Bioequivalence of tacrolimus formulations with different dynamic solubility and in-vitro dissolution profiles

This study was to evaluate the pharmacokinetics and bioequivalence of two tacrolimus formulations which had different in vitro drug release profiles. Dynamic solubility, in vitro dissolution profiles of the two formulations, and their influence on pharmacokinetics were examined. The male volunteers were randomly assigned to receive a single 1-mg capsule of the test or reference formulation and pharmacokinetic parameters were determined using a noncompartmental method. The two formulations released >85 % of tacrolimus in water within 30 min, which passed the criterion of evaluating the test formulation. However, the test formulation produced a faster initial release rate and plateaued in about 15 min, while the reference showed almost zero order initial release profiles. The AUC0-∞ values were 145.92 (reference) and 140.49 ng h/mL (test). The mean Cmax was 15.70 (reference) and 16.08 ng/mL (test) with Tmax values of 1.63 and 1.60 h, respectively. The t1/2 for the reference and test formulations was 29.12 and 27.85 h, respectively. Relative bioavailability was calculated to be 96.28 %. The point estimates for the mean ratio of the test to reference for the AUC0-t and Cmax were 0.969 and 1.026, respectively, satisfying the criterion for bioequivalence. The results suggest that the test formulation is pharmacokinetically equivalent to the reference in terms of both rate and extent of absorption. Even though the in vitro dissolution profiles of the formulations might not be equivalent, the pharmacokinetics indicated bioequivalence. Therefore, when developing poorly soluble drugs, it might be beneficial to pay attention to the dynamic solubility as well as dissolution profiles.