(M1230-08-53) Impact of Porosity on Drug Release from PLGA Microspheres

Purpose: Poly (lactic-co-glycolic acid) (PLGA) microspheres have been used to control in vivo drug release and are used in many marketed long acting injectable (LAI) products. Development of these products is challenging as the PLGA polymer shows inherent heterogeneity (molecular weight, blockiness and monomer ratio) which can affect product performance (1). Additionally, microstructural differences in these microspheres, resulting from formulation and processing parameters, contribute to differences in product performance (2). PLGA microspheres often show triphasic release profiles. Although monophasic, and biphasic release profiles can also occur, particularly for hydrophilic drugs. In triphasic drug release, an initial burst release is followed by a lag phase where only a small amount of drug release takes place and then a secondary drug release phase ensues. The lag phase has been ascribed to the time dependent processes of water penetration into the microspheres which results in polymer degradation, creation & enlargement of pores, and drug solubilization, all of which generate osmotic pressure. The aim of this study is to understand drug release behavior from microspheres with varying extents of porosity and utilize this as a means of achieving desired release profiles. For example, continuous drug release may result in improved therapeutic outcomes for some conditions. Triamcinolone acetonide was selected as the model drug for this study.

Methods: Porous microspheres were prepared via a modified solvent evaporation method to include a porogen (Fig. 1). The PLGA (75:25, 0.28 dL/g) was solubilized in 1 gm of dichloromethane (DCM) and 200 mg triamcinolone acetonide were solubilized in 800 μL of benzyl alcohol. 7 mL aqueous phase was prepared with different concentrations of porogen (0.25%, 1% and 2%)  in 1% w/v PVA (30-70 kDa) solution. Both phases were mixed and homogenized at 3600 rpm for 30 seconds. The resultant o/w emulsion was then dispersed into 100 mL of 0.1% w/v PVA solution for solvent extraction with continuous stirring at 400 rpm. The microspheres were then sieved through a 200 μm mesh size sieve followed by a 20 μm mesh size sieve to remove any extra-large or extra-small particles. The collected microspheres were washed and lyophilized for removal of residual solvent and moisture. The microspheres were analyzed as follows: particle size analysis was performed using a Mastersizer®; drug loading and encapsulation efficiency were analyzed via HPLC; microstructure analysis (SEM) and release studies were performed using USP type IV apparatus with dissolution media PBS 7.4 and a flow rate of 8 mL/min).

Results: Three formulations with different porogen concentration were prepared (0.25% w/v PTA, 1% w/v PTA and 2% PTA). There was a trend of increase in particle size with increase in porogen concentration which can be ascribed to the increased porous structural volume (Fig. 2a). The drug loading was fairly similar for all formulations (~15 - 20 % w/v). However, reduced drug loading was observed for the microspheres with higher porosity (1% w/v PTA and 2% PTA) (Fig. 2b). This is considered to be a result of drug leaching out of these microspheres during the solvent evaporation stage. The SEM images showed increased porosity on and near the surface with increase in porogen concentration (Fig. 3a). The surfaces and inner cores were porous throughout. The 2% w/v PTA microspheres deformed easily on compression demonstrating the plasticizer effect of ammonium bicarbonate at higher concentration. The release study demonstrated little or no lag phase for all the formulations (Fig. 3b). The 0.25% w/v porogen formulation showed relatively constant drug release over a one month period. The high standard deviations observed in the release profile of the 2% w/v PTA formulation may be due to the high concentration of salt reducing the glass transition temperature (Tg) of PLGA causing these microspheres to be excessively leaky. The lower Tg increases polymer chain mobility allowing structural rearrangement during the degradation phase. This process is unlikely to be uniform across the particles and therefore may account for the observed high standard deviations in the release profile.

Conclusion: Microspheres with different microstructures were successfully prepared via solvent evaporation using ammonium bicarbonate as a porogenic agent. SEM images confirmed the high porosity of microspheres with increased porogen concentration. The addition of pyrogen at low concentration may be useful to achieve constant drug release levels, avoiding the occurrence of lag phase. Further work will be conducted to fine tune the porogen concentration for constant drug release for a range of drug molecules. This approach can be applied to address the lag phase that occurs with many PLGA formulations and is typically undesirable. High concentrations of porogens increase PLGA plasticity resulting in rapid and erratic drug release and therefore should be avoided.

References: 1. Mylonaki J, J Control Release, 286, 2018, 231-239
2. Gu B, J Control Release. 2016 Apr 28;228:170-178

Acknowledgements: The authors acknowledge the University of Connecticut Pfizer Distinguished Chair Funds for supporting this research.

Figure 1. Schematic representation of the modified solvent evaporation method for preparation of porous microspheres.

Figure 2. (a) Particle size analysis for different formulations prepared with varying concentrations of porogen (mean + SD, n=3); and (b) % drug loading and encapsulation efficiency of various formulations (mean + SD, n=3).

Figure 3. (a) SEM images of various formulations at different magnifications; and (b) in vitro drug release for formulations prepared with different porogen concentration using USP type IV apparatus with dissolution media PBS 7.4 and flow rate 8 mL/min) (mean + SD, n=3)