Can serum progesterone be used to decide altrenogest supplementation discontinuation in problem pregnancies in mares?

Use of oral altrenogest as exogenous progesterone (P4) in mares in an effort to maintain high-risk pregnancies is a widespread practice. Treatment is sometimes initiated as early as 2 days after ovulation. Timing when it can safely be stopped is frequently uncertain and therefore, supplementation is often continued until 100-120 days, when the placental secretion of progestagens has taken over. The aim of this study was to determine if altrenogest interfered with progesterone assay and if timing of altrenogest administration discontinuation could be based on the blood concentration of P4.
Data were obtained from 7 healthy standard breed mares in seasonal anoestrus and in dioestrus. Mares were considered anoestrus when no follicles larger than 15 mm in diameter or corpus luteum (CL) were found at 2 consequent ultrasound examinations 7 days apart. They were confirmed dioestrus based on an observed ovulation and the presence of a CL 2 days after. Mares were treated with 0.044 mg/kg of altrenogest (Regumate®, Intervet, Boxmeer, Netherlands) orally once per day during 12 days beginning at any time for the anestrus group and at day 2 after ovulation for the dioestrus group. Blood was collected at day 0, 6, 12, 18 right before altrenogest administration and the serum was stored at -20°C until analysis. P4 concentration was obtained by radioimmunoassay and altrenogest by liquid chromatography-mass spectrometry. Differences in P4 and in altrenogest concentrations between different sampling days were determined by Friedman non parametric test.
Concentrations of altrenogest before treatment were almost not detectable (negative control). At day 6, they were significantly higher (p<0.05), and were back to basal, almost not detectable values at day 18, in both groups. P4 concentration was basal with values < 0.4 ng/ml all through the experiment in the anoestrus group. In the dioestrus group, P4 concentration was variable from one mare to another, but it was significantly higher in mid-dioestrus (day 6) as compared to the time of the impending next oestrus (day12).
No cross-reaction between P4 and altrenogest assays was observed as illustrated by the fact that concentration of P4 during anoestrus remained basal with or without supplementation. Despite the treatment, the variation of P4 over time was as expected in both groups. Altrenogest was no longer detectable 6 days after the last administration.
Further studies with larger numbers and sampling in dioestrus to compare P4 levels in treated and untreated mares to investigate the potential effect of altrenogest on endogenous P4 production by the CL are needed. However, our experiment shows that P4 levels assayed by RIA can be trusted to evaluate if the secondary CL’s have become functional and can thus maintain the pregnancy while oral altrenogest supplementation is discontinued.