Effect of altrenogest on endogenous progesterone during early pregnancy in recipient mares

Introduction: Altrenogest is commonly utilised for hormonal pregnancysupport with undefined effects. We investigated effects of altrenogest on endogenous progesterone (P4) in embryo recipient mares (RM) during early pregnancy by cross-sectional study.

Materials And Methods: Thirty RM received 150mg depot altrenogest from transfer, at 7-day intervals, through to 42 days (ALT), a further 31 RM received no altrenogest (CON). P4 assayed by amplified ELISA at 7-day intervals, starting 7 days after transfer. Data analysed by total (tP4, ng/ml) and corrected (cP4) by number of corpora lutea (CL). Statistical comparisons by Kruskal-Wallis ANOVA and Friedman repeated measures; values presented as median and 95%CI. Odds ratios (OR) calculated to evaluate associations.

Results: In ALT, cP4 was lower on days 21, 28 and 35 compared to days 14 and 42 (p<0.01). In CON, compared to day 14, cP4 was lower on days 21, 28, 35 (p<0.01) and day 42 (p=0.026). Secondary CL occurred in 17.24% CON and 26.67% ALT (OR 1.55; 0.45-5.28, p=0.487); tP4 was lower in ALT 4.8 (3.9-6.3), compared with CON 6.0 (5.4-7.3, p=0.04), and in ALT 4.7 (3.9-5.3), compared with CON 5.7 (5.1-7.4, p=0.02) at 21 and 28 days respectively. This held true for cP4 at 21 (ALT 3.7; 2.8-4.3 versus CON 4.7; 3.7-6.6, p=0.03) and 28 days (ALT 3.4; 2.5-4.3 versus CON 5.0; 4.0-5.8, p=0.005) where effect of multiple ovulations was nullified. In 40% of ALT, tP4 was <3.0 at 1 time-point compared to 3.2% in CON. The odds of a mare having a tP4 concentration <3.0 at any time-point was greater for ALT (OR 11.2; 1.37-91.83, p=0.024). CONCLUSIONS: Altrenogest led to suppressed endogenous P4 production with greatest magnitude at 28 days of gestation; we hypothesise that this is mediated via negative feedback of altrenogest on pituitary LH release and effective reduction of its luteotrophic influence.