Size optimization of synthetic graft copolymers for in vivo angiogenesis imaging

Angiogenesis is a critical step in tumor development and more than 25 angio genesis inhibitors are currently in clinical trials. Noninvasive in vivo im aging of angiogenesis represents a unique opportunity of repeatedly quantit ating microvascular parameters prior to and during anti-angiogenic treatmen ts. While several imaging tracers have been proposed for MR and nuclear ima ging, there does not exist any consensus of what constitutes an ideal size of an imaging agent. A series of synthetic pegylated DOTA derivatized graft copolymers (30, 60, 120 kDa) were synthesized and their in vivo behavior t ested in two breast cancer models differing in vascular endothelial growth factor (VEGF) expression. Polymers were labeled with different lanthanides (Eu, Gd, Dy) and absolute blood and tumor concentrations were determined by ICP-AES measurements. DOTA and the 30 kDa polymers underwent renal clearan ce resulting in low plasma levels. Slow leakage across neovasculature into tumor interstitium was clearly dependent on the molecular mass of all teste d agents in MCF-7 tumors. However, a cutoff was observed with minimal extra vasation occurring at and above 120 kDa in well differentiated MCF-7 tumors . VEGF overexpression caused detectable differences in extravasation of all polymers, including the 120 kDa compound. We conclude that large molecular weight contrast agents with a molecular mass of <120 kDa extravasate from experimental tumor neovasculature and may not be an accurate marker for mea suring true blood volume fractions when in vivo imaging is performed in the steady state.