Modern pharmaceutics has moved far beyond conventional tablets and injections. Advanced drug delivery systems are designed to improve therapeutic efficacy, patient compliance, and site-specific drug action while minimizing adverse effects. Unit 3 focuses on three important delivery platforms—Transdermal Drug Delivery Systems (TDDS), Gastroretentive Drug Delivery Systems (GRDDS), and Nasopulmonary Drug Delivery Systems—each offering unique solutions to challenges associated with drug absorption and bioavailability.
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Transdermal Drug Delivery Systems (TDDS)
Introduction to TDDS
Transdermal drug delivery systems are designed to deliver drugs across the skin into systemic circulation at a controlled rate. By bypassing the gastrointestinal tract and hepatic first-pass metabolism, TDDS offer sustained plasma drug levels and improved patient adherence, especially for chronic therapies.
Drug Permeation Through the Skin
The skin acts as a natural barrier, with the stratum corneum being the primary obstacle to drug permeation. Drugs can penetrate the skin through three main pathways: transcellular, intercellular, and appendageal routes. Successful permeation depends on the drug’s lipophilicity, molecular size, and concentration gradient across the skin.
Factors Affecting Skin Permeation
Several factors influence transdermal absorption, including skin hydration, thickness, temperature, blood flow, and the physicochemical properties of the drug. Formulation-related factors such as drug concentration, type of polymer, and presence of enhancers also play a critical role.
Permeation Enhancers
Permeation enhancers are substances that temporarily alter the skin barrier to improve drug transport. These include solvents, surfactants, fatty acids, and terpenes. While effective, their safety and reversibility must be carefully evaluated to avoid skin irritation or damage.
Components and Formulation Approaches
A typical TDDS consists of a drug reservoir or matrix, polymeric membrane, adhesive layer, backing layer, and release liner. Formulation strategies include reservoir systems, matrix systems, and drug-in-adhesive systems, each offering different release characteristics.
Gastroretentive Drug Delivery Systems (GRDDS)
Introduction to GRDDS
Gastroretentive drug delivery systems are designed to prolong the residence time of dosage forms in the stomach. This is particularly beneficial for drugs that are absorbed mainly in the upper gastrointestinal tract, are unstable in intestinal pH, or require local gastric action.
Advantages and Limitations
GRDDS improve bioavailability, reduce dosing frequency, and enhance therapeutic effectiveness. However, variability in gastric emptying time, patient posture, and food intake can affect system performance.
Floating Drug Delivery Systems
Floating systems have low density and remain buoyant on gastric fluids, allowing prolonged gastric retention. These systems often use gas-generating agents or swellable polymers to maintain flotation.
High-Density Systems
High-density systems sink to the bottom of the stomach and resist gastric emptying. Their effectiveness depends on achieving sufficient density without compromising patient comfort.
Inflatable and Gastroadhesive Systems
Inflatable systems expand in the stomach after ingestion, preventing passage through the pylorus. Gastroadhesive systems use bioadhesive polymers to adhere to the gastric mucosa, enhancing retention time. Both approaches are useful for drugs requiring extended gastric exposure.
Applications of GRDDS
GRDDS are widely applied in drugs used for Helicobacter pylori infections, antacids, antibiotics, and drugs with narrow absorption windows, improving therapeutic outcomes.
Nasopulmonary Drug Delivery Systems
Introduction to Nasal and Pulmonary Routes
The nasal and pulmonary routes provide rapid drug absorption due to their large surface area, high vascularization, and avoidance of first-pass metabolism. These routes are especially valuable for both local and systemic drug delivery.
Nasal Drug Delivery Systems
Nasal drug delivery offers quick onset of action and is useful for peptides, proteins, and vaccines. Formulation challenges include mucociliary clearance, enzymatic degradation, and limited dose volume.
Pulmonary Drug Delivery Systems
Pulmonary delivery targets drugs directly to the lungs, making it ideal for respiratory disorders such as asthma and chronic obstructive pulmonary disease. It is also explored for systemic delivery of macromolecules.
Dry Powder Inhalers (DPIs)
DPIs deliver micronized drug particles to the lungs through patient inhalation. Particle size, flow properties, and device design significantly influence drug deposition.
Metered Dose Inhalers (MDIs)
MDIs use propellants to deliver a precise dose of medication in aerosol form. Proper coordination between actuation and inhalation is critical for effective therapy.
Nebulizers
Nebulizers convert liquid formulations into fine aerosols suitable for inhalation, making them ideal for pediatric, geriatric, and critically ill patients who cannot use inhalers effectively.
Nasal Sprays
Nasal sprays deliver drugs directly to the nasal mucosa for local or systemic action. Formulations must ensure appropriate viscosity, spray pattern, and stability.