Influence of vehicle on kinetics of exogenous progesterone administered either by subcutaneous and intramuscular routes to sheep

This study aimed to determine the best vehicle and administration route for progesterone administration in sheep. In a first replicate, single intramuscular doses of 25 mg progesterone were administered to ewes previously ovariectomized, either in propylene glycol (group IM-PG, n = 6) or olive oil (group IM-OO, n = 5). In a second replicate, the same solutions of progesterone were administered subcutaneously to the same ewes (groups SC-PG, n = 6, and SC-OO, n = 5). In the present study, the best pharmacokinetic results of a single dose of 25 mg of progesterone were obtained, both using PG and OO as vehicles, by the subcutaneous route. Thus, progesterone remained in plasma for a longer time after subcutaneous administration in PG than in OO (t1/2β: 60.65 ± 13.07 vs. 27.51 ± 3.59 h; P < 0.05); the mean residence time being higher in SC-PG than in SC-OO group (88.99 ± 18.36 vs. 41.04 ± 5.31 h; P < 0.05). However, both vehicles allowed maintained plasma levels ⩾0.5 ng/ml for at least 30 h, so any of these treatments may be efficiently used for administration of exogenous progesterone.