Notes on Topical Preparations for the Skin

Prevalence of Atopic Eczema

  • Atopic eczema has shown increasing prevalence from the 1940s to 2000s, with a significant percentage of children affected.

Definitions

  • Topical: Refers to applications made to any external body surface for localized effects, specifically focusing on the skin and scalp in this context.
    • Common terms: Eye, nasal mucosa, mouth (buccal mucosa), rectal mucosa, vaginal mucosa, skin, scalp.
    • Labeling: "For external use only."

Rationale for Topical Formulations

  • Reasons for Local Treatment:
    • Enhances the barrier function of the skin.
    • Enables targeted drug delivery to specific skin layers, e.g., for treating fungal infections in the outermost skin or for eczema needing deeper treatment.
    • Fewer side effects compared to oral medications.

Improving Skin Barrier Function

  • Formulations:
    • Sunscreens: Protect against UV radiation.
    • Antibiotics: Provide protection against infections in abrasions.
    • Emollients: Add moisture, beneficial for dry skin conditions.
    • Moisturisers: Protect against drying from atmospheric conditions.
    • Barrier creams: Shield skin from harsh chemicals, e.g., for hair stylists.

Skin Structure

  • Layers of the Skin:
    • Stratum Corneum: The outer layer of the epidermis.
    • Epidermis: Skin cells originate in the stratum basale layer.
    • Dermis: Contains blood vessels, sweat glands, hair follicles, and sebaceous glands.
    • Subcutaneous Layer: Fatty tissue providing cushioning for the skin.

Topical Dosage Forms Selection

  • Considerations:
    • Identify whether treatment targets the skin surface, stratum corneum, viable epidermis, dermis, or skin appendages like hair and nails.

Types of Topical Dosage Forms

  • Semisolids: Ointments, pastes, creams, lotions, gels, liniments, collodions, plasters.
  • Liquid Forms: Topical solutions, soaps/shampoos, tinctures, powders, aerosols/foams, topical patches.

Occlusive vs Non-Occlusive Formulations

  • Occlusive Preparations: Prevent moisture loss, increase hydration, provide effective drug penetration (good for dry skin but bad for infections).
    • E.g., Ointments provide a significant barrier.
  • Non-Occlusive Preparations: Allow better skin breathing and are less greasy, suitable for infected skin.

Ointments

  • Characteristics:
    • Semisolid, external application to skin/mucous membranes.
    • Composed primarily of petroleum-based hydrocarbons (e.g., soft paraffin).
  • Types:
    • Hydrocarbon Ointments: Include soft paraffin; used for skin hydration and protection.
    • Water-Soluble Ointments: e.g., Macrogol ointments; flexible with controlled polymer length.

Properties of Hydrocarbon Ointments

  • Properties:
    • Occlusive, emollient, hydrate skin, difficult to remove with water alone, and greasy in texture.
    • Ideal when drug stability in water is a concern.

Properties of Water-Soluble Ointments

  • Characteristics:
    • Washable, non-greasy, and less occlusive than hydrocarbon ointments.

Topical Formulations - Pastes

  • Definition: Stiffer than ointments due to solid material content (20-50% solids).
  • Uses: Provides protective barrier, prevents dehydration, e.g., nappy rash prevention.

Topical Formulations - Creams

  • Construction: Composed of oil and water phases (o/w or w/o).
  • Functionality: Good spreadability, high patient acceptability; requires emulsifiers and preservatives.

Examples of Creams

  • O/W Creams: Aqueous creams with preservatives and emulsifying agents.
  • W/O Creams: Richer creams such as those containing lanolin or beeswax.

Topical Formulations - Lotions

  • Definition: Liquid preparations with suspended insoluble solids; low viscosity for easy application.

Topical Formulations - Liniments

  • Characteristics: Similar to lotions but designed to be rubbed in for pain relief.

Gels

  • Composition and Function: Combination of liquid and solid phases via gelling agents (natural or synthetic); often used for their transparency and hydrophilic properties.

Additional Types of Formulations

  • Collodions: Sticky formulation that dries to form a film, protecting skin wounds.
  • Transdermal vs Topical Delivery:
    • Topical aims for localized action; transdermal aims for systemic delivery.

Transdermal Delivery

  • Mechanism: Adhesive patches that penetrate skin, avoiding first-pass metabolism; useful for various medications (e.g., fentanyl, nicotine).

Conclusion**

  • Testing and Quality Control: Regulatory aspects of topical and transdermal formulations ensuring efficacy and safety.