Identification of Cathepsin L as the Molecular Target of Hydroxychloroquine With Chemical Proteomics.

Hydroxychloroquine (HCQ) and chloroquine have been utilized as antimalarial drugs for decades. Recently, these compounds were reported to inhibit various viruses utilizing the endosomal entry pathway. However, their direct molecular targets in host cells remain elusive. In this study, we developed a clickable photo-crosslinking probe in combination with proteomic approaches to identified cathepsin L (CTSL) as the binding target of HCQ. Extensive biochemical and in silico analyses were conducted to validate the HCQ-CTSL interactions. HCQ significantly inhibited the protease activity of CTSL and suppressed CTSL-dependent coronavirus entry in cells that support endosomal entry pathway. These findings not only reveal the underlying mechanism of how HCQ inhibits endosomal viral entry but also guide the rational use of HCQ against other emerging infectious agents.