Studies on the effects of aromatase inhibitors in model systems for breast cancer

Abstract

Based on measurement of estrogen receptor concentrations in their tumors, about 75% of postmenopausal breast cancer patients have hormone-dependent disease. Endocrine therapy which reduces the effects of estrogens is an important strategy for treating these patients. While antiestrogen therapy has become an accepted practice, clinical utilization of selective aromatase (estrogen synthetase) inhibitor only began in the last few years. It is difficult to evaluate new regimens for breast cancer treatment since all patients receive standard treatment first. In order to compare the effects of three aromatase inhibitors, 4-hydroxyandrostenedione (4-OHA), CGS 16949A and aminoglutethimide (AG), the compounds were investigated both in vitro and in vivo using a new model system developed for this purpose. In JEG-3 cells which express aromatase, these inhibitors showed different effects on the enzyme. Although all inhibited aromatase activity competitively, pretreatment with 4-OHA induced profound and sustained inhibition (inactivation of the enzyme) via an irreversible interaction at the active site. CGS 16949A was also found to cause irreversible inhibition of aromatase but by a different mechanism from that of 4-OHA. In contrast, the inhibitory effect of AG was completely reversible and caused an increase in aromatase activity when AG was removed after pretreatment. In MCF-7 human breast cancer cells transfected with aromatase gene (MCF-7Ca), growth was stimulated by aromatizable androgen, androstenedione ({dollar}\Delta\sp4{dollar}A) in cell culture. 4-OHA, CGS 16949A and AG were not cytotoxic but inhibited {dollar}\Delta\sp4{dollar}A stimulated growth of MCF-7Ca cells at the concentrations which caused {dollar}>{dollar}90% inhibition of aromatase activity. A new model for postmenopausal breast cancer was established by inoculating the MCF-7Ca cells with Matrigel into ovariectomized nude mice. Tumors developed from the cells and their growth was dependent on estrogen synthesized by intratumoral aromatization of supplemented {dollar}\Delta\sp4{dollar}A. Both aromatase inhibitors and antiestrogen inhibited tumor growth. Use of this model demonstrated that aromatase inhibitors are highly effective in inhibiting estrogen dependent processes and may be valuable agents for treating breast cancer.