SYNERGISTIC DECREASE OF CLONAL PROLIFERATION, INDUCTION OF DIFFERENTIATION, AND APOPTOSIS OF ACUTE PROMYELOCYTIC LEUKEMIA-CELLS AFTER COMBINED TREATMENT WITH NOVEL 20-EPI VITAMIN-D-3 ANALOGS AND 9-CIS RETINOICACID

Patients with acute promyelocytic leukemia (APL) usually relapse after all-trans retinoic acid (RA) treatment because this therapy fails to eradicate the malignant clone. Our data showed that KH 1060 and other 20-epi vitamin D-3 analogs alone were potent inhibitors of clonal grow th of NB4 cells, an APL cell line (ED(50) similar to 5 x 10(-11) M). T he combination of KH 1060 and 9-cis-RA synergistically and irreversibl y enhanced this effect. Neither KH 1060 nor 9-cis-RA (10(-6) M, 3 d) w ere strong inducers of differentiation of NB4 cells. However, 98% of t he cells underwent differentiation to a mature phenotype with features of both granulocytes and monocytes after exposure to a combination of both compounds. Apoptosis only increased after incubation of NB4 cell s with 9-cis-RA alone (28%) or with a combination of 9-cis-RA plus KH1 060 (32%). Immunohistochemistry showed that the bcl-2 protein decrease d from nearly 100% of the wild-type NB4 cells to 2% after incubation w ith a combination of KH 1060 and 9-cis-RA, and the bar protein increas ed from 50% of wild-type NB4 cells to 92% after culture with both anal ogs (5 x 10(-7) M, 3 d). Western blot analysis paralleled these result s. Studies of APL cells from one untreated individual paralleled our r esults with NB4 cells. Taken together, the data demonstrated that near ly all of the NB4 cells can be irreversibly induced to differentiate t erminally when exposed to the combination of KH 1060 and 9-cis-RA.