Breast cancer is the leading cause of death among women worldwide. When breast cancer spreads to other parts of the body, the prognosis is poor. Currently, there is no treatment for metastatic breast cancer. Antibody drug conjugates (ADCs) are new anti-cancer drugs composed of antibodies, linkers and cytotoxic agents. The monoclonal antibodies in the ADC target specific antigens, and the cytotoxic agent kills cancer cells without harming surrounding healthy cells. Ado-trastuzumab Emtansine (T-DM1), Fam-trastuzumab Deruxtecan (T-DXd) and Sacituzumab govitecan (SG) are FDA-approved ADCs for the treatment of breast cancer. T-DM1 and T-DXd target human epidermal growth factor receptor 2 (HER2), while SG targets trophoblast surface antigen 2 (TROP-2). Datopotamab deruxtecan (Dato-DXd) is an investigational ADC targeting TROP2. In the TROPION-Breast01 Phase III trial, the drug showed a clinically meaningful improvement in progression-free survival in breast cancer patients. Both TROP-2 and HER2 are involved in cancer cell proliferation. In addition to HER2 and TROP2, there are other proteins involved in tumor progression that could serve as important targets for ADCs.

What is the Structure of an ADC?

ADC drugs are composed of three parts: monoclonal antibodies, linkers and drug carriers. They bind to target antigens on the surface of tumor cells through antibodies, form early endosomes in cells by endocytosis/internalization, and then mature into late endosomes and Fusion with lysosomes. ADC drugs are cleaved in lysosomes and release the drug cargo, ultimately leading to cell apoptosis or death. The specific components of ADC drugs and their mechanisms of action determine the stability, cytotoxicity, pharmacokinetics and anti-tumor activity of the drug.

• Antibodies are crucial for the specific binding between target antigen and ADC. The ideal ADC antibody must have high affinity for binding to the target antigen; the ability to promote internalization; low immunogenicity; and long half-life. Currently, most of the antibodies used by known ADC drugs are immunoglobulin G (IgG) antibodies.
• The structure and chemical properties of the linker directly affect the pharmacokinetics, drug-to-antibody ratio (DAR) and degree of bystander effect of ADC. The ideal linker should not induce ADC aggregation; should not interfere with the toxicity of cytotoxic drugs; be stable enough to ensure that cytotoxic drugs are not released prematurely; and be able to accurately cleave and release the drug at the target site.
• The payload is a cytotoxic drug used by ADC to kill tumor cells. It needs to be highly stable in systemic circulation and lysosomes and has the ability to efficiently kill tumor cells at low concentrations.

Our Antibody-Drug Conjugates (ADCs)

ADC Targets in Breast Cancer

Nectin Cell Adhesion Molecule 4 (Nectin-4)

Nectin-4 is a cell surface protein that plays an important role in cell-cell adhesion. It is overexpressed in breast cancer and other malignancies and promotes cancer cell proliferation and metastasis. Enfortumab vedotin (EV), a nectin-4-targeting ADC, has been approved by the FDA for the treatment of urothelial cancer. The drug has also been approved in the EU and China. The EV-202 Phase 2 trial is ongoing to test the efficacy of EVs in breast cancer and other nectin-4-expressing cancers.

Tissue Factor (TF)

TF is a transmembrane protein mainly involved in blood coagulation. Scientists have discovered that this protein can promote cancer progression and metastasis. Tisotumab vedotin (TV), a TF-targeting ADC, has received accelerated approval from the FDA for the treatment of cervical cancer. However, this ADC has not yet been tested in breast cancer patients. XB002 is another ADC targeting TF. Scientists found in preclinical studies that the drug was effective against different types of tumors. XB002 caused reversible adverse reactions such as dry eye and non-infectious conjunctivitis in patients with solid tumors in phase I clinical trials.

Fig. 1. Mechanism of action of Tisotumab vedotin (Cancer Manag Res. 2023, 15: 1063-1072).

Mesothelin

Mesothelin is another cell surface protein that plays an important role in cell adhesion. This cell surface protein is mainly expressed in mesothelial cells of the conjunctiva, pericardium, and cornea. It is also overexpressed in cancer and associated with poor prognosis. Anetumab ravtansine (AR) and RC88 are two ADCs that target mesothelin. In Phase I clinical trials, Anetumab ravtansine caused manageable adverse effects, such as keratitis, fatigue, and anorexia, in patients with solid tumors. Both ADCs are being tested in phase 1/2 basket trials in triple-negative breast cancer and other solid tumors.

LIV-1

LIV-1 is a transmembrane protein that transports zinc into cells and is overexpressed in breast tumors. LIV-1 expression is associated with cancer metastasis and lymph node involvement. Ladiratuzumab vedotin (LV or SGN-LIV1A) is an ADC consisting of a monoclonal antibody targeting LIV1. This ADC can induce immunogenic cell death, which may be beneficial for patients undergoing immunotherapy. The drug's efficacy is being tested in a Phase 1 clinical trial in patients with LIV1-positive breast cancer. Preliminary results from the clinical trial showed an overall response rate (ORR) of 32% in breast cancer patients. Median progression-free survival (PFS) was 11.3 weeks. This drug can cause nausea, hair loss, fatigue, and peripheral neuropathy in patients. Preliminary results from a Phase Ib/II trial show an ORR of 35% in patients with triple-negative breast cancer who received the combination of ladiratuzumab vedotin and pembrolizumab.

Receptor Tyrosine Kinase-like Orphan Receptors 1 and 2

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) plays an important role in nervous system development. It is overexpressed in triple-negative breast cancer and has shown antitumor activity against this breast cancer subtype in xenograft models. NBE-002 is an anti-ROR1 ADC being tested in patients with solid tumors in a Phase 1/2 clinical trial. Receptor tyrosine kinase-like orphan receptor 2 (ROR2) plays an important role in the development of the skeletal system. It also promotes breast cancer metastasis, leading to poor prognosis. Ozuriftamab vedotin (BA3021) is a conditionally active biological (CAB) ADC targeting ROR2. It is also being tested in a Phase 1/2 clinical trial.

Human Epidermal Growth Factor Receptor 3 (HER3)

HER3 belongs to the human epidermal growth factor receptor (HER) family. The tyrosine kinase activity of HER3 is weak and cannot transduce signals. Therefore, it forms heterodimeric complexes with hepatocyte growth factor receptor (HGFR), fibroblast growth factor receptor 2 (FGFR2), and other receptor tyrosine kinases (RTKs). It is overexpressed in breast cancer and other cancers such as melanoma and head and neck cancer. Patritumab deruxtecan is an ADC targeting HER3 that has been tested in breast cancer patients in a Phase 1/2 clinical trial. The ORR for patients with HR-positive/HER3 high & low/HER2-negative breast cancer who received this drug was 30.1%. No patient showed complete response. The drug also causes adverse effects such as decreased platelet and neutrophil counts and anemia.

Fig. 2. Mechanism of action of HER2- and HER3-targeting ADCs (Cancers. 2021, 13(5): 1047).

Globohexaosylceramide (Globo-H)

Globo-H is a carbohydrate antigen expressed in cancer cells. OBI-999 is an anti-Globo-H ADC that has been tested in patients with solid tumors in a Phase 1 clinical trial. Patients received the drug intravenously at varying doses and found the drug to be well tolerated at doses up to 1.2 mg/kg. The drug causes adverse reactions such as anemia and neutropenia.

Folate Receptor Alpha (FRα)

FRα is a membrane-bound protein involved in folate transport. It is expressed in triple-negative breast cancer. MORAb-202 is an ADC consisting of a monoclonal antibody targeting FRα. In xenograft mouse models, the drug inhibited proliferation of FRα-expressing cell lines. In a phase 1 clinical trial in which the drug was administered to patients with solid tumors, common adverse reactions observed in patients were neutropenia and leukopenia.

B7 Homolog 4 Protein (B7-H4)

B7-H4 is an immune checkpoint protein that inhibits the function of activated T cells. It is expressed in breast cancer and other tumors such as endometrial and cholangiocarcinomas. AZD8205 and SGN-B7H4V are two novel ADCs targeting FRα. Both ADCs showed antitumor activity in mouse models. A Phase 1/2 clinical trial is underway to evaluate the safety of AZD8205 in tumors such as breast cancer. SGN-B7H4V is also currently being tested in Phase 1 clinical trials in breast cancer and other tumors.

ADC Development for Targets

BOC Sciences provides target-specific ADC development services, involving the design, synthesis and optimization of ADCs for specific targets. We can help select the appropriate ADC target for a specific disease or condition. We also provide a one-stop shop for drug payload selection, linker design and conjugation technology. In addition, BOC Sciences conducts comprehensive ADC optimization and characterization studies to evaluate ADC stability, pharmacokinetics, and efficacy in preclinical and clinical settings.

ADCs, as a new type of anticancer drug, have gained great attention in the past few decades because they can selectively deliver cytotoxic small molecules to targeted cancer cells while protecting healthy cells. ADCs approved for the treatment of breast cancer, including T-DM1 and T-DXd, have brought significant clinical benefits to patients with advanced breast cancer, leading to the development of more and more ADCs for the treatment of breast cancer. An increasing number of ADCs have been approved by the FDA for the treatment of solid tumors, and progress has been made in the development of innovative ADCs capable of treating a variety of malignancies. Overall, ADC may continue to be the fastest growing class of drugs and may become a first-line treatment option for cancer patients including breast cancer in the future. Whether used as monotherapy or in combination with other anti-cancer drugs in development, ADCs can bring breakthrough benefits to patients with advanced breast cancer.