Medicines Design & Manufacture - Topical Medicines Summary

Learning Outcomes

  • Understand the structure of the skin in relation to topical formulations.
  • Recognize a variety of drugs delivered via topical formulations and their targeted skin regions.
  • Differentiate between classes of semi-solid preparations used as emollients and topical vehicles.
  • Describe the two recognized mechanisms of emolliation.

Structure of the Skin

Functions of the Skin
  • Protection Against:
    • Pollution
    • Temperature
    • Humidity
    • Radiation
  • Roles:
    • Protects internal organs
    • Limits passage of chemicals
    • Stabilizes blood pressure and temperature
    • Mediates sensory perceptions (heat, cold, touch, pain)
    • Expresses emotions, indicators of human traits (color, hair, texture)
Skin Damage
  • Physical: Cuts, bruises, burns, radiation exposure
  • Chemical: Bites, stings, detergents, pollutants
  • Biological: Microorganisms (bacteria, fungi, viruses)
Skin Regions
  • Epidermis:
    • Multilayered; thickness varies (0.8 mm on palms, 6 mm on eyelids)
    • Stratum Corneum: dense, dead layer, swells in water (3-5 mm thick)
  • Dermis:
    • Matrix of connective tissue (collagen, elastin)
    • Contains nerves, blood vessels, and skin appendages
  • Subcutaneous Tissue (Hypoderm):
    • Fat providing cushioning and thermal barrier, synthesizes chemicals
Skin Appendages
  • Eccrine Sweat Glands: Produce sweat, regulate temperature, secrete substances (pH 4.0-6.8)
  • Apocrine Sweat Glands: Develop at hair follicles, milky/oily secretion that is metabolized by bacteria to create body odor
  • Sebaceous Glands: Secrete oily sebum (glycerides, fatty acids, cholesterol) into hair follicles; present over most skin except lips and palms

Drug Delivery Approaches

  • Manipulating the Stratum Corneum:
    • Antibacterials help combat infection
    • Sunscreens protect against UV radiation
    • Emollients restore skin pliability
  • Direct Delivery: Deliver drugs to viable skin tissues without alternative routes
  • Transdermal Systems: Provide systemic therapy for conditions like motion sickness and hormonal therapy
Routes of Penetration
  • Transdermal: Drug dissolves and diffuses
  • Transappendageal: Involves pilosebaceous units, eccrine glands
  • Systemic Delivery: Via circulation for effective treatment

Treatments for Skin Diseases

Stratum Corneum Treatments
  • Emollients: Hydrate and soothe skin; frequently applied for dry or scaling disorders
    • Example Products: Aqueous Cream BP, Neutrogena™
Treatments for Skin Appendages
  • Hyperhidrosis: Antiperspirants containing aluminium salts (e.g., Anhydrol Forte™)
  • Acne: Topical exfoliants and antibiotics (e.g., benzoyl peroxide)
  • Fungal Diseases: Topical imidazoles for localized dermatophytes
Treatments for Viable Epidermis/Dermis
  • Drug delivery effectiveness: Use of pro-drugs
  • Anti-inflammatories: Steroidal and NSAIDs like Ketoprofen gel
  • Anaesthetics: Local pain relief; can cause hypersensitivity
  • Antipruritics: Relief from itching in conditions like eczema
  • Antihistamines: Effective for insect bites

Topical Formulations

Liquid Preparations
  • Soaks/Baths: Active ingredients in aqueous solutions; e.g., Oilatum™
  • Lotions: Aqueous solutions that evaporate to soothe skin
Ointments
  • Greasy, semisolid formulations: Hydrocarbon and fat bases
Creams
  • Oily Creams: Contain w/o emulsifying agents; effective for thicker applications
  • Aqueous Creams: Contain o/w emulsifying agents for lighter formulations
Solid/Semisolid Preparations
  • Zinc and Salicylic Acid Paste (Lassar’s Paste): Treatment for hyperkeratotic skin; contains zinc oxide, salicylic acid, starch, and liquid paraffin
  • Emulsifying Ointment BP: Combination of emulsifying wax and paraffin; indicated for dry skin disorders

Recommended Further Reading

  • Barry, B.W., ‘Transdermal drug delivery’ in Aulton's Pharmaceutics, 3rd ed., pp.565-571
  • Williams, A.C., ‘Topical and transdermal drug delivery’ in Aulton's Pharmaceutics, 4th ed., pp.675-680.
  • Williams, A.C., ‘Topical and transdermal drug delivery’ in Aulton's Pharmaceutics, 5th ed., pp.715-720.