The Past, Present, and Future in the Diagnosis of a Neglected Sexually Transmitted Infection: Trichomoniasis

More than one million curable sexually transmitted infections occur every day. Trichomonas vaginalis is one of the main infections responsible for these epidemiological data; however, the diagnosis of this protozoan is still mainly based on microscopic and culture identification. The commercialization of immunological tests and the development of molecular techniques have improved the sensitivity of classical methods. Nevertheless, the fact that trichomoniasis is a neglected parasitic infection hinders the development of novel techniques and their implementation in routine diagnosis. This review article shows the different methods developed to identify T. vaginalis in population and the difficulties in diagnosing male and asymptomatic patients. The importance of including this parasite in routine gynecological screening, especially in pregnant women, and the importance of considering T. vaginalis as an indicator of high-risk sexual behavior are also discussed.

Promising hit compounds against resistant trichomoniasis: Synthesis and antiparasitic activity of 3-(ω-aminoalkoxy)-1-benzyl-5-nitroindazoles

A series of 11 3-(ω-aminoalkoxy)-1-benzyl-5-nitroindazoles (2-12) has been prepared starting from 1-benzyl-5-nitroindazol-3-ol 13, and evaluated against sensitive and resistant isolates of the sexually transmitted protozoan Trichomonas vaginalis. Compounds 2, 3, 6, 9, 10 and 11 showed trichomonacidal profiles with IC₅₀ < 20 µM against the metronidazole-sensitive isolate. Moreover, all these compounds submitted to cytotoxicity assays against mammalian cells exhibited low non-specific cytotoxic effects, except compounds 3 and 9 which displayed moderate cytotoxicity (CC₅₀ = 74.7 and 59.1 µM, respectively). Those compounds with trichomonacidal effect were also evaluated against a metronidazole-resistant culture. Special mention deserve compounds 6 and 10, which displayed better IC₅₀ values (1.3 and 0.5 µM respectively) than that of the reference drug (IC₅₀ MTZ = 3.0 µM). The high activity of these compounds against the resistant isolate reinforces the absence of cross-resistance with the reference drug. The remarkable trichomonacidal results against resistant T. vaginalis isolates suggest the interest of 3-(ω-aminoalkoxy)-1-benzyl-5-nitroindazoles to be considered as good prototypes to continue in the development of new drugs with enhanced trichomonacidal activity, aiming to increase the non-existent drugs to face clinical resistance efficiently for those patients in whom therapy with 5-nitroimidazoles is contraindicated.

Antichagasic and trichomonacidal activity of 1-substituted 2-benzyl-5-nitroindazolin-3-ones and 3-alkoxy-2-benzyl-5-nitro-2H-indazoles

Two series of new 5-nitroindazole derivatives, 1-substituted 2-benzylindazolin-3-ones (6-29, series A) and 3-alkoxy-2-benzyl-2H-indazoles (30-37, series B), containing differently functionalized chains at position 1 and 3, respectively, have been synthesized starting from 2-benzyl-5-nitroindazolin-3-one 5, and evaluated against the protozoan parasites Trypanosoma cruzi and Trichomonas vaginalis, etiological agents of Chagas disease and trichomonosis, respectively. Many indazolinones of series A were efficient against different morphological forms of T. cruzi CL Brener strain (compounds 6, 7, 9, 10 and 19-21: IC50 = 1.58-4.19 μM for epimastigotes; compounds 6, 19-21 and 24: IC50 = 0.22-0.54 μM for amastigotes) being as potent as the reference drug benznidazole. SAR analysis suggests that electron-donating groups at position 1 of indazolinone ring are associated with an improved antichagasic activity. Moreover, compounds of series A displayed low unspecific toxicities against an in vitro model of mammalian cells (fibroblasts), which were reflected in high values of the selectivity indexes (SI). Compound 20 was also very efficient against amastigotes from Tulahuen and Y strains of T. cruzi (IC50 = 0.81 and 0.60 μM, respectively), showing low toxicity towards cardiac cells (LC50 > 100 μM). In what concerns compounds of series B, some of them displayed moderate activity against trophozoites of a metronidazole-sensitive isolate of T. vaginalis (35 and 36: IC50 = 9.82 and 7.25 μM, respectively), with low unspecific toxicity towards Vero cells. Compound 36 was also active against a metronidazole-resistant isolate (IC50 = 9.11 μM) and can thus be considered a good prototype for the development of drugs directed to T. vaginalis resistant to 5-nitroimidazoles.

Sequestration of host-CD59 as potential immune evasion strategy of Trichomonas vaginalis

Trichomonas vaginalis is known to evade complement-mediated lysis. Because the genome of T. vaginalis does not possess DNA sequence with homology to human protectin (CD59), a complement lysis restricting factor, we tested the hypothesis that host CD59 acquisition by T. vaginalis organisms mediates resistance to complement killing. This hypothesis was based on the fact that trichomonads are known to associate with host proteins. No CD59 was detected on the surface of T. vaginalis grown in serum-based medium using as probe anti-CD59 monoclonal antibody (MAb). We, therefore, infected mice intraperitoneally with live T. vaginalis, and trichomonads harvested from ascites were tested for binding of CD59. Immunofluorescence showed that parasites had surface CD59. Furthermore, as mouse erythrocytes (RBCs) possess membrane-associated CD59, and trichomonads use RBCs as a nutrient source, organisms were co-cultured with murine RBCs for one week. Parasites were shown to have detectable surface CD59. Importantly, live T. vaginalis with bound CD59 were compared with batch-grown parasites without surface-associated CD59 for sensitivity to complement in human serum. Trichomonads without surface-bound CD59 had a higher level of killing by complement than did parasites with surface CD59. These data show that host CD59 acquired onto the surface by live T. vaginalis may be an alternative mechanism for complement evasion. We describe a novel strategy by T. vaginalis consistent with host protein procurement by this parasite to evade the lytic action of complement.

Synthesis and in vitro and in vivo biological evaluation of substituted nitroquinoxalin-2-ones and 2,3-diones as novel trichomonacidal agents

Two series of ten novel 7-nitroquinoxalin-2-ones and ten 6-nitroquinoxaline-2,3-diones with diverse substituents at positions 1 and 4 were synthesized and evaluated against the sexually transmitted parasite Trichomonas vaginalis. Furthermore, diverse molecular and drug-likeness properties were analyzed to predict the oral bioavailability following the Lipinski's "rule of five". 7-Nitroquinoxalin-2-one derivatives displayed moderate to high in vitro activity while the efficiency of most nitroquinoxaline-2,3-diones was rather low; both kinds of compounds did not show cytotoxic effects in mammalian cells. 7-Nitro-4-(3-piperidinopropyl)quinoxalin-2-one 9 achieved the highest trichomonacidal activity (IC50 = 18.26 μM) and was subsequently assayed in vivo in a murine model of trichomonosis. A 46.13% and a 50.70% reduction of pathogenic injuries were observed in the experimental groups treated orally during 7 days with 50 mg/kg and 100 mg/kg doses. The results obtained in the biological assays against T. vaginalis indicate that compounds with ω-(dialkylamino)alkyl substituents and a keto group at positions 4 and 2 of quinoxaline ring, respectively, provide interesting structural cores to develop novel prototypes to enhance the nitroquinoxalinones activity as trichomonacidal agents with interesting ADME properties according to virtual screening analysis.

Effect of Anapsos® in a murine model of experimental trichomoniasis

Immunomodulator effect of Anapsos (Polypodium leukotomas extract) in NMRI (US Naval Medical Research Institute) outbred mice infected by the intraperitoneal route with 10(7) Trichomonas vaginalis has been tested. Gross histopathologic changes in abdominal organs and mortality rate, as a consequence of the pathogenicity of the protozoa and the immune response of the host, were evaluated. Among the different treatment regimes assayed, Anapsos at doses of 20 mg/Kg/day administered for 10 days before infection decreases the parasite pathogenicity index (PI) in the treated animals when compared to those of the untreated control group. The immunosuppressor treatments with azathioprine (100 mg/Kg/day x 1), cyclophosphamide (100 mg/Kg/day x 1), and FK-506 (10 mg/Kg/day x 10) significantly decreased the PI, while an immunostimulant treatment with glycophosphopeptical (13 mg/Kg/day x 10) increased it. These assays have shown the usefulness of the murine model of experimental trichomoniasis for the study of immunomodulator activity of natural or synthetic drugs.

Morphological and molecular identification of Tetratrichomonas flagellates from the giant anteater (Myrmecophaga tridactyla)

A tetratrichomonad flagellate found in the diarrhoeic faeces of a 5 years-old male giant anteater (Myrmecophaga tridactyla) was characterised by morphological and genetic analysis. This protozoan presents four anterior flagella of unequal length and a recurrent flagellum attached to the undulating membrane without a free end portion, and a broad axostyle projection. Numerous vacuoles of different sizes containing bacteria and digestion products were found. The complete sequence of the DNA coding for the 16S rRNA-ITS1-5.8S rRNA-ITS2 region was also obtained in order to compare this isolate with other tetratrichomonad species. The sequence obtained was identical to others previously obtained by other researchers from bovines and turtles (Geochelone sp.). It is not easily explainable how the same organism could be found in such different hosts and locations; however these results indicate that some tetratrichomonad species could have a wide host range and could survive in a wide range of environmental conditions.

The Schistosoma bovis Sb14-3-3ζ recombinant protein cross-protects against Schistosoma mansoni in BALB/c mice

Current control programs against schistosomiasis could be reinforced through the use of an effective vaccine. Schistosome 14-3-3 proteins have been proposed as candidates for vaccine against the respective infections, and were seen to elicit high protection levels against Schistosoma bovis in a previous work done by our group. We have therefore investigated the protective capacity of the 14-3-3 protein from S. bovis - Sb14zeta - against Schistosoma mansoni in mice. In addition, we have addressed the influence of the co-administration of three different immunomodulators with the 14-3-3 polypeptide. Protection was high when the Sb14zeta protein was combined in two independent experiments with the AA2829 and PAL immunomodulatory molecules as regards both the reduction of worm numbers (mean: 64.8%) and egg loads in liver (mean: 73.9%) or intestine (mean: 71.5%). In contrast, the degree of protection achieved with the Sb14zeta-CpG vaccine was very low (14.9% reduction in worm numbers, and 46.6% and 32% reduction in liver and intestinal egg loads). The immune responses observed in the vaccinated animals showed that the production of IFNgamma and the absence of IL-4, accompanied by a strong humoral response, are insufficient to elicit protection against S. mansoni.

Immunomodulation of the Response to Excretory/Secretory Antigens of Fasciola hepatica by Anapsos® in Balb/C Mice and Rat Alveolar Macrophages

It is known that excretory/secretory antigens of Fasciola hepatica (ESFh) trigger a Th2-like immune response. Anapsos (A) is an aqueous hydrosoluble extract obtained from the rhizomes of the fern Polypodium leucotomos that has shown immunomodulator effects in some parasitic infections and immunological disorders. In this work we assess the effect of Anapsos and ESFh and Quillaja saponaria extract (Qs) on BALB/c mice and rat alveolar macrophages. Anapsos modulates the response of mice immunized with ESFh, decreasing IgG antibodies in A+ESFh- and A+Qs+ESFh-treated mice and triggering high levels of gammaIFN in spleen cell culture in comparison with ESFh- and Qs + ESFh--treated groups. Moreover, Anapsos showed statistically significant inhibitory effects on the nitrite production by rat alveolar macrophages prestimulated with lipopolysaccharide (LPS) as well as ESFh antigen in comparison with macrophages stimulated only with LPS. The application of ESFh and Anapsos combined avoids this inhibitory effect. Thus, Anapsos modulates the immune response against ESFh in naive mice and on the nitrite production in prestimulated rat aveolar macrophages.

Trichinella: Differing effects of antigens from encapsulated and non-encapsulated species on in vitro nitric oxide production

Trichinellosis is a cosmopolitan zoonotic disease affecting a wide variety of animals, including man. Non-encapsulated and encapsulated species diverge with respect to their developmental strategies. Little is known at the molecular level about parasite-derived mediators responsible for host muscle cell transformation occurring during trichinellosis. In this context, host-parasite relationships in Trichinella-infected animals could be related to different host-immune and cell mediators, e.g. nitric oxide (NO). Here, we investigate the stimulatory/inhibitory role of L1 antigens from four encapsulated (T. spiralis, T. britovi, T. nelsoni and T. nativa) and one non-encapsulated (T. pseudospiralis) Trichinella species on NO production from rat macrophages in vitro. Our results demonstrate that encapsulated and non-encapsulated Trichinella species differ in their capacity to stimulate the secretion of NO from host macrophages. Biological significance of these differences should be further assessed in the available experimental models.

Predicting antitrichomonal activity: A computational screening using atom-based bilinear indices and experimental proofs

Existing Trichomonas vaginalis therapies are out of reach for most trichomoniasis people in developing countries and, where available, they are limited by their toxicity (mainly in pregnant women) and their cost. New antitrichomonal agents are needed to combat emerging metronidazole-resistant trichomoniasis and reduce the side effects associated with currently available drugs. Toward this end, atom-based bilinear indices, a new TOMOCOMD-CARDD molecular descriptor, and linear discriminant analysis (LDA) were used to discover novel, potent, and non-toxic lead trichomonacidal chemicals. Two discriminant functions were obtained with the use of non-stochastic and stochastic atom-type bilinear indices for heteroatoms and H-bonding of heteroatoms. These atomic-level molecular descriptors were calculated using a weighting scheme that includes four atomic labels, namely atomic masses, van der Waals volumes, atomic polarizabilities, and atomic electronegativities in Pauling scale. The obtained LDA-based QSAR models, using non-stochastic and stochastic indices, were able to classify correctly 94.51% (90.63%) and 93.41% (93.75%) of the chemicals in training (test) sets, respectively. They showed large Matthews' correlation coefficients (C); 0.89 (0.79) and 0.87 (0.85), for the training (test) sets, correspondingly. The result of predictions on the 15% full-out cross-validation test also evidenced the robustness and predictive power of the obtained models. In addition, canonical regression analyses corroborated the statistical quality of these models (R(can) of 0.749 and of 0.845, correspondingly); they were also used to compute biological activity canonical scores for each compound. On the other hand, a close inspection of the molecular descriptors included in both equations showed that several of these molecular fingerprints are strongly interrelated with each other. Therefore, these models were orthogonalized using the Randić orthogonalization procedure. These classification functions were then applied to find new lead antitrichomonal agents and six compounds were selected as possible active compounds by computational screening. The designed compounds were synthesized and tested for in vitro activity against T. vaginalis. Out of the six compounds that were designed, and synthesized, three molecules (chemicals VA5-5a, VA5-5c, and VA5-12b) showed high to moderate cytocidal activity at the concentration of 10 microg/ml, other two compounds (VA5-8pre and VA5-8) showed high cytocidal and cytostatic activity at the concentration of 100 microg/ml and 10 microg/ml, correspondingly, and the remaining chemical (compound VA5-5e) was inactive at these assayed concentrations. Nonetheless, these compounds possess structural features not seen in known trichomonacidal compounds and thus can serve as excellent leads for further optimization of antitrichomonal activity. The LDA-based QSAR models presented here can be considered as a computer-assisted system that could potentially significantly reduce the number of synthesized and tested compounds and increase the chance of finding new chemical entities with antitrichomonal activity.

Molecular characterization of Trichinella genotypes by inter-simple sequence repeat polymerase chain reaction (ISSR-PCR)

A bulk analysis of inter-simple sequence repeat-polymerase chain reaction (ISSR-PCR) provides a quick, reliable, and highly informative system for DNA banding patterns that permit species identification. The present study evaluates the applicability of this system to Trichinella species identification. After a single amplification carried out on a single larva with the primer 816([CA]nRY) under high stringency conditions, which provide high reproducibility, we were able to identify by consistent banding patterns 5 sibling species: Trichinella spiralis (ISS48), 2 Trichinella britovi isolates (ISS11 and ISS86), Trichinella murrelli (ISS35), Trichinella nativa (ISS71), Trichinella nelsoni (ISS29); 3 additional Trichinella genotypes: T8 (ISS149), T9 (ISS408 and ISS409), and T6 (ISS34); and the nonencapsulated species Trichinella pseudospiralis (ISS13). Moreover, 33 new Trichinella isolates from 2 zoogeographical regions were unequivocally identified. All Trichinella isolates have shown an identical pattern with those produced by the reference strain. According to these data, we have demonstrated that ISSR-PCR is a robust technique that emerges as a useful new application for the molecular identification of Trichinella isolates in epidemiological studies.

A computer-based approach to the rational discovery of new trichomonacidal drugs by atom-type linear indices

Computational approaches are developed to design or rationally select, from structural databases, new lead trichomonacidal compounds. First, a data set of 111 compounds was split (design) into training and predicting series using hierarchical and partitional cluster analyses. Later, two discriminant functions were derived with the use of non-stochastic and stochastic atom-type linear indices. The obtained LDA (linear discrimination analysis)-based QSAR (quantitative structure-activity relationship) models, using non-stochastic and stochastic descriptors were able to classify correctly 95.56% (90.48%) and 91.11% (85.71%) of the compounds in training (test) sets, respectively. The result of predictions on the 10% full-out cross-validation test also evidenced the quality (robustness, stability and predictive power) of the obtained models. These models were orthogonalized using the Randic orthogonalization procedure. Afterwards, a simulation experiment of virtual screening was conducted to test the possibilities of the classification models developed here in detecting antitrichomonal chemicals of diverse chemical structures. In this sense, the 100.00% and 77.77% of the screened compounds were detected by the LDA-based QSAR models (Eq. 13 and Eq. 14, correspondingly) as trichomonacidal. Finally, new lead trichomonacidals were discovered by prediction of their antirichomonal activity with obtained models. The most of tested chemicals exhibit the predicted antitrichomonal effect in the performed ligand-based virtual screening, yielding an accuracy of the 90.48% (19/21). These results support a role for TOMOCOMD-CARDD descriptors in the biosilico discovery of new compounds.

A linear discrimination analysis based virtual screening of trichomonacidal lead-like compounds: Outcomes of in silico studies supported by experimental results

A computational (virtual) screening test to identify potential trichomonacidals has been developed. Molecular structures of trichomonacidal and non-trichomonacidal drugs were represented using stochastic and non-stochastic atom-based quadratic indices and a linear discrimination analysis (LDA) was trained to classify molecules regarding their antiprotozoan activity. Validation tests revealed that our LDA-QSAR models recognize at least 88.24% of trichomonacidal lead-like compounds and suggest using this methodology in virtual screening protocols. These classification functions were then applied to find new lead antitrichomonal compounds. In this connection, the biological assays of eight compounds, selected by computational screening using the present models, give good results (87.50% of good classification). In general, most of the compounds showed high activity against Trichomonas vaginalis at the concentration of 100 microg/ml and low cytotoxicity to this concentration. In particular, two heterocyclic derivatives (VA7-67 and VA7-69) maintained their efficacy at 10 microg/ml with an important trichomonacidal activity (100.00% of reduction), but it is remarkable that the compound VA7-67 did not show cytotoxic effects in macrophage cultivations. This result opens a door to a virtual study considering a higher variability of the structural core already evaluated, as well as of other chemicals not included in this study.

Effect of piroxicam, metamizol, and S-adenosylmethionine in a murine model of experimental trichomoniasis

Biological effects of piroxicam, metamizol, and S-adenosylmethionine (S-AMET) have been tested in NMRI mice infected intraperitoneally with Trichomonas vaginalis. An intraperitoneal treatment during ten preinfection days with piroxicam (10 mg/Kg/day), or metamizol (275 mg/Kg/day), but not with S-AMET (117 mg/Kg/day) induced a significant decrease of abdominal lesions and mortality, assessed by means of a pathogenicity index. The trichomonicidal activity of piroxicam, metamizol, and S-AMET was tested in vitro at the concentration of 300 microM, but found ineffective. These assays have shown the usefulness of the experimental trichomoniasis model for the study of the immunomodulating activity of synthetic drugs.

Vaccination of mice and sheep with Fh12 FABP from Fasciola hepatica using the new adjuvant/immunomodulator system ADAD

We evaluate the ability of a Fasciola hepatica FABP native antigen (Fh12) with a new vaccination system called ADAD to protect mice and sheep against an experimental F. hepatica infection. The vaccination protocol consists of a set of two injections. The first injection contains a micelle in which two components are included, saponin from Quillaja saponaria (Qs) and/or Anapsos (A) a Polypodium leucotomos hydroalcoholic extract, both emulsified in a non-mineral oil (Montanide) in a water/oil emulsion (30/70). This is subcutaneously injected to achieve the "adaptation" of the immune system to subsequent stimuli. The second injection contains in addition the Fh12 antigen. Two different experiments were carried out using two mouse strains (BALB/c and CD-1). Mice vaccinated with Qs+A+Fh12 presented a survival rate of 40%, when compared with control groups. Furthermore, we evaluated the efficiency of the vaccination in sheep against an experimental F. hepatica challenge. The vaccinated sheep presented lower fluke recovery (24.5%), number of eggs in bile fluid (58.1%) and faeces (40.3%) than control groups. The recovered flukes were shorter (32.7%), immature (34.0%) and with lower body mass (31.6%) than non-complete vaccinated sheep. Thus, the new ADAD system could be a good alternative for future vaccination experiments against fasciolosis.

Trichomonas vaginalis: Determination of Acid Phosphatase Activity as a Pharmacological Screening Procedure

A simple method to screen trichomonacides, based on the quantification of acid phosphatase (AP) activity, has been designed. Using p-nitrophenyl phosphate as chromogenic substrate, we first determined the optimal conditions for enzyme reaction. After seeding, a linear correlation between number of trichomonads and optical densities at 405 nm was obtained at 24 hr but not at 48 hr. Then, the inhibitory effect of metronidazole was assessed both by microscope counts and by AP determination. Similar values for 50% inhibitory concentrations (2.6 microM), with 95% confidence limits of 1.91-3.33 for microscopic and 2.21-3.05 for colorimetric method, were obtained. We concluded that the colorimetric method described in this investigation is suitable for pharmacological studies and for the screening of new, potential antitrichomonal agents.

Modulation by Polypodium leucotomos extract of cytokine patterns in experimental trichomoniasis model

The immunomodulating effects of Anapsos, an aqueous hydrosoluble extract obtained from the rhizomes of the fern Polypodium leucotomos, on both pathogenicity and cytokine levels in serum (IFN-gamma/IL-4) were assayed in a Trichomonas vaginalis experimental model (BALB/c mice infected with 10(7) trichomonads and examined at day 15 after infection). Doses of 20 mg/kg/day administered for 10 days before the infection with the parasite induced a decrease of the experimental pathogenicity approximately 10-20% compared to controls. Gross histopathologic changes at abdominal organs and mortality rate, as a consequence of pathogenicity of the protozoa and the immune response of the host, were evaluated. IFN-gamma and IL-4 cytokines were determined on days -5, 0, 5, 10, and 15 postinfection by indirect ELISA. Treatment with PAL before infection modulates and downregulates the IFN-gamma concentration, while anticipates and upregulates the IL-4 level. The assays performed have showed the utility of the murine model of experimental trichomoniasis for the evaluation of immunomodulatory activity of synthetic or natural products.

Relationship between biological behaviour and randomly amplified polymorphic DNA profiles of Trypanosoma cruzi strains

Once known some biological characteristics of six Trypanosoma cruzi strains, randomly amplified polymorphic DNA (RAPD) analysis was made. Cluster analysis by UPGMA (unweighted pair group method analysis) was then applied both to biological parameters and RAPD profiles. Inspection of the UPGMA phenograms indicates identical clusters, so supporting that usefulness of biological parameters to characterization of T. cruzi strains still remains.

Biological characterization of Trypanosoma cruzi strains

Biological parameters of five Trypanosoma cruzi strains from different sources were determined in order to know the laboratory behaviour of natural populations. The parameters evaluated were growth kinetics of epimastigotes, differentiation into metacyclic forms, infectivity in mammalian cells grown in vitro and parasite susceptibility to nifurtimox, benznidazole and gentian violet. Differences in transformation to metacyclic, in the percentage of infected cells as well as in the number of amastigotes per cell were observed among the strains. Regarding to pharmacological assays, Y strain was the most sensitive to the three assayed compounds. These data demonstrate the heterogeneity of natural populations of T. cruzi, the only responsible of infection in humans.

Setting of a colorimetric method to determine the viability of Trypanosoma cruzi epimastigotes

The method most commonly used in screening of drugs for the treatment of Chagas' disease, microscopic counting of viable trypanosomes, is time-consuming, labor-intensive, and dependent on the observer. Although the tetrazolium dye [MTT; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay is comparatively quick and accurate, it requires careful attention in design as well as in interpretation of the results. Therefore, we examined under various conditions the sensitivity and specificity of the MTT assay versus microscopic counting for determination of the viability of Trypanosoma cruzi for drug-screening purposes. We tested different concentrations of MTT in phenazine methosulfate (PMS) against T. cruzi epimastigotes of the Y strain in different stages of logarithmic growth. In our model, in tests of benznidazole and nifurtimox the optimal concentration of MTT was 2.5 mg/ml of PMS and the optimal incubation period was 75 min. This method detected parasite concentrations of approx. 500,000 epimastigotes/ml (P<0.01), and the linear correlation between absorbance values and numbers of epimastigotes per milliliter was very strong (approx. R = 0.99). The present MTT assay results in faster determination of the activity of compounds, is more objective, and enables testing of several drugs simultaneously.