Breakout session summary from AAPS/CRS joint workshop on critical variables in the in vitro and in vivo performance of parenteral sustained release products

Long-acting PLGA microspheres: advances in excipient and product analysis toward improved product understanding

Poly(lactic-co-glycolic acid) (PLGA) microspheres are a sustained-release drug delivery system with several successful commercial products used for the treatment of a variety of diseases. By utilizing PLGA polymers with different compositions, therapeutic agents can be released over durations varying from several weeks to several months. However, precise quality control of PLGA polymers and a fundamental understanding of all the factors associated with the performance of PLGA microsphere formulations remains challenging. This knowledge gap can hinder product development of both innovator and generic products. In this review, variability of the key release controlling excipient (PLGA), as well as advanced physicochemical characterization techniques for the PLGA polymer and PLGA microspheres are discussed. The relative merits and challenges of different in vitro release testing methods, in vivo pharmacokinetic studies, and in vitro-in vivo correlation development are also summarized. This review is intended to provide an in-depth understanding of long-acting microsphere products and consequently facilitate the development of these complex products.

IVIVC of Octreotide in PLGA-Glucose Microsphere Formulation, Sandostatin® LAR

In vitro-in vivo correlation (IVIVC) analysis reveals a relationship between in vitro release and in vivo pharmacokinetic response of the drug of interest. Sandostatin LAR Depot (SLD) for endocrine tumors and acromegaly is a sustained-release formulation of octreotide, a cyclic oligomer of 8 amino acids, which prolongs therapeutic efficacy and enhances medication compliance of octreotide. Since the efficacy of SLD is dependent on the pharmacokinetic characteristics of octreotide released from a biodegradable matrix polymer, poly(lactide-co-glycolide)-glucose, of SLD, the IVIVC of SLD is critical for predicting an in vivo behavior of the octreotide. In this study, in vitro release of octreotide from SLD was investigated using the release test media each containing 0.02% or 0.5% surfactant and having different pH values of 7.4 and 5.5. In vivo pharmacokinetic profiles of SLD were determined by LC-MS/MS analysis of the systemic blood concentration of octreotide after the SLD injection to rodents. In IVIVC analysis, the Weibull model was adopted as a drug release model for biodegradable microsphere formulation. The IVIVC analyses revealed the in vitro release test condition of SLD with the highest IVIV correlation coefficient. By applying the in vitro release data to the model derived from the IVIVC analysis, pharmacokinetic parameters of SLD could be predicted with the prediction error of ± 10 ~ 15%. IVIVC analysis and pharmacokinetic prediction model of SLD in our study can be an efficient tool for the development of long-acting pharmaceutical dosage forms.

In vitro-in vivo correlation of PLGA microspheres: Effect of polymer source variation and temperature

Development of Level A in vitro-in vivo correlations (IVIVCs) remains challenging for complex long-acting parenterals, such as poly(lactic-co-glycolic acid) PLGA microspheres. The nature of the PLGA polymer excipient has a dominant influence on the performance of PLGA microspheres. These microsphere systems typically exhibit multiphasic in vitro/in vivo release/absorption characteristics and may also show interspecies differences (animal model versus human data). These issues contribute to the difficulties in the development of IVIVCs for PLGA microsphere systems. To gain a better understanding of how to achieve IVIVCs for PLGA microspheres, microsphere formulations with similar as well as different release characteristics were prepared using PLGAs from different sources. Efforts were then made to establish IVIVCs for these formulations using in vitro release profiles obtained at both 37°C (human body temperature) and 39°C (rabbit body temperature) with in vivo data obtained from an animal model (rabbit). Risperidone was selected as the model drug; microsphere formulations were prepared under the same processing methods using apparently similar PLGAs from different sources. Owning to the different physicochemical properties of the PLGAs (such as inherent viscosity, monomer ratio (L/G ratio) and blockiness), the formulations exhibited significant differences in critical quality attributes (such as particle size, particle size distribution, porosity and pore size) and consequently had different in vitro and in vivo performance. IVIVCs were developed and it was shown that model predictability improved when IVIVCs were established using those formulations with comparable release characteristics. In addition, IVIVCs were established with T_scaling factors close to 1 using in vitro release profiles acquired at 39°C, emphasizing the importance of considering the body temperature in understanding interspecies differences. The present work provides a comprehensive understanding of the impact of the PLGA source variation on IVIVC development and predictability for complex long-acting parenterals such as PLGA microspheres.

Rahman N, Wong TW, Chee CF. Challenges and Complications of Poly(lactic-co-glycolic acid)-Based Long-Acting Drug Product Development

Poly(lactic-co-glycolic acid) (PLGA) is one of the preferred polymeric inactive ingredients for long-acting parenteral drug products that are constituted of complex formulations. Despite over 30 years of use, there are still many challenges faced by researchers in formulation-related aspects pertaining to drug loading and release. Until now, PLGA-based complex generic drug products have not been successfully developed. The complexity in developing these generic drug products is not just due to their complex formulation, but also to the manufacturing process of the listed reference drugs that involve PLGA. The composition and product attributes of commercial PLGA formulations vary with the drugs and their intended applications. The lack of standard compendial methods for in vitro release studies hinders generic pharmaceutical companies in their efforts to develop PLGA-based complex generic drug products. In this review, we discuss the challenges faced in developing PLGA-based long-acting injectable/implantable (LAI) drug products; hurdles that are associated with drug loading and release that are dictated by the physicochemical properties of PLGA and product manufacturing processes. Approaches to overcome these challenges and hurdles are highlighted specifically with respect to drug encapsulation and release.

Prediction of subcutaneous drug absorption - do we have reliable data to design a simulated interstitial fluid?

For many years subcutaneous (SC) administration has represented the main route for delivering biopharmaceuticals. However, little information exists about the milieu in the subcutaneous tissue, especially about the properties/composition of the fluid present in this tissue, the interstitial fluid (ISF), which is one of the key elements for the drug release and absorption. Better knowledge on SC ISF composition, properties and dynamics may provide better insight into in vivo drug performance. In addition, a simulated SC ISF, which allows better prediction of in vivo absorption of drugs after subcutaneous administration based on in vitro release experiments, would help to improve formulation design, and reduce the number of animal studies and clinical trials required to obtain marketing authorization. To date, a universal medium for predicting drug solubility/release in the interstitial space does not exist. This review provides an overview of the currently available information on composition and physicochemical properties of SC ISF and critically discusses different isolation techniques in the context of information that could be gained from the isolated fluid. Moreover, it surveys current in vitro release media aiming to mimic SC ISF composition and highlights information gaps that need to be filled for designing a meaningful artificial SC ISF.

Application of UV dissolution imaging to pharmaceutical systems

UV-vis spectrometry is widely used in the pharmaceutical sciences for compound quantification, alone or in conjunction with separation techniques, due to most drug entities possessing a chromophore absorbing light in the range 190-800 nm. UV dissolution imaging, the scope of this review, generates spatially and temporally resolved absorbance maps by exploiting the UV absorbance of the analyte. This review aims to give an introduction to UV dissolution imaging and its use in the determination of intrinsic dissolution rates and drug release from whole dosage forms. Applications of UV imaging to non-oral formulations have started to emerge and are reviewed together with the possibility of utilizing UV imaging for physical chemical characterisation of drug substances. The benefits of imaging drug diffusion and transport processes are also discussed.

Seaweed Components as Potential Modulators of the Gut Microbiota

Macroalgae, or seaweeds, are a rich source of components which may exert beneficial effects on the mammalian gut microbiota through the enhancement of bacterial diversity and abundance. An imbalance of gut bacteria has been linked to the development of disorders such as inflammatory bowel disease, immunodeficiency, hypertension, type-2-diabetes, obesity, and cancer. This review outlines current knowledge from in vitro and in vivo studies concerning the potential therapeutic application of seaweed-derived polysaccharides, polyphenols and peptides to modulate the gut microbiota through diet. Polysaccharides such as fucoidan, laminarin, alginate, ulvan and porphyran are unique to seaweeds. Several studies have shown their potential to act as prebiotics and to positively modulate the gut microbiota. Prebiotics enhance bacterial populations and often their production of short chain fatty acids, which are the energy source for gastrointestinal epithelial cells, provide protection against pathogens, influence immunomodulation, and induce apoptosis of colon cancer cells. The oral bioaccessibility and bioavailability of seaweed components is also discussed, including the advantages and limitations of static and dynamic in vitro gastrointestinal models versus ex vivo and in vivo methods. Seaweed bioactives show potential for use in prevention and, in some instances, treatment of human disease. However, it is also necessary to confirm these potential, therapeutic effects in large-scale clinical trials. Where possible, we have cited information concerning these trials.

Accelerated in vitro release testing method for a long-acting peptide-PLGA formulation

Poly (lactic-co-glycolic acid) (PLGA), a biocompatible and biodegradable polymer, is one of the most commonly used vehicles for controlled-release (CR) implantable dosage forms. Drug molecules formulated in such CR vehicles are released slowly over an extended period of time - often months to years - posing challenges for batch release and quality control testing. Thus, reliable and reproducible accelerated testing methods are required to bridge this gap during early formulation development. This work describes the development of an accelerated in vitro release testing method to predict the real-time in vitro release of a synthetic peptide from a 6-month CR PLGA implant formulation. While accelerated methods have been previously reported for PLGA-based formulations, this work describes a unique case of an aggregation-prone peptide, which required careful attention to the impact of different conditions on both release kinetics and peptide stability. This method describes a suitable combination of release conditions that could help in understanding the release profiles of such peptides prone to aggregation. Parameters including pH, buffer species, temperature, and addition of organic co-solvents and surfactants were evaluated separately and in combination for their ability to achieve complete peptide release within 2 weeks while accurately recapitulating release rate, profile and peptide stability. The accelerated release method that gave the best agreement with real-time release was a mixed media of co-solvent (5% tetrahydrofuran), surfactant (5% TritonX-100) and elevated temperature (50 °C) in a neutral buffer (PBS pH 7.4). This optimized accelerated release method achieved complete release of the peptide load within 14-21 days compared to 3- to 6-months of real-time release and could discriminate critical differences in release behavior between different CR formulations to guide formulation and process development.

Muscle Tissue as a Surrogate for In Vitro Drug Release Testing of Parenteral Depot Microspheres

Despite the importance of drug release testing of parenteral depot formulations, the current in vitro methods still require ameliorations in biorelevance. We have investigated here the use of muscle tissue components to better mimic the intramuscular administration. For convenient handling, muscle tissue was used in form of a freeze-dried powder, and a reproducible process of incorporation of tested microspheres to an assembly of muscle tissue of standardized dimensions was successfully developed. Microspheres were prepared from various grades of poly(lactic-co-glycolic acid) (PLGA) or ethyl cellulose, entrapping flurbiprofen, lidocaine, or risperidone. The deposition of microspheres in the muscle tissue or addition of only isolated lipids into the medium accelerated the release rate of all model drugs from microspheres prepared from ester-terminated PLGA grades and ethyl cellulose, however, not from the acid-terminated PLGA grades. The addition of lipids into the release medium increased the solubility of all model drugs; nonetheless, also interactions of the lipids with the polymer matrix (ad- and absorption) might be responsible for the faster drug release. As the in vivo drug release from implants is also often faster than in simple buffers in vitro, these findings suggest that interactions with the tissue lipids may play an important role in these still unexplained observations.

Fabrication of PLGA in situ forming implants and study on their correlation of in vitro release profiles with in vivo performances

In this study, a novel PLGA in situ forming implants (ISFIs) were fabricated and methods for testing the in vitro release profiles were also developed. The correlations between in vitro release profiles and in vivo performances (in vitro-in vivo correlation, IVIVC) were also studied. PLGA with different molecular weights were selected as the polymeric matrix. Biocompatible N-methy1-2-pyrrolidone (NMP) or glyceryl triacetate (GTA) were used as the solvents with the ratios of NMP/GTA from 60/40 (vol/vol) to 20/80 (vol/vol). Eprinomectin (EPR) was chosen as the model therapeutic. In vitro release profiles of the EPR-loaded PLGA ISFIs were investigated using various methods (i.e. 'tubule' sample-and-separate and dialysis method). Sprague-Dawley rats were used to study the in vivo pharmacokinetics of EPR-loaded PLGA ISFIs. The release data obtained via 'tubule' sample-separate method had a good IVIVC (Level A, R² > 0.99). These results showed that the 'tubule' sample-separate method was capable of discriminating the EPR-loaded ISFIs which were equivalent in formulation composition with manufacturing differences. Meanwhile, this method could be used to predict the in vivo performances of ISFIs in the investigated animal model.

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.

In-vitro drug release testing of parenteral formulations via an agarose gel envelope to closer mimic tissue firmness

Current in vitro drug-release testing of the sustained-release parenterals represents the in vivo situation insufficiently. In this work, a thin agarose hydrogel layer surrounding the tested dosage form was proposed to mimic the tissue. The method was applied on implantable formulations of different geometries (films, microspheres, and cylindrical implants); prepared from various polymers (several Resomer® grades or ethyl cellulose) and loaded with different model drugs: flurbiprofen, lidocaine or risperidone. The hydrogel layer did not possess any retarding effect on the released drug and acted as a physical restriction to swelling and/or plastic deformation of the tested dosage forms. This led to a different surface area available for drug-release compared with testing in release medium alone and correspondingly to significantly different release profiles of the majority of the formulations obtained between the two methods (e.g. t50% = 18 days in pure release medium vs. t50% = 26 days in gel-setup for risperidone loaded Resomer® 503 H films or t50% = 7 days vs. t50% = 19 days for risperidone loaded Resomer® 503 H microspheres). The limited space for swelling and the rigidity of the agarose gel might mimic the tight encapsulation of the dosage form in the tissue better than the conventional liquid medium.

In vitro-in vivo correlation of parenteral PLGA microspheres: Effect of variable burst release

Development of IVIVCs is a very complicated process, especially for complex drug products such as parenteral PLGA microspheres with multiphasic drug release characteristics. Specifically, microspheres that exhibit an initial burst release phase are even more challenging since the in vitro and in vivo burst release phases may not be comparable if drug absorption is rate-limiting at this stage. Therefore, the objectives of the present work were: 1) to investigate the predictability of developed IVIVCs for the in vivo burst release phase based on the in vitro burst release phase of the formulations; and 2) to evaluate the impact of variable burst release on the predictability of the developed IVIVCs for two different types of microsphere-based drug products. Accordingly, Risperdal Consta® (Risperidone) and Lupron Depot® (Leuprolide acetate, LA) were selected as model products. Compositionally equivalent risperidone and LA formulations with variable burst release phases were prepared with manufacturing process changes (such as solvent systems and mixing methods). The prepared microspheres exhibited differences in critical physicochemical properties (such as particle size, porosity, average pore diameter, and drug distribution) and hence differences in their in vitro release characteristics (such as variable burst release and release rate). The in vitro and in vivo (rabbit model (intramuscular injection) burst release were similar for the risperidone microspheres but were significantly different for the LA microspheres. This had an impact on the complexity of the developed IVIVC models. Level A IVIVCs with the ability to predict various types of burst release were developed using time scaling and shifting factors. Moreover, it was observed that IVIVCs developed using formulations with less variation in burst release had better predictability and vice-versa. Thus, the present research has provided a comprehensive understanding of the impact of the burst release phase on the development, complexity, and predictability of IVIVCs for complex parenteral microspheres containing a variety of therapeutic molecules.

A method of elevated temperatures coupled with magnetic stirring to predict real time release from long acting progesterone PLGA microspheres

The object of the study was to develop a quick and reproducible accelerated in vitro release method to predict and deduce the function of the real time (37 °C) release for long acting PLGA microspheres. The method could be described in several steps. First, the release of the microspheres were studied using the sample and separate method at 37 °C with normal orbital shaking and elevated temperatures with magnetic stirring to further accelerate the release. Second, the most similar profile at elevated temperatures with the real time release was chosen with the help of the n value in the fitted Korsmeyer-Peppas Function. Third, the Weibull function and conversion ratio were used to deduce the function of real time release according to the chosen profile at elevated temperatures. The key point in this study was to provide a quick and precise method to predict the real time release for long acting progesterone PLGA microspheres. So the elevated temperatures coupled with magnetic stirring were used to accelerate the release further, and when there have many similar release profiles with the real time release at elevated temperatures, releasing time at elevated temperatures and the R2 of the final deduced function will be used to help choosing the most similar release profile with the real time release. Four different types of progesterone PLGA microspheres were used to verify the method, and all the deduced function correlated well with the real time releases, for R 2  = 0.9912, 0.9781, 0.9918 and 0.9972, respectively.

Mechanistic analysis of triamcinolone acetonide release from PLGA microspheres as a function of varying in vitro release conditions

In vitro tests for controlled release PLGA microspheres in their current state often do not accurately predict in vivo performance of these products during formulation development. Here, we introduce a new mechanistic and multi-phase approach to more clearly understand in vitro-in vivo relationships, and describe the first "in vitro phase" with the model drug, triamcinolone acetonide (Tr-A). Two microsphere formulations encapsulating Tr-A were prepared from PLGAs of different molecular weights and end-capping (18kDa acid-capped and 54kDa ester-capped). In vitro release kinetics and the evidence for controlling mechanisms (i.e., erosion, diffusion, and water-mediated processes) were studied in four release media: PBST pH 7.4 (standard condition), PBST pH 6.5, PBS+1.0% triethyl citrate (TC), and HBST pH 7.4. The release mechanism in PBST was primarily polymer erosion-controlled as indicated by the similarity of release and mass loss kinetics. Release from the low MW PLGA was accelerated at low pH due to increased rate of hydrolysis and in the presence of the plasticizer TC due to slightly increased hydrolysis and much higher diffusion in the polymer matrix. TC also increased release from the high MW PLGA due to increased hydrolysis, erosion, and diffusion. This work demonstrates how in vitro conditions can be manipulated to change not only rates of drug release from PLGA microspheres but also the mechanism(s) by which release occurs. Follow-on studies in the next phases of this approach will utilize these results to compare the mechanistic data of the Tr-A/PLGA microsphere formulations developed here after recovery of microspheres in vivo. This new approach based on measuring mechanistic indicators of release in vitro and in vivo has the potential to design better, more predictive in vitro release tests for these formulations and potentially lead to mechanism-based in vitro-in vivo correlations.

In vitro simulation of distribution processes following intramuscular injection

There is an urgent need for in vitro dissolution test setups for intramuscularly applied dosage forms. Especially biorelevant methods are needed to predict the in vivo behavior of newly developed dosage forms in a realistic way. There is a lack of knowledge regarding critical in vivo parameters influencing the release and absorption behavior of an intramuscularly applied drug. In the presented work the focus was set on the simulation of blood perfusion and muscle tissue. A solid agarose gel, being incorporated in an open-pored foam, was used to mimic the gel phase of muscle tissue and implemented in a flow through cell. An aqueous solution of fluorescein sodium was injected. Compared to recently obtained in vivo results the distribution of the model substance was very slow. Furthermore an agarose gel of lower viscosity an open-pored foam and phosphate buffer saline pH 7.4 were implemented in a multi-channel-ceramic membrane serving as a holder for the muscle imitating material. Blood simulating release medium was perfused through the ceramic membrane including filling materials. Transport of the dissolved fluorescein sodium was, in case of the gel, not only determined by diffusion but also by convective transport processes. The more realistic the muscle simulating materials were constituted the less reproducible results were obtained with the designed test setups.

A reproducible accelerated in vitro release testing method for PLGA microspheres

The objective of the present study was to develop a discriminatory and reproducible accelerated in vitro release method for long-acting PLGA microspheres with inner structure/porosity differences. Risperidone was chosen as a model drug. Qualitatively and quantitatively equivalent PLGA microspheres with different inner structure/porosity were obtained using different manufacturing processes. Physicochemical properties as well as degradation profiles of the prepared microspheres were investigated. Furthermore, in vitro release testing of the prepared risperidone microspheres was performed using the most common in vitro release methods (i.e., sample-and-separate and flow through) for this type of product. The obtained compositionally equivalent risperidone microspheres had similar drug loading but different inner structure/porosity. When microsphere particle size appeared similar, porous risperidone microspheres showed faster microsphere degradation and drug release compared with less porous microspheres. Both in vitro release methods investigated were able to differentiate risperidone microsphere formulations with differences in porosity under real-time (37 °C) and accelerated (45 °C) testing conditions. Notably, only the accelerated USP apparatus 4 method showed good reproducibility for highly porous risperidone microspheres. These results indicated that the accelerated USP apparatus 4 method is an appropriate fast quality control tool for long-acting PLGA microspheres (even with porous structures).

In vitro-in vivo correlation of parenteral risperidone polymeric microspheres

The objective of the present study was to determine whether an in vitro-in vivo correlation (IVIVC) can be established for polymeric microspheres that are equivalent in formulation composition but prepared with different manufacturing processes. Risperidone was chosen as a model therapeutic and poly(lactic-co-glycolic acid) (PLGA) with similar molecular weight as that used in the commercial product Risperdal® Consta® was used to prepare risperidone microspheres. Various manufacturing processes were investigated to produce the risperidone microspheres with similar drug loading (approx. 37%) but distinctly different physicochemical properties (e.g. porosity, particle size and particle size distribution). In vitro release of the risperidone microspheres was investigated using different release testing methods (such as sample-and-separate and USP apparatus 4). In vivo pharmacokinetic profiles of the risperidone microsphere formulations following intramuscular administration were determined using a rabbit model. Furthermore, the obtained pharmacokinetic profiles were deconvoluted using the Loo-Riegelman method and the calculated in vivo release was compared with the in vitro release of these microspheres. Level A IVIVCs were established and validated for the compositionally equivalent risperidone microspheres based on the in vitro release data obtained using USP apparatus 4. The developed IVIVCs demonstrated good predictability and were robust. These results showed that the developed USP apparatus 4 method was capable of discriminating PLGA microspheres that are equivalent in formulation composition but with manufacturing differences and predicting their in vivo performance in the investigated animal model.

Drug delivery system innovation and Health Technology Assessment: Upgrading from Clinical to Technological Assessment

Health Technology Assessment (HTA) is a multidisciplinary health political instrument that evaluates the consequences, mainly clinical and economical, of a health care technology; the HTA aim is to produce and spread information on scientific and technological innovation for health political decision making process. Drug delivery systems (DDS), such as nanocarriers, are technologically complex but they have pivotal relevance in therapeutic innovation. The HTA process, as commonly applied to conventional drug evaluation, should upgrade to a full pharmaceutical assessment, considering the DDS complexity. This is useful to study more in depth the clinical outcome and to broaden its critical assessment toward pharmaceutical issues affecting the patient and not measured by the current clinical evidence approach. We draw out the expertise necessary to perform the pharmaceutical assessment and we propose a format to evaluate the DDS technological topics such as formulation and mechanism of action, physicochemical characteristics, manufacturing process. We integrated the above-mentioned three points in the Evidence Based Medicine approach, which is data source for any HTA process. In this regard, the introduction of a Pharmaceutics Expert figure in the HTA could be fundamental to grant a more detailed evaluation of medicine product characteristics and performances and to help optimizing DDS features to overcome R&D drawbacks. Some aspects of product development, such as manufacturing processes, should be part of the HTA as innovative manufacturing processes allow new products to reach more effectively patient bedside. HTA so upgraded may encourage resource allocating payers to invest in innovative technologies and providers to focus on innovative material properties and manufacturing processes, thus contributing to bring more medicines in therapy in a sustainable manner.

A Review of In Vitro Drug Release Test Methods for Nano-Sized Dosage Forms

This review summarizes the methods used to study real-time (37°C) drug release from nanoparticulate drug delivery systems and establish an IVIVC. Since no compendial standards exist, drug release is currently assessed using a variety of methods including sample and separate (SS), continuous flow (CF), dialysis membrane (DM) methods, and a combination thereof, as well as novel techniques like voltametry and turbidimetry. This review describes the principle of each method along with their advantages and disadvantages, including challenges with set-up and sampling. The SS method allows direct measurement of drug release with simple set-up requirements, but sampling is cumbersome. With the CF method, sampling is straightforward but the set-up is time consuming. Set-up as well as sampling is easier with the DM, but it may not be suitable for drugs that bind to the membrane. Novel methods offer the possibility of real-time drug release measurement but may be restricted to certain types of drugs. Of these methods, Level A IVIVCs have been obtained with dialysis, alone or in combination with the sample and separate technique. Future efforts should focus on developing mathematical models that describe drug release mechanisms as well as facilitate formulation development of nano-sized dosage forms.

In Vitro Dissolution Testing Strategies for Nanoparticulate Drug Delivery Systems: Recent Developments and Challenges

Nanoparticulate systems have emerged as prevalent drug delivery systems over the past few decades. These delivery systems (such as liposomes, emulsions, nanocrystals, and polymeric nanocarriers) have been extensively used to improve bioavailability, prolong pharmacological effects, achieve targeted drug delivery, as well as reduce side effects. Considering that any unanticipated change in product performance of such systems may result in toxicity and/or change in vivo efficacy, it is essential to develop suitable in vitro dissolution/release testing methods to ensure product quality and performance, and to assist in product development. The present review provides an overview of the current in vitro dissolution/release testing methods such as dialysis, sample and separate, as well as continuous flow methods. Challenges and future directions in the development of standardized and biorelevant in vitro dissolution/release testing methods for novel nanoparticulate systems are discussed.

Accelerated in-vitro release testing methods for extended-release parenteral dosage forms

OBJECTIVES

This review highlights current methods and strategies for accelerated in-vitro drug release testing of extended-release parenteral dosage forms such as polymeric microparticulate systems, lipid microparticulate systems, in-situ depot-forming systems and implants.

KEY FINDINGS

Extended-release parenteral dosage forms are typically designed to maintain the effective drug concentration over periods of weeks, months or even years. Consequently, 'real-time' in-vitro release tests for these dosage forms are often run over a long time period. Accelerated in-vitro release methods can provide rapid evaluation and therefore are desirable for quality control purposes. To this end, different accelerated in-vitro release methods using United States Pharmacopeia (USP) apparatus have been developed. Different mechanisms of accelerating drug release from extended-release parenteral dosage forms, along with the accelerated in-vitro release testing methods currently employed are discussed.

SUMMARY

Accelerated in-vitro release testing methods with good discriminatory ability are critical for quality control of extended-release parenteral products. Methods that can be used in the development of in-vitro-in-vivo correlation (IVIVC) are desirable; however, for complex parenteral products this may not always be achievable.

In-vitro dissolution methods for controlled release parenterals and their applicability to drug-eluting stent testing

Objectives

Dissolution testing is a powerful tool for the characterization of dosage form performance in vitro under standardized conditions. In spite of the increasing number of parenterally administered medicinal products, currently there are no compendial dissolution test methods designed especially for these types of dosage forms. In addition to classical drug delivery systems, drug/device combination products, such as drug-eluting stents, are being used increasingly.

Key findings

This review describes the current methods that are used most often for in-vitro dissolution testing of parenteral dosage forms, i.e. the ‘sample and separate’ methods, the ‘dialysis’ methods, and the ‘flow-through’ methods, with a special emphasis on whether these methods can be used for drug-eluting stent testing. In the light of current regulatory requirements and with the exploding costs of preclinical and clinical development, test systems that include biorelevant parameters and are predictive of in-vivo performance are increasingly important. Published attempts to take biorelevant conditions into consideration in the design of dissolution test apparatus developed for parenteral dosage forms, including a method that was designed to emulate the embedding and flow-conditions at the site of stent implantation, have been outlined in this review.

Summary

In spite of the large quantity of highly potent controlled release parenteral products marketed today, there is still a lack of suitable methods for in vitro dissolution testing for these dosage forms especially with regard to biorelevant testing conditions. For dosage forms implanted into tissues it seems of major importance to reproduce the transport forces which are predominant in vivo (diffusive versus convective) in the in-vitro experimental setup.

Unintended potential impact of perfect sink conditions on PLGA degradation in microparticles

Yet, no standardized test method for drug release measurements from PLGA-based microparticles has been generally agreed on, or described by the regulatory authorities. Often, perfect sink conditions are provided in vitro to avoid artificial drug saturation effects. However, the maintenance of such conditions might strongly affect PLGA degradation. The involved physicochemical processes are complex and the potential impact of perfect sink conditions is not yet well understood. Differently sized, highly porous, carbamazepine- and ibuprofen-loaded PLGA microparticles were prepared by a W/O/W emulsion solvent extraction/evaporation technique. The initial drug loading was intentionally low (3-4%) so that the two drugs were molecularly dispersed within the polymeric matrices (monolithic solutions). This was important to be able to exclude potential limited drug solubility effects on the resulting release kinetics. Drug release into phosphate buffer pH 7.4 was measured under perfect sink conditions. SEC, DSC and SEM were used to characterize polymer degradation. The decrease in the average polymer molecular weight, glass transition temperature as well as changes in the inner and outer morphology of the PLGA microparticles were strongly affected by the bulk fluid's volume. In the case of the poorly water-soluble drug carbamazepine, much lower "microparticle mass:phosphate buffer volume" ratios were required to maintain perfect sink conditions, resulting in stable pH values within the bulk fluid, slower PLGA degradation and, thus, lower drug release rates. Thus, great care has to be taken when defining the conditions for in vitro drug release measurements from PLGA-based microparticles, avoiding potentially artificial conditions for polymer degradation.