2.3. Evaluation of protease inhibition and structureactivity relationships. According to previously described procedures, the cathepsin inhibition assays were performed with an appropriate chromogenic peptide substrate. We used Z-Phe-Arg-pNA and Z-Arg-Arg-pNA for human CatL and human CatB, respectively. To monitor the activity of human CatS, Bz-Phe-Val-Arg-pNA was established as a chromogenic substrate and its Km value was determined to be 61.9 µM. All final compounds were tested at a concentration of 10 µM (Table ). In cases of more than 30% protease inhibition over 60 min, the compounds were investigated at five different concentrations; the slopes of the progress curves were plotted versus inhibitor concentrations, and Ki values were obtained by non-linear regression. A SARS-CoV-2 M pro activity assay with a recently developed fluorogenic substrate (Boc-Abu-Tle-Leu-Gln-AMC) was employed, compounds were screened at a concentration of 10 µM, and for selected active representatives, Ki values were determined (Table ). i A Ki value of >5 µM referred to the residual activity of M pro in the presence of 10 µM of the test compound being higher than 70%.