Commentary on individual bioequivalence by M.-L. Chen

Generic products of antiepileptic drugs: a perspective on bioequivalence and interchangeability

Most antiepileptic drugs (AEDs) are currently available as generic products, yet neurologists and patients are reluctant to switch to generics. Generic AEDs are regarded as bioequivalent to brand AEDs after meeting the average bioequivalence criteria; consequently, they are considered to be interchangeable with their respective brands without loss of efficacy and safety. According to the U.S. Food and Drug Administration (FDA) the present bioequivalence requirements are already so rigorous and constrained that there is little possibility that generics that meet regulatory bioequivalence criteria could lead to therapeutic problems. So is there a scientific rationale for the concerns about switching patients with epilepsy to bioequivalent generics? Herein we discuss the assessment of bioequivalence and propose a scaled-average bioequivalence approach where scaling of bioequivalence is carried out based on brand lot-to-lot variance as an alternative to the conventional bioequivalence test as a means to determine whether switching patients to generic formulations, or vice versa, is a safe and effective therapeutic option. Meeting the proposed scaled-average bioequivalence requirements will ensure that when an individual patient is switched, he or she has fluctuations in plasma levels similar to those from lot-to-lot of the brand reference levels and thus should make these generic products safely switchable without change in efficacy and safety outcomes.

Generic products of antiepileptic drugs (AEDs): is it an issue?

The availability of generic products of antiepileptic drugs (AEDs) has raised the following concerns: (1) Do generic AEDs work as well as brand AEDs in terms of their efficacy, safety and quality? (2) Can generic AEDs be used as substitutions for brand AEDs? and (3) Can generic products of AEDs be used interchangeably? The traditional average bioequivalence analysis addresses concern 1 but does not provide a complete adequate response to concerns 2 and 3. Drug interchangeability can be classified as drug prescribability or drug switchability. Drug prescribability refers to the situation where a patient is treated for the first time so that either a brand or a bioequivalent generic AED can be chosen. Drug switchability refers to the situation in which a brand AED is switched to a bioequivalent generic product of the same AED. The traditional average bioequivalence approach is sufficient to evaluate the prescribability of generic products, but does not ensure the switchability between prescribable formulations. The necessity of assuring switchability of two formulations can be addressed by individual bioequivalence. While the switch to generic AEDs is well tolerated by many patients and in general cost-effective, seizure control should not be sacrificed on the basis of cost alone, as the major end point in treating epilepsy with AEDs is seizure control without side effects. Until we have individual (within patient) bioequivalence data on generic AEDs and/or the tools to a priori identify the subset of patients susceptible to the generic switch, a switch of AED products in seizure-free patients is not recommended.

Some statistical considerations on the FDA draft guidance for individual bioequivalence

In December of 1997, the FDA proposed a draft guidance for future in vivo bioequivalence studies. The guidance suggested specific criteria for new drug sponsors to show individual bioequivalence (IBE). The criteria use a mixed-scaling aggregate strategy. It has been generally accepted that, under some particular situations, the proposed criteria would result in a relaxation of the current bioequivalence standard set by the average bioequivalence (ABE) criterion. Here we study the magnitude of this relaxation under three scenarios: when the conditions for an ABE investigation are met; when the drugs are highly variable, and when the experiments are poorly conducted. The magnitude of relaxation we report here may be surprisingly large to many. For example, when a drug is highly variable (with the intrasubject coefficient of variation reaching 40 per cent), the allowable limit for the ratio of the formulation means could reach 55-180 per cent in an IBE investigation. In comparison, the usual allowable limit in an ABE investigation is 80-125 per cent. Our investigation raises doubts on whether the implied standard of the new proposed IBE criteria would adequately ensure switchability in highly variable drugs.