What is Hydroxychloroquine and What Does It Do in the Body
Hydroxychloroquine (HCQ) is a 4-aminoquinoline antimalarial and immunomodulatory drug that is FDA-approved for treating malaria, rheumatoid arthritis, systemic lupus erythematosus (SLE), and chronic discoid lupus erythematosus 1.
Mechanism of Action
In Malaria
HCQ concentrates in the acid vesicles of the Plasmodium parasite and inhibits heme polymerization, while also interfering with parasite DNA 1. It is active against erythrocytic forms of chloroquine-sensitive strains but does not affect gametocytes or liver-stage forms 1.
In Rheumatic Diseases
The exact mechanisms underlying HCQ's anti-inflammatory and immunomodulatory effects are not fully known 1. However, the drug works through multiple pathways:
- Interferes with lysosomal activity and autophagy
- Disrupts antigen presentation by dendritic cells
- Inhibits toll-like receptor signaling
- Reduces transcription of pro-inflammatory cytokines
- Modulates co-stimulatory molecules on immune cells 2
Pharmacokinetics
HCQ has unique pharmacologic properties that are critical to understand:
- Absorption: 74% bioavailability with peak levels at 3-4 hours 1
- Distribution: Extensively distributed to tissues with a very large volume of distribution 1
- Half-life: Extraordinarily long at 40-50 days with chronic use (up to 123.5 days in plasma) 1
- Excretion: 16-30% cleared unchanged by the kidneys 1
- Metabolism: Metabolized to desethylhydroxychloroquine (DHCQ), the major metabolite 1
This extremely long half-life means drug levels remain detectable for 3 months after stopping, and steady-state is only reached after many weeks of dosing 1.
Critical Precautions
1. Retinal Toxicity (Most Important Long-Term Risk)
The most critical precaution is irreversible retinal toxicity, which increases dramatically with doses >5.0 mg/kg/day based on actual body weight 3, 4.
Key dosing principles:
- Keep dose ≤5.0 mg/kg/day based on actual body weight (not ideal body weight, which causes overdosing in thin patients) 3, 4
- At ≤5.0 mg/kg/day, retinopathy develops in <2% of patients within 10 years 3
- Doses >5.0 mg/kg dramatically increase both population risk and annual incremental risk 4
- For chloroquine, the equivalent safe dose is ≤2.3 mg/kg/day 3, 4
Screening requirements:
- Baseline ophthalmologic examination before starting or within first year 3
- Annual screening starting after 5 years of use for low-risk patients 3, 4
- Earlier and more frequent screening for high-risk patients (renal disease, doses >5.0 mg/kg/day, concurrent tamoxifen use, pre-existing retinal disease) 4
- Use spectral-domain optical coherence tomography (SD-OCT) as primary screening tool 4
Warning signs patients must report immediately:
- Decreased vision
- Difficulty with dark adaptation
- Visual field changes 1
2. Cardiac Toxicity
Serious and potentially fatal cardiac effects have been reported 1:
- Cardiomyopathy (can be irreversible)
- QRS and QTc prolongation
- Ventricular arrhythmias including torsade de pointes
- Cardiac arrest
Patients must seek immediate medical attention for:
- Fast or irregular heartbeat
- Lightheadedness or dizziness
- Syncope 1
3. Renal Disease
Renal impairment is a major risk factor because HCQ is 16-30% renally cleared 1. Patients with renal disease:
- Have unpredictably high blood drug levels 4
- Require dose reduction 1
- Need more frequent screening 4
- Are at significantly increased risk of retinopathy 4
This is particularly important in SLE patients, where renal involvement is common 4.
4. Hepatic Disease
Dose reduction is necessary in patients with hepatic impairment 1.
5. Severe Skin Reactions
Life-threatening skin reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported 1. Patients should seek immediate care for:
- Blisters on skin, eyes, lips, or mouth
- Itching or burning with or without fever 1
6. Skeletal Muscle Toxicity
Muscle weakness, atrophy, and myopathy can occur 1. Patients should report any muscle weakness promptly 1.
7. Neuropsychiatric Effects
New or worsening depression, suicidal thoughts, or mood changes require immediate medical attention 1.
8. Overdose
HCQ overdose is life-threatening with onset within 1-3 hours 1:
- Severe hypotension is common
- Cardiac toxicity (arrhythmias, cardiac arrest)
- Severe hypokalemia from intracellular shift
- CNS depression, seizures, coma
- Hemodialysis and hemoperfusion are NOT effective 1
- Contact poison control (1-800-221-2222) immediately 1
9. Special Populations
Geriatric patients:
- Start with lowest recommended dose
- Higher risk due to decreased renal function 1
Pregnancy and lactation:
- Generally considered safe at doses ≤400 mg/day
- No consistent increase in congenital malformations
- Reduces SLE flares and preeclampsia risk
- Compatible with breastfeeding 5
Administration
Take with food or milk to reduce gastrointestinal side effects 1. Do not crush or divide tablets 1.
Common Pitfalls to Avoid
Dosing by ideal body weight instead of actual body weight - This causes overdosing in thin patients and increases retinopathy risk 3, 4
Failing to adjust dose in renal disease - These patients have unpredictably high drug levels 4, 1
Stopping screening after 5 years - Risk continues to increase with duration, especially after decades of use 4
Stopping HCQ for borderline ophthalmologic findings - The goal is to safely maintain this valuable medication as long as possible 3
Not recognizing that blood levels are unreliable - Blood HCQ levels do not reliably predict efficacy or toxicity 4