19-nor Vitamin-D Analogs: A New Class of Potent Inhibitors of Proliferation and Inducers of Differentiation of Human Myeloid Leukemia Cell Lines

Abstract

We have studied the in vitro biological activities and mechanisms of action of 1,25-dihydroxyvitamin D3 (1,25D3) and nine potent 1,25D3 analogs on proliferation and differentiation of myeloid leukemia cell lines (HL-60, retinoic acid-resistant HL-60 [RA-res HL-60], NB4 and Kasumi-1). The common novel structural motiff for almost all the analogs included removal of C-19 (19-nor); each also had unsaturation of the side chain. All the compounds were potent; for example, the concentration of analogs producing a 50% clonal inhibition (ED50) ranged between 1 × 10−9 to 4 × 10−11 mol/L when using the HL-60 cell line. The most active compound [1,25(OH)2-16,23E-diene-26-trifluoro-19-nor-cholecalciferol (Ro 25-9716)] had an ED50 of 4 × 10−11mol/L; in contrast, the 1,25D3 produced an ED50of 10−9 mol/L with the HL-60 target cells. Ro 25-9716 (10−9 mol/L, 3 days) was a strong inducer of myeloid differentiation because it caused 92% of the HL-60 cells to express CD11b and 75% of these cells to reduce nitroblue tetrazolium (NBT). This compound (10−8 mol/L, 4 days) also caused HL-60 cells to arrest in the G1 phase of the cell cycle (88% cells in G1v 48% of the untreated control cells). The p27kip-1, a cyclin-dependent kinase inhibitor which is important in blocking the cell cycle, was induced more quickly and potently by Ro 25-9716 (10−7 mol/L, 0 to 5 days) than by 1,25D3, suggesting a possible mechanism by which these analogs inhibit proliferation of leukemic growth. The NB4 promyelocytic leukemia cells cultured with the Ro 25-9716 were also inhibited in their clonal proliferation (ED50, 5 × 10−11mol/L) and their expression of CD11b was enhanced (80% positive [10−9 mol/L, 4 days] v 27% untreated NB4 cells). Moreover, the combination of Ro 25-9716 (10−9mol/L) and all-trans retinoic acid (ATRA, 10−7 mol/L) induced 92% of the NB4 cells to reduce NBT, whereas only 26% of the cells became NBT positive after a similar exposure to the combination of 1,25D3 and ATRA. Surprisingly, Ro 25-9716 also inhibited the clonal growth of poorly differentiated leukemia cell lines (RA-res HL-60 [ED50, 4 × 10−9 mol/L] and Kasumi-1 [ED50, 5 × 10−10 mol/L]). For HL-60 cells, Ro 25-9716 markedly decreased the percent of the cells in S phase of the cell cycle and increased the expression of the cyclin-dependent kinase inhibitor, p27kip-1. In summary, 19-nor vitamin D3 compounds strongly induced differentiation and inhibited clonal proliferation of various myeloid leukemia cell lines, suggesting a therapeutic niche for their use in myeloid leukemia.