PET Imaging of the EPR Effect in Tumor Xenografts Using Small 15 nm Diameter Polyethylene Glycols Labeled with Zirconium-89.

The goal was to develop and characterize a companion diagnostic for the releasable PEG∼SN-38 oncology drug, PLX038, that would identify tumors susceptible to high accumulation of PLX038. PEG conjugates of the zirconium ligand desferroxamine B (DFB) of similar size and charge to PLX038 were prepared that contained one or four DFB, as well as one that contained three SN-38 moieties and one DFB. Uptake and associated kinetic parameters of the Zr-labeled nanocarriers were determined in tumor and normal tissues in mice using μPET/CT imaging. The data were fit to physiologically based pharmacokinetic models to simulate the mass-time profiles of distribution of conjugates in the tissues of interest. The time-activity curves for normal tissues showed high levels at the earliest time of measurement due to vascularization, followed by a monophasic loss. In tumors, levels were initially lower than in normal tissues but increased to 9% to 14% of injected dose over several days. The efflux half-life in tumors was very long, approximately 400 hours, and tumor levels remained at about 10% injected dose 9 days after injection. Compared with diagnostic liposomes, the PEG nanocarriers have a longer serum half-life, are retained in tumors at higher levels, remain there longer, and afford higher tumor exposure. The small PEG nanocarriers studied here show properties for passive targeting of tumors that are superior than most nanoparticles and might be effective probes to identify tumors susceptible to similar size therapeutic nanocarriers such as PLX038.