Antiuterotrophic effects of a pure antioestrogen, ICI 182,780: magnetic resonance imaging of the uterus in ovariectomized monkeys.

ICI 182,780 is a potent specific pure antioestrogen which may offer advantages in breast cancer treatment compared with partial agonists like tamoxifen. To characterize further the potency and efficacy of ICI 182,780, its effects on the uterus of ovariectomized, oestrogen-treated monkeys (Macaca nemestrina) have been measured using magnetic resonance imaging (MRI). Quantitative MRI allows accurate non-invasive repetitive measurements of endometrial and myometrial volume following hormonal treatments, using each animal as its own control. Single i.m. injections of a long-acting oil-based formulation of ICI 182,780 sustained blockade of oestradiol action on the monkey uterus in a dose-dependent manner for 3-6 weeks. Repeated injections of 4 mg ICI 182,780/kg at 4-weekly intervals provided increasingly effective blockade of uterine proliferation. In a short-acting formulation, ICI 182,780 also completely blocked the trophic action of oestradiol, administered concurrently, in ovariectomized monkeys. Similarly, ICI 182,780 caused involution of the uterus stimulated by prior treatment with oestradiol. The rate and extent of uterine involution in monkeys treated with ICI 182,780 was similar to that seen following oestrogen withdrawal. These studies demonstrate that ICI 182,780 is a fully effective pure antioestrogen in a primate.