(T1030-07-38) SubQ Environment Impact on In Vitro Drug Release of PLGA Microspheres

Purpose: PLGA-based formulations are the predominant marketed long-acting biodegradable injectables for subcutaneous (SubQ) administration, with the majority of these being microspheres (MS) [1]. As numerous process and formulation variables (e.g., physicochemical properties of the polymer) influence the product performance, conducting bioavailability and bioequivalence studies, during the development of novel and generic products is essential to ensure therapeutic efficacy and patient safety. However, these studies are both expensive and time-consuming. Additionally, developing an in vitro release test for long acting injectables is challenging due to the lack of established regulatory guidelines on the appropriate setup and release medium to use. Standard drug release studies are mainly conducted in conventional media, for example in PBS at pH 7.4; though, these conditions are oversimplified and inadequate for assessing product performance under physiologically relevant conditions. The objective of this study was to examine how subcutaneous macromolecules affect the release profile of PLGA microspheres and to assess the ability of the novel Subcutaneous Injection Site Simulator (Scissor) in vitro system to discern the performance of MS prepared with different PLGA polymers.

Methods: Two MS formulations were prepared by membrane emulsification/solvent evaporation method (Micropore Technologies Ltd, UK) and were loaded with flurbiprofen, selected as model lipophilic drug. MS 50 and MS 75 were formulated using an end-capped PLGA having a lactide/glycolide ratio of 50:50 (Mw ~39 KDa, Tg ~40 °C), or 75:25 (Mw ~ 16 KDa, Tg ~37 °C), respectively. Formulation and process parameters (i.e., PLGA concentration, injection speed, stirring rate, concentration of the stabilizer in the aqueous phase) were optimized to have the target MS size of about 50 µm and a narrow size distribution. Drug quantification was made by HPLC-UV and drug loading and encapsulation efficiency (EE%) were calculated. The MS release was performed using the Scissor, a dialysis-based chamber simulating the SubQ injection site and immersed in a bicarbonate buffer that mimics the infinite sink of the body. The SubQ tissue consists of a three-dimensional network, the “extracellular matrix (ECM)”, predominantly composed of collagen and glycosaminoglycans, such as hyaluronic acid (HA) and chondroitin sulphate [2]. To evaluate the impact of these macromolecules on the MS performance, the inner chamber was filled with a combination of simulated ECM components, varying in the absence (HA-XR) or presence (HA-XR collagen) of collagen. Briefly, 20 mg of MS were suspended in 0.5 mL of saline solution and injected into the Scissor injection site at 34 ± 0.5 °C. An aliquot of buffer was taken from the outer chamber at predetermined time points up to 7 days and flurbiprofen was quantified by HPLC-UV.

Results: MS with D₅₀ = 30-90 μm and the highest yield were achieved by setting the injection speed at 0.5 ml/min, the stirring rate at 330 rpm, using 12% (w/v) PLGA and 2% (w/v) HPMC as a stabilizer. The drug loading for MS 50 and MS 75 was ~12% and ~17%, and EE% was 60% and 75%, respectively. The release profiles obtained with MS 50 in the HA-XR and HA-XR collagen were similar (Figure 1), with the percentage of flurbiprofen released after 7 days being 21.3% and 19.7%, respectively. The impact of the presence of collagen was different for the MS 75 formulation. Specifically, the release of flurbiprofen after 7 days was 4.4% in the collagen-free matrix, whereas a 15.6% was released in the HA-XR collagen. Furthermore, the release of MS 75 is much slower compared to MS 50 in the collagen-free matrix. This difference can be attributed to the higher lipophilicity of PLGA 75:25.

Conclusion: The Scissor’s system demonstrated good discriminatory power, highlighting the distinct behaviour of MS depending on the polymer used. Moreover, the presence of collagen appears to have a polymer-dependent impact on flurbiprofen release. Given the importance of establishing a robust in vitro release method, that closely mimics the conditions at the subcutaneous injection site, these findings aim to clarify and enhance our understanding of the environment's influence on drug release. Developing such a method can reduce the need for extensive in vivo studies and support the advancement of subcutaneous formulation development.

References: [1] Selmin F. et al., Drug Discovery Today 25, 321–329 (2020).
[2] Kinnunen H.M. et al., Journal of Controlled Release 214 (2015) 94–102.

Figure 1 - Cumulative % of flurbiprofen released from formulation MS 50 in presence (green lines) or absence (blue lines) of collagen. The solid lines represent the mean profile (n=2), while the dashed lines represent individual profiles.

Figure 2 - Cumulative % of flurbiprofen released from formulation MS 75 in presence (green lines) or absence (blue lines) of collagen. The solid lines represent the mean profile (n=2), while the dashed lines represent individual profiles.