JNTU B.Pharmacy IV NOVEL DRUG DELIVERY SYSTEMS Previous Question Paper (September-2021 - R17)

The development of controlled release (CR) formulations depends significantly on the physicochemical properties of the drug candidate. These properties dictate the drug's release mechanism, absorption profile, and overall therapeutic efficacy.

Key physicochemical properties include:

• Aqueous Solubility: Drugs with low aqueous solubility are often candidates for CR systems to maintain a consistent dissolution rate. For highly soluble drugs, rate-controlling polymers or matrices are required to prevent dose dumping.
• Partition Coefficient (log P): This measure of lipophilicity determines the drug's ability to cross biological membranes. A moderate log P is ideal; very high lipophilicity may result in poor dissolution, while very low lipophilicity may hinder membrane permeability.
• Molecular Weight and Size: Smaller molecules generally exhibit higher diffusion coefficients through polymer matrices. Large molecules may require specific delivery technologies like osmotic pumps or prodrug approaches.
• pKa and Ionization: The degree of ionization at physiological pH affects solubility and permeability. Only the unionized form of a drug typically crosses biological membranes via passive diffusion.
• Drug Stability: The drug must be stable in the gastrointestinal environment and within the delivery system itself. Susceptibility to enzymatic degradation or chemical hydrolysis requires protective coatings or specific formulation strategies.
• Protein Binding: High plasma protein binding can extend the half-life of a drug, which may influence the design of the release rate to maintain steady-state concentrations.
• Dose Size: Drugs requiring high daily doses are often unsuitable for CR formulations because the resulting dosage form would be too large for patient compliance.
• Biological Half-life: Drugs with a short half-life (typically 2 to 6 hours) are ideal candidates for CR systems, as these formulations aim to maintain therapeutic levels over an extended period, reducing the frequency of administration.

Polymers are high molecular weight macromolecules composed of repeating structural units (monomers) connected by covalent bonds. They are fundamental to controlled release drug delivery systems (CRDDS) as they dictate the drug release kinetics, stability, and biocompatibility of the dosage form.

Classification of Polymers:

1. Based on Source: Natural (e.g., cellulose, chitosan, alginate, gelatin), Synthetic (e.g., polylactic acid, polyglycolic acid, PEG), and Semisynthetic (e.g., HPMC, carboxymethyl cellulose).
2. Based on Degradability: Biodegradable (polymers that break down into non-toxic metabolites via hydrolysis or enzymatic action, such as PLA, PLGA) and Non-biodegradable (polymers that remain intact, such as ethyl cellulose, PVC, silicone).
3. Based on Structure: Linear, branched, or cross-linked polymers.
4. Based on Response: Stimuli-responsive or 'smart' polymers (e.g., pH-sensitive, temperature-sensitive polymers) that alter their structure in response to environmental changes.

Properties of Polymers for CRDDS:

• Biocompatibility and Non-toxicity: Polymers must be inert, non-immunogenic, and non-carcinogenic to avoid adverse tissue reactions.
• Mechanical Strength: Sufficient structural integrity is required to maintain the dosage form shape and control drug diffusion.
• Permeability: The polymer must allow for controlled diffusion of the drug molecule through its matrix or membrane.
• Solubility and Swelling: The ability to hydrate and swell (hydrogels) is critical for controlling release rates in aqueous environments.
• Stability: Polymers must be chemically stable and compatible with the active pharmaceutical ingredient (API) during manufacturing and storage.

Advantages of Polymers in CRDDS:

• Controlled Release Kinetics: Polymers allow for zero-order or sustained release profiles, maintaining therapeutic plasma levels for extended periods.
• Targeted Delivery: Specific polymers can be functionalized to target drugs to particular tissues or cells, reducing systemic side effects.
• Protection of API: Polymers shield sensitive drugs (e.g., proteins, peptides) from degradation in the gastrointestinal tract or enzymatic inactivation.
• Patient Compliance: By reducing the frequency of dosing (e.g., once-daily administration), polymers significantly improve patient adherence to therapy.
• Versatility: Polymers can be engineered into various dosage forms, including implants, microspheres, hydrogels, and transdermal patches.