Bioequivalence of venlafaxine modified-release capsule revisited with an innovative approach using experimental and predictive models.

Background: To evaluate the venlafaxine:O-desmethyl venlafaxine (active metabolite) in vivo formation ratio (MR) in three independent bioequivalence (BE) studies consisting of single-dosed (under fasted and fed conditions) and multiple-dosed clinical trials on healthy subjects. The pooled data pharmacokinetic (PK) analysis demonstrates a model to conduct enantiomer/racemate/active metabolite bioanalysis for regulatory submission of bioavailability/bioequivalence (BA/BE) studies using an interesting MR concept.

Results: BE was established for all three studies. Moreover, the venlafaxine:O-desmethyl venlafaxine MR for C(max) and AUC(last) differed by more than 50% for fasted and fed single-dosed studies, while pooled data analysis found the MR for C(max) to be approximately 0.63 and the AUC to be approximately 0.36 for both test and reference drugs. However, negligible variation was observed for both rate and extent of drug and active metabolite absorption into the systemic circulation at steady state, as the MR for both C(max) and AUC was approximately 0.62.

Conclusions: The applications/consequences of the above results are immense. First, an achiral assay for venlafaxine and O-desmethyl venlafaxine estimation in human plasma has been justified for the regulatory acceptance of BA/BE studies, supported with both single- and multiple-dosed PK data showing negligible variation in terms of MR at C(max). Second, the current investigation shows the MR to be within ±10% when compared with the single-dosed reported study on a western population. Third, the racial effect would not lead to any significant clinical outcome using an interchangeable venlafaxine 150-mg capsule manufactured by Ranbaxy with an Efexor 150-mg capsule manufactured by Wyeth. Furthermore, a decision tree is proposed to evaluate if a racemate or an enantiomer drug and active metabolite bioanalysis should be executed for BA/BE regulatory submission using respective achiral or chiral assays when the drug moiety is a racemate or an enantiomer, formulated in modified-release dosage forms.