Design and synthesis of silicon-containing steroid sulfatase inhibitors possessing pro-estrogen antagonistic character

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.

Preparation and properties of silicon-containing benzoxazine with high thermal stability

A silicon-containing benzoxazine (PDpsp-a) was synthesized from bis( p-hydroxyphenyl)diphenylsilane, aniline, and paraformaldehyde. The structure of the monomer was supported by 1H-NMR and FTIR spectra. The curing behavior of benzoxazine was evaluated by differential scanning calorimetry and in-situ FTIR spectra. The thermal properties were studied by MDSC, TGA, and Py-GC/MS. The results indicated that the characteristic peak of oxazine ring began to disappear when the temperature was heated to 180°C and completely disappeared at 260°C. The polybenzoxazine (PDpsp-a) possessed a high glass transition temperatures (174°C) and had good thermal stability (T10 = 420°C). In the pyrolysates of polybenzoxazine (PDpsp-a), no silicon-containing compounds, no phenol species, and more benzene were detected, we speculated that the Ar-Si bond would fracture with the increase of temperature. The benzene was volatilized from the system as a pyrolysis product and the silicon could react with oxygen to form siloxanes remained in the carbon residue in the form of siloxane compounds. The formed silica layer could endow the silicon-containing polybenzoxazine high thermal degradation stability and high char yield.

Recent progress in the development of steroid sulphatase inhibitors - examples of the novel and most promising compounds from the last decade

The purpose of this review article is to provide an overview of recent achievements in the synthesis of novel steroid sulphatase (STS) inhibitors. STS is a crucial enzyme in the biosynthesis of active hormones (including oestrogens and androgens) and, therefore, represents an extremely attractive molecular target for the development of hormone-dependent cancer therapies. The inhibition of STS may effectively reduce the availability of active hormones for cancer cells, causing a positive therapeutic effect. Herein, we report examples of novel STS inhibitors based on steroidal and nonsteroidal cores that contain various functional groups (e.g. sulphamate and phosphorus moieties) and halogen atoms, which may potentially be used in therapies for hormone-dependent cancers. The presented work also includes examples of multitargeting agents with STS inhibitory activities. Furthermore, the fundamental discoveries in the development of the most promising drug candidates exhibiting STS inhibitory activities are highlighted.

Lead optimization of 4-(thio)-chromenone 6-O-sulfamate analogs using QSAR, molecular docking and DFT - a combined approach as steroidal sulfatase inhibitors

Aromatase and steroidal sulfatase (STS) are steroidogenic enzyme that increases the concentration of estrogens in circulation, a primary factor leading to breast cancer. At molecular level, 87% of STS is expressed and an inhibitor targeting STS could decrease the level of estrogens. In an attempt to identify the chemical structural requirement targeting placental STS inhibition, 26 compounds with pIC₅₀ ranging from 4.61 to 9.46 were subjected to computational studies including Quantitative Structural-Activity Relationship (QSAR), MolecularDocking followed by Density Functional Theory (DFT) studies. A robust and predictable model were developed with good R² (0.834) and cross-validated correlation coefficient value Q² _LOO (0.786) explaining the relationship quantitatively. The regression graphs suggests that the STS inhibition was greatly dependent on the electro topological state of an atom, sum of the atom type E-state (SdssC), maximum E-states for strong hydrogen bond acceptors (maxHBa) and basic group count descriptor (BCUTp-1h). Furthermore, docking results showed favorable interactions of sulfamate analogs with catalytically important amino acid residues such as LEU74, VAL101, and VAL486. The interactions of the best active compound 3j when compared with standard Irosustat show similar binding energies. DFT studies further confirm the presence of HOMO orbital centered on chromenone ring further highlighting its importance for receptor ligand hydrophobic interaction. The study reveals that substitution of thio in chromenone nucleus and introduction of adamantyl substitution at second position are favorable in inhibiting the enzyme STS.

Synthesis and thermal properties of silicon-containing benzoxazine

A novel benzoxazine, containing silicon (Si) in the main chain and bonded to two benzene ring, was synthesized from aniline, bis( p-hydroxyphenyl)dimethylsilane, and paraformaldehyde. The structure was characterized by proton nuclear magnetic resonance and Fourier transform infrared (FTIR) spectra. The curing behavior of the benzoxazine was evaluated by differential scanning calorimeter and in situ FTIR. The thermal stability of the resulting polybenzoxazine was studied by thermogravimetric analysis under nitrogen and air atmospheres. The results indicated that the Si-containing polybenzoxazine possessed significantly higher initial degradation temperature and char yield than conventional bisphenol A/aniline-based polybenzoxazine.

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.

Ru(ii)-Pheox-catalyzed Si–H insertion reaction: construction of enantioenriched carbon and silicon centers

We established a highly enantioselective Si–H insertion reaction to construct chiral centers at the carbon and silicon atoms, using a Ru(ii)–Pheox catalyst.

Synthesis and evaluation of raloxifene derivatives as a selective estrogen receptor down-regulator

Estrogen receptors (ERs) play a major role in the growth of human breast cancer cells. A selective estrogen receptor down-regulator (SERD) that acts as not only an inhibitor of ligand binding, but also induces the down-regulation of ER, would be useful for the treatment for ER-positive breast cancer. We previously reported that tamoxifen derivatives, which have a long alkyl chain, had the ability to down-regulate ERα. With the aim of expanding range of the currently available SERDs, we designed and synthesized raloxifene derivatives, which had various lengths of the long alkyl chains, and evaluated their SERD activities. All compounds were able to bind ERα, and RC10, which has a decyl group on the amine moiety of raloxifene, was shown to be the most potent compound. Our findings suggest that the ligand core was replaceable, and that the alkyl length was important for controlling SERD activity. Moreover, RC10 showed antagonistic activity and its potency was superior to that of 4,4'-(heptane-4,4-diyl)bis(2-methylphenol) (18), a competitive antagonist of ER without SERD activity. These results provide information that will be useful for the development of promising SERDs candidates.

Synthesis and in vitro growth inhibitory activity of novel silyl- and trityl-modified nucleosides

Seventeen silyl- and trityl-modified (5'-O- and 3',5'-di-O-) nucleosides were synthesized with the aim of investigating the in vitro antiproliferative activities of these nucleoside derivatives. A subset of the compounds was evaluated at a fixed concentration of 100μM against a small panel of tumor cell lines (HL-60, K-562, Jurkat, Caco-2 and HT-29). The entire set was also tested at varying concentrations against two human glioma lines (U373 and Hs683) to obtain GI50 values, with the best results being values of ∼25μM.

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.