Conceptually new deltanoids (vitamin D analogs) inhibit multistage skin tumorigenesis

Development of vitamin D analogs (deltanoids) as chemopreventive agents requires separation of desirable antiproliferative and pro-differentiating activities from the undesirable calcemic activity also found in the hormone calcitriol (1α,25-dihydroxyvitamin D3). Therefore, several conceptually new deltanoids were synthesized with modifications to the 1α- and/or 25-hydroxyl groups, positions traditionally considered essential for stimulating biological responses. In this study, 1β-hydroxymethyl-3-epi-25-hydroxyvitamin D3, a non-calcemic CH2 homolog of the natural hormone with antiproliferative activity in vitro, was ineffective as an inhibitor of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced induction of ornithine decarboxylase activity in mouse epidermis. However, a hybrid analog incorporating not only the calcemia-ablating 1β-hydroxymethyl alteration, but potentiating C,D ring 16-unsaturation and side chain 24,24-fluorination and 26,27-homologation was found to be as effective as calcitriol. Several non-calcemic 24- or 25-t-butyl sulfones, some containing side chain fluorination but all lacking the 25-hydroxyl group, were also shown to be active in this assay. Three sulfones and the 1β-hydroxymethyl hybrid were evaluated as inhibitors of multistage carcinogenesis in mouse skin. Female CD-1 mice were initiated with a single dose of 7,12-dimethylbenz[a]anthracene and then promoted twice weekly for 20 weeks with TPA. Deltanoids were applied topically 30 min before TPA. Unlike calcitriol, none of the atypical deltanoids affected body weight gain in these animals. Minimal effects on urinary calcium excretion were observed following chronic treatment with these analogs. All deltanoids inhibited the incidence and multiplicity of papilloma formation, with the hybrid analog showing the greatest efficacy. With this deltanoid, tumor incidence was significantly reduced by 28% and tumor multiplicity by 63%. These results, coupled with the rich chemical diversity available in side chain sulfur-containing deltanoids, particularly when combined with A ring modifications such as 1β-hydroxylalkyl groups, provide important new advances in the fundamental understanding of chemical structure–biological activity relationships as well as more potent and safe vitamin D analogs for cancer chemoprevention and other medicinal uses.