Design, Synthesis, Biological Activity, and Structural Analysis of Novel Des-C-Ring and Aromatic-D-Ring Analogues of 1α,25-Dihydroxyvitamin D₃

Abstract

The toxic calcemic effects of the natural hormone 1α,25-dihydroxyvitamin D3 (1,25D3, 1,25-dihydroxycholecalciferol) in the treatment of hyperproliferative diseases demand the development of highly active and noncalcemic vitamin D analogues. We report the development of two highly active and noncalcemic analogues of 1,25D3 that lack the C-ring and possess an m-phenylene ring that replaces the natural D-ring. The new analogues (3a, 3b) are characterized by an additional six-carbon hydroxylated side chain attached either to the aromatic nucleus or to the triene system. Both compounds were synthesized by the Pd-catalyzed tandem cyclization/cross coupling approach starting from alkyne 6 and diphenol 8. Key steps include a stereoselective Cu-assisted addition of a Grignard reagent to an aromatic alkyne and a Takai olefination of an aromatic aldehyde. The new compounds are noncalcemic and show transcriptional and antiproliferative activities similar to 1,25D3. Structural analysis revealed that they induce a large conformational rearrangement of the vitamin D receptor around helix 6.

Further Studies on the Highly Active Des-C-Ring and Aromatic-D-Ring Analogues of 1α,25-Dihydroxyvitamin D3 (Calcitriol): Refinement of the Side Chain

“…To further explore the potential binding of these analogues, molecular dynamics (MD) calculations were performed. In our study, we used the YASARA suite , with the Amber14 ForceField and a simulation time of 20 ns comparable to our previous study of des-C-ring-aryl-D-ring compounds . The results for the different analogues varied significantly in this study.…”

“…For comparison with previous synthetic approaches to similar compounds, , we chose the Wittig–Horner coupling between ketone 5a and the phosphine oxide anion 4b as the key step for the stereoselective formation of the respective vitamin D trienes of target compounds 2 and 3 (Scheme ) with the expectation that the final target compounds could be separated by chromatography from the corresponding unnatural triene counterparts. Ketone 5a was envisioned to arise from corresponding alkyne 6 .…”

“…Compound 2 forms tighter interactions with zIle299 and zMet300 (Figure A), similar to its parental compound PG-136 . The fluorine atoms at the terminus of the side chain allow analogue 2 to interact more strongly with the residues of helix 3, loops 6–7, and Cter, thus further stabilizing the active conformation and explaining the more potent activity of compound 2 compared to that of PG-136 . In order to maintain the hydrogen bonds formed by the 25-OH group, compound 3 that incorporates a rigidified alkyne side chain requires a different conformation of the aromatic ring and its side chains line the opposite side of the pocket (Figure A).…”

“…VDR binding was evaluated as previously described . The activity of each compound is shown as a percentage, in which the activity of 1 was normalized to 100%.…”

“…The His-tagged zVDR LBD (156-453) was expressed and purified as previously described . The concentrated purified proteins were incubated with a 2-fold excess of ligand and a 3-fold excess of the coactivator NCOA2 NR2 (KHKILHRLLQDSS) peptide.…”