19-Nor-1,25(OH)₂D₂ (a novel, noncalcemic vitamin D analogue), combined with arsenic trioxide, has potent antitumor activity against myeloid leukemia

Recently, we reported that a novel, noncalcemic vitamin B analogue (19-nor-1,25(OH)₂D₂; paricalcitol) had anticancer activity. In this study, we explored if paricalcitol enhanced anticancer effects of other clinically useful drugs in vitro against a large variety of cancer cells. Paricalcitol, when combined with As₂O₃, showed a markedly enhanced anti-proliferative effect against acute myeloid leukemia (AMI) cells. This combination induced monocytic differentiation of NB-4 acute promyelocytic leukemia (APL) cells and HL-60 AML cells and caused both to undergo apoptosis associated with down-regulation of Bcl-2 and Bcl-x _L. Paricalcitol induced monocytic differentiation of U937 AML cells, which was partially blocked by inducing expression of APL-related PML-retinoic acid receptor α (RARα) chimeric protein in the U937 cells containing a Zn²⁺-ioducible expression vector coding for this fusion protein (PR9 cells). Exposure to As₂O₃ decreased levels of PML-RARα in PR9 cells, and the combination of paricalcitol and As ₂O₃ enhanced their monocytic differentiation in parallel with the As₂O₃-mediated decrease of PML-RARα. Furthermore, As₂O₃ increased the transcriptional activity of paricalcitol probably by increasing intracellelar levels of paricalcitol by decreasing the function of the mitochondrial enzyme 25-hydroxyvitamin D ₃-24-hydroxylase, which functions to metabolize the active vitamin D in cells. In summary, the combination of paricalcitol and As₂O ₃ potently decreased growth and induced differentiation and apoptosis of AML cells. This probably occurred by As₂O₃ decreasing levels of both the repressive PML-RARα fusion protein and the vitamin D metabolizing protein, 25-hydtoxyvitamin D₃-24-hydroxyase, resulting in increased activity of paricalcitol. The combination of both of these Food and Drug Administration-approved drugs should be considered for treatment of nu-traits retinoic acid-resistant APL patients as well as those with other types of AML.