However, what is gratifying is that from 2007 to 2016, the mortality rate of breast cancer has been decreasing at a rate of 1.8% per year. This is mainly due to the advancement of medical technology. The current treatment of breast cancer mainly includes surgery, radiotherapy, chemotherapy, hormone therapy and targeted therapy. Among them, targeted therapy not only provides patients with a variety of options, but also greatly improves the survival of patients.
1. Patterns of antibody therapy for breast cancer
The antibodies currently approved or in clinical use for the treatment of breast cancer can be divided into three categories:
1) Naked antibodies targeting tumor surface antigens
This type of antibody mainly blocks tumor cell growth related signal pathways by binding to HER2 or other antigens on the tumor surface, thereby inhibiting tumor growth, or killing tumor cells through antibody-mediated ADCC.
2) Antibody Drug Conjugates
This type of antibody also targets the antigen on the surface of tumor cells, but the antibody is coupled with small molecule toxicants or radiopharmaceuticals. After the antibody binds to the antigen on the tumor cell surface, it enters the tumor cell through endocytosis, and release small molecule poison or radioactive drugs through lysosome or enzyme, so as to kill the tumor cells.
3) Immune checkpoint inhibitory antibodies represented by PD-1 and PD-L1 antibodies
These antibodies inhibit the inhibitory PD-1 or PD-L1 of the immune system, and then stimulate the immune response of the body's immune system, and ultimately kill or inhibit tumor cells through the immune system.
2. Monoclonal Antibodies & Bispecific Antibodies in breast cancer therapy
HER2 is expressed in 15-20% of breast cancer patients, so this target is an ideal target for breast cancer treatment. Currently, two monoclonal antibodies targeting HER2 (trastuzumab and pertuzumab) have been approved for the treatment of breast cancer, targeting the fourth and second domains of HER2, respectively.
At the same time, there are currently several antibodies against HER2 under clinical evaluation, such as Margetuximab, MCLA128 and ZW25.
Margetuximab is a monoclonal antibody targeting the fourth domain of HER2. Unlike Trastuzumab, this antibody modifies Fc (F243L/R292P/Y300L/L235V/P396L) to enhance antibody binding to CD16A, thereby increasing the ADCC effect mediated by the antibody, and at the same time the antibody reduces the affinity for CD32B, thus enhancing the body's immune response. The drug has been approved by the FDA in December 2020 for the treatment of adult metastatic HER2-positive breast cancer in combination with chemotherapy.
MCLA 128 is a bispecific antibody targeting HER2 and HER3. After the antibody binds to HER3, the antibody can block the binding of NRG1 protein to HER3. HER2/HER3 dimerization can be blocked even at high NRG1 protein levels. Therefore, it can block the activation of the downstream signals of HER2/HER3, thereby inhibiting the growth of tumor cells. In addition, the antibody modified Fc to enhance the ADCC effect mediated by the antibody, and further enhance the killing of tumor cells. The antibody is currently in 1/2 clinical trials.
ZW25 is a bispecific antibody developed based on Zymeworks' AzymetricTM platform. It can simultaneously bind to two non-overlapping epitopes (second domain and fourth domain) of HER2, which is called dual complementary binding. This unique design produces multiple mechanisms of action, including dual HER2 signal blockade, increased endocytosis of antibodies to increase the binding and removal of HER2 protein from the cell surface, and effective effector functions. The antibody is currently in clinical phase 2 and was introduced by BeiGene in China.
Antibodies that target other antigens
In addition to the HER2 target, there are other targets currently under development for the treatment of breast cancer, such as VEGF, PRLR (prolactin receptor), Pobo1 receptor and so on.
3. ADC for breast cancer treatment
Compared with naked antibodies, the ADC field has more targets for the treatment of breast cancer.
In addition to HER2, there are glycoprotein non-metastatic, trophoblast cell surface antigen, CA6 (sialylated MUC1), LIV-1 (a multiple transmembrane protein), and PTK7 (Protein tyrosine kinase 7), LAMP-1 (lysosomal surface-associated protein), P-cadherin (a cell membrane surface glycoprotein) and ephrinA4 (a member of the PTKs family).
Currently, two ADC drugs targeting HER2 have been approved by FDA for marketing -- Enhertu and Kadcyla.
Trastuzumab Emtansine (Kadcyla)
Trastuzumab, the small molecule cytotoxin DM1 is a microtubule inhibitor. TDM1 targets the fourth domain of the HER2 receptor, leading to intracellular release of cytotoxic catabolites containing DM1 through receptor-mediated internalization and subsequent lysosomal degradation. Binding of DM1 to tubulin disrupts the intracellular microtubule network, leading to cell cycle arrest and apoptotic cell death.
In 2013, the FDA approved Trastuzumab emtansine (Kadcyla) for the treatment of HER2-positive metastatic breast cancer patients who had previously received trastuzumab and taxane alone or in combination. In 2019, the FDA approved Trastuzumab emtansine (Kadcyla) as an adjunctive therapy for HER2-positive early-stage breast cancer patients with residual invasive disease after neoadjuvant therapy with taxane and trastuzumab. Similar to Trastuzumab emtansine (Kadcyla), Medi-4276, XMT-1522, and ARX788 are all ADC drugs targeting HER2, which carries microtubule inhibitors, and are currently under clinical evaluation.
Trastuzumab Deruxtecan (Enhertu)
Trastuzumab Deruxtecan (Enhertu) is constructed from the humanized HER2 monoclonal antibody trastuzumab via a peptide-based linker coupled to a new topoisomerase I inhibitor camptothecin derivative (DX-8951 derivative DXd) . It binds to HER2 on the surface of tumor cells through trastuzumab, enters tumor cells through HER2-mediated endocytosis, and is phagocytosed by intracellular lysosomes. Subsequently, the lysosomal enzyme cleaves the polypeptide linker, releases the loaded DXd, inhibits the activity of topoisomerase I, and then induces DNA damage and cell apoptosis, and exerts an effective anti-tumor effect.
Sacituzumab govitecan is an ADC drug formed by coupling a humanized IgG1 antibody targeting the TROP-2 antigen and the metabolically active product SN-38 of the chemotherapeutic drug irinotecan (a topoisomerase I inhibitor). It was approved by the FDA in April 2020 for the treatment of metastatic triple-negative breast cancer (TNBC) in patients who have received at least two therapies. It is the first ADC drug targeting TROP-2 that approved by FDA for the treatment of patients with metastatic triple-negative breast cancer.
TROP-2 is a cell surface glycoprotein encoded and expressed by the TACSTD2 gene. Its expression in normal tissues is very low, and it is overexpressed in a variety of malignant tumors. It mainly promotes tumor cell growth, proliferation and metastasis by regulating calcium ion signal pathway, cyclin expression and reducing fibronectin adhesion. TROP2 can also interact with β-catenin in the Wnt signaling cascade, and play a role in the transcription of nuclear oncogenes and cell proliferation.
SAR566658 is an ADC drug targeting CA6 (tumor-associated glycosylated MUC-1). It is composed of a huDS6 antibody that binds to CA6 and DM4 (Maytansine-derived microtubule inhibitor). In a phase I clinical trial involving 114 patients, 60% of patients in the 190 mg/m2 and 90 mg/m2 dose groups on the first and eighth days had tumor regression. In the 150 mg/m2 and 120 mg/m2 dose groups in the biweekly administration group, 35% of patients experienced tumor regression.
In addition to the ADC drugs introduced above, there are currently many ADC drugs targeting different targets of breast cancer in clinical trials, which will not be described in detail here. See the table below for specific information.
4. Advances in the treatment of breast cancer with PD-1 and PD-L1 antibodies
PD-1 (CD279) is a member of the CD28 family, which mainly plays an immunosuppressive role. It also plays an important role in the immune escape of tumor cells.
PD-1 is expressed on a variety of immune cells, including NKT cells, B cells and DC cells. The interaction of PD-1 and its ligand PD-L1 (CD274/B7-H1) can inhibit the proliferation and survival of T lymphocytes, and can promote the programmed death of T cells in the tumor microenvironment. Factors inhibiting PD-1 or PD-L1 can effectively stimulate the body's immune response and inhibit or kill tumor cells through the immune system.
At present, a number of PD-1 and PD-L1 related antibodies have been approved for marketing, and they have also achieved good efficacy in different indications. Their research progress and efficacy in breast cancer are shown in the following table.
The development of antibody technology has brought great success to the treatment of breast cancer. At present, there are mainly three types of antibody-related drugs that are being clinically explored for the treatment of breast cancer.
The first category is antibodies targeting tumor surface antigens, including monoclonal antibodies such as trastuzumab and pertuzumab that have been approved for marketing, and bispecific antibodies such as MCLA128 targeting Her2 and Her3 and ZW25 targeting different domains of Her2.
The second category is ADC drugs that target tumor cell surface antigens. At present, three ADC drugs have been approved for the treatment of breast cancer, two of which target Her2 (Enhertu and Kadcyla), and one targets TROP- 2 (Sacituzumab govitecan), and there are many ADC drugs targeting other targets under clinical evaluation, such as CA6, LIV-1, PTK7, LAMP-1, P-cadherin and ephrin A4.
The third category is an antibody that activates the body's immune system, PD-1 or PD-L1. These antibodies use immune-related cells to kill tumor cells by activating the immune system.
Of course, in addition to the above-mentioned antibody monotherapy for breast cancer, there are currently a variety of combinations of drugs under exploration, including trastuzumab and pertuzumab combined with small molecule drugs, PD-1 antibody combined with paclitaxel and other drugs.
Therefore, the treatment of antibodies in the field of breast cancer can be described as a hundred flowers blooming! We also hope that more and more treatment options will come out to bring good news to breast cancer patients.
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 A Review of Therapeutic Antibodies in Breast Cancer
 Novel antibody–drug conjugates for triple negative breast cancer
 A bifunctional fusion protein increases PRLR-positive breastcancer cell death in co-culture with natural killer cells
 DS-8201a, A Novel HER2-Targeting ADC with a Novel DNA Topoisomerase IInhibitor, Demonstrates a Promising Antitumor Efficacy with Differentiationfrom T-DM1
 Global Antibody-drug Conjugates (ADCs): Approvals & Clinical Trails Review
 ADC Drugs with New Targets Clinical Pipeline Review
 Progresses Of ADC Technology For Cancer Therapy
 The History Of ADC Drugs Development
 History and Development of Antibody Drug Conjugates (ADCs)
 ADCs Against Cancer: Clinical Landscape and Challenges