Evaluation of HER2-targeted Therapy for Gastric Cancer and Establishment and Verification of Monitoring System

1.Background China is a high incidence area of digestive tract cancer. Human epidermal growth factor receptor-2 (HER2) is a transmembrane receptor like protein that binds to cell membrane. It has tyrosine kinase activity and participates in cell growth, activation and proliferation by activating downstream signaling pathways. The positive expression of HER2 in gastric cancer was defined as pathological immunohistochemistry HER2 (3 +) or HER2 (2 +) with fish detection of gene amplification. The positive rate of HER2 in Chinese patients with gastric cancer is about 12-13% . The effective rate of chemotherapy combined with anti-HER2 targeted therapy for HER2 positive advanced gastric cancer was 47%, and the median survival time was extended to 13.8 months . However, HER2 heterogeneity directly affects the outcome of anti HER2 treatment . Therefore, compared with biopsy pathology, it is urgent to explore noninvasive, systemic and repeatable methods to evaluate HER2 status of systemic lesions. PET is the abbreviation of positron emission tomography. However, the application of 18F-FDG in tumor has some limitations, such as false positive or false negative, which can not effectively distinguish tumor and inflammation. The uptake of 18F-FDG by tumor is easily affected by pathological or physiological factors, such as local blood flow, oxygen content and necrosis . In recent years, researchers have devoted themselves to the development of new specific tumor imaging agents, among which human epidermal growth factor receptor 2 (HER2) imaging agent is a research hotspot.

Affibody, also known as "artificial antibody", is a type of engineered scaffold proteins, based on nonimmune protein affinity ligands. Compared with the antibody, the small size affibody has some unique advantages: 1) It has small molecular weight and simple structure, which could be synthesized and modified by peptide synthesizer. 2) It has high selectivity and affinity, but low nonspecific binding rate. 3) It has strong tissue permeability. Affibody quickly accumulates in the tumors and rapidly excretes from normal organs (blood, muscle etc.). And images with high contrast could be obtained only at few hours after administration. In recent years, the application of HER2 targeted affibody (ZHER2:342) and its analogues (ZHER2:2891 and ZHER2:2395) in tumor molecular imaging is beginning to attract people's attention. Gabriela Kramer-Marek compared the imaging performance of 18F-FBEM-cys-ZHER2:342 and 18F-FDG in nude mice with MDA-MB-231 tumor and lung metastasis. 18F-FBEM-cys-ZHER2:342 successfully targeted HER2 positive lung metastases, while 18F-FDG not detected metastasis for the interfered by peripheral inflammation and normal tissues.

68Ga is a new type of positron nuclide, which is produced by 68Ge-68Ga generator. The mother nuclide 68Ge is a long-lived radioisotope with a half-life of 271 days, which can be stored in the generator for about one year.

68Ga is easy to obtain, and the high purity 68Ga can be refresh after 4 hours, which is conducive to multi imaging on the same day. The physical half-life of 68Ga is 68 min, and 89% of the decay process is positron decay, Emax is 1.92 MeV, and the remaining 11% is electron capture. Compared with cyclotron, 68Ge-68Ga generator is in small size, does not need special site with radiation shielding structure, and has lower requirements for equipment operation and maintenance personnel. Compared with 18F, 68Ga has the advantages of low cost, simple preparation and fast labeling. Therefore, 68Ga labeled HER2 affinity probe is of great significance for in vivo monitoring of HER2 status. The 68Ga labeled affinity probe 68Ga-ABY-002 (68Ga-DOTA-ZHER2:342-pep) was prepared by Uppsala University in Sweden. The result showed that 68Ga-ABY-002 successfully detected lung metastases from breast cancer. However, there was no good detection effect of liver metastases for high uptake of the normal abdominal tissue, especially the liver background.

2 .The Work and Conditions of Our Center 68Ga-HER2-affibody-BCH was prepared by the department of nuclear medicine of Peking University Cancer Hospital. The time is only 30 minutes and the yield is 80% ± 5%. The radiochemical purity was more than 95%. Micro-PET imaging showed high uptake in tumors with ovarian cancer and breast cancer. In view of the convenient synthesis and good imaging performance, 68Ga-HER2-affibody-BCH is expected to be widely used in clinical diagnosis, differentiation and efficacy detection of HER2 positive tumors.