1,25-dihydroxyvitamin D₃ (1,25D3) is implicated in many cellular functions, including cell proliferation and differentiation, thus exerting potential antitumor effects. A major limitation for therapeutic use of 1,25D3 are potent calcemic activities. Therefore, synthetic analogs of 1,25D3 for use in anticancer therapy should retain cell differentiating potential, with calcemic activity being reduced. To obtain this goal, the analogs should effectively activate transcription of genes responsible for cell differentiation, leaving the genes responsible for calcium homeostasis less active. In order to better understand this phenomenon, we selected a series of structurally related 19-nor analogs of 1,25D (PRI-5100, PRI-5101, PRI-5105, and PRI-5106) and tested their activities in blood cells and in cells connected to calcium homeostasis. Affinities of analogs to recombinant vitamin D receptor (VDR) protein were not correlated to their pro-differentiating activities. Moreover, the pattern of transcriptional activities of the analogs was different in cell lines originating from various vitamin D-responsive tissues. We thus hypothesized that receptors which participate in transport of the analogs to the cells might contribute to the observed differences. In order to study this hypothesis, we produced renal cells with knock-out of the megalin gene. Our results indicate that megalin has a minor effect on semi-selective activities of vitamin D analogs.

Continuing our structure-activity studies on the vitamin D analogs with the altered intercyclic seco-B-ring fragment, we designed compounds possessing dienyne system conjugated with the benzene D ring. Analysis of the literature data and the docking experiments seemed to indicate that the target compounds could mimic the ligands with a good affinity to the vitamin D receptor (VDR). Multi-step synthesis of the C/D-ring building block of the tetralone structure was achieved and its enol triflate was coupled with the known A-ring fragments, possessing conjugated enyne moiety, using Sonogashira protocol. The structures of the final products were confirmed by NMR, UV and mass spectroscopy. Their binding affinities for the full-length human VDR were determined and it was established that compound substituted at C-2 with exomethylene group showed significant binding to the receptor. This analog was also able to induce monocytic differentiation of HL-60 cells.