A new series of vitamin D analogs is highly active for clonal inhibition, differentiation, and induction of WAF1 in myeloid leukemia

The active form of vitamin D₃ [1α,25-dihydroxyvitamin-D₃ (1α,25(OH)₂D₃)] modulates the proliferation and differentiation of hematopoietic cells. Analogs of 1α,25(OH)₂D₃ that have greater potency may have the potential as adjuvant therapy for high-risk patients in remission for acute myelogenous leukemia (AML) and myelodysplastic syndromes. A new generation of 11 analogs of 1α,25(OH)₂D₃ has been synthesized, and we examined their affects on the human leukemic cell line HL-60. This cell line provides a sensitive monitor of activity of the 1α,25(OH)₂D₃ analogs. All the compounds were potent, producing a 50% clonal inhibition (ED₅₀) in the range of 10^-8 to 10^-11 mol/L; nine of the 11 analogs had ED₅₀, at concentrations that were at least 10-fold lower than those for the parental 1,25(OH)₂D₃. The most active compound [cmpd LA, (22R)-1α,25-(OH)₂- 16,22,23-triene-D₃] had an ED₅₀ of 2 x 10^-11 mol/L; it was also tested on clonogenic cells from patients with AML, and it achieved an ED₅₀ of approximately 6 x 10^-11 mol/L, while 1α,25(OH)₂D₃ produced an ED₅₀ of approximately 10^-8 mol/L on the same population of cells. Five different cell surface markers were examined on HL-60 cells exposed to the 1α,25(OH)₂D₃ analogs: HLA-DR and CD11b were induced by all of the compounds; CD13 was induced by six of the 12 compounds, including 1,25(OH)₂D₃; CD14 was strongly induced by all compounds; and CD38 was induced rather weakly by nine of 12 analogs. WAF1/CIP1/p21, a cyclin- dependent kinase inhibitor (CDKI), which is important in blocking the cell cycle, was examined by Western blot and was found to be induced by all of the compounds, suggesting a possible mechanism by which these analogs inhibit leukemic growth. The induction of WAF1 occurred at concentrations of vitamin D analogs as low as 10^-10 mol/L. This structure-function study showed that a new series of 1α,25(OH)₂D₃ analogs was active in clonal inhibition, as well as induction of differentiation and WAF1 expression of HL-60 cells. The key structural motifs included C-16 double bond, double and/or triple bonds in the side chain, lengthening of the side chain, 20-epi-conformation of the side chain, replacement of six hydrogens at the end of the side chain with fluorines, and the removal of C-19. Consideration should be given to further in vivo testing of toxicity and efficacy to move toward a clinical trial, especially in a setting of minimal residual disease.