Lung cancer patients are more vulnerable to COVID-19 infection. Treatment of patients with lung cancer during the current COVID-19 pandemic is challenging and development of drugs for COVID-19 and lung cancer is urgently needed. Cathepsin L plays key role in lung cancer progression, invasion and more so, endocytosis of SARS-Cov-2 virus into the lung epithelial cells. Importantly, patients with KrasG12V mutation show agreessive disease and high-resistance to chemotherapy due to over-expression of Cathepsin L. Thus, Cathepsin L is a common target for lung cancer and COVID-19 infection. Chymostatin is a known cathepsin L inhibitor, here we screened it for dual inhibition of COVID-19 Mpro and lung cancer patients with COVID-19. In vitro COVID-19 Mpro fluorometric assay was performed to evaluate its inhibitory efficacy by chymostatin. Chymostatin showed dose-dependent inhibition of COVID-19 Mpro activity with IC50 of 15.81 μ M (P<0.0001). Isothermal titration calorimetry based binding studies determined that chymostatin strongly bound to COVID-19 Mpro (KD=22.45 μ M). CD spectrum analysis demonstrated that chymostatin (10 μ M)-induced changes in the secondary structure of COVID-19 Mpro. A unit cell crystal parameter COVID-19 Mpro co-crystalized with chymostatin (a = 49.926 Å, b = 108.71 Å, c = 56.49 Å, α = 90.0°, β = 102.7553°, γ = 90.0°;unit cell volume: 299.03 Ao) was determined using X-ray diffractometer. Molecular docking studies demonstrated that chymostatin interacts with active site amino acids (Cys145). Pro-apoptotic effect of chymostatin was evaluated on human H441 lung cancer cells and mouse lung normal epithelial cells using Annexin V/propidium iodide staining. Chymostatin did not show any cytotoxicity on normal mouse lung epithelial cells up to 100 μ M, where as it inhibited proliferation of human H441 lung cancer cells (KrasG12V mutant) with IC50 of 12 μ M. In conclusion, preliminary in vitro studies demonstrated that naturally occurring cathepsin L inhibitor chymostatin is a SARS-CoV-2 Mpro inhibitor and exhibited anticancer activity against lung cancer cells. The results of the study warranted detailed studies to develop as drugs for lung cancer patients during COVID-19 pandemic (supported by VPR-PHF, the University of Oklahoma Health Sciences Centre for financial support through COVID-19 seed grant program).
Chymostatin, a cathepsin L inhibitor, inhibits lung cancer cell proliferation and COVID-19 Mpro in vitro