PREVENTION OF ESTRUS IN BITCHES BY SUBCUTANEOUS IMPLANTATION OF CHLORMADINONE ACETATE

The efficacy and clinical safety of chlormadinone acetate (CAP) in pre venting estrus were assessed in bitches on condition that CAP was subc utaneously implanted in silastic silicon rubber. To evaluate the long- term efficacy of implantation, 19 bitches were divided into 4 groups g iven doses of 2.5, 5, 10 and 25 mg/kg, respectively. Although estrus w as observed within 13 months after implantation in all of the bitches given CAP in the 2.5 mg/kg dose, and in 13 to 15 months in 3 of the 5 bitches given the 5 mg/kg dose, it was prevented for at least 24 month s in all of the bitches given doses of 10 mg/kg or more. Plasma proges terone levels remained low throughout the period of estrus prevention, indicating a close correlation with the effect of CAP. The mean body weight of the bitches in groups receiving higher doses increased sligh tly over the course of the experiment. Except this, no clinical, hemat ology or biochemistry abnormalities were found in any of the treated b itches. Another 6 bitches were given 10 to 30 mg/kg of CAP, but the im plants were removed to observe the recurrence of estrus and to measure the amount of CAP in the removed implant. Estrus recurred after remov al even in the bitch given 30 mg/kg. The concentrations of CAP in the plasma and the amounts of CAP remaining in the implants demonstrated t he sustained release of CAP from the implants. The concentrations of C AP in the individual bitches indicated that the lowest concentration e ffective in preventing estrus is 0.7 ng/ml. Subcutaneous implantation of CAP thus proved to be safe and effective in preventing estrus in bi tches for prolonged periods.