Pharmaceutical Formulations of Anthralin

1. Introduction

Anthralin (also known as dithranol) is a synthetic anthraquinone derivative that has been used for more than a century in the treatment of inflammatory skin disorders, especially psoriasis. Its therapeutic effect originates from the inhibition of DNA synthesis, suppression of protein kinase C, and modulation of cytokine release. Because anthralin is a potent irritant, modern pharmaceutical development focuses on delivering an effective dose while minimizing local inflammation and staining.

2. Physicochemical Characteristics Relevant to Formulation

• Solubility: Anthralin is practically insoluble in water (0.01mg/mL) but readily dissolves in organic solvents such as ethanol, propylene glycol, and dimethyl sulfoxide.
• Stability: It is photosensitive and oxidizes in the presence of air, turning brown to black. Moisture accelerates degradation.
• pKa: Approximately 5.5, allowing modest ionisation in weakly acidic media.
• Melting point: 170175C, indicating suitability for semisolid dosage forms.

These properties dictate the need for protective packaging, use of antioxidants, and selection of suitable vehicle systems.

3. Conventional SemiSolid Formulations

3.1 Ointments

Traditional anthralin ointments contain 0.12% w/w anthralin dispersed in a petroleumbased base (white soft paraffin, liquid paraffin, and emulsifying agents). The high lipid content limits drug release but reduces irritation by forming a barrier on the skin.

3.2 Creams

Creams are oilinwater emulsions that improve the spreadability and aesthetic acceptability of anthralin. Typical compositions involve:

• Ethanol or propylene glycol (solubilizer, 510%).
• Liquid paraffin (emollient, 1020%).
• Polysorbate 80 (emulsifier, 12%).
• Water phase with preservatives (e.g., phenoxyethanol).

Adding ethanolbased cosolvents accelerates drug dissolution, giving a more rapid onset of action.

3.3 Gels

Carbomer, hydroxypropyl methylcellulose (HPMC), or poloxamer gels allow a clear, lowstaining presentation. A typical gel includes 0.51% anthralin dissolved in ethanol, then neutralised with triethanolamine to achieve a pH around 5.5, which balances stability and skin tolerance.

4. Advanced Delivery Systems

4.1 Liposomal Formulations

Liposomes encapsulate anthralin in phospholipid bilayers, protecting it from oxidation and reducing skin irritation. Small unilamellar vesicles (80150nm) prepared by thinfilm hydration followed by extrusion have demonstrated:

• Increased drug retention in the stratum corneum.
• Reduced erythema scores by ~30% compared with conventional ointments.

4.2 Nanoparticle Dispersions

Solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) using stearic acid, glyceryl monostearate, or cetyl alcohol have been evaluated. Advantages include:

• Controlled release over 812hours.
• Improved photostability when coated with antioxidant layers (vitaminE, BHT).

4.3 Microemulsions

Oilinwater microemulsions containing isopropyl myristate, Transcutol P, and a surfactant blend (Cremophor EL / Tween 80) produce thermodynamically stable, transparent systems. The high interfacial area promotes rapid drug permeation while maintaining low irritancy.

4.4 Solid Dosage Forms for Local Application

Pressthrough tablets (PTT) and medicated patches have been explored for controlled release. PTTs incorporate anthralin in a matrix of ethyl cellulose and lactose, providing a dose of 0.5mg per tablet that dissolves upon contact with skin moisture.

5. Excipients Selected to Mitigate Irritation and Staining

6. Stability Considerations

Because anthralin degrades under light and oxygen, formulation strategies include:

• Packaging in ambercolored, airtight tubes or aluminiumfoil sachets.
• Inclusion of chelating agents such as EDTA (0.01%) to sequester metal ions that catalyse oxidation.
• Storage at 28C for liquid systems; semisolids are stable for up to 24months at room temperature when protected from light.

7. Clinical Implications of Formulation Choice

Clinical trials comparing 0.1% anthralin ointment with a 0.1% liposomal gel reported:

• Similar Psoriasis Area and Severity Index (PASI) reduction (45%).
• Significantly lower patientreported burning sensation (mean VAS score 1.2 vs. 3.4).

For extensive plaque psoriasis, a stepwise approach is recommended: start with a lowconcentration (0.05%) cream, increase to 0.5% gel as tolerance improves, and consider nanoparticle gels for refractory lesions.

8. Regulatory Landscape

Anthralin is classified as a prescriptiononly medicine in most jurisdictions. The United States FDA lists it under Dermatologicals Topical Antipsoriatics. European Medicines Agency (EMA) requires a documented oxidative stability test (10% degradation after 6months at 40C/75% RH). Any novel delivery system must demonstrate bioequivalence to the reference ointment and provide a complete safety dossier.

9. Future Directions

Research is focusing on:

• Hybrid hydrogelliposome composites that combine the comfort of hydrogels with the protective capacity of liposomes.
• Digital dosing devices that dispense a precise microgram amount, reducing waste and staining.
• Combination products that pair anthralin with lowdose corticosteroids or vitamin D analogues to achieve synergistic efficacy while limiting adverse effects.

Advances in rheology modifiers (e.g., xanthan gum, carbopol940) are also being explored to finetune the spreadability of highpotency formulations without compromising drug release.

10. Key Takeaways

• Anthralins poor water solubility and irritancy drive the need for carefully designed semisolid or nanocarrier systems.
• Modern formulations aim to balance potency (0.052% w/w) with patient comfort, employing antioxidants, pH control, and protective packaging.
• Liposomal and nanoparticle carriers show promise in reducing staining and erythema while maintaining therapeutic outcomes.
• Regulatory acceptance hinges on demonstrated stability, bioequivalence, and safety data.