9.1 Principles of Drug Delivery

Drug delivery systems revolutionize medicine by optimizing how drugs reach their targets. These systems enhance therapeutic effects, minimize side effects, and improve patient compliance through controlled release, targeted delivery, and enhanced bioavailability.

From oral pills to high-tech nanoparticles, various routes and methods exist for getting drugs into the body. Understanding bioavailability, pharmacokinetics, and the challenges in drug delivery design is crucial for developing effective treatments.

Principles of Drug Delivery

Principles of drug delivery systems

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• Drug delivery systems aim to:
  • Enhance therapeutic efficacy by optimizing drug concentration at the target site
  • Minimize side effects by reducing drug exposure to non-target tissues
  • Improve patient compliance through convenient and less frequent dosing
• Key principles:
  • Controlled release
    • Sustained release: Prolonged drug release over an extended period (transdermal patches)
    • Pulsatile release: Drug release in pulses at specific time intervals (insulin pumps)
  • Targeted delivery
    • Site-specific delivery: Directing drugs to specific tissues or organs (antibody-drug conjugates)
    • Cell-specific delivery: Targeting drugs to specific cell types (nanoparticles)
  • Enhanced bioavailability
    • Increasing the fraction of the administered dose that reaches systemic circulation (lipid-based formulations)

Routes of drug administration

• Oral route
  • Advantages: Convenient, non-invasive, cost-effective
  • Limitations: First-pass metabolism, variable absorption, gastrointestinal irritation
• Parenteral route (injection)
  • Intravenous (IV): Rapid onset, 100% bioavailability, precise dosing
  • Intramuscular (IM): Suitable for depot formulations, slower absorption than IV
  • Subcutaneous (SC): Suitable for self-administration, slower absorption than IM
• Transdermal route
  • Advantages: Avoids first-pass metabolism, provides sustained release
  • Limitations: Limited to potent drugs, skin irritation, slow onset
• Pulmonary route
  • Advantages: Large absorptive surface area, rapid onset, avoids first-pass metabolism
  • Limitations: Short duration of action, variable absorption, device dependence
• Nasal route
  • Advantages: Rapid absorption, avoids first-pass metabolism
  • Limitations: Limited drug absorption, nasal irritation, short duration of action

Bioavailability and pharmacokinetics concepts

• Bioavailability
  • Definition: Fraction of the administered dose that reaches systemic circulation unchanged
  • Factors affecting bioavailability: Route of administration, drug formulation, first-pass metabolism
• Pharmacokinetics
  1. Absorption: Process of drug entry into the body
  2. Distribution: Movement of drug from the bloodstream to tissues
  3. Metabolism: Chemical alteration of the drug by the body (liver enzymes)
  4. Excretion: Elimination of the drug and its metabolites from the body (kidneys)
• Pharmacodynamics
  • Mechanism of action: How the drug interacts with its target to produce a pharmacological effect (receptor binding)
  • Dose-response relationship: Relationship between drug concentration and its therapeutic effect
  • Therapeutic index: Ratio of the median lethal dose ($LD_{50}$) to the median effective dose ($ED_{50}$)

Challenges in drug delivery design

• Physicochemical properties of the drug
  • Solubility: Affects absorption and bioavailability (Biopharmaceutics Classification System)
  • Stability: Determines storage conditions and shelf life (refrigeration)
  • Permeability: Influences the ability of the drug to cross biological membranes (Lipinski's Rule of Five)
• Biological barriers
  • Blood-brain barrier: Limits drug entry into the central nervous system
  • Gastrointestinal tract: Presents challenges for oral drug delivery (pH, enzymes)
  • Skin: Serves as a barrier for transdermal drug delivery
• Patient factors
  • Age, weight, and gender
  • Genetic variations affecting drug metabolism and response (cytochrome P450 enzymes)
  • Comorbidities and concomitant medications (drug-drug interactions)
• Formulation considerations
  • Excipients: Inactive ingredients that improve drug stability, solubility, and delivery (polymers)
  • Drug loading and release kinetics
  • Targeted delivery approaches (liposomes, nanoparticles)
• Safety and regulatory requirements
  • Biocompatibility of delivery systems
  • Toxicity and immunogenicity
  • Clinical trials and regulatory approval processes (FDA)