Altrenogest For Equines

Altrenogest is used to suppress estrus in mares in order to facilitate induction of normal estrous cycle activity. It is also used in mares to synchronize scheduled breeding activity, thus improving breeding efficiency. In the suppression of estrous behavior in mares, this is usually related to mares that are used as performance horses.

Altrenogest is a progestin of the 19-nortestosterone group. As a veterinary medication, it is used extensively in the reproductive management of mares to control the reproductive cycle, help maintain pregnancy, and help with behavior modification. Altrenogest has been demonstrated to maintain pregnancy in oophorectomized mares and may be of benefit in mares that abort due to subtherapeutic progestin levels.

Altrenogest is typically administered orally by applying directly on the base of the mare’s tongue or on the mare’s feed. Altrenogest injection cost has been of some concern to horse owners when used for the management of behavioral issues in mares, but for the synchronization of estrus, altrenogest is generally considered to be a cost-effective medication.

Pharmacokinetics

The pharmacokinetics of altrenogest have been studied in horses. After oral dosing of 44 mg/kg PO, peak levels usually occur within 15-30 minutes postadministration; 24 hours postadministration, levels were below the level of quantification.¹ Elimination half-lives are approximately 2.5-4 hours. Altrenogest appears to be primarily eliminated in the urine. Peak urine levels occur 3-6 hours after oral administration. Urine levels were detectable up to 12 days postadministration.²

Contraindications & Warnings

Altrenogest is contraindicated in mares having a previous or current history of uterine inflammation (ie, acute, subacute, or chronic endometritis). Natural or synthetic gestagen therapy may exacerbate existing low-grade or “smoldering” uterine inflammation into a fulminating uterine infection in some instances (package insert; Regu-Mate®). When used to maintain pregnancy, it should be discontinued within 24 hours of the due date.³ Pregnant mares with gastrointestinal diseases may not absorb altrenogest adequately, thus injectable progesterone is preferred.⁴ Altrenogest should not be used in horses intended for food purposes and should not enter water courses, as this may be dangerous for fish and other aquatic organisms.

Since it has been determined that altrenogest can disrupt the hormonal activity of other species (such as humans) via contact, the use of protective gloves when handling altrenogest are recommended and that a physician be contacted immediately in the event of accidental contact. According to the available literature, altrenogest should not be handled by women of child-bearing age, and should not be administered to immature animals, animals with diabetes, uterine infection, a history of vaginal discharge or reproductive abnormalities, or animals where the owner or managers suspect they are pregnant.⁵

Where to buy Altrenogest

Altrenogest is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. Compounded medications of this type have become of increasing popularity among veterinarians due to the variety of concentrations available. Altrenogest 0.22% (2.2 mg/mL) injectable for equines is available in oil solution in 150-mL and 1000-mL bottles (Regu-Mate®, Altren®, OvaMed®, Rx). This medication is banned in racing animals in some countries.²

Altrenogest Injections For Horses

Altrenogest 60 mg/ml (50mL) injectable by NexGen is effective in normally cycling mares for minimizing the necessity for estrus detection, for the synchronization of estrus, and permitting scheduled breeding. It is also effective in suppressing estrus expression in show mares or mares to be raced.

¹Machnik M, et al. Pharmacokinetics of altrenogest in horses. J Vet Phamacol Ther. 2007;30:86-90.

²Plumb’s Veterinary Drugs.

³Baker TW, Davidson AF. Obstetrical Emergencies I.  Western Veterinary Conference 2012 2012. 2012. 

⁴Volkmann D. Rational Use of Progestagen Therapy During Pregnancy.  Western Veterinary Conference 2010 2010. 2010. 

⁵equi-med.com.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.