Evaluation of Parameters Impacting Drug Susceptibility in Intracellular Trypanosoma cruzi Assay Protocols

In order to understand the key parameters influencing drug susceptibility, different Trypanosoma cruzi assay protocols were evaluated using a comparative assay design. The assays compared in this study were an image-based intracellular T. cruzi assay quantified through an image-mining algorithm and an intracellular assay utilizing a β-galactosidase-expressing T. cruzi strain. Thirty-one reference compounds known to exhibit activities against intracellular T. cruzi were used as benchmarks. Initial comparison using EC(50) values from two assays showed a very poor correlation, with an R(2) value of 0.005. Nitroheterocyclics and CYP51 inhibitors were inactive in an image-based assay, but were highly active in a colorimetric assay. In order to identify the differentiating factor, we synchronized the compound–parasite incubation times or the sequential cell and compound seeding schemes between assays, but the correlation remained low. A high correlation (R(2) = 0.86) was observed only after both compound incubation time and cell seeding were synchronized between assays. Further analysis of EC(50) and maximum inhibition values showed that nitroheterocyclics and CYP51 inhibitors exhibit relatively large deviations in activity between experimental protocols routinely used for in vitro intracellular T. cruzi assays. These findings suggest that the factors mentioned are critical when designing an intracellular T. cruzi assay.