Estradiol-17[beta] Pharmacokinetics and Histological Assessment of the Ovaries and Uterine Horns following Intramuscular Administration of Estradiol Cypionate in Feral Cats

Feral cats (Felis catus) have a devastating impact on Australian native fauna. Several programs exist to control their numbers through lethal removal, using tools such as baiting with toxins. Adult male cats are especially difficult to control. We hypothesized that one way to capture these male cats is to lure them using female cats. As female cats are seasonal breeders, a method is needed to artificially induce reproductive (estrous) behavior so that they could be used for this purpose year-round (i.e., regardless of season). Estrogens can be given to cats to induce estrous behavior, but it is not known for how long this elevates the blood concentrations of estradiol. Following the administration of a long-acting estrogen, estradiol cypionate, to nine captive feral cats, blood estrogen concentrations remained elevated for several days. This information will be useful to ongoing studies that are investigating ways to reduce the abundance of feral cats in Australia, especially adult male cats. The control of feral cats (Felis catus) in Australia is a key biological conservation issue. Male cats are more difficult to control than female cats. Collared and tagged female cats displaying estrous behavior have been considered as a way to lure male cats and reveal their locations. As female cats are seasonal breeders, artificial induction of estrous behavior following the administration of a long-acting estrogen could improve their use for this purpose. Estradiol cypionate was intramuscularly administered to nine entire non-pregnant female feral cats, of unknown estrous status, at 0.1, 0.3, or 0.5 mg/kg. Mean peak serum concentrations of estradiol-17[beta] were 365 pg/mL (0.1 mg/kg), 1281 pg/mL (0.3 mg/kg), and 1447 pg/mL (0.5 mg/kg). The time-course of estradiol-17[beta] concentrations after various doses of estradiol cypionate was assessed using non-compartmental and non-linear mixed-effects methods. At the highest-studied dose (0.5 mg/kg), the 50th percentile of estradiol-17[beta] concentrations exceeded 0.1 ng/mL for 11.8 days, and 0.05 ng/mL for 14.6 days. The duration increased with increasing dose. No signs of toxicity were noticed in any cat during the study. This information will be useful to ongoing studies that are investigating ways to reduce the abundance of feral cats in Australia, especially adult male cats. Keywords: conservation; invasive species; mathematical modelling; pharmacology; reproduction; wildlife