Altrenogest
Altrenogest is an orally active synthetic progestogen used chiefly to suppress and coordinate estrous activity in mares and sexually mature cycling gilts. Temporary progestational exposure allows the return to estrus to be planned after withdrawal in appropriately selected animals. It can also suppress estrous behavior in mares when that is the intended clinical objective.
Separate specialist applications involve inadequate progestational support in pregnancy. These reproductive uses depend on the diagnosis, ovarian activity and gestational state of the animal.
Group
Synthetic progestogen; reproductive management
Mechanism of Action
Altrenogest activates progesterone receptors and reproduces important progestational effects. Negative feedback at the hypothalamic-pituitary axis suppresses gonadotropin support of estrous activity, reducing expression of estrus and modifying the timing of ovarian events during treatment.
After withdrawal, removal of this inhibitory signal permits reproductive activity to resume in responsive cycling animals. This creates a scheduling effect rather than an absolute induction of fertility in every reproductive state; ovarian activity, season and prior cycling status influence the response.
At the reproductive tract, progestational signaling promotes secretory endometrial changes and reduces spontaneous uterine contractility. These tissue effects provide the rationale for selected pregnancy-support uses, while also explaining why reproductive-tract disease and a confirmed indication are important to case selection.
Dose Rates
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Estrus suppression for synchronization | 0.044 mg/kg | PO | Every 24 h | Up to 15 days; Do not use in: Pregnancy, Uterine inflammation or infection |
| Long-term suppression of estrous behavior | 0.044 mg/kg | PO | Every 24 h | Do not use in: Pregnancy, Uterine inflammation or infection; Reported extended regimen; reassess duration and competition eligibility individually. |
| Documented progesterone deficiency — pregnancy support | 0.044 mg/kg | PO | Every 24 h | Do not use in: Active uterine infection without a specialist treatment plan; Extra-label specialist regimen. Described endpoints include day 60 or later with endogenous progesterone above 4 ng/mL, day 120, or continued support through pregnancy; select one case-specific plan. |
| Selected pregnancy-loss prevention | 0.088 mg/kg | PO | Every 24 h | Separate extra-label pregnancy-support regimen; not a replacement for the estrus-suppression schedule. |
| Placentitis-associated pregnancy support | 22–44 mg/animal | PO | Every 24 h | Extra-label adjunct within a complete placentitis treatment plan; equivalent to 10–20 mL of the 2.2 mg/mL solution. |
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Documented luteal insufficiency — reported volume regimen A | 0.0970033954 mg/kg | PO | Every 24 h | Do not use in: First trimester when avoidable, Uterine infection; Equivalent to 2 mL per 100 lb of the 2.2 mg/mL equine solution. This is a distinct reported regimen, not a midpoint of the other schedules. |
| Luteal insufficiency or premature labor — reported volume regimen B | 0.0485016977 mg/kg | PO | Every 24 h | Do not use in: Uterine infection; Equivalent to 0.1 mL per 10 lb of the 2.2 mg/mL solution; maintain as a separate reported regimen. |
| Selected premature-labor management — pregnancy maintenance | 0.088 mg/kg | PO | Every 24 h | Do not use in: Uterine infection; Reported when tocolytics do not control myometrial activity. Withdraw 2–3 days before predicted whelping and monitor fetal viability. |
Pigs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Estrus synchronization in sexually mature cycling gilts | 15 mg/animal | PO | Every 24 h | Up to 14 days; Do not use in: Pregnancy, No previous estrous cycle, Current or previous uterine inflammation; Use Matrix label volume: 6.8 mL per gilt once daily on an individual portion of feed; 15 mg is the nominal label amount. |
Indications
- Suppression and coordination of estrus in mares to facilitate scheduled breeding.
- Temporary suppression of estrous behavior in selected mares.
- Synchronization of estrus in sexually mature gilts that have experienced at least one estrous cycle.
- Specialist support of documented or strongly supported luteal insufficiency and selected premature-labor or pregnancy-loss circumstances in mares and bitches.
Contraindications
- Do not use routine estrus-suppression treatment in pregnant animals; labelled pregnancy contraindications must be distinguished from separately selected extra-label pregnancy-support cases.
- Avoid use in animals with current or previous uterine inflammation, endometritis or chronic uterine infection.
- Do not use equine-labelled product in horses intended for human consumption.
Interactions
- Rifampin may increase progestin metabolism and reduce activity; the veterinary magnitude of this interaction is not established.
Adverse effects
- Overt adverse effects are uncommon at labelled equine doses. Small, inconsistent changes in calcium, potassium and hepatic enzyme measurements have been described.
- Progestational treatment can exacerbate uterine infection; pyometra is a concern in bitches.
Toxicity
A reliable acute toxic-dose threshold for the target veterinary species is not established. Overdose or unintended human exposure requires prompt assessment; monitor hematology when indicated.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Regu-Mate 2.2 mg/mL oral solution Equine-labelled oil solution, 0.22%; not for horses intended for food. Specialist extra-label pregnancy-support use is distinct from routine labelled estrus suppression. | Oral Liquid | 2.2 mg / mL | PO |
| Matrix 2.2 mg/mL oral solution Swine-labelled product for sexually mature gilts that have already cycled. Use its own 6.8 mL dosing directions, approved dosing device and withdrawal instructions; do not treat it as an unrestricted multispecies product. | Oral Liquid | 2.2 mg / mL | PO |
Monitoring and Administration
Administer the equine oral solution directly at the base of the tongue or on the usual grain ration; give the complete daily amount. Protect the light-sensitive solution in a tightly closed, light-resistant container at the product storage temperature.
Monitor estrous behavior and ovarian activity. Estrus commonly follows withdrawal within 2–5 days in mares and 4–9 days in appropriately selected gilts.
Document luteal insufficiency and exclude infectious causes before pregnancy-support treatment. Monitor fetal viability by ultrasonography and maternal uterine health; progesterone assays do not normally cross-react with altrenogest.
For pregnancy support, determine the treatment endpoint from the indication and expected parturition date. In bitches, the reported 0.088 mg/kg regimen is withdrawn 2–3 days before predicted whelping.
Altrenogest is absorbed through intact skin. Use vinyl, neoprene or nitrile gloves; latex gloves are not protective. Avoid contamination of dosing equipment and clothing. Wash skin exposure promptly with soap and water; flush ocular exposure for 15 minutes and seek medical care.
Pregnant or potentially pregnant people must not handle the product. Additional handler exclusions include thromboembolic disease or history, cerebrovascular/coronary disease, breast or estrogen-dependent neoplasia, unexplained vaginal bleeding and prior estrogen-associated tumors.
Matrix is a gilt product: individually top-dress the labelled amount on feed for 14 consecutive days. Its label specifies 6.8 mL, described as 15 mg; the milligram figure is nominal. The US label specifies a 21-day slaughter withdrawal. Apply the locally applicable label and withdrawal rules; do not extrapolate this to all food-animal uses.