Characterisation of Novel Drug Delivery Systems

The comprehensive characterisation of nanoparticulate and vesicular delivery systems requires a distinct analytical panel beyond that applied to...

The comprehensive characterisation of nanoparticulate and vesicular delivery systems requires a distinct analytical panel beyond that applied to conventional dosage forms, reflecting the unique physical properties and stability considerations of colloidal systems. Particle size, expressed as the Z-average diameter, is most commonly determined by Dynamic Light Scattering using instruments such as the Malvern Zetasizer, with acceptance targets generally below 200 nanometres for intravenous administration and below 500 nanometres for oral delivery, and findings are frequently corroborated by Transmission Electron Microscopy to provide direct morphological confirmation. The Polydispersity Index, also derived from Dynamic Light Scattering, quantifies the breadth of the particle size distribution, with values below 0.2 indicating a narrow, essentially monodisperse population, values up to approximately 0.3 generally regarded as acceptable, and values exceeding 0.5 indicative of a broad, potentially unstable distribution requiring formulation refinement.

Zeta potential, a measure of the electrostatic charge at the particle surface derived from electrophoretic mobility measurements, provides a key indicator of colloidal stability, since particles bearing a sufficiently strong surface charge, conventionally at least thirty millivolts in magnitude, resist aggregation through electrostatic repulsion, while particles stabilised primarily through steric hindrance, for instance by a PEGylated surface coating, may remain colloidally stable at somewhat lower zeta potential magnitudes, around twenty millivolts, owing to the additional steric contribution to stability.

Encapsulation Efficiency, conventionally abbreviated EE per cent, quantifies the proportion of total drug successfully incorporated within the nanocarrier, calculated as the difference between total and free drug content divided by total drug content, and is typically determined indirectly by separating unencapsulated free drug through ultrafiltration or dialysis followed by high-performance liquid chromatography or ultraviolet quantification of the drug remaining in the filtrate. Drug Loading, a related but distinct parameter, expresses the mass of drug incorporated as a proportion of the total nanoparticle mass, and together these two parameters determine the practical dose of nanocarrier material a patient must receive to achieve a given therapeutic dose of drug.

In-vitro release from nanoparticulate systems is typically assessed using a dialysis bag method, employing a membrane with a molecular weight cut-off around twelve kilodaltons immersed in phosphate-buffered saline at pH 7.4 under sink conditions, with sampling extended over twenty-four to seventy-two hours to capture the often prolonged release kinetics characteristic of these systems. Finally, colloidal stability is assessed by monitoring particle size, polydispersity index, and zeta potential over one to three months at a range of storage temperatures, typically 4, 25, and 40 degrees Celsius, together with resistance to freeze-thaw cycling, since aggregation and Ostwald ripening represent the principal physical instability risks confronting nanoparticulate formulations during long-term storage.

Review Questions

1. Compare the preparation methods and applications of PLGA nanoparticles, SLN, and NLC.

2. Explain how PEGylation extends liposomal circulation time and describe the significance of Doxil as a clinical precedent.

3. Distinguish microspheres from microcapsules and describe the mechanisms of drug release from each.

4. Explain the principle of self-emulsification underlying SEDDS and SMEDDS and the role of the pseudo-ternary phase diagram.

5. List the key characterisation parameters used to evaluate nanoparticulate drug delivery systems and explain the significance of each.