Parenteral Administration of Progestin Nestorone® to Lactating Cynomolgus Monkeys: An Ideal Hormonal Contraceptive at Lactation?

Abstract

Lactational amenorrhea ideally should provide a means of contraception to augment the interval between pregnancies, but its duration varies widely, and a considerable number of fully nursing mothers resume ovulating within 6 months of giving birth. Ideally, using a lactational hormonal contraceptive should be effective without causing problems in lactation or for the nursing infant. Nestorone (NES) is a good candidate, because its parenteral use would ensure effective contraception in nursing mothers and because the mother’s and infant’s liver should rapidly metabolize the hormone. This study evaluated NES in five cynomolgus monkeys and their nursing infants. Implants that release about 40 mg of NES daily in vitro were implanted subcutaneously 3 to 4 months after delivery and were removed after 4 weeks. Levels of NES and other hormones were estimated by radioimmunoassay in blood and milk samples. Maternal serum NES levels averaged 337 pmol/liter while implants were in place, and the average level in milk was 586 pmol/liter. Concentrations in serum and breast milk correlated to a significant degree (Fig. 1). Nestorone was not detected in any infant serum sample (lower limit of detection, 13 pmol/liter). Prolactin levels decreased from 41.4 to 26.3 ng/ml during implant use, a significant change. Neither estradiol nor progesterone was detected in maternal serum. Infants continued to gain weight during the 4-week implantation period. These findings suggest that NES is close to an ideal contraceptive for use during lactation. Hum Reprod 1999;14:1993–1997