Hydroxychloroquine

The exact mechanisms of hydroxychloroquine are unknown. It has been shown that hydroxychloroquine accumulates in the lysosomes of the malaria parasite, raising the pH of the vacuole. This activity interferes with the parasite's ability to proteolyse hemoglobin, preventing the normal growth and replication of the parasite. Hydroxychloroquine can also interfere with the action of parasitic heme polymerase, an enzyme that uses ferriprotoporphyrin IX (FP) released from hemoglobin as a substrate to form beta-hematin. By reducing the activity of heme polymerase without inhibiting the release of FP, hydroxychloroquine leads to the accumulation of FP in a toxic form.

Hydroxychloroquine accumulation in human organelles also raise their pH, which inhibits antigen processing, prevents the alpha and beta chains of the major histocompatibility complex (MHC) class II from dimerizing, inhibits antigen presentation of the cell, and reduces the inflammatory response. Elevated pH in the vesicles may alter the recycling of MHC complexes so that only the high affinity complexes are presented on the cell surface. Self peptides bind to MHC complexes with low affinity and so they will be less likely to be presented to autoimmune T cells. Hydroxychloroquine also reduces the release of cytokines like interleukin-1 and tumor necrosis factor, possibly through inhibition of Toll-like receptors.

The raised pH in endosomes, prevent virus particles (such as SARS-CoV and SARS-CoV-2) from utilizing their activity for fusion and entry into the cell.

Hydroxychloroquine inhibits terminal glycosylation of ACE2, the receptor that SARS-CoV and SARS-CoV-2 target for cell entry. ACE2 that is not in the glycosylated state may less efficiently interact with the SARS-CoV-2 spike protein, further inhibiting viral entry.