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 F-18-FDG. Copper-64 (Cu-64) 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 tumorspecific Cu-64 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 Cu-64 resulted in >95% of the Cu-64 being chelated by the immunoconjugate. Specific activities of at least 10 mu Ci/mu g (1 Ci = 37 GBq) were routinely achieved, and no additional purification was required after Cu-64 labeling. Solid-phase radioimmunoassays 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 GID-2-positive tumors, with minimal uptake in GD-2-negative tumors and nontarget tissues such as liver. The Cu-64-SarAr-mAb system described here is potentially applicable to Cu-64-PET imaging with a broad range of antibody or peptide-based imaging agents.