19-nor-26,27-bishomo-Vitamin D₃ Analogs: A Unique Class of Potent Inhibitors of Proliferation of Prostate, Breast, and Hematopoietic Cancer Cells

Abstract

Vitamin D₃ [1,25-dihydroxyvitamin-D₃ (1,25(OH)₂D₃)] modulates the proliferation and differentiation of many cell types. Analogs of 1,25(OH)₂D₃ that have greater potency may become adjuvant therapy for breast and prostate cancers, myelodysplastic syndrome, acute myelogenous leukemia in remission and other cell types, especially in the setting of low disease burden. A new class of analogs of 1,25(OH)₂D₃ has been synthesized that has a novel 19-nor motif, as well as incorporating many structural elements previously shown to increase potency. These analogs were examined for their effects on prostate cancer cell lines (PC-3, LN-CaP, and DU 145), a human breast cell line (MCF-7), and an acute myeloid leukemia cell line (HL-60). Dose-response clonogenic studies showed that each of these analogs had more potent antiproliferative activities against the cancer cells than 1,25(OH)₂D₃, and 1,25-(OH)₂-16,23Z-diene-26,27-bishomo-19-nor-D₃ (Ro 27-2014) was the most potent analog [10-fold increased activity compared to 1,25(OH)₂D₃]. Further studies were performed using Ro 27-2014. Pulse-exposure studies showed that a 5-day pulse-exposure to Ro 27-2014 (10^-7 m) in liquid culture was adequate to achieve a 50% inhibition of MCF-7 clonal growth in soft agar in the absence of the analog, suggesting that the growth inhibition mediated by the analog was irreversible. Cell cycle analyses using MCF-7 cells showed that Ro 27-2014 (10^-7 m for 4 days) induced a significant increase in the number of cells in G₀-G₁ (72.8 ± 8.9% versus 49.9 ± 3.5% in control cells), with a concomitant decrease in the percent of cells in S phase (13.1 ± 6.2% versus 35.8 ± 3.5% in control cells). The chief toxicity of vitamin D₃ compounds is hypercalcemia, and therefore, we examined calcemic activity of Ro 27-2014 in mice and found it not to induce hypercalcemia at doses of 0.05 µg i.p. three times per week. In contrast, the same dose of a 19-nor vitamin D₃ compound with 6 fluorines on the side chain (1,25-(OH)₂-16-ene-23-yne-26,27-F₆-19-nor-D₃), although also having potent anticancer activity, caused severe hypercalcemia (18 mg/dl). In summary, 19-nor vitamin D₃ compounds with desaturation and lengthening of their side chains result in a series of compounds with a good therapeutic index, having potent anticancer activity and low toxicity.