Exploration of vitamin D receptor pharmacodynamic features under tumoral and normal cells microenviroments: a molecular modeling study

RIBONE, S. R.; CERUTTI, J.P.; VITALE, C.; FERRONATO, M. J.; CURINO, A. C.; QUEVEDO, M. A.

Salta

Workshop; 1st Workshop on Drug Discovery; 2019

Sociedad Argentina de Investigación Bioquímica y Biología Molecular

The vitamin D receptor (VDR) is a member of the nuclear receptor (NR) superfamily that binds different isotypes of retinoid X corregulators. The involvement of VDR in antiproliferative and antitumoral cellular pathways has attracted much scientific attention, mostly as a therapeutic target to treat cancer. Unfortunately, Calcitriol (CTR), a recognized agonist of VDR, cannot be used as antiproliferative agent due to its hypercalcemic effects. Among the pharmacophoric contacts established by CTR, the interactions with residues His305 and His397 of VDR are particularly relevant, forming a network of hydrogen bond interactions through the hydroxyl group present in the ligand sidechain. We have already reported the synthesis and evaluation of several CTR analogues, obtained by modification of its sidechain, with some compounds exhibiting interesting antiproliferative and/or selectivity properties, with associated hypercalcemic effects. In this context, the possibility of obtaining CTR analogs with selective antiproliferative effect towards tumoral cells constitutes a particularly interesting aspect. We have previously reported that differences in the pH environment between tumoral and normal cells may represent a key physicochemical property driving this selectivity. In this work, we present molecular modeling studies (i.e. constant pH molecular dynamics, molecular docking, classical molecular dynamics and free energy of binding analysis) performed in order to elucidate at an atomistic level the molecular basis behind the potential of CTR analogs to reach tumoral cells selectivity. In this way, we found that His305 and His397 residues exhibited a specific tautomeric configuration, which is required for the genomic effects of VDR. This feature is closely related to an hydrogen bond network conecting the ligand binding site and the activating factor 2 (AF-2) domain, which in turn produces an allosteric control of VDR agonism. Also, differential acid-base properties were observed for His305 and His397 as evidenced by constant pH molecular dynamic simulations (CpHMD). In this way, His305 exhibited an increased acidity (pKa 5.5) compared to His397(pKa 6.6) and to typical histidine residues. This feature favors the pharmacodynamic interaction of CTR analogs possesing electron-rich groups as part of the sidechain, thus conferring them a higher affinity for VDR at pH 5.5 (tumoral environment) compared to the VDR receptor modeled at pH 7.4 (normal cells environment). On the other hand, non-selective compounds (such as CTR), exhibits a homologous interaction patterns with VDR at both pH conditions.To the best of our knowledge, the results presented constitutes the first evidence related to physicochemical features of VDR that may represent a key molecular feature for the design of new CTR analogs exhibiting not only improved potency and lowered hypercalcemic effects, but also high selectivity indexes.