Pharmacoresistance: Modern Concept and Basic Data Derived from Human Brain Tissue

Multiple studies have established that approximately one third of patients with epilepsy have seizures that cannot be controlled satisfactorily with antiepileptic drugs (AEDs), but the data have not conclusively determined whether pharmacoresistance develops as a result of the disease process or exists at the time of the initial seizure (Fig. 1). The mechanism by which it occurs has not been firmly established, although it is believed to be a multifactorial process. Evidence suggests that drug resistance is present in the early stage of the disease, a concept that is potentially beneficial to patients. Epileptic seizures and epilepsies—therapeutic situation. The rationale for such a concept is clear from both a clinical and scientific perspective. Greater emphasis on identifying the AED most suited to the particular patient on the first attempt, when the likelihood of achieving seizure control is greatest, is one possible consequence. Recognition of the importance of maintaining a consistent course of therapy, with the optimal balance of efficacy, tolerability, and safety, and recognizing that even minor changes in dosage could result in seizure relapse, is another (1). It is crucial to develop an individual management strategy from the outset, based on a thorough assessment of patient-related factors (e.g., seizure type and epilepsy syndrome classification, comorbidities, and medical, psychosocial, and family history) and an understanding of the mechanisms of action, side effects profile, and interactions of various AEDs. Patients whose first AED trial is unsuccessful have only about a 10% chance of responding to future medication regimens (2), suggesting that initial failure is a strong predictor of refractory epilepsy. In a recent study of 470 drug-naïve patients, for example, 47% responded to initial treatment, primarily with a modest to moderate dose of carbamazepine (CBZ), valproate (VPA), or lamotrigine (LTG) (1). An additional 13% achieved seizure freedom with a second AED trial. Only 1% responded well to a third attempt at monotherapy, and an additional 3% achieved seizure control with a two-drug combination. Although there is emerging evidence that refractory seizures often respond to three-drug AED regimens, that was not the case for any patients in this study group (1,3) (Fig. 2). Antiepileptic drug (AED) regimens. Probability of seizure freedom in newly diagnosed patients with epilepsy according to the number of AED regimens. Dotted lines indicate 95% confidence intervals. Reprinted from ref. 1 with permission of Lippincott, Williams & Wilkins. The authors of the study recommend a staged approach to epilepsy management. Under such a system, a second AED is substituted for an initial antiepileptic agent that fails to improve seizure control or is poorly tolerated, while a second AED is added if the first is well tolerated and provides considerable improvement but fails to achieve complete seizure remission. Earlier consideration of surgical management, recommended for any patient who fails to respond to two generally well-tolerated treatment regimens, is another potential consequence of a concept of early pharmacoresistance (1). Although surgery is routinely delayed, often for more than two decades, because of the attendant risk of failure or worsened cognitive deficits, such an approach overlooks the perils of remaining in a pharmacoresistant state for many years. Long periods of refractory epilepsy are associated with a range of difficulties in psychosocial function, including dependent behavior, lifestyle restrictions, poor academic achievement, and a decline in self-esteem (1,4). The likelihood of cognitive impairment increases with seizure frequency, duration, and severity. Exposure to multiple AEDs often has an adverse effect on cognitive function as well, primarily resulting in impaired attention, vigilance, and psychomotor speed (5). The risks associated with antiepileptic agents are highest in patients subject to rapid initiation of drugs, higher doses of the medication, and higher anticonvulsant blood levels and polypharmacy, although seizure reduction may result in improved cognition, offsetting the untoward effects of the drugs. Some researchers also theorize that failed trials with AEDs contribute to the development of a pharmacoresistant state (6). The basis for this hypothesis lies in animal studies in which the administration of AEDs (i.e., LTG or CBZ) during kindling development has been found to reduce the efficacy of these drugs in fully kindled seizures (7, 8). Patients with intractable seizures also have a standardized mortality rate approximately five times that of members of the general population. The increased death rate is primarily because of the risk of sudden unexpected death in epilepsy (SUDEP) (9), but an elevated risk of suicide is also a factor (10). The incidence of injury and head trauma is also higher in patients with uncontrolled seizures (11). Approximately 20% of patients with refractory epilepsy are excellent candidates for epilepsy surgery, with a high likelihood of achieving seizure remission with minimal adverse effects. In patients with temporal lobe epilepsy (TLE), febrile convulsions in childhood, unilateral hippocampal sclerosis, and temporal hypometabolism are associated with a highly favorable postoperative prognosis, with a rate of Class I outcomes in the 80–90% range (12, 13). In EEG studies, predictors of positive outcomes include unilateral spikes restricted to the temporal lobe, late propagation and bilateralization of discharge (14), anterior temporal ictal onset (15), and ongoing discharge at the site of ictal onset until seizure termination. In such cases, there is little rationale for the delay of ≥15–22 years, which is the norm today (16, 17). In addition to the use of magnetic resonance imaging (MRI), positron emission tomography (PET) scan, and video EEG to assess eligibility for surgery, intraoperative MRI, an emerging technique now being used experimentally, increases the likelihood of positive outcomes. In resective surgery, it makes it possible not only to determine immediately whether the entire epileptogenic zone has been removed but also to enlarge the resection, if necessary, without subsequent surgery (18). Ironically, earlier consideration of surgical management has potential benefits for patients who are determined to be poor candidates for surgery as well. Combination therapy is recommended early in the disease process for such individuals, with AED selection based on potentially complementary mechanisms of action. A reassessment of the seizure diagnosis may also be indicated (5), as well as a review of medication compliance and an investigation of any lifestyle problems, such as substance abuse, that could be interfering with treatment (1). Vagal nerve stimulation (VNS), a new and relatively safe treatment modality for refractory epilepsy, should also be considered. The overall focus for such patients should be on improving the quality of life by detecting and treating any existing mood disorders and minimizing adverse effects of AEDs, as well as by reducing the frequency of seizures (16). Scientifically, a concept of pharmacoresistance remains problematic. Studies of human drug resistance are limited, as is an understanding of how, when, and where drug resistance in epilepsy develops. Knowledge of the process by which structural alterations occur after congenital and acquired brain injury is also crucial to clinicians' ability to provide neuroprotection, either by stopping the processes by which drug resistance develops or, ideally, preventing the development of epileptogenesis itself (19). A better understanding of the mechanisms leading to pharmacoresistance might make it easier to find the right drug or the right combination and sequence of AEDs as well. This review of human drug resistance in epilepsy focuses on three independent processes: the development of Ammon's Horn Sclerosis (AHS), the major neuropathological substrate in patients with TLE, the overexpression of multidrug transporters, and the alteration of drug targets. AHS (also known as mesial temporal epilepsy and hippocampal sclerosis), a common maldevelopmental disorder associated with TLE, is detected in ∼65% of patients with TLE (20). It has yet to be determined conclusively whether AHS is the cause or the consequence of chronic epileptic seizures and pharmacoresistant TLE. Likewise it is uncertain whether a predisposition is required to trigger the structural alterations that are believed to create a drug-resistant network. The use of a kindling model in animal studies has shown that neuronal damage and reorganization can be secondary to induced seizures; ongoing reorganization has also been detected in animals with spontaneous seizures (6,21). Biochemical, imaging, and neuropsychological studies similarly suggest that the structural alterations may be progressive, but it is not known how often this is the case. Although AHS is believed to be acquired, the point at which the structural reconstruction occurs has not been firmly established. Evidence suggests that the reorganization of the hippocampus occurs very early in life, but because epilepsy surgery is typically delayed, often for more than two decades, it has not been possible to verify this in studies of human brain tissue. However, molecular neuropathological studies of the developmental aspects of AHS have revealed abnormal organization of the dentate granule cell layer in young individuals with early onset of hippocampal seizures. In addition, most patients with AHS have a history of febrile seizures beginning early in life (20). It is not certain whether the associated cognitive deficits could be prevented by performing surgery at an early stage of the development of mesial temporal sclerosis. Segmental loss of pyramidal neurons in the CA1, CA3, and CA4 fields (sparing the CA2 sector), dispersion of the granule cell layer with limited loss of granular neurons of the gyrus dentatus, and reactive gliosis are the primary histopathological hallmarks of AHS (20,22). (The process underlying the regional selectivity is poorly understood.) As cells die, a recurrent excitation response develops, producing a neuron circuit with a lower seizure threshold. However, the cause of cell death remains uncertain. One hypothesis involves mitochondrial dysfunction, which has been the focus of recent investigation as a possible cause of pharmacoresistance and observed in the seizure foci of human and experimental epilepsy. When kainate was administered systemically to rats to induce seizures, for example, mitochondrial superoxide production in the hippocampus increased, often preceding neuronal death (23). An increase in 8-Hydroxy-2-deoxyguanosine, an oxidative lesion of DNA, was also observed in the rat hippocampus after kainate administration. In addition, experimental studies conducted on human brain tissue derived from patients who underwent epilepsy surgery revealed a loss in the glucose oxygen capacity of cells in the CA3 region, reflecting an alteration of the mitochondria in the area close to the primary epileptogenic zone and a breakdown of the cells' energy supply associated with epileptic attacks (22). The findings suggest that not only does mitochondrial oxygen-mediated oxidative stress have a leading role in seizure-induced neuronal death, but also antioxidants protective of mitochondrial targets may be a useful adjunct in the management of seizure disorders. Recent studies of the granule cell dispersion that characterizes AHS have focused on the persistence of Cajal-Retzius cells as a likely cause of the hippocampal reorganization, a finding that is highlighted in imaging studies showing markedly different patterns in the gyrus dentatus of a patient with AHS and the gyrus dentatus of a control. Located outside the gyrus prenatally, Cajal-Retzius cells are initially abundant. By birth, the number of cells is substantially diminished, and by adulthood, few remain. However, in patients with AHS, these cells persist, providing further evidence of an early insult (20). Although AEDs do not all have the same mechanisms of action, the majority of patients with refractory seizures are resistant to many anticonvulsants and, often, to virtually all of them. This suggests a nonspecific action, such as seizure-induced overexpression of multidrug–resistant proteins, as the primary cause of pharmacoresistant epilepsy (24). Research has focused on the gene P-glycoprotein (PGP) (MDR-1) and the multidrug resistance-associated protein (MRP-1), as well as on additional members of the MRP family (MRP-1 through MRP-7 have been identified) (2). The proteins were shown to correlate with cellular resistance of anticancer drugs before being investigated as a means of resistance to anticonvulsive agents. Drugs whose action occurs within the brain, including AEDs, are generally made to be highly lipophilic, a quality that makes it possible for them to easily penetrate the blood–brain barrier (BBB) via the lipid membranes (Table 1). In individuals with pharmacoresistant epilepsy, multidrug transporter proteins are believed to function as an active defense mechanism, preventing lipophilic substances from reaching the brain through either the BBB or blood-cerebrospinal fluid barrier (24). The proteins, which are thought to be located in the apical cell membrane of capillary endothelial cells, achieve that end by acting as outwardly directed efflux pumps that transfer the part of the drug that enters the cells by diffusion back into the blood. Studies of human brain tissue have shown that gene expression has taken place in glial cells and neurons that do not normally express these proteins (25), forming a pathway of pharmacoresistance that is independent of the origin of the epilepsy (e.g., the focal cortical dysplasia, hippocampal sclerosis, and dysembryoplastic neuroepithelial tumors) and lowering the interstitial concentration of AEDs in the vicinity of the epileptogenic pathology (26). Multidrug transporters respond differently to different AEDs. Animal studies of various drugs' rate of entry from blood to cerebrospinal fluid (CSF) reveal a significant correlation between the penetration rate and the degree of the drug's lipid solubility, which appears to be the primary factor affecting an anticonvulsant's entry rate (2). Intracellular phenytoin (PHT) levels in an MDR-1 expressing neuroectodermal cell line were found to be only one fourth of the level in MDR-1 negative cells, which suggests that PGP has a major role in the export of that drug (2,25). Numerous animal and experimental studies have established the role of PGP in the active transport of many AEDs, including gabapentin, topiramate, CBZ, phenobarbital, felbamate, and LTG. MRPs have been identified as active transporters of PHT and CBZ as well, and as the only known transporter of VPA (27, 28). Unlike most AEDs, and indeed most centrally active compounds, VPA (a medium-chain fatty acid that is not highly lipid soluble at physiologic pH) does not penetrate into the CSF by simple diffusion. Inhibitors of PGP and the MRPs are undergoing clinical evaluation of their ability to reverse or prevent drug resistance in cancer patients (29). Increasing evidence of the primary role of overexpression of these multidrug transporters in refractory epilepsy indicates a need to test such inhibitors in epileptic patients as well. An altered pharmacosensitivity of drug targets is the focus of additional studies, which have identified the voltage-gated sodium channels in hippocampal neurons as the major target of first-line AEDs. Sodium currents and the interaction of voltage- and ligand-gated ion channels are critical to neuronal excitation and, thus, to epileptogenicity (30). Researchers hypothesize that, in patients with chronic epilepsy, the density and distribution and the molecular structure and function of the ion channels are altered, thereby preventing the changes that normally occur to produce the anticonvulsive action of the drug. In animal experiments, this alteration has been observed after seizures. Studies of genetic animal models of epilepsy have also shown evidence of ion-channel mutations. Additional studies of epileptic patients and their families, as well as investigations of human brain tissue, further indicate that alteration of voltage-gated sodium channels is a pathogenetic mechanism common to some forms of epilepsy. For example, generalized epilepsy with febrile seizures plus (GEFS+) type 1, a syndrome characterized by febrile convulsions in childhood, followed by generalized seizures in adulthood, was recently discovered to be linked to a mutation of the β1 sodium channel subunit gene sodium channel density SCN1b on chromosome 19q (31). Normally, sodium channels, or voltage-operated transmembraneous proteins, are selectively permeable to sodium ions on depolarization but remain closed in their resting states. The process begins with a voltage change (depolarization), which opens the gate, allowing the sodium ion to enter. Rapid depolarization follows and the gate closes, remaining inactivated until it recovers and is ready for reactivation. The principal current responsible for the depolarization phase, and thus an essential current for neuronal excitation and epileptiform activity, is the voltage-dependent fast inactivating sodium current (INa). The neuronal excitability is modulated, however, by a persistent nonactivating sodium current (INaP) (30). It is not certain whether these currents are by the same Evidence of altered expression or function has been established in a of experimental studies in animals as well as in human epileptic brain tissue. In hippocampal kindling was found to substantially change sodium current sodium current after the generalized the voltage of of to more in both the and thereby of sodium channels at resting membrane In kindled VPA is in the CBZ is (30). Sodium voltage-gated channels, can be into and have different In studies of human tissue as a result of epilepsy surgery, focal epilepsy to be associated with alterations in sodium channel The between two sodium channel and was in hippocampal and temporal lobe tissue associated with seizures than in control an effect that may be A in was associated with lobe epilepsy Although the sodium channel is a target of many AEDs, experimental studies provide evidence that different drugs with voltage-gated ion channels and produce current alterations at different times and in different For example, PHT appears to primarily by reducing an effect achieved even in concentration by a of the to Sodium is also the mechanism of action of CBZ and but both of these agents cause more of a delay in from (30). The action of an of also on the of the In rats made epileptic by voltage-gated sodium channels a effect from CBZ and PHT with the drug's effect in A study conducted with brain tissue from two patients who underwent surgery different to the same AED as well. In the derived from a patient with drug-resistant epilepsy, CBZ effect on the sodium channel or on the of the neurons (Fig. However, in the tissue taken from a patient with chronic epilepsy, the were and the response to the drug was Additional conducted on patients with a known resistance to CBZ that the drug has effect on seizures human epilepsy. Reprinted from ref. by permission of & As this review has cell loss and the overexpression of multidrug transporters, and altered drug targets all to be factors in the development of Although evidence suggests that it is present in the early stage of the disease, to the and mechanisms in of finding a means of providing new AED an additional chance for patients with refractory seizures. the of a concept of early pharmacoresistance are It for a understanding of the and interactions of AEDs and a emphasis on the importance of the AED that is suited to the individual patient from the many patients with newly diagnosed epilepsy achieve seizure control with the first anticonvulsant and are thus likely to remain on the medication for an the drug should be for as well as A concept of early pharmacoresistance is further by evidence that a response to the initial AED trial is a and the most predictor of (1). A second drug trial is as either as another course of or with the addition of a complementary but brain imaging is recommended at this early stage of epilepsy as well. a structural such as AHS is surgery should be without further