Letrozole administration during ovarian stimulation: intrafollicular concentrations, effects on steroidogenesis and pregnancy outcome-a secondary analysis.

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusions: 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.