Investigations into microsporidian methionine aminopeptidase type 2: a therapeutic target for microsporidiosis

Identification of Potential Druggable Targets and Structure-Based Virtual Screening for Drug-like Molecules against the Shrimp Pathogen Enterocytozoon hepatopenaei

Enterocytozoon hepatopenaei (EHP) causes slow growth syndrome in shrimp, resulting in huge economic losses for the global shrimp industry. Despite worldwide reports, there are no effective therapeutics for controlling EHP infections. In this study, five potential druggable targets of EHP, namely, aquaporin (AQP), cytidine triphosphate (CTP) synthase, thymidine kinase (TK), methionine aminopeptidase2 (MetAP2), and dihydrofolate reductase (DHFR), were identified via functional classification of the whole EHP proteome. The three-dimensional structures of the proteins were constructed using the artificial-intelligence-based program AlphaFold 2. Following the prediction of druggable sites, the ZINC15 and ChEMBL databases were screened against targets using docking-based virtual screening. Molecules with affinity scores ≥ 7.5 and numbers of interactions ≥ 9 were initially selected and subsequently enriched based on their ADMET properties and electrostatic complementarities. Five compounds were finally selected against each target based on their complex stabilities and binding energies. The compounds CHEMBL3703838, CHEMBL2132563, and CHEMBL133039 were selected against AQP; CHEMBL1091856, CHEMBL1162979, and CHEMBL525202 against CTP synthase; CHEMBL4078273, CHEMBL1683320, and CHEMBL3674540 against TK; CHEMBL340488, CHEMBL1966988, and ZINC000828645375 against DHFR; and CHEMBL3913373, ZINC000016682972, and CHEMBL3142997 against MetAP2.The compounds exhibited high stabilities and low binding free energies, indicating their abilities to suppress EHP infections; however, further validation is necessary for determining their efficacy.

A Perspective on the Molecular Identification, Classification, and Epidemiology of Enterocytozoon bieneusi of Animals

The microsporidian Enterocytozoon bieneusi is an obligate intracellular pathogen that causes enteric disease (microsporidiosis) in humans and has been recorded in a wide range of animal species worldwide. The transmission of E. bieneusi is direct and likely occurs from person to person and from animal to person via the ingestion of spores in water, food, or the environment. The identification of E. bieneusi is usually accomplished by molecular means, typically using the sequence of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA. Currently, ~820 distinct genotypes of E. bieneusi have been recorded in at least 210 species of vertebrates (mammals, birds, reptiles, and amphibians) or invertebrates (insects and mussels) in more than 50 countries. In this chapter, we provide a perspective on (1) clinical aspects of human microsporidiosis; (2) the genome and DNA markers for E. bieneusi as well as molecular methods for the specific and genotypic identification of E. bieneusi; (3) epidemiological aspects of E. bieneusi of animals and humans, with an emphasis on the genotypes proposed to be zoonotic, human-specific, and animal-specific; and (4) future research directions to underpin expanded molecular studies to better understand E. bieneusi and microsporidiosis.

Microsporidial Stromal Keratitis: Epidemiological Features, Slit-Lamp Biomicroscopic Characteristics, and Therapy

PURPOSE

Microsporidial stromal keratitis is a rare form of infectious keratitis, with only 7 cases reported in the United States to date. This study was performed to evaluate risk factors, clinical features, and response to therapy.

METHODS

A retrospective review of the medical records of all patients diagnosed with microsporidial stromal keratitis seen in the practices of the authors between 1999 and 2020 was performed. Diagnosis was determined by cytology or histopathology in corneal specimens. Risk factors, presence or absence of distinctive clinical features, and response to medical and surgical therapies were recorded.

RESULTS

Nine patients-7M:2F, aged 7 to 99 years-with microsporidial stromal keratitis were identified. Exposures to recreational water and hymenopteran insect bites, both epidemiologically linked risk factors for systemic microsporidial infection, were identified in our patients. Presence of stromal edema with features of disciform keratitis and a distinctive granular keratitis were observed in 6 of 9 and 5 of 9 patients, respectively. Poor response to medical therapy was noted. Penetrating keratoplasty was effective in curing the infection. Final visual acuity was 20/40 or better in 6 of 9 patients.

CONCLUSIONS

In patients with slowly progressive keratitis, history of exposure to recreational water or hymenopteran insects should be sought. In patients with corneal edema consistent with disciform keratitis, with evolution to a granular keratitis, microsporidia should be considered in the differential diagnosis. In cases of established microsporidial stromal keratitis, penetrating keratoplasty should be considered if prompt response to medical therapy is not noted.

Enterocytozoon bieneusi of animals-With an 'Australian twist'

Enterocytozoon bieneusi is a microsporidian microorganism that causes intestinal disease in animals including humans. E. bieneusi is an obligate intracellular pathogen, typically causing severe or chronic diarrhoea, malabsorption and/or wasting. Currently, E. bieneusi is recognised as a fungus, although its exact classification remains contentious. The transmission of E. bieneusi can occur from person to person and/or animals to people. Transmission is usually via the faecal-oral route through E. bieneusi spore-contaminated water, environment or food, or direct contact with infected individuals. Enterocytozoon bieneusi genotypes are usually identified and classified by PCR-based sequencing of the internal transcribed spacer region (ITS) of nuclear ribosomal DNA. To date, ~600 distinct genotypes of E. bieneusi have been recorded in ~170 species of animals, including various orders of mammals and reptiles as well as insects in >40 countries. Moreover, E. bieneusi has also been found in recreational water, irrigation water, and treated raw- and waste-waters. Although many studies have been conducted on the epidemiology of E. bieneusi, prevalence surveys of animals and humans are scant in some countries, such as Australia, and transmission routes of individual genotypes and related risk factors are poorly understood. This article/chapter reviews aspects of the taxonomy, biology and epidemiology of E. bieneusi; the diagnosis, treatment and prevention of microsporidiosis; critically appraises the naming system for E. bieneusi genotypes as well as the phylogenetic relationships of these genotypes; provides new insights into the prevalence and genetic composition of E. bieneusi populations in animals in parts of Australia using molecular epidemiological tools; and proposes some areas for future research in the E. bieneusi/microsporidiosis field.

Methionine aminopeptidases with short sequence inserts within the catalytic domain are differentially inhibited: Structural and biochemical studies of three proteins from Vibrio spp

Methionine aminopeptidases (MetAPs) have been recognized as drug targets and have been extensively studied for discovery of selective inhibitors. MetAPs are essential enzymes in all living cells. While most prokaryotes contain a single gene, some prokaryotes and all eukaryotes including human have redundancy. Due to the similarity in the active sites of the MetAP enzyme between the pathogens and human limited the success of discovering selective inhibitors. We recently have discovered that MetAPs with small inserts within the catalytic domain to have different susceptibilities against some inhibitors compared to those that do not have. Using this clue we used bioinformatic tools to identify new variants of MetAPs with inserts in pathogenic species. Two new isoforms were identified in Vibrio species with two and three inserts in addition to an isoform without any insert. Multiple sequence alignment suggested that inserts are conserved in several of the Vibrio species. Two of the three inserts are common between two and three insert isoforms. One of the inserts is identified to have "NNKNN" motif that is similar to well-characterized quorum sensing peptide, "NNWNN". Another insert is predicted to have a posttranslational modification site. Three Vibrio proteins were cloned, expressed, purified, enzyme kinetics established and inhibitor screening has been performed. Several of the pyridinylpyrimidine derivatives selectively inhibited MetAPs with inserts compared to those that do not have, including the human enzyme. Crystal structure and molecular modeling studies provide the molecular basis for selective inhibition.

Therapeutic targets for the treatment of microsporidiosis in humans

INTRODUCTION

Microsporidia have been increasingly reported to infect humans. The most common presentation of microsporidiosis is chronic diarrhea, a significant mortality risk in immune-compromised patients. Albendazole, which inhibits tubulin, and fumagillin, which inhibits methionine aminopeptidase type 2 (MetAP2), are the two main therapeutic agents used for treatment of microsporidiosis. In addition, to their role as emerging pathogens in humans, microsporidia are important pathogens in insects, aquaculture, and veterinary medicine. New therapeutic targets and therapies have become a recent focus of attention for medicine, veterinary, and agricultural use. Areas covered: Herein, we discuss the detection and symptoms of microsporidiosis in humans and the therapeutic targets that have been utilized for the design of new drugs for the treatment of this infection, including triosephosphate isomerase, tubulin, MetAP2, topoisomerase IV, chitin synthases, and polyamines. Expert opinion: Enterocytozoon bieneusi is the most common microsporidia in human infection. Fumagillin has a broader anti-microsporidian activity than albendazole and is active against both Ent. bieneusi and Encephaliozoonidae. Microsporidia lack methionine aminopeptidase type 1 and are, therefore, dependent on MetAP2, while mammalian cells have both enzymes. Thus, MetAP2 is an essential enzyme in microsporidia and new inhibitors of this pathway have significant promise as therapeutic agents.

Molecular characterization of Nosema bombycis methionine aminopeptidase 2 (MetAP2) gene and evaluation of anti-microsporidian activity of Fumagilin-B in silkworm Bombyx mori

Nosema bombycis is a spore-forming parasite causing microsporidiosis in silkworm Bombyx mori. Methionine aminopeptidase 2 (MetAP2), an essential gene of N. bombycis, is a target for the anti-microsporidian drug Fumagillin, an antibiotic derived from Aspergillus fumigatus. In this study, a 1077 bp full-length cDNA of the MetAP2 gene of N. bombycis was cloned and characterized. Furthermore, the expression study of the MetAP2 gene revealed a ubiquitous expression during all the developmental stages of the silkworm B. mori. The phylogenetic analysis of the MetAP2 gene of N. bombycis revealed the MetAP2 gene sequences to be highly conserved in nature. The present study also includes the validation of the anti-microsporidian drug Fumagillin against the MetAP2 gene of N. bombycis. The findings revealed that Fumagilin-B could also suppress the N. bombycis multiplication in the silkworm B. mori, thereby proving the therapeutic role of Fumagillin against microsporidian infection. This is the first-ever report regarding the characterization of the MetAP2 gene in the Indian isolate of N. bombycis and also towards the usage of Fumagillin in the control of microsporidiosis in B. mori.

Successful Treatment of Disseminated Anncaliia algerae Microsporidial Infection With Combination Fumagillin and Albendazole

Anncaliia algerae myositis is a life-threatening, emerging microsporidiosis among immunocompromised hosts. We report a case of disseminated A algerae infection in a man previously treated with alemtuzumab. Due to failure of albendazole-based therapy, fumagillin was added as a novel approach to management, with a good clinical response and patient survival.

In Vitro Gene Silencing of the Fish Microsporidian Heterosporis saurida by RNA Interference

Heterosporis saurida, a microsporidian parasite of lizardfish, Saurida undosquamis, causes severe economic losses in marine aquaculture. Among the novel approaches being explored for treatment of parasitic infections in aquaculture is small interfering RNA molecules. The aim of the present study was to investigate the efficiency of using siRNA to knock down expression of specific genes of H. saurida in vitro. For this purpose, siRNAs specific for ATP/ADP antiporter 1 and methionine aminopeptidase II genes were designed and tested using a previously developed in vitro cultivation model. Silencing of H. saurida target genes was assessed and the efficacy of using siRNA for inhibition of gene expression was measured by quantitative real-time polymerase chain reaction (PCR). Silencing of ATP/ADP antiporter 1 or methionine aminopeptidase II by siRNA reduced H. saurida infection levels in EK-1 cells 40% and 60%, respectively, as measured by qRT-PCR and spore counts. Combined siRNA treatment of both ATP/ADP antiporter 1 and methionine aminopeptidase II siRNAs was more effective against H. saurida infection as seen by the 16S rRNA level and spore counts. Our study concluded that siRNA could be used to advance development of novel approaches to inhibit H. saurida and provide an alternative approach to combat microsporidia.

Molecular and biochemical characterization of methionine aminopeptidase of Babesia bovis as a potent drug target

Aminopeptidases are increasingly being investigated as therapeutic targets in various diseases. In this study, we cloned, expressed, and biochemically characterized a member of the methionine aminopeptidase (MAP) family from Babesia bovis (B. bovis) to develop a potential molecular drug target. Recombinant B. bovis MAP (rBvMAP) was expressed in Escherichia coli (E. coli) as a glutathione S-transferase (GST)-fusion protein, and we found that it was antigenic. An antiserum against the rBvMAP protein was generated in mice, and then a native B. bovis MAP was identified in B. bovis by Western blot assay. Further, an immunolocalization assay showed that MAP is present in the cytoplasm of the B. bovis merozoite. Analysis of the biochemical properties of rBvMAP revealed that it was enzymatically active, with optimum activity at pH 7.5. Enhanced enzymatic activity was observed in the presence of divalent manganese cations and was effectively inhibited by a metal chelator, ethylenediaminetetraacetic acid (EDTA). Moreover, the enzymatic activity of BvMAP was inhibited by amastatin and bestatin as inhibitors of MAP (MAPi) in a dose-dependent manner. Importantly, MAPi was also found to significantly inhibit the growth of Babesia parasites both in vitro and in vivo; additionally, they induced high levels of cytokines and immunoglobulin (IgG) titers in the host. Therefore, our results suggest that BvMAP is a molecular target of amastatin and bestatin, and those inhibitors may be drug candidates for the treatment of babesiosis, though more studies are required to confirm this.

Parasitic diarrheal disease: drug development and targets

Diarrhea is the manifestation of gastrointestinal infection and is one of the major causes of mortality and morbidity specifically among the children of less than 5 years age worldwide. Moreover, in recent years there has been a rise in the number of reports of intestinal infections continuously in the industrialized world. These are largely related to waterborne and food borne outbreaks. These occur by the pathogenesis of both prokaryotic and eukaryotic organisms like bacteria and parasites. The parasitic intestinal infection has remained mostly unexplored and under assessed in terms of therapeutic development. The lack of new drugs and the risk of resistance have led us to carry out this review on drug development for parasitic diarrheal diseases. The major focus has been depicted on commercially available drugs, currently synthesized active heterocyclic compounds and unique drug targets, that are vital for the existence and growth of the parasites and can be further exploited for the search of therapeutically active anti-parasitic agents.

Expression and biochemical characterization of a type I methionine aminopeptidase of Plasmodium vivax

Methionine aminopeptidases (MetAPs), ubiquitous enzymes that play an important role in nascent protein maturation, have been recognized as attractive targets for the development of drugs against pathogenic protozoa including Plasmodium spp. Here, we characterized partial biochemical properties of a type I MetAP of Plasmodium vivax (PvMetAP1). PvMetAP1 had the typical amino acid residues essential for metal binding and substrate binding sites, which are well conserved in the type I MetAP family enzymes. Recombinant PvMetAP1 showed activity in a broad range of neutral pHs, with optimum activity at pH 7.5. PvMetAP1 was stable under neutral and alkaline pHs, but was relatively unstable under acidic conditions. PvMetAP1 activity was highly increased in the presence of Mn(2+), and was effectively inhibited by a metal chelator, EDTA. Fumagillin and aminopeptidase inhibitors, amastatin and bestatin, also showed an inhibitory effect on PvMetAP1. The enzyme had a highly specific hydrolytic activity for N-terminal methionine. These results collectively suggest that PvMetAP1 belongs to the family of type I MetAPs and may play a pivotal role for the maintenance of P. vivax physiology by mediating protein maturation and processing of the parasite.

Development of a novel in vitro method for drug development for fish; application to test efficacy of antimicrosporidian compounds

Few drugs are approved for treating diseases caused by parasites in minor species such as fish. This is due, in part, to the expense of drug development and to the comparatively small market. In vivo effectiveness trials for antiparasitic drugs are costly, time consuming and require ethics approval, therefore an in vitro screening approach is a cost-effective alternative to finding promising drug candidates. We developed an in vitro testing system to test antimicrosporidial compounds against a microsporidian pathogen Heterosporis saurida. Five antiparasitic compounds, albendazole, fumagillin, TNP-70, nitazoxanide and lufenuron, were assayed for antimicrosporidial activity. All compounds reduced the number of H saurida spores in infected cells when applied at a concentration that did not appear to be toxic to the host cells. Albendazole inhibited replication of H saurida by >60 per cent, fumagillin and its analogue TNP-470 inhibited H saurida >80 per cent, nitazoxanide and lufenuron inhibited growth >70 per cent. The data suggest that both fumagillin and its analogous TNP-70 hold the best promise as therapeutic agents against H saurida. The ability to use fish cell cultures to assess drugs against H saurida demonstrates an approach that may be helpful to evaluate other drugs on different microsporidia and host cells.

Tyrosine nitration moderates the peptidase activity of human methionyl aminopeptidase 2

Methionyl aminopeptidase 2 (MetAP2) plays an important role in the regulation of angiogenesis. This study examined whether nitration of MetAP2 alters its enzymatic activity in vitro. The activity of unmodified, nitrated and oxidised MetAP2 was assessed and it was found that nitration significantly reduced its ability to cleave a chromogenic substrate. Mass spectrometry analysis identified Tyr336 as a nitrated residue in MetAP2. Structural and evolutionary analysis indicate that this is an important residue for MetAP2 activity. Combined, the results show that the activity of MetAP2 is reduced by nitration and raise the possibility that nitration of MetAP2 is a mechanism contributing to endothelial dysfunction.

Characterization of the biochemical properties of two methionine aminopeptidases of Cryptosporidium parvum

We identified two methionine aminopeptidases of Cryptosporidium parvum (CpMetAP1 and CpMetAP2) and characterized the biochemical properties of the recombinant enzymes. CpMetAP1 and CpMetAP2 belong to the type I and type II MetAP subfamilies, respectively. Both CpMetAPs have typical amino acid residues essential for metal binding and substrate binding sites, which are conserved in the MetAP family. Bacterially expressed recombinant CpMetAP1 and CpMetAP2 showed similar biochemical properties including a broad optimal pH range (pH 7.5-8.5) with maximum activity at pH 8.0. The two enzymes were stable under neutral and alkaline pHs but were relatively unstable under acidic conditions. The activities of CpMetAP1 and CpMetAP2 increased highly in the presence of Mn(2+) and Co(2+). CpMetAP1 and CpMetAP2 were effectively inhibited by the metal chelators, EDTA and 1,10-phenanthroline, and were partially inhibited by the aminopeptidase inhibitors, amastatin and bestatin. Fumagillin also showed an inhibitory effect on both CpMetAPs.

Metallo-aminopeptidase inhibitors

Aminopeptidases are enzymes that selectively hydrolyze an amino acid residue from the N-terminus of proteins and peptides. They are important for the proper functioning of prokaryotic and eukaryotic cells, but very often are central players in the devastating human diseases like cancer, malaria and diabetes. The largest aminopeptidase group include enzymes containing metal ion(s) in their active centers, which often determines the type of inhibitors that are the most suitable for them. Effective ligands mostly bind in a non-covalent mode by forming complexes with the metal ion(s). Here, we present several approaches for the design of inhibitors for metallo-aminopeptidases. The optimized structures should be considered as potential leads in the drug discovery process against endogenous and infectious diseases.

Structure of a microsporidian methionine aminopeptidase type 2 complexed with fumagillin and TNP-470

Microsporidia are protists that have been reported to cause infections in both vertebrates and invertebrates. They have emerged as human pathogens particularly in patients that are immunosuppressed and cases of gastrointestinal infection, encephalitis, keratitis, sinusitis, myositis and disseminated infection are well described in the literature. While benzimidazoles are active against many species of microsporidia, these drugs do not have significant activity against Enterocytozoon bieneusi. Fumagillin and its analogues have been demonstrated to have activity invitro and in animal models of microsporidiosis and human infections due to E. bieneusi. Fumagillin and its analogues inhibit methionine aminopeptidase type 2. Encephalitozoon cuniculi MetAP2 (EcMetAP2) was cloned and expressed as an active enzyme using a baculovirus system. The crystal structure of EcMetAP2 was determined with and without the bound inhibitors fumagillin and TNP-470. This structure classifies EcMetAP2 as a member of the MetAP2c family. The EcMetAP2 structure was used to generate a homology model of the E. bieneusi MetAP2. Comparison of microsporidian MetAP2 structures with human MetAP2 provides insights into the design of inhibitors that might exhibit specificity for microsporidian MetAP2.

Efficacy of gamma interferon and specific antibody for treatment of microsporidiosis caused by Encephalitozoon cuniculi in SCID mice

Microsporidia are eukaryotic, obligate, intracellular protists that are emerging pathogens in immunocompromised hosts, including AIDS patients and organ transplant recipients. The efficacy of gamma interferon (IFN-gamma) for the treatment of microsporidiosis caused by Encephalitozoon cuniculi was studied by means of adoptive transfer and IFN-gamma administration in SCID mice. While the adoptive transfer of CD4(+) T cells from immunocompetent mice prolonged survival of SCID mice infected perorally with E. cuniculi, survival was not improved by adoptive transfer of CD4(+) T lymphocytes from IFN-gamma-deficient mice. The protective effect of IFN-gamma was confirmed in cytokine therapy experiments in which SCID mice receiving IFN-gamma survived significantly longer than mice receiving mock injections. The administration of serum containing specific antibodies against E. cuniculi was found to prolong the survival of concurrently IFN-gamma-treated SCID mice. The data presented in this study suggest that IFN-gamma is potentially useful as a cytokine therapy for microsporidiosis, especially in CD4(+) T-cell-deficient patients.

Microsporidiosis: current status

PURPOSE OF REVIEW

Microsporidiosis is an emerging and opportunistic infection associated with a wide range of clinical syndromes in humans. This review highlights the research on microsporidiosis in humans during the previous 2 years.

RECENT FINDINGS

The reduced and compact microsporidian genome has generated much interest for better understanding the evolution of these parasites, and comparative molecular phylogenetic studies continue to support a relationship between the microsporidia and fungi. Through increased awareness and improved diagnostics, microsporidiosis has been identified in a broader range of human populations that, in addition to persons with HIV infection, includes travelers, children, organ transplant recipients, and the elderly.

SUMMARY

Effective commercial therapies for Enterocytozoon bieneusi, the most common microsporidian species identified in humans, are still lacking, making the need to develop tissue culture and small animal models increasingly urgent. Environmental transport modeling and disinfection strategies are being addressed for improving water safety. Questions still exist about whether microsporidia infections remain persistent in asymptomatic immune-competent individuals, reactivate during conditions of immune compromise, or may be transmitted to others at risk, such as during pregnancy or through organ donation. Reliable serological diagnostic methods are needed to supplement polymerase chain reaction or histochemistry when spore shedding may be sporadic.

System for expression of microsporidian methionine amino peptidase type 2 (MetAP2) in the yeast Saccharomyces cerevisiae

Microsporidia are parasitic protists of all classes of vertebrates and most invertebrates. They recently emerged as important infections in various immunosuppressed and immunocompetent patient populations. They are also important veterinary and agricultural pathogens. Current therapies for microsporidiosis include benzimidazoles, which bind tubulin-inhibiting microtubule assembly, and fumagillin and its derivatives, which bind and inhibit methionine amino peptidase type 2 (MetAP2). Benzimidazoles are not active against Enterocytozoon bieneusi, the most common cause of human microsporidiosis. Fumagillin is active against most microsporidia, including E. bieneusi, but thrombocytopenia has been a problem in clinical trials. There is a pressing need for more-specific microsporidian MetAP2 inhibitors. To expedite and facilitate the discovery of safe and effective MetAP2 inhibitors, we have engineered Saccharomyces cerevisiae to be dependent on Encephalitozoon cuniculi MetAP2 (EcMetAP2) for its growth, where EcMetAP2 is harbored on an episomal uracil-selectable tetracycline-regulated plasmid. We have also constructed a leucine-selectable tetracycline-regulated expression plasmid into which any MetAP2 gene can be cloned. By utilizing a 5-fluoroorotic acid-mediated plasmid shuffle in the EcMetAP2 yeast strain, a yeast strain can be generated whose growth is dependent on MetAP2 from any organism. The level of heterologous MetAP2 gene expression can be controlled by the addition of tetracycline to the growth medium. These yeast strains should permit high-throughput screening for the identification of new inhibitors with high specificity and activity toward microsporidian MetAP2.

Antimicrosporidial activities of fumagillin, TNP-470, ovalicin, and ovalicin derivatives in vitro and in vivo

Therapies for microsporidiosis in humans are limited, and fumagillin, which appears to be the most broadly effective antimicrosporidial drug, is considered to be moderately toxic. The purpose of this study was to apply an in vitro drug screening assay for Encephalitozoon intestinalis and Vittaforma corneae and an in vivo athymic mouse model of V. corneae infection to assess the efficacy of TNP-470 (a semisynthetic analogue of fumagillin), ovalicin, and eight ovalicin derivatives. TNP-470, ovalicin, and three of the ovalicin derivatives inhibited both E. intestinalis and V. corneae replication by more than 70% in vitro. Another three of the ovalicin derivatives inhibited one of the two microsporidian species by more than 70%. None of the treated athymic mice survived the V. corneae infection, but they did survive statistically significantly longer than the untreated controls after daily treatment with fumagillin administered at 5, 10, and 20 mg/kg of body weight subcutaneously (s.c.), TNP-470 administered at 20 mg/kg intraperitoneally (i.p.), or ovalicin administered at 5 mg/kg s.c. Of two ovalicin derivatives that were assessed in vivo, NSC 9665 given at 10 mg/kg i.p. daily also statistically significantly prolonged survival of the mice. No lesions associated with drug toxicity were observed in the kidneys or livers of uninfected mice treated with these drugs at the highest dose of 20 mg/kg daily. These results thus support continued studies to identify more effective fumagillin-related drugs for treating microsporidiosis.