Evaluation of [^111/114mIn]CHX-A"-DTPA-Z_HER2:342, an Affibody ligand conjugate for targeting of HER2-expressing malignant tumors

Aim. Radionuclide imaging of the HER2 receptor, which is a target for trastuzumab therapy, can provide important diagnostic information. Further, targeting radionuclide therapy might be an option for treatment of HER2 expressing tumors. The phage-display selected Affibody ligand Z_HER2:342, which binds to HER2 with an affinity of 22 pM, may here play an important role. The small size of the Z_HER2:342, 7.5 kDa, enables quick tumor localization and fast blood clearance. Earlier, successful targeting of HER2-expressing xenografts using Z_HER2:342 labeled using [¹¹¹In]benzy1-DTPA was reported. By changing to the CHX-A"-DTPA chelator, the stability and labeling kinetics of the radiometal-Z_HER2:342 conjugate can be improved. The aim of this study was to evaluate the labeling of the CHX-A"-DTPA-Z_HER2:342 conjugate with ¹¹¹In for diagnostic imaging and with ^114mIn for locoregional radionuclide therapy. Methods. The isothiocyanate derivative of CHX-A"-DTPA was coupled to Z_HER2:342 in alkaline conditions at 37 °C. The conjugate was labeled with both ¹¹¹In and ^114mIn and evaluated in vitro and in vivo. Results. Labeling with ¹¹¹In and ¹¹⁴In provided >95% yield after 30 min at RT. Specific radioactivity was 0.5 and 12 MBq/nmol, for ^114mIn and ¹¹¹In, respectively. The radiolabeled conjugates demonstrated specific binding to HER2 expressing SKOV-3 cells. In mice bearing SKOV-3 xenografts, the tumor uptake of [¹¹¹In]CHX-A"-DTPA-Z_HER2:342 4 h postinjection was 10.3±3.6% IA/g and tumor-to-blood ratio about 190. Conclusion. [¹¹¹In]CHX-A"-DTPA-Z_HER2:342 is a promising candidate for the visualization of HER2 expression in malignant tumors. Labeled with ^114mIn it could also be used for locoregional treatment of HER2 expressing tumors.