A 280-day tolerance-efficacy study with mibolerone for suppression of estrus in the cat

Mibolerone, a synthetic anabolic steroid, prevented estrus in domesticated cats when orally given a daily dose of 50 microng over a 180-day period. Doses of 20 microng daily and 50 microng given once a week failed to prevent estrus. Treatment with the 50-microng dose each day for 6 months had no apparent effects on subsequent estrus, mating, queening, or litter size. Kittens born to queens which had been treated did not have obvious developmental defects. Systemic metabolic changes produced by treatment were detected only in thyroid function, as revealed in dose- and time-related changes in serum cholesterol concentrations, thyroid gland weights, and thyroid histology. Clinical evidence of thyroid dysfunction was not apparent during the 6 months of treatment. Clinical and microscopic evidence of slight masculinization was apparent in cats after 3 months of treatment with 20 or 50 microng per day. Masculinizing changes consisted of thickening of the cervical dermis and clitoral hypertrophy. Behavioral changes were not observed. The apparent mechanism of action of mibolerone in the cat is the suppression of the release of pituitary luteinizing hormone.