A new PET imaging agent for monitoring HER2 status using Trastuzumab antibody conjugated with a novel ⁸⁹ Zr-chelator.

Objectives: HER2 overexpression on the surface of breast cancer cells correlate with a poor prognosis. The development of molecular imaging tracers targeting HER2 and their application in the clinic to improve characterization of breast cancer has been a long sought after objective. Hence, we designed a new PET imaging agent for whole-body PET imaging for monitoring HER2 status using Trastuzumab (Tmab) antibody conjugated with a novel bifunctional chelator with 4 N-hydroxy-N-methyl succinamide pendant arms (HMSA) for complexing ⁸⁹Zr. We investigated the potential of ⁸⁹Zr-HMSA-Tmab to visualize human breast tumors by small animal PET imaging. A comparative study with ⁸⁹Zr-labeled Tmab conjugated with the current standard desferrioxamine (DFO) was performed.

Methods: Commercially available p-isothiocyanatobenzyl-DFO and HMSA, synthesized in house, were conjugated to Tmab surface lysines using standard procedures. Labeling was achieved using neutralized ⁸⁹Zr-oxalate (pH 7.0) mixed with 0.5 M HEPES (pH 7.4) and DFO- or HMSA-Tmab in sodium acetate (pH 5.5) was added and incubated at room temperature for 1 h with a final pH at 7.0. Purification of the radiolabeled conjugates was performed by centrifugal filtration. The chelator:antibody ratio was determined by isotopic dilution. In vitro and ex vivo stabilities were evaluated using ITLC, SDS-PAGE, and a turbidity assay. In cellulo characterization was accomplished by incubating JIMT-1 (low HER2+ expression, Tmab-resistant) breast cancer cell line with the ⁸⁹Zr-Tmab conjugates. PET imaging was performed in NOD/SCID mice (n≥8 per group) implanted with JIMT-1 orthotopic tumors. Mice were injected with 3mg/kg (2.0 ± 1.0 MBq) of the ⁸⁹Zr-Tmab conjugates when tumors reached a volume of 30 mm³. Imaging was performed on day 72 h and 144 h post injection.

Results: ⁸⁹Zr-HMSA-Tmab had 5-fold increase of molar activity and superior solubility (low aggregation) compared to ⁸⁹Zr-DFO-Tmab. Radiochemical purities (95%) and chelator:antibody ratios were similar for both conjugates. In addition, ⁸⁹Zr-HMSA-Tmab had increased stability in vitro and ex vivo. In cellulo characterization showed an increase in ⁸⁹Zr-HMSA-Tmab internalization and cellular accumulation. PET imaging and biodistribution demonstrated that both ⁸⁹Zr-HMSA-Tmab and ⁸⁹Zr-DFO-Tmab accumulate similarly in JIMT-1 orthotopic tumors and had similar biodistribution profile. Conclusion: Stability and solubility are significant components of a clinically successful radiolabeled conjugate. ⁸⁹Zr-HMSA-Tmab had similar tumor targeting properties compared to ⁸⁹Zr-DFO-Tmab. Therefore, HMSA is a novel chelator that has important advantages over DFO. Our data suggest that HMSA could be used advantageously to develop other antibody conjugates for PET imaging. Research Support: Cancer Research Society and Sherbrooke Molecular Imaging Centre