Specific binding of modified ZD6474 (Vandetanib) monomer and its dimer with VEGF receptor-2

Tumor angiogenesis is an important component of cancer biology driven in part by the hypothesis that tumor vessel growth is a necessary requirement for tumor growth. Angiogenesis does not only depend on endothelial cell invasion and proliferation, it also requires pericyte coverage of vascular sprouts for vessel stabilization. These processes are coordinated by vascular endothelial growth factor (VEGF). Vandetanib, also known as ZD6474, is an orally bioavailable small molecule tyrosine kinase inhibitor of multiple growth factors that is an antagonist of the vascular endothelial growth factor receptor-2 (VEGFR-2). ZD6474 was purchased and modified to add a fluorescent dye (6-FAM). A linker was used to couple the ZD6474 monomer to create dimers, and similarly linked to FAM fluorescent dye. The two compounds were compared using human endothelial cell (HUVECs) and the cancer cell lines U-87-MG and MDA-MB-231. We compared cellular uptake and binding specificity, as well as effects on cellular viability and angiogenesis in a series of in vitro and in vivo studies. ZD6474 dimer demonstrated improved uptake in HUVECs and other VEGFR expressing cells over ZD6474 monomer in vitro. Therapeutic effects were mixed, with in vitro studies showing the dimer having the same or weaker effects compared to the monomer, while an in vivo study using pseudotumors (matrigel plug assay) seemed to indicate stronger effects could be obtained from the dimer. Finally, in biodistribution study in a xenograft tumor model, the dimer accumulated 20× greater concentration in the tumor than in the liver, spleen, or kidneys, and also 20× greater than the accumulation of the monomer or the dye alone, all at 24 h following compound injection. This study provides a rationale for the evaluation of dimeric ZD6474 as a potent imaging agent of angiogenic activity in vivo.