Monotherapy trials: prerequisite data

Is it possible for an antiepileptic drug (AED) to be effective as add-on therapy in refractory epilepsy but ineffective as monotherapy for the same seizure type(s)? If the answer is 'no', why not award a new AED a monotherapy licence once it has been shown to be effective as adjunctive treatment in placebo controlled, dose-ranging studies in patients with difficult-to-control epilepsy? The recent comparative study between carbamazepine and remacemide, however, suggests that subtle pharmacokinetic/pharmacodynamic interactions between established and new AEDs can indicate efficacy in add-on studies that does not necessarily transfer to monotherapy. There is some evidence that an AED whose primary mechanism of action does not involve blockade of voltage-dependent sodium channels may do less well than carbamazepine in terms of efficacy end-points in a double-blind, head-to-head comparison. These observations lead to the conclusion that monotherapy trials are, indeed, required. They should be undertaken after proof of efficacy has been obtained using a single short presurgical AED withdrawal study backed up by a substantive dose-ranging phase III efficacy trial. There is no reason to recommend earlier assessment since the clinical need for new AEDs is in refractory epilepsy. The subsequent monotherapy trial programme should contain elements utilising comparative and withdrawal designs. Sponsors should be able to seek a licence for their drug either as a first choice treatment in newly diagnosed epilepsy or as substitution monotherapy once treatment with at least one other AED has failed.