Ketanserin, an antidepressant, exerts its antileishmanial action via inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) enzyme of Leishmania donovani

Drug repurposing for parasitic protozoan diseases

Protozoan parasites are major hazards to human health, society, and the economy, especially in equatorial regions of the globe. Parasitic diseases, including leishmaniasis, malaria, and others, contribute towards majority of morbidity and mortality. Around 1.1 million people die from these diseases annually. The lack of licensed vaccinations worsens the worldwide impact of these diseases, highlighting the importance of safe and effective medications for their prevention and treatment. However, the appearance of drug resistance in parasites continuously affects the availability of medications. The demand for novel drugs motivates global antiparasitic drug discovery research, necessitating the implementation of many innovative ways to maintain a continuous supply of promising molecules. Drug repurposing has come out as a compelling tool for drug development, offering a cost-effective and efficient alternative to standard de novo approaches. A thorough examination of drug repositioning candidates revealed that certain drugs may not benefit significantly from their original indications. Still, they may exhibit more pronounced effects in other disorders. Furthermore, certain medications can produce a synergistic effect, resulting in enhanced therapeutic effectiveness when given together. In this chapter, we outline the approaches employed in drug repurposing (sometimes referred to as drug repositioning), propose novel strategies to overcome these hurdles and fully exploit the promise of drug repurposing. We highlight a few major human protozoan diseases and a range of exemplary drugs repurposed for various protozoan infections, providing excellent outcomes for each disease.

Clotrimazole causes membrane depolarization and induces sub G0 cell cycle arrest in Leishmania donovani

Clotrimazole is an FDA approved drug and is widely used as an antifungal agent. An extensive body of research is available about its mechanism of action on various cell types but its mode of killing of Leishmania donovani parasites is unknown. L. donovani causes Visceral Leishmaniasis which is a public health problem with limited treatment options. Its present chemotherapy is expensive, has adverse effects and is plagued with drug resistance issues. In this study we have explored the possibility of repurposing clotrimazole as an antileishmanial drug. We have assessed its efficacy on the parasites and attempted to understand its mode of action. We found that it has a half-maximal inhibitory concentration (IC50) of 35.75 ± 1.06 μM, 12.75 ± 0.35 μM and 73 ± 1.41 μM in promastigotes, intracellular amastigotes and macrophages, respectively. Clotrimazole is 5.73 times more selective for the intracellular amastigotes as compared to the mammalian cell. Effect of clotrimazole was reduced by ergosterol supplementation. It leads to impaired parasite morphology. It alters plasma membrane permeability and disrupts plasma membrane potential. Mitochondrial function is compromised as is evident from increased ROS generation, depolarized mitochondrial membrane and decreased ATP levels. Cell cycle analysis of clotrimazole treated parasites shows arrest at sub-G0 phase suggesting apoptotic mode of cell death.

Metabolic Pathways of Leishmania Parasite: Source of Pertinent Drug Targets and Potent Drug Candidates

Leishmaniasis is a tropical disease caused by a protozoan parasite Leishmania that is transmitted via infected female sandflies. At present, leishmaniasis treatment mainly counts on chemotherapy. The currently available drugs against leishmaniasis are costly, toxic, with multiple side effects, and limitations in the administration route. The rapid emergence of drug resistance has severely reduced the potency of anti-leishmanial drugs. As a result, there is a pressing need for the development of novel anti-leishmanial drugs with high potency, low cost, acceptable toxicity, and good pharmacokinetics features. Due to the availability of preclinical data, drug repurposing is a valuable approach for speeding up the development of effective anti-leishmanial through pointing to new drug targets in less time, having low costs and risk. Metabolic pathways of this parasite play a crucial role in the growth and proliferation of Leishmania species during the various stages of their life cycle. Based on available genomics/proteomics information, known pathways-based (sterol biosynthetic pathway, purine salvage pathway, glycolysis, GPI biosynthesis, hypusine, polyamine biosynthesis) Leishmania-specific proteins could be targeted with known drugs that were used in other diseases, resulting in finding new promising anti-leishmanial therapeutics. The present review discusses various metabolic pathways of the Leishmania parasite and some drug candidates targeting these pathways effectively that could be potent drugs against leishmaniasis in the future.

Molecular Basis of the Leishmanicidal Activity of the Antidepressant Sertraline as a Drug Repurposing Candidate

Drug repurposing affords the implementation of new treatments at a moderate cost and under a faster time-scale. Most of the clinical drugs against Leishmania share this origin. The antidepressant sertraline has been successfully assayed in a murine model of visceral leishmaniasis. Nevertheless, sertraline targets in Leishmania were poorly defined. In order to get a detailed insight into the leishmanicidal mechanism of sertraline on Leishmania infantum, unbiased multiplatform metabolomics and transmission electron microscopy were combined with a focused insight into the sertraline effects on the bioenergetics metabolism of the parasite. Sertraline induced respiration uncoupling, a significant decrease of intracellular ATP level, and oxidative stress in L. infantum promastigotes. Metabolomics evidenced an extended metabolic disarray caused by sertraline. This encompasses a remarkable variation of the levels of thiol-redox and polyamine biosynthetic intermediates, as well as a shortage of intracellular amino acids used as metabolic fuel by Leishmania Sertraline killed Leishmania through a multitarget mechanism of action, tackling essential metabolic pathways of the parasite. As such, sertraline is a valuable candidate for visceral leishmaniasis treatment under a drug repurposing strategy.

Repurposing as a strategy for the discovery of new anti-leishmanials: the-state-of-the-art

Leishmaniasis is a vector-borne neglected tropical disease caused by protozoan parasites of the genus Leishmania for which there is a paucity of effective viable non-toxic drugs. There are 1·3 million new cases each year causing considerable socio-economic hardship, best measured in 2·4 million disability adjusted life years, with greatest impact on the poorest communities, which means that desperately needed new antileishmanial treatments have to be both affordable and accessible. Established medicines with cheaper and faster development times may hold the cure for this neglected tropical disease. This concept of using old drugs for new diseases may not be novel but, with the ambitious target of controlling or eradicating tropical diseases by 2020, this strategy is still an important one. In this review, we will explore the current state-of-the-art of drug repurposing strategies in the search for new treatments for leishmaniasis.

Glycyrrhizic acid attenuates growth of Leishmania donovani by depleting ergosterol levels

In the present study, glycyrrhizic acid (GA) the main component of Glycyrrhiza glabra was evaluated for its efficacy as antileishmanial agent and its mode of action explored. GA inhibits promastigotes and intracellular amastigotes in a dose dependent manner at an IC₅₀ value of 34 ± 3.0 μM and 20 ± 4.2 μM respectively. GA was non-toxic against THP-1 macrophage host cell line. GA was found to inhibit recombinant Leishmania donovani HMG-CoA reductase (LdHMGR) enzyme at the half-maximum inhibitory concentration of 24 ± 4.3 μM indicating the sensitivity and specificity of GA towards the enzyme. However, GA could cause only 30% reduction in HMGR activity when measured in Leishmania promastigotes treated with 34 μM of GA. Interestingly western blot analysis revealed fivefold reduced HMGR expression in GLA treated promastigotes. To further study the mode of action of GA, we used transgenic parasites overexpressing LdHMGR. Results indicated that ∼2 fold resistance was exhibited by LdHMGR overexpressing promastigotes to GA with an IC₅₀ value of 74 μM compared to the wild type parasite. This explained the specific binding of GA to LdHMGR enzyme. There was ∼2 fold depletion in ergosterol levels in wild type promastigotes compared to the HMGR overexpressors. This data was further validated by exogenous supplementation of GA treated cells with ergosterol and 40% reversal of growth inhibition was observed. The results obtained suggested that GA kills the parasite by affecting sterol biosynthetic pathway, especially by inhibiting the L. donovani HMGR and altering ergosterol levels. The finding from the current study shows that GA is a potential antileishmanial chemotherapeutic agent.

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Glycyrrhizic acid (GA) kills both promastigote and amastigote forms of Leishmania donovani (Ld).

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It inhibits recombinant LdHMGR enzyme and native promastigote enzyme.

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It depletes ergosterol levels in Leishmania promastigotes.

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Ergosterol supplementation partially rescues GA inhibited promastigotes.

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HMGR overexpressors are 2 fold resistant to GA.

Recombinant 3-Hydroxy 3-Methyl Glutaryl-CoA Reductase from Candida glabrata (Rec-CgHMGR) Obtained by Heterologous Expression, as a Novel Therapeutic Target Model for Testing Synthetic Drugs

The enzyme 3-hydroxy-3-methyl-glutaryl CoA reductase (HMGR) is a glycoprotein of the endoplasmic reticulum that participates in the mevalonate pathway, the precursor of cholesterol in human and ergosterol in fungi. This enzyme has three domains: transmembrane, binding, and soluble. In this study, we expressed and purified the soluble fraction of the HMGR enzyme from Candida glabrata (CgHMGR) in an Escherichia coli heterologous system and used it as a model for studying its inhibitory activity. The soluble fraction of CgHMGR was fused to the maltose binding protein (MBP), purified, and characterized. Optimal pH was 8.0, and its optimal temperature activity was 37 °C. The k _m and V _max for the HMG-CoA were 6.5 μM and 2.26 × 10^-3 μM min^-1, respectively. Recombinant CgHMGR was inhibited by simvastatin presenting an IC₅₀ at 14.5 μM. In conclusion, our findings suggest that the recombinant HMGR version from C. glabrata may be used as a study model system for HMGR inhibitors such as statins and newly synthesized inhibitor compounds that might be used in the treatment of hypercholesterolemia or mycosis.

Benzodiazines: recent synthetic advances

This review provides a comprehensive overview of the recent developments in synthetic strategies for benzodiazines, important scaffolds in medicinal chemistry.

Visceral leishmaniasis: Revisiting current treatments and approaches for future discoveries

The current treatments for visceral leishmaniasis are old and toxic with limited routes of administration. The emergence of drug-resistant Leishmania threatens the efficacy of the existing reservoir of antileishmanials, leading to an urgent need to develop new treatments. It is particularly important to review and understand how the current treatments act against Leishmania in order to identify valid drug targets or essential pathways for next-generation antileishmanials. It is equally important to adapt newly emerging biotechnologies to facilitate the current research on the development of novel antileishmanials in an efficient fashion. This review covers the basic background of the current visceral leishmaniasis treatments with an emphasis on the modes of action. It briefly discusses the role of the immune system in aiding the chemotherapy of leishmaniasis, describes potential new antileishmanial drug targets and pathways, and introduces recent progress on the utilization of high-throughput phenotypic screening assays to identify novel antileishmanial compounds.

Antileishmanial effect of mevastatin is due to interference with sterol metabolism

Visceral leishmaniasis (VL) is one of the most severe forms of leishmaniasis which is fatal if left untreated. Sterol biosynthetic pathway in Leishmania is currently being explored for its therapeutic potential. In the present study, we have evaluated the antileishmanial efficacy of mevastatin, a known inhibitor of 3-hydroxy-3-methyl glutaryl-CoA reductase (HMGR) enzyme. Mevastatin inhibited Leishmania donovani promastigotes and intracellular amastigotes with an 50% inhibitory concentration (IC50) value of 23.8 ± 4.2 and 7.5 ± 1.1 μM, respectively, without exhibiting toxicity towards host cell line. Mevastatin also inhibited recombinant L. donovani HMGR (LdHMGR) enzyme activity with an IC50 value of 42.2 ± 3.0 μM. Kinetic analysis revealed that the inhibition of recombinant LdHMGR activity by mevastatin was competitive with HMG-CoA. Mevastatin-treated parasites exhibited 66% reduction in ergosterol levels with respect to untreated parasites. Incubation of mevastatin-treated L. donovani promastigotes with ergosterol resulted in revival of cell growth, whereas cholesterol supplementation failed to cause reversal in cell death. To further prove the specificity of mevastatin for HMGR enzyme, HMGR-overexpressing parasites were used which showed almost threefold resistance to mevastatin. It also induced morphological changes in the parasite accompanied by lipid body accumulation. Hence, antileishmanial effect of mevastatin was due to the inhibition of HMGR, which eventually leads to reduction in ergosterol levels and hence parasite death. The present study may have implications in the treatment of visceral form of leishmaniasis.

Mianserin, an antidepressant kills Leishmania donovani by depleting ergosterol levels

In the present study, we have investigated the antileishmanial potential of mianserin, an antidepressant. Mianserin was found to inhibit both the promastigote and amastigote forms of the parasite in a dose dependant manner. The IC50 values for promastigotes and amastigotes were 21 μM and 46 μM respectively. Interestingly, mianserin failed to inhibit THP-1 differentiated macrophages up to 100 μM concentration thus, exhibiting parasite selectivity. When mianserin was incubated with recombinant Leishmania donovani 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) enzyme, it exhibited an IC50 value of 19.8 μM. Inhibition kinetics revealed competitive mode of enzyme inhibition as the Km increased with no change in Vmax. Further structural investigation of enzyme-inhibitor interaction revealed quenching of HMGR tryptophan intrinsic fluorescence with a K(sv) value of 3.025±0.37 M(-1) and an apparent binding constant of 0.0954 mM. We further estimated ergosterol levels which is a major component of Leishmania cell membrane. It is synthesized by HMGR enzyme, the first rate limiting enzyme of the sterol biosynthetic pathway. Analysis of ergosterol levels by HPLC revealed ∼2.5-fold depletion in mianserin treated promastigotes with respect to untreated parasites. This data was further validated by exogenous supplementation of mianserin treated cells with ergosterol and cholesterol. Reversal of growth inhibition was observed only upon ergosterol addition though it was refractory to cholesterol supplementation. Overall, our results demonstrate the possibility of repositioning of an antidepressant for the treatment of Visceral Leishmaniasis.