Small-Animal PET of Tumor Angiogenesis Using a 76Br-Labeled Human Recombinant Antibody Fragment to the ED-B Domain of Fibronectin

The aim of this study was to image the extra domain B (ED-B) of fibronectin, an angiogenesis-related target, in solid tumors using small-animal PET. Toward this aim, an ED-B fibronectin-binding human antibody derivative (L19-SIP) was labeled with 76Br via an enzymatic approach. Biodistribution and imaging studies were performed in human teratoma–bearing mice for up to 48 h after injection.Methods: L19-SIP was labeled with 76Br using bromo-peroxidase/H2O2. The stability of the labeled antibody was tested both in vitro and in vivo. Biodistribution and small-animal imaging studies (PET andCT) were performed in F9-bearing 129/ sv mice (n 5 3 or 4). Results: The enzymatic radiobromination approach afforded the labeled antibody in high yield (.55%) un-der mild reaction conditions. 76Br-L19-SIP stability in mouse se-rum proved to be similar to that of the 125I-labeled analog (.80% of intact material at 48 h after injection). Fast and specific in vivo targeting was obtained in tumors and other organs expressing ED-B fibronectin (i.e., ovaries and uterus). However, slow renal clearance and persistent activity predominately in blood and stomach suggests partial 76Br-L19-SIP debromination in vivo. This debromination was confirmed in a metabolism study in nor-mal mice. The F9 tumors were clearly imaged by small-animal PET at each considered time point, starting at 5 h up to 48 h after injection. Conclusion: 76Br-L19-SIP specifically accumulated at the target site, enabling detailed small-animal PET of tumor neovasculature. Therefore, targeting the angiogenesis-associated expression of ED-B fibronectin can be a valuable tool for tumor detection using molecular imaging with PET