Topical bioequivalence: Experimental and regulatory considerations following formulation complexity

Documenting topical bioequivalence can be an extremely complex process, which is intrinsically dependent on the formulation technological features. According to EMA guideline, for simple formulations, BE may be demonstrated by documenting the qualitative (Q1), quantitative (Q2), microstructure (Q3) and performance (Q4) equivalence. Nevertheless, when addressing complex semisolids, equivalence regarding local availability should also be demonstrated. The purpose of this study is to pursue this strategy using two opposite scenarios: a simple dimetindene maleate 1 mg/g gel formulation and a diclofenac diethylammonium 23.2 mg/g emulgel, representing a complex formulation. For both formulations, Q1/Q2 test (TP) and reference products (RP) were used. Rheology, in vitro release (IVRT) and in vitro permeation methods (IVPT) were developed and validated for both products. For the dimetindene formulation, equivalence pertaining to Q4 was established. However, high variability was observed for some rheology endpoints, especially for the different RP batches. Therefore, equivalence could not be established for Q3 as per EMA requirements. Can some rheology endpoints be waived? Can we establish reasonable criteria that are overall feasible for generic manufacturers and at the same time safe for the patient? An attempt was made to propose a wider acceptance range based on the inter-batch variability of the RP. For that, the rationale presented in the EMA guideline on bioequivalence for highly variable products was used. For the diclofenac formulation, Q3 equivalence was likewise not established. Q4 equivalence was only found for some batch combinations and when applying a wider acceptance criterion (75-133%). Furthermore, IVPT equivalence also failed to be demonstrated. Nevertheless, since the TP displays an equivalent pharmacokinetic profile compared to the RP, the observed Q3, Q4 and local availability differences are not expected to be clinically significant. This study draws attention to an effective search to determine the most appropriate strategy for assessing topical bioequivalence on a case-by-case basis.