Acivicin: a highly active potential chemotherapeutic agent against visceral leishmaniasis

Decreased glutamate transport in acivicin resistant Leishmania tarentolae

Studies of drug resistance in the protozoan parasites of the genus Leishmania have been helpful in revealing biochemical pathways as potential drug targets. The chlorinated glutamine analogue acivicin has shown good activity against Leishmania cells and was shown to target several enzymes containing amidotransferase domains. We selected a Leishmania tarentolae clone for acivicin resistance. The genome of this resistant strain was sequenced and the gene coding for the amidotransferase domain-containing GMP synthase was found to be amplified. Episomal expression of this gene in wild-type L. tarentolae revealed a modest role in acivicin resistance. The most prominent defect observed in the resistant mutant was reduced uptake of glutamate, and through competition experiments we determined that glutamate and acivicin, but not glutamine, share the same transporter. Several amino acid transporters (AATs) were either deleted or mutated in the resistant cells. Some contributed to the acivicin resistance phenotype although none corresponded to the main glutamate transporter. Through sequence analysis one AAT on chromosome 22 corresponded to the main glutamate transporter. Episomal expression of the gene coding for this transporter in the resistant mutant restored glutamate transport and acivicin susceptibility. Its genetic knockout led to reduced glutamate transport and acivicin resistance. We propose that acivicin binds covalently to this transporter and as such leads to decreased transport of glutamate and acivicin thus leading to acivicin resistance.

Studies of drug resistance in the protozoan parasites of the genus Leishmania have been helpful in revealing biochemical pathways as potential drug targets. Here we report on the characterization at the genomics and metabolomics levels of a L. tarentolae strain made resistant to acivicin, an analogue of glutamine with activity against this parasite. We found that resistance to acivicin is accompanied by a reduced uptake and intracellular levels of glutamate and that both are expected to share the same transporter. Through gene overexpression and disruption studies we identified the main amino acid transporter responsible for glutamate uptake.

Antileishmanial assessment of isoxazole derivatives against L. donovani

A chemical library comprising substituted 3-nitroisoxazoles and 3-aminoisoxazoles was prepared and screened for their antileishmanial activity against L. donovani. As compared to Miltefosine, the standard drug used in bioassays, several compounds displayed remarkably better inhibition of the promastigote and amastigote stages of parasites. The in vivo evaluation of a few compounds in a golden hamster model showed significant reduction of the parasite load post treatment via the intraperitoneal route by several compounds. The preliminary pharmacokinetic evaluation of a representative compound 4mf via the oral route, however, indicated high systemic clearance from the body.

Identification of potent chemotypes targeting Leishmania major using a high-throughput, low-stringency, computationally enhanced, small molecule screen

Patients with clinical manifestations of leishmaniasis, including cutaneous leishmaniasis, have limited treatment options, and existing therapies frequently have significant untoward liabilities. Rapid expansion in the diversity of available cutaneous leishmanicidal chemotypes is the initial step in finding alternative efficacious treatments. To this end, we combined a low-stringency Leishmania major promastigote growth inhibition assay with a structural computational filtering algorithm. After a rigorous assay validation process, we interrogated ∼200,000 unique compounds for L. major promastigote growth inhibition. Using iterative computational filtering of the compounds exhibiting >50% inhibition, we identified 553 structural clusters and 640 compound singletons. Secondary confirmation assays yielded 93 compounds with EC₅₀s ≤ 1 µM, with none of the identified chemotypes being structurally similar to known leishmanicidals and most having favorable in silico predicted bioavailability characteristics. The leishmanicidal activity of a representative subset of 15 chemotypes was confirmed in two independent assay formats, and L. major parasite specificity was demonstrated by assaying against a panel of human cell lines. Thirteen chemotypes inhibited the growth of a L. major axenic amastigote-like population. Murine in vivo efficacy studies using one of the new chemotypes document inhibition of footpad lesion development. These results authenticate that low stringency, large-scale compound screening combined with computational structure filtering can rapidly expand the chemotypes targeting in vitro and in vivo Leishmania growth and viability.

Leishmaniasis is a parasitic disease with cutaneous, mucocutaneous and visceral clinical manifestations, depending on the Leishmania spp. and human host. Globally, there are 350 million people at risk of leishmaniasis, but current treatment options rely predominantly on ancient pentavalent antimonials, which have the potential to cause serious systemic toxicity. Our research focuses on the rapid expansion of potential anti-leishmanial compounds that could function as novel chemical structures for future drug development and offer additional therapeutic options to patients with leishmaniasis. We combined high throughput screening methodologies with computational algorithms and multiple confirmatory assay formats to identify and characterize new potent L. major promastigote growth inhibitors, including one that displays in vivo activity without toxicity to human cells. Our use of a large, broadly distributed compound library enabled the identification of these new chemotypes. In addition, since this chemical library is publicly available and annotated, we were able to cross-query archived bioassays and to identify new molecular targets that may be involved in L. major growth and viability as well as identify new protein targets for future leishmanicidal drug discovery.

Purine and pyrimidine metabolism in Leishmania

Purines and pyrimidines are indispensable to all life, performing many vital functions for cells: ATP serves as the universal currency of cellular energy, cAMP and cGMP are key second messenger molecules, purine and pyrimidine nucleotides are precursors for activated forms of both carbohydrates and lipids, nucleotide derivatives of vitamins are essential cofactors in metabolic processes, and nucleoside triphosphates are the immediate precursors for DNA and RNA synthesis. Unlike their mammalian and insect hosts, Leishmania lack the metabolic machinery to make purine nucleotides de novo and must rely on their host for preformed purines. The obligatory nature of purine salvage offers, therefore, a plethora of potential targets for drug targeting, and the pathway has consequently been the focus of considerable scientific investigation. In contrast, Leishmania are prototrophic for pyrimidines and also express a small complement of pyrimidine salvage enzymes. Because the pyrimidine nucleotide biosynthetic pathways of Leishmania and humans are similar, pyrimidine metabolism in Leishmania has generally been considered less amenable to therapeutic manipulation than the purine salvage pathway. However, evidence garnered from a variety of parasitic protozoa suggests that the selective inhibition of pyrimidine biosynthetic enzymes offers a rational therapeutic paradigm. In this chapter, we present an overview of the purine and pyrimidine pathways in Leishmania, make comparisons to the equivalent pathways in their mammalian host, and explore how these pathways might be amenable to selective therapeutic targeting.

A review of natural products with antileishmanial activity

Infections caused by protozoa of the genus Leishmania are a major worldwide health problem, with high endemicity in developing countries. The incidence of the disease has increased since the emergence of AIDS. In the absence of a vaccine, there is an urgent need for effective drugs to replace/supplement those in current use. The plant kingdom is undoubtedly valuable as a source of new medicinal agents. The present work constitutes a review of the literature on plant extracts and chemically defined molecules of natural origin showing antileishmanial activity. The review refers to 101 plants, their families, and geographical distribution, the parts utilized, the type of extract and the organism tested. It also includes 288 compounds isolated from higher plants and microorganisms, classified into appropriate chemical groups. Some aspects of recent antileishmanial-activity-directed research on natural products are discussed.

Inactivation of the amidotransferase activity of carbamoyl phosphate synthetase by the antibiotic acivicin

Carbamoyl phosphate synthetase (CPS) from Escherichia coli catalyzes the formation of carbamoyl phosphate from 2 mol of ATP, bicarbonate, and glutamine. CPS was inactivated by the glutamine analog, acivicin. In the presence of ATP and bicarbonate the second-order rate constant for the inactivation of the glutamine-dependent activities was 4.0 x 10(4) m(-1) s(-1). In the absence of ATP and bicarbonate the second-order rate constant for inactivation of CPS was reduced by a factor of 200. The enzyme was protected against inactivation by the inclusion of glutamine in the reaction mixture. The ammonia-dependent activities were unaffected by the incubation of CPS with acivicin. These results are consistent with the covalent labeling of the glutamine-binding site located within the small amidotransferase subunit. The binding of ATP and bicarbonate to the large subunit of CPS must also induce a conformational change within the amidotransferase domain of the small subunit that enhances the nucleophilic character of the thiol group required for glutamine hydrolysis. The acivicin-inhibited enzyme was crystallized, and the three-dimensional structure was determined by x-ray diffraction techniques. The thiol group of Cys-269 was covalently attached to the dihydroisoxazole ring of acivicin with the displacement of a chloride ion.

Antileishmanial activity of hamycin: a polyene antibiotic

Hamycin, a polyene antibiotic, now in extensive use in the treatment of candidiasis and otomycosis, is found to be remarkably effective in killing Leishmania donovani promastigotes in a liquid medium at a concentration of 0.2 microgram/ml. The glucose stimulated respiration and the uptake of 2-deoxy-D[U-14C]-glucose was inhibited in cells treated with the drug at a growth inhibitory concentration. An immediate release of isotopic glucose from preloaded cells could be demonstrated after exposure to hamycin. All the above effects could be effectively prevented in the presence of ergosterol. The primary site of action of hamycin on L. donovani promastigote cells appears to be membrane sterols that result in the loss of the permeability barrier to small metabolites. The lower minimum inhibitory concentration of hamycin compared to other established drugs warrants further study in the context of increasing reports of clinical resistance to pentavalent antimonials.