Positron emission tomography (PET) imaging of neuroblastoma and melanoma with ⁶⁴Cu-SarAr immunoconjugates

The advancement of positron emission tomography (PET) depends on the development of new radiotracers that will complement ¹⁸F-FDG. Copper-64 (⁶⁴Cu) is a promising PET radionuclide, particularly for antibody-targeted imaging, but the high in vivo lability of conventional chelates has limited its clinical application. The objective of this work was to evaluate the novel chelating agent SarAr (1-N-(4-aminobenzyl)-3, 6,10,13,16,19-hexaazabicyclo[6.6.6]-eicosane-1,8-diamine) for use in developing a new class of tumor-specific ⁶⁴Cu radiopharmaceuticals for imaging neuroblastoma and melanoma. The anti-GD2 monoclonal antibody (mAb) 14.G2a, and its chimeric derivative, ch14.18, target disialogangliosides that are overexpressed on neuroblastoma and melanoma. Both mAbs were conjugated to SarAr using carbodiimide coupling. Radiolabeling with ⁶⁴Cu resulted in >95% of the ⁶⁴Cu being chelated by the immunoconjugate. Specific activities of at least 10 μCi/μg (1 Ci = 37 GBq) were routinely achieved, and no additional purification was required after ⁶⁴Cu labeling. Solid-phase radioimmuno-assays and intact cell-binding assays confirmed retention of bioactivity. Biodistribution studies in athymic nude mice bearing s.c. neuroblastoma (IMR-6, NMB-7) and melanoma (M21) xenografts showed that 15-20% of the injected dose per gram accumulated in the tumor at 24 hours after injection, and only 5-10% of the injected dose accumulated in the liver, a lower value than typically seen with other chelators. Uptake by a GD2-negative tumor xenograft was significantly lower (<5% injected dose per gram). MicroPET imaging confirmed significant uptake of the tracer in GD-2-positive tumors, with minimal uptake in GD-2-negative tumors and nontarget tissues such as liver. The ⁶⁴Cu-SarAr-mAb system described here is potentially applicable to ⁶⁴Cu-PET imaging with a broad range of antibody or peptide-based imaging agents.