Transdermal Drug Delivery (Patches)

Transdermal Drug Delivery (Patches)

1. Definition & scope

• A transdermal drug delivery system (TDDS), popularly a patch or transdermal therapeutic system (TTS), is a discrete, self-contained dosage form that, when applied to intact skin, delivers a drug across the skin (percutaneous absorption) into the systemic circulation at a controlled, predictable rate over an extended period (hours to days) (Shargel 8e Ch.9, pp.29–32; KDT 8e Ch.1, pp.12–13).
• The skin is the largest and most accessible organ of the body; the patch exploits it as a port for systemic delivery, not merely for local (topical) effect — this distinguishes transdermal (systemic intent) from topical/cutaneous dosage forms (ointments, creams, gels) whose intent is local action on the skin itself (Shargel 8e Ch.9, p.29; KDT 8e Ch.1, p.11).
• A transdermal patch is conceptually an extended-release (ER) dosage form delivered by a non-oral route — it provides a ≥2-fold reduction in dosing frequency versus immediate-release and aims for zero-order input analogous to a slow intravenous infusion (Shargel 8e Ch.9, pp.3–4, 29).
• Cutaneous (transdermal) route is a systemic route in KDT's taxonomy because, although applied to skin, the therapeutic target is the bloodstream; the prototype is glyceryl trinitrate (GTN) applied as ointment or patch for angina (KDT 8e Ch.1, p.11).
• Scope of this topic (per RGUHS-class PYQ): skin structure & permeation routes; physicochemical factors governing transdermal flux; patch architectures (reservoir vs matrix/drug-in-adhesive); permeation enhancers (chemical + physical — iontophoresis, microneedles, sonophoresis); marketed systems; advantages, limitations, adverse effects. It is not a generic NDDS or oral modified-release overview.

Terminology note: TDDS = transdermal drug delivery system; TTS = transdermal therapeutic system (synonymous, originally an ALZA trademark generalised into common use); patch = the marketed product. All three are used interchangeably below.