Bioequivalence study of Prontogest 100 mg/2 mL solution for intramuscular injection versus reference progesterone injection, USP 500 mg/10 mL under fasting conditions: a randomized, open-label, two-sequence, four-period, fully replicated crossover study in healthy postmenopausal women.

Intramuscular progesterone remains an important formulation when prolonged systemic exposure is needed in gynecology and reproductive medicine. Demonstrating bioequivalence between a generic product and the reference formulation is necessary to support therapeutic interchangeability. The objectiveof thisstudy is to compare the pharmacokinetics, bioequivalence, safety, and tolerability of Prontogest 100 mg/2 mL intramuscular injection with the reference progesterone injection after a single dose under fasting conditions. In this open-label, randomized, single-dose, two-treatment, two-sequence, four-period fully replicated crossover study, healthy postmenopausal women received the test or reference product according to TRTR or RTRT sequences, separated by 15-day washout periods. Blood samples were collected up to 72 h after each dose, and plasma progesterone concentrations were measured using a validated LC-MS/MS method. The primary pharmacokinetic endpoints were baseline-corrected Cmax and AUC0-t, analyzed on the log-transformed scale by ANOVA. Because the within-reference variability for Cmax was 20.84%, conventional average bioequivalence criteria were applied. Of 42 screened participants, 26 were randomized and dosed, 23 completed all four periods, and 24 were included in the pharmacokinetic/statistical analysis. Mean test and reference values were comparable for Cmax (44.39 vs 45.33 ng/mL), AUC0-t (1030.34 vs 1028.52 ng·h/mL), and AUC0-∞ (1109.65 vs 1194.29 ng·h/mL), with identical median Tmax values of 8.3 h. The geometric mean ratio (test/reference) was 94.85% for Cmax and 99.98% for AUC0-t, with both 90% confidence intervals falling within the accepted 80.00-125.00% bioequivalence range. Eleven mild treatment-emergent adverse events occurred in 9 participants, mainly headache, with no serious adverse events or clinically significant abnormalities. Prontogest 100 mg/2 mL was demonstrated to be bioequivalent to the reference progesterone injection after a single intramuscular dose in healthy postmenopausal women and was generally well tolerated. Registration no. in (Clinicaltrials.gov): NCT07495007.