The antitumor effects of 22-oxa-1,25-dihydroxy-vitamin D-3 (OCT), a vitamin D-3 analogue, were evaluated on 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumors. The combined effects of OCT (0.3 mu g/kg) with tamoxifen (0.5 mg/kg) medroxyprogesterone acetate (MPA) 2.5 mg/kg), or a new aromatase inhibitor, CGS 16949A (0.8 mg/kg) were also evaluated. OCT significantly suppressed the growth of tumors without hypercalcemia in a dose dependent manner at the fourth week from the start of treatment. Tumor size in the OCT+CGS 16949A group was significantly decreased compared with that in the OCT or CGS 16949A alone. However, there was no significant difference in tumor size between OCT alone and combined therapy with tamoxifen or MPA. We conclude that a single administration of OCT, which does not cause hypercalcemia, is effective for breast cancer and that a combination of OCT and aromatase inhibitor, CGS 16949A augments the antitumor effect on tumors compared to each single agent.

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