24-Oxo metabolites of vitamin D3 analogues: disassociation of their prominent antileukemic effects from their lack of calcium modulation.

The seco-steroid hormone, 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] inhibits proliferation and induces differentiation of malignant cells including those of the hematopoietic system. The 24-oxo metabolite of 1,25(OH)(2)D(3) also has prominent antiproliferative activities against various cancer cells. We chemically synthesized five novel 24-oxo vitamin D(3) analogues and evaluated their abilities both to inhibit clonal growth and induce differentiation of myeloid leukemia cells and to cause hypercalcemia. The 1alpha,25-dihydroxy-16-ene-D(3) [1,25(OH)(2)-16-ene-D(3)] and 1alpha,25-dihydroxy-16-ene-19-nor-D(3) [1,25(OH)(2)-16-ene-19-nor-D(3)] and their 24-oxo metabolites showed greater potency than 1,25(OH)(2)D(3) in their abilities to inhibit clonal proliferation of HL-60, NB4, and U937 leukemic cell lines as measured by methylcellulose soft-gel assay. Their inhibition of clonal growth was irreversible as analyzed by pulse exposure studies. The synthetic analogues also had greater potency than 1,25(OH)(2)D(3) to induce differentiation of HL-60 and NB4 cells as measured by generation of superoxide, nonspecific esterase production, and induction of CD11b and CD14 cell surface antigens and to increase the proportion of these cells in the G(0)-G(1) phase of the cell cycle. For most assays, the 24-oxo metabolite was slightly more potent than the unmodified analogue, and 50% activity was usually found in the nanomolar range. These analogues and their 24-oxo metabolites also inhibited fresh leukemic cell clonal proliferation. Expression of p27(KIP1), a cyclin-dependent kinase inhibitor that plays an important role in blocking the cell cycle, was found by Western blot analysis to be induced by the analogues and their 24-oxo metabolites in both HL-60 and U937 cells, suggesting a possible mechanism by which these analogues inhibit leukemic growth. Notably, the calcemic activity tested by injections of 1alpha,25-dihydroxy-16-ene-24-oxo-19-nor-D(3) in mice was at least 12-fold less than 1alpha,25(OH)(2)-16-ene-19-nor-D(3). Taken together, chemically synthesized 24-oxo metabolites of 1alpha,25(OH)(2)-16-ene-D(3) and 1alpha,25(OH)(2)-16-ene-19-nor-D(3) irreversibly inhibited proliferation and induced differentiation of acute myeloid leukemia cells with minimal toxicity; these compounds may have a role in the maintenance phase of therapy for acute myeloid leukemia.