Development and evaluation of 89Zr-trastuzumab for clinical applications.

Objectives: Due to the suitable physical characteristics of ⁸⁹Zr as a PET radionuclide and affinity of Trastuzumab monoclonal antibody against HER2, [⁸⁹Zr]Zr-Trastuzumab was prepared and went through preclinical evaluations for ultimate human applications.

Methods: ⁸⁹Zr was produced by using ⁸⁹Y(p,n)⁸⁹Zr reaction at a 30 MeV cyclotron (radionuclide purity>99.9%, specific activity of 17 GBq/µg). p-SCN-Bn-Deferoxamine (DFO); was conjugated to trastuzumab, followed by labeling with ⁸⁹Zr in oxalate form at optimized condition. Cell binding, internalization and, radioimmuno-activity assays were studied using HER2+ BT474 and HER2- CHO cell lines. Finally, the biodistribution of the radioimmunoconjugate was assessed in normal and HER2+ BT474 tumor-bearing mice using tissue counting and imaging at different intervals after injection. Also, a woman with HER2-positive metastatic breast cancer under treatment with Herceptin underwent both [⁸⁹Zr]Zr-Trastuzumab and, [¹⁸F]FDG PET/CTs.

Results: ⁸⁹Zr was produced with high radionuclidic and radiochemical purities (>99%) and [⁸⁹Zr]Zr-DFO-Trastuzumab was prepared with radiochemical purity of >98% and specific activity of 9.85 GBq/µmol. The radioimmunoconjugate was stable both in PBS buffer and in human serum for at least 48 h. The radioimmunoactivity assay demonstrated about 70% of [⁸⁹Zr]Zr-DFO-Trastuzumab is bound to the BT474 cells at the number of 250×10⁶ cells. Cell binding studies showed that about 28% of radioimmunoconjugate is attached to BT474 cells after 90 min. Internalization studies showed that 50% of [⁸⁹Zr]Zr-Trastuzumab is internalized to BT474 cells only in 6 h. The biodistribution study of the labeled compound in normal mice demonstrated the same pattern of the monoclonal antibodies which is entirely different from the biodistribution of free ⁸⁹Zr. Biodistribution and imaging studies in tumor-bearing mice showed the significant uptake values of [⁸⁹Zr]Zr-Trastuzumab in tumor sites. [⁸⁹Zr]Zr-Trastuzumab PET/CT revealed metastatic lesions documented previously with [¹⁸F]FDG PET/CT scan in a woman with breast cancer who was under treatment with Herceptin. Although the [¹⁸F]FDG PET/CT scan had better quality images, the valuable and unique advantage of [⁸⁹Zr]Zr-Trastuzumab PET/CT is delineating HER2+ metastasis, which is essential in diagnosis and HER2-based treatments.

Conclusion: The prepared [⁸⁹Zr]Zr-Trastuzumab has a high potential radio-pharmaceutical for immune-PET imaging of the patients with HER2+ tumors.