THE BIOLOGICAL-ACTIVITY OF NONSTEROIDAL VITAMIN-D HORMONE ANALOGS LACKING BOTH THE C-RINGS AND D-RINGS

1 alpha,25-dihydroxyvitamin D is a key calcium-regulating hormone but also displays potent differentiating and antiproliferative activities on many cell types. The structural requirements of this secosteroid ho rmone have been extensively studied for the A-ring and side chain, whe reas relatively little is known about the requirements of the natural CD-ring structure for the vitamin D-like biological activity. We have embarked on a vast program in which derivatives were synthesized and e valuated characterized by profound structural changes in the central C /D-region. This first series of nonsteroidal analogs consists of l)-2- butenylidene)-4-methylenecyclohexane-1,3-diol (KS 176) and derivatives thereof. These analogs are characterized by the absence of normal C-a nd D-rings and by the presence of an unnatural five-membered ring whic h we call the E-ring. KS 176 with the otherwise natural side chain str ucture of 1 alpha,25(OH)(2)D-3 has between 10 and 30% of the biologica l activity of 1 alpha,25(OH)(2)D-3 when tested in vitro (prodifferenti ating effects on HL-60 and MG-63; antiproliferating activity on MCF-7 and keratinocytes) but has minimal in vivo calcemic effects. Introduct ion of several side chain modifications created analogs with increased intrinsic noncalcemic biological properties, whereas their calcemic p otency remains very low. These data demonstrate that the full CD-rings are not mandatory for the biological activity of 1 alpha,25(OH)(2)D-3 since they can be replaced by a new ring structure which generates an appropriate spacing of the A-seco B-rings in relation to the side cha in. The biological activity of these nonsteroidal analogs probably inv olves a classical genomic activation since they are also active in tra nsfection assays using an osteocalcin vitamin D responsive element cou pled to a human growth hormone reporter gene.