Alternative chelator for ⁸⁹Zr radiopharmaceuticals: radiolabeling and evaluation of 3,4,3-(LI-1,2-HOPO)

Abstract

Zirconium-89 is an effective radionuclide for antibody-based positron emission tomography (PET) imaging because its physical half-life (78.41 h) matches the biological half-life of IgG antibodies. Desferrioxamine (DFO) is currently the preferred chelator for (89)Zr(4+); however, accumulation of (89)Zr in the bones of mice suggests that (89)Zr(4+) is released from DFO in vivo. An improved chelator for (89)Zr(4+) could eliminate the release of osteophilic (89)Zr(4+) and lead to a safer PET tracer with reduced background radiation dose. Herein, we present an octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO) as a potentially superior alternative to DFO. The HOPO ligand formed a 1:1 Zr-HOPO complex that was evaluated experimentally and theoretically. The stability of (89)Zr-HOPO matched or surpassed that of (89)Zr-DFO in every experiment. In healthy mice, (89)Zr-HOPO cleared the body rapidly with no signs of demetalation. Ultimately, HOPO has the potential to replace DFO as the chelator of choice for (89)Zr-based PET imaging agents.

Figure 1

Structures of the currently used chelator for ⁸⁹Zr, DFO, and the newly investigated alternative chelator, HOPO.

Figure 2

HPLC chromatogram of the co-injection of radioactive ⁸⁹Zr-HOPO and nonradioactive Zr-HOPO. The ∼30 s separation between the two peaks is due to the sequential setup of the UV and radioactive detectors.

Figure 3

General chemical structures and optimized DFT structures for (I) the Zr-HOPO complex; (II) a complex of an alternative HOPO-based ligand [3,3,3-(LI-1,2-HOPO)] with Zr; (III) a previously studied positively charged Zr-DFO complex; (IIIa) a new, more stable conformation of the same charged Zr-DFO complex; and (IV) a neutral, uncharged Zr-DFO complex.

Figure 4

Radio-TLC profiles of the ⁸⁹Zr-HOPO radiolabeling reaction over time at different concentrations of ligand. Two peaks are observed, with the first peak (the kinetic product) converting to the second peak (the thermodynamic product) over time. The initial area ratio and separation of the two peaks are dependent upon the concentration of the ligand.

Figure 5

Coronal PET images of ⁸⁹Zr-HOPO. Healthy mice were administered ⁸⁹Zr-HOPO (260 μCi [9.6 MBq] in 0.9% sterile saline) via tail vein injection and imaged between 10 min and 24 h after injection. The gall bladder (a), gut (b), and bladder (c) can be visualized. The ⁸⁹Zr-HOPO complex undergoes rapid renal clearance followed by slower hepatobiliary clearance. No uptake of ⁸⁹Zr in the bone is observed.

Figure 6

Biodistribution of ⁸⁹Zr-HOPO and ⁸⁹Zr-DFO in select organs. Healthy, athymic nude mice were injected with either ⁸⁹Zr-HOPO or ⁸⁹Zr-DFO (24–35 μCi [0.89–1.29 MBq] in 0.9% sterile saline) via the tail vein, sacrificed at specified time points, and necropsied. The concentration of radioactivity in the chosen organs is expressed as %ID/g and presented as an average value from four animals ± standard deviation. Bl = blood, GB = gall bladder, L = liver, LI = large intestines, SI = small intestines, K = kidney, Bo = bone. ∗ indicates P values of <0.05.

Figure 7

Blood clearance of ⁸⁹Zr-HOPO and ⁸⁹Zr-DFO in healthy, athymic nude mice (n = 4) over time. Inset shows a zoomed graph for further detail.