Lysyl-tRNA synthetase as a drug target in malaria and cryptosporidiosis.

Malaria and cryptosporidiosis, caused by apicomplexan parasites, remain major drivers of global child mortality. New drugs for the treatment of malaria and cryptosporidiosis, in particular, are of high priority; however, there are few chemically validated targets. The natural product cladosporin is active against blood- and liver-stage Plasmodium falciparum and Cryptosporidium parvum in cell-culture studies. Target deconvolution in P. falciparum has shown that cladosporin inhibits lysyl-tRNA synthetase (PfKRS1). Here, we report the identification of a series of selective inhibitors of apicomplexan KRSs. Following a biochemical screen, a small-molecule hit was identified and then optimized by using a structure-based approach, supported by structures of both PfKRS1 and C. parvum KRS (CpKRS). In vivo proof of concept was established in an SCID mouse model of malaria, after oral administration (ED90 = 1.5 mg/kg, once a day for 4 d). Furthermore, we successfully identified an opportunity for pathogen hopping based on the structural homology between PfKRS1 and CpKRS. This series of compounds inhibit CpKRS and C. parvum and Cryptosporidium hominis in culture, and our lead compound shows oral efficacy in two cryptosporidiosis mouse models. X-ray crystallography and molecular dynamics simulations have provided a model to rationalize the selectivity of our compounds for PfKRS1 and CpKRS vs. (human) HsKRS. Our work validates apicomplexan KRSs as promising targets for the development of drugs for malaria and cryptosporidiosis.

Novel lactate dehydrogenase inhibitors with in vivo efficacy against Cryptosporidium parvum.

Cryptosporidium parvum is a highly prevalent zoonotic and anthroponotic protozoan parasite that causes a diarrheal syndrome in children and neonatal livestock, culminating in growth retardation and mortalities. Despite the high prevalence of C. parvum, there are no fully effective and safe drugs for treating infections, and there is no vaccine. We have previously reported that the bacterial-like C. parvum lactate dehydrogenase (CpLDH) enzyme is essential for survival, virulence and growth of C. parvum in vitro and in vivo. In the present study, we screened compound libraries and identified inhibitors against the enzymatic activity of recombinant CpLDH protein in vitro. We tested the inhibitors for anti-Cryptosporidium effect using in vitro infection assays of HCT-8 cells monolayers and identified compounds NSC158011 and NSC10447 that inhibited the proliferation of intracellular C. parvum in vitro, with IC50 values of 14.88 and 72.65 µM, respectively. At doses tolerable in mice, we found that both NSC158011 and NSC10447 consistently significantly reduced the shedding of C. parvum oocysts in infected immunocompromised mice's feces, and prevented intestinal villous atrophy as well as mucosal erosion due to C. parvum. Together, our findings have unveiled promising anti-Cryptosporidium drug candidates that can be explored further for the development of the much needed novel therapeutic agents against C. parvum infections.

Lactate dehydrogenase and malate dehydrogenase: Potential antiparasitic targets for drug development studies.

Parasitic diseases remain a major public health concern for humans, claiming millions of lives annually. Although different treatments are required for these diseases, drug usage is limited due to the development of resistance and toxicity, which necessitate alternative therapies. It has been shown in the literature that parasitic lactate dehydrogenases (LDH) and malate dehydrogenases (MDH) have unique pharmacological selective and specificity properties compared to other isoforms, thus highlighting them as viable therapeutic targets involved in aerobic and anaerobic glycolytic pathways. LDH and MDH are important therapeutic targets for invasive parasites because they play a critical role in the progression and development of parasitic diseases. Any strategy to impede these enzymes would be fatal to the parasites, paving the way to develop and discover novel antiparasitic agents. This review aims to highlight the importance of parasitic LDH and MDH as therapeutic drug targets in selected obligate apicoplast parasites. To the best of our knowledge, this review presents the first comprehensive review of LDH and MDH as potential antiparasitic targets for drug development studies.

Genetic modification of the diarrhoeal pathogen Cryptosporidium parvum.

Recent studies into the global causes of severe diarrhoea in young children have identified the protozoan parasite Cryptosporidium as the second most important diarrhoeal pathogen after rotavirus. Diarrhoeal disease is estimated to be responsible for 10.5% of overall child mortality. Cryptosporidium is also an opportunistic pathogen in the contexts of human immunodeficiency virus (HIV)-caused AIDS and organ transplantation. There is no vaccine and only a single approved drug that provides no benefit for those in gravest danger: malnourished children and immunocompromised patients. Cryptosporidiosis drug and vaccine development is limited by the poor tractability of the parasite, which includes a lack of systems for continuous culture, facile animal models, and molecular genetic tools. Here we describe an experimental framework to genetically modify this important human pathogen. We established and optimized transfection of C. parvum sporozoites in tissue culture. To isolate stable transgenics we developed a mouse model that delivers sporozoites directly into the intestine, a Cryptosporidium clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system, and in vivo selection for aminoglycoside resistance. We derived reporter parasites suitable for in vitro and in vivo drug screening, and we evaluated the basis of drug susceptibility by gene knockout. We anticipate that the ability to genetically engineer this parasite will be transformative for Cryptosporidium research. Genetic reporters will provide quantitative correlates for disease, cure and protection, and the role of parasite genes in these processes is now open to rigorous investigation.

Mycophenolic anilides as broad specificity inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitors.

Inosine-5'-monophosphate dehydrogenase (IMPDH) is a potential target for microorganisms. However, identifying inhibitor design determinants for IMPDH orthologs continues to evolve. Herein, a series of mycophenolic anilide inhibitors of Cryptosporidium parvum and human IMPDHs are reported. Furthermore, molecular docking of 12 (e.g. SH-19; CpIMPDH Ki,app = 0.042 ± 0.015 µM, HsIMPDH2 Ki,app = 0.13 ± 0.05 µM) supports different binding modes with the two enzymes. For CpIMPDH the inhibitor extends into a pocket in an adjacent subunit. In contrast, docking suggests the inhibitor interacts with Ser276 in the NAD binding site in HsIMPDH2, as well as an adjacent pocket within the same subunit. These results provide further guidance for generating IMPDH inhibitors for enzymes found in an array of pathogenic microorganisms, including Mycobacterium tuberculosis.

Side chain rotameric changes and backbone dynamics enable specific cladosporin binding in Plasmodium falciparum lysyl-tRNA synthetase.

Cladosporin (CLD) is a fungal metabolite that kills the malaria parasite via inhibiting its cytoplasmic lysyl-tRNA synthetase (KRS) and abrogating protein translation. Here we provide structural and drug selectivity analyses on CLD interacting residues in apo and holo KRSs from Plasmodium falciparum, Homo sapiens, Cryptosporidium parvum, and Mycobacterium ulcerans. We show that both gross and subtle alterations in protein backbone and sidechains drive the active site structural plasticity that allows integration of CLD in KRSs. The ligand-induced fit of CLD in PfKRS is marked by closure and stabilization of three disordered loops and one alpha helix. However, these structural rearragements are not evident in KRS-CLD complexes from H. sapiens, C. parvum, or M. ulcerans. Strikingly, CLD fits into the MuKRS active site due to a remarkable rotameric alteration in its clash-prone methionine residue that provides accommodation for the methyl moiety in CLD. Although the high concentrations of drugs used for Hs, Cp, and MuKRS-CLD complexes in co-crystallization studies enable elucidation of a structural framework for understanding drug binding in KRSs, we propose that these data should be concurrently assessed via biochemical studies of potency and drug selectivity given the poor cell-based activity of CLD against human and bacterial cells. Our comprehensive analyses of KRS-CLD interactions, therefore, highlight vital issues in structure-based drug discovery studies.

The Action of the Hexokinase Inhibitor 2-deoxy-d-glucose on Cryptosporidium parvum and the Discovery of Activities against the Parasite Hexokinase from Marketed Drugs.

Cryptosporidium parvum is one of the major species causing mild to severe cryptosporidiosis in humans and animals. We have previously observed that 2-deoxy-d-glucose (2DG) could inhibit both the enzyme activity of C. parvum hexokinase (CpHK) and the parasite growth in vitro. However, the action and fate of 2DG in C. parvum was not fully investigated. In the present study, we showed that, although 2DG could be phosphorylated by CpHK to form 2DG-6-phosphate (2DG6P), the anti-cryptosporidial activity of 2DG was mainly attributed to the action of 2DG on CpHK, rather than the action of 2DG or 2DG6P on the downstream enzyme glucose-6-phosphate isomerase (CpGPI) nor 2DG6P on CpHK. These observations further supported the hypothesis that CpHK could serve as a drug target in the parasite. We also screened 1,200 small molecules consisting of marketed drugs against CpHK, from which four drugs were identified as CpHK inhibitors with micromolar level of anti-cryptospordial activities at concentrations nontoxic to the host cells (i.e. hexachlorphene, thimerosal, alexidine dihydrochloride, and ebselen with EC50  = 0.53, 1.77, 8.1 and 165 µM, respectively). The anti-CpHK activity of the four existing drugs provided us new reagents for studying the enzyme properties of the parasite hexokinase.

Repurposing existing drugs: identification of irreversible IMPDH inhibitors by high-throughput screening.

Inosine 5'-monophosphate dehydrogenase (IMPDH) is an essential enzyme for the production of guanine nucleotides. Disruption of IMPDH activity has been explored as a therapeutic strategy for numerous purposes, such as for anticancer, immunosuppression, antiviral, and antimicrobial therapy. In the present study, we established a luciferase-based high-throughput screening system to identify IMPDH inhibitors from our chemical library of known bioactive small molecules. The screening of 1400 compounds resulted in the discovery of three irreversible inhibitors: disulfiram, bronopol, and ebselen. Each compound has a distinct chemical moiety that differs from other reported IMPDH inhibitors. Further evaluation revealed that these compounds are potent inhibitors of IMPDHs with kon values of 0.7 × 104 to 9.3 × 104 M-1·s-1. Both disulfiram and bronopol exerted similar degree of inhibition to protozoan and mammalian IMPDHs. Ebselen showed an intriguing difference in mode of inhibition for different IMPDHs, with reversible and irreversible inhibition to each Cryptosporidium parvum IMPDH and human IMPDH type II, respectively. In the preliminary efficacy experiment against cryptosporidiosis in severe combined immunodeficiency (SCID) mouse, a decrease in the number of oocyst shed was observed upon the oral administration of disulfiram and bronopol, providing an early clinical proof-of-concept for further utilization of these compounds as IMPDH inhibitors.

Identification of adaptive inhibitors of Cryptosporidium parvum fatty acyl-coenzyme A synthetase isoforms by virtual screening.

Cryptosporidiosis is a significant cause of gastroenteritis in both humans and livestock in developing countries. The only FDA-approved drug available against the same is nitazoxanide, with questionable efficacy in malnourished children and immunocompromised patients. Recent in vitro studies have indicated the viability of Triacsin C as a potential drug candidate, which targets the parasite's long-chain fatty acyl coenzyme A synthetase enzyme (LC-FACS), a critical component of the fatty acid metabolism pathway. We have used this molecule as a baseline to propose more potent versions thereof. We have applied a combined approach of substructure replacement, literature search, and database screening to come up with 514 analogs of Triacsin C. A virtual screening protocol was carried out which lead us to identify a potential hit compound. This was further subjected to a 100-ns molecular dynamics simulation in complex to determine its stability and binding characteristics. After which, the ADME/tox properties were predicted to assess its viability as a drug. The molecule R134 was identified as the best hit due to its highest average binding affinity, stability in complex when subjected to MD simulations, and reasonable predicted ADMET (Absorption, Distribution, Metabolism, Excretion and Toxicity) properties comparable to those of the Triacsin C parent molecule. We have proposed R134 as a putative drug candidate against the Cryptosporidium parvum LC-FACS enzyme isoforms, following an in silico protocol. We hope the results will be helpful when planning future in vitro experiments for identifying drugs against Cryptosporidium.

Structurally Linked Dynamics in Lactate Dehydrogenases of Evolutionarily Distinct Species.

We present new findings about how primary and secondary structure affects the role of fast protein motions in the reaction coordinates of enzymatic reactions. Using transition path sampling and committor distribution analysis, we examined the difference in the role of these fast protein motions in the reaction coordinate of lactate dehydrogenases (LDHs) of Apicomplexa organisms Plasmodium falciparum and Cryptosporidium parvum. Having evolved separately from a common malate dehydrogenase ancestor, the two enzymes exhibit several important structural differences, notably a five-amino acid insertion in the active site loop of P. falciparum LDH. We find that these active site differences between the two organisms' LDHs likely cause a decrease in the contribution of the previously determined LDH rate-promoting vibration to the reaction coordinate of P. falciparum LDH compared to that of C. parvum LDH, specifically in the coupling of the rate-promoting vibration and the hydride transfer. This effect, while subtle, directly shows how changes in structure near the active site of LDH alter catalytically important motions. Insights provided by studying these alterations would prove to be useful in identifying LDH inhibitors that specifically target the isozymes of these parasitic organisms.

Inhibition of Calcium-Dependent Protein Kinase 1 (CDPK1) In Vitro by Pyrazolopyrimidine Derivatives Does Not Correlate with Sensitivity of Cryptosporidium parvum Growth in Cell Culture.

Cryptosporidiosis is a serious diarrheal disease in immunocompromised patients and malnourished children, and treatment is complicated by a lack of adequate drugs. Recent studies suggest that the natural occurrence of a small gatekeeper residue in serine threonine calcium-dependent protein kinase 1 (CDPK1) of Cryptosporidium parvum might be exploited to target this enzyme and block parasite growth. Here were explored the potency with which a series of pyrazolopyrimidine analogs, which are selective for small gatekeeper kinases, inhibit C. parvum CDPK1 and block C. parvum growth in tissue culture in vitro. Although these compounds potently inhibited kinase activity in vitro, most had no effect on parasite growth. Moreover, among those that were active against parasite growth, there was a very poor correlation with their 50% inhibitory concentrations against the enzyme. Active compounds also had no effect on cell invasion, unlike the situation in Toxoplasma gondii, where these compounds block CDPK1, prevent microneme secretion, and disrupt cell invasion. These findings suggest that CPDK1 is not essential for C. parvum host cell invasion or growth and therefore that it is not the optimal target for therapeutic intervention. Nonetheless, several inhibitors with low micromolar 50% effective concentrations were identified, and these may affect other essential targets in C. parvum that are worthy of further exploration.

Differential Gene Expression and Protein Localization of Cryptosporidium parvum Fatty Acyl-CoA Synthetase Isoforms.

Cryptosporidium parvum is unable to synthesize fatty acids de novo, but possesses three long-chain fatty acyl-CoA synthetase (CpACS) isoforms for activating fatty acids. We have recently shown that these enzymes could be targeted to kill the parasite in vitro and in vivo. Here, we demonstrated that the CpACS genes were differentially expressed during the parasite life cycle, and their proteins were localized to different subcellular structures by immunofluorescence and immuno-electron microscopies. Among them, CpACS1 displayed as an apical protein in sporozoites and merozoites, but no or little presence during the intracellular merogony until the release of merozoites, suggesting that CpACS1 probably functioned mainly during the parasite invasion and/or early stage of intracellular development. Both CpACS2 and CpACS3 proteins were present in all parasite life cycle stages, in which CpACS2 was present in the parasite and the parasitophorous vacuole membranes (PVM), whereas CpACS3 was mainly present in the parasite plasma membranes with little presence in the PVM. These observations suggest that CpACS2 and CpACS3 may participate in scavenging and transport of fatty acids across the PVM and the parasite cytoplasmic membranes, respectively.

Expression of Cryptosporidium parvum thioredoxin peroxidase in COS-7 cells confers radioprotection.

Cryptosporidium parvum is one of the most radioresistant organisms identified to date. In a previous study, we found that thioredoxin peroxidase (CpTPx) was significantly upregulated in this species following exposure to high dose (10 kGy) of γ-irradiation. To assess the potential of CpTPx to confer radioprotection in mammalian cells, it was expressed in COS-7 African green monkey kidney cells (CpTPx-COS7). For comparison, the thioredoxin peroxidase of Cryptosporidium muris (CmTPx) was also expressed in these cells (CmTPx-COS7 cells), which has been confirmed to have lesser antioxidant activity than CpTPx in the previous study. Notably, the survival rates of CpTPx-COS7 cells were significantly higher (12-22%) at 72 h after 8 Gy irradiation than CmTPx-COS7 or non-transfected COS-7 (ntCOS-7) counterparts. In addition, CpTPx revealed a 50% of ROS reduction in irradiated CpTPx-COS7 cells, while γ-H2AX DNA damage marker expression was not significantly changed. Furthermore, the amount of apoptosis only increased to about 120% after 2-8 Gy irradiation compared to 200-300% increase observed in ntCOS-7 cells. CmTPx was shown to have antioxidant and DNA damage protection activities; however, these activities were always lower than those of CpTPx. These results suggest that the potent antioxidant and protective activities of CpTPx are well conserved in this cell-based system and that CpTPx contributed to the radioprotection of mammalian cells through its exceptional antioxidant activity.

Amelioration of Cryptosporidium parvum infection in vitro and in vivo by targeting parasite fatty acyl-coenzyme A synthetases.

BACKGROUND: Cryptosporidium is emerging as 1 of the 4 leading diarrheal pathogens in children in developing countries. Its infections in patients with AIDS can be fatal, whereas fully effective treatments are unavailable. The major goal of this study is to explore parasite fatty acyl-coenzyme A synthetase (ACS) as a novel drug target. METHODS: A colorimetric assay was developed to evaluate biochemical features and inhibitory kinetics of Cryptosporidium parvum ACSs using recombinant proteins. Anticryptosporidial efficacies of the ACS inhibitor triacsin C were evaluated both in vitro and in vivo. RESULTS: Cryptosporidium ACSs displayed substrate preference toward long-chain fatty acids. The activity of parasite ACSs could be specifically inhibited by triacsin C with the inhibition constant Ki in the nanomolar range. Triacsin C was highly effective against C. parvum growth in vitro (median inhibitory concentration, 136 nmol/L). Most importantly, triacsin C effectively reduced parasite oocyst production up to 88.1% with no apparent toxicity when administered to Cryptosporidium-infected interleukin 12 knockout mice at 8-15 mg/kg/d for 1 week. CONCLUSIONS: The findings of this study not only validated Cryptosporidium ACS (and related acyl-[acyl-carrier-protein]-ligases) as pharmacological targets but also indicate that triacsin C and analogues can be explored as potential new therapeutics against the virtually untreatable cryptosporidial infection in immunocompromised patients.

Characterization of biochemical properties of a selenium-independent glutathione peroxidase of Cryptosporidium parvum.

Glutathione peroxidase (GPx; EC 1.11.1.9) is an important antioxidant enzyme that catalyses the reduction of organic and inorganic hydroperoxides to water in oxygen-consuming organisms, using glutathione as an electron donor. Here, we report the characterization of a GPx of Cryptosporidium parvum (CpGPx). CpGPx contained a standard UGU codon for cysteine instead of a UGA opal codon for seleno-cysteine (SeCys) at the active site, and no SeCys insertion sequence (SECIS) motif was identified within the 3'-untranslated region (UTR) of CpGPx, which suggested its selenium-independent nature. In silico and biochemical analyses indicated that CpGPx is a cytosolic protein with a monomeric structure. Recombinant CpGPx was active over a wide pH range and was stable under physiological conditions. It showed a substrate preference against organic hydroperoxides, such as cumene hydroperoxide and t-butyl hydroperoxide, but it also showed activity against inorganic hydroperoxide, hydrogen peroxide. Recombinant CpGPx was not inhibited by potassium cyanide or by sodium azide. The enzyme effectively protected DNA and protein from oxidative damage induced by hydrogen peroxide, and was functionally expressed in various developmental stages of C. parvum. These results collectively suggest the essential role of CpGPx for the parasite's antioxidant defence system.

CDPKs of Cryptosporidium parvum--stage-specific expression in vitro.

Cryptosporidium parvum is a zoonotic agent that bears a high risk for the health of particularly immunocompromised humans and animals. As currently available drugs and therapies against cryptosporidiosis do not turn out satisfactory, more intensive research on the control of this parasite is necessary. The genus Cryptosporidium is unique within the phylum Apicomplexa as its localisation is intracellular but extracytoplasmatic. Infection of host cells is initially a parasite-driven process, but the signalling events and their downstream actions within Cryptosporidium are poorly understood. Calcium-dependent protein kinases (CDPKs) are probably involved in the regulation of invasion and egress. Previously described in plants, algae and other Apicomplexa, CDPKs are not found in vertebrates. They are thus promising targets for pharmaceutical intervention. While CDPK1 is well characterised in Toxoplasma gondii (TgCDPK1) and Plasmodium falciparum (PfCDPK1), only little information exists about the expression and function of CDPK in C. parvum. Here, we describe results of the in silico analysis of seven CpCDPKs. Five CpCDPKs contain potential sites for N-myristoylation and N-palmitoylation. In a nested 3' rapid amplification of cDNA ends (RACE)-PCR, expression of six CpCDPKs resulted in distinct bands in infected cell cultures and extracts of freshly excysted sporozoites. The length of the 3' untranslated region (3' UTR) is described as well. Our results indicate CDPK expression to be stage specific on the mRNA level.

A novel calcium-dependent protein kinase inhibitor as a lead compound for treating cryptosporidiosis.

Cryptosporidium parasites infect intestinal cells, causing cryptosporidiosis. Despite its high morbidity and association with stunting in the developing world, current therapies for cryptosporidiosis have limited efficacy. Calcium-dependent protein kinases (CDPKs) are essential enzymes in the biology of protozoan parasites. CDPK1 was cloned from the genome of Cryptosporidium parvum, and potent and specific inhibitors have been developed based on structural studies. In this study, we evaluated the anti-Cryptosporidium activity of a novel CDPK1 inhibitor, 1294, and demonstrated that 1294 significantly reduces parasite infection in vitro, with a half maximal effective concentration of 100 nM. Pharmacokinetic studies revealed that 1294 is well absorbed, with a half-life supporting daily administration. Oral therapy with 1294 eliminated Cryptosporidium parasites from 6 of 7 infected severe combined immunodeficiency-beige mice, and the parasites did not recur in these immunosuppressed mice. Mice treated with 1294 had less epithelial damage, corresponding to less apoptosis. Thus, 1294 is an important lead for the development of drugs for treatment of cryptosporidiosis.

The Cryptosporidium signaling kinase CDPK5 plays an important role in male gametogenesis and parasite virulence.

The protozoan parasite Cryptosporidium is a leading cause of diarrhea in young children. The parasite's life cycle involves a coordinated and timely progression from asexual to sexual stages, leading to the formation of the transmissible oocyst. Underlying molecular signaling mechanisms orchestrating sexual development are not known. Here, we describe the function of a signaling kinase in Cryptosporidium male gametogenesis. We reveal the expression of Cryptosporidium parvum calcium-dependent protein kinase 5 (CDPK5) during male gamete development and its important role in the egress of mature gametes. Genetic ablation of this kinase results in viable parasites, indicating that this gene is dispensable for parasite survival. Interestingly, cdpk5 deletion decreases parasite virulence and impacts oocyst shedding in immunocompromised mice. Using phosphoproteomics, we identify possible CDPK5 substrates and biological processes regulated by this kinase. Collectively, these findings illuminate parasite cell biology by revealing a mechanism controlling male gamete production and a potential target to block disease transmission.

TKL family kinases in human apicomplexan pathogens.

Apicomplexan parasites are the primary causative agents of many human diseases, including malaria, toxoplasmosis, and cryptosporidiosis. These opportunistic pathogens undergo complex life cycles with multiple developmental stages, wherein many key steps are regulated by phosphorylation mechanisms. The genomes of apicomplexan pathogens contain protein kinases from different groups including tyrosine kinase-like (TKL) family proteins. Although information on the role of TKL kinases in apicomplexans is quite limited, recent studies have revealed the important role of this family of proteins in apicomplexan biology. TKL kinases in these protozoan pathogens show unique organization with many novel domains thus making them attractive candidates for drug development. In this mini review, we summarize the current understanding of the role of TKL kinases in human apicomplexan pathogens' (Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum) biology and pathogenesis.

A cysteine protease inhibitor rescues mice from a lethal Cryptosporidium parvum infection.

Cryptosporidiosis, caused by the protozoan parasite Cryptosporidium parvum, can stunt infant growth and can be lethal in immunocompromised individuals. The most widely used drugs for treating cryptosporidiosis are nitazoxanide and paromomycin, although both exhibit limited efficacy. To investigate an alternative approach to therapy, we demonstrate that the clan CA cysteine protease inhibitor N-methyl piperazine-Phe-homoPhe-vinylsulfone phenyl (K11777) inhibits C. parvum growth in mammalian cell lines in a concentration-dependent manner. Further, using the C57BL/6 gamma interferon receptor knockout (IFN-γR-KO) mouse model, which is highly susceptible to C. parvum, oral or intraperitoneal treatment with K11777 for 10 days rescued mice from otherwise lethal infections. Histologic examination of untreated mice showed intestinal inflammation, villous blunting, and abundant intracellular parasite stages. In contrast, K11777-treated mice (210 mg/kg of body weight/day) showed only minimal inflammation and no epithelial changes. Three putative protease targets (termed cryptopains 1 to 3, or CpaCATL-1, -2, and -3) were identified in the C. parvum genome, but only two are transcribed in infected mammals. A homology model predicted that K11777 would bind to cryptopain 1. Recombinant enzymatically active cryptopain 1 was successfully targeted by K11777 in a competition assay with a labeled active-site-directed probe. K11777 exhibited no toxicity in vitro and in vivo, and surviving animals remained free of parasites 3 weeks after treatment. The discovery that a cysteine protease inhibitor provides potent anticryptosporidial activity in an animal model of infection encourages the investigation and development of this biocide class as a new, and urgently needed, chemotherapy for cryptosporidiosis.