Dapson: Effect in Leprosy and Skin Diseases

Dapson (diaminodiphenylsulfone, brand names Dapson Fatol and generics) is a sulfone antibiotic with additional anti-inflammatory effects. In Germany, dapson is established in several indications: as a component of multidrug therapy for leprosy, in the treatment of dermatitis herpetiformis Duhring, as a reserve agent for Pneumocystis pneumonia, and in some inflammatory skin diseases with neutrophilic components.

In clinical practice, dapson is predominantly prescribed by dermatologists and infectious disease specialists. The substance has a challenging safety profile: hemolytic anemia, methemoglobinemia, and rare but severe hypersensitivity reactions such as DRESS syndrome or Stevens Johnson syndrome require careful patient selection, G6PD deficiency screening before starting therapy, and close monitoring.

Mechanism of Action

Dapson is structurally related to sulfonamides and inhibits the bacterial enzyme dihydropteroate synthase. This blocks folate synthesis in bacteria and stops the growth of especially Mycobacterium leprae and Pneumocystis jirovecii. Clinically, dapson acts bacteriostatically.

In addition to its antibacterial effect, dapson has an anti-inflammatory component that is used in dermatology. It inhibits the function of neutrophilic granulocytes, reduces their migration, and suppresses the release of myeloperoxidase and leukotrienes. These effects explain its efficacy in diseases with neutrophilic inflammation such as dermatitis herpetiformis, cutaneous lupus erythematosus, or pyoderma gangraenosum.

Pharmacokinetically, dapson shows high oral bioavailability of over 90 percent. The half-life is 20 to 30 hours, allowing once or twice daily administration. Metabolism occurs predominantly in the liver through N-acetylation and CYP3A4-mediated N-hydroxylation. The metabolite hydroxylamine is responsible for methemoglobin formation and hemolytic effects.

Indications

• Leprosy as a component of WHO multidrug therapy together with rifampicin and clofazimine
• Dermatitis herpetiformis Duhring, a bullous skin disease associated with celiac disease, with dramatic symptom improvement
• Pneumocystosis (PCP) for prophylaxis or treatment in HIV-positive patients and in case of trimethoprim-sulfamethoxazole intolerance
• Cutaneous lupus erythematosus when other therapies fail
• Pyoderma gangraenosum, bullous pemphigoid, linear IgA dermatosis, and other neutrophilic dermatoses
• Acne vulgaris in topical form (5 or 7.5 percent gel) in moderate inflammatory acne
• Off-label for toxoplasmosis prophylaxis in HIV-positive patients

Dapson is not first-line for simple bacterial infections. For sulfonamide-sensitive pathogens, other substances are better suited. Self-medication is not available.

Dosage and Administration

Leprosy: 100 mg daily orally, in combination with rifampicin 600 mg monthly (supervised) and clofazimine according to WHO regimen.

Dermatitis herpetiformis: Start with 50 mg per day, increase to 100 to 200 mg per day depending on effect. A gluten-free diet reduces requirements long-term.

Pneumocystosis prophylaxis: 100 mg per day or 50 mg twice daily. Treatment of manifest pneumocystosis 100 mg per day in combination with trimethoprim or pyrimethamine.

Acne topical: 5 or 7.5 percent gel once or twice daily on affected skin areas.

Oral administration: with or without food, adequate water. Swallow tablets whole.

Renal insufficiency: with eGFR below 60 ml per minute, caution and close observation are needed due to cumulative toxicity. Hepatic insufficiency: dose adjustment required in severe impairment.

Duration of therapy: in leprosy several months to years according to WHO regimen, in dermatitis herpetiformis long-term (often years), in pneumocystosis prophylaxis as long as immunosuppression persists.

Side Effects

Very common: hemolytic anemia with fatigue, pallor, and shortness of breath, especially at higher doses. Methemoglobinemia with cyanosis, headaches, dizziness, and exertional dyspnea.

Common: headaches, nausea, vomiting, dizziness, rash, pruritus, peripheral neuropathy with long-term therapy.

Rare but relevant: dapson hypersensitivity syndrome with fever, rash, eosinophilia, and organ involvement (liver, kidney, lung), often 4 to 8 weeks after starting therapy. DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms) and Stevens Johnson syndrome as very rare but life-threatening reactions.

G6PD deficiency: in glucose-6-phosphate dehydrogenase deficiency there is a significantly increased risk of severe hemolytic crises. G6PD testing is mandatory before therapy, in case of deficiency dapson should only be used after strict indication.

With long-term therapy: regular monitoring of blood count, methemoglobin, liver enzymes, and renal function.

With topical application: skin dryness, erythema, pruritus, temporary skin irritation. Systemic effects are very rare.

Drug Interactions

• Trimethoprim: synergistic effect in pneumocystosis, concurrent use is established. Risk of hyperkalemia and methemoglobinemia additive.
• Rifampicin: strong CYP3A4 induction, significantly reduces dapson levels, higher doses may be necessary.
• Probenecid: inhibits renal secretion and increases levels.
• Other methemoglobin-forming substances (phenytoin, local anesthetics, nitrates): additive methemoglobinemia, caution advised.
• Other myelotoxic agents (methotrexate, azathioprine): additive bone marrow suppression.
• Antiretroviral therapy: in HIV patients careful therapy planning in specialized centers, especially with protease inhibitor comedication.
• Live vaccines: caution when used as an immunomodulator (off-label).

Special Notes

Pregnancy: can be used in leprosy and dermatitis herpetiformis with clear indication. Caution in third trimester due to methemoglobinemia in newborns, folic acid supplementation. Breastfeeding: passage into breast milk, hemolysis in infants possible, especially with G6PD deficiency in the child. Use must be decided individually.

Children: possible in pediatric indications for leprosy and dermatitis herpetiformis, weight-adjusted.

G6PD test: mandatory before any therapy. In case of deficiency, alternative therapies should be preferred, in exceptional cases low dose with close hemolysis monitoring.

HLA typing: in patients of Asian origin, HLA B 13:01 testing can estimate the risk of dapson hypersensitivity syndrome.

Before starting therapy: complete blood count, differential blood count, methemoglobin, bilirubin, liver values, kidney values, G6PD status, if applicable HLA B 13:01.

Monitoring: weekly blood count checks during dose escalation, later monthly. If signs of hypersensitivity syndrome (fever, rash, eosinophilia) occur, discontinue immediately.

Lifestyle in dermatitis herpetiformis: consistent gluten-free diet reduces symptoms and long-term required dapson dose. Nutritional counseling is essential.

Fitness to drive: limited if dizziness or signs of anemia occur, individual assessment needed.

You might also be interested in

• Rifampicin, component of leprosy multidrug therapy
• Methotrexate, alternative immunosuppressant in bullous dermatoses
• Nicotinamide, in combination for bullous pemphigoid
• Doxycycline, tetracycline in combination for dermatological indications
• Adapalene, topical retinoid for acne

Frequently Asked Questions

Sources

• Gelbe Liste, Dapson active ingredient profile
• BfArM, Federal Institute for Drugs and Medical Devices
• WHO, Guidelines for treatment of leprosy
• AWMF, Guidelines for dermatitis herpetiformis and pneumocystosis