The in vitro trypanocidal activity of N-substituted p-benzoquinone imines: assessment of biochemical structure-activity relationships using the Hansch approach.

It has previously been found that naphthoquinones can potentiate the rate of hydrogen peroxide production by mitochondrial preparations of Trypanosoma brucei brucei and that organisms treated with naphthoquinones are more susceptible to lysis, especially in the presence of compounds such as heme, which promote the homolytic cleavage of hydrogen peroxide. We have evaluated the lytic effect of various N-substituted p-benzoquinone imines both in vitro and in vivo and have attempted to correlate their structure with trypanocidal activity using the Hansch approach. While none of the compounds tested proved to be active in vivo, all caused the lysis of trypanosomes in vitro. The parameters that correlated best with trypanocidal activity were the conditional redox potential, the lipophilicity of the substituent attached to the nitrogen atom and the number of active hydrogens on the quinonoid ring. These findings suggest two possible modes of action, which may in fact be related. Conjugate nucleophilic addition and/or oxidative damage could be responsible for lysis of the parasites. These same compounds were previously found to be active against the ascitic sarcoma 180 in mice. The strong correlation between antineoplastic activity in vivo and trypanocidal activity in vitro suggests a similar mode of action in both cases. Further studies aimed at developing a quinonelike compound that will be active against trypanosomes in vivo are now in progress.