Preparation of HER2 radioligand ⁹⁹Tc^m-ABH2 and its in vivo imaging in breast cancer-xenografted mice

Objective: To prepare ⁹⁹Tcm-human epidermal growth factor receptor 2 (HER2) affibody (ABH2) and explore its feasibility as an imaging agent in HER2-positive breast cancer. Methods: Sodium glucoheptonate and SnCl₂·2H₂0 were used to label ABH2 with ⁹⁹Tc^m. The labeling yield and radiochemical purity of ⁹⁹Tc^m-ABH2 were determined. The stability of ⁹⁹Tc^m-ABH2 was tested in PBS and serum. The equilibrium disassociation constant (K_d) of ⁹⁹Tc^m-ABH2 was measured with MBA-MD-361 breast cancer cells. SPECT/CT imaging was carried out at 1.0 h and 4.5 h after injection of 37 MBq ⁹⁹Tc^m-ABH2 in nude mice (n = 4) xenografted with MBA-MD-361 breast cancer. The T/NT (liver, brain, lung, heart, bone and muscle) ratios were deduced from SPECT/CT acquired data. For the blocking experiment, 200 μg ABH2 was injected intravenously before ⁹⁹Tc^m-ABH2 injection. SPECT/CT imaging was performed in the same way. The T/NT ratios were compared between non-blocked and blocked groups by one-way analysis of variance. Results: The labeling yield of ⁹⁹Tc^m-ABH2 was above 99%. ⁹⁹Tc^m-ABH2 was substantially stable in PBS and serum; the radiochemical purity was (95.0 ± 1.0)% after incubation with serum at 37 ℃ for 6.0 h. The K_d of ⁹⁹Tc^m-ABH2 was 1.7 nmol/L. The radioactive uptake in cancer was visualized at 1.0 h and 4.5 h after injection of ⁹⁹Tc^m-ABH2 in HER2 positive MDA-MB-361 breast cancer. ⁹⁹Tc^m-ABH2 was cleared out mainly through urinary system. The ratios of tumor to liver, lung, brain, heart, muscle and bone were 1.81 ± 0.60, 8.95 ± 1.13, 20.08 ± 6.12, 7.61 ± 0.56, 10.62 ± 1.78, 11.42 ± 2.07, respectively at 4.5 h after ⁹⁹Tc^m-ABH2 injection. After ABH2 blocking, the corresponding T/NT ratios were 0.60 ± 0.23, 3.05 ± 1.38, 5.24 ± 2.17, 2.42 ± 1.02, 8.16 ± 2.66, 2.76 ± 0.48 (F = 29.38, P < 0.05) respectively. Conclusion: ⁹⁹Tc^m-ABH2 could be synthesized with high purity, and this new agent could be used to image HER2-positive breast cancer specifically. (authors)