Drug Delivery Systems

Introduction

Drug delivery: science of transporting therapeutic molecules to target sites. Goal: maximize efficacy, minimize toxicity, reduce required dose. Challenge: deliver unstable drugs, cross biological barriers (blood-brain barrier), target specific cells. Solutions: encapsulation, modification, engineered transport.

"Drug delivery is the forgotten half of drug discovery. A perfect drug is useless if it cannot reach its target. The future of medicine lies in intelligent delivery systems." -- Biomedical engineer

Drug Delivery Challenges

Stability

Proteins: degraded by proteases (stomach, liver). RNA: unstable, degraded by nucleases. pH-sensitive: stomach acid inactivates some drugs. Solution: encapsulation protects.

Absorption

Poor permeability: large molecules cannot cross cell membranes. Hydrophobic drugs: cannot dissolve in aqueous environment. Solution: formulation enhancers, transporters.

Barriers

Blood-brain barrier: excludes most drugs (protect CNS). Tumor microenvironment: dense stroma prevents penetration. Vascular: extravasation difficult. Solutions: specific targeting, transporter exploitation.

Off-Target Effects

Systemic distribution: drug hits unintended sites causing side effects. Example: chemotherapy kills healthy cells. Solution: targeting to cancer cells only.

Liposomes and Vesicles

Structure

Lipid bilayer spheres: hydrophobic core, hydrophilic exterior. Encapsulates hydrophobic drugs in bilayer, hydrophilic in aqueous core. Biocompatible: lipids similar to cell membranes.

Types

Unilamellar: single bilayer (small, ~100 nm). Multilamellar: concentric bilayers (variable size). Liposomal assembly: thin-film hydration, sonication, extrusion.

Modifications

PEGylation: surface coating prevents immune recognition (longer circulation). Targeting ligands: antibodies, peptides direct to specific cells. Triggers: pH, temperature, light-sensitive.

Clinical Examples

Doxil: liposomal doxorubicin (chemotherapy). Reduced cardiotoxicity vs. free drug. FDA approved: 1995. Cost: expensive ($50,000+ per treatment).

Nanoparticles

Types

Polymeric: PLGA, PLA nanoparticles. Metal: gold, silver (photothermal therapy). Lipid: lipid nanoparticles (mRNA vaccines). Ceramic: silica, hydroxyapatite.

Size and Properties

Diameter: 1-100 nm. Passive targeting: smaller nanoparticles penetrate deeper, accumulate in leaky tumor vasculature (EPR effect). Surface area: high, enables many drug molecules per particle.

Synthesis

Polymeric: emulsion-solvent evaporationMetal: chemical reduction, nucleationLipid: self-assembly, microfluidicsControl: size, shape, surface chemistry

Advantages

Stability: protects drugs. Bioavailability: enhanced absorption. Targeting: surface modifications direct delivery. Cellular uptake: endocytosis facilitates entry.

Polymer-Based Systems

Natural Polymers

Chitosan: cationic, from shellfish. Alginate: anionic, from seaweed. Gelatin: from collagen. Advantage: biocompatible, biodegradable. Disadvantage: variability, limited control.

Synthetic Polymers

PLGA: widely used, degradation tunable. PVA: water-soluble, film-forming. PCL: slow degradation. Advantage: controllable properties. Disadvantage: less biocompatible than natural.

Hybrid Systems

Combine natural + synthetic: gelatin-PLGA. Benefits both: biocompatibility + control. Increasingly popular for advanced applications.

Degradation

Hydrolytic: water breaks bonds. Enzymatic: cells degrade. Time course: days to years (tuned by chemistry). Drug release: controlled by degradation rate.

Sustained Release

Goal

Release drug over prolonged period (days-months). Reduce dosing frequency. Maintain therapeutic level. Reduce side effects (no peaks/troughs).

Mechanisms

Diffusion: drug diffuses through polymer matrix. Erosion: polymer degrades, drug released. Combined: both occur. Rate determined by polymer properties, drug loading.

Release Kinetics

Zero-order: constant rate (ideal, difficult to achieve)First-order: exponential decay (typical)Burst: initial rapid release (minimize with coating)

Examples

Depo-Provera: injected monthly (sustained progesterone). Ocusert: inserts (sustained pilocarpine). Goal: patient convenience, improved compliance.

Targeted Delivery

Passive Targeting

EPR effect: tumors have leaky vessels, enhanced nanoparticle accumulation. Size < 100 nm preferentially extravasates. Not targeted but "passive" accumulation in tumors.

Active Targeting

Surface ligands: antibodies to cancer antigens. Receptors: folic acid (cancer cells overexpress receptors). Peptides: cell-penetrating sequences. Require receptor identification.

Dual Targeting

Multi-ligand: increase specificity. Example: antibody + folic acid. Requires multiple receptor overexpression. Higher specificity but more complex.

Challenges

Heterogeneity: cancer cells express different levels. Antigen masking: antibodies blocked by stroma. Immunogenicity: antibodies trigger immune response. Cost: expensive.

Transdermal Delivery

Advantage

Avoid first-pass metabolism (liver breaks down drug). Steady-state plasma levels. Patient convenience (patches). Reduced side effects (lower dose).

Barrier

Stratum corneum: lipid-rich, hydrophobic. Blocks most drugs. Strategy: permeation enhancers (DMSO, ethanol, surfactants) disrupt barrier.

Penetration Enhancers

Chemical: increase drug solubility, disrupt lipids. Electrical: iontophoresis uses electrical gradient. Mechanical: microneedles puncture stratum corneum. Thermal: heat increases diffusion.

Drug Requirements

Small molecular weight: < 500 Da. Lipophilic: balance hydrophilic/hydrophobic. Examples: nicotine, estrogen, fentanyl patches. Not suitable for large molecules (proteins).

Oral Drug Delivery

Challenges

pH variation: stomach acidic (~1.5), small intestine neutral (~7). Proteolysis: enzymes degrade proteins/peptides. Poor absorption: large molecules. Hepatic metabolism: first-pass effect.

Solutions

Enteric coating: protects from stomach acid, releases in intestine. Protease inhibitors: prevent degradation. Absorption enhancers: tight junction modulators. Prodrugs: inactive forms activated after absorption.

Formulations

Tablets, capsules: simple, cheap. Microencapsulation: protect from degradation. Nanoparticles: enhanced absorption. Gels: increased residence time, bioadhesion.

Bioavailability

Oral often < 10% (hepatic metabolism, poor absorption). Comparison: IV (100%), sublingual (rapid, avoid first-pass), intramuscular (complete absorption).

Parenteral Routes

Advantages

100% bioavailability: avoids GI degradation, first-pass. Rapid onset: IV immediate. Patient compliance: directly administered.

Routes

Intravenous (IV): fastest but requires expertise, infection risk. Intramuscular (IM): slower, depot effect possible. Subcutaneous (SC): self-administered, slower. Intradermal: targeted to skin, small volumes.

Injectable Formulations

Solutions: simplest, immediate release. Suspensions: microcrystals, prolonged release. Emulsions: oil-in-water, improved stability. Liposomes, nanoparticles: advanced delivery.

Depots

Microspheres: injected IM, sustained release over weeks-months. Example: Lupron (leuprolide) monthly or 3-month injection. Reduces dosing frequency, improves compliance.

Pharmacokinetics

ADME

Absorption: how drug enters body. Distribution: where it goes. Metabolism: how it's broken down. Elimination: how it exits (renal, fecal, respiratory).

Half-Life

Time for concentration to drop to half. Short t1/2: frequent dosing needed. Long t1/2: less frequent dosing. Optimal: match dosing interval to t1/2.

Drug Interactions

Metabolism inhibition: reduces clearance, increases blood level. Induction: increases clearance, reduces efficacy. Clinical: monitor drug levels, adjust dose.

Clinical Applications

Cancer

Liposomal doxorubicin: reduced cardiotoxicity. Albumin-bound paclitaxel: improved solubility. Gold nanoparticles: photothermal therapy under investigation.

mRNA Vaccines

COVID-19: Pfizer/BioNTech, Moderna. Lipid nanoparticles deliver mRNA. Revolutionary technology: enabled rapid vaccine development. High stability, immunogenicity.

Gene Therapy

Viral vectors: natural delivery mechanisms. Non-viral: lipoplexes, polyplexes. Challenges: immune response, off-target effects. Approved therapies emerging.

Chronic Diseases

Diabetes: insulin pumps, inhalable insulin. Osteoporosis: monthly/yearly injections (bisphosphonates). Hypertension: once-daily pills (sustained-release formulations).

References

• Langer, R. "New Methods of Drug Delivery." Science, vol. 249, no. 4976, 1990, pp. 1527-1533.
• Allen, T. M., and Cullis, P. R. "Drug Delivery Systems: Entering the Mainstream." Science, vol. 303, no. 5665, 2004, pp. 1818-1822.
• Torchilin, V. P. "Recent Advances with Liposomes as Pharmaceutical Carriers." Nature Reviews Drug Discovery, vol. 4, 2005, pp. 145-160.
• Shi, J., Kantoff, P. W., et al. "Cancer Nanomedicine: Progress, Challenges and Opportunities." Nature Reviews Cancer, vol. 17, 2017, pp. 20-37.
• Mitragotri, S., Burke, P. A., and Langer, R. "Overcoming the Challenges in Administering Biopharmaceuticals." Nature Reviews Drug Discovery, vol. 13, 2014, pp. 655-672.