C-6 functionalized analogs of 25-hydroxyvitamin D 3 and 1α,25-dihydroxyvitamin D 3: synthesis and binding analysis with vitamin D-binding protein and vitamin D receptor

Abstract

In this article, we describe the development of a general synthetic strategy to functionalize the C-6 position of vitamin D 3 and its biologically important metabolites, i.e. 25-hydroxyvitamin D 3 (25-OH-D 3) and 1α,25-dihydroxyvitamin D 3 [1,25(OH) 2D 3]. We employed Mazur’s cyclovitamin D method to synthesize vitamin D 3 analogs with several functionalities at the C-6 position. In addition, we synthesized 6-(3-hydroxypropyl) and 6-[(2-bromoacetoxy)propyl] derivatives of 25-OH-D 3 15 and 16, respectively, and 6-(3-hydroxypropyl) derivative of 1,25(OH) 2D 3 17. Competitive binding assays of 15– 17 with human serum vitamin D-binding protein showed that all these analogs specifically bound to this protein, although with significantly lower affinity than the 25-OH-D 3, the strongest natural binder, but with comparable affinity with 1,25(OH) 2D 3, the hormone. On the other hand, 6-[3-hydroxypropyl], 1α,25-dihydroxyvitamin D 3 17 did not show any specific binding for recombinant nuclear vitamin D receptor. These results indicated that the region containing the C-6 position of the parent seco-steroid [1,25(OH) 2D 3] may be an important recognition marker towards vitamin D receptor binding. Information, delineated in this article, will be important for evaluating structure-activity relationship in synthetic analogs of vitamin D and its metabolites.