEDs achieving such often very beneficial seizure reduction in individuals with chronic refractory epilepsy (Table 1)? More specifically, what is the clinical evidence that altered drug targets or excessive transport of AEDs away from the epileptogenic target tissue are responsible for reduced sensitivity of the epileptogenic tissue to the effect of AEDs?

CLINICAL EVIDENCE FOR THE TARGET AND MULTIDRUG TRANSPORTER HYPOTHESES OF INTRACTABLE EPILEPSY

The evidence in humans for either a defective drug- target mechanism or a reduced brain drug penetration be- cause of increased drug-efflux transport capacity comes from samples of brain resected during epilepsy surgery in patients with surgically amenable epilepsy syndromes and many disabling pharmacoresistant seizures. The re- sults, therefore, do not automatically apply to the large number of patients with infrequent or less disabling drug- resistant seizures who as a consequence are not undergo- ing surgery. As shown earlier, adding a modern AED may achieve short-term seizure control in 5–8% of patients with partial epilepsy previously refractory to many AEDs and result in reduction of previous seizure frequency of ≥50% in as many as 51% of patients (see Table 1). In addition, a number of AEDs such as OXC, LTG, and TPM can be used as monotherapy in patients with refractory partial epilepsy (10). A proof-of-principle trial or at least a pilot trial in surgical candidates with coadministration of AEDs and a Pgp inhibitor showing a better seizure outcome in patients with refractory epilepsy is needed as suggestive evidence for the clinical relevance of the multidrug-transporter hy- potheses (Table 7). Now we have one single case report, in which the calcium channel blocker verapamil, which is a known inhibitor of Pgp, was used as add-on treatment in a patient with refractory partial epilepsy. The addition of verapamil reduced seizure frequency and improved sub- jective well-being (134). A randomized controlled trial with a more selective Pgp inhibitor is needed to support this observation. The addition of verapamil may increase the plasma concentration of AEDs such as CBZ, or may have intrinsic antiepileptic activity independent of its role on Pgp (143). As long as we depend on brain tissue for

Epilepsia, Vol. 46, No. 6, 2005

examination of transporter mechanisms, by default, we are able to test the hypothesis only by examination of re- sected brain samples taken from patients with refractory epilepsy undergoing epilepsy surgery. We have currently no way to test whether a patient previously refractory to a given AED who responds to substitution or addition of another AED does so because the efficacious AED is not affected by upregulated multidrug transporters (in contrast to impaired brain penetration for the previous AED). In addition, we have no compelling evidence that the AED tissue concentration is reduced in patients not responding to an individual AED. Although the alterations in mul- tidrug resistance proteins clearly warrants further investi- gation, no conclusive evidence exists that increased brain expression of such efflux transporters is a major source of pharmacoresistant epilepsy. In view of this limited proof that multidrug transporters are functionally important in human drug-resistant epilepsy, new techniques for study- ing the function of these transporters in the epileptic brain are urgently needed. One promising technique in this re- spect is positron emission tomography (PET), using ra- diolabeled probes for Pgp function, thus allowing study of how Pgp affects uptake of drugs into the brain (144). Studies in experimental animals have indicated that it is possible to assess Pgp function in the BBB and its effect on the uptake and binding of drugs within the intact brain, by using suitable Pgp modulators labeled with positron emit- ters. Provided that radiopharmaceuticals (and Pgp modu- lators) can be developed for human use, the role of Pgp in drug-refractory epilepsy may be explored in patients, including exploration of the relation between polymor- phisms of transporter genes and the pharmacokinetics of test compounds within the brain (144).

Even if the AED reaches the site of action in the brain at sufficient concentrations, reduced sensitivity to the molec- ular target may form another obstacle to the therapeutic ac- tion, as suggested by the reduced target-sensitivity theory. As soon as suitable pharmacologic agents are available that can reverse the reduced target sensitivity or are tai- lored to act directly on the altered target, the target hypoth- esis can be tested in clinical trials of surgical candidates. If surgical samples would be able to show differences in target sensitivity among AEDs, it might explain why pa- tients not responding to CBZ or PHT are able to respond to OXC and vice versa (145). Unless and until such clinical– experimental correlations are available, it is very difficult to evaluate the clinical relevance of the target hypothesis. Taking into account that both altered target and transport mechanisms may coexist for the same AED, as pointed out in Table 6, for example in the case of CBZ, it is difficult to understand why newer AEDs, which are only affected by one of these mechanisms, are not more efficacious than CBZ. None of the modern AEDs for partial epilepsy is more efficacious than CBZ (146). This fact alone suggests that other mechanisms of refractoriness must be operative

in addition. In spite of several limitations outlined earlier, both the poor focus penetration and the insensitive-target hypotheses have significantly improved our understand- ing of the complex mechanisms of pharmacoresistance in partial epilepsy and to develop concepts for proof-of- principle animal experiments and clinical studies. More specifically, these hypotheses have offered possible ex- planations for the occurrence of various clinical patterns of pharmacoresistance in epilepsy.

With respect to the patterns of drug resistance in epilepsy discussed in this review, theoretically two of the three patterns shown in Fig. 2 could be explained, at least in part, by the drug-target and multidrug-transporter hypotheses. Thus in view of animal data convincingly demonstrating that seizures can induce overexpression of multidrug transporters in the brain, a high seizure fre- quency before onset of AED therapy could be associated with de novo AED resistance because of upregulation of drug-efflux transporters in epileptogenic brain tissue. In- deed, the number of seizures before onset of therapy and a high seizure frequency belong to the clinical factors that have been associated with intractability (7). Recur- rent seizures and status epilepticus also have been shown to induce drug-target alterations in the brain of experimen- tal animals, so that such alterations also likely are involved in de novo resistance. Apart from such acquired (induced) alterations in drug targets or multidrug transporters, these alterations may be intrinsic (constitutive) (e.g., because of genetic polymorphisms), and thus could contribute to de novo AED resistance. The second, less frequent pat- tern (i.e., progression from remission to resistance) may result from the morphologic and functional alterations in brain targets and epileptic circuits associated with pro- gression of epilepsy despite AED treatment (147–149). Once seizures reoccur because of such progression, they are likely to induce expression of drug-efflux transporters, so that both drug-target and drug-transporter alterations may be involved in this second pattern of resistance, too. The third pattern of resistance with intermediate periods of remission (Fig. 2) is difficult to explain on the basis of current neurobiologic theories.

CONCLUSIONS

Although AEDs are very useful in blocking seizures, many patients do not respond adequately to these agents. A challenge for the scientific community is to determine the reasons for these AED failures and develop novel treat- ment strategies to overcome obstacles to seizure control. Any neurobiologic theory of drug resistance in epilepsy must explain why some patients with seemingly identi- cal seizure types or epilepsy will enter long-term remis- sion, whereas in others, AEDs cannot control seizures. Different patterns of pharmacoresistance exist. In most patients, the AEDs do not work at the time of onset,

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874 D.SCHMIDTANDW.LO ̈SCHER

whereas in others, pharmacoresistance develops over time, and in still others, significant seizure reduction is seen with the introduction of each modern AED in the man- agement of previously drug-resistant cases. Although it would be na ̈ıve to assume that two putative biologic theo- ries (which are relatively simple) are able to explain fully the clinical observation of de novo, progressive, or re- versible drug resistance, and the wax-and-wane course in some patients, these theories at least provide hypotheses to be tested in future studies. Although novel biologic mech- anisms have been postulated recently, largely surrounding the “target” and “transporter” hypotheses, which are re- viewed here, one must conclude that we need more hard data for final evaluation. In the absence of an obvious link between the “clinical” and “biologic” mechanisms, it is tempting to speculate that genetic and structural de- fects affect (in an as yet unexplained way or ways) drug target and drug transport or other AED-related mecha- nisms and result in constitutive (de novo) drug resistance. In some patients, disturbed plasticity or compromised en- dogenous antiepileptic mechanisms may lead to progres- sion into postnatally acquired drug resistance. In other pa- tients, intact plasticity or endogenous antiepileptic mech- anisms may attenuate mechanisms of drug resistance and allow the brain to respond to AEDs. If these speculations are correct, the mechanisms of pharmacoresistance may be intertwined with epileptogenic mechanisms, and rever- sal of pharmacoresistance may be functionally related to plasticity mechanisms and endogenous antiepileptogenic functions. Whatever the exact mechanisms may be, to en- hance our understanding about the mechanisms of phar- macoresistance in epilepsy, studies on brain tissue from drug-resistant patients and suitable experimental models of intractable epilepsy are required. Clinical trials with agents that may attenuate or reverse diminished target sensitivity or excessive transport mechanisms are needed to provide evidence on the relevance of putative mech- anisms in the wide spectrum of clinical presentations of drug-resistant epilepsy in humans. We also need more ev- idence from studies on epileptic brain tissue that AED target alterations and overexpression of multidrug trans- porters may be important mechanisms of pharmacoresis- tance. Furthermore, the long-term, progressive changes in neural networks that eventually provoke spontaneous and recurring seizures may lead to reduced pharmacosensitiv- ity. Studies in the kindling model of pharmacoresistant TLE have indicated that both genetic and disease-related factors may be involved in development of pharmacore- sistance (55).

Drug resistance is not unique to epilepsy or neurology, but occurs in other conditions such as arthritis or can- cer and may involve similar mechanisms in these differ- ent conditions. For instance, drug transport–based mecha- nisms are among the most intensively studied mechanisms of pharmacoresistance in oncology (150,151). As in on-

cology, study of the basis of drug resistance in epilepsy may allow prediction of poor response to AED treatment and should offer new rational approaches to treatment, for instance, by design of AEDs that are not targets for brain-expressed resistance mechanisms (101,136).

Acknowledgment: We thank Professor Matti Sillanpa ̈ a ̈ (Turku, Finland) for helpful discussions on the prevalence of drug-resistant epilepsy, and Ingeborg Wimmer (Nu ̈rnberg, Ger- many) for help with the literature search.

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