Antiproliferative Activity of Double Point Modified Analogs of 1,25-Dihydroxyvitamin D2 Against Human Malignant Melanoma Cell Lines.

Vitamin D is a lipid soluble steroid hormone with pleiotropic biological properties, including regulation of cell proliferation, differentiation and apoptosis. As to these desirable anticancer actions, 1,25-dihydroxyvitamins D and analogs have been reported to inhibit the proliferation and to induce differentiation of a wide variety of cancer cell types, including human malignant melanoma. However, there is a need for novel and more efficacious vitamin D analogs, and how best to design such is still an open issue. A series of double point modified (DPM) analogs of 1,25-dihydroxyvitamin D2 (1,25(OH)2D2) induced differentiation of the vitamin D receptor (VDR) positive A375 and VDR negative SK-MEL 188b human malignant melanoma cell lines. Surprisingly, the dose of 1,25(OH)2D2 required to inhibit the proliferation of the A375 melanoma cell line by was several fold lower than that required in the case of 1,25(OH)2D3. To evaluate the impact of the modification in the side chain (additional 22-hydroxyl) and in the A-ring (5,6-trans modification), the regular side-chain of vitamin D2 or D3 was retained in the structure of our analogs. As expected, 5,6-trans modification was advantageous to enhancing the anti-proliferative activity of analogs, but not as a single point modification (SPM). Very unexpectedly, the additional 22-hydroxyl in the side-chain reduced significantly the anti-proliferative activity of both the natural and 5,6-trans series analogs. Finally, an induction of pigmentation in melanoma SK-MEL 188b cells was observed to sensitized cells to the effect of vitamin D analogs.

Biological Evaluation of Double Point Modified Analogues of 1,25-Dihydroxyvitamin D2 as Potential Anti-Leukemic Agents.

Structurally similar double-point modified analogues of 1,25-dihydroxyvitamin D2 (1,25D2) were screened in vitro for their pro-differentiating activity against the promyeloid cell line HL60. Their affinities towards human full length vitamin D receptor (VDR) and metabolic stability against human vitamin D 24-hydroxylase (CYP24A1) were also tested. The analogues (PRI-1730, PRI-1731, PRI-1732, PRI-1733 and PRI-1734) contained 5,6-trans modification of the A-ring and of the triene system, additional hydroxyl or unsaturation at C-22 in the side chain and reversed absolute configuration (24-epi) at C-24 of 1,25D2. As presented in this paper, introduction of selected structural modifications simultaneously in two distinct parts of the vitamin D molecule resulted in a divergent group of analogues. Analogues showed lower VDR affinity in comparison to that of the parent hormones, 1,25D2 and 1,25D3, and they caused effective HL60 cell differentiation only at high concentrations of 100 nM and above. Unexpectedly, introducing of a 5,6-trans modification combined with C-22 hydroxyl and 24-epi configuration switched off entirely the cell differentiation activity of the analogue (PRI-1734). However, this analogue remained a moderate substrate for CYP24A1, as it was metabolized at 22%, compared to 35% for 1,25D2. Other analogues from this series were either less (12% for PRI-1731 and PRI-1733) or more (52% for PRI-1732) resistant to the enzymatic deactivation. Although the inactive analogue PRI-1734 failed to show VDR antagonism, when tested in HL60 cells, its structure might be a good starting point for our design of a vitamin D antagonist.

Convergent synthesis of double point modified analogs of 1α,25-dihydroxyvitamin D2 for biological evaluation.

There is a long lasting controversy over the biological activity of vitamin D2 as compared to vitamin D3 in terms of maintaining of calcium homeostasis and raising the level of circulating 25-OH-D. To shed more light on this relationship we synthesized 1α,25-dihydroxyvitamin D2, by a novel convergent strategy, to compare this compound directly with the activity of 1α,25-dihydroxyvitamin D3. The same synthetic strategy also provided a series of (5E,7E) geometric isomers of the natural 1α,25-dihydroxyvitamin D2 as well as a series of double point modified analogs of its (24R)-epimer, including C-22 hydroxy derivatives. The structure of the new analogs was determined by 1H and 13C NMR as well as by mass spectrometry. The influence of (5E,7E) modification, alone or in combination with additional modifications in the side chain, on the activity profile and metabolic deactivation of analogs of 1α,25-dihydroxyvitamin D2 still remains unknown. (5E,7E) modification in the structure of new analogs of 1α,25-dihydroxyvitamin D2 is expected to give analogs with no influence on calcium level, as was previously obtained for the analogs of 1α,25-dihydroxyvitamin D3. Investigation of the affinities for the vitamin D receptor and cell differentiation, transcriptional and calcium activities of the most active form of vitamin D2 and of (5E,7E) analogs, compared to 1α,25-dihydroxyvitamin D3, is underway in the collaborating laboratories.

Double point modified analogs of vitamin d as potent activators of vitamin D receptor.

Rational design, chemical synthesis, structural analysis, molecular modeling and biological evaluation are reviewed for all the double point modified vitamin D analogs that have been developed as potential therapeutics over the last several years. The idea of double modifications was based on the 3D structure of the ligand binding domain of the model of the vitamin D receptor. It was recently proved that structural modifications in the two remote parts of the vitamin D molecule might have additive biological effects resulting in an increased functional activity and lowered calcemic side effect. Recent in vivo experiments clearly demonstrated the potential use of these analogs in new therapeutic areas such as autoimmune and hyper-proliferative diseases, including cancer and the systemic treatment of psoriasis. Although some of these analogs are already approaching clinical trials, the molecular mechanism of action and their improved efficiency still remain to be fully understood. In this review the key steps of the convergent synthetic strategies that combine the modified A-ring and the CD-ring fragment carrying the altered side-chain are presented. The advantages of using the natural alicyclic and acyclic precursors are demonstrated as well as all the modern synthetic methodologies used for combining structural fragments. The results of molecular mechanics modeling are critically examined as well as the advantages and limitations of the use of the models of vitamin D proteins for the docking experiments and the design of new analogs. The potential use of advanced structural approaches, including high resolution X-ray crystallography, is discussed as to the prospect of providing a better understanding of the observed activity of modified analogs. Biological profiles in vitro and in vivo for groups of analogs are presented in a new tabular form to illustrate structure activity relationships.