Pharmacokinetics of Two Formulations of Altrenogest Administered to Mares

Altrenogest is a synthetic progestin that suppresses reproductive behaviours and assists pregnancy maintenance in female horses. Two formulations are available, a 'weekly' intramuscular injection and a daily oral formulation. Altrenogest administration has returned positive swabs for steroids; consequently, using injectable altrenogest in racing mares is prohibited. Oral administration may be permitted in race mares if there is one clear day between dosing and racing. The only pharmacokinetic data available were generated from geldings. Therefore, to assist veterinarians and analysts in determining accurate dosing and detection intervals, pharmacokinetic analysis using mares is required. Blood samples were taken from 10 mares pretreatment to obtain baseline concentrations. Mares were administered altrenogest, either oral (PO; 0.044 mg/kg; daily for 15 days) or intramuscular (IM; 0.3 mg/kg; twice; Days 0 and 7). On the first and last treatment day, blood samples were taken at designated times post dosing. After a 3-week washout, mares received the alternative treatment with sampling repeated. At the initial dose, for IM administration mean (± SD) plasma altrenogest C_max was 18.0 ± 6.6 ng/mL at 7.9 ± 3.9 h compared with PO dosing 13.2 ± 5.8 ng/mL at 0.8 ± 0.8 h. Plasma C_max on the final day was significantly higher (p = 0.002 [IM]; p = 0.006 [PO]). At 24 h post final oral treatment, mean (± SD) plasma altrenogest was 1.0 ± 0.8 ng/mL and at 48 h were 0.65 ± 0.5 ng/mL. Plasma concentrations well exceeding this may indicate that the one clear day rule or dosage recommendations have not been adhered to.