Studies of structure–transport relationships in biodegradable polymer microspheres for drug delivery using NMR cryodiffusometry

NMR cryoporometry characterisation studies of the relation between drug release profile and pore structural evolution of polymeric nanoparticles

PLGA/PLA polymeric nanoparticles could potentially enhance the effectiveness of convective delivery of drugs, such as carboplatin, to the brain, by enabling a more sustained dosage over a longer time than otherwise possible. However, the link between the controlled release nanoparticle synthesis route, and the subsequent drug release profile obtained, is not well-understood, which hinders design of synthesis routes and availability of suitable nanoparticles. In particular, despite pore structure evolution often forming a key aspect of past theories of the physical mechanism by which a particular drug release profile is obtained, these theories have not been independently tested and validated against pore structural information. Such validation is required for intelligent synthesis design, and NMR cryoporometry can supply the requisite information. Unlike conventional pore characterisation techniques, NMR cryoporometry permits the investigation of porous particles in the wet state. NMR cryoporometry has thus enabled the detailed study of the evolving, nanoscale structure of nanoparticles during drug release, and thus related pore structure to drug release profile in a way not done previously for nanoparticles. Nanoparticles with different types of carboplatin drug release profiles were compared, including burst release, and various forms of delayed release. ESEM and TEM images of these nanoparticles also provided supporting data showing the rapid initial evolution of some nanoparticles. Different stages, within a complex, varying drug release profile, were found to be associated with particular types of changes in the nanostructure which could be distinguished by NMR. For a core-coat nanoparticle formulation, the development of smaller nanopores, following an extended induction period with no structural change, was associated with the onset of substantial drug release. This information could be used to independently validate the rationale for a particular synthesis method. Hence, the specific reasons for the effectiveness of the synthesis route, for obtaining core-coat nanoparticles with delayed release, have been elucidated.

Probing pore connectivity in random porous materials by scanning freezing and melting experiments

Freezing and melting behavior of nitrobenzene confined to pores of Vycor porous glass with random pore structure has been studied by means of nuclear magnetic resonance cryoporometry. The two transitions are found to reveal a broad hysteresis. To get deeper insight into the mechanisms leading to this phenomenon, scanning experiments exploiting temperature reversal upon incomplete freezing or melting have been performed. In this way, it was found that different cooling and warming histories result in different solid-liquid configurations within the pore system. Further evolution of the thus-attained configurations with changing temperature unveiled important information about the transition paths. In particular, these experiments indicated the occurrence of a pronounced pore blocking for freezing, resulting in freezing transitions via invasion percolation. The melting, on the other hand, is found to occur homogeneously over the whole pore network.

The initial release of cisplatin from poly(lactide-co-glycolide) microspheres

PLGA microspheres loaded with cisplatin were produced using a single emulsion method. A semi-empirical model, with bi-exponential terms, was found to give a better fit to the drug release profiles compared to a mono-exponential model. This model suggests that there are two separate fractions of drug present in the depot. A fraction of the drug is located near/at the surface of the depot, and is readily released during immersion in buffer. A second fraction of drug is entrapped deeper within the depot and is subsequently released. It was also found that the initial release of cisplatin from PLGA microsphere is highly diffusion-controlled and the classical Higuchi model provides a good fit. From studies of water diffusion using PFG-NMR, results suggested that 50:50 PLGA microsphere was most susceptible to swelling and this might have promoted the faster initial drug release. Results from NMR cryoporometry also indicated that the developed PLGA microspheres could have "ink-bottle" pores.