Small-animal PET of tumor angiogenesis using a 76Br-labeled human recombinant antibody fragment to the ED-B domain of fibronectin

Abstract

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 bromoperoxidase/H2O2. The stability of the labeled antibody was tested both in vitro and in vivo. Biodistribution and small-animal imaging studies (PET and CT) were performed in F9-bearing 129/ sv mice (n = 3 or 4). Results: The enzymatic radiobromination approach afforded the labeled antibody in high yield (>55%) under mild reaction conditions. 76Br-L19-SIP stability in mouse serum 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 normal 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. Copyright © 2007 by the Society of Nuclear Medicine, Inc.