Transdermal Drug Delivery Systems Overview

Terminology

• Percutaneous refers to processes that occur through the skin.
• Transdermal involves delivering substances into or through the skin for systemic effects.
• Transdermal Drug Delivery Systems (TDDSs) transport drugs across the skin into the bloodstream without retaining them in dermal layers.

Advantages of Transdermal Drug Delivery Systems

• Avoids gastrointestinal (GI) variations associated with oral therapy.
• Bypasses first-pass metabolism in the liver.
• Allows for continuous medication delivery, enabling easy termination of treatment.
• Facilitates multiday therapy with a single application.
• Reduces risks and inconveniences linked to parenteral methods.
• Prolongs the activity of drugs with short half-lives.
• Serves as an alternative route when oral administration is inappropriate.
• Enhances patient compliance due to ease of use.

Disadvantages of Transdermal Drug Delivery Systems

• Only potent drugs can effectively utilize TDDS due to skin impermeability.
• Suitable only for moderately lipophilic, uncharged drugs with low molecular weight (MW).
• Risk of contact dermatitis in some patients.
• Formulations, components, and the drug itself must be non-irritating and non-sensitizing.

Structure of the Skin

• Epidermis: Outer layer consisting mainly of the stratum corneum.
• Dermis: Layer beneath the epidermis.
• Fatty subcutaneous tissue: Deepest skin layer that supports skin structure.
• Skin appendages: Include hair follicles and glands.
• Stratum corneum: 10-15 µm thick; acts as a semipermeable membrane consisting of dried, dead epidermal cells.

Pathways of Transdermal Permeation

• The skin functions analogously to a membrane, influencing drug diffusion based on several factors.

Factors Affecting Drug Passage Through Skin

• Drug properties such as hydrophilicity or lipophilicity influence penetration levels.
• pKa: Ionization can diminish the ability of drugs to permeate.
• Molecular weight (MW) range of 100-800 is ideal for skin permeation; specific ideal MW is 400.
• Particle size affects penetration speed:
  • Molecules smaller than 10 µm remain on the skin surface.
  • Particles sized 3-10 µm may concentrate in hair follicles.

Resistance to Transdermal Absorption

• Resistance arises from multiple skin components including:
  • Vehicle (formulation),
  • Stratum corneum,
  • Epidermis,
  • Dermis.
• Total resistance is additive and calculated as ( R_{\text{total}} = R_{\text{vehicle}} + R_{\text{stratum corneum}} + R_{\text{epidermis}} + R_{\text{dermis}} ).
• The mathematical expression for drug diffusion considers diffusion coefficient, partition coefficient, surface area, and drug concentrations in sebum and systemic circulation.

Explore the advantages and disadvantages of Transdermal Drug Delivery Systems (TDDSs) in this quiz. Understand how TDDSs work and the implications for drug delivery compared to traditional methods. Test your knowledge on the benefits and limitations associated with this innovative approach to medication administration.