Bioequivalence Evaluation of Topical Metronidazole Products Using Dermal Microdialysis in New Zealand Rabbits

Comparative assessment of cutaneous pharmacokinetics (cPK) by dermal microdialysis (dMD) appears to be suitable to evaluate the bioequivalence (BE) of topical dermatological drug products applied to the skin (TDDPs). Although dMD studies in the literature have reported inconclusive BE assessments, we have addressed several methodological deficiencies to improve dMD's capability to assess BE between reference (R) and approved generic (referred to as test (T)) gel and cream products of metronidazole (MTZ). The 90% confidence interval (CI) of the geometric mean ratios for the Ln(AUC_0-24) and Ln(C_max) endpoints was centered within the BE limits of 80-125%. The CIs extended outside this range as the proof-of-principle study was not statistically powered to demonstrate BE (N = 7 rabbits). A power analysis suggests that, with the variability observed in this study, 21 rabbits for the cream and 11 rabbits for the gel would be sufficient to support an evaluation of BE with the 2 probe replicates we used, and only 10 and 5 rabbits would be sufficient to power the study for the cream and gel, respectively, if 4 probe replicates are used for each treatment per rabbit. These results indicate that dMD when properly controlling variables can be used to support BE assessments for TDDPs.