Synthesis and pharmacological characterization of Disila-AM80 (Disila-tamibarotene) and Disila-AM580, silicon analogues of the RARalpha-selective retinoid agonists AM80 (Tamibarotene) and AM580

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Silicon switch: Carbon-silicon Bioisosteric replacement as a strategy to modulate the selectivity, physicochemical, and drug-like properties in anticancer pharmacophores

Bioisosterism is one of the leading strategies in medicinal chemistry for the design and modification of drugs, consisting in replacing an atom or a substituent with a different atom or a group with similar chemical properties and an inherent biocompatibility. The objective of such an exercise is to produce a diversity of molecules with similar behavior while enhancing the desire biological and pharmacological properties, without inducing significant changes to the chemical framework. In drug discovery and development, the optimization of the absorption, distribution, metabolism, elimination, and toxicity (ADMETox) profile is of paramount importance. Silicon appears to be the right choice as a carbon isostere because they possess very similar intrinsic properties. However, the replacement of a carbon by a silicon atom in pharmaceuticals has proven to result in improved efficacy and selectivity, while enhancing physicochemical properties and bioavailability. The current review discusses how silicon has been strategically introduced to modulate drug-like properties of anticancer agents, from a molecular design strategy, biological activity, computational modeling, and structure-activity relationships perspectives.

Structural development of silicon-containing retinoids: structure-activity relationship study of the hydrophobic pharmacophore of retinobenzoic acids using silyl functionalities

We designed and synthesized a series of retinobenzoic acids bearing various silyl functionalities in order to explore in detail the structure-activity relationship (SAR) at the hydrophobic moiety of retinoids. Among the synthesized compounds, 24c bearing a t -butyldimethylsilyl (TBS) group at the hydrophobic site exhibited potent retinoid activity comparable to that of the lead compound Am555S ( 4 ). Compound 24c exhibited transcription-promoting activity towards all three subtypes of retinoic acid receptor (RAR), but showed the highest activity towards RARγ, in contrast to the high RARα-selectivity of Am80 ( 3 ) and Am555S ( 4 ). The SARs presented here should be helpful in the development of subtype-selective retinoids, and in particular 24c might be a promising lead compound for new RARγ ligands.

Effects of AM80 compared to AC261066 in a high fat diet mouse model of liver disease

The roles of retinoids in nonalcoholic fatty liver disease (NAFLD) remain unclear and a better understanding may lead to therapies that prevent or limit NAFLD progression. We examined the actions of retinoic acid receptor (RAR) agonists- AM80 for RARα and AC261066 for RARβ2- in a murine model of NAFLD. We fed wild type C57Bl/6 mice a chow or a 45% high fat diet (HFD) for 12 weeks, followed by 4 additional weeks with the HFD+AM80; HFD+AC261066; or HFD. The HFD+AM80 group showed greater hyperglycemia and glucose intolerance compared to other groups. Histopathological evaluation of the livers showed the highest degree of steatosis, triglycerides levels, and inflammation, assessed by F4/80 staining, in the HFD+AM80-treated compared to the HFD, the HFD+AC261066, and chow-fed mice. Liver vitamin A (retinol (ROL)) and retinyl palmitate levels were markedly lower in all HFD groups compared to chow-fed controls. HFD+AC261066-treated mice showed higher levels of a key intracellular ROL transporter, retinol-binding protein-1 (RBP1) compared to the HFD and HFD+AM80 groups. In conclusion, these data demonstrate that the selective RARα agonist AM80 exacerbates HFD-induced NAFLD and hyperglycemia. These findings should inform future studies examining the therapeutic potential of RAR agonists in HFD-related disorders.

Progress in the medicinal chemistry of silicon: C/Si exchange and beyond

Application of silyl functionalities is one of the most promising strategies among various ‘elements chemistry’ approaches for the development of novel and distinctive drug candidates. Replacement of one or more carbon atoms of various biologically active compounds with silicon (so-called sila-substitution) has been intensively studied for decades, and is often effective for alteration of activity profile and improvement of metabolic profile. In addition to simple C/Si exchange, several novel approaches for utilizing silicon in medicinal chemistry have been suggested in recent years, focusing on the intrinsic differences between silicon and carbon. Sila-substitution offers great potential for enlarging the chemical space of medicinal chemistry, and provides many options for structural development of drug candidates.

Expanding the chemical space of hydrophobic pharmacophores: the role of hydrophobic substructures in the development of novel transcription modulators

Interactions between biologically active compounds and their targets often involve hydrophobic interactions, and hydrophobicity also influences the pharmacokinetic profile.

Development of silicon-containing bis-phenol derivatives as androgen receptor antagonists: selectivity switching by C/Si exchange

We previously reported that bis-phenol derivatives, including LG190178 (3a), possess not only vitamin D receptor (VDR) agonistic activity, but also androgen receptor (AR) antagonistic activity. Here, we describe the design, synthesis and evaluation of silicon-containing bis-phenol derivatives, with the objective of obtaining increased selectivity toward VDR or AR. We found that replacement of the quaternary carbon in the bis-phenol skeleton with silicon increased AR-antagonistic activity and reduced VDR-agonistic activity, that is, the AR selectivity of the silicon-containing compounds was higher than that of corresponding carbon compounds. To our knowledge, this is the first report of nuclear receptor (NR) selectivity switching by sila-substitution (C/Si exchange). Among the compounds synthesized, AR-selective ligand (S,R)-3b exhibited more potent anti-androgenic activity (IC50=0.072 μM) than hydroxyflutamide, a well-known androgen antagonist (IC50=1.4 μM), in SC-3 cell proliferation assay. These results suggest that sila-substitution is a useful approach for structural development of selective AR ligands.

Disila-analogues of the synthetic retinoids EC23 and TTNN: synthesis, structure and biological evaluation

Silicon chemistry offers the potential to tune the effects of biologically active organic molecules. Subtle changes in the molecular backbone caused by the exchange of a carbon atom for a silicon atom (sila-substitution) can significantly alter the biological properties. In this study, the biological effects of a two-fold sila-substitution in the synthetic retinoids EC23 (4-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-ylethynyl)benzoic acid (4a)) and TTNN (6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-2-naphthoic acid (7a)) as well as their corresponding analogues with an indane instead of a 1,2,3,4-tetrahydronaphthalene skeleton (compounds 5a and 8a) were investigated. Two-fold C/Si exchange in 4a, 5a, 7a and 8a leads to the silicon-analogues disila-EC23 (4b), 5b, disila-TTNN (7b) and 8b, which contain a 1,2,3,4-tetrahydro-1,4-disilanaphthalene (4b, 7b) or 1,3-disilaindane skeleton (5b, 8b). Exchange of the SiCH(2)Si moiety of 5b for an SiOSi fragment leads to the disiloxane 6 (2-oxa-1,3-disilaindane skeleton). The EC23 derivative 5a, the TTNN derivative 8a and the silicon-containing analogues 4b, 5b, 6, 7b and 8b were synthesised, and the biological properties of the C/Si pairs 4a/4b, 5a/5b, 7a/7b and 8a/8b and compound 6 were evaluated in vivo using RAR isotype-selective reporter cells. EC23 (4a) and its derivatives disila-EC23 (4b), 5a, 5b and 6 are very potent RAR agonists, which are even more potent than the powerful reference compound TTNPB. Disila-substitution of EC23 (4a) and 5a leads to a moderate decrease in RARα activation, whereas the RARβ,γ activation is almost not affected. In contrast, two-fold C/Si exchange in the weak retinoid agonist TTNN (7a) and 8a resulted in considerably different effects: a significant increase (7a→7b) and almost no change (8a→8b) in transcription activation potential for all three RAR isotypes. Disila-TTNN (7b) can be regarded as a powerful RARβ,γ-selective retinoid.