p-SCN-Bn-HOPO: A Superior Bifunctional Chelator for ⁸⁹Zr ImmunoPET

Abstract

Zirconium-89 has an ideal half-life for use in antibody-based PET imaging; however, when used with the chelator DFO, there is an accumulation of radioactivity in the bone, suggesting that the 89Zr4+ cation is being released in vivo. Therefore, a more robust chelator for 89Zr could reduce the in vivo release and the dose to nontarget tissues. Evaluation of the ligand 3,4,3-(LI-1,2-HOPO) demonstrated efficient binding of 89Zr4+ and high stability; therefore, we developed a bifunctional derivative, p-SCN-Bn-HOPO, for conjugation to an antibody. A Zr-HOPO crystal structure was obtained showing that the Zr is fully coordinated by the octadentate HOPO ligand, as expected, forming a stable complex. p-SCN-Bn-HOPO was synthesized through a novel pathway. Both p-SCN-Bn-HOPO and p-SCN-Bn-DFO were conjugated to trastuzumab and radiolabeled with 89Zr. Both complexes labeled efficiently and achieved specific activities of approximately 2 mCi/mg. PET imaging studies in nude mice with BT474 tumors (n = 4) showed good tumor uptake for both compounds, but with a marked decrease in bone uptake for the 89Zr-HOPO-trastuzumab images. Biodistribution data confirmed the lower bone activity, measuring 17.0%ID/g in the bone at 336 h for 89Zr-DFO-trastuzumab while 89Zr-HOPO-trastuzumab only had 2.4%ID/g. We successfully synthesized p-SCN-Bn-HOPO, a bifunctional derivative of 3,4,3-(LI-1,2-HOPO) as a potential chelator for 89Zr. In vivo studies demonstrate the successful use of 89Zr-HOPO-trastuzumab to image BT474 breast cancer with low background, good tumor to organ contrast, and, importantly, very low bone uptake. The reduced bone uptake seen with 89Zr-HOPO-trastuzumab suggests superior stability of the 89Zr-HOPO complex.

Detection of HER2-Positive Metastases in Patients with HER2-Negative Primary Breast Cancer Using 89Zr-Trastuzumab PET/CT

“…When loosely chelated, 89 Zr is known to be a bone seeker (17). Indeed, evidence suggests that when 89 Zr is chelated to antibodies with desferrioxamine, radioactivity accumulates in the bone (18). Thus, non-specific binding of potentially free 89 Zr at sites of osseous turnover associated with bone metastases could be the reason for the false positive osseous foci in this study.…”

89Zr-Trastuzumab PET/CT for Detection of Human Epidermal Growth Factor Receptor 2–Positive Metastases in Patients With Human Epidermal Growth Factor Receptor 2–Negative Primary Breast Cancer

“…When loosely chelated, 89 Zr is known to be a bone seeker [11]. Indeed, evidence suggests that when 89 Zr is chelated to antibodies with desferrioxamine, radioactivity accumulates in the bone [12]. Thus, non-specific binding of potentially free 89 Zr at sites of osseous turnover associated with bone metastases could result in false-positive osseous foci.…”

Comparison of the octadentate bifunctional chelator DFO*-pPhe-NCS and the clinically used hexadentate bifunctional chelator DFO-pPhe-NCS for 89Zr-immuno-PET

“…Although the resulting slightly impaired stability of 89 Zr-DFO-mAb does not really hamper image quality, it is clear from preclinical studies that in time, some 89 Zr 4+ becomes released from the conjugate and accumulates in the bones [13–16]. For example, Perk et al compared the biodistribution of the conjugates 89 Zr-DFO-cetuximab, 88 Y-DOTA-cetuximab and 177 Lu-DOTA-cetuximab in tumor-bearing nude mice [14].…”

“…Some chelators contain hydroxamate moieties, like the linear and macrocyclic tetrahydroxamate chelators reported by Guerard et al, and the trihydroxamate chelators reported by Boros et al and Zhai et al [17–20]. Other examples do not contain hydroxamate moieties like p -SCN-Bn-H6phospha, 3,4,3-(LI-1,2-HOPO), and 3-hydroxypyridin-2-one (2,3-HOPO) [16, 21–23]. However, none of these developments have led to a bifunctional chelator that outperforms 89 Zr-DFO-mAb complexes on all abovementioned aspects of an optimal chelating agent.…”