Gehring, R., Martinez, M. Assessing product bioequivalence for extended-release formulations and drugs with long half-lives. J. vet. Pharmacol. Therap.35 (Suppl. 1), 3–9.
When a drug product remains in the body for a duration of hours or days, estimating the extent of drug exposure is relatively straightforward. For immediate release (IR) dosage forms, the peak drug concentration (Cmax) and time to peak concentrations (Tmax) are typically adequate for comparing product rates of drug absorption. However, unique complexities need to be addressed when a drug remains in the system for a duration of months or years. Specifically, if the long duration of exposure is attributable to formulation effects, Cmax and Tmax may not be adequate to compare the rate of drug absorption, especially when there are multiphasic absorption processes. In this case, it may be appropriate to use partial (segmented areas) for comparing product profiles. However, the decision of how the curves are segmented is not straightforward. Points to consider are discussed in this manuscript. Alternatively, if the long half-life is due to the pharmacokinetics (PK) of the active pharmaceutical ingredient and not to formulation effects, Cmax and Tmax may be appropriate metrics for the rate of absorption but decisions regarding the duration of blood sampling needed to capture the AUC should be based upon an assessment of the inherent variability in drug absorption and elimination. Furthermore, it is important to be confident that the absorption phase is complete. In this manuscript, we explore some of the difficulties associated with determining optimal metrics for comparing the rate and extent of product release for long half life drugs when the long duration of exposure is attributable to either the inherent drug PK or to formulation effects.