Comparison of the in vivo biodistributions of αvβ6-binding agents for PET imaging applications.

Expression of the αvβ6 integrin is upregulated on multiple solid epithelial tumors while this integrin is largely absent on the healthy human epithelium. In the present study, we sought to compare the in vivo biodistributions of three αvβ6-selective agents that could be used for the positron emission tomography (PET)-based detection of malignancies expressing this integrin. A streptavidin-coupled αvβ6-binding peptide (streptavidin-A20), an anti-αvβ6 monoclonal antibody (620W.7) and an αvβ6-targeted protein, derived from a modified adenovirus 5 receptor-binding domain (Ad5.KO1.A20), were radiolabeled with [89Zr] and intravenously injected into mice bearing bilateral αvβ6-positive and -negative melanoma xenografts. Micro-PET imaging was performed at 0.33, 24, 48, 72, and (620W.7 group only) 144 h post-injection. Streptavidin-A20-based radiotracers preferentially accumulated in αvβ6-positive tumors, achieving >3.4:1 A375-β6:A375 activity ratios between 24 and 72 h post-injection, with off-target uptake restricted to the kidneys. 620W.7 exhibited roughly 2-fold greater accumulation in A375-β6 than A375 tumors, with lower levels of renal retention and greater activity detected in the liver and spleen. Despite having demonstrated selective binding to αvβ6-expressing cells in vitro, the Ad5.KO1.A20 protein failed to selectively traffic to tumors expressing this integrin in vivo. Nevertheless, streptavidin-A20 and 620W.7 demonstrated promise as αvβ6-selective agents for in vivo applications.