|Trade names||Plaquenil, others|
|By mouth (tablets)|
|Bioavailability||Variable (74% on average); Tmax = 2–4.5 hours|
|Elimination half-life||32–50 days|
|Excretion||Mostly Kidney (23–25% as unchanged drug), also biliary (<10%)|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||335.872 g/mol g·mol−1|
|3D model (JSmol)|
|(what is this?)|
Hydroxychloroquine (HCQ), sold under the brand name Plaquenil among others, is a medication used for the prevention and treatment of certain types of malaria. Specifically it is used for chloroquine-sensitive malaria. Other uses include treatment of rheumatoid arthritis, lupus, and porphyria cutanea tarda. It is taken by mouth.
Common side effects include vomiting, headache, changes in vision and muscle weakness. Severe side effects may include allergic reactions. It appears to be safe in pregnancy but this use has not been well studied. Hydroxychloroquine is in the antimalarial and 4-aminoquinoline families of medication.
Hydroxychloroquine was approved for medical use in the United States in 1955. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. The wholesale cost in the developing world is about $5.40 to 7.44 per month. In the United Kingdom this dose costs the NHS about £5.15. In the United States the wholesale cost of a month of treatment is about US$25 as of 2019. In 2016 it was the 135th most prescribed medication in the United States with more than 4 million prescriptions.
Hydroxychloroquine is widely used in the treatment of post-Lyme arthritis. It may have both an anti-spirochaete activity and an anti-inflammatory activity, similar to the treatment of rheumatoid arthritis.
For short-term treatment of acute malaria, adverse effects can include abdominal cramps, diarrhea, heart problems, reduced appetite, headache, nausea and vomiting.
For prolonged treatment of lupus or arthritis, adverse effects include the acute symptoms, plus altered eye pigmentation, acne, anaemia, bleaching of hair, blisters in mouth and eyes, blood disorders, convulsions, vision difficulties, diminished reflexes, emotional changes, excessive coloring of the skin, hearing loss, hives, itching, liver problems or liver failure, loss of hair, muscle paralysis, weakness or atrophy, nightmares, psoriasis, reading difficulties, tinnitus, skin inflammation and scaling, skin rash, vertigo, weight loss, and occasionally urinary incontinence. Hydroxychloroquine can worsen existing cases of both psoriasis and porphyria.
One of the most serious side effects is a toxicity in the eye (generally with chronic use). People taking 400 mg of hydroxychloroquine or less per day generally have a negligible risk of macular toxicity, whereas the risk begins to go up when a person takes the medication over 5 years or has a cumulative dose of more than 1000 grams. The daily safe maximum dose for eye toxicity can be computed from one's height and weight using this calculator. Cumulative doses can also be calculated from this calculator. Macular toxicity is related to the total cumulative dose rather than the daily dose. Regular eye screening, even in the absence of visual symptoms, is recommended to begin when either of these risk factors occurs.
Toxicity from hydroxychloroquine may be seen in two distinct areas of the eye: the cornea and the macula. The cornea may become affected (relatively commonly) by an innocuous cornea verticillata or vortex keratopathy and is characterized by whorl-like corneal epithelial deposits. These changes bear no relationship to dosage and are usually reversible on cessation of hydroxychloroquine.
The macular changes are potentially serious. Advanced retinopathy is characterized by reduction of visual acuity and a "bull's eye" macular lesion which is absent in early involvement.
Hydroxychloroquine generally does not have significant interactions with other medications, but care should be taken if combined with medication altering liver function as well as aurothioglucose (Solganal), cimetidine (Tagamet) or digoxin (Lanoxin). HCQ can increase plasma concentrations of penicillamine which may contribute to the development of severe side effects. It enhances hypoglycemic effects of insulin and oral hypoglycemic agents. Dose altering is recommended to prevent profound hypoglycemia. Antacids may decrease the absorption of HCQ. Both neostigmine and pyridostigmine antagonize the action of hydroxychloroquine.
Due to rapid absorption, symptoms of overdose can occur within a half an hour after ingestion. Overdose symptoms include convulsions, drowsiness, headache, heart problems or heart failure, difficulty breathing and vision problems.
Hydroxychloroquine has similar pharmacokinetics to chloroquine, with rapid gastrointestinal absorption and elimination by the kidneys. Cytochrome P450 enzymes (CYP2D6, 2C8, 3A4 and 3A5) metabolize hydroxychloroquine to N-desethylhydroxychloroquine.
Antimalarials are lipophilic weak bases and easily pass plasma membranes. The free base form accumulates in lysosomes (acidic cytoplasmic vesicles) and is then protonated, resulting in concentrations within lysosomes up to 1000 times higher than in culture media. This increases the pH of the lysosome from 4 to 6. Alteration in pH causes inhibition of lysosomal acidic proteases causing a diminished proteolysis effect. Higher pH within lysosomes causes decreased intracellular processing, glycosylation and secretion of proteins with many immunologic and nonimmunologic consequences. These effects are believed to be the cause of a decreased immune cell functioning such as chemotaxis, phagocytosis and superoxide production by neutrophils. HCQ is a weak diprotic base that can pass through the lipid cell membrane and preferentially concentrate in acidic cytoplasmic vesicles. The higher pH of these vesicles in macrophages or other antigen-presenting cells limits the association of autoantigenic (any) peptides with class II MHC molecules in the compartment for peptide loading and/or the subsequent processing and transport of the peptide-MHC complex to the cell membrane.
Mechanism of action
Hydroxychloroquine increases lysosomal pH in antigen-presenting cells. In inflammatory conditions, it blocks toll-like receptors on plasmacytoid dendritic cells (PDCs). Hydroxychloroquine, by decreasing TLR signaling, reduces the activation of dendritic cells and the inflammatory process. Toll-like receptor 9 (TLR 9) recognizes DNA-containing immune complexes and leads to the production of interferon and causes the dendritic cells to mature and present antigen to T cells, therefore reducing anti-DNA auto-inflammatory process.
In 2003 a novel mechanism was described wherein hydroxychloroquine inhibits stimulation of the toll-like receptor (TLR) 9 family receptors. TLRs are cellular receptors for microbial products that induce inflammatory responses through activation of the innate immune system.
As with other quinoline antimalarial drugs, the mechanism of action of quinine has not been fully resolved. The most accepted model is based on hydrochloroquinine and involves the inhibition of hemozoin biocrystallization, which facilitates the aggregation of cytotoxic heme. Free cytotoxic heme accumulates in the parasites, causing their deaths.
Brand names of hydroxychloroquine include Plaquenil, Hydroquin, Axemal (in India), Dolquine, Quensyl, Quinoric.
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