Potent Inhibitors of Plasmodial Serine Hydroxymethyltransferase (SHMT) Featuring a Spirocyclic Scaffold.

With the discovery that serine hydroxymethyltransferase (SHMT) is a druggable target for antimalarials, the aim of this study was to design novel inhibitors of this key enzyme in the folate biosynthesis cycle. Herein, 19 novel spirocyclic ligands based on either 2-indolinone or dihydroindene scaffolds and featuring a pyrazolopyran core are reported. Strong target affinities for Plasmodium falciparum (Pf) SHMT (14-76 nm) and cellular potencies in the low nanomolar range (165-334 nm) were measured together with interesting selectivity against human cytosolic SHMT1 (hSHMT1). Four co-crystal structures with Plasmodium vivax (Pv) SHMT solved at 2.2-2.4 Šresolution revealed the key role of the vinylogous cyanamide for anchoring ligands within the active site. The spirocyclic motif in the molecules enforces the pyrazolopyran core to adopt a substantially more curved conformation than that of previous non-spirocyclic analogues. Finally, solvation of the spirocyclic lactam ring of the receptor-bound ligands is discussed.

Conformational Aspects in the Design of Inhibitors for Serine Hydroxymethyltransferase (SHMT): Biphenyl, Aryl Sulfonamide, and Aryl Sulfone Motifs.

Malaria remains a major threat to mankind due to the perpetual emergence of resistance against marketed drugs. Twenty-one pyrazolopyran-based inhibitors bearing terminal biphenyl, aryl sulfonamide, or aryl sulfone motifs were synthesized and tested towards serine hydroxymethyltransferase (SHMT), a key enzyme of the folate cycle. The best ligands inhibited Plasmodium falciparum (Pf) and Arabidopsis thaliana (At) SHMT in target, as well as PfNF54 strains in cell-based assays in the low nanomolar range (18-56 nm). Seven co-crystal structures with P. vivax (Pv) SHMT were solved at 2.2-2.6 Šresolution. We observed an unprecedented influence of the torsion angle of ortho-substituted biphenyl moieties on cell-based efficacy. The peculiar lipophilic character of the sulfonyl moiety was highlighted in the complexes with aryl sulfonamide analogues, which bind in their preferred staggered orientation. The results are discussed within the context of conformational preferences in the ligands.

Mechanism of Allosteric Inhibition of the Enzyme IspD by Three Different Classes of Ligands.

Enzymes of the nonmevalonate pathway of isoprenoid biosynthesis are attractive targets for the development of herbicides and drugs against infectious diseases. While this pathway is essential for many pathogens and plants, mammals do not depend on it for the synthesis of isoprenoids. IspD, the third enzyme of the nonmevalonate pathway, is unique in that it has an allosteric regulatory site. We elucidated the binding mode of phenylisoxazoles, a new class of allosteric inhibitors. Allosteric inhibition is effected by large conformational changes of a loop region proximal to the active site. We investigated the different roles of residues in this loop by mutation studies and identified repulsive interactions with Asp291 and Asp292 to be responsible for inhibition. Crystallographic data and the response of mutant enzymes to three different classes of allosteric inhibitors provide an in-depth understanding of the allosteric mechanism. The obtained mutant enzymes show selective resistance to allosteric inhibitors and provide conceptually valuable information for future engineering of herbicide-resistant crops. We found that the isoprenoid precursors IPP and DMAPP are natural inhibitors of Arabidopsis thaliana IspD; however, they do not seem to bind to the allosteric site.

Antimalarial Inhibitors Targeting Serine Hydroxymethyltransferase (SHMT) with in Vivo Efficacy and Analysis of their Binding Mode Based on X-ray Cocrystal Structures.

Target-based approaches toward new antimalarial treatments are highly valuable to prevent resistance development. We report several series of pyrazolopyran-based inhibitors targeting the enzyme serine hydroxymethyltransferase (SHMT), designed to improve microsomal metabolic stability and to identify suitable candidates for in vivo efficacy evaluation. The best ligands inhibited Plasmodium falciparum (Pf) and Arabidopsis thaliana (At) SHMT in target assays and PfNF54 strains in cell-based assays with values in the low nanomolar range (3.2-55 nM). A set of carboxylate derivatives demonstrated markedly improved in vitro metabolic stability (t1/2 > 2 h). A selected ligand showed significant in vivo efficacy with 73% of parasitemia reduction in a mouse model. Five new cocrystal structures with PvSHMT were solved at 2.3-2.6 Å resolution, revealing a unique water-mediated interaction with Tyr63 at the end of the para-aminobenzoate channel. They also displayed the high degree of conformational flexibility of the Cys364-loop lining this channel.

An amphiphilic graft copolymer-based nanoparticle platform for reduction-responsive anticancer and antimalarial drug delivery.

Medical applications of anticancer and antimalarial drugs often suffer from low aqueous solubility, high systemic toxicity, and metabolic instability. Smart nanocarrier-based drug delivery systems provide means of solving these problems at once. Herein, we present such a smart nanoparticle platform based on self-assembled, reduction-responsive amphiphilic graft copolymers, which were successfully synthesized through thiol-disulfide exchange reaction between thiolated hydrophilic block and pyridyl disulfide functionalized hydrophobic block. These amphiphilic graft copolymers self-assembled into nanoparticles with mean diameters of about 30-50 nm and readily incorporated hydrophobic guest molecules. Fluorescence correlation spectroscopy (FCS) was used to study nanoparticle stability and triggered release of a model compound in detail. Long-term colloidal stability and model compound retention within the nanoparticles was found when analyzed in cell media at body temperature. In contrast, rapid, complete reduction-triggered disassembly and model compound release was achieved within a physiological reducing environment. The synthesized copolymers revealed no intrinsic cellular toxicity up to 1 mg mL(-1). Drug-loaded reduction-sensitive nanoparticles delivered a hydrophobic model anticancer drug (doxorubicin, DOX) to cancer cells (HeLa cells) and an experimental, metabolically unstable antimalarial drug (the serine hydroxymethyltransferase (SHMT) inhibitor (±)-1) to Plasmodium falciparum-infected red blood cells (iRBCs), with higher efficacy compared to similar, non-sensitive drug-loaded nanoparticles. These responsive copolymer-based nanoparticles represent a promising candidate as smart nanocarrier platform for various drugs to be applied to different diseases, due to the biocompatibility and biodegradability of the hydrophobic block, and the protein-repellent hydrophilic block.

Inhibitors of plasmodial serine hydroxymethyltransferase (SHMT): cocrystal structures of pyrazolopyrans with potent blood- and liver-stage activities.

Several of the enzymes related to the folate cycle are well-known for their role as clinically validated antimalarial targets. Nevertheless for serine hydroxymethyltransferase (SHMT), one of the key enzymes of this cycle, efficient inhibitors have not been described so far. On the basis of plant SHMT inhibitors from an herbicide optimization program, highly potent inhibitors of Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) SHMT with a pyrazolopyran core structure were identified. Cocrystal structures of potent inhibitors with PvSHMT were solved at 2.6 Å resolution. These ligands showed activity (IC50/EC50 values) in the nanomolar range against purified PfSHMT, blood-stage Pf, and liver-stage P. berghei (Pb) cells and a high selectivity when assayed against mammalian cell lines. Pharmacokinetic limitations are the most plausible explanation for lack of significant activity of the inhibitors in the in vivo Pb mouse malaria model.

Deorphaning pyrrolopyrazines as potent multi-target antimalarial agents.

The discovery of pyrrolopyrazines as potent antimalarial agents is presented, with the most effective compounds exhibiting EC50 values in the low nanomolar range against asexual blood stages of Plasmodium falciparum in human red blood cells, and Plasmodium berghei liver schizonts, with negligible HepG2 cytotoxicity. Their potential mode of action is uncovered by predicting macromolecular targets through avant-garde computer modeling. The consensus prediction method suggested a functional resemblance between ligand binding sites in non-homologous target proteins, linking the observed parasite elimination to IspD, an enzyme from the non-mevalonate pathway of isoprenoid biosynthesis, and multi-kinase inhibition. Further computational analysis suggested essential P. falciparum kinases as likely targets of our lead compound. The results obtained validate our methodology for ligand- and structure-based target prediction, expand the bioinformatics toolbox for proteome mining, and provide unique access to deciphering polypharmacological effects of bioactive chemical agents.

Identification of 1,3-diiminoisoindoline carbohydrazides as potential antimalarial candidates.

A series of inhibitors of plant enzymes of the non-mevalonate pathway from herbicide research efforts at BASF were screened for antimalarial activity in a cell-based assay. A 1,3-diiminoisoindoline carbohydrazide was found to inhibit the growth of Plasmodium falciparum with an IC(50) value <100 nM. Synthesis of a variety of derivatives allowed an improvement of the initial antimalarial activity down to IC(50) =18 nM for the most potent compound, the establishment of a structure-activity relationship, and the evaluation of the cytotoxic profile of the diiminoisoindolines. Furthermore, interesting configurational and conformational aspects for this class of compounds were studied by computational and X-ray crystal structure analysis. Some of the compounds can act as tridentate ligands, forming 2:1 ligand-iron(III) complexes, which also display antimalarial activity in the nanomolar IC(50) range, paired with low cytotoxicity.