对脂质体进行聚乙二醇化（PEGylation）修饰是延长其体内循环时间，提高递药效率的常规手段之一。然而PEG无法在体内有效降解，长期生物安全性并未得到证实；给药后诱导体内产生大量PEG抗体，降低药物递送效率，并可能引发其他的免疫反应。因此寻找替代PEG化的修饰手段成为许多研究者的共识。近年来一种被称为PAS化（PASylation）的技术被应用到蛋白质和多肽药物改良中，其原理为亲水性氨基酸脯氨酸P，丙氨酸A和丝氨酸S组成的聚多肽PAS可形成类似PEG的无规则卷曲构象。PAS化可延长蛋白质和多肽药物半衰期，且其多肽结构利于生物降解。PAS化作为一种较为新颖且前景广阔的技术，在国内外尚未见其脂质体修饰的相关报道。本项目拟构建一类由亲水性多肽序列PAS修饰的脂质体，提供一种新型的向脂质体表面引入亲水聚合物基团进行修饰并制备长循环脂质体的研究思路，并探讨其对脂质体的安全性及递药效率的影响。

PEGylation is widely applied in fabricating liposomal drugs due to the fact that it can prolong in vivo circulation time and improve drug delivery efficiency. However, PEG cannot degrade completely in vivo and its long-term biosafety has not been validated yet. Besides, anti-PEG immunity also remains an issue since it could induce accelerated blood clearance effect (ABC effect) which lowers drug delivery efficiency and other immune responses as well. In recent years people have brought up the idea of replacing PEGylation with PASylation, which is a proper combination of prolines (P), alanines (A) and serines (S). PAS peptide sequence could adopt random coil structure in aqueous buffers which is very similar to PEG, but the peptide sequence is more easily degraded in vivo which would be potentially more safe than PEG. Until now there have been no reports on its liposomal modification, therefore we propose to establish liposomes modified with PAS peptide sequence to explore the possibility of introducing PAS peptide-based hydrophilic group onto liposomes and investigate its effect on the safety and delivery efficiency of liposomes.