Development of Positron Emission Tomography Probe of 64Cu-labeled Anti-c-kit 12A8 Fab to Measure Protooncogene C-kit Expression

Introduction: C-kit is an important diagnostic and therapeutic target molecule for several malignancies, and c-kit-targeted drugs have been used clinically. Because abundant c-kit expression in tumors is a prerequisite for successful c-kit-targeted therapy, imaging of c-kit expression is expected to play a pivotal role in the therapeutic decision for each patient. We evaluated 64Cu-labeled Fab of anti-c-kit antibody 12A8 as a positron emission tomography (PET) imaging probe. Methods: 111In- or 125I-labeled 12A8 Fab were evaluated in vitro by cell binding, competitive inhibition, and cellular internalization assays and in vivo by biodistribution in mice bearing c-kit-expressing and -non-expressing tumors. Next, Fab fragment was labeled with the positron emitter 64Cu and evaluated by PET. Results: Radiolabeled 12A8 Fab showed specific binding to c-kit-expressing cells with high affinity, and internalized into cells after binding to c-kit on cell surface. Although tumor accumulation of [111In]Fab was lower than that of [111In]IgG, the faster blood clearance of [111In]Fab provided higher tumor-to-blood ratio at 6 h postinjection onwards. Blood clearance of 64Cu-labeled 12A8 Fab was slower than that of [111In]Fab, but PET using [64Cu]Fab clearly visualized the tumor at 6 h postinjection onwards. Conclusion: The 64Cu-labeled 12A8 Fab could be used for c-kit-specific PET imaging and might help in selecting appropriate patients for c-kit-targeted treatments