Specific endothelial binding and tumor uptake of radiolabeled angiostatin

Angiostatin (AS) is a potent antiangiogenic agent which inhibits tumor growth through specific action on proliferating endothelial cells. Imaging of radiolabeled AS would enhance our knowledge on the pharmacokinetics of AS and might provide useful information relating to tumor neovasculature. We therefore investigated the potential of radiolabeled AS as a novel tumor imaging agent. Human angiostatin was radioiodine labeled using the lactoperoxidase method. Competition binding studies showed a dose-dependent inhibition of ¹²⁵I-AS binding to endothelial cells by excess unlabeled AS, and a displacement curve demonstrated that specific binding was dose dependent and saturable, with a K_d value of 169 nM. Gel analysis showed that ¹²⁵I-AS remained stable in serum for up to 24 h without significant degradation. Intravenously injected ¹²⁵I-AS in rats was cleared from the blood in an exponential fashion. Biodistribution data from human colon cancer-bearing Balb/C nude mice showed high uptake in the kidneys, stomach, liver, and lungs. Tumor uptake was 3.2±0.7, 2.6±0.2, and 1.7±0.2%ID/g at 2, 4, and 9 h after injection, respectively. Tumor to muscle count ratio increased from 3.1±0.5 at 2 h to 4.4±0.5 at 9 h. Serial scintigraphy from 1 to 5 h after ¹²³I-AS injection demonstrated high uptake in the kidneys and bladder, consistent with renal excretion. There was clear demarcation of tumor by 1 h, with gradual increase in contrast over time (4-h tumor to contralateral thigh ratio =4.7±1.1). Thus, radioiodine-labeled angiostatin binds specifically to endothelial cells and has potential as a novel tumor imaging agent.