Letrozole administration during ovarian stimulation: intrafollicular concentrations, effects on steroidogenesis and...

To determine intrafollicular concentrations of letrozole (LTZ) and key steroids in women undergoing ovarian stimulation (OS) with or without LTZ co-treatment, and to explore associated clinical outcomes.

Follicular fluid (FF) collected at oocyte pickup from 30 women participating in the RIOT-B study, a randomized controlled trial comparing OS with recombinant FSH (150 IU/day) combined with either LTZ (5 mg/day; n = 15) or placebo (n = 15). Concentrations of LTZ and steroid were quantified by LC-MS/MS. Associations between FF composition and clinical outcomes were evaluated.

At oocyte pickup, mean FF LTZ concentration was 144 ± 12 nmol/L (nM) (range 51-270). FF 17β-estradiol (E₂) concentrations were similar between groups (≈900 nM), whereas testosterone, androstenedione, DHEA, and 17OH-progesterone (17OH-P₄) were markedly elevated in the LTZ group (p < 0.001-0.0001). In contrast, P₄ levels remained unchanged. Among LTZ-treated women, higher FF concentrations of LTZ and testosterone were significantly associated with failure to conceive. No relationship was observed between FF LTZ levels and BMI or gonadotropin dose.

Intrafollicular estrogen levels remain preserved during LTZ administration, accompanied by pronounced androgen accumulation and elevated 17OH-P₄. These findings suggest complex, cell-specific effects of aromatase inhibition on follicular steroidogenesis and raise questions regarding the optimal dosing and mechanistic rationale for LTZ co-treatment in assisted reproduction.