I. What is the deal with chloroquine?
Let’s start with a review of some virology basics. Coronaviruses are enveloped RNA viruses, meaning they are coated in a piece of host cell membrane. There are several families of Coronaviruses which differ in the receptors they use for host entry, but several papers in the last decade or so have revealed an important role for endocytosis in viral entry. Through this process the entire virus (envelope and all) is internalized, and the endocytosed virion therefore ends up inside a membrane-bound compartment in the cell (an endosome) from which it must escape before making mischief in the nucleus. This trafficking and escape process generally depends on acidification of the endosome and/or fusion with acidified lysosomes.
OK. So what does this have to do with chloroquine (CQ)? CQ belongs to a class of agents known as “cationic amphiphilic drugs” (CADs) which share some peculiar structural features - most notably an amino group on one end. When these drugs encounter acidified compartments, their amino groups become protonated and the molecules become trapped. The end result is that CADs accumulate to very high concentrations in normally acidified compartments like the endosome, lysosome, and trans-Golgi apparatus and have a variety of effects on the enzymes and macromolecules that reside there. For example, this phenomenon is the mechanistic basis for phospholipidosis (“foamy macrophages”) in patients treated with the cationic amphiphilic drug amiodarone. Each CAD induces unique but overlapping changes in lysosomal and Golgi metabolism in complex ways specific to its chemical composition. It should be noted here that the exact effects of CQ on lysosomes and endosomes remains an area of active research.
Nevertheless, it has been reported for decades that chloroquine, amiodarone, and other CADs can interfere with the replication of a diverse array of enveloped viruses including influenza, Ebola virus, HIV, Dengue, Zika, and HCV in vitro. On the other hand, in vivo assays and clinical trials have been less promising – chloroquine failed to prevent influenza infection in a clinical trial (Paton, et. al. Lancet Inf Dis. 2011) and, if anything, increased viral load in an HIV trial (Paton, et. al. JAMA 2012). And, concerningly, at least one group reported increased influenza replication in the presence of CQ (Wu, et. al. J Med Vir 2015).
II. Why do people think CQ/HCQ might be effective against SARS-Cov-2?
The majority of evidence supporting the use of CQ and its derivative, hydroxychloroquine (HCQ), come from studies of the original SARS-Cov which emerged in 2002. In the ensuing years, basic virology studies established that SARS-Cov depends on endosomal escape, that it buds from the Golgi apparatus, and that its receptor (ACE2) is itself glycosylated in the Golgi. Thus, CQ was a rational drug to test. Vincent and colleagues established the efficacy of CQ in inhibiting SARS-Cov replication and provided evidence that impaired endosomal acidification as well as impaired ACE2 glycosylation might be responsible. Similar data were reported by other groups (Keyrts, et. al. Biochem Biophys Res Commun. 2004).
Based on these data, Wang and colleagues published two papers in Feb (Wang, et. al. Cell Research 2020) and March (Liu, et. al. Cell Discovery 2020) of this year examining the effects of CQ and HCQ (more widely available and less toxic) on the novel SARS-Cov-2 in vitro. CQ dramatically inhibits SARS-Cov-2 replication at low micromolar concentrations, while HCQ inhibits replication at ~10uM. Importantly, these concentrations are at least 10-fold lower than reported cytotoxic doses – though it is virtually certain normal cellular physiology is being perturbed to some extent.
What about clinical evidence? As of this writing (March 21), there are no rigorous clinical data specifically demonstrating therapeutic benefit of CQ or HCQ in prevention or treatment of COVID-19 save for a “publication” (Gao, et. al. BioScience Trends 2020) which states without evidence that CQ has proven effective in >100 patients. There are, however, >20 active clinical trials (most in China) and it is likely that data will emerge in the coming weeks. Which brings us to ...