Comparison of two altrenogest preparations in mares: A pilot study

Altrenogest is a synthetic progestin used in reproductive management of horses. Aims were to characterize and compare plasma altrenogest concentrations associated with weekly injectable versus daily oral preparations of altrenogest treatments. We hypothesized that an injectable preparation would not maintain concentrations higher than an oral preparation given once weekly. Two mares with no in vivo source of progesterone received either Treatment A (150 mg intramuscular altrenogest on days 0, 7, and 14) or Treatment B (0.044 mg/kg body weight of oral altrenogest daily for 3 weeks).After a 3-week washout period, mares received the opposite treatment for 3 weeks. Blood was collected prior to treatments and on days 0, 1, 4, 7, 8, 11, 14, 15, 18, and 21. High-performance liquid chromatography-tandem mass spectrometry was used for altrenogest assays. Mean minimum plasma concentrations after Treatments A and B were 1.51 ± 0.65 and 2.38 ± 1.53 ng/ml, respectively. Plasma altrenogest concentrations were significantly higher in Treatment A than Treatment B at 24 hours post treatment (days 1, 8, and 15). No significant difference between plasma altrenogest concentrations was detected on days 4 and 7 of each week. Finding that plasma altrenogest concentrations were higher for Treatment A at all time points except days 7, 14, and 21 supported our hypothesis. An injectable altrenogest preparation may be considered a more practical and safer solution for long-term treatment; based on this study, it was comparable to a proven daily oral altrenogest treatment for a minimum of 4 days.