1-ALPHA,25-DIHYDROXY-20-EPI-VITAMIN-D-3 - AN EXTRAORDINARILY POTENT INHIBITOR OF LEUKEMIC-CELL GROWTH IN-VITRO

We have evaluated seven recently synthesized vitamin D-3 analogs for t heir abilities to inhibit clonal growth of leukemic cells, to induce l eukemic cell differentiation, to stimulate clonal growth of normal mye loid committed stem cells, and to transactivate a reporter gene having a 1,25(OH)(2)D-3 response element (VDRE). The 1,25(OH)(2)-20-epi-D-3 showed extraordinary activity; at 10(-11) mol/L it inhibited clonal gr owth of 87% of HL-60 myeloblast cells, 60% of S-LB1 cells (human T-cel l lymphotropic virus type 1 [HTLV-1]-immortalized human T-lymphocyte c ell line) and 50% of leukemic clonogenic cells (colony-forming unit-le ukemia) obtained from patients with acute myelogenous leukemia. No eff ect of either 1,25(OH)(2)D-3 or 1,25(OH)(2)-20-epi-D-3 was observed on the clonal proliferation of an HTLV-1-immortalized human T-lymphocyte cell line (Ab-VDR) having nonfunctional 1,25 (OH)(2)D-3 cellular rece ptors (VDR). The abilities of 1,25(OH)(2)-20-epi-D-3 to induce differe ntiation of HL-60 cells, as measured by generation of superoxide and n onspecific esterase production, was less than its antiproliferative ac tivities. This analog stimulated colony-forming unit-granulocyte-macro phage growth from normal human bone marrow. To gain insights into the remarkable antileukemic activities of 1,25(OH)(2)-20-epi-D-3, we exami ned its ability to enter HL-60 cells, bind to the VDR, and interact wi th a transfected VDRE attached upstream of a TK promoter-driven report er gene (chloramphenicol acetyl transferase [CAT]). The 1,25(OH)(2)-20 -epi-D-3 potently increased CAT activity (>16-fold, as compared with c ells transfected with control receptor having no VDRE); paradoxically, 1,25(OH)(2)-20-epi-D-3 was of equal potency to 1,25(OH)(2)D-3 in tran sactivating the VDRE-containing reporter gene, even though the analog had a 1,000-fold greater antileukemic effect as compared with 1,25(OH) (2)D-3. In summary, we have identified an extremely potent 1,25(OH)(2) D-3 analog with antiproliferative and differentiating effects on leuke mic cells and that may be clinically useful. This analog appears to ge nerate biologic responses via the classical VDR pathway, but further s tudies are required to elucidate the mechanism by which this analog pr oduces its prominent activities. (C) 1994 by The American Society of H ematology.