Turning stealth liposomes into cationic liposomes for anticancer drug delivery

Targeting the anticancer agents selectively to cancer cells is desirable to improve the efficacy and to reduce the side effects of anticancer therapy. Previously reported passive tumor targeting by PEGylated liposomes (stealth liposomes) have resulted in their higher tumor accumulation. However their interaction with cancer cells has been minimal due to the steric hindrance of the PEG coating. This dissertation reports two approaches to enhance the interaction of stealth liposomes with cancer cells. First, we designed a lipid-hydrazone-PEG conjugate that removes the PEG coating at acidic pH as in the tumor interstitium. However, such a conjugate was highly unstable on shelf. Targeting the anticancer agents selectively to cancer cells is desirable to improve the efficacy and to reduce the side effects of anticancer therapy. Previously reported passive tumor targeting by PEGylated liposomes (stealth liposomes) have resulted in their higher tumor accumulation. However their interaction with cancer cells has been minimal due to the steric hindrance of the PEG coating. This dissertation reports two approaches to enhance the interaction of stealth liposomes with cancer cells. First, we designed a lipid-hydrazone-PEG conjugate that removes the PEG coating at acidic pH as in the tumor interstitium. However, such a conjugate was highly unstable on shelf. Second we developed lipids with imidazole headgroups. Such lipids can protonate to provide positive charges on liposome surface at lowered pH. Additionally, negatively charged PEGylated phospholipids can cluster with the protonated imidazole lipids to display excess positive charges on the surface of the liposomes, thus enhancing their interaction with negatively charged cancer cells. We prepared convertible liposome formulations I, II and III consisting of one of the three imidazole-based lipids DHI, DHMI and DHDMI with estimated pKa values of 5.53, 6.2 and 6.75, respectively. Zeta potential measurement confirmed the increase of positive surface charge of such liposomes at lowered pHs. DSC studies showed that at pH 6.0 formulation I formed two lipid phases, whereas the control liposome IV remained a one-phase system at pHs 7.4 and 6.0. The interaction of such convertible liposomes with negatively charged model liposomes mimicking biomembranes at lowered pH was substantiated by 3-4 times increase in average sizes of the mixture of the convertible liposomes and the model liposomes at pH 6.0 compared to pH 7.4. The doxorubicin-loaded convertible liposomes show increased cytotoxicity in B16F10 (murine melanoma) and Hela cells at pH 6.0 as compared to pH 7.4. Liposome III shows the highest cell kill at pH 6.0 for both the cells. The control formulation IV showed no difference in cytotoxicity at pH 7.4 and 6.0. Uptake of convertible liposome II by B16F10 cells increased by 57 % as the pH was lowered from 7.4 to 6.0