Semisolid Dosage Forms (I)

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Topical Drug Delivery

  • Skin is used for both topical (local) and systemic (transdermal) drug delivery.

  • Topical delivery involves applying a formulation to treat the skin directly.

  • Topical formulations deliver active substances to the skin or enhance its barrier properties.

  • Transdermal delivery involves drugs penetrating the skin into systemic circulation.

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Topical Semisolid Preparations

  • Common semisolid preparations: creams, ointments, pastes, gels.

  • Semisolid status: neither completely solid nor liquid.

  • Recent additions: topical patches and sprays for drug delivery.

  • Preparations may be medicated (contain drugs) or unmedicated (no drug).

  • Bases can be natural or synthetic, influencing hydrophilic/hydrophobic properties.

  • May contain excipients like preservatives, antioxidants, stabilizers, emulsifiers, thickeners, and enhancers.

  • Formulations for broken skin must be sterile.

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Properties of Topical Formulations

  • Emollient: softens or soothes skin.

  • Occlusion: closure state; occlusives can have emollient effects.

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Semisolid Dosage Forms

  • Categories of semisolid preparations:

    • Medicated preparations: contain therapeutic agents (APIs).

    • Unmedicated preparations: serve as protectants or lubricants (emollients, humectants, occlusives).

  • Definition of cosmetics and drugs as per Federal Food, Drug, and Cosmetic Act (FD&C Act).

  • An example: Moisturizers with sunscreen can be both cosmetic and drug.

  • Types of semisolid forms:

    • Creams

    • Ointments

    • Pastes

    • Gels

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Emulsion

  • Two-phase systems with one phase dispersed in another (internal in continuous).

  • Immiscible phases require an emulsifying agent for formation (e.g., surfactant).

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Ointments

  • Semisolid preparations for external application on skin or mucous membranes.

  • Used for emollient properties, skin lesion protection, and active ingredient delivery.

  • Classifications of ointment bases:

    • Hydrocarbon bases (oleaginous)

    • Absorption bases

    • Water miscible bases (emulsion)

    • Water-soluble bases

  • Bases act as carriers or vehicles.

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Hydrocarbon Bases (Oleaginous, Water-Free)

  • Immiscible with water and not absorbed by the skin.

  • Occlusive properties retain moisture, enhancing drug penetration.

  • Preferred for dry lesions, though greasiness may stain clothing.

  • Examples: mineral oil, white petrolatum, paraffin, waxes.

  • Levigation: reduces particle size of powders using a liquid agent.

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Absorption Bases

  • Absorb water to form water-in-oil (w/o) emulsions, remain semisolid.

  • Less occlusive and easier to spread than hydrocarbon bases.

  • Good emollients that hydrate skin.

  • Examples: Anhydrous Lanolin, Beeswax.

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Water Miscible Bases (Emulsifying Bases)

  • Anhydrous, semisolid bases containing oil-in-water emulsifiers.

  • Form o/w emulsions upon adding aqueous solutions.

  • Example: Hydrophilic Ointment (water-washable).

  • Delivers good miscibility with skin exudates.

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Water-Soluble Bases

  • Contain no oleaginous components, completely water-washable.

  • Comprise mixtures of liquid and solid macrogols.

  • Vary in consistency based on molecular weight.

  • Macrogols are polymers with limited water absorption capacity.

  • Example: Polyethylene Glycol Ointment, which combines PEG3350 and PEG400.

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Selection of Appropriate Ointment Base

  • Considerations for base selection:

    1. Desired drug release rate.

    2. Need for moisture occlusion.

    3. Drug stability in base.

    4. Impact of drug on ointment consistency.

    5. Ease of removal with water.

    6. Skin surface characteristics.