Bicyclic azetidines kill the diarrheal pathogen Cryptosporidium in mice by inhibiting parasite phenylalanyl-tRNA synthetase

Pyridopyrimidinones as a new chemotype of calcium dependent protein kinase 1 (CDPK1) inhibitors for Cryptosporidium

The protozoan protein kinase calcium-dependent protein kinase 1 (CDPK1) has emerged as a potential therapeutic target for the treatment of cryptosporidiosis. A focused screen of known kinase inhibitors identified a pyridopyrimidinone as a new chemotype of Cryptosporidium parvum (Cp) CDPK1 inhibitors. Structural comparison of CpCDPK1 to two representative human kinases, RIPK2 and Src, revealed differences in the positioning of the αC-helix that was used in the design of a potent pyridopyrimidinone-based CpCDPK1 inhibitor 7 (a.k.a. UH15-16, IC50 = 10 nM), which blocked the growth of three C. parvum strains (EC50 = 12-40 nM) as well as C. hominis (EC50 = 85 nM) in HCT-8 host cells. Pharmacokinetic and tissue distribution analyses indicated that 7 had low systemic exposure after oral administration, but high gastrointestinal concentration, as well as good Caco-2 cell permeability. Finally, 7 demonstrated partial efficacy in an IL-12 knock-out mouse model of acute cryptosporidiosis.

Cryptosporidium PI(4)K inhibitor EDI048 is a gut-restricted parasiticidal agent to treat paediatric enteric cryptosporidiosis

Diarrhoeal disease caused by Cryptosporidium is a major cause of morbidity and mortality in young and malnourished children from low- and middle-income countries, with no vaccine or effective treatment. Here we describe the discovery of EDI048, a Cryptosporidium PI(4)K inhibitor, designed to be active at the infection site in the gastrointestinal tract and undergo rapid metabolism in the liver. By using mutational analysis and crystal structure, we show that EDI048 binds to highly conserved amino acid residues in the ATP-binding site. EDI048 is orally efficacious in an immunocompromised mouse model despite negligible circulating concentrations, thus demonstrating that gastrointestinal exposure is necessary and sufficient for efficacy. In neonatal calves, a clinical model of cryptosporidiosis, EDI048 treatment resulted in rapid resolution of diarrhoea and significant reduction in faecal oocyst shedding. Safety and pharmacological studies demonstrated predictable metabolism and low systemic exposure of EDI048, providing a substantial safety margin required for a paediatric indication. EDI048 is a promising clinical candidate for the treatment of life-threatening paediatric cryptosporidiosis.

Cryptosporidium lysyl-tRNA synthetase inhibitors define the interplay between solubility and permeability required to achieve efficacy

Cryptosporidiosis is a diarrheal disease caused by infection with Cryptosporidium spp. parasites and is a leading cause of death in malnourished children worldwide. The only approved treatment, nitazoxanide, has limited efficacy in this at-risk patient population. Additional safe therapeutics are urgently required to tackle this unmet medical need. However, the development of anti-cryptosporidial drugs is hindered by a lack of understanding of the optimal compound properties required to treat this gastrointestinal infection. To address this knowledge gap, a diverse set of potent lysyl-tRNA synthetase inhibitors was profiled to identify optimal physicochemical and pharmacokinetic properties required for efficacy in a chronic mouse model of infection. The results from this comprehensive study illustrated the importance of balancing solubility and permeability to achieve efficacy in vivo. Our results establish in vitro criteria for solubility and permeability that are predictive of compound efficacy in vivo to guide the optimization of anti-cryptosporidial drugs. Two compounds from chemically distinct series (DDD489 and DDD508) were identified as demonstrating superior efficacy and prioritized for further evaluation. Both compounds achieved marked parasite reduction in immunocompromised mouse models and a disease-relevant calf model of infection. On the basis of these promising data, these compounds have been selected for progression to preclinical safety studies, expanding the portfolio of potential treatments for this neglected infectious disease.

Emerging therapeutic avenues against Cryptosporidium: A comprehensive review

Cryptosporidium is among the top causes of life-threatening diarrheal infection in public health and livestock sectors. Despite its high prevalence and economic importance, currently, there is no vaccine. Control of this protozoan is difficult due to the excretion of many resistant oocysts in the feces of the infected host, which contaminate the environment. Paromomycin shows inconsistent results and isn't considered a reliable therapy for cryptosporidiosis. Nitazoxanide (NTZ), the only FDA-approved drug against this parasite, is less productive in impoverished children and PLWHA (people living with HIV/AIDS). The absence of mitochondria and apicoplast, its unique location inside enterocytes separated by parasitophorous vacuole, and, most importantly, challenges in its genetic manipulations are some hurdles to the drug-discovery process. A library of compounds has been tested against Cryptosporidium during in vitro and in vivo trials. However, there has still not been sufficient success in finding the drug of choice against this parasite. Recent genome editing technologies based on CRISPR/Cas-9 have explored the functions of the vital genes by producing transgenic parasites that help to screen a collection of compounds to find target-specific drugs, provided the sufficient availability of in vitro culturing platforms, efficient transfection methods, and analytic techniques. The use of herbal remedies against Cryptosporidium is also an emerging area of interest with sufficient clinical success due to enhanced concern regarding anthelmintic resistance. Here, we highlighted present treatment options with their associated limitations, the use of genetic tools and natural products against it to find safe, effective, and inexpensive drugs to control the ever-increasing global burden of this disease.

Identification of potent and orally efficacious phosphodiesterase inhibitors in Cryptosporidium parvum-infected immunocompromised male mice

Cryptosporidium parvum and C. hominis are parasites that cause life-threatening diarrhea in children and immunocompromised people. There is only one approved treatment that is modestly effective for children and ineffective for AIDS patients. Here, screening 278 compounds from the Merck KGaA, Darmstadt, Germany collection and accelerated follow-up enabled by prior investigation of the compounds identifies a series of pyrazolopyrimidine human phosphodiesterase (PDE)-V (hPDE-V) inhibitors with potent anticryptosporidial activity and efficacy following oral administration in C. parvum-infected male mice. The lead compounds affect parasite host cell egress, inhibit both C. parvum and C. hominis, work rapidly, and have minimal off-target effects in a safety screening panel. Interestingly, the hPDE-V inhibitors sildenafil and the 4-aminoquinoline compound 7a do not affect Cryptosporidium. C. parvum expresses one PDE (CpPDE1) continuously during asexual growth, the inhibited life stage. According to homology modeling and docking, the lead compounds interact with CpPDE1. Bulkier amino acids (Val900 and His884) in the CpPDE1 active site replace alanines in hPDE-V and block sildenafil binding. Supporting this, sildenafil kills a CRISPR-engineered Cryptosporidium CpPDE1 V900A mutant. The CpPDE1 mutation also alters parasite susceptibility to pyrazolopyrimidines. CpPDE1 is therefore a validated pyrazolopyrimidine molecular target to exploit for target-based optimization for improved anticryptosporidial development.

Treating cryptosporidiosis: A review on drug discovery strategies

Despite several decades of research on therapeutics, cryptosporidiosis remains a major concern for human and animal health. Even though this field of research to assess antiparasitic drug activity is highly active and competitive, only one molecule is authorized to be used in humans. However, this molecule was not efficacious in immunocompromised people and the lack of animal therapeutics remains a cause of concern. Indeed, the therapeutic arsenal needs to be developed for both humans and animals. Our work aims to clarify research strategies that historically were diffuse and poorly directed. This paper reviews in vitro and in vivo methodologies to assess the activity of future therapeutic compounds by screening drug libraries or through drug repurposing. It focuses on High Throughput Screening methodologies (HTS) and discusses the lack of knowledge of target mechanisms. In addition, an overview of several specific metabolic pathways and enzymatic activities used as targets against Cryptosporidium is provided. These metabolic processes include glycolytic pathways, fatty acid production, kinase activities, tRNA elaboration, nucleotide synthesis, gene expression and mRNA maturation. As a conclusion, we highlight emerging future strategies for screening natural compounds and assessing drug resistance issues.

ATP mimetics targeting prolyl-tRNA synthetases as a new avenue for antimalarial drug development

The prolyl-tRNA synthetase (PRS) is an essential enzyme for protein translation and a validated target against malaria parasite. We describe five ATP mimetics (L95, L96, L97, L35, and L36) against PRS, exhibiting enhanced thermal stabilities in co-operativity with L-proline. L35 displays the highest thermal stability akin to halofuginone, an established inhibitor of Plasmodium falciparum PRS. Four compounds exhibit nanomolar inhibitory potency against PRS. L35 exhibits the highest potency of ∼1.6 nM against asexual-blood-stage (ABS) and ∼100-fold (effective concentration [EC50]) selectivity for the parasite. The macromolecular structures of PfPRS with L95 and L97 in complex with L-pro reveal their binding modes and catalytic site malleability. Arg401 of PfPRS oscillates between two rotameric configurations when in complex with L95, whereas it is locked in one of the configurations due to the larger size of L97. Harnessing such specific and selective chemical features holds significant promise for designing potential inhibitors and expediting drug development efforts.

Intestinal cDC1s provide cues required for CD4+ T cell-mediated resistance to Cryptosporidium

Cryptosporidium is an enteric pathogen and a prominent cause of diarrheal disease worldwide. Control of Cryptosporidium requires CD4+ T cells, but how protective CD4+ T cell responses are generated is poorly understood. Here, Cryptosporidium parasites that express MHCII-restricted model antigens were generated to understand the basis for CD4+ T cell priming and effector function. These studies revealed that parasite-specific CD4+ T cells are primed in the draining mesenteric lymph node but differentiate into Th1 cells in the gut to provide local parasite control. Although type 1 conventional dendritic cells (cDC1s) were dispensable for CD4+ T cell priming, they were required for CD4+ T cell gut homing and were a source of IL-12 at the site of infection that promoted local production of IFN-γ. Thus, cDC1s have distinct roles in shaping CD4+ T cell responses to an enteric infection: first, to promote gut homing from the mesLN, and second, to drive effector responses in the intestine.

Discovery of novel anti-infective agents

Academic and other non-profit institutions have a long-term vision to improve human health where commercial interests can be limited for profit organizations. Medicinal chemistry to these diseases with no commercial benefit needs is well suited in the academic environment and this chapter outlines some work conducted at Calibr-Skaggs around antibiotic drug development that has led to initiation of multiple clinical trials over the last decade.

Bicyclic Pyrrolidine Inhibitors of Toxoplasma gondii Phenylalanine t-RNA Synthetase with Antiparasitic Potency In Vitro and Brain Exposure

Previous studies have shown that bicyclic azetidines are potent and selective inhibitors of apicomplexan phenylalanine tRNA synthetase (PheRS), leading to parasite growth inhibition in vitro and in vivo, including in models of Toxoplasma infection. Despite these useful properties, additional optimization is required for the development of efficacious treatments of toxoplasmosis from this inhibitor series, in particular, to achieve optimal exposure in the brain. Here, we describe a series of PheRS inhibitors built on a new bicyclic pyrrolidine core scaffold designed to retain the exit-vector geometry of the isomeric bicyclic azetidine core scaffold while offering avenues to sample diverse chemical space. Relative to the parent series, bicyclic pyrrolidines retain reasonable potency and target selectivity for parasite PheRS vs host. Further structure-activity relationship studies revealed that the introduction of aliphatic groups improved potency and ADME and PK properties, including brain exposure. The identification of this new scaffold provides potential opportunities to extend the analogue series to further improve selectivity and potency and ultimately deliver a novel, efficacious treatment of toxoplasmosis.

Etiological surveillance of viral diarrhea from 2017 to 2019 in Zhangzhou city, Fujian province, China

Background

Viral diarrhea is one of the major causes of morbidity and mortality in children. This study aimed to conduct etiological surveillance of viral diarrhea in Zhangzhou city, Fujian province, China, from 2017 to 2019 to identify the prevalence, distribution, and characteristics of viral pathogens causing gastrointestinal infections in the region.

Methods

Stool samples were collected from patients with acute diarrhea in Zhangzhou city, Fujian province, China, from 2017 to 2019. Rotavirus, norovirus, astrovirus, and adenovirus were detected using fluorescence immunochromatography assay.

Results

Of the total 5,627 samples that were collected, at least one of the viruses (rotavirus, norovirus, astrovirus and adenovirus) was found to be positive in 1,422 samples. Rotavirus, norovirus, astrovirus, and adenovirus, were detected in 53.73, 16.68, 15.52, and 14.97%, respectively. Mixed infections were determined in 17.65% of the positive samples. The predominant mixed infections observed were a combination of norovirus and astrovirus, followed by rotavirus and norovirus, and rotavirus and astrovirus. The highest positive rate was observed in the 12-23-month group for rotavirus and adenovirus, while a significantly higher positive rate was observed for norovirus and astrovirus in the 6-11-month group.

Conclusion

These findings from this etiological surveillance highlight the significant burden of viral diarrhea in Zhangzhou city, with rotavirus being the predominant pathogen. The identification of common mixed infections provides insights into the complex nature of viral diarrhea transmission. Target interventions and public health strategies should be implemented, particularly during the winter and spring seasons, to prevent and control the spread of viral pathogens causing gastrointestinal infections in this region.

Pyroptosis Tuning in Intestinal Cryptosporidiosis via the Natural Histone Deacetylase Inhibitor Romidepsin

Cryptosporidium is an opportunistic protozoan, with many species of cross-human infectivity. It causes life-threatening diarrhoea in children and CD4-defective patients. Despite its limited efficacy, nitazoxanide remains the primary anti-cryptosporidial drug. Cryptosporidium infects the intestinal brush border (intracellular-extracytoplasmic) and down-regulates pyroptosis to prevent expulsion. Romidepsin is a natural histone deacetylase inhibitor that triggers pyroptosis. Romidepsin's effect on cryptosporidiosis was assessed in immunocompromised mice via gasdermin-D (GSDM-D) immunohistochemical expression, IFN-γ, IL-1β and IL-18 blood levels by ELISA, and via parasite scanning by modified Ziehl-Neelsen staining and scanning electron microscopy (SEM). Oocyst deformity and local cytokines were also assessed in ex vivo ileal explants. Following intraperitoneal injection of romidepsin, oocyst shedding significantly reduced at the 9th, 12th and 15th d.p.i. compared with infected-control and drug-control (nitazoxanide-treated) mice. H&E staining of intestinal sections from romidepsin-treated mice showed significantly low intestinal scoring with marked reduction in epithelial hyperplasia, villous blunting and cellular infiltrate. SEM revealed marked oocyst blebbing and paucity (in vivo and ex vivo) after romidepsin compared with nitazoxanide. Regarding pyroptosis, romidepsin triggered significantly higher intestinal GSDM-D expression in vivo, and higher serum/culture IFN-γ, IL-1β and IL-18 levels in romidepsin-treated mice than in the control groups. Collectively, in cryptosporidiosis, romidepsin succeeded in enhancing pyroptosis in the oocysts and infected epithelium, reducing infection and shifting the brush border towards normalisation.

Bicyclic pyrrolidine inhibitors of Toxoplasma gondii phenylalanine t-RNA synthetase with antiparasitic potency in vitro and brain exposure

Previous studies have shown that bicyclic azetidines are potent and selective inhibitors of apicomplexan phenylalanine tRNA synthetase (PheRS), leading to parasite growth inhibition in vitro and in vivo, including in models of Toxoplasma infection. Despite these useful properties, additional optimization is required for the development of efficacious treatments of toxoplasmosis from this inhibitor series, in particular to achieve sufficient exposure in the brain. Here, we describe a series of PheRS inhibitors built on a new bicyclic pyrrolidine core scaffold designed to retain the exit-vector geometry of the isomeric bicyclic azetidine core scaffold while offering avenues to sample diverse chemical space. Relative to the parent series, bicyclic pyrrolidines retain reasonable potency and target selectivity for parasite PheRS vs. host. Further structure-activity relationship studies revealed that the introduction of aliphatic groups improved potency, ADME and PK properties, including brain exposure. The identification of this new scaffold provides potential opportunities to extend the analog series to further improve selectivity and potency and ultimately deliver a novel, efficacious treatment of toxoplasmosis.

Anti-Cryptosporidial Drug-Discovery Challenges and Existing Therapeutic Avenues: A "One-Health" Concern

Cryptosporidiosis is the leading cause of life-threatening diarrheal infection, especially in infants. Oocysts contaminate the environment, and also, being a zoonotic disease, cryptosporidiosis is a threat to One Health. Nitazoxanide is the only FDA-approved drug, effective only in immunocompetent adults, and is not safe for infants. The absence of mitochondria and apicoplast, the presence of an electron-dense band (ED band), hindrances in its genetic and phenotypic manipulations, and its unique position inside the host cell are some challenges to the anti-cryptosporidial drug-discovery process. However, many compounds, including herbal products, have shown efficacy against Cryptosporidium during in vitro and in vivo trials. Still, the "drug of choice" against this protozoan parasite, especially in immunocompromised individuals and infants, has not yet been explored. The One-Health approach addresses this issue, focusing on the intersection of animal, human, and environmental health. The objective of this review is to provide knowledge about novel anti-cryptosporidial drug targets, available treatment options with associated limitations, and possible future shifts toward natural products to treat cryptosporidiosis. The current review is organized to address the treatment and prevention of cryptosporidiosis. An anti-cryptosporidial drug that is effective in immunocompromised individuals and infants is a necessity of our time.

Genetic crosses within and between species of Cryptosporidium

Parasites and their hosts are engaged in reciprocal coevolution that balances competing mechanisms of virulence, resistance, and evasion. This often leads to host specificity, but genomic reassortment between different strains can enable parasites to jump host barriers and conquer new niches. In the apicomplexan parasite Cryptosporidium, genetic exchange has been hypothesized to play a prominent role in adaptation to humans. The sexual lifecycle of the parasite provides a potential mechanism for such exchange; however, the boundaries of Cryptosporidium sex are currently undefined. To explore this experimentally, we established a model for genetic crosses. Drug resistance was engineered using a mutated phenylalanyl tRNA synthetase gene and marking strains with this and the previously used Neo transgene enabled selection of recombinant progeny. This is highly efficient, and genomic recombination is evident and can be continuously monitored in real time by drug resistance, flow cytometry, and PCR mapping. Using this approach, multiple loci can now be modified with ease. We demonstrate that essential genes can be ablated by crossing a Cre recombinase driver strain with floxed strains. We further find that genetic crosses are also feasible between species. Crossing Cryptosporidium parvum, a parasite of cattle and humans, and Cryptosporidium tyzzeri a mouse parasite resulted in progeny with a recombinant genome derived from both species that continues to vigorously replicate sexually. These experiments have important fundamental and translational implications for the evolution of Cryptosporidium and open the door to reverse- and forward-genetic analysis of parasite biology and host specificity.

Intestinal cDC1s provide IL-12 dependent and independent functions required for CD4+ T cell-mediated resistance to Cryptosporidium

Cryptosporidium is an enteric pathogen that is a prominent cause of diarrheal disease. Control of this infection requires CD4+ T cells, though the processes that lead to T cell-mediated resistance have been difficult to assess. Here, Cryptosporidium parasites that express MHCII-restricted model antigens were generated to dissect the early events that influence CD4+ T cell priming and effector function. These studies highlight that parasite-specific CD4+ T cells are primed in the draining mesenteric lymph node (mesLN) and differentiate into Th1 cells in the gut, where they mediate IFN-γ-dependent control of the infection. Although type 1 conventional dendritic cells (cDC1s) were not required for initial priming of CD4+ T cells, cDC1s were required for CD4+ T cell expansion and gut homing. cDC1s were also a major source of IL-12 that was not required for priming but promoted full differentiation of CD4+ T cells and local production of IFN-γ. Together, these studies reveal distinct roles for cDC1s in shaping CD4+ T cell responses to enteric infection: first to drive early expansion in the mesLN and second to drive effector responses in the gut.

Cost-effective In Vivo and In Vitro Mouse Models for Evaluating Anticryptosporidial Drug Efficacy: Assessing Vorinostat, Docetaxel, and Baicalein

BACKGROUND

Cryptosporidiosis is a significant diarrheal disease in humans and animals. Immunodeficient mice are the primary small animal models, but their high costs and specialized breeding/housing requirements limit in vivo drug testing. Numerous anticryptosporidial lead compounds identified in vitro remain untested in vivo.

METHODS

Cryptosporidium tyzzeri, a natural mouse parasite closely related to Cryptosporidium parvum and Cryptosporidium hominis, was isolated to establish an infection model in immunocompetent mice. The model was validated using classic anticryptosporidial drugs (paromomycin and nitazoxanide) and then employed to assess the efficacy of 3 new leads (vorinostat, docetaxel, and baicalein). An in vitro culture of C. tyzzeri was also developed to complement the animal model.

RESULTS

Chronic C. tyzzeri infection was established in chemically immunosuppressed wild-type mice. Paromomycin (1000 mg/kg/d) and nitazoxanide (100 mg/kg/d) demonstrated efficacy against C. tyzzeri. Vorinostat (30 mg/kg/d), docetaxel (25 mg/kg/d), and baicalein (50 mg/kg/d) were highly effective against C. tyzzeri infection. In vitro, nitazoxanide, vorinostat, docetaxel, and baicalein exhibited low to submicromolar efficacy against C. tyzzeri.

CONCLUSIONS

Novel in vivo and in vitro models have been developed for cost-effective anticryptosporidial drug testing. Vorinostat, docetaxel, and baicalein show potential for repurposing and/or optimization for developing new anticryptosporidial drugs.

Targeting Aminoacyl tRNA Synthetases for Antimalarial Drug Development

Infections caused by malaria parasites place an enormous burden on the world's poorest communities. Breakthrough drugs with novel mechanisms of action are urgently needed. As an organism that undergoes rapid growth and division, the malaria parasite Plasmodium falciparum is highly reliant on protein synthesis, which in turn requires aminoacyl-tRNA synthetases (aaRSs) to charge tRNAs with their corresponding amino acid. Protein translation is required at all stages of the parasite life cycle; thus, aaRS inhibitors have the potential for whole-of-life-cycle antimalarial activity. This review focuses on efforts to identify potent plasmodium-specific aaRS inhibitors using phenotypic screening, target validation, and structure-guided drug design. Recent work reveals that aaRSs are susceptible targets for a class of AMP-mimicking nucleoside sulfamates that target the enzymes via a novel reaction hijacking mechanism. This finding opens up the possibility of generating bespoke inhibitors of different aaRSs, providing new drug leads.

Genetic crosses within and between species of Cryptosporidium

Parasites and their hosts are engaged in rapid coevolution that balances competing mechanisms of virulence, resistance, and evasion. This often leads to host specificity, but genomic reassortment between different strains can enable parasites to jump host barriers and conquer new niches. In the apicomplexan parasite Cryptosporidium genetic exchange has been hypothesized to play a prominent role in adaptation to humans. The sexual lifecycle of the parasite provides a potential mechanism for such exchange; however, the boundaries of Cryptosporidium sex are currently undefined. To explore this experimentally, we established a model for genetic crosses. Drug resistance was engineered using a mutated phenylalanyl tRNA synthetase gene and marking strains with this and the previously used Neo transgene enabled selection of recombinant progeny. This is highly efficient, and genomic recombination is evident and can be continuously monitored in real time by drug resistance, flow cytometry, and PCR mapping. Using this approach multiple loci can now be modified with ease. We demonstrate that essential genes can be ablated by crossing a Cre recombinase driver strain with floxed strains. We further find that genetic crosses are also feasible between species. Crossing C. parvum, a parasite of cattle and humans, and C. tyzzeri a mouse parasite resulted in progeny with a recombinant genome derived from both species that continues to vigorously replicate sexually. These experiments have important fundamental and translational implications for the evolution of Cryptosporidium and open the door to reverse- and forward- genetic analysis of parasite biology and host specificity.

Mode of action studies confirm on-target engagement of lysyl-tRNA synthetase inhibitor and lead to new selection marker for Cryptosporidium

Introduction

Cryptosporidiosis is a leading cause of diarrheal-associated morbidity and mortality, predominantly affecting children under 5 years old in low-and-middle-income countries. There is no effective treatment and no vaccine. New therapeutics are emerging from drug discovery efforts. It is critical that mode of action studies are performed alongside drug discovery to ensure the best clinical outcomes. Unfortunately, technology to identify and validate drug targets for Cryptosporidium is severely lacking.

Methods

We used C. parvum lysyl-tRNA synthetase (CpKRS) and DDD01510706 as a target-compound pair to develop both chemical and genetic tools for mode of action studies for Cryptosporidium. We adapted thermal proteome profiling (TPP) for Cryptosporidium, an unbiased approach for target identification.

Results

Using TPP we identified the molecular target of DDD01510706 and confirm that it is CpKRS. Genetic tools confirm that CpKRS is expressed throughout the life cycle and that this target is essential for parasite survival. Parasites genetically modified to over-express CpKRS or parasites with a mutation at the compound-binding site are resistant to treatment with DDD01510706. We leveraged these mutations to generate a second drug selection marker for genetic modification of Cryptosporidium, KRSR. This second selection marker is interchangeable with the original selection marker, NeoR, and expands the range of reverse genetic approaches available to study parasite biology. Due to the sexual nature of the Cryptosporidium life cycle, parental strains containing different drug selection markers can be crossed in vivo.

Discussion

Selection with both drug markers produces highly efficient genetic crosses (>99% hybrid progeny), paving the way for forward genetics approaches in Cryptosporidium.

Structure-activity relationship of BMS906024 derivatives for Cryptosporidium parvum growth inhibition

BMS906024, a γ-secretase inhibitor that blocks Notch signaling, was previously shown to inhibit Cryptosporidium parvum growth in vitro. A structure-activity relationship (SAR) analysis of BMS906024 reported herein demonstrates the importance of the stereochemistry of the C-3 benzodiazepine and the succinyl β-substituent. However, concomitant removal of the succinyl α-substituent and switching the primary amide with secondary amides was tolerated. For example, 32 (SH287) inhibited C. parvum growth in HCT-8 host cells with an EC50 = 6.4 nM and an EC90 = 16 nM; however, blocking C. parvum growth with BMS906024 derivatives was correlative with inhibition of Notch signaling, highlighting that additional SAR analysis will be needed to separate these two activities.

Drug discovery for parasitic diseases: powered by technology, enabled by pharmacology, informed by clinical science

While prevention is a bedrock of public health, innovative therapeutics are needed to complement the armamentarium of interventions required to achieve disease control and elimination targets for neglected diseases. Extraordinary advances in drug discovery technologies have occurred over the past decades, along with accumulation of scientific knowledge and experience in pharmacological and clinical sciences that are transforming many aspects of drug R&D across disciplines. We reflect on how these advances have propelled drug discovery for parasitic infections, focusing on malaria, kinetoplastid diseases, and cryptosporidiosis. We also discuss challenges and research priorities to accelerate discovery and development of urgently needed novel antiparasitic drugs.

Tyrosine Kinase Inhibitors Display Potent Activity against Cryptosporidium parvum

The protozoan parasite Cryptosporidium is a leading cause of diarrheal disease (cryptosporidiosis) and death in young children. Cryptosporidiosis can be life-threatening in individuals with weak immunity such as HIV/AIDS patients and organ transplant recipients. There is currently no effective drug to treat cryptosporidiosis in the pediatric and immunocompromised population. Therefore, there is an urgent need to expedite the drug discovery process in order to develop new and effective therapies to reduce the global disease burden of cryptosporidiosis. In this study, we employed a drug repurposing strategy to screen a library of 473 human kinase inhibitors to determine their activity against Cryptosporidium parvum. We have identified 67 new anti-cryptosporidial compounds using phenotypic screening based on a transgenic C. parvum strain expressing a luciferase reporter. Further, dose-response assays led to the identification of 11 hit compounds that showed potent inhibition of C. parvum at nanomolar concentration. Kinome profiling of these 11 prioritized hits identified compounds that displayed selectivity in targeting specific families of kinases, particularly tyrosine kinases. Overall, this study identified tyrosine kinase inhibitors that hold potential for future development as new drug candidates against cryptosporidiosis. IMPORTANCE The intestinal parasite Cryptosporidium parvum is a major cause of diarrhea-associated morbidity and mortality in children, immunocompromised people, and young ruminant animals. With no effective drug available to treat cryptosporidiosis in humans and animals, there is an urgent need to identify anti-parasitic compounds and new targets for drug development. To address this unmet need, we screened a GSK library of kinase inhibitors and identified several potent compounds, including tyrosine kinase inhibitors, that were highly effective in killing C. parvum. Overall, our study revealed several novel compounds and a new family of kinases that can be targeted for anti-cryptosporidial drug development.

Structural characterization of glutamyl-tRNA synthetase (GluRS) from Plasmodium falciparum

Aminoacyl-tRNA synthetases (aaRSs) are essential enzymes in protein translation machinery that provide the charged tRNAs needed for protein synthesis. Over the past decades, aaRSs have been studied as anti-parasitic, anti-bacterial, and anti-fungal drug targets. This study focused on the cytoplasmic glutamyl-tRNA synthetase (GluRS) from Plasmodium falciparum, which belongs to class Ib in aaRSs. GluRS unlike most other aaRSs requires tRNA to activate its cognate amino acid substrate L-Glutamate (L-Glu), and fails to form an intermediate adenylate complex in the absence of tRNA. The crystal structures of the Apo, ATP, and ADP-bound forms of Plasmodium falciparum glutamyl-tRNA synthetase (PfGluRS) were solved at 2.1 Å, 2.2 Å, and 2.8 Å respectively. The structural comparison of the Apo- and ATP-bound holo-forms of PfGluRS showed considerable conformational changes in the loop regions around the ATP-binding pocket of the enzyme. Biophysical characterization of the PfGluRS showed binding of the enzyme substrates L-Gluand ATP.. The sequence and structural conservation were evident across GluRS compared to other species. The structural dissection of the PfGluRS gives insight into the critical residues involved in the binding of ATP substrate, which can be harvested to develop new antimalarial drugs.

Targeting prolyl-tRNA synthetase via a series of ATP-mimetics to accelerate drug discovery against toxoplasmosis

The prolyl-tRNA synthetase (PRS) is a validated drug target for febrifugine and its synthetic analog halofuginone (HFG) against multiple apicomplexan parasites including Plasmodium falciparum and Toxoplasma gondii. Here, a novel ATP-mimetic centered on 1-(pyridin-4-yl) pyrrolidin-2-one (PPL) scaffold has been validated to bind to Toxoplasma gondii PRS and kill toxoplasma parasites. PPL series exhibited potent inhibition at the cellular (T. gondii parasites) and enzymatic (TgPRS) levels compared to the human counterparts. Cell-based chemical mutagenesis was employed to determine the mechanism of action via a forward genetic screen. Tg-resistant parasites were analyzed with wild-type strain by RNA-seq to identify mutations in the coding sequence conferring drug resistance by computational analysis of variants. DNA sequencing established two mutations, T477A and T592S, proximal to terminals of the PPL scaffold and not directly in the ATP, tRNA, or L-pro sites, as supported by the structural data from high-resolution crystal structures of drug-bound enzyme complexes. These data provide an avenue for structure-based activity enhancement of this chemical series as anti-infectives.

Treatment of cryptosporidiosis: nitazoxanide yes, but we can do better

INTRODUCTION

Cryptosporidiosis was initially recognized as an important cause of diarrhea in AIDS patients. It has been underdiagnosed in other populations. Recent studies have highlighted the importance of Cryptosporidium as a cause of diarrhea and malnutrition in young children in resource-poor countries and an emerging pathogen in organ-transplant recipients.

AREAS COVERED

Nitazoxanide is FDA approved for treatment of cryptosporidiosis in immunocompetent people. However, it is less effective in HIV and transplant patients and malnourished children. In transplant recipients, there is emerging data on antiparasitic combinations for cryptosporidiosis, including combinations of nitazoxanide, azithromycin, and in one case rifaximin. High-throughput phenotypic screens have identified some potential treatments. Among them, clofazimine was no better than placebo in a trial in AIDS patients. There have also been efforts to develop drug versus specific parasite targets. However, in part due to safety issues, none of these compounds have advanced into clinical trials.

EXPERT OPINION

Development of new and more efficacious therapies for cryptosporidium is imperative. Current approve therapy is far from optimal and lacks efficacy in high-risk populations, such as, patients living with HIV. Additionally, there is limited data on patients with other types of immunosuppression (Transplanted, autoimmune conditions, etc).

Past, current, and potential treatments for cryptosporidiosis in humans and farm animals: A comprehensive review

The intracellular protozoan parasite of the genus Cryptosporidium is among the leading causes of waterborne diarrheal disease outbreaks throughout the world. The parasite is transmitted by ingestion of infective oocysts that are highly stable in the environment and resistant to almost all conventional disinfection methods and water treatments. Control of the parasite infection is exceedingly difficult due to the excretion of large numbers of oocysts in the feces of infected individuals that contaminate the environment and serve as a source of infection for susceptible hosts including humans and animals. Drug development against the parasite is challenging owing to its limited genetic tractability, absence of conventional drug targets, unique intracellular location within the host, and the paucity of robust cell culture platforms for continuous parasite propagation. Despite the high prevalence of the parasite, the only US Food and Drug Administration (FDA)-approved treatment of Cryptosporidium infections is nitazoxanide, which has shown moderate efficacy in immunocompetent patients. More importantly, no effective therapeutic drugs are available for treating severe, potentially life-threatening cryptosporidiosis in immunodeficient patients, young children, and neonatal livestock. Thus, safe, inexpensive, and efficacious drugs are urgently required to reduce the ever-increasing global cryptosporidiosis burden especially in low-resource countries. Several compounds have been tested for both in vitro and in vivo efficacy against the disease. However, to date, only a few experimental compounds have been subjected to clinical trials in natural hosts, and among those none have proven efficacious. This review provides an overview of the past and present anti-Cryptosporidium pharmacotherapy in humans and agricultural animals. Herein, we also highlight the progress made in the field over the last few years and discuss the different strategies employed for discovery and development of effective prospective treatments for cryptosporidiosis.

Exploration of aminoacyl-tRNA synthetases from eukaryotic parasites for drug development

Parasitic diseases result in considerable human morbidity and mortality. The continuous emergence and spread of new drug-resistant parasite strains is an obstacle to controlling and eliminating many parasitic diseases. Aminoacyl-tRNA synthetases (aaRSs) are ubiquitous enzymes essential for protein synthesis. The design and development of diverse small molecule, drug-like inhibitors against parasite-encoded and expressed aaRSs have validated this enzyme family as druggable. In this work, we have compiled the progress to date towards establishing the druggability of aaRSs in terms of their biochemical characterization, validation as targets, inhibitor development, and structural interpretation from parasites responsible for malaria (Plasmodium), lymphatic filariasis (Brugia,Wuchereria bancrofti), giardiasis (Giardia), toxoplasmosis (Toxoplasma gondii), leishmaniasis (Leishmania), cryptosporidiosis (Cryptosporidium), and trypanosomiasis (Trypanosoma). This work thus provides a robust framework for the systematic dissection of aaRSs from these pathogens and will facilitate the cross-usage of potential inhibitors to jump-start anti-parasite drug development.

Identification of potent anti-Cryptosporidium new drug leads by screening traditional Chinese medicines

Cryptosporidium spp. are gastrointestinal opportunistic protozoan parasites that infect humans, domestic animals, and wild animals all over the world. Cryptosporidiosis is the second leading infectious diarrheal disease in infants less than 5 years old. Cryptosporidiosis is a common zoonotic disease associated with diarrhea in infants and immunocompromised individuals. Consequently, cryptosporidiosis is considered a serious economic, veterinary, and medical concern. The treatment options for cryptosporidiosis are limited. To address this problem, we screened a natural product library containing 87 compounds of Traditional Chinese Medicines for anti-Cryptosporidium compounds that could serve as novel drug leads and therapeutic targets against C. parvum. To examine the anti-Cryptosporidium activity and half-maximal inhibitory doses (EC50) of these compounds, we performed in vitro assays (Cryptosporidium growth inhibition assay and host cell viability assay) and in vivo experiments in mice. In these assays, the C. parvum HNJ-1 strain was used. Four of the 87 compounds (alisol-A, alisol-B, atropine sulfate, and bufotalin) showed strong anti-Cryptosporidium activity in vitro (EC50 values = 122.9±6.7, 79.58±13.8, 253.5±30.3, and 63.43±18.7 nM, respectively), and minimum host cell cytotoxicity (cell survival > 95%). Furthermore, atropine sulfate (200 mg/kg) and bufotalin (0.1 mg/kg) also showed in vivo inhibitory effects. Our findings demonstrate that atropine sulfate and bufotalin are effective against C. parvum infection both in vitro and in vivo. These compounds may, therefore, represent promising novel anti-Cryptosporidium drug leads for future medications against cryptosporidiosis.

An update on Cryptosporidium biology and therapeutic avenues

Cryptosporidium species has been identified as an important pediatric diarrheal pathogen in resource-limited countries, particularly in very young children (0–24 months). However, the only available drug (nitazoxanide) has limited efficacy and can only be prescribed in a medical setting to children older than one year. Many drug development projects have started to investigate new therapeutic avenues. Cryptosporidium’s unique biology is challenging for the traditional drug discovery pipeline and requires novel drug screening approaches. Notably, in recent years, new methods of oocyst generation, in vitro processing, and continuous three-dimensional cultivation capacities have been developed. This has enabled more physiologically pertinent research assays for inhibitor discovery. In a short time, many great strides have been made in the development of anti-Cryptosporidium drugs. These are expected to eventually turn into clinical candidates for cryptosporidiosis treatment in the future. This review describes the latest development in Cryptosporidium biology, genomics, transcriptomics of the parasite, assay development, and new drug discovery.

Live imaging of the Cryptosporidium parvum life cycle reveals direct development of male and female gametes from type I meronts

Cryptosporidium is a leading infectious cause of diarrhea around the world associated with waterborne outbreaks, community spread, or zoonotic transmission. The parasite has significant impact on early childhood mortality, and infection is both a consequence and cause of malnutrition and stunting. There is currently no vaccine, and treatment options are very limited. Cryptosporidium is a member of the Apicomplexa, and, as typical for this, protist phylum relies on asexual and sexual reproduction. In contrast to other Apicomplexa, including the malaria parasite Plasmodium, the entire Cryptosporidium life cycle unfolds in a single host in less than 3 days. Here, we establish a model to image life cycle progression in living cells and observe, track, and compare nuclear division of asexual and sexual stage parasites. We establish the length and sequence of the cell cycles of all stages and map the developmental fate of parasites across multiple rounds of invasion and egress. We propose that the parasite executes an intrinsic program of 3 generations of asexual replication, followed by a single generation of sexual stages that is independent of environmental stimuli. We find no evidence for a morphologically distinct intermediate stage (the tetraploid type II meront) but demonstrate direct development of gametes from 8N type I meronts. The progeny of each meront is collectively committed to either asexual or sexual fate, but, importantly, meronts committed to sexual fate give rise to both males and females. We define a Cryptosporidium life cycle matching Tyzzer’s original description and inconsistent with the coccidian life cycle now shown in many textbooks.

Inhibition of Plasmodium falciparum phenylalanine tRNA synthetase provides opportunity for antimalarial drug development

Bicyclic azetidine compounds possess antimalarial activity via targeting of the cytoplasmic Plasmodium falciparum (Pf) protein translation enzyme phenylalanine-tRNA synthetase (cFRS). These drugs kill parasites both in vitro and in vivo, including the blood, liver, and transmission developmental stages. Here we present the co-crystal structure of PfcFRS with a potent inhibitor, the bicyclic azetidine BRD7929. Our studies reveal high-affinity binding of BRD7929 with PfcFRS along with exquisite specificity compared with the human enzyme, leading in turn to potent and selective inhibition of the parasite enzyme. Our co-crystal structure shows that BRD7929 binds in the active site in the α subunit of PfcFRS, where it occupies the amino acid site, an auxiliary site, and partially the ATP site. This structural snapshot of inhibitor-bound PfcFRS thus provides a platform for the structure-guided optimization of novel antimalarial compounds.

Double drugging of prolyl-tRNA synthetase provides a new paradigm for anti-infective drug development

Toxoplasmosis is caused by Toxoplasma gondii and in immunocompromised patients it may lead to seizures, encephalitis or death. The conserved enzyme prolyl-tRNA synthetase (PRS) is a validated druggable target in Toxoplasma gondii but the traditional ‘single target–single drug’ approach has its caveats. Here, we describe two potent inhibitors namely halofuginone (HFG) and a novel ATP mimetic (L95) that bind to Toxoplasma gondii PRS simultaneously at different neighbouring sites to cover all three of the enzyme substrate subsites. HFG and L95 act as one triple-site inhibitor in tandem and form an unusual ternary complex wherein HFG occupies the 3’-end of tRNA and the L-proline (L-pro) binding sites while L95 occupies the ATP pocket. These inhibitors exhibit nanomolar IC₅₀ and EC₅₀ values independently, and when given together reveal an additive mode of action in parasite inhibition assays. This work validates a novel approach and lays a structural framework for further drug development based on simultaneous targeting of multiple pockets to inhibit druggable proteins.

Among infectious diseases, parasitic diseases are a major cause of death and morbidity. Toxoplasmosis is caused by an infection of the apicomplexan parasite Toxoplasma gondii. In immunocompromised patients Toxoplasmosis may lead to seizures, encephalitis or death. Novel therapeutics for human parasites are constantly needed. In recent years, the aminoacyl-tRNA synthetase (aaRS) enzyme family has been validated as a drug target for several parasitic infections. The Toxoplasma gondii prolyl-tRNA synthetase inhibitor halofuginone (HFG) has been validated earlier but here we show that an ATP-mimic called L95 is a potent inhibitor and can bind to the target enzyme in the presence of HFG. Thus, the two inhibitors described in this study simultaneously occupy all three natural substrate (ATP, L-amino acid and 3’-end of tRNA) binding pockets and thereby inhibit the enzyme leading to parasite death. This unprecedented double drugging of a pathogen enzyme may delay resistance mutation generation and this approach opens the path to multi-drugging of validated parasite proteins.

Advances in therapeutic and vaccine targets for Cryptosporidium: Challenges and possible mitigation strategies

Cryptosporidium is known to be the second most common diarrheal pathogen in children, causing potentially fatal diarrhea and associated with long-term growth stunting and cognitive deficits. The only Food and Drug Administration-approved treatment for cryptosporidiosis is nitazoxanide, but this drug has not shown potentially effective results in susceptible hosts. Therefore, a safe and effective drug for cryptosporidiosis is urgently needed. Cryptosporidium genome sequencing analysis may help develop an effective drug, but both in vitro and in vivo approaches to drug evaluation are not fully standardized. On the other hand, the development of partial immunity after exposure suggests the possibility of a successful and effective vaccine, but protective surrogates are not precise. In this review, we present our current perspectives on novel cryptosporidiosis therapies, vaccine targets and efficacies, as well as potential mitigation plans, recommendations and perceived challenges.

Bicyclic azetidines target acute and chronic stages of Toxoplasma gondii by inhibiting parasite phenylalanyl t-RNA synthetase

Toxoplasma gondii commonly infects humans and while most infections are controlled by the immune response, currently approved drugs are not capable of clearing chronic infection in humans. Hence, approximately one third of the world’s human population is at risk of reactivation, potentially leading to severe sequelae. To identify new candidates for treating chronic infection, we investigated a series of compounds derived from diversity-oriented synthesis. Bicyclic azetidines are potent low nanomolar inhibitors of phenylalanine tRNA synthetase (PheRS) in T. gondii, with excellent selectivity. Biochemical and genetic studies validate PheRS as the primary target of bicyclic azetidines in T. gondii, providing a structural basis for rational design of improved analogs. Favorable pharmacokinetic properties of a lead compound provide excellent protection from acute infection and partial protection from chronic infection in an immunocompromised mouse model of toxoplasmosis. Collectively, PheRS inhibitors of the bicyclic azetidine series offer promise for treatment of chronic toxoplasmosis.

Current treatments for toxoplasmosis are limited by adverse reactions and inability to cure chronic infections dominated by semi-dormant cyst forms. Here the authors demonstrate the potential of small molecule inhibitors of PheRS for controlling acute and chronic toxoplasmosis.

Host cells with transient overexpression of MDR1 as a novel in vitro model for evaluating on-target effect for activity against the epicellular Cryptosporidium parasite

OBJECTIVES

To rapidly generate host cells with resistance to multiple compounds for differentiating drug action on parasite target or the host cell target (i.e. on-target or off-target effect) against the zoonotic enteric parasite Cryptosporidium parvum.

METHODS

Transient overexpression of a multidrug resistance protein 1 (MDR1) gene in host cells (HCT-8 cell line) was explored to increase drug tolerance of the host cells to selected anti-cryptosporidial leads. In vitro cytotoxicity and anti-cryptosporidial efficacy of selected compounds were evaluated on the parasite grown in WT parental and transiently transfected HCT-8 cells. The approach was based on the theory that, for an epicellular parasite receiving consistent exposure to compounds in culture medium, overexpressing MDR1 in HCT-8 cells would increase drug tolerance of host cells to selected compounds but would not affect the anti-cryptosporidial efficacy if the compounds acted solely on the parasite target and the drug action on host cell target played no role on the antiparasitic efficacy.

RESULTS

Six known anti-cryptosporidial leads were tested. Transient overexpression of MDR1 increased drug tolerance of HCT-8 cells on paclitaxel, doxorubicin HCl and vincristine sulphate (2.11- to 2.27-fold increase), but not on cyclosporin A, daunorubicin HCl and nitazoxanide. Increased drug tolerance in host cells had no effect on antiparasitic efficacy of paclitaxel, but affected that of doxorubicin HCl.

CONCLUSIONS

Data confirmed that, at efficacious concentrations, paclitaxel acted mainly on the parasite target, while doxorubicin might act on both parasite and host cell targets. This model can be employed for studying the action of additional anti-cryptosporidial leads, and adapted to studying drug action in other epicellular pathogens. The limitation of the model is that the anti-cryptosporidial leads/hits need to be MDR1 substrates.

Emerging treatment options for cryptosporidiosis

PURPOSE OF REVIEW

Substantial progress has been made recently on the development of new therapeutics for cryptosporidiosis, an infection by the protozoan parasite Cryptosporidium that is associated with diarrhea, malnutrition, growth stunting, cognitive deficits, and oral vaccine failure in children living in low-resource settings.

RECENT FINDINGS

Various drug discovery approaches have generated promising lead candidates. The repurposed antimycobacterial drug clofazimine was tested in Malawian HIV patients with cryptosporidiosis but was ineffective. Target-based screens identified inhibitors of lysyl-tRNA synthetase, phenylalanyl-tRNA synthetase, methionyl-tRNA synthetase, and calcium-dependent protein kinase 1. Phenotypic screens led to discovery of a phosphatidylinositol 4-kinase inhibitor, the piperazine MMV665917, and the benzoxaborole AN7973. The relationship between pharmacokinetic properties and in-vivo efficacy is gradually emerging. A pathway to clinical trials, regulatory approval, and introduction has been proposed but additional work is needed to strengthen the route.

SUMMARY

Several lead compounds with potent activity in animal models and a favorable safety profile have been identified. A sustained effort will be required to advance at least one to clinical proof-of-concept studies. The demonstrated risk of resistance indicates multiple candidates should be advanced as potential components of a combination therapy.

The Biology of the Intestinal Intracellular Parasite Cryptosporidium

Cryptosporidium emerged as a leading global cause of severe diarrheal disease in children. The parasite occupies a unique intracellular niche at the brush border of intestinal epithelial cells, where it undergoes a complex sexual life cycle. How this life cycle unfolds and how host and parasite interact remain largely to be discovered. A series of technical advances now offer genetic and immunological tools for mechanistic investigation of the parasite. Here we introduce the pathogen and disease and highlight important questions to tackle onward. We invite scientists to consider this versatile parasite model to probe the biology and immunology of the intestine.

Spontaneous Selection of Cryptosporidium Drug Resistance in a Calf Model of Infection

The intestinal protozoan Cryptosporidium is a leading cause of diarrheal disease and mortality in young children. There is currently no fully effective treatment for cryptosporidiosis, which has stimulated interest in anticryptosporidial development over the last ∼10 years, with numerous lead compounds identified, including several tRNA synthetase inhibitors. Here, we report the results of a dairy calf efficacy trial of the methionyl-tRNA (Cryptosporidium parvum MetRS [CpMetRS]) synthetase inhibitor 2093 and the spontaneous emergence of drug resistance. Dairy calves experimentally infected with Cryptosporidium parvum initially improved with 2093 treatment, but parasite shedding resumed in two of three calves on treatment day 5. Parasites shed by each recrudescent calf had different amino acid-altering mutations in the gene encoding CpMetRS (CpMetRS), yielding either an aspartate 243-to-glutamate (D243E) or a threonine 246-to-isoleucine (T246I) mutation. Transgenic parasites engineered to have either the D243E or T246I CpMetRS mutation using CRISPR/Cas9 grew normally but were highly 2093 resistant; the D243E and T246I mutant-expressing parasites, respectively, had 2093 half-maximal effective concentrations (EC₅₀s) that were 613- and 128-fold that of transgenic parasites with wild-type CpMetRS. In studies using recombinant enzymes, the D243E and T246I mutations shifted the 2093 IC₅₀ >170-fold. Structural modeling of CpMetRS based on an inhibitor-bound Trypanosoma brucei MetRS crystal structure suggested that the resistance mutations reposition nearby hydrophobic residues, interfering with compound binding while minimally impacting substrate binding. This is the first report of naturally emerging Cryptosporidium drug resistance, highlighting the need to address the potential for anticryptosporidial resistance and establish strategies to limit its occurrence.

Opportunities and Challenges in Developing a Cryptosporidium Controlled Human Infection Model for Testing Antiparasitic Agents

Cryptosporidiosis is a leading cause of moderate-to-severe diarrhea in low- and middle-income countries, responsible for high mortality in children younger than two years of age, and it is also strongly associated with childhood malnutrition and growth stunting. There is no vaccine for cryptosporidiosis and existing therapeutic options are suboptimal to prevent morbidity and mortality in young children. Recently, novel therapeutic agents have been discovered through high-throughput phenotypic and target-based screening strategies, repurposing malaria hits, etc., and these agents have a promising preclinical in vitro and in vivo anti-Cryptosporidium efficacy. One key step in bringing safe and effective new therapies to young vulnerable children is the establishment of some prospect of direct benefit before initiating pediatric clinical studies. A Cryptosporidium controlled human infection model (CHIM) in healthy adult volunteers can be a robust clinical proof of concept model for evaluating novel therapeutics. CHIM could potentially accelerate the development path to pediatric studies by establishing the safety of a proposed pediatric dosing regimen and documenting preliminary efficacy in adults. We present, here, perspectives regarding the opportunities and perceived challenges with the Cryptosporidium human challenge model.

Recent advances in the synthesis and reactivity of azetidines: strain-driven character of the four-membered heterocycle

Azetidines represent one of the most important four-membered heterocycles used in organic synthesis and medicinal chemistry. The reactivity of azetidines is driven by a considerable ring strain, while at the same the ring is significantly more stable than that of related aziridines, which translates into both facile handling and unique reactivity that can be triggered under appropriate reaction conditions. Recently, remarkable advances in the chemistry and reactivity of azetidines have been reported. In this review, we provide an overview of the synthesis, reactivity and application of azetidines that have been published in the last years with a focus on the most recent advances, trends and future directions. The review is organized by the methods of synthesis of azetidines and the reaction type used for functionalization of azetidines. Finally, recent examples of using azetidines as motifs in drug discovery, polymerization and chiral templates are discussed.

Current status and challenges in drug discovery against the globally important zoonotic cryptosporidiosis

The zoonotic cryptosporidiosis is globally distributed, one of the major diarrheal diseases in humans and animals. Cryptosporidium oocysts are also one of the major environmental concerns, making it a pathogen that fits well into the One Health concept. Despite its importance, fully effective drugs are not yet available. Anti-cryptosporidial drug discovery has historically faced many unusual challenges attributed to unique parasite biology and technical burdens. While significant progresses have been made recently, anti-cryptosporidial drug discovery still faces a major obstacle: identification of systemic drugs that can be absorbed by patients experiencing watery diarrhea and effectively pass through electron-dense (ED) band at the parasite-host cell interface to act on the epicellular parasite. There may be a need to develop an in vitro assay to effectively screen hits/leads for their capability to cross ED band. In the meantime, non-systemic drugs with strong mucoadhesive properties for extended gastrointestinal exposure may represent another direction in developing anti-cryptosporidial therapeutics. For developing both systemic and non-systemic drugs, a non-ruminant animal model exhibiting diarrheal symptoms suitable for routine evaluation of drug absorption and anti-cryptosporidial efficacy may be very helpful.

Use-case scenarios for an anti-Cryptosporidium therapeutic

Cryptosporidium is a widely distributed enteric parasite that has an increasingly appreciated pathogenic role, particularly in pediatric diarrhea. While cryptosporidiosis has likely affected humanity for millennia, its recent "emergence" is largely the result of discoveries made through major epidemiologic studies in the past decade. There is no vaccine, and the only approved medicine, nitazoxanide, has been shown to have efficacy limitations in several patient groups known to be at elevated risk of disease. In order to help frontline health workers, policymakers, and other stakeholders translate our current understanding of cryptosporidiosis into actionable guidance to address the disease, we sought to assess salient issues relating to clinical management of cryptosporidiosis drawing from a review of the literature and our own field-based practice. This exercise is meant to help inform health system strategies for improving access to current treatments, to highlight recent achievements and outstanding knowledge and clinical practice gaps, and to help guide research activities for new anti-Cryptosporidium therapies.

Molecular Epidemiology of Human Cryptosporidiosis in Low- and Middle-Income Countries

Cryptosporidiosis is one of the most important causes of moderate to severe diarrhea and diarrhea-related mortality in children under 2 years of age in low- and middle-income countries. In recent decades, genotyping and subtyping tools have been used in epidemiological studies of human cryptosporidiosis. Results of these studies suggest that higher genetic diversity of Cryptosporidium spp. is present in humans in these countries at both species and subtype levels and that anthroponotic transmission plays a major role in human cryptosporidiosis. Cryptosporidium hominis is the most common Cryptosporidium species in humans in almost all the low- and middle-income countries examined, with five subtype families (namely, Ia, Ib, Id, Ie, and If) being commonly found in most regions. In addition, most Cryptosporidium parvum infections in these areas are caused by the anthroponotic IIc subtype family rather than the zoonotic IIa subtype family. There is geographic segregation in Cryptosporidium hominis subtypes, as revealed by multilocus subtyping. Concurrent and sequential infections with different Cryptosporidium species and subtypes are common, as immunity against reinfection and cross protection against different Cryptosporidium species are partial. Differences in clinical presentations have been observed among Cryptosporidium species and C. hominis subtypes. These observations suggest that WASH (water, sanitation, and hygiene)-based interventions should be implemented to prevent and control human cryptosporidiosis in low- and middle-income countries.

Cryptosporidium: Host-Parasite Interactions and Pathogenesis

Purpose of Review

Cryptosporidium spp. (C. hominis and C. parvum) are a major cause of diarrhea-associated morbidity and mortality in young children globally. While C. hominis only infects humans, C. parvum is a zoonotic parasite that can be transmitted from infected animals to humans. There are no treatment or control measures to fully treat cryptosporidiosis or prevent the infection in humans and animals. Our knowledge on the molecular mechanisms of Cryptosporidium-host interactions and the underlying factors that govern infectivity and disease pathogenesis is very limited.

Recent Findings

Recent development of genetics and new animal models of infection, along with progress in cell culture platforms to complete the parasite lifecycle in vitro, is greatly advancing the Cryptosporidium field.

Summary

In this review, we will discuss our current knowledge of host-parasite interactions and how genetic manipulation of Cryptosporidium and promising infection models are opening the doors towards an improved understanding of parasite biology and disease pathogenesis.

Simple Secondary Amines Inhibit Growth of Gram-Negative Bacteria through Highly Selective Binding to Phenylalanyl-tRNA Synthetase

Antibiotics to treat drug-resistant Gram-negative infections are urgently needed but challenging to discover. Using a cell-based screen, we identified a simple secondary amine that inhibited the growth of wild-type Escherichia coli and Acinetobacter baumannii but not the growth of the Gram-positive organism Bacillus subtilis. Resistance mutations in E. coli and A. baumannii mapped exclusively to the aminoacyl-tRNA synthetase PheRS. We confirmed biochemically that the compound inhibited PheRS from these organisms and showed that it did not inhibit PheRS from B. subtilis or humans. To understand the basis for the compound's high selectivity for only some PheRS enzymes, we solved crystal structures of E. coli and A. baumannii PheRS complexed with the inhibitor. The structures showed that the compound's benzyl group mimics the benzyl of phenylalanine. The other amine substituent, a 2-(cyclohexen-1-yl)ethyl group, induces a hydrophobic pocket in which it binds. Through bioinformatic analysis and mutagenesis, we show that the ability to induce a complementary hydrophobic pocket that can accommodate the second substituent explains the high selectivity of this remarkably simple molecular scaffold for Gram-negative PheRS. Because this secondary amine scaffold is active against wild-type Gram-negative pathogens but is not cytotoxic to mammalian cells, we suggest that it may be possible to develop it for use in combination antibiotic therapy to treat Gram-negative infections.

Quarterly picks from the editors

The Science Translational Medicine editors highlight interesting translational ties across select articles published recently in the Science family of journals.