Studies in Chemotherapy

Laboratory Selection of Trypanosomatid Pathogens for Drug Resistance

The selection of parasites for drug resistance in the laboratory is an approach frequently used to investigate the mode of drug action, estimate the risk of emergence of drug resistance, or develop molecular markers for drug resistance. Here, we focused on the How rather than the Why of laboratory selection, discussing different experimental set-ups based on research examples with Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The trypanosomatids are particularly well-suited to illustrate different strategies of selecting for drug resistance, since it was with African trypanosomes that Paul Ehrlich performed such an experiment for the first time, more than a century ago. While breakthroughs in reverse genetics and genome editing have greatly facilitated the identification and validation of candidate resistance mutations in the trypanosomatids, the forward selection of drug-resistant mutants still relies on standard in vivo models and in vitro culture systems. Critical questions are: is selection for drug resistance performed in vivo or in vitro? With the mammalian or with the insect stages of the parasites? Under steady pressure or by sudden shock? Is a mutagen used? While there is no bona fide best approach, we think that a methodical consideration of these questions provides a helpful framework for selection of parasites for drug resistance in the laboratory.

Drug resistance in trypanosomes: selective interference with trypanocidal action

Selective reversal of the trypanocidal action of carboxylated arsenicals by p-aminobenzoic acid and of melaminyl arsenicals and diamidines by melamine has been demonstrated in vivo and in vitro. The structural specificity of these reversal phenomena is high, and suggests preferential adsorption of the antagonist during a reversible primary drug fixation stage. Thiols antagonized neutral, carboxylated and melaminyl aromatic arsenicals equally, but not diamidines; p-aminobenzoic acid antagonism is specific for carboxylated arsenicals, and melamine antagonizes only the melaminyl arsenicals and the diamidines. These reversals reflect the pattern of crossresistance behaviour and suggest that cellular structures associated with a reversible stereospecific drug adsorption phase are modified during the development of resistance.

Drug resistance in trapanososmes: cross-resitance analyses

Eight strains of Trypanosoma rhodesiense, made resistant respectively to atoxyl, butarsen, acriflavine, stilbamidine, Surfen C, suramin, and pontamine sky blue 5BX, have been examined for cross-resistance to representatives of nine structurally dissimilar groups of trypanocide. On the basis of their predominant ionic form at blood pH, these groups are considered in three main classes: (a) feebly ionized (neutral aromatic arsenicals), (b) ionized as cations (melaminyl arsenicals and antimonials, acridine derivatives, diguanidines and diamidines, 6-aminoquinoline and 6-aminocinnoline derivatives, phenanthridinium derivatives, triphenylmethane dyes), and (c) ionized as anions (carboxylated aromatic arsenicals and sulphonated naphthylamine derivatives). The results are discussed in relation to those of other workers and to possible modes of trypanocidal drug action. Cross-resistance behaviour is not wholly explicable on an ionic basis; the results suggest that stereospecific structural changes associated with initial drug uptake occur in resistant trypanosomes.

Drug resistance in trypanosomes: effects of metabolic inhibitiors, pH and oxidation-reduction potential on normal and resistant Trypanosoma rhodesiense

A wide variety of metabolic inhibitors tested in vitro for trypanocidal activity on normal and drug-resistant strains of Trypanosoma rhodesiense showed no relation between acquired drug resistance and changes in specific enzymatic function. Oxidation-reduction potential is an important factor in trypanocidal action but is not obviously related to the development of resistance. The dependence on pH of the trypanocidal action of ionizing drugs against both normal and resistant trypanosomes supports the postulate that the development of resistance involves physical changes in cell structures associated with the uptake of drug.

Glucantime susceptibility of Leishmania promastigotes under variable growth conditions

Growth inhibition of Leishmania promastigotes by glucantime was compared in three different media. Glucantime inhibited the growth of Leishmania cultured in complex medium but did not affect parasite growth when added to cells cultured in defined or semi-defined media. Supplementation of the complex medium with biopterin partially reversed the glucantime effect in sensitive strains, although the addition of folic acid or oleic acid did not alter the activity of glucantime. Differences in fatty acid composition were observed between strains showing different degrees of glucantime susceptibility.

Trypanosoma brucei: five commonly used trypanocides assayed in vitro with a mammalian feeder layer system for cultivation of bloodstream forms

The in vitro activity of five commonly used trypanocides on bloodstream forms of Trypanosoma brucei brucei TC 221 was examined in 24-well culture plates in the presence of bovine fibroblast feeder layer cells. The minimum effective concentrations determined were as follows: Berenil 1.0 microgram/ml; Samorin 10.0 micrograms/ml; Antrycide dimethylsulfate 0.1 microgram/ml; Arsobal 0.01 microgram/ml; Naganol 1.0 microgram/ml. Contrary to values obtained with other in vitro assays, minimum effective concentrations obtained here were within the range of drug levels reached in blood, plasma, or serum of humans and animals after treatment with curative doses. The trypanocidal activity of Naganol in this assay was of particular interest, since Naganol has been hitherto considered to be inactive in vitro.