In addition, MPI8 is potent toward cathepsin L, but not toward any other tested proteases that can exert dual functions, with high selectivity in inhibiting both cellular MPro and cathepsin L over cathepsins B and K. Due to its high selectivity toward cathepsin L that reduces potential toxicity toward host cells and high cellular and antiviral potency, MPI8 is a promising candidate for preclinical and clinical investigations for treating COVID-19. Here, CTSL is linked to COVID-19.