An In Vitro Assay for Trypanocidal Activity *

The method of testing the therapeutic action of drugs by injecting them into infected laboratory animals, although an indispensable procedure, is not entirely adequate if the object of the inquiry is to obtain information regarding small quantitative differences between related compounds and their mechanism of action. Does a drug, as such, act directly on the parasites or only after it has undergone some change in the body; or does it act by stimulating a host reaction? Considerations of this nature lead to the belief that information of value might be obtained from an examination of the trypanocidal power of various drugs in vitro, thereby eliminating many factors dependent upon the vertebrate host. Before the work of Yorke and his group, 6, 7,8 such in vitro studies on trypanosomes were almost invariably made at room temperature, and the period of observation was limited to a few hours. These limitations were imposed by conditions of experimentation which did not suffice to maintain the parasites in the control observations, where no drug was involved, for more than a very short period. The medium in which the drugs were diluted and the trypanosomes suspended was usually physiological saline or some slight modification thereof; and under such conditions trypanosomes can not survive for more than a few hours at 37°C. These facts were appreciated as early as 1912 when it was demonstrated that failure of atoxyl to act in vitro was attributable to culture methods.8 By using defibrinated guinea-pig blood, early workers were able to prolong culture viability to five hours at 220 C. and were able to.show that two and a half hours were required for atoxyl activity. Several succeeding investigators improved the method but all found it impossible to maintain pathogenic trypanosomes at 370 C.7 Yorke and his co-workers succeeded in culturing trypanosomes at 370 C. in such a way that the number of viable organisms was about the same at the end of 24 hours as at the start of the experiment.6 The