Pharmacokinetics and effects of 17β-estradiol and progesterone implants in ovariectomized rats

For the pharmacokinetic evaluation of Silastic capsules, ovariectornized (OVX) rats were implanted subcutaneously with this dosage form containing 17 beta-estradiol (5, 10, 15, or 20% in cholesterol, where 5% 17 beta-estradiol equals 0.4 mg) or progesterone (20, 40, 110, or 220 mg of crystalline progesterone). The time-course of serum 17 beta-estradiol and progesterone released from these capsules in the OVX rat is characterized by an initial increase in serum hormone levels followed by a decline and then an apparent steady-state that persists from 7 to 24 days postimplant. Both hormones have large clearance values (total clearance is 97.7 L/day for 17 beta-estradiol and 20.9 L/clay for progesterone). For 17 beta-estradiol and progesterone only, 11% of the dose was released from the implant after 24 days. Thus, the Silastic membrane represents the rate controlling barrier for these hormones. The relationship between graded doses of 17 beta-estradiol or progesterone and serum concentration was linear. Neither tail flick latencies measured at 48, 52.5, and 55 degrees C nor the antinociceptive potency of morphine (ED50 values) were altered by continuous administration to steady-state of graded doses of 17 beta-estradiol or progesterone. We demonstrate how a dose-dependent analysis of some of the behavioral effects of 17 beta-estradiol or progesterone can be conducted at steady-state serum hormone concentrations. Perspective: we describe a method to obtain sustained serum levels of estrogen or progesterone and the consequences of these sustained hormone levels on acute thermal nociception and the antinociceptive response to morphine. This rat model of hormone replacement may provide insights into the role of these hormones in pathological pain states.