Enhancement of the antitumor efficacy of the antiprogestin, onapristone, by combination with the antiestrogen, ICI 164384.

So far, no combination of endocrine treatments has been routinely used in the therapy of breast Cancer. It was, therefore, our interest to determine whether the combination of the antiprogestin, onapristone (ON), and the pure antiestrogen, ICI 164384 (ICI) might provide a more effective therapy than either monotherapy in experimental mammary tumors containing both estrogen and progesterone receptors. In the MXT-mammary tumor of the mouse, ON (5 mg/kg) administered for 3 weeks exerted an ovariectomy-like antitumor effect (56% inhibition), whereas ICI (30 mg/kg) was weakly effective (28% inhibition). The combination of ON and ICI was, however, distinctly more effective than the monotherapies or ovariectomy, causing 78% inhibition. A similar potentiation of antitumor effect by the combination was manifested in the dimethylbenzanthracene-induced mammary tumor of the rat when ON (5 mg/kg) and ICI (30 mg/kg) were administered once daily for 4 weeks (s.c.). The remission rates of tumors found after treatment with ICI, ON, the combination and ovariectomy (complete and partial remission) were 15%, 46%, 71% and 100% respectively. In the animals bearing DMBA-induced tumors, treatment with ON alone significantly increased the serum levels of luteinizing hormone and prolactin, but caused only a slight increase in the peripheral levels of estradiol and progesterone. ON had no appreciable effect on the uterine and ovarian weights. ICI reduced the uterine weight and the serum progesterone level. In the combination with ON, ICI reversed the effect of ON on the progesterone level without influencing the luteinizing hormone and prolactin levels. These findings suggest that the augmentation of antitumor effectiveness by the combination of two antihormones can be ascribed not only to their effects at estrogen- and progesterone-receptor-binding sites, but also to the decrease in the peripheral level of progesterone. Thus, an appropriate combination of antiprogestin and pure antiestrogen may be useful in the management of breast cancer.