Mechanisms of uterine specificity of vaginal progesterone

Endometrial transformations achieved by vaginal progesterone exceed those normally expected from the circulating concentrations obtained, this suggests some degree of direct vagina to uterus transport. We speculate on the different mechanisms involved in uterine specificity of vaginal progesterone and report data of a preliminary randomized study comparing progesterone concentrations in serum and endometrial tissue obtained from hysterectomy specimens after vaginal or i.m. administration. Eight post-menopausal women undergoing transabdominal hysterectomy were randomized to receive either vaginal progesterone gel, 90 mg, or i.m. progesterone formulation, 50 mg, at 08.00 and 20.00 on the day before surgery and at 06.00 on the day of surgery. Venous blood samples for progesterone measurement were drawn at 08.00 on the day before surgery and during the surgery. Endometrial progesterone concentrations were markedly higher in women who received vaginal progesterone (1.38 +/- 0.66 and 0.38 +/- 0.19 ng/mg protein, for vaginal and i.m. groups respectively) (P < 0.02) despite lower serum concentrations (4.17 < 0.56 and 32.32 +/- 11.06 ng/ml, for vaginal and i.m. groups respectively) (P < 0.001). The vaginal route induces endometrial progesterone concentrations that far exceed those expected from the serum progesterone concentrations achieved.