Skin and Drug Delivery Systems Summary

Anatomy of the Skin

  • Layers of the skin:
    • Stratum Corneum:
    • Outer layer; 40% water, 40% proteins, 20% lipids
    • Comprises 15-25 layers of flat nonliving cells
    • Semipermeable membrane allowing passive diffusion of drugs
    • Epidermis
    • Dermis (vascularized): Supports systemic absorption (via hair follicles, sweat and sebaceous glands)

Routes of Drug Penetration

  • Intercellular (between cells)
  • Intracellular (Transcellular) (through cells)
  • Transappendageal (Transfollicular) (via hair follicles)

Drug Penetration of the Skin

  • Influencing factors:
    • Physicochemical properties: particle size, pH (skin ~5.5), oil-water partition coefficient
    • Passive diffusion primarily through stratum corneum:
    • Drug concentration and pharmaceutical excipients (vehicles, penetration enhancers)
    • Skin condition affects penetration:
    • Normal skin acts as a barrier, while disorders (e.g., eczema) can increase penetration
    • Age and skin condition (damaged skin, temperature) also impact absorption

Drug Delivery Systems

  • Transdermal Drug Products: Deliver drugs into systemic circulation (e.g., nitroglycerin ointment)
    • Skin is not the target organ, requires penetration enhancers
  • Topical Drug Products: Target the skin for treating dermal issues (e.g., antibiotics), requiring prolonged retention for therapeutic effect

Topical Product Classifications

  • Pharmacologic:
    • Antibiotics, antifungal, anti-infective, keratogenic, keratolytic, etc.
  • Physicochemical:
    • Liquids (solutions, suspensions, emulsions), semi-solids (ointments, creams, gels), and solids (powders)

Ointments

  • Types:
    • Medicated (active ingredients)
    • Unmedicated (e.g., protectants, emollients, lubricants, occlusives)
  • Ideal ointment base: stable, compatible, non-toxic
  • Incorporation Methods:
    • Levigation (reducing solid particle size)
    • Fusion (melting components together)

Emulsion Bases

  • W/O Emulsions:
    • Contain water, allow incorporation of more water
  • O/W Emulsions:
    • Easily washed off, non-occlusive, hydrophilic ointments

Other Product Forms

  • Pastes: Thicker than ointments, higher solid content; not suitable for hairy areas
  • Lotions: Treat dry skin, can be medicated for systemic effects
  • Liniments: Rubbing solutions for penetrating action or irritation
  • Collodions: Liquid preparations for occlusive coating with rapid evaporation

Proper Use of Topical Products

  • Clean and dry affected area before application
  • Typical dosage 1-3 mg/cm²; bandages usually not recommended
  • Educate patients on administration method, frequency, and potential reactions.