M.C. Davies

The preparation of sub-200 nm poly(lactide-co-glycolide) microspheres for site-specific drug delivery

Abstract The biodistribution of injected colloidal carriers for targeted delivery will be highly dependent upon their size and surface properties. This paper describes investigations on the preparation of sub-200 nm poly(lactide-co-glycolide) (PLGA) microspheres by the variation of processing parameters using the solvent evaporation technique. Smaller particle sizes were found to be favoured by a two stage emulsification process, a low PLGA concentration and an increased surfactant concentration. In the latter case, it would appear that the viscosity of the continuous phase is a crucial factor. The process of particle size reduction could further be complimented by using a surfactant of lower molecular weight. Employing such procedures, microspheres as low as 90 nm in diameter of low polydispersity were prepared in a reproducible manner.

Surface plasmon resonance for real time in situ analysis of protein adsorption to polymer surfaces.

The adsorption of a range of plasma proteins to metal and polymer surfaces has been examined using surface plasmon resonance (SPR). The adsorption of proteins was initially studied on the SPR silver sensor surface, and then on a model polystyrene film spun coated directly onto this substrate. In both cases, reproducible adsorption profiles for albumin were attained which compared well with corresponding atomic force microscopy (AFM) and ellipsometry data on protein monolayer packing and thickness respectively. The SPR data revealed the influence of concentration on both protein adsorption kinetics and the time for formation of a monolayer coating. SPR data also highlighted different adsorption kinetics and final monolayer SPR angle shift values for three plasma proteins which have been interpreted in terms of their molecular dimensions and orientation at the polymer interface. AFM data confirmed the presence of a closely packed protein layer for all three protein systems. These studies are discussed in terms of employing SPR in the study of protein interactions at surfaces which are important in the design and evaluation of novel biomedical polymeric materials.

Structure and behaviour in hydrophilic matrix sustained release dosage forms: 2. NMR-imaging studies of dimensional changes in the gel layer and core of HPMC tablets undergoing hydration

Abstract The swelling of hydrating HPMC tablets has been studied by NMR microscopy. The technique is non-invasive and has allowed both dimensional changes in the core and in the developing surface hydrated layer to be studied. Hydration at the edges of the tablet occurred to a greater extent than in the centre of the table surfaces, giving rise to a convex shaped hydrated layer. Gel layer development occurred to the same extent in both axial and radial directions, and was similar in all HPMC types. The predominantly axial swelling reported for all HPMC types, was shown to result almost equally from growth of the hydrated surface gel layer and expansion of the ungelled tablet core. The smaller axial expansion observed in E4M tablets was not a result of slower gel layer growth, but was entirely due to a smaller expansion of the core.

Preparation of sub-100 nm human serum albumin nanospheres using a pH-coacervation method.

Human serum albumin (HSA) nanospheres of about 100 nm diameter were prepared using a pH-coacervation method whereby acetone was added to an HSA solution (pH 9.0). The particles obtained were cross-linked by glutaraldehyde. Increasing the pH of the HSA solution resulted in a gradual rise in the particle size of the resultant nanospheres. A higher cross-linking efficiency was obtained with increased glutaraldehyde concentration and cross-linking time. No significant differences in surface properties, as determined by zeta potential measurements, were recorded between particles prepared from HSA solutions with different pH. The nanospheres were quite stable over 4 days in both phosphate buffer saline (PBS) solution (pH 7.4) and rat serum, but degraded rapidly over 6 hours when incubated in PBS solution containing trypsin.

Control of staphylococcal adhesion to polystyrene surfaces by polymer surface modification with surfactants.

The adherence of three clinical isolates of Staphylococcus epidermidis to model polystyrene surfaces was studied in vitro using epifluorescent image analysis. A series of 16 Pluronic surfactants (A-B-A block copolymers where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO)) were used as surface modifiers for the model polystyrene surfaces. Substantial reductions (up to 97%) in bacterial adhesion levels were achieved with all copolymers tested, irrespective of the PPO or PEO block lengths. It appears likely that such treatments create a sterically stabilized surface with adsorbed PEO chains, conferring nonspecific anti-adhesive properties which can limit bacterial attachment.

Competitive protein adsorption as observed by surface plasmon resonance

The competitive nature of protein adsorption has been investigated in situ by surface plasmon resonance (SPR) analysis. The adsorption from blood plasma solutions of albumin, fibrinogen and immunoglobulin-G (IgG), to a polystyrene surface was investigated as part of concentration- and time-dependent studies, to observe the sequential adsorption of the three proteins at the surface. Adsorption of plasma solutions at a range of concentrations or incubation times was performed and the resulting surfaces were probed by the addition of an appropriate antibody to the protein surface. The process was repeated for each antigen leading to a surface concentration profile of each protein with respect to plasma concentration and plasma incubation time. The SPR was able to detect changes in the relative surface concentration of each component demonstrating that the proteins residence time at the interface was dependent upon its molecular weight, bulk concentration and surface affinity. All ri,hts reserved

Detection and determination of surface levels of poloxamer and PVA surfactant on biodegradable nanospheres using SSIMS and XPS

The surface chemical characterisation of sub-200 nm poly(DL-lactide co-glycolide) nanospheres has been carried out using the complementary analytical techniques of static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS). The nanospheres, which are of interest for site-specific drug delivery, were prepared using an emulsification-solvent evaporation technique with poly(vinyl alcohol), Poloxamer 407 and Poloxamine 908 respectively as stabilisers. The presence of surfactant molecules on the surface of cleaned biodegradable colloids was confirmed and identified on a qualitative molecular level (SSIMS) and from a quantitative elemental and functional group analysis (XPS) perspective. SSIMS and XPS data were also used in combination with electron microscopy to monitor the effectiveness of cleaning procedures in removing poorly bound surfactant molecules from the surface of nanospheres. The findings are discussed with respect to the development of nanoparticle delivery systems, particularly the composition of the surface for extending blood circulation times and achieving site-specific deposition.

Blind reconstruction of scanning probe image data

Scanning probe microscopy has proven to be an invaluable tool for the investigation of surface topography; however, the finite geometry of the imaging tip can often distort image data and complicate metrological investigations of surface features. Here, the derivation of a computational procedure for the estimation of the geometry of the scanning probe from the topographic image data alone is presented. The properties of the tip function extracted from such data permit an assessment of the sample‐related information content of an image. The technique is demonstrated by its application to simulated scanning probe microscopy image data, where its performance can be assessed, and by its application to experimental image data obtained from the scanning force microscope.

Biodegradable polymeric microparticles for drug delivery and vaccine formulation: the surface attachment of hydrophilic species using the concept of poly(ethylene glycol) anchoring segments.

Poly(ethylene glycol)-dextran (PEG-DEX) conjugates have been used as a combined stabilizer and surface modifier to produce resorbable poly(DL-lactide-co-glycolide) (PLG) microparticles by an emulsification/solvent evaporation technique. The use of PEG or dextran polymers alone was incapable of producing microparticles. Particle size measurements revealed smaller mean particle sizes (480 nm) and improved polydispersity when using a 1.2% PEG substituted conjugate relative to a 9% substituted material (680 nm). PLG microparticles modified by post-adsorbed PEG-DEX conjugates flocculated in 0.01 M salt solutions, whereas PLG microparticles prepared using PEG-DEX as a surfactant were stable in at least 0.5 M NaCl solutions. Surface modification of PLG microparticles was confirmed by zeta potential measurements and surface analysis using X-ray photoelectron spectroscopy. The presence of surface exposed dextran was confirmed by an immunological detection method using a dextran-specific antiserum in an enzyme-linked immunosorbent assay. The findings support a model in which the PEG component of the PEG-DEX conjugate provides an anchor to the microparticle surface while the dextran component extends from the particle surface to contribute a steric stabilization function. This approach offers opportunities for attaching hydrophilic species such as targeting moieties to biodegradable microparticles to improve the interaction of drug carriers and vaccines with specific tissue sites.

Steric stabilization of microspheres with grafted polyethylene oxide reduces phagocytosis by rat Kupffer cells in vitro.

Sterically stabilized polyethylene oxide-polystyrene copolymer microspheres, (PS-PEO) and charge stabilized polystyrene (PS) microspheres of similar size (1 micron) were prepared in order to compare their uptake by cultured rat Kupffer cells isolated by centrifugal elutriation. The uptake of the sterically stabilized particles was found to be much less than that for the charge stabilized control. The uptake of microspheres stabilized with covalently grafted PEO was lower or equivalent to that of control microspheres stabilized by the adsorption of the non-ionic PEO-polypropylene oxide (PPO-PEO) surfactant Poloxamer 238 or Methoxy-PEO. Phagocytic uptake by Kupffer cells at low and body temperature (8 degrees C and 37 degrees C) demonstrated that PS-PEO particles showed both low adherence and low metabolic uptake. The adsorption of PEO, as Poloxamer 238, to particles with covalently attached or grafted PEO resulted in a synergistic reduction in uptake that was greater than the individual effects of grafting and adsorption alone (P less than or equal to 0.001). It is suggested that this combination produces a more effective steric barrier on the particle surface with the Poloxamer adsorbing to the surface between the grafted PEO chains. The relevance to drug targeting/carrier systems is discussed.