Susceptibilities of Ugandan Plasmodium falciparum Isolates to Proteasome Inhibitors

The proteasome is a promising target for antimalarial chemotherapy. We assessed ex vivo susceptibilities of fresh Plasmodium falciparum isolates from eastern Uganda to seven proteasome inhibitors: two asparagine ethylenediamines, two macrocyclic peptides, and three peptide boronates; five had median IC50 values, 100 nM. TDI8304, a macrocylic peptide lead compound with drug-like properties, had a median IC50 of 16 nM. Sequencing genes encoding the β2 and β5 catalytic proteasome subunits, the predicted targets of the inhibitors, and five additional proteasome subunits, identified two mutations in b2 (I204T, S214F), three mutations in b5 (V2I, A142S, D150E), and three mutations in other subunits. The b2 S214F mutation was associated with decreased susceptibility to two peptide boronates, with IC50s of 181 nM and 2635 nM against mutant versus 62 nM and 477 nM against wild type parasites for MMV1579506 and MMV1794229, respectively, although significance could not be formally assessed due to the small number of mutant parasites with available data. The other b2 and b5 mutations and mutations in other subunits were not associated with susceptibility to tested compounds. Against culture-adapted Ugandan isolates, two asparagine ethylenediamines and the peptide proteasome inhibitors WLW-vinyl sulfone and WLL-vinyl sulfone (which were not studied ex vivo) demonstrated low nM activity, without decreased activity against b2 S214F mutant parasites. Overall, proteasome inhibitors had potent activity against P. falciparum isolates circulating in Uganda, and genetic variation in proteasome targets was uncommon.