The hypothesis is that activation of MMP-9 causes EE barrier dysfunction and the treatment with HCQ inhibits MMP-9 and preserves EE barrier/juxtacrine, and synchronous endothelial-myocyte coupling as also recently seen in the COVID-19 patients wherein high levels of MMPs including the MMP-9 are associated with the disease severity [75,76,77,78,79]. Here, MMP9 is linked to COVID-19.