Vaginal administration of 17-alpha hydroxyprogesterone caproate appears to be safe in non-pregnant female rats and rabbits

Intramuscular (250 mg once weekly) or subcutaneous (275 mg once weekly) injections of 17-hydroxyprogesterone caproate (17-OHPC) has been utilised to prevent recurent spontaneous preterm birth in pregnant women with a short cervix or those with a prior preterm birth but its efficacy in these conditions has been questioned. It is unclear whether adequate concentrations of 17-OHPC reach the suspected target organs such as the cervix and uterus following either IM or SC administration.The objective of this study was to determine feasibility and safety of vaginal administration of 17-OHPC in adult female Sprague Dawley rats and female New Zealand rabbits.Gels containing 17-OHPC were administered intravaginally to rats and rabbits for 10 consecutive days. After the last dose, serial blood samples and terminal uterine and adipose tissue samples were collected. 17-OHPC concentrations were measured by LC-MS-MS.Tissue histology showed that intravaginal administration of 17-OHPC was safe. There was no renal or hepatic toxicity as measured by liver function and kidney function tests.Intravaginal administration of 17-OHPC resulted in low systemic plasma concentrations but substantially higher uterus and adipose tissue concentrations of 17-OHPC. Composition of the formulation affected the tissue distribution of 17-OHPC.Future studies warrant further evaluation of the effect and safety of daily intravaginal administration of 17-OHPC gel in pregnant animals.