Biorelevant and screening dissolution methods for minocycline hydrochloride microspheres intended for periodontal administration.

Currently, there is no compendial-level method to assess dissolution of particulate systems administered in the periodontal pocket. This work seeks to develop dissolution methods for extended release poly(lactic-co-glycolic acid) (PLGA) microspheres applied in the periodontal pocket. Arestin®, PLGA microspheres containing minocycline hydrochloride (MIN), is indicated for reduction of pocket depth in adult periodontitis. Utilizing Arestin® as a model product, two dissolution methods were developed: a dialysis set-up using USP apparatus 4 and a novel apparatus fabricated to simulate in vivo environment of the periodontal pocket. In the biorelevant method, the microspheres were dispersed in 250 µL of simulated gingival crevicular fluid (sGCF) which was enclosed in a custom-made dialysis enclosure. sGCF was continuously delivered to the device at a biorelevant flow rate and was collected daily for drug content analysis using UPLC. Both methods could discriminate release characteristics of a panel of MIN-loaded PLGA microspheres that differed in composition and process conditions. A mechanistic model was developed, which satisfactorily explained the release profiles observed using both dissolution methods. The developed methods may have the potential to be used as routine quality control tools to ensure batch-to-batch consistency and to support evaluation of bioequivalence for periodontal microspheres.

The effect of PLGA molecular weight differences on risperidone release from microspheres.

The objective of the present study was to investigate the effect of molecular weight differences of poly (lactic-co-glycolic acid) (PLGA) on the in vitro release profile of risperidone microspheres. Four different PLGA molecular weights were investigated and all the microsphere formulations were prepared using the same manufacturing process. Physicochemical properties (particle size, drug loading, morphology and molecular weight) as well as in vitro degradation profiles of the prepared microspheres were investigated in addition to in vitro release testing. The in vitro release tests were performed using a previously developed flow through cell (USP apparatus 4) method. The particle size of the four prepared microsphere formulations varied, however there were no significant differences in the drug loading. Interestingly, the in vitro release profiles did not follow the molecular weight of the polymers used. Instead, the drug release appeared to be dependent on the glass transition temperature of the polymers as well as the porosity of the prepared formulations. The catalytic effect of risperidone (an amine drug) on PLGA during manufacturing and release testing, minimized the differences in the molecular weights of the four formulations, explaining the independence of the release profiles on PLGA molecular weight.

Positive Charge of "Sticky" Peptides and Proteins Impedes Release From Negatively Charged PLGA Matrices.

The influence of electrostatic interactions and/or acylation on release of charged ("sticky") agents from biodegradable polymer matrices was systematically characterized. We hypothesized that release of peptides with positive charge would be hindered from negatively charged poly(lactic-co-glycolic acid) (PLGA) microparticles. Thus, we investigated release of peptides with different degrees of positive charge from several PLGA microparticle formulations, with different molecular weights and/or end groups (acid- or ester-terminated). Indeed, release studies revealed distinct inverse correlations between the amount of positive charge on peptides and their release rates from each PLGA microparticle formulation. Furthermore, we examined the case of peptides with net charge that changes from negative to positive within the pH range observed in degrading microparticles. These charge changing peptides displayed counterintuitive release kinetics, initially releasing faster from slower degrading (less acidic) microparticles, and releasing slower from the faster degrading (more acidic) microparticles. Importantly, trends between agent charge and release rates for model peptides also translated to larger, therapeutically relevant proteins and oligonucleotides. The results of these studies may improve future design of controlled release systems for numerous therapeutic biomolecules exhibiting positive charge, ultimately reducing time-consuming and costly trial and error iterations of such formulations.

In vitro characterization of a sustained-release formulation for enfuvirtide.

Although approved by the U.S. Food and Drug Administration, enfuvirtide is rarely used in combination antiretroviral therapies (cART) to treat HIV-1 infection, primarily because of its intense dosing schedule that requires twice-daily subcutaneous injection. Here, we describe the development of enfuvirtide-loaded, degradable poly(lactic-co-glycolic) acid microparticles that provide linear in vitro release of the drug over an 18-day period. This sustained-release formulation could make enfuvirtide more attractive for use in cART.

"Zero-dimensional" single-walled carbon nanotubes.

The shorter, the more dispersible: An iterative, emulsion-based shortening technique has been used to reduce the length of single-walled carbon nanotubes (SWNTs) to the same order of magnitude as their diameter (ca. 1 nm), thus achieving an effectively "zero-dimensional" structure with improved dispersibility and, after hydroxylation, long-term water solubility. Finally, zero-dimensional SWNTs were positively identified using mass spectrometry for the first time.

The effect of monomer order on the hydrolysis of biodegradable poly(lactic-co-glycolic acid) repeating sequence copolymers.

The effect of sequence on copolymer properties is rarely studied despite the precedent from Nature that monomer order can create materials of significant diversity. Poly(lactic-co-glycolic acid) (PLGA), one of the most important biodegradable copolymers, is widely used in an unsequenced, random form for both drug delivery microparticles and tissue engineering matrices. Sequenced PLGA copolymers have been synthesized and fabricated into microparticles to study how their hydrolysis rates compare to those of random copolymers. Sequenced PLGA microparticles were found to degrade at slower, and often more constant, rates than random copolymers with the same lactic to glycolic acid ratios as demonstrated by molecular weight decrease, lactic acid release, and thermal property analyses. The impact of copolymer sequence on in vitro release was studied using PLGA microparticles loaded with model agent rhodamine-B. These assays established that copolymer sequence affects the rate of release and that a more gradual burst release can be achieved using sequenced copolymers compared to a random control.

A retrospective mathematical analysis of controlled release design and experimentation.

The development and performance evaluation of new biodegradable polymer controlled release formulations relies on successful interpretation and evaluation of in vitro release data. However, depending upon the extent of empirical characterization, release data may be open to more than one qualitative interpretation. In this work, a predictive model for release from degradable polymer matrices was applied to a number of published release data in order to extend the characterization of release behavior. Where possible, the model was also used to interpolate and extrapolate upon collected released data to clarify the overall duration of release and also kinetics of release between widely spaced data points. In each case examined, mathematical predictions of release coincide well with experimental results, offering a more definitive description of each formulation's performance than was previously available. This information may prove particularly helpful in the design of future studies, such as when calculating proper dosing levels or determining experimental end points in order to more comprehensively evaluate a controlled release system's performance.

A unified mathematical model for the prediction of controlled release from surface and bulk eroding polymer matrices.

A unified model has been developed to predict release not only from bulk eroding and surface eroding systems but also from matrices that transition from surface eroding to bulk eroding behavior during the course of degradation. This broad applicability is afforded by fundamental diffusion/reaction equations that can describe a wide variety of scenarios including hydration of and mass loss from a hydrolysable polymer matrix. Together, these equations naturally account for spatial distributions of polymer degradation rate. In this model paradigm, the theoretical minimal size required for a matrix to exhibit degradation under surface eroding conditions was calculated for various polymer types and then verified by empirical data from the literature. An additional set of equations accounts for dissolution- and/or degradation-based release, which are dependent upon hydration of the matrix and erosion of the polymer. To test the model's accuracy, predictions for agent egress were compared to experimental data from polyanhydride and polyorthoester implants that were postulated to undergo either dissolution-limited or degradation-controlled release. Because these predictions are calculated solely from readily attainable design parameters, it seems likely that this model could be used to guide the design controlled release formulations that produce a broad array of custom release profiles.

Differences between chemisorbed and physisorbed biomolecules on particle deposition to hydrophobic surfaces.

This study examines differences between chemisorbed and physisorbed biomolecules on bacterial adhesion to both hydrophobic and hydrophilic surfaces that are biologically nonspecific. Bacteria-sized latex microspheres were used as a simplified model in order to study these factors that affect microbial adhesion. Two biomolecules (protein A, poly-D-lysine) were covalently bound to microspheres in order to study the effect of proteins on particle filtration rates in columns packed with glass beads. When poly-D-lysine or protein A was covalently bonded to the microspheres, sticking coefficients (a) for the microspheres increased by up to an order of magnitude as compared with uncoated latex microspheres. The glass packing beads were then made hydrophobic by covalently attaching silane groups with different carbon-chain lengths (0.2, 1.2, and 2.8 nm). Sticking coefficients forthe uncoated microspheres on these silanized packing beads (alpha = 0.15 at 1 mM ionic strength; 0.76 at 100 mM) were larger than those on uncoated glass packing beads (0.02 at 1 mM; 0.15 at 100 mM). In addition, adhesion increased with ionic strength on both hydrophobic and hydrophilic surfaces. Physical adsorption gave different results. When either dextran or protein A was physically adsorbed to both the microspheres and the column, no appreciable change in adhesion was observed. Covalently attaching protein A to the microspheres increased their hydrophobicity, but sticking coefficients were large regardless of the substrate hydrophobicity as a result of biomolecule-surface interactions. This study demonstrates that, at high ionic strength, covalently attached hydrophobic species give much higher sticking coefficients for particles than do physically adsorbed species.