1. Clinical Overview
Dithranol (Anthralin) is a synthetic derivative of chrysarobin, used topically as a first-line treatment for chronic, stable plaque psoriasis. It is a potent reducing agent and antimitotic agent that normalizes keratinocyte differentiation and reduces hyperproliferation. The 1.15% w/w concentration is considered a high-strength preparation, typically used for short-contact therapy in a hospital or supervised outpatient setting due to its high potential for skin irritation and staining.
| Onset | Duration | Bioavailability |
|---|---|---|
| Clinical improvement is typically observed within 1-2 weeks of consistent application. | The therapeutic effect persists for several weeks to months after a course of treatment, depending on disease severity and maintenance therapy. | Topical bioavailability is negligible (<1%) when applied to intact psoriatic plaques, as it is minimally absorbed systemically. |
2. Mechanism of Action
Dithranol's exact mechanism is multifactorial. It inhibits mitochondrial respiration and oxidative phosphorylation, leading to reduced ATP production in hyperproliferative keratinocytes. It generates reactive oxygen species (ROS) which contribute to its antiproliferative and anti-inflammatory effects. It also modulates immune responses by inhibiting neutrophil and monocyte chemotaxis and reducing the production of pro-inflammatory cytokines like IL-6 and IL-8.
3. Indications & Uses
- Chronic, Stable Plaque Psoriasis (Psoriasis Vulgaris)
- Scalp Psoriasis (in specific formulations)
4. Dosage & Administration
Adult Dosage: Apply a thin layer once daily. For 1.15% strength, use SHORT-CONTACT THERAPY (SCT): Apply precisely to psoriatic plaques, leave on for 10-30 minutes (as tolerated and prescribed), then wash off thoroughly with lukewarm water. Duration of contact is titrated based on tolerance.
Administration: 1. Wear gloves or use an applicator. 2. Apply strictly to the plaque, avoiding surrounding healthy skin. 3. Petroleum jelly can be applied to the normal skin around the plaque for protection. 4. After the prescribed contact time, wash off completely. 5. Wash hands thoroughly after application. 6. Stains on skin are temporary but can persist for days.
5. Side Effects
Common side effects may include:
- Staining of skin (brown/purple discoloration)
- Staining of hair, fabrics, and bathroom surfaces
- Localized irritation, burning, or stinging sensation
- Erythema (redness) of treated plaques and surrounding skin
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Other Topical Irritants (e.g., Salicylic Acid, Tazarotene, High-potency Steroids) | Increased risk of severe skin irritation, burning, and dermatitis. | Major |
| UV Therapy (PUVA, NB-UVB) | Dithranol can be photosensitizing. Concurrent use may increase risk of severe burns. Usually used in sequential regimens (e.g., Goeckerman regimen: UVB + Dithranol) under strict supervision. | Major |
| Topical Coal Tar | May be used in combination regimens (e.g., Ingram regimen) but requires careful monitoring for excessive irritation. | Moderate |
| Methotrexate, Ciclosporin (Systemic) | No direct pharmacokinetic interaction. However, used as sequential or rotational therapy for psoriasis management. | None |
7. Patient Counselling
- DO apply only to the thick, scaly plaques.
- DO use the exact contact time prescribed by your doctor.
- DO wash hands thoroughly after application (even if using gloves).
- DO protect surrounding normal skin with petroleum jelly.
- DO wear old clothing and use old bed linens during treatment.
- DON'T apply on the face, armpits, groin, or genitals.
- DON'T use on broken, raw, or oozing skin.
- DON'T cover the area with airtight bandages (occlusion).
- DON'T take hot showers immediately after application.
8. Toxicology & Storage
Overdose: Topical Overdose: Severe irritation, burning pain, erythema, edema, vesiculation, ulceration, and chemical burns. Systemic Overdose (theoretical): Nausea, vomiting, diarrhea, abdominal pain, hematuria, renal tubular damage, and CNS effects like vertigo.
Storage: Store below 25°C. Protect from light and moisture. Keep the tube tightly closed. Keep out of reach of children. Do not freeze.