APN/CD13-targeting as a strategy to alter the tumor accumulation of liposomes.

Targeting angiogenic vasculature has been validated as a viable approach for cancer imaging and therapy. The tumor vasculature-specific ligand asparagine-glycine-arginine (NGR) peptide targets the isoform of aminopeptidase N (APN, also referred to as CD13) that is expressed on the endothelial cells in angiogenic vessels such as the neovasculature in tumors. APN/CD13 has become widely recognized as a rational target for therapeutic development and several NGR-conjugated agents are now in pre-clinical and clinical development. In the current study, a CT image-based approach is used to evaluate the in vivo performance of several NGR conjugated liposome formulations that vary in terms of NGR density and PEG spacer arm length. Indeed, for the first time it is demonstrated that CT imaging can be used for quantitative and longitudinal assessment of the pharmacokinetics and biodistribution of an actively targeted liposome formulation containing an iodinated agent. In comparison to conventional methods, the CT image guided drug delivery approach enables visualization of the intratumoral distribution of liposomes and quantification of the fraction of tumor occupied by the vesicles over time. This study is the first to use CT for molecular imaging.