MicroPET imaging of HER2+ tumors using F-18-labeled anti-HER2 diabody

Objectives Rapid blood clearance of antibody fragments such as diabodies supports their use as imaging agents for targeted tumor detection. Current radiolabeling methods for immunoPET involve lengthy conjugations and therefore long-lived isotopes. A one-pot, remote-controlled method for synthesis of N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) was developed. [¹⁸F]SFB was conjugated to anti-HER2 diabody and evaluated in a HER2 tumor system.

Methods [¹⁸F]SFB was synthesized using a one-pot microwave reactor system, then conjugated to anti-HER2 diabody, followed by spin column purification. HER2+ (MCF7-HER2 transfected, human breast cancer) and HER2-negative (C6, rat glioma) cells were used for immunoreactivity in vitro and tumor targeting in vivo (subcutaneous xenografts in nude mice). Tumor targeting was evaluated by microPET and ex vivo biodistribution.

Results [¹⁸F]SFB labeling of anti-HER2 diabody was repeated > 4 times. RCY was 4-8% after 45 min of reaction. Labeled diabody selectively bound positive cells in vitro(immunoreactivity 32 – 67%) and targeted positive tumors in vivo (mean tumor:blood = 1.9, n = 3). MicroPET images showed tumor targeting at 4 hours post-injection.

Conclusions A novel, HER2-targeting immunoPET radiotracer was developed. The simplification of ¹⁸F labeling allows pairing of the in vivo half life of the fragment with the radioactive half life of the isotope. A rapid and simplified radiolabeling procedure combined with suitable in vivo kinetics for high-contrast, same-day imaging supports this approach as a general method for fast and feasible generation of immunoPET tracers.

Research Support California Breast Cancer Research Program #14GB-0157, NIH CA 086386, CA 119367.