2.1.1. Importance of Charge Neutralization for Passive Targeting Although neutral non-PEGylated radiolabeled liposomes were shown to accumulate in human tumors [63], PEGylation is AZD8055 molecular weight required for effective tumor localization. PEGylation protected against aggregation of assemblies made with cationic lipids, enhanced their tumor uptake, and decreased their accumulation in the liver [64]. Campbell et al. compared the biodistribution of negatively charged liposomes (−20mV) and positively

charged liposomes (+31mV) after intravenous injection to tumor-bearing mice [65]. While liver was the major destination for both formulations with more than Inhibitors,research,lifescience,medical 50% of the injected dose, positively charged liposomes showed lower spleen accumulation and higher lung accumulation. Interestingly, in tumors, positively charged liposomes showed higher association with tumor blood vessels than negatively charged

ones. Levchenko et al. proposed the modulation of positively and negatively Inhibitors,research,lifescience,medical charged liposomes biodistribution by different opsonins [66]. Inhibitors,research,lifescience,medical Moreover, neutral PEGylated liposomes encapsulating doxorubicin showed superior therapeutic activity compared to cationic ones the decreased antitumor efficacy was correlated with reduced blood circulation and tumor accumulation of cationic liposomes [67]. A critical correlation between negative liposome charge and uptake by liver and spleen has been reported [66]; charge shielding by PEG decreased liver uptake and prolonged blood circulation. Finally, Huang and coworkers reported abolishment of liver uptake of cationic liposomes after their neutralization by postinsertion of DSPE-PEG leading to an increased tumor accumulation Inhibitors,research,lifescience,medical [68]. 2.1.2. Importance of Prior Administration/Accelerated Blood Clearance (ABC) Cancer treatments usually imply repeated administration of the same therapeutic agent to previously treated (predosed) patients. Administration of radiolabeled PEGylated liposomes to animals pretreated with a first dose of PEGylated

liposomes revealed a drastic decrease of their blood concentration 4h after injection from 50% of the injected dose for Inhibitors,research,lifescience,medical naive animals to 0.6% of the injected dose for predosed animals [69]. Noteworthy, after the second administration, PEGylated liposomes were cleared from the circulation very rapidly (decrease in half-life from 2.4h to 0.1h) crotamiton and this decreased blood residency was mirrored by increased accumulation in liver and spleen, supporting the accelerated blood clearance of liposomes after their second administration. This phenomenon is termed accelerated blood clearance (ABC). ABC is dependent on the time after initial injection: no ABC was reported for PEGylated liposomes injected daily or with injection intervals less than 5 days in rats whereas a one week interval induced accelerated blood clearance in the same study [69]. This delay reflects the two phases of ABC [70, 71].