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 (<i>T</i>. <i>b</i>.) 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 (<i>K</i>_i < 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

Identification of Potent and Selective Cathepsin S Inhibitors Containing Different Central Cyclic Scaffolds

Starting from the weakly active dual CatS/K inhibitor <b>5</b>, structure-based design supported by X-ray analysis led to the discovery of the potent and selective (>50 000-fold vs CatK) cyclopentane derivative <b>22</b> by exploiting specific ligand–receptor interactions in the S2 pocket of CatS. Changing the central cyclopentane scaffold to the analogous pyrrolidine derivative <b>57</b> decreased the enzyme as well as the cell-based activity significantly by 24- and 69-fold, respectively. The most promising scaffold identified was the readily accessible proline derivative (e.g., <b>79</b>). This compound, with an appealing ligand efficiency (LE) of 0.47, included additional structural modifications binding in the S1 and S3 pockets of CatS, leading to favorable in vitro and in vivo properties. Compound <b>79</b> reduced IL-2 production in a transgenic DO10.11 mouse model of antigen presentation in a dose-dependent manner with an ED₅₀ of 5 mg/kg