Evaluation of the physicochemical characteristics of pH sensitive liposome formulations for effective endosomal escape and cytosolic delivery.

Introduction: The idea of using pH-sensitive liposomes to improve intracellular delivery of anticancer drugs has been gaining traction. Various types of pH-responsive lipids materials are being evaluated for this purpose. However, finding out the optimal composition for efficient cytosolic delivery of a biologically active material usually requires performing cell uptake or efficacy study that may involve considerable resources. The purpose of this study was to elaborate a physiochemical test enables to mimics the endosomal environment (acidic pH) to investigate the impact of the acidification of endosomal lumen on the physicochemical properties of pH-sensitive liposomes.
Method: Two classes of pH sensitive lipids, namely fusogenic (DOPE) and ionizable (DODAP) were selected to prepare Calcein loaded liposomes (figure1). Liposomes were then analyzed by dynamic light scattering (DLS) for physicochemical aspects (size and polydispersity) in HEPES buffer pH 7.4 and in various citrate buffer (pH 6.5, 5.5, and 4.5) simulating physiological and endosomal environment, respectively. Content release rates of the liposomes as a function of pH were monitored by using calcein fluorescence dequenching assay.
Results: The results evidenced distinct structural and release behavior for each class of lipids. At endosomal pH, liposomes with DODAP exhibit superior cargo release characteristics than DOPE formulation, which is consistent with DLS analysis, which reveals drastic increases in both size and PDI in acidic conditions (Figure 1). This characteristic could be the key for triggering endosomal escape and subsequent cytosolic delivery of the cargo.
Conclusion: In sum, this test is helpful for the rational selection of different components of pH-sensitive liposome formulations to facilitate the process of endosomal escape and thereby achieve effective cytosolic release.