Structure-Activity Relationships of Antiplasmodial Pantothenamide Analogues Reveal a New Way by Which Triazoles Mimic Amide Bonds

Chemical synthesis and enzymatic late-stage diversification of novel pantothenate analogues with antiplasmodial activity

The emergence of resistance to nearly every therapeutic agent directed against malaria-causing Plasmodium parasites emphasises the dire need for new antimalarials. Despite their high potency and low cytotoxicity in vitro, the clinical use of pantothenamides is hindered by pantetheinase-mediated hydrolysis in human serum. We herein report the chemical synthesis and biological activity of a new series of pantothenamide analogues in which the labile amide group is replaced with an isoxazole ring. In addition, we utilised, for the first time, enzymatic late-stage diversification to generate additional isoxazole-containing pantothenamide-mimics. Thirteen novel isoxazole-containing pantothenamide-mimics were generated, several of which display nanomolar antiplasmodial activity against Plasmodium falciparum and are non-toxic to human cells in vitro. Although the derivatives generated via late-stage diversification are less potent than the parent compounds, the most potent still exerted its activity via a mechanism that interferes with the pantothenate-utilising process and appears to be nontoxic to human cells. This increases the appeal of using late-stage diversification to modify pantothenamide-mimics, potentially leading to compounds with improved antiplasmodial and/or pharmacological properties.

Triazole hybrid compounds: A new frontier in malaria treatment

Reviewing the advancements in malaria treatment, the emergence of triazole hybrid compounds stands out as a groundbreaking development. Combining the advantages of triazole and other moieties, these hybrid compounds offer a new frontier in the battle against malaria. Their potential as effective antimalarial agents has captured the attention of researchers and holds promise for overcoming the challenges posed by drug-resistant malaria strains. We focused on their broad spectrum of antimalarial activity of diverse hybridized 1,2,3-triazoles and 1,2,4-triazoles, structure-activity relationship (SAR), drug-likeness, bioavailability and pharmacokinetic properties reported since 2018 targeting multiple stages of the Plasmodium life cycle. This versatility makes them highly effective against both drug-sensitive and drug-resistant strains of P. falciparum, making them invaluable tools in regions where resistance is prevalent. The synergistic effects of combining the triazole moiety with other pharmacophores have resulted in even greater antimalarial potency. This approach has the potential to circumvent existing resistance mechanisms and provide a more sustainable solution to malaria treatment. While triazole hybrid compounds show great promise, further research and clinical trials are warranted to fully evaluate their safety, efficacy and long-term effects. As research progresses, these compounds can potentially revolutionize the field and contribute to global efforts to eradicate malaria, ultimately saving countless lives worldwide.

Itaconate: an antimicrobial metabolite of macrophages

Itaconate is a conjugated 1,4-dicarboxylate produced by macrophages. This small molecule has recently received increasing attention due to its role in modulating the immune response of macrophages upon exposure to pathogens. Itaconate has also been proposed to play an antimicrobial function; however, this has not been explored as intensively. Consistent with the latter, itaconate is known to show antibacterial activity in vitro and was reported to inhibit isocitrate lyase, an enzyme required for survival of bacterial pathogens in mammalian systems. Recent studies have revealed bacterial growth inhibition under biologically relevant conditions. In addition, an antimicrobial role for itaconate is substantiated by the high concentration of itaconate found in bacteria-containing vacuoles, and by the production of itaconate-degrading enzymes in pathogens such as Salmonella enterica ser. Typhimurium, Pseudomonas aeruginosa, and Yersinia pestis. This review describes the current state of literature in understanding the role of itaconate as an antimicrobial agent in host–pathogen interactions.

Pantothenate and CoA biosynthesis in Apicomplexa and their promise as antiparasitic drug targets

The Apicomplexa phylum comprises thousands of distinct intracellular parasite species, including coccidians, haemosporidians, piroplasms, and cryptosporidia. These parasites are characterized by complex and divergent life cycles occupying a variety of host niches. Consequently, they exhibit distinct adaptations to the differences in nutritional availabilities, either relying on biosynthetic pathways or by salvaging metabolites from their host. Pantothenate (Pan, vitamin B5) is the precursor for the synthesis of an essential cofactor, coenzyme A (CoA), but among the apicomplexans, only the coccidian subgroup has the ability to synthesize Pan. While the pathway to synthesize CoA from Pan is largely conserved across all branches of life, there are differences in the redundancy of enzymes and possible alternative pathways to generate CoA from Pan. Impeding the scavenge of Pan and synthesis of Pan and CoA have been long recognized as potential targets for antimicrobial drug development, but in order to fully exploit these critical pathways, it is important to understand such differences. Recently, a potent class of pantothenamides (PanAms), Pan analogs, which target CoA-utilizing enzymes, has entered antimalarial preclinical development. The potential of PanAms to target multiple downstream pathways make them a promising compound class as broad antiparasitic drugs against other apicomplexans. In this review, we summarize the recent advances in understanding the Pan and CoA biosynthesis pathways, and the suitability of these pathways as drug targets in Apicomplexa, with a particular focus on the cyst-forming coccidian, Toxoplasma gondii, and the haemosporidian, Plasmodium falciparum.

A novel heteromeric pantothenate kinase complex in apicomplexan parasites

Coenzyme A is synthesised from pantothenate via five enzyme-mediated steps. The first step is catalysed by pantothenate kinase (PanK). All PanKs characterised to date form homodimers. Many organisms express multiple PanKs. In some cases, these PanKs are not functionally redundant, and some appear to be non-functional. Here, we investigate the PanKs in two pathogenic apicomplexan parasites, Plasmodium falciparum and Toxoplasma gondii. Each of these organisms express two PanK homologues (PanK1 and PanK2). We demonstrate that PfPanK1 and PfPanK2 associate, forming a single, functional PanK complex that includes the multi-functional protein, Pf14-3-3I. Similarly, we demonstrate that TgPanK1 and TgPanK2 form a single complex that possesses PanK activity. Both TgPanK1 and TgPanK2 are essential for T. gondii proliferation, specifically due to their PanK activity. Our study constitutes the first examples of heteromeric PanK complexes in nature and provides an explanation for the presence of multiple PanKs within certain organisms.

1,2,3-Triazoles as Biomimetics in Peptide Science

Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules mimicking peptides is of great importance for the development of new biologically active molecules. Therefore, replacing the amide bond in a peptide with a heterocyclic bioisostere, such as the 1,2,3-triazole ring, can be considered an effective solution for the synthesis of biologically relevant peptidomimetics. These 1,2,3-triazoles may have an interesting biological activity, because they behave as rigid link units, which can mimic the electronic properties of amide bonds and show bioisosteric effects. Additionally, triazole can be used as a linker moiety to link peptides to other functional groups.

Exploring Heteroaromatic Rings as a Replacement for the Labile Amide of Antiplasmodial Pantothenamides

Malaria-causing Plasmodium parasites are developing resistance to antimalarial drugs, providing the impetus for new antiplasmodials. Although pantothenamides show potent antiplasmodial activity, hydrolysis by pantetheinases/vanins present in blood rapidly inactivates them. We herein report the facile synthesis and biological activity of a small library of pantothenamide analogues in which the labile amide group is replaced with a heteroaromatic ring. Several of these analogues display nanomolar antiplasmodial activity against Plasmodium falciparum and/or Plasmodium knowlesi, and are stable in the presence of pantetheinase. Both a known triazole and a novel isoxazole derivative were further characterized and found to possess high selectivity indices, medium or high Caco-2 permeability, and medium or low microsomal clearance in vitro. Although they fail to suppress Plasmodium berghei proliferation in vivo, the pharmacokinetic and contact time data presented provide a benchmark for the compound profile likely required to achieve antiplasmodial activity in mice and should facilitate lead optimization.

Toward a Stable and Potent Coenzyme A-Targeting Antiplasmodial Agent: Structure-Activity Relationship Studies of N-Phenethyl-α-methyl-pantothenamide

Pantothenamides (PanAms) are potent antiplasmodials with low human toxicity currently being investigated as antimalarials with a novel mode of action. These structural analogues of pantothenate, the vitamin precursor of the essential cofactor coenzyme A, are susceptible to degradation by pantetheinase enzymes present in serum. We previously discovered that α-methylation of the β-alanine moiety of PanAms increases their stability in serum and identified N-phenethyl-α-methyl-pantothenamide as a pantetheinase-resistant PanAm with potent, on-target, and selective antiplasmodial activity. In this study, we performed structure-activity relationship investigations to establish whether stability and potency can be improved further through alternative modification of the scissile amide bond and through substitution/modification of the phenyl ring. Additionally, for the first time, the importance of the stereochemistry of the α-methyl group was evaluated in terms of stability versus potency. Our results demonstrate that α-methylation remains the superior choice for amide modification, and that while monofluoro-substitution of the phenyl ring (that often improves ADME properties) was tolerated, N-phenethyl-α-methyl-pantothenamide remains the most potent analogue. We show that the 2S,2'R-diastereomer is far more potent than the 2R,2'R-diastereomer and that this cannot be attributed to preferential metabolic activation by pantothenate kinase, the first enzyme of the coenzyme A biosynthesis pathway. Unexpectedly, the more potent 2S,2'R-diastereomer is also more prone to pantetheinase-mediated degradation. Finally, the results of in vitro studies to assess permeability and metabolic stability of the 2S,2'R-diastereomer suggested species-dependent degradation via amide hydrolysis. Our study provides important information for the continued development of PanAm-based antimalarials.

The coenzyme A biosynthetic pathway: A new tool for prodrug bioactivation

Prodrugs account for more than 5% of pharmaceuticals approved worldwide. Over the past decades several prodrug design strategies have been firmly established; however, only a few functional groups remain amenable to this approach. The aim of this overview is to highlight the use of coenzyme A (CoA) biosynthetic enzymes as a recently explored bioactivation scheme and provide information about its scope of utility. This emerging tool is likely to have a strong impact on future medicinal and biological studies as it offers promiscuity, orthogonal selectivity, and the capability of assembling exceptionally large molecules.