Controlled Drug Delivery Systems & Polymers
This unit establishes the foundational principles of controlled drug delivery. It covers terminology, rationale, advantages/disadvantages, and criteria for selecting drug candidates for CDDS. It details design approaches based on diffusion, dissolution, and ion-exchange principles, alongside the critical physicochemical and biological properties of drugs. The unit also introduces polymers—their classification, properties, and vital applications in formulating controlled release systems.
Syllabus & Topics
- 1CDDS – Terminology & Rationale: Conventional dosage forms (immediate release) result in peaks and valleys in plasma drug concentration. If peak > Minimum Toxic Concentration (MTC) = toxicity. If valley < Minimum Effective Concentration (MEC) = sub-therapeutic. Controlled Drug Delivery System (CDDS): Delivers the drug at a predetermined rate to maintain a constant, optimal therapeutic drug level (ideally zero-order kinetics). Sustained Release (SR): Prolongs action but release rate isn’t strictly constant (often 1st order). Rationale for CDDS: (1) Maintain therapeutic levels for extended periods. (2) Reduce dosing frequency (e.g., TDS to OD). (3) Increase patient compliance. (4) Reduce fluctuations in plasma level (reduce side effects). (5) Improve efficiency of treatment.
- 2Advantages & Selection of Drug Candidates: Advantages: Improved efficacy, reduced toxicity, improved patient compliance, potential for targeted delivery, more constant plasma levels. Disadvantages: High cost of formulation, risk of dose dumping (sudden release of large drug amount if system fails), difficult to withdraw drug in case of toxicity, unpredictable GI transit time affects release. Selection of Drug Candidates: (1) Half-life: Ideal is 2-8 hours. Very short (<2h) requires too large a dose; very long (>8h) inherently sustained, doesn’t need CDDS. (2) Dose size: Should be small (<500mg) so the formulated product isn’t too large to swallow. (3) Therapeutic index (TI): Drugs with narrow TI are good candidates (e.g., theophylline) to avoid peaks. (4) Absorption: Should be uniformly absorbed throughout the GI tract. (5) Metabolism: Drugs with extensive first-pass metabolism are poor candidates. (6) Aqueous solubility: Extremes (very low or very high) make CDDS formulation difficult.
- 3Approaches to Design – Diffusion Controlled: Release rate is determined by the diffusion of drug molecules through a polymer. Two main types: (1) Reservoir Systems: A drug core is enclosed by a water-insoluble polymer membrane. Water diffuses in, dissolves drug, drug diffuses out. Release is constant (zero-order) as long as the core solution is saturated. Formula: Fick’s first law. Risk: Membrane rupture leads to dose dumping. (2) Matrix Systems: Drug is homogeneously dispersed/dissolved throughout a polymer matrix. Release occurs as drug at the surface dissolves, then internal drug diffuses through pores created. Release rate decreases over time (Higuchi kinetics; square root of time dependent) because the diffusion distance increases. Matrix can be hydrophilic (swells, e.g., HPMC) or lipophilic (waxy materials).
- 4Approaches to Design – Dissolution & Ion Exchange: Dissolution Controlled Systems: Drug release is controlled by the dissolution rate of a polymer coating or matrix. Applicable to highly water-soluble drugs. (1) Matrix/Monolith type: Drug interspersed in a slowly dissolving matrix. (2) Encapsulation/Coating type: Drug particles or granules are coated with polymers of varying thickness or different dissolution rates (e.g., Spansules). Slower dissolution of the coat delays release. Ion Exchange Resins: Cross-linked water-insoluble polymers with ionizable groups. Acidic drugs bind to basic resins, basic drugs bind to acidic resins. Drug-Resin complex is formulated. In the GI tract, endogenous ions (Na+, K+, Cl-) displace the drug from the resin. Release rate depends on the pH, ion concentration of GI fluid, and properties of the resin. Used extensively for liquid sustained-release suspensions.
- 5Physicochemical & Biological Properties Relevant to CDDS: Physicochemical properties: (1) Aqueous solubility and pKa: Dictate absorption and dissolution mechanics. (2) Partition coefficient (Lipophilicity): Affects ability to cross biological membranes. Extreme lipophilicity can lead to tissue trapping. (3) Stability: Drug must be stable in the GI environment (acidic stomach, enzymatic intestine) for the extended residence time. Biological properties: (1) Absorption: Must be good throughout the GI tract. (2) Distribution: High volume of distribution may require larger doses, complicating CDDS design. (3) Metabolism: Should not be significantly altered by the extended release profile. (4) Elimination half-life: 2-8 hours is ideal. (5) Margin of safety (Therapeutic Index): CDDS is excellent for narrow TI drugs.
- 6Polymers in Formulation of CDDS: Polymers: Large molecules composed of repeating structural units (monomers). They are the fundamental building blocks of almost all NDDS. Classification: (1) Origin: Natural (e.g., alginate, chitosan, gelatin, cellulose derivatives like HPMC), Synthetic (e.g., PEG, PVP, PLGA). (2) Biodegradability: Biodegradable (break down in the body into non-toxic products, e.g., PLA, PLGA, polycaprolactone – essential for implants), Non-biodegradable (e.g., silicone, ethyl cellulose). (3) Response to environment: Smart/Stimuli-responsive polymers (change properties based on pH, temperature, magnetic field). Properties: Molecular weight, degree of cross-linking, crystallinity, hydrophilicity define the polymer’s behavior (swelling, degradation, drug permeability). Applications: Used as rate-controlling membranes (reservoir), matrix formers, mucoadhesives, coating materials for targeted release (enteric coating), and backbone for targeted carrier systems.
Learning Objectives
Exam Prep Questions
Q1. What is “Dose Dumping” in CDDS?
Dose dumping is a potentially dangerous phenomenon in controlled release formulations where an environmental factor (like taking the medication with alcohol, or a physical defect in the formulation’s membrane) causes the premature, rapid, and unintended release of the entire drug content. Because CDDS contain an amount of drug intended for 12–24 hours, dump release can lead to acute toxicity.
Q2. Why are drugs with a very short half-life (< 2 hours) or very long half-life (> 8 hours) generally not formulated as CDDS?
Drugs with very short half-lives would require excessively large amounts of the drug to maintain therapeutic levels for 12–24 hours. The resulting dosage form (tablet/capsule) would be too large for a patient to swallow safely.
Drugs with very long half-lives are already eliminated slowly from the body, achieving a natural sustained effect with conventional once-daily dosing; thus, putting them into a complex CDDS is unnecessary and costly.
Q3. What is the primary difference between a matrix system and a reservoir system in diffusion-controlled release?
In a Reservoir system, a central core of drug is completely enclosed by a rate-controlling polymer membrane; as long as the core is saturated, drug diffuses at a constant (zero-order) rate. However, if the membrane breaks, dose dumping occurs.
In a Matrix system, the drug is homogeneously mixed throughout the polymer structure. Release typically decreases over time (Higuchi kinetics) as drug from deeper layers takes longer to diffuse out. Matrix systems are generally safer as they do not suffer from catastrophic dose dumping if broken.