In vivo MR imaging for tumor-associated initial neovascularization by supramolecular contrast agents.

Microvascular imaging is required to understand tumor angiogenesis development; however, an appropriate whole-body imaging method has not yet been established. Here, we successfully developed a supramolecular magnetic resonance (MR) contrast agent for long-term whole-tissue observation in a single individual. Fluorescein- and Gd-chelate-conjugated polyethylene glycols (PEGs) were synthesized, and their structures were optimized. Spectroscopic and pharmacokinetic analyses suggested that the fluorescein-conjugated linear and 8-arm PEGs with a molecular weight of approximately 10 kDa were suitable to form a supramolecular structure to visualize the microvessel structure and blood circulation. Microvascular formation was evaluated in a glioma cell transplantation model, and neovascularization around the glioma tissue at 5 days was observed, with the contrast agent leaking out into the cancer tissue. In contrast, after 12 days, microvessel structures were formed inside the glioma tissue, but the agents did not leak out. These imaging data for the first time proved that the microvessels formed inside cancer tissues at the early stage are very leaky, but that they form continuous microvessels after 12 days.