Hydroxychloroquine

Introduction Overview

Generic Name

hydroxychloroquine

Brand Name

Plaquenil

Dosage Form

oral tablet

Drug Class

Antimalarial quinolines, Antirheumatics

Pharmacokinetics

Absorption:
Hydroxychloroquine is well absorbed from the gastrointestinal tract following oral administration, with bioavailability estimated at approximately 74%. Absorption occurs primarily in the small intestine, and peak plasma concentrations are typically reached within 3 to 4 hours post-dose. Food does not significantly affect absorption but taking the drug with a meal or milk is recommended to improve gastrointestinal tolerability. The drug exhibits variable first-pass metabolism and undergoes partial hepatic extraction.

Distribution:
Hydroxychloroquine demonstrates extensive tissue distribution due to its high affinity for melanin-containing tissues as well as lysosomes within cells. It accumulates substantially in tissues such as the liver, spleen, lungs, and particularly in retinal pigment epithelium, which underlies the risk for retinopathy. The apparent volume of distribution (Vd) is very large, typically reported between 5,000 to 44,000 liters, reflecting significant sequestration in body compartments. The plasma protein binding is moderate (approximately 50%), predominantly to serum albumin.

Metabolism:
This drug undergoes partial hepatic metabolism through the cytochrome P450 enzyme system, involving CYP2C8, CYP3A4, and CYP2D6 isoenzymes. Hydroxychloroquine is metabolized to active metabolites such as desethylhydroxychloroquine. The metabolic pathways contribute to both the therapeutic and toxic effects, including retinal toxicity risk. Variability in metabolism may be influenced by genetic polymorphisms, hepatic function, and concomitant medications.

Elimination:
Hydroxychloroquine has a prolonged elimination half-life ranging from approximately 30 to 50 days after chronic dosing, attributable to its extensive tissue binding and slow release. The drug is excreted both renally and via feces; about 20-25% is excreted unchanged in the urine. Renal impairment can significantly decrease clearance, warranting dose adjustments or increased monitoring. The prolonged half-life informs the weekly dosing regimen when used for malaria prophylaxis. Monitoring for accumulation is critical in long-term use to mitigate adverse effects, particularly retinopathy.

Dosage Forms

Available Dosage Forms:
- Oral tablets: 200 mg hydroxychloroquine sulfate equivalent to 155 mg hydroxychloroquine base per tablet.

Dosages for Adults and Adolescents:
- Malaria Prophylaxis: 400 mg salt (310 mg base) orally once weekly. Weight-based dosing of 6.5 mg/kg salt (5 mg/kg base) once weekly, not exceeding 400 mg salt per dose.
- Malaria Treatment: Initial loading dose of 800 mg salt (620 mg base), followed by 400 mg salt (310 mg base) at 6, 24, and 48 hours post initial dose; total dose 2000 mg salt (1550 mg base).
- Systemic Lupus Erythematosus (SLE): 200 to 400 mg salt (155 to 310 mg base) per day orally divided into 1 or 2 doses. Doses above 400 mg/day not recommended due to toxicity risk.
- Rheumatoid Arthritis: Initial dose 400 to 600 mg salt (310 to 465 mg base)/day, divided in 1 or 2 doses; maintenance dose 200 to 400 mg salt (155 to 310 mg base)/day. Dose adjustment based on therapeutic response; treatment onset can take weeks to months.

Specific Patient Populations:
- Pediatric Malaria Prophylaxis: 6.5 mg/kg salt (5 mg/kg base) orally once weekly, maximum 400 mg salt per dose.
- Pediatric Malaria Treatment: Weight-based dosing following adult proportion, not to exceed adult maximum doses. Initial dose 13 mg/kg salt (10 mg/kg base), followed by 6.5 mg/kg salt (5 mg/kg base) at 6, 24, and 48 hours.
- Pregnant Women: Recommended for malaria treatment in chloroquine-sensitive regions, with caution advised; risks and benefits carefully evaluated.
- Renal Impairment: Dose adjustments and careful monitoring required due to decreased clearance and risk of toxicity.
- Use in lupus and rheumatoid arthritis not approved for individuals under 18 years old.

Clinical guidelines emphasize adherence to dosing schedules to maximize efficacy while monitoring for drug accumulation and toxicity, particularly in chronic conditions like lupus and rheumatoid arthritis.

Mechanism Of Action

Hydroxychloroquine exerts its antimalarial and immunomodulatory effects primarily through interference with lysosomal activity and autophagy in host cells as well as disruption of parasite intracellular processes. In malaria parasites, it accumulates in the acidic food vacuole where it inhibits heme polymerase activity, thereby preventing detoxification of toxic heme derivatives produced during hemoglobin digestion. This accumulation leads to parasite death by toxic heme buildup.

At the cellular level in autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis, hydroxychloroquine increases the pH within intracellular vacuoles, disrupting antigen processing and presentation via MHC class II molecules in antigen-presenting cells. This modulation diminishes T-cell activation and reduces pro-inflammatory cytokine production, including interleukin-1 and tumor necrosis factor-alpha, thus attenuating the autoimmune inflammatory response.

Hydroxychloroquine also interferes with toll-like receptor (TLR) signaling, particularly TLR7 and TLR9, by inhibiting their activation in plasmacytoid dendritic cells. This action reduces the production of interferon-alpha and other cytokines implicated in autoimmune pathogenesis. Furthermore, hydroxychloroquine’s ability to inhibit cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has been postulated, which may contribute to its anti-inflammatory effects.

Comparatively, hydroxychloroquine differs from chloroquine by having a better safety profile and lower toxicity. Research evidence demonstrates hydroxychloroquine's efficacy in reducing flares, improving survival, and decreasing corticosteroid requirements in lupus patients, underscoring its role as a disease-modifying antirheumatic drug. Ongoing studies continue to investigate its detailed molecular interactions and potential antiviral mechanisms.

Pharmacological Actions

Antimalarial activity via inhibition of parasite heme polymerase, leading to toxic heme accumulation and parasite death.
Immunomodulatory effects through alteration of lysosomal pH in antigen-presenting cells, inhibiting antigen processing and T-cell activation.
Inhibition of pro-inflammatory cytokines such as interleukin-1 and tumor necrosis factor-alpha, reducing systemic inflammation.
Suppression of toll-like receptor signaling (TLR7 and TLR9), decreasing interferon-alpha production in autoimmune diseases.
Modulation of autophagic pathways influencing immune cell function and survival.
Disease-modifying properties in systemic lupus erythematosus and rheumatoid arthritis by preventing immune system overactivation.
Partial inhibition of cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway, contributing to anti-inflammatory effects.
Extended half-life supporting weekly dosing for malaria prophylaxis.
Lower toxicity profile compared to chloroquine, with improved safety in chronic use.
Ability to cross cell membranes and accumulate in tissues with melanin, contributing to both efficacy and toxicity.

Therapeutic Or Clinical Uses

Treatment and prophylaxis of malaria caused by Plasmodium species sensitive to hydroxychloroquine, including P. falciparum, P. malariae, P. ovale, and chloroquine-sensitive P. vivax.
Management of systemic lupus erythematosus and discoid lupus erythematosus to reduce disease flares and improve symptoms.
Treatment of acute and chronic rheumatoid arthritis to decrease joint inflammation and slow disease progression.
Used as adjunct therapy in autoimmune disorders to reduce reliance on corticosteroids and other immunosuppressants.
Prophylaxis of malaria in individuals traveling to areas with chloroquine-sensitive malaria strains.
Treatment of uncomplicated malaria cases, especially in pregnancy in chloroquine-sensitive regions.
Reduction of thrombotic events and lipid abnormalities in patients with lupus.
Use in dermatologic manifestations of autoimmune diseases, such as cutaneous lupus and dermatomyositis.
Investigational use for other autoimmune and inflammatory conditions under clinical study.
Concomitant use with an 8-aminoquinoline (e.g., primaquine) for radical cure of P. vivax and P. ovale infections.

Side Or Adverse Effects

Retinopathy leading to irreversible retinal damage, visual disturbances, blurred vision, and potential vision loss, especially with high-dose or long-term use.
Cardiac toxicity including QT interval prolongation, risk of arrhythmias, and potentially fatal cardiomyopathy, exacerbated when combined with other QT-prolonging drugs (e.g., azithromycin).
Gastrointestinal symptoms such as nausea, vomiting, abdominal pain, anorexia, and weight loss.
Central nervous system effects including headache, dizziness, mood changes, seizures, and neuropsychiatric symptoms such as emotional lability and depression.
Dermatologic reactions including rash, pruritus, photosensitivity, and rare severe skin reactions such as Stevens-Johnson syndrome.
Hematologic abnormalities such as anemia, leukopenia, thrombocytopenia, and agranulocytosis.
Hypoglycemia manifesting as headaches, sweating, irritability, and tachycardia.
Muscle weakness, neuromyopathy, and loss of coordination with prolonged use.
Hearing disturbances including tinnitus and hearing loss.
Allergic reactions including urticaria, angioedema, and anaphylaxis.
Low blood cell counts leading to increased susceptibility to infections and bleeding.
Potential irreversible damage to auditory and vestibular function with chronic use.
Hair loss (alopecia) and pigmentation changes.
Elevated liver enzymes and rare hepatotoxicity.
Overdose may cause life-threatening cardiotoxicity, CNS depression, and death.

Precautions

Conduct baseline and periodic ophthalmologic examinations to monitor for retinopathy, especially in patients on long-term therapy or high doses.
Avoid use in patients with known hypersensitivity to hydroxychloroquine, chloroquine, or quinine derivatives.
Use cautiously in patients with preexisting retinal or vision disorders, kidney or liver impairment, to minimize risk of toxicity.
Monitor cardiac function, especially in patients with known heart disease or those receiving concomitant QT-prolonging medications.
Regular blood counts recommended to detect hematologic adverse effects early.
Avoid use in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency unless benefits outweigh risks.
Pregnant patients should be closely monitored; hydroxychloroquine may be used for malaria prophylaxis and treatment during pregnancy when clearly needed.
Use caution in patients with psoriasis or porphyria, which may be exacerbated by hydroxychloroquine.
Avoid concurrent use with antacids or kaolin-pectin preparations within 4 hours of dosing due to decreased absorption.
Patients should be advised to report any vision changes, muscle weakness, or cardiac symptoms promptly.
Not approved for use in pediatric patients with lupus or rheumatoid arthritis under 18 years of age.
Use with caution in patients with alcoholism due to potential for increased toxicity.
Dose adjustments and enhanced monitoring recommended for patients with renal impairment.
Avoid abrupt discontinuation to prevent disease flare in autoimmune conditions.
Educate patients on potential delayed onset of therapeutic effects and necessity of adherence.

Warnings

Risk of irreversible retinal damage and vision loss with prolonged use or high doses; necessitates regular ophthalmologic screening.
May cause serious cardiac adverse effects including arrhythmias and cardiomyopathy; heightened risk with co-administration of QT-prolonging agents (e.g., azithromycin).
Overdose can be fatal; prompt medical attention is critical in suspected cases.
Use in patients with pre-existing heart rhythm disorders (e.g., long QT syndrome) requires extreme caution or avoidance.
May cause severe hypoglycemia, potentially leading to unconsciousness.
Contraindicated in patients with known hypersensitivity to hydroxychloroquine or chloroquine.
Potential severe skin reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported.
Pregnant women are at increased risk of malaria-related complications; risk-benefit assessment necessary for treatment.
Plaquenil therapy may lead to muscle weakness and neuromyopathy, which can be progressive and severe.
Concomitant use with tamoxifen is associated with an increased risk of retinopathy.
May impair ability to perform hazardous activities such as driving due to blurred vision and dizziness.
Long-term use necessitates regular monitoring to detect toxicities early.
Drug accumulation may occur due to long elimination half-life; dose adjustments should be carefully managed.
Discontinue drug immediately if signs of visual disturbances or cardiac arrhythmias develop.
Monitoring and patient education regarding symptom recognition are imperative during therapy.

Contraindication

Hypersensitivity or allergy to hydroxychloroquine, chloroquine, or any component of the formulation.
Known retinopathy or pre-existing retinal damage from hydroxychloroquine or related drugs.
Patients with pre-existing long QT syndrome or other significant cardiac conduction abnormalities.
Children under 18 years of age for treatment of lupus or rheumatoid arthritis, as safety and efficacy have not been established.
Concurrent use with other drugs known to prolong QT interval without medical supervision.
History of severe drug-induced hypersensitivity reactions to antimalarial quinolines.
Patients with glucose-6-phosphate dehydrogenase deficiency for malaria treatment without careful risk assessment.
Severe hepatic or renal impairment where drug clearance would be significantly compromised without adequate monitoring.
Known psoriasis exacerbated by antimalarial therapy.
Use during breastfeeding unless specifically advised by a healthcare provider considering risks and benefits.
Use in patients with porphyria without careful supervision and consideration.
Avoid use in combination with tamoxifen due to increased risk of retinal toxicity.

Drug Interactions

Azithromycin: Increases risk of QT interval prolongation and potentially fatal cardiac arrhythmias when combined with hydroxychloroquine.
Cimetidine: May increase hydroxychloroquine plasma levels by inhibiting its metabolism, necessitating monitoring.
Cyclosporine: Potential for increased cyclosporine toxicity; monitoring required when co-administered.
Methotrexate: Combined use may enhance immunosuppressive effects and toxicity; close monitoring recommended.
Rifampicin: Induces hepatic metabolism and may decrease hydroxychloroquine plasma concentrations, reducing efficacy.
Praziquantel: Possible interaction altering praziquantel metabolism; use with caution.
Ampicillin: Potential pharmacokinetic interactions; clinical relevance unclear, monitor therapy.
Digoxin: Hydroxychloroquine may increase digoxin levels, increasing risk of digoxin toxicity.
Tamoxifen: Combined use increases risk of irreversible retinopathy and visual disturbances.
Heart rhythm medications (e.g., quinidine, procainamide): Additive QT prolongation effects increasing risk of torsades de pointes.
Other QT-prolonging agents including certain antidepressants and antipsychotics, heightening arrhythmia risks.
Antacids and kaolin-pectin: May reduce absorption of hydroxychloroquine if taken simultaneously; separate doses by at least 4 hours.
CYP3A4 inhibitors (e.g., ketoconazole, ritonavir): May increase hydroxychloroquine plasma concentrations; monitor for toxicity.
CYP3A4 inducers (e.g., carbamazepine, phenytoin): May reduce hydroxychloroquine efficacy by increasing metabolism.
Vaccines: May have reduced humoral response when used concomitantly; consult guidelines for immunization timing.

Preparations Of The Drug

Plaquenil tablets: Oral tablets containing 200 mg hydroxychloroquine sulfate (equivalent to 155 mg base) recommended for oral administration.
Sovuna: A related brand formulation of hydroxychloroquine sulfate tablets for malaria and autoimmune disease treatment.
Available as generic hydroxychloroquine sulfate tablets for the same indications and dosing regimens.
No parenteral or topical formulations routinely available; use limited to oral administration.
Formulated to be taken once daily or weekly depending on indication, with meals for improved gastrointestinal tolerability.
Dispensed in various package sizes suitable for short-term malaria treatment or chronic autoimmune disease therapy.
Shelf-stable at room temperature, protected from moisture, heat, and light.
Not available as chewable tablets, suspensions, or pediatric liquid formulations; dosing in children based on tablet fractioning and weight-based calculation.
Approved for prophylactic and treatment indications with detailed prescribing information available for each indication.
Accompanied by comprehensive medication guides to assist patients in safe administration and adherence.
Commercial preparations undergo quality control to ensure uniform hydroxychloroquine content and bioavailability.
May be supplied with patient information leaflets detailing precautions, side effect monitoring, and drug interactions.