Repurposing a Library of Human Cathepsin L Ligands: Identification of Macrocyclic Lactams as Potent Rhodesain and Trypanosoma brucei Inhibitors

Rhodesain
(RD) is a parasitic, human cathepsin L (hCatL) like cysteine protease
produced by Trypanosoma brucei (T. b.) species and a potential drug target
for the treatment of human African trypanosomiasis (HAT). A library
of hCatL inhibitors was screened, and macrocyclic lactams were identified
as potent RD inhibitors (Ki < 10 nM),
preventing the cell-growth of Trypanosoma brucei rhodesiense (IC50 < 400 nM). SARs addressing the S2 and S3 pockets
of RD were established. Three cocrystal structures with RD revealed
a noncovalent binding mode of this ligand class due to oxidation of
the catalytic Cys25 to a sulfenic acid (Cys–SOH) during crystallization.
The P-glycoprotein efflux ratio was measured and the in vivo brain
penetration in rats determined. When tested in vivo in acute HAT model,
the compounds permitted up to 16.25 (vs 13.0 for untreated controls)
mean days of survival.