Kit formulation for ^99mTc-labeling of recombinant anti-HER2 Affibody molecules with a C-terminally engineered cysteine

Introduction: Molecular imaging of human epidermal growth factor receptor type 2 (HER2)-expression in malignant tumors provides potentially important information for patient management. Affibody molecules have shown to be suitable tracers for imaging applications using single photon emission computed tomography or positron emission tomography. Results from an earlier evaluation of the application of site-specific ^99mTc-labeling of the Affibody molecule, Z_HER2:2395-C, were favorable. Methods: As a preparation for clinical application of this tracer, we have developed and evaluated a robust single-vial freeze-dried kit, allowing labeling of the Affibody molecule, Z_HER2:2395-C, with ^99mTc. Results: The composition of the kit [containing glucoheptonate, EDTA and tin(II)-chloride], as well as the protein amount and the pertechnetate volume were optimized for a high labeling yield (>90%) and minimal presence of reduced hydrolyzed technetium colloids (<1%). The specificity to HER2 receptors, the binding competence and the stability in phosphate-buffered saline and murine serum were verified in vitro. The shelf-life was also evaluated in vitro. , showing no reduction in labeling yield or binding capacity to HER2-expressing cells after over 400 days of storage of the single-vial freeze-dried kit. Conclusions: Z_HER2:2395-C labeled with ^99mTc using the lyophilized kit was stable and resulted in a favorable biodistribution in an in vivo evaluation in normal Naval Medical Research Institute mice.