Date of Award

2020

Document Type

Thesis

Degree Name

Master of Science (M.S.)

Department

Pharmaceutical and Chemical Sciences

First Advisor

Xin Guo

First Committee Member

Xiaoling Li

Second Committee Member

Joseph Harrison

Abstract

Solid tumors possess biological features that are different from those in healthy tissues, which provides opportunities of anticancer treatment by nanomedicines. Due to the presence of the fenestrated tumor vasculatures, nanomedicines can selectively accumulate in tumor tissues by the enhanced permeability and retention (EPR) effect. The acidic pH in tumor interstitium (pH 6.0-7.0) also provides a promising mechanism to trigger the nanomedicines to promote the cellular uptake of cargo drugs. The previously reported stealth liposomes coated with PEG are known to accumulate in tumors owing to their prolonged circulation time. The PEG coating on liposomes can hinder serum protein adsorption and thus prevent rapid elimination by the reticuloendothelial system, thus increasing the liposome circulation time. However, liposomal interaction with cancer cells can also be hindered by the PEG coating.

In order to improve the anticancer activity of stealth liposomes, novel synthetic imidazole-based lipids were introduced to the composition of stealth liposomes to develop the pH-sensitive imidazole-based convertible liposomes (ICL). At acidic pH, the imidazole-based lipids would protonate to acquire positive charges, thus clustering with the negatively charged PEGylated lipids. Such lipid-lipid electrostatic interaction would induce phase separation of the bilayer to generate a PEG-free domain that displays excess positive charges. Such newly converted, cationic liposomes at acidic pH in tumor interstitium would have better interaction with negatively charged cancer cells and/or enhanced drug release, therefore overcoming the drawback of traditional stealth liposomes.

After synthesizing the imidazole-based lipids DHI, DHMI and DHDMI, we constructed doxorubicin (DOX)-loaded ICL formulations. The physicochemical properties of ICL were characterized, and factors influencing such properties were explored. The pH-triggered acquisition of positive charges of ICL was confirmed by the elevation of ζ- potentials and aggregation with negatively charged model liposomes that mimic bio-membranes at acidic pH 6.0-7.0. Acidic pH-triggered release of ICL was confirmed by drug release assays. It was also found that although the incorporation of cholesterol can remarkably reduce the size and increase the encapsulation efficiency (EE) of ICL, it also hinders the pH-sensitivity of ICL. The morphology of ICL at both pH 7.4 and pH 6.0 was characterized under transmission electron microscopy (TEM), which showed morphological changes in response to acidic pH 6.0, which further supported the proposed pH-sensitivity of ICL.

Cytotoxicity assays on 3D MCS of HeLa, A549, MDA-MB-231 and MDA-MB-468 cell lines were conducted to evaluate the anticancer activity of ICL formulations. ICL formulations without cholesterol showed considerably enhanced anticancer activities against MCS compared with the non-sensitive stealth liposomes (NSL). However, incorporation of cholesterol decreased such activities. The IC50 values of cholesterol-free ICL and ICL with cholesterol against MCS strongly suggested that the pH-sensitivity introduced by the imidazole-based lipids would enhance the anticancer activity of stealth liposomes, while the hindrance of the pH-sensitivity by cholesterol would reduce such activities.

Taken together, ICL’s pH-sensitivity is correlated with their enhanced anticancer activity than non-sensitive stealth liposomes.

Pages

149

Available for download on Friday, September 24, 2021

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