CALCA and COVID-19: Collectively, most of the natural product-derived inhibitors targeting the spike S1 RBD:ACE2 receptor interaction have been investigated based on in silico simulation [29,39,40,41,42,43]; however, this study not only provides information on a building block of COVID-19 therapeutic agents with the MoA that inhibits SARS-CoV-2 cell entry based on the actual molecular interaction between KC and the spike S1 RBD/ACE2 receptor, it also suggests that further structural analysis of KC derivatives is warranted to obtain increased binding affinity for both proteins.