Although 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] has been shown to inhibit the growth of certain malignant cells, its hypercalcemic effect has prevented clinical application. We have recently developed a novel vitamin D3 analog, 22-oxa-1,25-(OH)2D3 (OCT), that is capable of promoting differentiation and inhibiting proliferation without inducing hypercalcemia. The present study was undertaken to determine whether OCT could be applied for the treatment of breast cancer with or without estrogen receptor (ER). OCT inhibited the proliferation of both ER-positive (MCF-7, T-47D, and ZR-75-1) and ER-negative breast cancer cells (MDA-MB-231 and BT-20) in vitro in a time- and dose-dependent manner, as determined by cell number and [3H]thymidine uptake. The antiproliferative effect was observed with a concentration as low as 10(-11) M OCT, and treatment of MCF-7 cells with 10(-8) M OCT for 8 days caused more than a 50% reduction in cell number compared with that of vehicle-treated cells. OCT was approximately 1 order of magnitude more potent than 1,25-(OH)2D3 in inhibiting the proliferation of MCF-7 cells. The in vivo effect of OCT was examined in athymic mice implanted with ER-negative MX-1 tumor, which was established as the xenograft derived from human breast carcinoma. Intratumor administration of OCT three times a week remarkably delayed the growth of MX-1 tumor in a time- and dose-dependent manner. The antitumor effect of 1 microgram/kg BW OCT was greater than that of 500 microgram/kg BW adriamycin, and the relative tumor weights in each group on day 26 were 29.7% and 50.5% of that in the vehicle-treated group, respectively. The effects of OCT and adriamycin were additive, and the relative tumor weight after 26 days of combined treatment was 21.7% of that in the vehicle-treated group. Oral administration of OCT was also effective, and the relative tumor weight in the OCT-treated group (1 microgram/kg BW) was 54.6 +/- 0.1% (mean +/- SEM) of that in the vehicle-treated group. Neither intratumor nor oral administration of OCT raised the serum calcium level in these animals. These results demonstrate that OCT is a potent inhibitor of the proliferation of breast cancer cells with or without ER and that OCT inhibits the growth of breast cancer in vivo without inducing hypercalcemia. We suggest that OCT may provide a new strategy for the treatment of breast carcinoma regardless of ER status.