Development of mass transport resistance in poly(lactide-co-glycolide) films and particles – A mechanistic study

The role of porosity in polyester microparticles for drug delivery

Polymer microparticles are a cornerstone in the field of injectable sustained delivery systems: They allow the entrapment of various types of hydrophobic or hydrophilic drugs including biopharmaceuticals. Microparticles can be prepared from the material of choice and tailored to specific target sizes. Importantly, they can retain the drug at the local administration site to achieve a sustained drug release for long-term therapeutic effects. This review focuses on the role of porosity of microparticles as a tremendously important property. Principles to prepare porous carriers via different techniques and additives are discussed, emphasizing that porosity is not a static property but can be dynamic, e.g., for particles from polylactide or poly(lactide-co-glycolide). Considering the contribution of porosity in the overall assessment of drug carrier systems, as well as their manipulation/alteration post-production such as by pore closing, will enlarge the understanding of polymer microparticles as an important class of modern pharmaceutical dosage forms.

Effect of processing parameters on characteristics of biodegradable extended-release microspheres containing leuprolide acetate

OBJECTIVE

Poly(lactic-co-glycolic acid) microsphere containing leuprolide acetate - an extended-release drug delivery system whose characteristics (i.e. loading capacity, particle size and initial burst phase) depend on processing parameters.

METHODS

Microspheres were prepared by water/oil/water double-emulsion solvent evaporation method; drug content in microspheres was determined by high-performance liquid chromatography (HPLC); peptide concentration in the release medium was measured by fluorescence spectrometer; particle size and particle size distribution were measured by laser diffraction method; interaction between poly(lactic-co-glycolic acid) (PLGA) and leuprolide acetate (LA) was determined by differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR).

RESULTS

DSC curves and assay results proved LA adsorption ability of PLGA film. FTIR spectra proved ionic interactions between positive charged LA molecules and negative charged PLGA chains in phosphate buffer pH 7.4. Ten processing parameters including LA concentration (mg/mL), PLGA concentration (mg/mL), W1/O ratio (v/v), the first homogenization time (min), the first homogenization speed (rpm), O/W2 ratio (v/v), PVA concentration of W2 phase (mg/ml), the second homogenization time (s), the volume of diluted solution (ml) and nitrogen aeration time (min) have impacts on loading capacity, particle size and initial burst phase of microspheres. The release exponent (n) of Korsmeyer-Peppas model was 0.3571 (lower than 0.43), indicating that Fickian diffusion manipulated release kinetics of initial burst phase.

CONCLUSIONS

Processing parameter modification contributes to small microspheres with high loading capacity and controlled initial burst phase.

Polymer microsphere inks for semi-solid extrusion 3D printing at ambient conditions

Extrusion-based 3D printing methods have great potential for manufacturing of personalized polymer-based drug-releasing systems. However, traditional melt-based extrusion techniques are often unsuitable for processing thermally labile molecules. Consequently, methods that utilize the extrusion of semi-solid inks under mild conditions are frequently employed. The rheological properties of the semi-solid inks have a substantial impact on the 3D printability, making it necessary to evaluate and tailor these properties. Here, we report a novel semi-solid extrusion 3D printing method based on utilization of a Carbopol gel matrix containing various concentrations of polymeric microspheres. We also demonstrate the use of a solvent vapor-based post-processing method for enhancing the mechanical strength of the printed objects. As our approach enables room-temperature processing of polymers typically used in the pharmaceutical industry, it may also facilitate the broader application of 3D printing and microsphere technologies in preparation of personalized medicine.

3D printed PLGA implants: How the filling density affects drug release

Different types of ibuprofen-loaded, poly (D,L lactic-co-glycolic acid) (PLGA)-based implants were prepared by 3D printing (Droplet Deposition Modeling). The theoretical filling density of the mesh-shaped implants was varied from 10 to 100%. Drug release was measured in agarose gels and in well agitated phosphate buffer pH 7.4. The key properties of the implants (and dynamic changes thereof upon exposure to the release media) were monitored using gravimetric measurements, optical microscopy, Differential Scanning Calorimetry, Gel Permeation Chromatography, and Scanning Electron Microscopy. Interestingly, drug release was similar for implants with 10 and 30% filling density, irrespective of the experimental set-up. In contrast, implants with 100% filling density showed slower release kinetics, and the shape of the release curve was altered in agarose gels. These observations could be explained by the existence (or absence) of a continuous aqueous phase between the polymeric filaments and the "orchestrating role" of substantial system swelling for the control of drug release. At lower filling densities, it is sufficient for the drug to be released from a single filament. In contrast, at high filling densities, the ensemble of filaments acts as a much larger (more or less homogeneous) polymeric matrix, and the average diffusion pathway to be overcome by the drug is much longer. Agarose gel (mimicking living tissue) hinders substantial PLGA swelling and delays the onset of the final rapid drug release phase. This improved mechanistic understanding of the control of drug release from PLGA-based 3D printed implants can help to facilitate the optimization of this type of advanced drug delivery systems.

3D printed PLGA implants: APF DDM vs. FDM

3D Printing offers a considerable potential for personalized medicines. This is especially true for customized biodegradable implants, matching the specific needs of each patient. Poly(lactic-co-glycolic acid) (PLGA) is frequently used as matrix former in biodegradable implants. However, yet relatively little is known on the technologies, which can be used for the 3D printing of PLGA implants. The aim of this study was to compare: (i) Arburg Plastic Freeforming Droplet Deposition Modeling (APF DDM), and (ii) Fused Deposition Modeling (FDM) to print mesh-shaped, ibuprofen-loaded PLGA implants. During APF DDM, individual drug-polymer droplets are deposited, fusing together to form filaments, which build up the implants. During FDM, continuous drug-polymer filaments are deposited to form the meshes. The implants were thoroughly characterized before and after exposure to phosphate buffer pH 7.4 using optical and scanning electron microscopy, GPC, DSC, drug release measurements and monitoring dynamic changes in the systems' dry & wet mass and pH of the bulk fluid. Interestingly, the mesh structures were significantly different, although the device design (composition & theoretical geometry) were the same. This could be explained by the fact that the deposition of individual droplets during APF DDM led to curved and rather thick filaments, resulting in a much lower mesh porosity. In contrast, FDM printing generated straight and thinner filaments: The open spaces between them were much larger and allowed convective mass transport during drug release. Consequently, most of the drug was already released after 4 d, when substantial PLGA set on. In the case of APF DDM printed implants, most of the drug was still entrapped at that time point and substantial polymer swelling transformed the meshes into more or less continuous PLGA gels. Hence, the diffusion pathways became much longer and ibuprofen release was controlled over 2 weeks.

How agarose gels surrounding PLGA implants limit swelling and slow down drug release

The aim of this study was to better understand to which extent and in which way the presence of an agarose gel (mimicking living tissue) around a PLGA [poly(lactic-co-glycolic acid)] implant affects the resulting drug release kinetics. Ibuprofen-loaded implants were prepared by hot melt extrusion. Drug release was measured upon exposure to phosphate buffer pH 7.4 in Eppendorf tubes, as well as upon inclusion into an agarose gel which was exposed to phosphate buffer pH 7.4 in an Eppendorf tube or in a transwell plate. Dynamic changes in the implants' dry & wet mass and dimensions were monitored gravimetrically and by optical macroscopy. Implant erosion and polymer degradation were observed by SEM and GPC. Different pH indicators were used to measure pH changes in the bulk fluids, gels and within the implants during drug release. Ibuprofen release was bi-phasic in all cases: A zero order release phase (~20% of the dose) was followed by a more rapid, final drug release phase. Interestingly, the presence of the hydrogel delayed the onset of the 2nd release phase. This could be attributed to the sterical hindrance of implant swelling: After a certain lag time, the degrading PLGA matrix becomes sufficiently hydrophilic and mechanically instable to allow for the penetration of substantial amounts of water into the system. This fundamentally changes the conditions for drug release: The latter becomes much more mobile and is more rapidly released. A gel surrounding the implant mechanically hinders system swelling and, thus, slows down drug release. These observations also strengthen the hypothesis of the "orchestrating" role of PLGA swelling for the control of drug release and can help developing more realistic in vitro release set-ups.

The effect of polymer blends on initial release regulation and in vitro-in vivo relationship of peptides loaded PLGA-Hydrogel Microspheres

The aim of this study was to resolve the lag time problem for peptides loaded PLGA-Hydrogel Microspheres (PLGA-gel-Ms) by blending low molecular PLGA (Mw. 1 kDa) into PLGA (Mw. 10 kDa) as an intrinsic porogen, and then assess the in vitro-in vivo relationship (IVIVR). Here, Goserelin acetate (GOS) was chosen as the model peptides. When compared to additional types of porogen, the intrinsic porogen avoided impurities remaining and protected the bioactivities of the peptides. By adding 10% PLGA (Mw. 1 kDa), the lag time was eliminated both in vitro and in vivo with a desirable EE (97.04% ± 0.51%). The release mechanisms were found to be: a) initial GOS release mainly controlled by pores diffusion and b) autocatalysis of PLGA (Mw. 1 kDa) which increased the quantity of aqueous pores, as revealed by SEM images. To solve the challenges caused by multiphasic release profiles, for the first time the Segmented phases IVIVR were proposed and developed, and showed improved linear fitting effects and supported the proposed release mechanisms. The application of PLGA blends could provide a new insight into PLGA microsphere initial release rate regulation.

Physicochemical Characterization and Pharmacokinetics of Agomelatine-Loaded PLGA Microspheres for Intramuscular Injection

PURPOSE

The aim of this study was to design agomelatine loaded long acting injectable microspheres, with an eventual goal of reducing the frequency of administration and improving patient compliance in treatment of depression.

METHODS

AGM-loaded microspheres were prepared by an O/W emulsion solvent evaporation method. The physicochemical properties and in vitro performance of the microspheres were characterized. The pharmacokinetics of different formulations with various particle sizes and drug loadings were evaluated.

RESULTS

AGM-loaded microspheres with drug loading of 23.7% and particle size of 60.2 μm were obtained. The in vitro release profiles showed a small initial burst release (7.36%) followed by a fast release, a period of lag time and a second accelerated release. Pore formation and pore closure were observed in vitro, indicating that the release of drug from microspheres is dominated by water-filled pores. Pharmacokinetic studies showed that AGM microspheres could release up to 30 days in vivo at a steady plasma concentration. As well, particle size and drug loading could significantly influence the in vivo release of AGM microspheres.

CONCLUSIONS

AGM-loaded microspheres are a promising carrier for the treatment of major depressant disorder.

Controlled drug release and hydrolysis mechanism of polymer-magnetic nanoparticle composite

Uniform and multifunctional poly(lactic acid) (PLA)-nanoparticle composite has enormous potential for applications in biomedical and materials science. A detailed understanding of the surface and interface chemistry of these composites is essential to design such materials with optimized function. Herein, we designed and investigated a simple PLA-magnetic nanoparticle composite system to elucidate the impact of nanoparticles on the degradation of polymer-nanoparticle composites. In order to have an in-depth understanding of the mechanisms of hydrolysis in PLA-nanoparticle composites, degradation processes were monitored by several surface sensitive techniques, including scanning electron microscopy, contact angle goniometry, atomic force microscopy, and sum frequency generation spectroscopy. As a second-order nonlinear optical technique, SFG spectroscopy was introduced to directly probe in situ chemical nature at the PLA-magnetic nanoparticle composite/aqueous interface, which allowed for the delineation of molecular mechanisms of various hydrolysis processes for degradation at the molecular level. The best PLA-NP material, with a concentration of 20% MNP in the composite, was found to enhance the drug release rate greater than 200 times while maintaining excellent controlled drug release characteristics. It was also found that during hydrolysis, various crystalline-like PLA domains on the surfaces of PLA-nanoparticle composites influenced various hydrolysis behaviors of PLA. Results from this study provide new insight into the design of nanomaterials with controlled degradation and drug release properties, and the underlined molecular mechanisms. The methodology developed in this study to characterize the polymer-nanoparticle composites is general and widely applicable.

Cyclic swelling as a phenomenon inherent to biodegradable polyesters

The aim of this study is to evaluate and describe the phenomenon and mechanism of the spontaneous cyclic swelling and deswelling of linear and branched aliphatic polyesters in the aqueous medium. The fluctuation of gel volume in one or several cycles as an inherent property of biodegradable and bioerodible materials has not yet been described. We have observed the process at linear and branched polyesters of aliphatic α-hydroxy acids. The period of duration of cycles was in order of hours to days, as influenced by the size of the bodies ranging from 25 to 1000 mg, the temperature in the range of 7°C-42°C, ionic strength, and pH value. The results demonstrated that swelling is accompanied by hydrolysis of ester bonds with the development of small water-soluble osmotically active molecules. After reaching a higher degree of swelling, the obstruction effect of the gel decreases and the diffusion of soluble degradation products from the body to the environment prevails. A decrease in osmotic pressure inside the body and a decrease in the hydrophilic character of the gel matrix result in deswelling by a collapse of the structure, probably due to hydrophobic interactions of nonpolar polyester chains.

Mathematical modeling of drug delivery from autocatalytically degradable PLGA microspheres--a review

PLGA microspheres are widely studied for controlled release drug delivery applications, and many models have been proposed to describe PLGA degradation and erosion and drug release from the bulk polymer. Autocatalysis is known to have a complex role in the dynamics of PLGA erosion and drug transport and can lead to size-dependent heterogeneities in otherwise uniformly bulk-eroding polymer microspheres. The aim of this review is to highlight mechanistic, mathematical models for drug release from PLGA microspheres that specifically address interactions between phenomena generally attributed to autocatalytic hydrolysis and mass transfer limitation effects. Predictions of drug release profiles by mechanistic models are useful for understanding mechanisms and designing drug release particles.

Extrusion printed polymer structures: a facile and versatile approach to tailored drug delivery platforms

A novel extrusion printing system was used to create drug delivery structures wherein dexamethasone-21-phosphate disodium salt (Dex21P) was encapsulated within a biodegradable polymer (PLGA) and water soluble poly(vinyl alcohol) (PVA) configurations. The ability to control the drug release profile through the spatial distribution of drug within the printed 3-dimensional structures is demonstrated. The fabricated configurations were characterised by optical microscopy and SEM to evaluate surface morphology. The results clearly demonstrate the successful encapsulation of dexamethasone within a laminated PLGA:PVA structure. The resulting drug release profiles from the structures show a two stage release profile with distinctly different release rates and minimal initial burst release observed. Dexamethasone release was monitored over a 4-month period. This approach clearly demonstrates that the extrusion printing technique provides a facile and versatile approach to fabrication of novel drug delivery platforms.