Syntheses and Pharmacological Properties of the Histaminic H1 Antagonists Sila-terfenadine-A, Sila-terfenadine-B, Disila-terfenadine, and Sila-fexofenadine: A Study on C/Si Bioisosterism

Recent Advances in the Biological Investigation of Organometallic Platinum-Group Metal (Ir, Ru, Rh, Os, Pd, Pt) Complexes as Antimalarial Agents

In the face of the recent pandemic and emergence of infectious diseases of viral origin, research on parasitic diseases such as malaria continues to remain critical and innovative methods are required to target the rising widespread resistance that renders conventional therapies unusable. The prolific use of auxiliary metallo-fragments has augmented the search for novel drug regimens in an attempt to combat rising resistance. The development of organometallic compounds (those containing metal-carbon bonds) as antimalarial drugs has been exemplified by the clinical development of ferroquine in the nascent field of Bioorganometallic Chemistry. With their inherent physicochemical properties, organometallic complexes can modulate the discipline of chemical biology by proffering different modes of action and targeting various enzymes. With the beneficiation of platinum group metals (PGMs) in mind, this review aims to describe recent studies on the antimalarial activity of PGM-based organometallic complexes. This review does not provide an exhaustive coverage of the literature but focusses on recent advances of bioorganometallic antimalarial drug leads, including a brief mention of recent trends comprising interactions with biomolecules such as heme and intracellular catalysis. This resource can be used in parallel with complementary reviews on metal-based complexes tested against malaria.

Me CAAC=N- : A Cyclic (Alkyl)(Amino)Carbene Imino Ligand

A cyclic (alkyl)(amino)carbene (CAAC) has been shown to react with a covalent azide similar to the Staudinger reaction. The reaction of Me CAAC with trimethylsilyl azide afforded the N-silylated 2-iminopyrrolidine (Me CAAC=NSiMe3 ), which was fully characterized. This compound undergoes hydrolysis to afford the 2-iminopyrrolidine and trimethylsiloxane which co-crystallize as a hydrogen-bonded adduct. The N-silylated 2-iminopyrrolidine was used to transfer the novel pyrrolidine-2-iminato ligand onto both main-group and transition-metal centers. The reaction of the tetrabromodiborane bis(dimethyl sulfide) adduct with two equivalents of Me CAAC=NSiMe3 afforded the disubstituted diborane. The reaction of Me CAAC=NSiMe3 with TiCl4 and CpTiCl3 afforded Me CAAC=NTiCl3 and Me CAAC=NTiCl2 Cp, respectively.

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.

Cyclopalladated organosilane–tethered thiosemicarbazones: novel strategies for improving antiplasmodial activity

Ferrocenyl- and aryl-derived cyclopalladated organosilane thiosemicarbazone complexes were synthesised via C–H bond activation and evaluated for antiplasmodial activity.

In vitro metabolism of haloperidol and sila-haloperidol: new metabolic pathways resulting from carbon/silicon exchange

The neurotoxic side effects observed for the neuroleptic agent haloperidol have been associated with its pyridinium metabolite. In a previous study, a silicon analog of haloperidol (sila-haloperidol) was synthesized, which contains a silicon atom instead of the carbon atom in the 4-position of the piperidine ring. In the present study, the phase I metabolism of sila-haloperidol and haloperidol was studied in rat and human liver microsomes. The phase II metabolism was studied in rat, dog, and human hepatocytes and also in liver microsomes supplemented with UDP-glucuronic acid (UDPGA). A major metabolite of haloperidol, the pyridinium metabolite, was not formed in the microsomal incubations with sila-haloperidol. For sila-haloperidol, three metabolites originating from opening of the piperidine ring were observed, a mechanism that has not been observed for haloperidol. One of the significant phase II metabolites of haloperidol was the glucuronide of the hydroxy group bound to the piperidine ring. For sila-haloperidol, the analogous metabolite was not observed in the hepatocytes or in the liver microsomal incubations containing UDPGA. If silanol (SiOH) groups are not glucuronidated, introducing silanol groups in drug molecules could provide an opportunity to enhance the hydrophilicity without allowing for direct phase II metabolism. To provide further support for the observed differences in metabolic pathways between haloperidol and sila-haloperidol, the metabolism of another pair of C/Si analogs was studied, namely, trifluperidol and sila-trifluperidol. These studies showed the same differences in metabolic pathways as between sila-haloperidol and haloperidol.

Silicon Analogues of the Retinoid Agonists TTNPB and 3-Methyl-TTNPB, Disila-TTNPB and Disila-3-methyl-TTNPB: Chemistry and Biology

Twofold sila-substitution (C/Si exchange) in the saturated ring of the tetrahydronaphthalene skeleton of the retinoid agonists TTNPB (1 a) and 3-methyl-TTNPB (2 a) leads to disila-TTNPB (1 b) and disila-3-methyl-TTNPB (2 b), respectively. The silicon compounds 1 b and 2 b were synthesized in multiple steps, and their identities were established by elemental analyses, multinuclear NMR experiments, and single-crystal X-ray diffraction studies. Like TTNPB (1 a) and 3-methyl-TTNPB (2 a), the analogous silicon-based arotinoids 1 b and 2 b are strong pan-RAR agonists and display the same strong differentiation and apoptosis-inducing activity in NB4 promyelocytic leukemia cells as the parent carbon compounds. These results are in keeping with the nearly isomorphous structures of 1 a and 1 b bound to the complex of the RARbeta ligand-binding domain with the nuclear receptor (NR) box 2 peptide of the SRC-1 coactivator. The contacts within the ligand-binding pocket are identical except for helix H11, for which two turns are shifted in the disila-TTNPB (1 b) complex. This study represents the first comprehensive structure-function analysis of a carbon/silicon switch in a signaling molecule and demonstrates that silicon analogues can have the same biological functionalities and conserved structures as their parent carbon compounds, and it illustrates at the same time that silicon analogues of biologically active compounds have the potential to induce alternative allosteric effects, as in the case of helix H11, which might allow for novel options in drug design.