Imaging cancer using PET — the effect of the bifunctional chelator on the biodistribution of a 64Cu-labeled antibody

Abstract

Introduction Use of copper radioisotopes in antibody radiolabeling is challenged by reported loss of the radionuclide from the bifunctional chelator used to label the protein. The objective of this study was to investigate the relationship between the thermodynamic stability of the 64Cu-complexes of five commonly used bifunctional chelators (BFCs) and the biodistribution of an antibody labeled with 64Cu using these chelators in tumor-bearing mice. Methods The chelators (p-NH2-Bn-NOTA, BAT-6, p-NH2-Bn-DOTA, DOTA, and SarAr) were conjugated to the anti-GD2 antibody ch14.18, and the conjugated antibody was labeled with 64Cu and injected into mice bearing subcutaneous human melanoma tumors (M21) (n = 3-5 for each study). Biodistribution data were obtained from PET images acquired at 1, 24 and 48 hours post-injection, and at 48 hours post-injection a full ex vivo biodistribution study was carried out. Results The biodistribution, including tumor targeting, was similar for all the radioimmunoconjugates. At 48 h post-injection, the only statistically significant differences in radionuclide uptake (p < 0.05) were between blood, liver, spleen and kidney. For example, liver uptake of [64Cu]ch14.18-p-NH2-Bn-NOTA was 4.74 ± 0.77 per cent of the injected dose per gram of tissue (%ID/g), and for [64Cu]ch14.18-SarAr was 8.06 ± 0.77 %ID/g. Differences in tumor targeting correlated with variations in tumor size rather than which BFC was used. Conclusions The results of this study indicate that differences in the thermodynamic stability of these chelator-Cu(II) complexes were not associated with significant differences in uptake of the tracer by the tumor. However, there were significant differences in tracer concentration in other tissues, including those involved in clearance of the radioimmunoconjugate (e.g., liver and spleen).

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

“…In contrast, we found little difference between the macrocyclic immunoconjugates with all showing high labeling efficiency and >95% stability at 24 h and >94.9% at 48 h. The labeled conjugates could be used without additional purification. We speculate that the comparatively high labeling efficiency and stability we find compared to others 21,72,73 may be related to our use of fraction 9 from the 64 Cu purification.…”

“…Numerous chelators have been reported for complexing copper [5][6][7][8][9][10][11][12][13][14][15][16] and several of these have been functionalized to allow attachment to antibodies [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and are now commercially available. The choice of antibody/BFC combination will affect efficacy as an imaging or therapy agent.…”

“…The choice of BFC can influence the biodistribution of radioactivity in vivo [18][19][20][21][22][23][24][35][36][37][38][39][40][41] , with biodistribution related to the in vivo stability of the 64 Cu 2+ -BFC complex. Generally high liver activity is assumed to be indicative of instability of the 64 Cu 2+ -BFC complex in vivo.…”

“…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.…”

Radio Pet Therapy of Healthy Wistar Rats with &lt;sup&gt;64&lt;/sup&gt;CuCl&lt;sub&gt;2&lt;/sub&gt; Labeled to &lt;sup&gt;64&lt;/sup&gt;Cu(AcAc)&lt;sub&gt;2&lt;/sub&gt; Supported in Functionalized TiO&lt;sub&gt;2&lt;/sub&gt; Nanoparticles

“…The system showed very good performance with no interference from the magnetic field and reported energy resolution of ~15%. All crystal pixels were clearly resolved, and success simultaneous mouse imagine was performed [9] [10] [11]. A series of new hexadentate and pentadentate chelators were designed and synthetized as chelators 64 Cu.…”