Human epidermal growth factor receptor 2 (abbreviated as Her2, ERBB2, HER2/neu or c-erbB2) is a protein encoded by ERBB2 gene. In normal cells, Her2 has a very low expression level; but Her2 is highly expressed during the period of embryonic development, and is very important in the regulation of cell proliferation, differentiation, development, adhesion and migration (Gutierrez, C. and R. Schiff, HER2: biology, detection, and clinical implications. Arch Pathol Lab Med, 2011. 135(1): p. 55-62.).
Her2 belongs to the family of human epidermal growth factor receptor, and this family consists of 4 members: Her1 (EGFR), Her2, Her3 and Her4. Her2 has no specific ligand, and the activation of its downstream pathway depends on formation of homologous or heterologous dimers (Gutierrez et al, Arch Pathol Lab Med, 2011. 135(1): p. 55-62.). Human epidermal growth factors are all locate on cell surface, and have a similar structure: one extracellular domain (ECD) binding to a ligand, one single transmembrane α-helix transmembrane domain and one intracellular region that consists of an intracellular membrane-proximal domain, a tyrosine kinase catalytic domain and a tyrosine-rich C-terminal tail domain playing a regulatory role (Eccles, Int J Dev Biol, 2011. 55(7-9): p. 685-96). The extracellular domain (ECD) of human epidermal growth factor can further be separated into 4 subdomains, i.e., regions I, II, III and IV, in which regions II and IV are cysteine-rich domains and participate in dimerization and activation of the receptor.
Overexpression of Her2 may results in disorders of cell normal functions, and usually closely relates to tumor genesis and development. The homologous or heterologous polymerization of Her2 may lead to phosphorylation of tyrosine residues of the receptor, and initiate many signal pathways and causes cell proliferation and tumor genesis. As a biomarker for prognosis and prediction, amplification or overexpression of Her2 gene occurs in about 15-30% breast cancer and 10-30% gastric/esophageal cancer. Overexpression of Her2 may also be observed in other tumors such as ovary, endometrium, bladder, lung, colon, and head-neck tumors.
In breast cancer, Her2 is commonly recognized as a predictive factor and a therapeutic target. Since Her2 has no specific ligand, its antibodies usually inhibit tumor cells by blocking dimerization and activation of the receptor and mediating killing effect of immune system. At present, Trastuzumab and Pertuzumab are the main Her2-targeted therapeutic antibodies commercially available.
In 1998, FDA approved a Her2-targeting humanization monoclonal antibody, trastuzumab (also called as HERCEPTIN®; humanization degree 95%) of Genentech Inc. This antibody recognizes Her2 extracellular domain IV juxtamembrane epitope, and its antigen affinity constant can be up to 0.1 nmol/L. Trastuzumab recognizes the epitope consisting of the 3 loops (557-561, 570-573 and 593-603) at the C-terminal of section IV. Because the epitope may be close to or directly interact with the binding domain of its dimerization partner, trastuzumab's binding to the epitope may induce steric hindrance inhibiting the dimerization process. In addition, trastuzumab's binding may also protect the extracellular domain of the Her2 receptor from the attack by proteinase for hydrolysis.
The mechanisms of action of trastuzumab may include: immune-induced bioactivities (antibody dependent cell-mediated cytotoxicity (ADCC) and Natural killer cell activity), inducing the internalization of Her2 receptor, inhibiting DNA repair, breaking PI3K pathway, activating p27kip1 induced G1 cycle stoppage, stimulating cancer cell apoptosis and inhibiting the activation of intracellular p95 domain off of the extracellular domain of the receptor[4,5]. Among them, there have been reports about trastuzumab induced immuno-mediated therapeutic bioactivities. In particular, ADCC plays an important role, as it was shown in a BT474 xenograft mouse model, when the Fc receptor was knocked out, the inhibition rate of cancer growth was reduced from 96% to 29% (Nat Med, 2000, 6:443-6). Kohrt et al (J Clin Invest, 2012. 122(3): 1066-75) report that stimulation of natural killer cells with a CD137-specific antibody enhances trastuzumab efficacy in xenotransplant models of breast cancer.
Trastuzumab is currently used as a first-line drug for treatment of breast cancer, and is effective in treatment of metastatic breast cancer with Her2 overexpression, and its objective reflection rate of single drug first-line treatment is 30-50%; but it has unsatisfied effect in treating metastatic breast cancer with lower Her2 expression, and resistance has been developed in a number of patients for whom the antibody is initially effective within 1 year. This may be related to shielding of antigen epitopes or abnormal activation of receptor signaling pathway caused by changes of some gene expressions in tumor cells. In addition, Her2 together with other members (Her1, Her3 and Her4) of the family can form ligand-dependent or ligand-independent heterologous dimers, thereby activating downstream pathways, and then resulting proliferation of tumor cells, while trastuzumab cannot inhibit formation of heterologous dimers, so this may be one of reasons for the development of resistance.
Pertuzumab (PERJETA®) was approved by FDA for marketing in USA in 2012, and has certain curative effects on advanced prostate cancer, non-small cell lung cancer, ovarian cancer and breast cancer, but its curative effects still depend on Her2 expression level.
Pertuzumab recognizes key sites for heterologous dimerization of Her2 extracellular domain II, and the epitope recognized thereby are located in segment 245-311 of II subregion center, and key residues are H245, V286, S288, L295, H296 and K311. In which, L295, H296 are key sites for mediating heterologous dimerization of Her2 and Her3, and L295A/H296A double mutation can completely block heterologous dimerization of Her2/Her3 (Franklin, M. C., et al., Insights into ErbB signaling from the structure of the ErbB2-pertuzumab complex. Cancer Cell, 2004. 5(4): p. 317-28.). Hence, Pertuzumab can be used for effectively inhibiting the formation of Her2/Her3 heterologous dimer, but does not show obvious inhibition effects on the formation of EGFR/Her2 heterologous dimer.
At present, there is a need for developing new anti-HER2 antibodies.