Effect of polymer source on in vitro drug release from PLGA microspheres.

Determination of the qualitative (Q1) sameness of poly (lactic-co-glycolic acid) (PLGA) polymers can be very challenging due to PLGA being a random copolymer with inherent heterogeneity. Performance variation of PLGA microsphere drug product as a result of altered PLGA characteristics has been recognized as a critical limiting factor in product development. It has been reported that PLGA characteristics and degradation profiles are sensitive to minor differences in the manufacturing and control processes. Accordingly, the objectives of the present research were: 1) to determine minor differences in the physicochemical properties (such as inherent viscosity/molecular weight (Mw), blockiness, and glass transition temperature (Tg)) and the hydrolytic degradation profiles of PLGA polymers from different sources; and 2) to investigate the impact of any differences determined in (1) on the physicochemical properties (Q3) and in vitro release of leuprolide acetate microspheres. PLGA polymers were purchased from three different sources with similar inherent viscosity/Mw, monomer (Lactide/Glycolide) ratio, and end group as per the manufacturers' certificate of analysis (COA). These PLGA polymers were evaluated using the same in-house methods and showed differences in their properties such as Mw and blockiness. Three compositionally equivalent leuprolide acetate microspheres were prepared via a solvent evaporation method using the three PLGA polymers from different sources. The prepared microspheres showed differences in their physicochemical properties (such as particle size, porosity and average pore diameter) as well as in their in vitro drug release characteristics (burst effect and release rate). These results indicate that polymer source related variations have the potential to significantly impact the Q3 sameness and therapeutic performance of long-acting PLGA microspheres. The fundamental understanding gained on polymer properties will make a critical contribution to the development of quality control strategies as well as to future regulatory guidance on the evaluation of such complex drug products.

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

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

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.

Development of Level A in vitro-in vivo correlations for peptide loaded PLGA microspheres.

Peptide loaded PLGA microsphere products are more complex in terms of manufacturing, drug release characteristics as well as release mechanism compared to small molecule loaded PLGA microsphere products. This is due to the complex structure of peptides, their hydrophilicity, charged state, large size and potential for instability. Moreover, therapeutic peptides are highly potent and therefore, any unintended change in the microsphere release profile may lead to undesirable side effects and toxicity. Accordingly, the objectives of the present work were: 1) to evaluate the impact of minor manufacturing changes on the quality and performance of peptide microspheres; and 2) to investigate the feasibility of developing Level A in vitro-in vivo correlations (IVIVCs) for peptide microspheres. Compositionally equivalent leuprolide acetate (LA) microspheres prepared with minor manufacturing changes (solvent system/homogenization speed) showed significant differences in their physicochemical properties (such as pore size, total porosity, particle size and surface distribution of peptide on the prepared microspheres). This, in turn, resulted in significant alteration in the release characteristics. Peptide-polymer interaction, in vitro degradation and microsphere morphology studies were conducted to facilitate understanding of the differences in the drug release characteristics. A rabbit model was used to determine the pharmacokinetic profiles of all the prepared formulations. The obtained in vivo release profiles showed the same rank order as the in vitro release profiles but with low burst release and overall faster in vivo release rates. The low in vivo burst release is considered to be due to the masking effect of the absorption phase from the intramuscular site, and this complicated the development of an IVIVC. Despite these challenges, an affirmative Level A IVIVC over the entire release profile was successfully developed in a rabbit model for peptide microspheres for the first time. The developed IVIVC was also predictive of the RLD product, Lupron Depot®. This work highlights the feasibility of developing IVIVCs for complex parenteral drug products such as peptide microspheres. In conclusion, these results indicate the sensitivity of peptide release, and hence, the safety and efficacy of highly potent peptide microspheres, to minor manufacturing changes. Accordingly, development of IVIVCs for such complex drug products is highly desirable.

Effect of minor manufacturing changes on stability of compositionally equivalent PLGA microspheres.

The physicochemical properties and drug release characteristics of Q1/Q2 equivalent microspheres are sensitive to minor manufacturing changes, which may alter their stability under different storage-conditions. This may be undesirable due to the presence of a substantial amount of drug in microsphere products. Hence, the objective of the present work was to investigate the impact of minor manufacturing changes on the stability of Q1/Q2 equivalent microspheres under various storage conditions. Two Q1/Q2 equivalent risperidone microsphere formulations prepared with minor manufacturing changes (solvent system etc.) showed differences in their physicochemical properties (size, morphology, porosity etc.), drug release characteristics and hence, storage stability. Overall, both formulations were stable under long-term storage conditions (4 °C/ambient humidity). However, under the intermediate storage conditions (25 °C/ambient humidity), only formulation 1 was stable while formulation 2 showed significant polymer degradation, particle aggregation and alteration in the drug release characteristics. Lastly, under accelerated storage conditions (40 °C/ambient humidity vs 75% RH), the extent of polymer degradation, morphological changes and alteration of drug release characteristics of formulation 2 was significantly higher compared to that of formulation 1. Thus, minor manufacturing changes have the potential to significantly alter the storage stability and, hence, the quality and performance of complex drug products such as microspheres.

Development of in vitro-in vivo correlation of parenteral naltrexone loaded polymeric microspheres.

Establishment of in vitro-in vivo correlations (IVIVCs) for parenteral polymeric microspheres has been very challenging, due to their complex multiphase release characteristics (which is affected by the nature of the drug) as well as the lack of compendial in vitro release testing methods. Previously, a Level A correlation has been established and validated for polymeric microspheres containing risperidone (a practically water insoluble small molecule drug). The objectives of the present study were: 1) to investigate whether a Level A IVIVC can be established for polymeric microspheres containing another small molecule drug with different solubility profiles compared to risperidone; and 2) to determine whether release characteristic differences (bi-phasic vs tri-phasic) between microspheres can affect the development and predictability of IVIVCs. Naltrexone was chosen as the model drug. Three compositionally equivalent formulations of naltrexone microspheres with different release characteristics were prepared using different manufacturing processes. The critical physicochemical properties (such as drug loading, particle size, porosity, and morphology) as well as the in vitro release characteristics of the prepared naltrexone microspheres and the reference-listed drug (Vivitrol®) were determined. The pharmacokinetics of the naltrexone microspheres were investigated using a rabbit model. The obtained pharmacokinetic profiles were deconvoluted using the Loo-Riegelman method, and compared with the in vitro release profiles of the naltrexone microspheres obtained using USP apparatus 4. Level A IVIVCs were established and validated for predictability. The results demonstrated that the developed USP 4 method was capable of detecting manufacturing process related performance changes, and most importantly, predicting the in vivo performance of naltrexone microspheres in the investigated animal model. A critical difference between naltrexone and risperidone loaded microspheres is their respective bi-phasic and tri-phasic release profiles with varying burst release and lag phase. These variations in release profiles affect the development of IVIVCs. Nevertheless, IVIVCs have been established and validated for polymeric microspheres with different release characteristics.

Accelerated in vitro release testing method for naltrexone loaded PLGA microspheres.

The objective of the present study was to develop a discriminatory and reproducible accelerated release testing method for naltrexone loaded parenteral polymeric microspheres. The commercially available naltrexone microsphere product (Vivitrol®) was used as the testing formulation in the in vitro release method development, and both sample-and-separate and USP apparatus 4 methods were investigated. Following an in vitro drug stability study, frequent media replacement and addition of anti-oxidant in the release medium were used to prevent degradation of naltrexone during release testing at "real-time" (37°C) and "accelerated" (45°C), respectively. The USP apparatus 4 method was more reproducible than the sample-and-separate method. In addition, the accelerated release profile obtained using USP apparatus 4 had a shortened release duration (within seven days), and good correlation with the "real-time" release profile. Lastly, the discriminatory ability of the developed accelerated release method was assessed using compositionally equivalent naltrexone microspheres with different release characteristics. The developed accelerated USP apparatus 4 release method was able to detect differences in the release characteristics of the prepared naltrexone microspheres. Moreover, a linear correlation was observed between the "real-time" and accelerated release profiles of all the formulations investigated, suggesting that the release mechanism(s) may be similar under both conditions. These results indicate that the developed accelerated USP apparatus 4 method has the potential to be an appropriate fast quality control tool for long-acting naltrexone PLGA microspheres.

Recent advances in testing of microsphere drug delivery systems.

INTRODUCTION: This review discusses advances in the field of microsphere testing. AREAS COVERED: In vitro release-testing methods such as sample and separate, dialysis membrane sacs and USP apparatus IV have been used for microspheres. Based on comparisons of these methods, USP apparatus IV is currently the method of choice. Accelerated in vitro release tests have been developed to shorten the testing time for quality control purposes. In vitro-in vivo correlations using real-time and accelerated release data have been developed, to minimize the need to conduct in vivo performance evaluation. Storage stability studies have been conducted to investigate the influence of various environmental factors on microsphere quality throughout the product shelf life. New tests such as the floating test and the in vitro wash-off test have been developed along with advancement in characterization techniques for other physico-chemical parameters such as particle size, drug content, and thermal properties. EXPERT OPINION: Although significant developments have been made in microsphere release testing, there is still a lack of guidance in this area. Microsphere storage stability studies should be extended to include microspheres containing large molecules. An agreement needs to be reached on the use of particle sizing techniques to avoid inconsistent data. An approach needs to be developed to determine total moisture content of microspheres.