16.2 Polymers in drug delivery systems

Polymeric drug delivery systems revolutionize how medications reach their targets in the body. These systems use specially designed polymers to control drug release, improve bioavailability, and reduce side effects. From nanoparticles to hydrogels, they offer versatile solutions for various therapeutic needs.

Key factors in developing these systems include polymer properties, drug-polymer interactions, and release kinetics. While challenges like scalability and regulatory hurdles exist, the future looks promising with personalized medicine and smart delivery systems on the horizon.

Polymeric Drug Delivery Systems

Properties of polymers for drug delivery

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• Biocompatibility ensures polymers are non-toxic and non-immunogenic, minimizing adverse reactions in the body (PEG, PLGA)
• Biodegradability allows polymers to degrade into non-toxic byproducts, eliminating the need for surgical removal (polycaprolactone, polyanhydrides)
• Controlled drug release enables sustained release over an extended period and targeted delivery to specific sites (hydrogels, microspheres)
• Versatility in design allows polymers to be tailored to meet specific drug delivery requirements with tunable properties such as size, shape, and surface characteristics (block copolymers, dendrimers)

Types of polymeric delivery systems

• Polymer-drug conjugates
  • Covalently bind drugs to polymer backbone
  • Enhance drug solubility and stability
  • Release drug through chemical or enzymatic cleavage (PEG-paclitaxel, HPMA-doxorubicin)
• Polymeric micelles
  • Self-assemble into nanostructures with hydrophobic core and hydrophilic shell
  • Encapsulate poorly water-soluble drugs
  • Release drug through diffusion or polymer degradation (PEG-PCL, PEG-PLGA)
• Polymeric nanoparticles
  • Form solid colloidal particles with drug dispersed or encapsulated
  • Enhance drug bioavailability and stability
  • Release drug through diffusion, erosion, or stimuli-responsive mechanisms (PLGA nanoparticles, chitosan nanoparticles)
• Hydrogels
  • Create three-dimensional cross-linked polymer networks
  • Swell in aqueous media and entrap drugs
  • Release drug through diffusion or polymer degradation (PEG-based hydrogels, alginate hydrogels)

Factors in drug release kinetics

• Polymer composition and molecular weight affect degradation rate and drug release profile, with higher molecular weight generally slowing drug release (PLA vs PLGA)
• Drug-polymer interactions influence drug loading capacity and release behavior, with strong interactions potentially resulting in slower drug release (ionic interactions, hydrogen bonding)
• Porosity and surface area impact drug release, with higher porosity and surface area accelerating release by allowing greater water penetration and drug diffusion (mesoporous silica nanoparticles)
• Environmental factors such as pH, temperature, and ionic strength can trigger drug release, with stimuli-responsive polymers exploiting these factors for controlled release (pH-sensitive polymers, thermoresponsive polymers)

Challenges vs prospects in polymer delivery

• Challenges include:
  1. Scalability and reproducibility in manufacturing
  2. Regulatory hurdles and safety concerns
  3. Variability in drug loading and release kinetics
  4. Potential for drug instability during storage and delivery
• Future prospects involve:
  • Personalized medicine through tailored drug delivery systems
  • Combination therapy with multiple drugs in a single system (co-delivery of chemotherapeutics)
  • Stimuli-responsive and smart polymeric systems (glucose-responsive insulin delivery)
  • Targeted delivery to specific tissues or cells (antibody-conjugated nanoparticles)
  • Improved patient compliance through less frequent dosing (long-acting injectable formulations)