RNA Polymerase 1 inhibitors against African trypanosomes in vitro and in mice

Therapy of human African trypanosomiasis, fatal if not treated, has been greatly improved by orally available and safer fexinidazole, but reliable cure of late-stage central nervous system (CNS) infection remains a challenge. African trypanosomes are unique among eukaryotes in having an RNA Polymerase 1 that transcribes not only rRNA but also mRNA that encodes the abundant variable surface glycoprotein (VSG) that coats the cell membrane of bloodstream-form parasites and allows them to evade host immune defenses. RNA Pol 1 inhibitor BMH-21, an intercalator not associated with DNA damage and a lead for new cancer therapy, has reported activity against the synthesis of rRNA and VSG mRNA in Trypanosoma brucei brucei. We evaluated a library of BMH-21 analogs against bloodstream-form T. b. brucei in vitro and found limited tolerance for structural modification. Striking and time-dependent bimodal dose-response curves indicate these compounds have a complex mechanism of action. Most potent against T. b. brucei was benzopyridoquinazoline Compound 2, with an EC₅₀ of 84 nM. Analysis of in vitro pharmacokinetic-pharmacodynamic (PK-PD) relationships by a hollow fiber model system revealed that in T. b. brucei, the kinetic driver of Compound 2 is drug concentration, a finding that was confirmed by dose fractionation in infected mice and that highlights the utility of the in vitro system for assessing antitrypanosomal PK-PD. Although Compound 2 accumulates 20-fold in mouse brain tissue vs plasma, efficacy against CNS T. b. brucei in mice was limited. RNA Pol 1 remains an attractive target for developing new and more selectively toxic antitrypanosomal agents.