Department of Pharmaceutical Sciences, The Biointerfaces Institute, University of Michigan, 2800 Plymouth Rd, Ann Arbor, MI 48109, USA.
Department of Pharmaceutical Sciences, The Biointerfaces Institute, University of Michigan, 2800 Plymouth Rd, Ann Arbor, MI 48109, USA; Department of Biomedical Engineering, University of Michigan, 2200 Bonisteel Blvd, Ann Arbor, MI 48109, USA.
J Control Release. 2022 Dec;352:438-449. doi: 10.1016/j.jconrel.2022.08.043. Epub 2022 Nov 1.
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.
聚（乳酸-共-乙醇酸）（PLGA）微球水相孔隙内的 pH 值，通常称为微气候 pH 值（μpH），在体外已被广泛评估，结果表明其通常对 pH 敏感的包封药物分子有害。然而，体外μpH 值是否代表实际的体内值，长期以来一直是一个悬而未决的问题。在此，我们首次通过结合两种先前经过验证的技术——笼式植入系统和共聚焦激光扫描显微镜，定量描绘了降解 PLGA 微球内部的体内μpH 值分布动力学。PLGA（50/50、Mw=24-38 kDa、酸端封端和酯端封端）微球制剂，分别含有和不含有包封艾塞那肽的制剂，这是一种已知在 pH>4.5 时不稳定的 pH 敏感肽，通过笼式植入物皮下给予大鼠，持续 6 周。结果与两种不同的体外条件进行了比较。引人注目的是，体内μpH 值的发展与低微球浓度的体外条件（1μm 尼龙袋）相似，但与传统的高微球浓度样品分离条件非常不同。前者条件下释放介质中稳定的外部 pH 值的改善可能是一个重要因素。通过质谱评估微球内剩余艾塞那肽的稳定性，发现其主要通过 pH 依赖性酰化而稳定降解，其趋势与μpH 值的变化略有平行。该方法可用于阐明体内 PLGA 包封药物的 pH 触发不稳定性，并改善用于评估一般 PLGA 微球性能的体内预测性体外条件。
