Parenteral and Alternative Drug Delivery
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Lesson 2 of 9
Notes
Parenteral Drug Delivery
Parenteral routes bypass the GI tract, providing 100% bioavailability (for IV) and rapid onset. Routes: (1) Intravenous (IV) โ direct systemic delivery; onset seconds-minutes; used for emergencies, drugs with poor oral bioavailability, or large volumes; risk of air embolism, infection, phlebitis; (2) Intramuscular (IM) โ depot absorption from muscle; slower onset; suitable for oily solutions, suspensions, and depot formulations; (3) Subcutaneous (SC) โ absorption from subcutaneous fat; used for insulin, heparin, vaccines; (4) Intradermal โ into dermis; used for allergy testing and BCG vaccine.
Parenteral formulations must be sterile (free from viable microorganisms), pyrogen-free (endotoxin-free), isotonic (osmolality 280-320 mOsm/kg to prevent haemolysis or cell shrinkage), and non-particulate. Sterilisation methods: autoclaving (121ยฐC steam, 15 min, most reliable), dry heat, gamma irradiation, sterile filtration (0.22 ฮผm membrane for heat-labile products).
Transdermal Drug Delivery
Transdermal patches deliver drugs through the skin into systemic circulation. Advantages: avoids first-pass metabolism, maintains steady-state plasma concentrations, improves adherence, easily reversible. The stratum corneum is the main barrier; drugs suitable for transdermal delivery are lipophilic, low molecular weight (<500 Da), and low dose requirements. Examples: fentanyl patch (72-h analgesia), glyceryl trinitrate patch (angina prophylaxis), nicotine patch, oestradiol patch. Penetration enhancers (e.g. ethanol, propylene glycol) disrupt stratum corneum lipid structure to improve flux.
Inhalation Drug Delivery
Inhalation delivers drugs directly to the lungs, providing local effect with minimal systemic exposure (for asthma/COPD) or rapid systemic absorption (highly vascularised alveolar surface). Devices: pressurised metered-dose inhalers (pMDI โ propellant-driven, requires hand-breath coordination), dry powder inhalers (DPI โ breath-actuated, no propellant), nebulisers (aerosolise liquid for inspiration in hospital or home use). Optimal particle size for deep lung deposition is 1-5 ฮผm (mass median aerodynamic diameter). Particles >5 ฮผm deposit in oropharynx (increased oral candidiasis with inhaled corticosteroids); <1 ฮผm exhaled.