Mechanistic insights into cytochrome P450 inhibition by progesterone and its impact on fluoxetine pharmacokinetics

Progesterone (PR) is an essential steroid hormone regulating female reproductive function, whereas fluoxetine (FLU), a selective serotonin reuptake inhibitor (SSRI), serves as a first-line antidepressant for antenatal use. The two agents may undergo cytochrome P450 (CYP)-mediated interactions, threatening maternal and fetal safety. This study integrated cocktail incubation, in vitro / in vivo assays, and mechanistic static models (MSM) to characterize PR-modulated CYP activity and FLU-associated drug-drug interactions (DDI). In human/rat liver and human placental microsomes, PR exerted reversible competitive inhibition on human hepatic CYP2C9 (IC 50 = 26.16 ± 3.89 μM, K i = 34.64 μM) and rat CYP2C11 (IC 50 = 20.81 ± 3.49 μM, K i = 38.30 μM), with no inhibitory effect on placental CYPs. It suppressed FLU metabolism in rat liver via mixed inhibition (IC 50 = 16.56 ± 1.50 μM). In vivo , intramuscular progesterone (IMP) elevated rat FLU AUC 0-t by 57.4% and C max by 123.7%, while lowering CL/F by 42.1%. MSM predicted evident perinatal DDI risks for high-dose IMP (AUCR up to 1.53), driven by altered hepatic first-pass metabolism. Though the general population faces low risks, close therapeutic monitoring is mandatory for pregnant women. This work underscores the delivery route, physiological status, and patient subgroups in DDI evaluation, supporting the safe combined use of PR and CYP2C9 substrates.