Polyamine and pentamidine metabolism in African trypanosomes

A pentamidine-resistant line of bloodstream Trypanosoma brucei brucei (S427/118) has been developed by stepwise selection in axenic culture in vitro. After 57 days of selection, the resistant line (S427/118/PR32) was able to grow normally in 32 ng/ml (54 pM) pentamidine with an IC₅₀ value of 105 ng/ml (177 pM), which is 26-times higher than that of the parental strain. Post-mitochondrial supernatant extracts of both strains were unable to metabolize [³H]pentamidine, whereas under identical conditions rat liver microsomes were able to convert >5% of the drug to hydroxylation products. Thus metabolic conversion of pentamidine does not appear to be involved in either the mode of action of or resistance to pentamidine. Pentamidine-sensitive trypanosomes exposed for 4 h in vivo to therapeutic doses of pentamidine (4 mg/kg) did not show any significant changes in either polyamine-, thiol- or S-adenosylmethionine metabolites, indicating that inhibition of S-adenosylmethionine decarboxylase is not involved in the trypanocidal action of the drug. However, a marked increase in basic amino acid content was noted. In particular, lysine content was increased 13-fold following exposure to pentamidine.