Treatment of Endometriosis in Monkeys: Effectiveness of Continuous Infusion of a Gonadotropin-Releasing Hormone Agonist Compared to Treatment with a Progestational Steroid*

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Abstract
The use of a GnRH agonist or a progestational steroid (levonorgestrel) for the treatment of endometriosis in monkeys was compared. Four monkeys with spontaneous endometriosis were treated for 6 months with a continuous infusion of a GnRH agonist (25 μg/day). Five animals with surgically induced endometriosis were treated with the same agonist for 3 months. An additional group of five monkeys with surgically induced endometriosis was treated orally with levonorgestrel (1 mg/kg.day), while a final group of four monkeys served as untreated controls. During agonist treatment, the four monkeys with spontaneous endometriosis gained body weight and had a greater than 80% decline in cyst size (representing a decline in secretory activity). Monkeys with surgically induced endometriosis had almost total resolution of endometrial lesions during agonist treatment, which was maintained throughout a 4-month posttreatment period. After initial stimulation at the onset of the GnRH agonist infusion, serum LH, FSH, estradiol, and progesterone levels decreased to near the levels of detection, where they remained until treatment was terminated. In comparison, levonorgestrel reduced endometrial lesion size, but the monkeys did not resume normal cycles as early as those treated with the agonist. Levonorgestrel-treated monkeys had normal serum LH and FSH levels, but low serum estradiol and progesterone levels. The results of this study indicate that either continuous infusion of a GnRH agonist or administration of levonorgestrel is effective for treating endometriosis in monkeys. The hormonal data suggest that the GnRH agonist acts at the level of the hypothalamus and pituitary, whereas levonorgestrel acts at the ovarian level.

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