‘How does methylene blue’s mode of action differ from chloroquine in treating malaria?‘
References the news article, https://newatlas.com/methylene-blue-dye-malaria/53280/?utm_source=quora&utm_medium=referral
While Chloroquine is chemically derived from Methylene Blue (, see below from 2), their anti-malarial actions appear to be quite different.
This answer briefly outlines
- What’s currently known (circa 2018) of the mechanisms of action (MOA) of Chloroquine and Methylene Blue.
- Shared history of Chloroquine and Methylene Blue, the latter a precursor to the former.
Chloroquine & Methylene Blue: Brief Look at What’s Known About How They Work
Chloroquine targets the asexual erythrocyte stages of the malaria parasite while latest studies suggest Methylene Blue is very effective against its post-erythrocytic sexual gametocyte stages (see below from).
In its human host’s red blood cells, the malaria parasite’s asexual stages (ring, trophozoite, schizont) stages survive by using essential amino acids released from degrading hemoglobin. To tap its essential nutrients, malaria trophozoites take in large amounts of hemoglobin into their digestion vacuoles when they are cycling through the red blood cell phase (intraerythrocytic cycle). Parasite hemoglobin degradation releases hematin (ferriprotoporphyrin), a heme byproduct found lethal to the parasites themselves, which they neutralize by biocrystallizing (polymerizing) heme into an insoluble polymer called hemozoin.
Chloroquine is a weak base and thus easily diffuses into lysosomes where it becomes protonated and is thus unable to diffuse out (, 5, 6). Once inside malaria parasite lysosomes, Chloroquine is hypothesized to prevent parasite digestion of heme by inhibiting hemozoin biocrystallization by binding hematin gamma-oxodimers. This ends up killing the parasite ( , 8; also see below from 9).
OTOH, Methylene Blue‘s MOA isn’t yet as clearly outlined. Interest in the 1990s focused on its effect on malarial parasite’senzyme.
The tripeptideis a that widely functions as a reducing agent in myriad cellular biochemical pathways. Studies suggest Glutathione can directly degrade hematin, which would help the malaria parasite and work against Chloroquine (10, 11).
Studies found Methylene Blue to apparently selectively block malarial, but not human, glutathione reductase (, 13) when tested at therapeutic doses ( ). Such Methylene Blue activity could even reverse Chloroquine resistance, a finding of undeniably important clinical relevance ( ).
However, Methylene Blue‘s effect on malarial glutathione reductase alone couldn’t explain its anti-malarial effect since glutathione reductase knockout malaria parasites were found equally susceptible to it ().
Recent reports showed Methylene Blue specifically targets all sexual gametocyte stages of malaria parasites both in vitro (, ) and in vivo ( , ). Though how it does this isn’t yet clear, this opens the possibility of preventing transmission back to mosquitoes and thereby help break malaria’s life cycle ( , , ).
Being cheap and easy to make at scale adds to Methylene Blue‘s value as an anti-malarial, especially in combination treatments which could overcome its ineffectiveness as a monotherapy as well as mitigate its side effects since lower doses would suffice.
Problem is Methylene Blue can be toxic to(G6PD) deficient people by increasing hemolysis and triggering anemia (24), a problem exacerbated by malaria selection pressure that manifests as G6PD deficiency and leads to abnormal hemoglobins variably protected from malaria. Thus G6PD deficiency tends to be higher in malaria-endemic regions. Safety in G6PD-deficient patients needs to be established first. A Thai clinical trial funded by the University of Oxford studied this issue but hasn’t posted or published results from it yet ( ).
Brief History Of Methylene Blue & Chloroquine
Methylene Blue is an aniline dye. It and others like it were 19th century synthetic products when scientists such asended up synthesizing dyes such as Mauve in their attempt to try to synthesize the antimalarial Quinine ( , 26) as an alternative to this ancient natural anti-malarial derived from the South American cinchona tree. Called by some ‘the unrivalled hero of 19th century chemical industry’, synthesized Methylene Blue (27). The likes of Robert Koch and Paul Ehrlich then began using such dyes as biological stains.
- In 1887 Polish pathologist, Czesław Chęciński, first reported that a combination of eosin and Methylene Blue applied to blood smears specifically stained what we now know to be malaria parasites (28).
- In 1890, also published a study where he reported staining malaria parasites in blood smears using a mixture of eosin and Methylene Blue (27).
- Ehrlich took this further by hypothesizing that if Methylene Blue could selectively stain malaria parasites, maybe it could also be selectively toxic to it (9, )
- Indeed, Ehrlich reported successfully treating two malaria patients with Methylene Blue (28, ).
Being a natural extract, Quinine was hobbled by limited supply so a synthetic alternative such as Methylene Blue was considered a boon. However, this perceived advantage proved short-lived, given its many more disadvantages,
- Not as effective as Quinine.
- Discolored skin, eyes, mucous membranes, urine.
- GI tract disturbances.
Following Ehlrich‘s anti-malaria research, many scientists at Bayer IG Farbenindustrie A.G. in Germany began synthesizing a wide variety of compounds. Some such as atabrine (quinacrine or) had anti-malarial activity though it too stained skin and eyes.
Johann ‘Hans’ Andersag () synthesized a 4-amino-quinoline in 1934 by replacing atabrine’s acridine ring with a quinoline (28). Called Resochin® for RESOrcinate of a 4-aminoCHINolin, IG Farben shelved it in 1935 as being slightly more toxic than atabrine after a test on just 4 neurosyphilis patients who were deliberately infected with malaria at a psychiatric clinic in Dusseldorf. Reoschin® thus languished for >10 years (28).
Licensed to the US Winthrop Chemical Company (6), Resochin®’s potent antimalarial properties were rediscovered by chance during WW II in a variation of the adage, ‘necessity is the mother of invention’. The Japanese invasion of Pearl Harbor blocked Allied access to Quinine. With Atabrine the only synthetic alternative, intense search for alternatives spurred the American Board for the Coordination of Malarial Studies (5) to screen some 16000 compounds, one being SN-7618 (SN=Survey Number). With trials that started in early 1944, by early 1946, tests on >5000 individuals clearly showed SN-7618 as far superior to atabrine. No coloring of skin nor eyes nor GI tract upsets. In November 1945 EK Marshall renamed it Chloroquine () and it soon made its way the world over as a pre-eminent anti-malarial and the rest is history.
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