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

Abstract

The advancement of positron emission tomography (PET) depends on the development of new radiotracers that will complement 18 F-FDG. Copper-64 ( 64 Cu) 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 64 Cu 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 64 Cu resulted in >95% of the 64 Cu being chelated by the immunoconjugate. Specific activities of at least 10 Ci/ g (1 Ci ‫؍‬ 37 GBq) were routinely achieved, and no additional purification was required after 64 Cu 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 GD-2-positive tumors, with minimal uptake in GD-2-negative tumors and nontarget tissues such as liver. The 64 Cu-SarAr-mAb system described here is potentially applicable to 64 Cu-PET imaging with a broad range of antibody or peptide-based imaging agents.

Comparison of 64Cu-Complexing Bifunctional Chelators for Radioimmunoconjugation: Labeling Efficiency, Specific Activity, and in Vitro/in Vivo Stability

“…Encapsulating hexaamine sarcophagine ligands, first prepared by Sargeson and coworkers [47][48][49][50][51] , form extraordinary stable complexes with copper(II) and benefit from rapid complexation rates 52,53 . Methods have been developed to introduce reactive functional groups to the ligand framework [54][55][56] , to allow the ligands to be conjugated to peptides and antibodies with a goal of synthesizing copper radiopharmaceuticals that benefit from the special properties of sarcophagine ligands 21,45,46,[57][58][59][60][61] . Each BFC was conjugated to the anti-CD20 antibody, rituximab, as a model antibody with clinical utility.…”

High-Contrast PET of Melanoma Using 18F-MEL050, a Selective Probe for Melanin with Predominantly Renal Clearance

“…In recent years, several other radiolabeled imaging probes have been evaluated for melanoma imaging, including methylene blue dye (10), monoclonal antibodies against melanoma-associated antigens (11,12), iodoamphetamine (13), a-melanocyte-stimulating hormone ana-logs (14,15), and benzamide (BZA)-based compounds (16,17). To date, iodinated BZA analogs have been among the most promising of the newer melanoma radiotracers for both diagnosis and therapeutic applications.…”

Molecular Imaging of Breast Cancer Tissue via Site-Directed Radiopharmaceuticals

“…Moreover, 67 Cu-containing radiopharmaceuticals possess therapeutic potential and provide in vivo pharmacokinetic profiles nearly identical to 64 Cu-containing drugs (Blower et al, 1996). 67 Cu has ideal physical characteristics that are well-suited for radionuclide therapy (β -, 100%, 121 keV, t 1/2 = 62 h) (Voss, et al, 2007). As such, 64 Cu can be used as a dose-determinant indicator in 67 Cu-based therapeutic regimens.…”

“…Since the liver is the anatomical location of copper metabolism, the stability of the copper chelate in a 64 Cu-containing radiopharmaceutical under physiological conditions is very important (Wadas et al, 2007). Increasing the stability of copper complexes in vivo has been a major focus in the field of nuclear medicine for some time (Di Bartolo et al, 2001 andGasser et al, 2008;Kukis et al, 1993;Voss et al, 2007;Pippin et al, 1991;Geraldes et al, 2000;Wieghardt et al, 1982;van der Merwe et al, 1985;Soluri et al, 2003;Maina et al, 2005;Monstein et al, 2006;Sun & Chen, 2007) as the development of stable copper complexes will allow for a variety of copper-based diagnostic and therapeutic strategies. Recent work in the field of copper radiochemistry has produced very promising results.…”