As I explained yesterday, the reason I'm interested in hydroxychloroquine and chloroquine is because I'm investigating whether these, like quercetin and EGCG, are antiviral by acting as zinc ionophores, in which case I might add quercetin and EGCG to the protocol in The Food and Supplement Guide for the Coronavirus.
A zinc ionophore is a compound that can bring zinc across a cellular membrane, thereby transporting the zinc into the cell, or into a specific location within the cell.
Although chloroquine has been shown to act as a zinc ionophore, I doubt this contributes to its antiviral activity against SARS-CoV, the coronavirus that causes SARS, or SARS-CoV-2, the coronavirus that causes COVID-19.
There are three big reasons for this:
- The concentrations shown to affect zinc transport are far higher than those required to kill the viruses.
- The zinc ionophore activity of chloroquine brings zinc into locations within the cell that would not be expected to kill the virus.
- Other effects that do occur at relevant concentrations have better support.
The zinc ionophore activity of chloroquine has only been shown for concentrations between 10-300 micromoles per liter (abbreviated uM here, a measure of the number of molecules or ions per liter). At 10 uM, it increased the transport of ionic zinc into the cell about 2.3-fold, and at 100-300 uM it increased it about 3.4-fold.
While the paper on chloroquine as a zinc ionophore didn't show that there is no zinc ionophore activity at 0.1-9.9 uM, the fact that it has only been shown at concentrations that are 100 times the minimum concentration required to kill all the virus and nine times the concentration required to kill half the virus makes it entirely unclear whether it has any effect at all at concentrations that are actually relevant.
The story gets worse, however.
When chloroquine brings zinc ions into the cell, they don't get distributed far and wide within the cell. Instead, they get stuck in a digestive organelle known as the lysosome.
Will lysosomal zinc kill SARS-CoV or SARS-CoV-2? Probably not.
Like many viruses, these ones enter the cell in little pockets of the cell membrane that invaginate, and then pinch off to form little bubbles. This process is called endocytosis (to bring within the cell), and the little bubble is called the endosome. Endosomes eventually fuse with lysosomes, where their contents will be digested and broken up into components that the cell needs. The viruses don't want to be digested and broken up. They will die. As a result, all viruses that enter through an endosome must escape the endosome before it fuses with a lysosome.
So, lysosomal zinc will not kill a virus. Viruses as a rule evade lysosomes.
Now, perhaps when chloroquine brings zinc into a cell, before it winds up in a lysosome it first travels through an endosome. And perhaps this endosomal zinc will kill the virus.
So, let's ask the question: how do SARS-CoV and SARS-CoV-2 escape the endosome, and could zinc kill them within the endosome or prevent them from escaping?
Zinc probably isn't directly toxic to these viruses. We know, for example, that coronavirus 229E survives fine on surfaces rich in zinc, but dies quickly on surfaces rich in copper. The copper kills this virus as well as SARS-CoV by generating oxidative stress, something zinc is very unlikely to do. While zinc inhibits a number of enzymes used by the virus, it's apparent lack of direct toxicity to the virus suggests that zinc isn't going to kill the virus just by being present in the endosome with it.
Might it prevent the virus from escaping the endosome?
At the time of writing, there are no published studies on how SARS-CoV-2 escapes the endosome, but SARS-CoV does so through the activity of the human enzyme cathepsin L. Although this enzyme can be inhibited by zinc in liver cells, the IC50 (the concentration required to inhibit the enzyme's activity by 50%) is around 50 micrograms per milliliter (ug/mL), which is 50 times the zinc concentration of plasma, and 50 times the zinc concentration that is probably present within an endosome when it first pinches off from a cell membrane. At 10 uM, which is the concentration at which chloroquine is 100% effective at stopping SARS-CoV infection, it roughly doubles the lysosomal zinc. But a doubling of the plasma concentration would push it to 2 ug/mL, which is still 25-fold lower than the IC50 against cathepsin L.
So, it seems very unlikely that endosomal zinc would prevent SARS-CoV or SARS-CoV-2 from escaping the endosome.
As I noted in The Food and Supplement Guide for the Coronavirus, there are two SARS-CoV enzymes known to be inhibited by zinc:
- Papain-like protease 2 (PLP2) at an IC50 of 1.3 uM.
- 3CL protease at a Ki of 1.1 uM. (Ki and IC50 are both measures of half-maximal inhibition; you can read about the difference here).
The half-maximal inhibition of these enzymes occurs close to 1 uM, which is 0.0654 ug/mL, meaning that zinc is about 765 times more potent of inhibiting SARS-CoV viral replication proteins than it is by inhibiting human cathepsin L.
These proteins are not active in the endosome. Instead, SARS-CoV escapes the endosome and begins using the cellular machinery to read its RNA transcripts and produce proteins. This happens in the cytosol, the main compartment of the cell, or on the cytosolic surface of the endoplasmic reticulum, a cellular compartment dedicated to the production and processing of proteins. The SARS-CoV proteins that are inhibited by zinc are found embedded in the host cell's endoplasmic reticulum facing the cytosol.
The best chance for inhibiting viral replication, then, lies in increasing cytosolic zinc, not endosomal zinc.
But chloroquine doesn't increase cytosolic zinc. It traps zinc in lysosomes, where it is irrelevant to viral replication.
So how does chloroquine kill SARS-CoV and SARS-CoV-2 in vitro? Here's what those in vitro papers found:
- It increases endosomal pH. Fusion of the virus with the endosome, and later escape of the virus from the endosome, can both be pH-dependent. Increasing endosomal pH appears to prevent fusion of SARS-CoV with the endosome, and to the extent it makes it in, might also prevent its escape into the cytosol. This is supported by the fact that ammonium chloride, another agent that increases endosomal pH, has the same effect.
- Chloroquine and ammonium chloride also raise the pH in the golgi apparatus, the compartment where sugars are added to proteins in a process known as glycosylation. ACE2, the protein on the cell surface that allows the entry of SARS-CoV and SARS-CoV-2 into the cell, is one of the proteins that are glycosylated in the golgi. Chloroquine and ammonium chloride both interrupt the glycosylation of ACE2. They do not affect the amount of ACE2 on the cell surface, but it is possible that the the virus is less able to dock to ACE2 when the protein hasn't been glycosylated.
Chloroquine raises the pH of the endosomes, lysosomes, and golgi. This is a clearly toxic effect of the drug on human cells, because it will broadly disrupt the ability of the cell to take in things from its environment, digest things that need to be broken down, and glycosylate things that need to be glycosylated. As a result, chloroquine also interferes with the glycosylation of antibodies, which most likely contributes to its ability to treat autoimmune conditions. However, it might also hurt the immune defense against viruses, which might underly why it acted as an antiviral against chikungunya virus in vitro but enhanced viral replication in vivo.
As noted yesterday, despite the routine use of chloroquine against COVID-19 in China and its incorporation into at least two national guidelines for COVID-19, there is as yet no evidence it is effective.
In any case, what, if anything does this say about quercetin? I'll let you know tomorrow.
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