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Chloroquine - Mechanism of Action, Uses & Side Effects

Overview

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Mechanism of Action

1. Hemoglobin digestion - The Plasmodium parasite digests the host red blood cell's hemoglobin inside its acidic food vacuole to obtain essential amino acids.
2. Toxic heme release - This digestion releases large amounts of free heme (ferriprotoporphyrin IX), which is toxic to the parasite itself.
3. Normal detoxification - To protect itself, the parasite normally polymerizes free heme into an insoluble, chemically inert pigment called hemozoin, which is safely sequestered in the food vacuole.
4. Chloroquine blocks this - Chloroquine, being a diprotic weak base, is concentrated in the acidic food vacuole by ion trapping. There, it specifically binds to free heme and prevents its polymerization into hemozoin. The resulting accumulation of toxic heme causes oxidative damage to parasite membranes, leading to lysis of both the parasite and the red blood cell.

The exact molecular details are not fully elucidated, but the heme-polymerization blockade is the primary accepted mechanism.

• Lippincott Illustrated Reviews: Pharmacology, p. 1171-1172
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, p. (block 18)

Immunomodulatory Mechanism (for non-malarial uses)

• Toll-like receptor (TLR) signaling
• Antigen processing and presentation
• Pro-inflammatory cytokine production (TNF-α, IL-1, IL-6)

Resistance Mechanism

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Pharmacokinetics

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Clinical Uses

1. Malaria

• Treatment of uncomplicated malaria caused by P. vivax, P. ovale, P. malariae, P. knowlesi, and chloroquine-sensitive strains of P. falciparum
• Drug of choice for P. ovale and P. malariae infections
• Prophylaxis for travelers to areas where chloroquine-sensitive malaria is still prevalent (some parts of Central America, Haiti, the Middle East)
• Standard prophylactic dose: 300 mg base (500 mg salt) orally, once weekly, beginning 1-2 weeks before travel and continued for 4 weeks after leaving the endemic area
• Note: For P. falciparum, artemisinin-based combination therapies (ACTs) have largely replaced chloroquine due to widespread resistance

2. Extraintestinal (Hepatic) Amebiasis

• Used as an adjunct for amoebic liver abscess when metronidazole is insufficient or not tolerated

3. Autoimmune / Rheumatic Diseases (mainly hydroxychloroquine)

• Rheumatoid arthritis
• Systemic Lupus Erythematosus (SLE)
• Discoid lupus
• Sjogren's syndrome (Hydroxychloroquine is generally preferred due to a safer side-effect profile)

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Side Effects

At Prophylactic / Low Doses (generally well tolerated)

• GI upset: nausea, vomiting, abdominal discomfort
• Pruritus - common, especially in dark-skinned individuals
• Headache
• Blurred vision / visual disturbances
• Skin rash / urticaria

With Prolonged / High-Dose Use

Acute Overdose / Toxicity

• Rapid parenteral administration is especially dangerous
• Features: hypotension, vasodilation, cardiac arrhythmias, cardiac arrest
• Neurological: confusion, convulsions, coma
• Parenteral doses >5 g are usually fatal
• Management: mechanical ventilation + epinephrine + diazepam

Contraindications / Cautions

• Psoriasis - may provoke an acute attack
• Porphyria - may trigger an acute attack
• Hepatic dysfunction
• Severe GI disease
• Neurological disorders
• Pregnancy: crosses placenta (use with caution; still used when benefits outweigh risks for malaria)

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Summary Table

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Chloroquine - Mechanism of Action, Uses & Side Effects

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1. Mechanism of Action

A. Antimalarial Mechanism (Primary)

Chloroquine also raises the vacuolar pH, amplifying heme toxicity and disrupting parasite metabolism.

B. Immunomodulatory Mechanism (for autoimmune uses)

• Inhibits antigen processing and presentation - macrophages and monocytes require an acidic pH for protein digestion and antigen processing; raising the pH disrupts this
• Disrupts MHC class II receptor assembly - stabilizes MHC molecules with invariant chains at higher pH, preventing displacement by autoantigens and downregulating antigen presentation
• Blocks endosomal Toll-like receptors (TLR3, 7, 8, 9) - preventing recognition of self-nucleic acids and suppressing type-I interferon production (particularly relevant in SLE)
• Reduces pro-inflammatory cytokines (TNF-α, IL-1, IL-6) from macrophages
• Stabilizes lysosomal membranes and decreases circulating immune complexes

C. Resistance Mechanism

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2. Pharmacokinetics

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3. Clinical Uses

Malaria

• Treatment of uncomplicated malaria due to P. vivax, P. ovale, P. malariae, P. knowlesi, and chloroquine-sensitive P. falciparum
• Drug of choice for P. ovale and P. malariae
• Chemoprophylaxis for travelers to areas with proven chloroquine-sensitive malaria (parts of Central America, Haiti, the Middle East)
  • Dose: 300 mg base (500 mg salt) orally once weekly, starting 1-2 weeks before travel, continuing 4 weeks after return
• Note: For P. falciparum, artemisinin-based combination therapies (ACTs) have largely replaced chloroquine due to near-universal resistance

Non-malarial Uses

• Hepatic (extraintestinal) amebiasis - used as adjunct or when metronidazole is insufficient
• Rheumatoid arthritis - disease-modifying (DMARD) via immunomodulatory mechanism (hydroxychloroquine preferred)
• Systemic Lupus Erythematosus (SLE) - reduces flares, protects against organ damage
• Discoid lupus erythematosus
• Sjogren's syndrome

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4. Adverse Effects

At Low / Prophylactic Doses (generally mild)

• GI upset: nausea, vomiting, abdominal pain, diarrhea
• Pruritus - common, especially in dark-skinned individuals
• Headache
• Blurred vision / visual disturbances
• Urticaria / skin rash

With Prolonged or High-Dose Use

Acute Overdose / Toxicity

• Cardiovascular: Hypotension, vasodilation, suppressed myocardial function, arrhythmias, cardiac arrest
• CNS: Confusion, convulsions, coma
• Management: Mechanical ventilation + epinephrine + diazepam

Contraindications / Cautions

• Psoriasis - may provoke acute exacerbation
• Porphyria - may trigger acute attack
• Hepatic dysfunction (use with caution)
• Severe GI or neurological disorders
• G6PD deficiency (risk of hemolysis)
• Avoid concurrent QT-prolonging drugs

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Quick Reference Summary

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Treatment of Severe and Complicated Falciparum Malaria

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Criteria for Severe/Complicated Falciparum Malaria

Note: Microscopy may be negative in severe infection due to sequestration. A strongly suspected case should be treated immediately without waiting for confirmation.

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General Principles

1. Admit to ICU - severe malaria requires intensive nursing care and close monitoring
2. Start parenteral antimalarial therapy immediately - do not delay for lab confirmation
3. Identify and treat all complications simultaneously
4. Switch to oral therapy as soon as the patient can tolerate it (after minimum 24 h parenteral treatment)
5. In endemic areas, also start broad-spectrum antibiotics empirically because bacterial co-infection/sepsis cannot be excluded clinically

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Specific Antimalarial Treatment

First-Line: IV Artesunate (Drug of Choice)

• 35% lower mortality than quinine in Southeast Asian adults (SEAQUAMAT trial)
• 22.5% reduction in mortality vs. quinine in African children (AQUAMAT trial)

• 2.4 mg/kg IV (or IM) on admission (time = 0)
• Then at 12 h and 24 h
• Then once daily until patient can take oral medication

• No cardiac toxicity (does not cause QT prolongation)
• No hypoglycemia
• Kills all erythrocytic stages (early ring forms to late trophozoites)
• Much faster parasite clearance (significant reduction within 12 h vs. 24-48 h with quinine)

• In children weighing <20 kg: dose is 3 mg/kg (higher weight-adjusted dose)
• In artemisinin-resistant areas (parts of Southeast Asia, East Africa): give artesunate plus quinine together at full doses
• Pre-referral rectal artesunate (10 mg/kg) can be given as a bridging measure in rural areas before IV access is available - shown to reduce mortality in children

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Second-Line: Parenteral Quinine (if artesunate unavailable)

• Loading dose: 20 mg of quinine salt/kg by slow IV infusion over 4 hours (or divided IM injection)
• Maintenance dose: 10 mg/kg 8-hourly (infused over 2-8 h); infusion rate must not exceed 5 mg/kg/hour
• Do NOT give the loading dose if the patient has already received quinine or mefloquine in the previous 24 hours

• Hypoglycemia (stimulates insulin secretion) - check blood glucose frequently
• QT prolongation / cardiac arrhythmias
• Cinchonism: tinnitus, high-tone hearing loss, nausea, headache

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Third Option: Artemether (IM)

• 3.2 mg/kg IM stat on admission, then 1.6 mg/kg/day IM
• Oil-based formulation; absorption is more erratic than artesunate
• Does not confer the same survival benefit as IV artesunate
• Used when IV access is unavailable and artesunate is not on hand

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Step-Down to Oral Therapy

• Full 3-day course of oral ACT (artemisinin-based combination therapy)
  • Artemether-lumefantrine (AL) - preferred in first trimester pregnancy
  • Artesunate-amodiaquine
  • Dihydroartemisinin-piperaquine (DHA-PPQ)
  • Artesunate-mefloquine (in Southeast Asia)
• Add single-dose primaquine 0.25 mg/kg as gametocytocide (to block transmission) - safe even in G6PD deficiency at this low dose; contraindicated in pregnancy

• Quinine 10 mg/kg TDS + Doxycycline 100 mg OD (or 3 mg/kg in children) for 7 days total (including parenteral days)
• In pregnant women and children <8 years: replace doxycycline with Clindamycin 10 mg/kg 12-hourly
• Alternatively, complete with area-specific ACT

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Summary Treatment Algorithm

SEVERE FALCIPARUM MALARIA SUSPECTED
            ↓
Confirm with RDT / thick blood smear (do NOT delay treatment)
            ↓
IV ARTESUNATE 2.4 mg/kg at 0h, 12h, 24h → then OD
  [If unavailable: IV Quinine 20 mg/kg loading → 10 mg/kg 8-hrly]
            ↓
Minimum 24 h parenteral treatment
            ↓
Patient tolerating oral medications?
  YES → Full 3-day oral ACT course + single-dose primaquine
  NO  → Continue parenteral up to maximum 7 days

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Supportive Management of Complications

Treatments Proven Ineffective or Harmful (DO NOT USE):

• High-dose glucocorticoids (steroids)
• Urea
• Heparin
• Dextran
• Desferrioxamine
• Anti-TNF-α antibodies
• High-dose phenobarbital (20 mg/kg)
• Large-volume albumin boluses
• Routine exchange transfusion

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Special Populations

Pregnancy

• Artemisinin derivatives are safe in all trimesters and preferred over quinine
• Quinine increases the risk of hypoglycaemia in pregnancy and should be avoided if possible
• Do NOT give primaquine in pregnancy
• Artemether-lumefantrine is the preferred ACT in first trimester

Children

• Artesunate dose: 3 mg/kg for children <20 kg
• Clindamycin replaces doxycycline (doxycycline contraindicated <8 years)
• Pre-referral rectal artesunate is particularly valuable

Artemisinin Resistance (Southeast Asia / East Africa)

• Parasites show slow clearance (half-life >5 h; clearance >3 days)
• Give artesunate + quinine together at full doses
• Triple ACT combinations (e.g., artesunate + mefloquine + piperaquine) under evaluation

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Pharmacotherapy of Immunosuppression in Organ Transplantation

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General Principles of Transplant Immunosuppression

1. HLA matching - Select the best ABO blood type-compatible, HLA-matched donor
2. Multitier therapy - Use several agents simultaneously, each at a different molecular target. Synergistic effects allow lower doses of each drug, limiting toxicity while maximizing efficacy
3. Induction + Maintenance - Greater immunosuppression is needed early (induction) to prevent acute rejection; maintenance doses are then reduced over time as the risk of rejection falls and drug toxicity risks rise
4. Investigate graft dysfunction - Every episode must be evaluated for rejection, drug toxicity, recurrence of original disease, or infection (these can coexist)
5. Dose reduction - Reduce or withdraw a drug when its toxicity exceeds its benefit

Sites of Action (Overview)

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Phase 1: Induction Therapy

A. Depleting Agents

1. Antithymocyte Globulin (ATG / Thymoglobulin) - Most Widely Used

• Polyclonal antibody (rabbit or equine) against human thymocytes
• Depletes CD3+ T cells from circulation
• Two FDA-approved preparations:
  • Lymphocyte Immune Globulin (Atgam) - equine derived
  • Antithymocyte Globulin (Thymoglobulin) - rabbit derived (more commonly used)
• Side effects: fever, chills, cytokine release syndrome; give glucocorticoids to blunt first-dose reactions; increased risk of infection and post-transplant lymphoproliferative disorder (PTLD)

2. Alemtuzumab (Campath-1H) - off-label in transplantation

• Humanized anti-CD52 monoclonal antibody
• CD52 is expressed widely on B cells, T cells, macrophages, and NK cells
• Produces prolonged lymphocyte depletion
• FDA-approved for CLL and MS; increasingly used off-label as induction in kidney, pancreas, and other organ transplants

3. Muromonab-CD3 (OKT3) - largely historical

• Murine anti-CD3 monoclonal antibody that blocks the T-cell receptor complex
• First-dose cytokine release syndrome is a major adverse effect
• Discontinued in the US; replaced by better-tolerated agents

B. Immune Modulators (Non-depleting)

Anti-IL-2 Receptor Monoclonal Antibodies

• Basiliximab (chimeric - 33% mouse/67% human)
• Daclizumab (humanized - withdrawn from market in some regions)
• Well tolerated; no cytokine storm; used in low-to-moderate immunological risk

C. For Antibody-Mediated (Humoral) Rejection

• Plasmapheresis - removes circulating anti-HLA antibodies
• Intravenous Immunoglobulin (IVIG) - immunomodulatory
• Rituximab - chimeric anti-CD20 mAb; depletes B lymphocytes

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Phase 2: Maintenance Therapy

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I. Calcineurin Inhibitors (CNIs) - BACKBONE of Maintenance

Mechanism (both drugs share the same final pathway):

1. TCR activation → rise in intracellular Ca²⁺
2. Ca²⁺ activates calcineurin (a phosphatase)
3. Calcineurin dephosphorylates NFAT (Nuclear Factor of Activated T-cells)
4. Dephosphorylated NFAT translocates to nucleus → activates transcription of IL-2, IL-4, and other lymphokines
5. IL-2 drives T-cell proliferation and activation of cytotoxic T lymphocytes

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A. Tacrolimus (FK506) - PREFERRED CNI in most centers

• Nephrotoxicity (acute and chronic interstitial fibrosis)
• Neurotoxicity: tremor, headache, insomnia, seizures
• Diabetogenicity - new-onset diabetes after transplant (NODAT)
• Hypertension, hyperkalemia
• Does NOT cause gingival hyperplasia or hirsutism (unlike cyclosporine)
• Alopecia and diarrhea (especially with MMF)

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B. Cyclosporine (Cyclosporin A) - older CNI, still used

• Nephrotoxicity - dose-limiting; acute (reversible vasoconstriction) and chronic (irreversible striped interstitial fibrosis)
• Hirsutism (hypertrichosis)
• Gingival hyperplasia
• Hypertension
• Hyperlipidemia (raises LDL and cholesterol)
• Hyperuricemia / gout
• Hepatotoxicity
• Neurotoxicity

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II. Antiproliferative / Antimetabolic Agents

A. Mycophenolate Mofetil (MMF) - PREFERRED antiproliferative

• GI toxicity: nausea, vomiting, diarrhea, abdominal pain (dose-limiting)
• Bone marrow suppression: leukopenia, anemia, thrombocytopenia
• Increased susceptibility to infections (especially CMV)
• Increased risk of malignancy (especially skin cancer)
• Teratogenic - strict contraception required

B. Azathioprine - older alternative to MMF

• Bone marrow suppression - leukopenia (most common), thrombocytopenia, anemia
• Increased infection risk (especially VZV, HSV)
• Hepatotoxicity
• Alopecia, GI toxicity, pancreatitis
• Increased risk of malignancy

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III. mTOR Inhibitors (Proliferation Signal Inhibitors)

A. Sirolimus (Rapamycin)

• Prophylaxis of organ rejection (usually combined with reduced-dose CNI + glucocorticoids)
• CNI-sparing regimens to preserve renal function in patients with CNI nephrotoxicity
• Drug-eluting stents (inhibits local intimal proliferation)
• LT (liver transplant) patients with HCC to reduce tumor recurrence

• Hyperlipidemia (dose-dependent - cholesterol + triglycerides) - requires treatment
• NOT directly nephrotoxic - but worsens renal function when combined with cyclosporine
• Impaired wound healing (avoid in early post-op period)
• Lymphocele (increased in renal transplant)
• Oral ulcers / stomatitis, mucositis
• Pneumonitis (interstitial lung disease)
• Anemia, leukopenia, thrombocytopenia
• Proteinuria (avoid if GFR <30% or existing proteinuria)

B. Everolimus

• Hydroxylated derivative of sirolimus; same mechanism via FKBP-12 → mTOR inhibition
• Better oral bioavailability than sirolimus
• Approved for kidney and liver transplantation
• Similar adverse effect profile to sirolimus

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IV. Glucocorticoids

• Bind intracellular glucocorticoid receptors → regulate gene transcription
• Curtail NF-κB activation
• Suppress pro-inflammatory cytokines: IL-1, IL-6
• Inhibit T cells from producing IL-2 and proliferating
• Inhibit cytotoxic T lymphocyte activation
• Reduce chemotaxis and lysosomal enzyme release from neutrophils/monocytes
• Broad anti-inflammatory effects - relatively little effect on humoral immunity

• Part of maintenance triple therapy (at low dose)
• High-dose "pulse" steroids (methylprednisolone IV) to treat established acute rejection
• Pretreat before ATG infusion to blunt cytokine release syndrome
• Early steroid withdrawal protocols are increasingly used (especially in pediatric patients) to reduce long-term morbidity

• Growth retardation in children
• Avascular necrosis of bone
• Osteoporosis / osteopenia
• Hyperglycemia / diabetes
• Hypertension
• Cataracts
• Poor wound healing, increased infection risk
• Cushingoid features

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V. Belatacept - Costimulation Blockade

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Phase 3: Treatment of Established Rejection

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Standard Triple Maintenance Regimen

Tacrolimus (CNI)
    +
Mycophenolate Mofetil (antiproliferative)
    +
Prednisolone (glucocorticoid)

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Complications of Long-term Immunosuppression

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