In vitro degradation and erosion behavior of commercial PLGAs used for controlled drug delivery.

Copolymers of lactic (or lactide) and glycolic (or glycolide) acids (PLGAs) are among the most commonly used materials in biomedical applications, such as parenteral controlled drug delivery, due to their biocompatibility, predictable degradation rate, and ease of processing. Besides manufacturing variables of drug delivery vehicles, changes in PLGA raw material properties can affect product behavior. Accordingly, an in-depth understanding of polymer-related "critical quality attributes" can improve selection and predictability of PLGA performance. Here, we selected 19 different PLGAs from five manufacturers to form drug-free films, submillimeter implants, and microspheres and evaluated differences in their water uptake, degradation, and erosion during in vitro incubation as a function of L/G ratio, polymerization method, molecular weight, end-capping, and geometry. Uncapped PLGA 50/50 films from different manufacturers with similar molecular weights and higher glycolic unit blockiness and/or block length values showed faster initial degradation rates. Geometrically, larger implants of 75/25, uncapped PLGA showed higher water uptake and faster degradation rates in the first week compared to microspheres of the same polymers, likely due to enhanced effects of acid-catalyzed degradation from PLGA acidic byproducts unable to escape as efficiently from larger geometries. Manufacturer differences such as increased residual monomer appeared to increase water uptake and degradation in uncapped 50/50 PLGA films and poly(lactide) implants. This dataset of different polymer manufacturers could be useful in selecting desired PLGAs for controlled release applications or comparing differences in behavior during product development, and these techniques to further compare differences in less reported properties such as sequence distribution may be useful for future analyses of PLGA performance in drug delivery.

Mechanistic evaluation of the initial burst release of leuprolide from spray-dried PLGA microspheres.

Spray-dried poly(lactic-co-glycolic acid) (PLGA) peptide-loaded microspheres have demonstrated similar long-term in vitro release kinetics compared to those produced by the solvent evaporation method and commercial products. However, the difficult-to-control initial burst release over the first 24 h after administration presents an obstacle to product development and establishing bioequivalence. Currently, detailed information about underlying mechanisms of the initial burst release from microspheres is limited. We investigated the mechanism and extent of initial burst release using 16 previously developed spray-dried microsphere formulations of the hormone drug, leuprolide acetate, with similar composition to the commercial 1-month Lupron Depot® (LD). The burst release kinetics was measured with a previously validated continuous monitoring system as well as traditional sample-and-separate methods. The changes in pore structure and polymer permeability were investigated by SEM imaging and the uptake of a bodipy-dextran probe. In vitro results were compared to pharmacokinetics in rats over the same interval. High-burst, spray-dried microspheres were differentiated in the well-mixed continuous monitoring system but reached an upper limit when measured by the sample-and-separate method. Pore-like occlusions observed by confocal microscopy in some formulations indicated that particle swelling may have contributed to probe diffusion through the polymer phase and showed the extensive internal pore structure of spray-dried particles. Continuous monitoring revealed a rapid primary (1°) phase followed by a constant-rate secondary (2°) release phase, which comprised ∼80% and 20% of the 24-hr release, respectively. The ratio of 1° phase duration (t1°) and the characteristic probe diffusion time (τ) was highly correlated to 1° phase release for spray dried particles. Of the four spray-dried formulations administered in vivo, three spray-dried microspheres with similar polymer density showed nearly ideal linear correlation between in vivo absorption and well-mixed in vitro release kinetics over the first 24 h. By contrast, the more structurally dense LD and a more-dense in-house formulation showed a slight lag phase in vivo relative to in vitro. Furthermore, in vitro dimensionless times (tburst/τ) were highly correlated with pharmacokinetic parameters for spray-dried microspheres but not for LD. While the correlation of increases in effective probe diffusion and 1° phase release strongly suggests diffusion through the polymer matrix as a major release mechanism both in vitro and in vivo, a fixed lower limit for this release fraction implies an alternative release mechanism. Overall, continuous monitoring release and probe diffusion appears to have potential in differentiating between leuprolide formulations and establishing relationships between in vitro release and in vivo absorption during the initial burst period.

Minimizing the initial burst of octreotide acetate from glucose star PLGA microspheres prepared by the solvent evaporation method.

Sandostatin long-acting release (SLAR) depot for 1-month controlled release of octreotide is a somatostatin analogue product that has been used extensively in the pharmacological treatment of acromegaly. The complexities in the SLAR coacervation manufacturing processes and the use of a unique glucose-starpoly(lactic-co-glycolic acid) (PLGA-glu) may have contributed to the lack of US FDA-approved generic products referencing SLAR in the USA. To address this challenge, we encapsulated octreotide acetate by the commonly used solvent evaporation method in microspheres of a similar composition to SLAR, including the use of a comparable PLGA-glu. Based on our previous study that identified key formulation variables to prepare octreotide acetate/PLGA-glu microspheres, including lowering initial peptide pH and introducing an annealing step post loading, here we added NaCl to the external water phase to further improve the formulation. The resulting microspheres exhibited highly similar release and stability performance in vitro to SLAR, including an exceptionally low initial burst. The very low initial burst was also confirmed by pharmacokinetics in rats. Full erosion behavior analysis (polymer MW, water uptake and mass loss) revealed a slightly faster degradation of SLAR than the solvent evaporation formulations. Analysis of kinetics of dry Tg of the formulations reflected (a) the elevated residual solvent in SLAR and was not duplicated in the solvent evaporation formulations, and (b) the slightly higher Tg of peptide loaded formulations relative to than blank microspheres, consistent with the interaction of the acetate salt of octreotide with linear PLGA chains in the PLGA-glu. These data indicate that it is possible to prepare peptide loaded microspheres by the solvent evaporation method with extraordinarily similar performance to microspheres, such as those in SLAR, that are prepared by the low-burst release coacervation method.

Mapping in vivo microclimate pH distribution in exenatide-encapsulated PLGA microspheres.

The pH inside the aqueous pores of poly(lactic-co-glycolic acid) (PLGA) microspheres, often termed microclimate pH (µpH), has been widely evaluated in vitro and shown to commonly be deleterious to pH-labile encapsulated drug molecules. However, whether the in vitro µpH is representative of the actual in vivo values has long been remained a largely unresolved issue. Herein we quantitatively mapped, for the first time, the in vivo µpH distribution kinetics inside degrading PLGA microspheres by combining two previously validated techniques, a cage implant system and confocal laser scanning microscopy. PLGA (50/50, Mw = 24-38 kDa, acid-end capped and ester-capped) microsphere formulations with and without encapsulating exenatide, a pH-labile peptide that is known to be unstable when pH > 4.5, were administered to rats subcutaneously via cage implants for up to 6 weeks. The results were compared with two different in vitro conditions. Strikingly, the in vivo µpH developed similarly to the low microsphere concentration in vitro condition with 1-µm nylon bags but very different from conventional high microsphere concentration sample-and-separate conditions. Improved maintenance of stable external pH in the release media for the former condition may have been one important factor. Stability of exenatide remaining inside microspheres was evaluated by mass spectrometry and found that it was steadily degraded primarily via pH-dependent acylation with a trend that slightly paralleled the changes in µpH. This methodology may be useful to elucidate pH-triggered instability of PLGA encapsulated drugs in vivo and for improving in vivo-predictive in vitro conditions for assessing general PLGA microsphere performance.

Coaxial electrospray of uniform polylactide core-shell microparticles for long-acting contraceptive.

Despite its high efficacy and good patient compliance, the only long-acting injectable (LAI) contraceptive currently available in the US, depot medroxyprogesterone acetate (DMPA), is limited by significant side effects and a delayed return to fertility for up to 10 months after its intended duration of action. To overcome these limitations, we sought to develop an injectable poly(D,l-lactide) (PLA) microparticle for sustained release of contraceptive hormone, etonogestrel (ENG). A one-step technique, coaxial electrospray method was applied to prepare uniform ENG loaded core-shell structured and slow-degrading PLA microparticles (ENG-cs-MPs) to provide release control while minimizing polymer content. By adjusting voltage, polymer concentration and flow rate of the coaxial jetting solution, the prepared ENG-cs-MPs exhibited uniformly small particle size with volume mean diameter of 14.7 ± 0.5 µm and a shell thickness of 2.5 ± 0.1 µm, high drug loading of ~54%, high encapsulation efficiency of ~99%, and initial 1-day burst release of just ~10%. Long-term in vitro release of ENG was continuous for more than 3 months without change of the shell structure in 6 months. In PK studies, ENG-cs-MPs achieved a steady and continuous drug release for approximately 3 months and then quickly tapered off within 3 weeks. Hence, ENG-cs-MPs prepared by the coaxial electrospray method may be useful as a LAI contraceptive with an improved PK profile relative to DMPA.