Improving release completeness from PLGA-based implants for the acid-labile model protein ovalbumin

Burst Release from In Situ Forming PLGA-Based Implants: 12 Effectors and Ways of Correction

In modern pharmaceutical technology, modified-release dosage forms, such as in situ formed implants, are gaining rapidly in popularity. These dosage forms are created based on a configurable matrix consisting of phase-sensitive polymers capable of biodegradation, a hydrophilic solvent, and the active substance suspended or dissolved in it. The most used phase-sensitive implants are based on a biocompatible and biodegradable polymer, poly(DL-lactide-co-glycolide) (PLGA).

OBJECTIVE

This systematic review examines the reasons for the phenomenon of active ingredient "burst" release, which is a major drawback of PLGA-based in situ formed implants, and the likely ways to correct this phenomenon to improve the quality of in situ formed implants with a poly(DL-lactide-co-glycolide) matrix.

DATA SOURCES

Actual and relevant publications in PubMed and Google Scholar databases were studied.

STUDY SELECTION

The concept of the review was based on the theory developed during literature analysis of 12 effectors on burst release from in situ forming implants based on PLGA. Only those studies that sufficiently fully disclosed one or another component of the theory were included.

RESULTS

The analysis resulted in development of a systematic approach called the "12 Factor System", which considers various constant and variable, endogenous and exogenous factors that can influence the nature of 'burst release' of active ingredients from PLGA polymer-based in situ formed implants. These factors include matrix porosity, polymer swelling, LA:GA ratio, PLGA end groups, polymer molecular weight, active ingredient structure, polymer concentration, polymer loading with active ingredients, polymer combination, use of co-solvents, addition of excipients, and change of dissolution conditions. This review also considered different types of kinetics of active ingredient release from in situ formed implants and the possibility of using the "burst release" phenomenon to modify the active ingredient release profile at the site of application of this dosage form.

ATB0,+-targeted nanoparticles initiate autophagy suppression to overcome chemoresistance for enhanced colorectal cancer therapy

Oxaliplatin (OXA) resistance remains the major obstacle to the successful chemotherapy of colorectal cancer (CRC). As a self-protection mechanism, autophagy may contribute to tumor drug resistance, therefore autophagy suppression could be regarded as a possible treatment option in chemotherapy. Cancer cells, especially drug-resistant tumor cells, increase their demand for specific amino acids by expanding exogenous supply and up-regulating de novo synthesis, to meet the needs for excessive proliferation. Therefore, it is possible to inhibit cancer cell proliferation through pharmacologically blocking the entry of amino acid into cancer cells. SLC6A14 (ATB^{0, +}) is an essential amino acid transporter, that is often abnormally up-regulated in most cancer cells. Herein, in this study, we designed oxaliplatin/berbamine-coloaded, ATB^0,+-targeted nanoparticles ((O+B)@Trp-NPs) to therapeutically target SLC6A14 (ATB^{0, +}) and inhibit cancer proliferation. The (O+B)@Trp-NPs utilize the surface-modified tryptophan to achieve SLC6A14-targeted delivery of Berbamine (BBM), a compound that is found in a number of plants used in traditional Chinese medicine, which could suppress autolysosome formation though impairing autophagosome-lysosome fusion. We verified the feasibility of this strategy to overcome the OXA resistance during colorectal cancer treatment. The (O+B)@Trp-NPs significantly inhibited the proliferation and decreased the drug resistance of resistant colorectal cancer cells. In vivo, (O+B)@Trp-NPs greatly suppressed the tumor growth in tumor-bearing mice, which is consistent with the in vitro data. This research offers a unique and promising chemotherapeutic treatment for colorectal cancer.

A Single Injection with Sustained-Release Microspheres and a Prime-Boost Injection of Bovine Serum Albumin Elicit the Same IgG Antibody Response in Mice

Although vaccination is still considered to be the cornerstone of public health care, the increase in vaccination coverage has stagnated for many diseases. Most of these vaccines require two or three doses to be administered across several months or years. Single-injection vaccine formulations are an effective method to overcome the logistical barrier to immunization that is posed by these multiple-injection schedules. Here, we developed subcutaneously (s.c.) injectable microspheres with a sustained release of the model antigen bovine serum albumin (BSA). The microspheres were composed of blends of two novel biodegradable multi-block copolymers consisting of amorphous, hydrophilic poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) (PCL-PEG-PCL) blocks and semi-crystalline poly(dioxanone) (PDO) blocks of different block sizes. In vitro studies demonstrated that the release of BSA could be tailored over a period of approximately four to nine weeks by changing the blend ratio of both polymers. Moreover, it was found that BSA remained structurally intact during release. Microspheres exhibiting sustained release of BSA for six weeks were selected for the in vivo study in mice. The induced BSA-specific IgG antibody titers increased up to four weeks after administration and were of the same magnitude as found in mice that received a priming and a booster dose of BSA in phosphate-buffered saline (PBS). Determination of the BSA concentration in plasma showed that in vivo release probably took place up to at least four weeks, although plasma concentrations peaked already one week after administration. The sustained-release microspheres might be a viable alternative to the conventional prime-boost immunization schedule, but a clinically relevant antigen should be incorporated to assess the full potential of these microspheres in practice.

Pharmacokinetic-pharmacodynamic model to predict drug concentrations and intraocular pressure lowering effect for a bimatoprost six-month slow-release system

INTRODUCTION

Ophthalmic drug product development and dosing regimen selection depend on animal eye drug concentration-effect relationships since human eye tissues cannot be sampled for drug quantification. This study hypothesized that a pharmacokinetic-pharmacodynamic (PK-PD) mathematical model developed based on dog studies can be applied to the human eye of different ages, based on physiological parameter adjustment, to predict drug concentrations and effects in response to a new 6-month slow-release, intracameral, intraocular pressure (IOP) lowering, anti-glaucoma bimatoprost implant.

METHODS

Using previously reported dog concentration-effect relationship data at various doses, and the physiological parameters of dog eye, a PK-PD model was designed to predict dog aqueous humor drug concentrations and IOP lowering effects simultaneously for a given dose. After validating the model using the dog IOP data, it was applied to the human eye.

RESULTS

Using a drug release rate constant of 0.0002 h-1, the model predicted the dog IOP lowering effect with an error less than 6% or less at various doses (Observed = 0.91*Predicted + 2.35; R2 = 0.98). Considering literature reported aqueous humor volumes and flow rates in old (over 60 years) and young (20 to 30 years) humans, aqueous humor elimination rate constant was estimated to be 0.9 and 0.68 h-1, respectively. The model when modified using the older human eye parameters, predicted the IOP lowering effects reported in a clinical trial with 63-year-old adults, with an error of 6.2% or less. The model, when used for young adult eye not previously tested in clinical trials, predicted lower drug concentrations and effects, possibly due to 54% higher aqueous humor volume relative to older adults. The model predicted an IOP reduction of 26.3 and 30.6%, at 10 and 15 microgram doses, respectively, in young adults.

CONCLUSIONS

The PK-PD model developed is useful for product design and patient dosing by predicting eye drug concentration and effect time-courses in response to implant administration at various doses, frequencies, and release rates.

PLGA implants for controlled drug release: Impact of the diameter

The aim of this study was to better understand the importance of the diameter of poly(lactic-co-glycolic acid) (PLGA)-based implants on system performance, in particular the control of drug release. Different types of ibuprofen-loaded implants were prepared by hot melt extrusion using a Leistritz Nano 16 twin-screw extruder. Drug release was measured in well agitated phosphate buffer pH7.4 bulk fluid and in agarose gels in Eppendorf tubes or transwell plates. Dynamic changes in the implants' dry & wet mass, volume, polymer molecular weight as well as inner & outer morphology were monitored using gravimetric analysis, optical macroscopy, gel permeation chromatography and scanning electron microscopy. The physical states of the drug and polymer were determined by DSC. Also pH changes in the release medium were investigated. Irrespective of the type of experimental set-up, the resulting absolute and relative drug release rates decreased with increasing implant diameter (0.7 to 2.8 mm). Bi-phasic drug release was observed in all cases from the monolithic solutions (ibuprofen was dissolved in the polymer): A zero order release phase was followed by a final, rapid drug release phase (accounting for 80-90% of the total drug dose). The decrease in the relative drug release rate with increasing system diameter can be explained by the increase in the diffusion pathway lengths to be overcome. Interestingly, also the onset of the final rapid drug release phase was delayed with increasing implant diameter. This can probably be attributed to the higher mechanical stability of thicker devices, offering more resistance to substantial entire system swelling.

Current trends in PLGA based long-acting injectable products: The industry perspective

INTRODUCTION

Poly (lactic-co-glycolic acid) (PLGA) has been used in many long-acting drug formulations, which have been approved by the US Food and Drug Administration (FDA). PLGA has unique physicochemical properties, which results in complexities in the formulation, characterization, and evaluation of generic products. To address the challenges of generic development of PLGA-based products, the FDA has established an extensive research program to investigate novel methods and tools to aid product development and regulatory review.

AREAS COVERED

This review article intends to provide a comprehensive review on physicochemical properties of PLGA polymer, characterization, formulation, and analytical aspects, manufacturing conditions on product performance, in-vitro release testing, and bioequivalence. Current research on formulation development as per QbD in vitro release testing methods, regulatory research outcomes, and bioequivalence.

EXPERT OPINION

The development of PLGA based long-acting injectables is promising and challenging when considering the numerous interrelated delivery-related factors. Achieving a successful formulation requires a thorough understanding of the critical interactions between polymer/drug properties, release profiles over time, up-to-date knowledge on regulatory guidance, and elucidation of the impact of multiple in vivo conditions to methodically evaluate the eventual clinical efficacy.

Microfluidic Production of Polymeric Core-Shell Microspheres for the Delayed Pulsatile Release of Bovine Serum Albumin as a Model Antigen

For many vaccines, multiple injections are required to confer protective immunity against targeted pathogens. These injections often consist of a primer administration followed by a booster administration of the vaccine a few weeks or months later. A single-injection vaccine formulation that provides for both administrations could greatly improve the convenience and vaccinee's compliance. In this study, we developed parenterally injectable core-shell microspheres with a delayed pulsatile release profile that could serve as the booster in such a vaccine formulation. These microspheres contained bovine serum albumin (BSA) as the model antigen and poly(dl-lactide-co-glycolide) (PLGA) with various dl-lactide:glycolide monomer ratios as the shell material. Highly monodisperse particles with different particle characteristics were obtained using a microfluidic setup. All formulations exhibited a pulsatile in vitro release of BSA after an adjustable lag time. This lag time increased with the increasing lactide content of the polymer and ranged from 3 to 7 weeks. Shell thickness and bovine serum albumin loading had no effect on the release behavior, which could be ascribed to the degradation mechanism of the polymer, with bulk degradation being the main pathway. Co-injection of the core-shell microspheres together with a solution of the antigen that serves as the primer would allow for the desired biphasic release profile. Altogether, these findings show that injectable core-shell microspheres combined with a primer are a promising alternative for the current multiple-injection vaccines.

The Role of the Mechanical, Structural, and Thermal Properties of Poly(l-lactide-co-glycolide-co-trimethylene carbonate) in the Development of Rods with Aripiprazole

In this work, we aimed to determine the role of the mechanical, structural, and thermal properties of poly(l-lactide-co-glycolide-co-trimethylene carbonate) (P(l-LA:GA:TMC)) with shape memory in the formulation of implantable and biodegradable rods with aripiprazole (ARP). Hot melt extrusion (HME) and electron beam (EB) irradiation were applied in the formulation process of blank rods and rods with ARP. Rod degradation was carried out in a PBS solution. HPLC; NMR; DSC; compression and tensile tests; molecular weight (M_n); water uptake (WU); and weight loss (WL) analyses; and SEM were used in this study. HME and EB irradiation did not influence the structure of ARP. The mechanical tests indicated that the rods may be safely implanted using a pre-filled syringe. During degradation, no unfavorable changes in terpolymer content were observed. A decrease in the glass transition temperature and the M_n, and an increase in the WU and the WL were revealed. The loading of ARP and EB irradiation induced earlier pore formation and more intense WU and WL changes. ARP was released in a tri-phasic model with the lag phase; therefore, the proposed formulation may be administered as a delayed-release system. EB irradiation was found to accelerate ARP release.

Polyanhydride nanoparticles stabilize pancreatic cancer antigen MUC4β

Pancreatic cancer (PC) is one of the most lethal malignancies and represents an increasing and challenging threat, especially with an aging population. The identification of immunogenic PC-specific upregulated antigens and an enhanced understanding of the immunosuppressive tumor microenvironment have provided opportunities to enable the immune system to recognize cancer cells. Due to its differential upregulation and functional role in PC, the transmembrane mucin MUC4 is an attractive target for immunotherapy. In the current study we characterized the antigen stability, antigenicity and release kinetics of a MUC4β-nanovaccine to guide further optimization and, in vivo evaluation. Amphiphilic polyanhydride copolymers based on 20 mol % 1,8-bis(p-carboxyphenoxy)-3,6-dioxaoctane and 80 mol % 1,6-bis(p-carboxyphenoxy)hexane were used to synthesize nanoparticles. Structurally stable MUC4β protein was released from the particles in a sustained manner and characterized by gel electrophoresis and fluorescence spectroscopy. Modest levels of protein degradation were observed upon release. The released protein was also analyzed by MUC4β-specific monoclonal antibodies using ELISA and showed no significant loss of epitope availability. Further, mice immunized with multiple formulations of combination vaccines containing MUC4β-loaded nanoparticles generated MUC4β-specific antibody responses. These results indicate that polyanhydride nanoparticles are viable MUC4β vaccine carriers, laying the foundation for evaluation of this platform for PC immunotherapy.

Development of mAb-loaded 3D-printed (FDM) implantable devices based on PLGA

The main objective of this work was to explore the feasibility to print monoclonal antibody (mAb)-loaded implantable systems using fused-deposition modelling (FDM) to build complex dosage form designs. Indeed, to our knowledge, this work is the first investigation of mAb-loaded devices using FDM. To make this possible, different steps were developed and optimized. A mAb solution was stabilized using trehalose (TRE), sucrose (SUC), hydroxypropyl-β-cyclodextrin (HP-β-CD), sorbitol or inulin (INU) in order to be spray dried (SD). Printable filaments were then made of poly(lactide-co-glycolide) (PLGA) and mAb powder (15% w/w) using hot melt extrusion (HME). The FDM process was optimized to print these filaments without altering the mAb stability. TRE was selected and associated to L-leucine (LEU) to increase the mAb stability. The stability was then evaluated considering high and low molecular weight species levels. The mAb-based devices were well-stabilized with the selected excipients during both the HME and the FDM processes. The 3D-printed devices showed sustained-release profiles with a low burst effect. The mAb-binding capacity was preserved up to 70% following the whole fabrication process. These promising results demonstrate that FDM could be used to produce mAb-loaded devices with good stability, affinity and sustained-release profiles of the mAb.

Hot melt extrusion: An emerging manufacturing method for slow and sustained protein delivery

With the rapid development of the biopharmaceutical industry, an increasing number of new therapeutic protein products (TPPs) have been approved by the FDA and many others are under pre-clinical and clinical evaluation. A major limitation of biopharmaceuticals is their limited half-life when administered systemically. A one-time, implantable, sustained protein delivery device would be advantageous in order to improve the quality of life of patients. Hot melt extrusion (HME) is a mature technology that has been extensively used for a broad spectrum of applications in the polymer and pharmaceutical industry and has achieved success as evidenced by a variety of FDA-approved commercial products. These commercial products are mostly for sustained delivery of small molecule therapeutics, leaving a significant gap for HME formulation of therapeutic proteins. With the increasing need of sustained TPP delivery, HME shows promise as a downstream processing method due to its high efficiency and economic value. Several challenges remain for the application of HME in protein delivery. Progress of HME for protein delivery, challenges encountered, and potential solutions will be detailed in this review article. This article is categorized under: Implantable Materials and Surgical Technologies > Nanomaterials and Implants Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Engineering microenvironment of biodegradable polyester systems for drug stability and release control

Polymeric systems made of poly(lactic acid) or poly(lactic-co-glycolic acid) are widely used for long-term delivery of small and large molecules. The advantages of poly(lactic acid)/poly(lactic-co-glycolic acid) systems include biodegradability, safety and a long history of use in US FDA-approved products. However, as drugs delivered by the polymeric systems and their applications become more diverse, the significance of microenvironment change of degrading systems on long-term drug stability and release kinetics has gained renewed attention. In this review, we discuss various issues experienced with acidifying microenvironment of biodegradable polymer systems and approaches to overcome the detrimental effects of polymer degradation on drug stability and release control.

Leveraging advances in chemistry to design biodegradable polymeric implants using chitosan and other biomaterials

The metamorphosis of biodegradable polymers in biomedical applications is an auspicious myriad of indagation. The utmost challenge in clinical conditions includes trauma, organs failure, soft and hard tissues, infection, cancer and inflammation, congenital disorders which are still not medicated efficiently. To overcome this bone of contention, proliferation in the concatenation of biodegradable materials for clinical applications has emerged as a silver bullet owing to eco-friendly, nontoxicity, exorbitant mechanical properties, cost efficiency, and degradability. Several bioimplants are designed and fabricated in a way to reabsorb or degrade inside the body after performing the specific function rather than eliminating the bioimplants. The objective of this comprehensive is to unfurl the anecdote of emerging biological polymers derived implants including silk, lignin, soy, collagen, gelatin, chitosan, alginate, starch, etc. by explicating the selection, fabrication, properties, and applications. Into the bargain, emphasis on the significant characteristics of current discernment and purview of nanotechnology integrated biopolymeric implants has also been expounded. This robust contrivance shed light on recent inclinations and evolution in tissue regeneration and targeting organs followed by precedency and fly in the ointment concerning biodegradable implants evolved by employing fringe benefits provided by 3D printing technology for building tissues or organs construct for implantation.

Silica particles incorporated into PLGA-based in situ-forming implants exploit the dual advantage of sustained release and particulate delivery

Poly (lactic-co-glycolic acid) (PLGA) in situ-forming implants are well-established drug delivery systems for controlled drug release over weeks up to months. To prevent initial burst release, which is still a major issue associated with PLGA-based implants, drugs attached to particulate carriers have been encapsulated. Unfortunately, former studies only investigated the resulting release of the soluble drugs and hence missed the potential offered by particulate drug release. In this study, we developed a system capable of releasing functional drug-carrying particles over a prolonged time. First, we evaluated the feasibility of our approach by encapsulating silica particles of different sizes (500 nm and 1 μm) and surface properties (OH or NH₂ groups) into in situ-forming PLGA implants. In this way, we achieved sustained release of particles over periods ranging from 30 to 70 days. OH-carrying particles were released much more quickly when compared to NH₂-modified particles. We demonstrated that the underlying release mechanisms involve size-dependent diffusion and polymer-particle interactions. Second, particles that carried covalently-attached ovalbumin (OVA) on their surfaces were incorporated into the implant. We demonstrated that OVA was released in association with the particles as functional entities over a period of 30 days. The released particle-drug conjugates maintained their colloidal stability and were efficiently taken up by antigen presenting cells. This system consisting of particles incorporated into PLGA-based in situ-forming implants offers the dual advantage of sustained and particulate release of drugs as a functional unit and has potential for future use in many applications, particularly in single-dose vaccines.

Injectable Lipid-Based Depot Formulations: Where Do We Stand?

The remarkable number of new molecular entities approved per year as parenteral drugs, such as biologics and complex active pharmaceutical ingredients, calls for innovative and tunable drug delivery systems. Besides making these classes of drugs available in the body, injectable depot formulations offer the unique advantage in the parenteral world of reducing the number of required injections, thus increasing effectiveness as well as patient compliance. To date, a plethora of excipients has been proposed to formulate depot systems, and among those, lipids stand out due to their unique biocompatibility properties and safety profile. Looking at the several long-acting drug delivery systems based on lipids designed so far, a legitimate question may arise: How far away are we from an ideal depot formulation? Here, we review sustained release lipid-based platforms developed in the last 5 years, namely oil-based solutions, liposomal systems, in situ forming systems, solid particles, and implants, and we critically discuss the requirements for an ideal depot formulation with respect to the used excipients, biocompatibility, and the challenges presented by the manufacturing process. Finally, we delve into lights and shadows originating from the current setups of in vitro release assays developed with the aim of assessing the translational potential of depot injectables.

Quality by design thinking in the development of long-acting injectable PLGA/PLA-based microspheres for peptide and protein drug delivery

Adopting the Quality by Design (QbD) approach in the drug development process has transformed from "nice-to-do" into a crucial and required part of the development, ensuring the quality of pharmaceutical products throughout their whole life cycles. This review is discussing the implementation of the QbD thinking into the production of long-acting injectable (LAI) PLGA/PLA-based microspheres for the therapeutic peptide and protein drug delivery. Various key elements of the QbD approaches are initially elaborated using Bydureon®, a commercial product of LAI PLGA/PLA-based microspheres, as a classical example. Subsequently, the factors influencing the release patterns and the stability of the peptide and protein drugs are discussed. This is followed by a summary of the state-of-the-art of manufacturing LAI PLGA/PLA-based microspheres and the related critical process parameters (CPPs). Finally, a landscape of generic product development of LAI PLGA/PLA-based microspheres is reviewed including some major challenges in the field.

Development of a Stable Respiratory Syncytial Virus Pre-Fusion Protein Powder Suitable for a Core-Shell Implant with a Delayed Release in Mice: A Proof of Concept Study

Currently, there is an increasing interest to apply pre-fusion (pre-F) protein of respiratory syncytial virus (RSV) as antigen for the development of a subunit vaccine. A pre-F-containing powder would increase the flexibility regarding the route of administration. For instance, a pre-F-containing powder could be incorporated into a single-injection system releasing a primer, and after a lag time, a booster. The most challenging aspect, obtaining the booster after a lag time, may be achieved by incorporating the powder into a core encapsulated by a nonporous poly(dl-lactic-co-glycolic acid) (PLGA) shell. We intended to develop a stable freeze-dried pre-F-containing powder. Furthermore, we investigated whether incorporation of this powder into the core-shell implant was feasible and whether this system would induce a delayed RSV virus-neutralizing antibody (VNA) response in mice. The developed pre-F-containing powder, consisting of pre-F in a matrix of inulin, HEPES, sodium chloride, and Tween 80, was stable during freeze-drying and storage for at least 28 days at 60 °C. Incorporation of this powder into the core-shell implant was feasible and the core-shell production process did not affect the stability of pre-F. An in vitro release study showed that pre-F was incompletely released from the core-shell implant after a lag time of 4 weeks. The incomplete release may be the result of pre-F instability within the core-shell implant during the lag time and requires further research. Mice subcutaneously immunized with a pre-F-containing core-shell implant showed a delayed RSV VNA response that corresponded with pre-F release from the core-shell implant after a lag time of approximately 4 weeks. Moreover, pre-F-containing core-shell implants were able to boost RSV VNA titers of primed mice after a lag time of 4 weeks. These findings could contribute to the development of a single-injection pre-F-based vaccine containing a primer and a booster.

In-situ forming implants loaded with chlorhexidine and ibuprofen for periodontal treatment: Proof of concept study in vivo

Control of infection and inflammation is crucial for the success of periodontal treatment. In this study, in-situ forming implants (ISFI) loaded with chlorhexidine dihydrochloride (CHX) and ibuprofen (IBU) were developed and tested to optimize periodontal treatment outcomes. Release profiles were promising. Exposure to 1.5% and 5.3% CHX-IBU loaded ISFI's release media decreased significantly the P. gingivalis growth up to 20-fold and 35-fold, respectively, after 48 h (p < 0.05). The metabolic activity assay of gingival epithelial cells (EC) demonstrated 1.5% CHX-IBU-loaded ISFI to be non-toxic, therefore, it was selected for further experimentation. Furthermore, significant down-regulation of TNF-α release (34% at 6 h and 43% at 24 h, p < 0.05) in P. gingivalis lipopolysaccharide (Pg-LPS) stimulated EC exposed to 1.5% CHX-IBU ISFI release medium was demonstrated by ELISA. In vivo, 1.5% CHX-IBU ISFI was injected into the periodontal pocket in an experimental periodontitis mouse model and the reduction in inflammation and improvement in periodontal wound healing was evaluated through inflammatory cell scoring and histomorphometry at 7- and 15-days post-treatment. The results indicate that CHX-IBU loaded ISFI could be efficient as adjuvant to periodontal therapy for the control of infection and inflammation. Moreover, other (e.g., pro-regenerative) drugs could be incorporated into ISFI to further improve periodontal treatment outcomes.

Controlling and quantifying the stability of amino acid-based cargo within polymeric delivery systems

In recent years, the rapid growth and availability of protein and peptide therapeutics has not only expanded the boundaries of modern science but has also revolutionized the practice of medicine today. The potential of such therapies, however, is greatly limited by the innate instabilities of proteins and peptides, which is further magnified during therapeutic formulation processing, transport, storage, and administration. In this paper, we will consider the unique stability challenges associated with protein/peptide polymeric delivery systems from an engineering approach oriented towards the quantification and modification of amino acid-based cargo stability. While a number of methods have been developed for the purposes of quantifying factors affecting protein and peptide stability, current measurement techniques remain largely limited in scope in regard to polymeric drug delivery systems. This paper will primarily describe the influence of water content, pH, and temperature on protein and peptide stability within polymer-based delivery systems. Moreover, we will review current instrumentation used to quantify factors affecting protein/peptide stability with respect to water content, pH, and temperature. Lastly, we will outline several recommendations to help guide future research efforts to develop methods more specific to quantifying protein/peptide stability within polymer-based delivery systems.

Investigations Concerning the Residence Time Distribution of Twin-Screw-Extrusion Processes as Indicator for Inherent Mixing

Over recent years Twin-Screw-Extrusion (TSE) has been established as a platform technology for pharmaceutical manufacturing. Compared to other continuous operation, one of the major benefits of this method is the combination of several unit operations within one apparatus. Several of these are linked to the Residence Time Distribution (RTD), which is typically expressed by the residence time density function. One relevant aspect for pharmaceutical processes is the mixing capacity, which is represented by the width of this distribution. In the frame of this study the influence of the mass flow, the temperature and the screw-barrel clearance were investigated for a constant barrel load (specific feed load, SFL). While the total mass flow as well as the external screw diameter affected the mixing performance, the barrel temperature had no influence for the investigated range. The determined results were additionally evaluated with respect to a fit to the Twin-Dispersion-Model (TDM). This model is based on the superimposition of two mixing functions. The correlations between varied process parameters and the obtained characteristic model parameters proved this general physical view on extrusion.

The mechanism behind the biphasic pulsatile drug release from physically mixed poly(dl-lactic(-co-glycolic) acid)-based compacts

Successful immunization often requires a primer, and after a certain lag time, a booster administration of the antigen. To improve the vaccinees' comfort and compliance, a single-injection vaccine formulation with a biphasic pulsatile release would be preferable. Previous work has shown that such a release profile can be obtained with compacts prepared from physical mixtures of various poly(dl-lactic(-co-glycolic) acid) types (Murakami et al., 2000). However, the mechanism behind this release profile is not fully understood. In the present study, the mechanism that leads to this biphasic pulsatile release was investigated by studying the effect of the glass transition temperature (Tg) of the polymer, the temperature of compaction, the compression force, the temperature of the release medium, and the molecular weight of the incorporated drug on the release behavior. Compaction resulted in a porous compact. Once immersed into release medium with a temperature above the Tg of the polymer, the drug was released by diffusion through the pores. Simultaneously, the polymer underwent a transition from the glassy state into the rubbery state. The pores were gradually closed by viscous flow of the polymer and further release was inhibited. After a certain period of time, the polymer matrix ruptured, possibly due to a build-up in osmotic pressure, resulting in a pulsatile release of the remaining amount of drug. The compression force and the molecular weight of the incorporated drug did not influence the release profile. Understanding this mechanism could contribute to further develop single-injection vaccines.

Impact of change of matrix crystallinity and polymorphism on ovalbumin release from lipid-based implants

The objectives of this study were to prepare lipid-based implants by hot melt extrusion (HME) for the prolonged release of ovalbumin (OVA), and to relate protein release to crystallinity and polymorphic changes of the lipid matrix. Two lipids, glycerol tristearate and hydrogenated palm oil, with different composition and degree of crystallinity were studied. Solid OVA was dispersed within the lipid matrixes, which preserved its stability during extrusion. This was partially attributed to a protective effect of the lipidic matrix. The incorporation of OVA decreased the mechanical strength of the implants prepared with the more crystalline matrix, glycerol tristearate, whereas it remained comparable for the hydrogenated palm oil because of stronger physical and non-covalent interactions between the protein and this lipid. This was also the reason for the faster release of OVA from the glycerol tristearate matrix when compared to the hydrogenated palm oil (8 vs. 28 weeks). Curing induced and increased crystallinity, and changes in the release rate, especially for the more crystalline matrix. In this case, both an increase and a decrease in release, were observed depending on the tempering condition. Curing at higher temperatures induced a melt-mediated crystallization and solid state transformation of the glycerol tristearate matrix and led to rearrangements of the inner structure with the formation of larger pores, which accelerated the release. In contrast, changes in the hydrogenated palm oil under the same curing conditions were less noticeable leading to a more robust formulation, because of less polymorphic changes over time. This study helps to understand the effect of lipid matrix composition and crystallinity degree on the performance of protein-loaded implants, and to establish criteria for the selection of a lipid carrier depending on the release profile desired.