Contraception with subdermal implants releasing the progestin ST-1435: a dose-finding study.

A new modified subdermal implant releasing the potent progestin ST-1435 was studied in eleven fertile-aged women. These implants have been developed for contraception and they have a life-time of two years. Three implant lengths of 4, 6 and 8 cm were tested to find the optimal steroid dose for inhibition of ovulation. Serum samples were collected twice per week during a six-week period every six months. The concentrations of serum ST-1435, estradiol and progesterone were determined by RIA. Ovulation was inhibited by all ST-1435 doses tested. The concentration of serum progesterone was below 6 nmol/l in all samples tested showing the absence of luteinization. The concentration of serum ST-1435 increased with increasing ST-1435 dose. Serum estradiol concentrations were quite variable, showing wide range and occasional high peak values typical of progestin treatment; the mean value of serum estradiol concentrations measured did not differ with different ST-1435 doses. The results of steroid determinations led to the conclusion that a single 4 cm subdermal implant is optimal for contraception. With this dosage level, ovulation is inhibited and side effects are minimized. Bleeding control was variable. No hormonal side effects due to the progestin ST-1435 were reported. This method, using a single 4 cm subcutaneous implant releasing the progestin ST-1435 with a life-time of two years, represents a promising alternative for inhibition of ovulation and contraception.