Hydroxychloroquine is a 4-aminoquinolone that is used to prevent and treat malaria, caused by parasitic protozoan from the Plasmodium genus. Hydroxychloroquine is also used to treat non-infectious diseases such as rheumatoid arthritis and lupus thanks to its immunomodulatory properties. Recent studies revealed that hydroxychloroquine has beneficial effects on COVID-19 patients, as the current evidence shows that hydroxychloroquine inhibits SARS CoV-2 entry in host cells by interfering with endocytosis and affecting the glycoprofile on the spike glycoprotein.

A chemotherapeutic agent that acts against erythrocytic forms of malarial parasites. Hydroxychloroquine appears to concentrate in food vacuoles of affected protozoa. It inhibits plasmodial heme polymerase. 

Clinical Use

Hydroxychloroquine is approved for the treatment of both systemic and cutaneous lupus erythematosus. Both chloroquine and quinacrine (Atabrine) are also effective in this skin disease. Low-dose chloroquine is used for the therapy of porphyria cutanea tarda in patients in whom phlebotomy has failed or is contraindicated. Other skin diseases in which the drugs are useful (after sunscreens and avoidance of sun exposure) include polymorphous light eruption and solar urticaria.    

In Rheumatic Arthritis:

Hydroxychloroquine lacks the myelosuppressive, hepatic, and renal toxicities seen with some other DMARDs, which simplifies monitoring. Its onset may be delayed for up to 6 weeks, but the drug should not be considered a therapeutic failure until after 6 months of therapy with no response.

Short-term toxicities include GI (nausea, vomiting, diarrhea), ocular (accommodation defects, benign corneal deposits, blurred vision, scotomas, night blindness, preretinopathy), dermatologic (rash, alopecia, skin pigmentation), and neurologic (headache, vertigo, insomnia) effects. Periodic ophthalmologic examinations are necessary for early detection of reversible retinal toxicity.

Indications

Hydroxychloroquine (Plaquenil), like chloroquine, is a 4-aminoquinoline derivative used for the suppressive and acute treatment of malaria. It also has been used for rheumatoid arthritis and discoid and systemic lupus erythematosus. Hydroxychloroquine has not been proved to be more effective than chloroquine. Adverse reactions associated with its use are similar to those described for chloroquine.The drug should not be used in patients with psoriasis or porphyria, since it may exacerbate these conditions.

Mechanism of action

Hydroxychloroquine, like chloroquine, is also used for treating acute forms of malaria caused by P. vivax, P. malariae, P. ovale, and also sensitive forms of P. falciparum. It is also effective and safe like chloroquine, although it does not have obvious advantages. The only advantage is that it is somewhat better tolerated. Its use is somewhat more limited than chloroquine. Synonyms of this drug are plaquenil, quensyl, toremonil, and others.