Plasma altrenogest concentrations in mares after intrarectal or intravaginal administration

Abstract

Altrenogest (Regu-Mate®) is labelled for daily oral administration and is used in mares to
suppress estrus behavior, maintain pregnancy and synchronize estrous cycles. It is a very potent progestin,
with potential risks associated with transdermal absorption for persons handling or administering this
compound. The objective was to determine plasma concentrations of altrenogest following rectal or
vaginal administration in mares. Altrenogest (0.044 mg/kg) was administered once daily for 5 days;
orally, vaginally, and transrectally in 7, 5 and 5 mares, respectively. Transrectal palpation and ultrasound
examinations were performed on Days 1, 3, and 5 to assess cervical tone, endometrial edema, and ovarian
structures. Venous blood was drawn at time 0 (prior to drug administration) and at 30 and 60 minutes
after drug administration. To determine altrenogest concentrations, plasma from each sample was
analyzed via ultra-performance liquid chromatography coupled to a triple quadruple mass spectrometer.
Although absorption of altrenogest occurred after vaginal or oral administration, plasma altrenogest
concentrations were less (p < 0.05) than those of orally treated mares. Additional studies are needed to
determine if plasma altrenogest concentrations after rectal or vaginal administration are sufficient to
suppress estrus behavior or maintain pregnancy.