Stereochemical modification of geminal dialkyl substituents on pantothenamides alters antimicrobial activity

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.

Discovery of antiplasmodial pyridine carboxamides and thiocarboxamides

Malaria continues to be a significant burden, particularly in Africa, which accounts for 95% of malaria deaths worldwide. Despite advances in malaria treatments, malaria eradication is hampered by insecticide and antimalarial drug resistance. Consequently, the need to discover new antimalarial lead compounds remains urgent. To help address this need, we evaluated the antiplasmodial activity of twenty-two amides and thioamides with pyridine cores and their non-pyridine analogues. Twelve of these compounds showed in vitro anti-proliferative activity against the intraerythrocytic stage of Plasmodium falciparum, the most virulent species of Plasmodium infecting humans. Thiopicolinamide 13i was found to possess submicromolar activity (IC50 = 142 nM) and was >88-fold less active against a human cell line. The compound was equally effective against chloroquine-sensitive and -resistant parasites and did not inhibit β-hematin formation, pH regulation or PfATP4. Compound 13i may therefore possess a novel mechanism of action.

Use of Strain-Release for the Diastereoselective Construction of Quaternary Carbon Centers

Herein, we describe the formation of quaternary carbon centers with excellent diastereoselectivity via a strain-release protocol. An organometallic species is generated by Cp*Rh(III)-catalyzed C-H activation, which is then coupled with strained bicyclobutanes (BCBs) and a prochiral carbon electrophile in a three-component reaction. This work illustrates a rare example of BCBs in transition metal catalysis and demonstrates their broad potential to access novel reaction pathways. The method developed exhibits ample functional group tolerance, and the products can be further transformed into valuable α-quaternary β-lactones. Preliminary mechanistic investigations suggest a twofold C-C bond cleavage sequence involving σ-bond insertion and an ensuing β-carbon elimination event.

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.

The Pathophysiological Role of CoA

The importance of coenzyme A (CoA) as a carrier of acyl residues in cell metabolism is well understood. Coenzyme A participates in more than 100 different catabolic and anabolic reactions, including those involved in the metabolism of lipids, carbohydrates, proteins, ethanol, bile acids, and xenobiotics. However, much less is known about the importance of the concentration of this cofactor in various cell compartments and the role of altered CoA concentration in various pathologies. Despite continuous research on these issues, the molecular mechanisms in the regulation of the intracellular level of CoA under pathological conditions are still not well understood. This review summarizes the current knowledge of (a) CoA subcellular concentrations; (b) the roles of CoA synthesis and degradation processes; and (c) protein modification by reversible CoA binding to proteins (CoAlation). Particular attention is paid to (a) the roles of changes in the level of CoA under pathological conditions, such as in neurodegenerative diseases, cancer, myopathies, and infectious diseases; and (b) the beneficial effect of CoA and pantethine (which like CoA is finally converted to Pan and cysteamine), used at pharmacological doses for the treatment of hyperlipidemia.

Antimalarial pantothenamide metabolites target acetyl-coenzyme A biosynthesis in Plasmodium falciparum

Malaria eradication is critically dependent on new therapeutics that target resistant Plasmodium parasites and block transmission of the disease. Here, we report that pantothenamide bioisosteres were active against blood-stage Plasmodium falciparum parasites and also blocked transmission of sexual stages to the mosquito vector. These compounds were resistant to degradation by serum pantetheinases, showed favorable pharmacokinetic properties, and cleared parasites in a humanized mouse model of P. falciparum infection. Metabolomics revealed that coenzyme A biosynthetic enzymes converted pantothenamides into coenzyme A analogs that interfered with parasite acetyl-coenzyme A anabolism. Resistant parasites generated in vitro showed mutations in acetyl-coenzyme A synthetase and acyl-coenzyme A synthetase 11. Introduction and reversion of these mutations in P. falciparum using CRISPR-Cas9 gene editing confirmed the roles of these enzymes in the sensitivity of the malaria parasites to pantothenamides. These pantothenamide compounds with a new mode of action may have potential as drugs against malaria parasites.

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.

Stable pantothenamide bioisosteres: novel antibiotics for Gram-positive bacteria

The emergence of multidrug resistant bacteria has prioritized the development of new antibiotics. N-substituted pantothenamides, analogs of the natural compound pantetheine, were reported to target bacterial coenzyme A biosynthesis, but these compounds have never reached the clinic due to their instability in biological fluids. Plasma-stable pantothenamide analogs could overcome these issues. We first synthesized a number of bioisosteres of the prototypic pantothenamide N7-Pan. A compound with an inverted amide bond (CXP18.6-012) was found to provide plasma-stability with minimal loss of activity compared to the parent compound N7-Pan. Next, we synthesized inverted pantothenamides with a large variety of side chains. Among these we identified a number of novel stable inverted pantothenamides with selective activity against Gram-positive bacteria such as staphylococci and streptococci, at low micromolar concentrations. These data provide future direction for the development of pantothenamides with clinical potential.

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

Pantothenamides are potent growth inhibitors of the malaria parasite Plasmodium falciparum. Their clinical use is, however, hindered due to the ubiquitous presence of pantetheinases in human serum, which rapidly degrade pantothenamides into pantothenate and the corresponding amine. We previously reported that replacement of the labile amide bond with a triazole ring not only imparts stability toward pantetheinases, but also improves activity against P. falciparum. A small library of new triazole derivatives was synthesized, and their use in establishing structure-activity relationships relevant to antiplasmodial activity of this family of compounds is discussed herein. Overall it was observed that 1,4-substitution on the triazole ring and use of an unbranched, one-carbon linker between the pantoyl group and the triazole are optimal for inhibition of intraerythrocytic P. falciparum growth. Our results imply that the triazole ring may mimic the amide bond with an orientation different from what was previously suggested for this amide bioisostere.

Antiplasmodial Mode of Action of Pantothenamides: Pantothenate Kinase Serves as a Metabolic Activator Not as a Target

N-Substituted pantothenamides (PanAms) are pantothenate analogues with up to nanomolar potency against blood-stage Plasmodium falciparum (the most virulent species responsible for malaria). Although these compounds are known to target coenzyme A (CoA) biosynthesis and/or utilization, their exact mode of action (MoA) is still unknown. Importantly, PanAms that retain the natural β-alanine moiety are more potent than other variants, consistent with the involvement of processes that are selective for pantothenate (the precursor of CoA) or its derivatives. The transport of pantothenate and its phosphorylation by P. falciparum pantothenate kinase (PfPanK, the first enzyme of CoA biosynthesis) are two such processes previously highlighted as potential targets for the PanAms' antiplasmodial action. In this study, we investigated the effect of PanAms on these processes using their radiolabeled versions (synthesized here for the first time), which made possible the direct measurement of PanAm uptake by isolated blood-stage parasites and PanAm phosphorylation by PfPanK present in parasite lysates. We found that the MoA of PanAms does not involve interference with pantothenate transport and that inhibition of PfPanK-mediated pantothenate phosphorylation does not correlate with PanAm antiplasmodial activity. Instead, PanAms that retain the β-alanine moiety were found to be metabolically activated by PfPanK in a selective manner, forming phosphorylated products that likely inhibit other steps in CoA biosynthesis or are transformed into CoA antimetabolites that can interfere with CoA utilization. These findings provide direction for the ongoing development of CoA-targeted inhibitors as antiplasmodial agents with clinical potential.

Triazole Substitution of a Labile Amide Bond Stabilizes Pantothenamides and Improves Their Antiplasmodial Potency

The biosynthesis of coenzyme A (CoA) from pantothenate and the utilization of CoA in essential biochemical pathways represent promising antimalarial drug targets. Pantothenamides, amide derivatives of pantothenate, have potential as antimalarials, but a serum enzyme called pantetheinase degrades pantothenamides, rendering them inactive in vivo In this study, we characterize a series of 19 compounds that mimic pantothenamides with a stable triazole group instead of the labile amide. Two of these pantothenamides are active against the intraerythrocytic stage parasite with 50% inhibitory concentrations (IC₅₀s) of ∼50 nM, and three others have submicromolar IC₅₀s. We show that the compounds target CoA biosynthesis and/or utilization. We investigated one of the compounds for its ability to interact with the Plasmodium falciparum pantothenate kinase, the first enzyme involved in the conversion of pantothenate to CoA, and show that the compound inhibits the phosphorylation of [¹⁴C]pantothenate by the P. falciparum pantothenate kinase, but the inhibition does not correlate with antiplasmodial activity. Furthermore, the compounds are not toxic to human cells and, importantly, are not degraded by pantetheinase.

A cross-metathesis approach to novel pantothenamide derivatives

Pantothenamides are known for their in vitro antimicrobial activity. Our group has previously reported a new stereoselective route to access derivatives modified at the geminal dimethyl moiety. This route however fails in the addition of large substituents. Here we report a new synthetic route that exploits the known allyl derivative, allowing for the installation of larger groups via cross-metathesis. The method was applied in the synthesis of a new pantothenamide with improved stability in human blood.

A pantetheinase-resistant pantothenamide with potent, on-target, and selective antiplasmodial activity

Pantothenamides inhibit blood-stage Plasmodium falciparum with potencies (50% inhibitory concentration [IC50], ∼20 nM) similar to that of chloroquine. They target processes dependent on pantothenate, a precursor of the essential metabolic cofactor coenzyme A. However, their antiplasmodial activity is reduced due to degradation by serum pantetheinase. Minor modification of the pantothenamide structure led to the identification of α-methyl-N-phenethyl-pantothenamide, a pantothenamide resistant to degradation, with excellent antiplasmodial activity (IC50, 52 ± 6 nM), target specificity, and low toxicity.