Identification of clinically relevant biomarkers of epileptogenesis - a strategic roadmap.

Onset of many forms of epilepsy occurs after an initial epileptogenic insult or as a result of an identified genetic defect. Given that the precipitating insult is known, these epilepsies are, in principle, amenable to secondary prevention. However, development of preventive treatments is difficult because only a subset of individuals will develop epilepsy and we cannot currently predict which individuals are at the highest risk. Biomarkers that enable identification of these individuals would facilitate clinical trials of potential anti-epileptogenic treatments, but no such prognostic biomarkers currently exist. Several putative molecular, imaging, electroencephalographic and behavioural biomarkers of epileptogenesis have been identified, but clinical translation has been hampered by fragmented and poorly coordinated efforts, issues with inter-model reproducibility, study design and statistical approaches, and difficulties with validation in patients. These challenges demand a strategic roadmap to facilitate the identification, characterization and clinical validation of biomarkers for epileptogenesis. In this Review, we summarize the state of the art with respect to biomarker research in epileptogenesis and propose a five-phase roadmap, adapted from those developed for cancer and Alzheimer disease, that provides a conceptual structure for biomarker research.

Strengthening the Case for Epilepsy Drug Development: Bridging Experiences from the Alzheimer's Disease Field-An Opinion.

Given the sheer number of drugs (over 20!) available for treatment of seizures, epilepsy can be considered one of the most successful areas in pharmaceutical development and especially for neuroscience. However, despite the large number of drug treatment options available for managing patients with epilepsy, there remains considerable unmet need. For example, the overall impact on seizure control has not been substantial with approximately 30% of patients remaining refractory or their seizures not adequately controlled. Also there is need for epilepsy prevention and for certain sub-populations with severe intractable epilepsy. High unmet need often drives new industry investment into therapeutic market opportunities, however the profound success of antiepileptic drugs has contributed to the hurdles for industry investment in new therapies for epilepsy. Furthermore, the payor environment has also changed with new challenges for evidence generation and demonstration of additive value above existing standard of care treatments. Challenges in translational science, in the clinical trial environment including cost and operational technical difficulty, and in the commercial environment have resulted in the pharmaceutical industry directing investments away from epilepsy into other therapeutic areas such as oncology and immunology as opportunities for higher probabilities of success and returns of investment. The neuroscience area in general is perceived a high risk area and a notable exception has been the active industry involvement in Alzheimer's disease (AD), especially for therapeutics that could modify the course or prevent AD. AD is a very high risk area with no successful efficacious treatments found to date despite recent failures, there remains promise that therapies are forthcoming. The promise is fueled by a number of innovative factors that reduced R&D challenges in the AD field and contributed to a high level of drug development activity and investment. This paper addresses hurdles facing epilepsy drug discovery and development and focuses on some key solutions that could be eased to facilitate industry interest. Similarities in drug development challenges provide opportunities that bridge experiences and learnings from AD to epilepsy. Overall, the epilepsy field is probably in a good position for advancing into the next generation therapeutics of antiepileptic drugs targeted for increased efficacy in refractory epilepsy and for antiepileptogenesis.

New avenues for anti-epileptic drug discovery and development.

Despite the introduction of over 15 third-generation anti-epileptic drugs, current medications fail to control seizures in 20-30% of patients. However, our understanding of the mechanisms mediating the development of epilepsy and the causes of drug resistance has grown substantially over the past decade, providing opportunities for the discovery and development of more efficacious anti-epileptic and anti-epileptogenic drugs. In this Review we discuss how previous preclinical models and clinical trial designs may have hampered the discovery of better treatments. We propose that future anti-epileptic drug development may be improved through a new joint endeavour between academia and the industry, through the identification and application of tools for new target-driven approaches, and through comparative preclinical proof-of-concept studies and innovative clinical trials designs.

Correlation analysis between anticonvulsant ED50 values of antiepileptic drugs in mice and rats and their therapeutic doses and plasma levels.

OBJECTIVE: Although there are several animal models of epilepsy, the extrapolation of antiepileptic drug (AEDs) performance to epileptic patients from anticonvulsant activity results in animals is not straightforward. Consequently, the aim of this work was to perform a correlation analysis between therapeutic daily doses (D) and average steady-state plasma concentrations (Css,av) of AEDs and their activity in common anticonvulsant animal models. METHODS: AED activity in anticonvulsant animal models was expressed as maximal electroshock seizure (MES) test ED50 values in mice and rats and ED50 values in audiogenic seizure-susceptible mice (AGS ED50). Data were examined, by use of linear and logarithmic approaches, for an association between Css,av (mg/L or micromol/L) and D (mg or mmol) for each AED in epileptic patients as the dependent variable (Y) and its MES ED50 in mice and rats and AGS ED50 in mice (mg/kg or micromol/kg) as the independent variable (X). RESULTS: Linear correlation analyses between Css,av (mg/L) and ED50 (mg/kg) for 11 AEDs gave the following correlation coefficients (R2): 0.68 (mice, MES); 0.73 (rat, MES); 0.64 (AGS). Switching the units from milligrams to micromoles improved the correlation significantly and gave the following R2 values: 0.88 (mice, MES); 0.90 (rat, MES); 0.76 (AGS). The linear correlation between Css,av and ED50 was better than that between D and ED50. CONCLUSIONS: The results of this analysis suggest that the relationship between Css,av and ED50 is useful in predicting target concentration ranges in humans. The Y intercepts of the Css,av-versus-ED50 and D-versus- ED50 plots were similar in all three animal models and ranged between 12 and 17 mg/L and between 570 and 890 mg, respectively, indicating that for all AEDs analyzed except valproic acid and ethosuximide, the therapeutic plasma concentration is in the range 10-20 mg/L.

Pharmacokinetic interactions of topiramate.

Topiramate is a new antiepileptic drug (AED) that has been approved worldwide (in more than 80 countries) for the treatment of various kinds of epilepsy. It is currently being evaluated for its effect in various neurological and psychiatric disorders. The pharmacokinetics of topiramate are characterised by linear pharmacokinetics over the dose range 100-800 mg, low oral clearance (22-36 mL/min), which, in monotherapy, is predominantly through renal excretion (renal clearance 10-20 mL/min), and a long half-life (19-25 hours), which is reduced when coadministered with inducing AEDs such as phenytoin, phenobarbital and carbamazepine. The absolute bioavailability, or oral availability, of topiramate is 81-95% and is not affected by food. Although topiramate is not extensively metabolised when administered in monotherapy (fraction metabolised approximately 20%), its metabolism is induced during polytherapy with carbamazepine and phenytoin, and, consequently, its fraction metabolised increases. During concomitant treatment with topiramate and carbamazepine or phenytoin, the (oral) clearance of topiramate increases 2-fold and its half-life becomes shorter by approximately 50%, which may require topiramate dosage adjustment when phenytoin or carbamazepine therapy is added or discontinued. From a pharmacokinetic standpoint, topiramate is a unique example of a drug that, because of its major renal elimination component, is not subject to drug interaction due to enzyme inhibition, but nevertheless is susceptible to clinically relevant drug interactions due to induction of its metabolism. Unlike old AEDs such as phenytoin and carbamazepine, topiramate is a mild inducer and, currently, the only interaction observed as a result of induction by topiramate is that with ethinylestradiol. Topiramate only increases the oral clearance of ethinylestradiol in an oral contraceptive at high dosages (>200 mg/day). Because of this dose-dependency, possible interactions between topiramate and oral contraceptives should be assessed according to the topiramate dosage utilised. This paper provides a critical review of the pharmacokinetic interactions of topiramate with old and new AEDs, an oral contraceptive, and the CNS-active drugs lithium, haloperidol, amitriptyline, risperidone, sumatriptan, propranolol and dihydroergotamine. At a daily dosage of 200 mg, topiramate exhibited no or little (with lithium, propranolol and the amitriptyline metabolite nortriptyline) pharmacokinetic interactions with these drugs. The results of many of these drug interaction studies with topiramate have not been published before, and are presented and discussed for the first time in this article.

Pharmacological characterization of the homomeric and heteromeric UNC-49 GABA receptors in C. elegans.

(1) UNC-49B and UNC-49C are gamma-aminobutyric acid (GABA) receptor subunits encoded by the Caenorhabditis elegans unc-49 gene. UNC-49B forms a homomeric GABA receptor, or can co-assemble with UNC-49C to form a heteromeric receptor. The pharmacological properties of UNC-49B homomers and UNC-49B/C heteromers were investigated in Xenopus oocytes. (2) The UNC-49 subunits are most closely related to the bicuculline- and benzodiazepine-insensitive RDL GABA receptors of insects. Consistent with this classification, bicuculline (10 micro M) did not inhibit, nor did diazepam (10 micro M) enhance UNC-49B homomeric or UNC-49B/C heteromeric receptors. (3) The UNC-49C subunit strongly affects the pharmacology of UNC-49B/C heteromeric receptors. UNC-49B homomers were much more picrotoxin sensitive than UNC-49B/C heteromers (IC(50)=0.9+/-0.2 micro M and 166+/-42 micro M, respectively). Pentobarbitone enhancement was greater for UNC-49B homomers compared to UNC-49B/C heteromers. Propofol (50 micro M) slightly enhanced UNC-49B homomers but slightly inhibited UNC-49B/C heteromers. Penicillin G (10 mM) inhibited UNC-49B homomers less strongly than UNC-49B/C heteromers (30% compared to 53% inhibition, respectively). (4) Several aspects of UNC-49 pharmacology were unusual. Picrotoxin sensitivity strongly correlates with dieldrin sensitivity, yet UNC-49B homomers were highly dieldrin resistant. The enhancing neurosteroid pregnanolone (5beta-pregnan-3alpha-ol-20-one; 10 micro M) strongly inhibited both UNC-49 receptors. Alphaxalone (10 micro M), another enhancing neurosteroid, did not affect UNC-49B homomers, but slightly inhibited UNC-49B/C heteromers. (5) UNC-49 subunits and mammalian GABA(A) receptor alpha, beta, and gamma subunit classes all share roughly the same degree of sequence similarity. Thus, although they are most similar to other invertebrate GABA receptors, the UNC-49 receptors share significant structural and pharmacological overlap with mammalian GABA(A) receptors.