Design and validation of selective and potent cathepsin S inhibitors as anti-cancer therapy

To further validate Cathepsin S as a molecular target we sought to design, through molecular modelling and synthesis, a potent, highly selective and cell active inhibitor superior to our current tool compound, I6. Analysis of the S2 pocket, and the gatekeeper residues, revealed a significant opportunity upon which selectivity for CatS over the CatL, B, and K could be achieved. A series of small molecules were synthesized (1a-f) to explore this opportunity with promising results, though CatV activity remained a significant problem. Further iterations of the P2-P3 linker group (CF3amine to amide) in combination with previous changes made in P2 position afforded small inhibitors with excellent selectivity profiles (2a-f). The biochemical potency of these new inhibitors was above the desired 10 nM threshold set out at the onset of the project for further screening. Within the literature discussing the development of CatK inhibitors, such as Odancatib, it has been reported that extension of the P3 group into the deep S3 pocket led to improvements in biochemical potency (3a-f). These changes were also believed to improve selectivity for CatK over Cathepsin family members, although use of 3-pyridyl group has been shown to erode CatK activity. Interestingly, incorporation of functionalised 3-pyridyl groups led to a significant jump in biochemical potency with the most active compounds displaying picomolar affinity. This series of inhibitors was taken forward and screened in our antigen presentation assay from which we identified QDD200081 which displays an improved level of potency over I6 by 9-fold.