Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that belongs to the ErbB family and is involved in angiogenesis, cell proliferation, metastasis, and inhibition of apoptosis. Studies have shown that EGFR protein is more frequently overexpressed in TNBC and is an independent prognostic indicator of disease-free survival (DFS) and overall survival (OS). The monoclonal antibody cetuximab combined with cisplatin chemotherapy has shown promising results in Phase Ⅱ studies, suggesting some subtypes of TNBC may be EGFR inhibition sensitive.

Vascular Endothelial Growth Factor (VEGF) is expressed by 30-60% in TNBC. It stimulates endothelial cell proliferation and migration, inhibits endothelial cell apoptosis, and promotes angiogenesis. One study found that TNBC patients had significantly elevated VEGF expression, and TNBC patients had shorter Disease-free survival (DFS) with non-TNBC patients. Angiogenesis is considered to be a key component driving tumor cell proliferation. Therefore, VEGF has also become a highly potential target for the treatment of TNBC. Results from a study of adjunctive bevacizumab (a monoclonal antibody targeting VEGF-A) in patients with TNBC suggest that bevacizumab may improve the efficacy of immunotherapy in patients with TNBC.

Fibroblast growth factor receptors (FGFRs) are a family of transmembrane receptors that play a role in several processes, including proliferation, angiogenesis, and migration. FGFR, its numerous FGF ligands, and signaling partners are often dysregulated in breast cancer progression and are one of the causes of resistance to treatment in breast cancer. In addition, FGFRs' genetic alterations may contribute to tumor progression and poor outcomes in breast cancer patients. Therefore, FGFR is also considered a potential therapeutic target for breast cancer.

Diverse studies have demonstrated that aberrant activation of the FGF/FGFR axis may be involved in the progression of the different subtypes of breast cancer. Among breast cancer patients, several subgroups have been shown to harbor genetic aberrations in FGFRs, including amplification of FGFR1, FGFR2, and FGFR4 and mutations in FGFR2 and FGFR4. Although FGFR1 amplification is associated with a poorer prognosis in hormone receptor-positive BC, its role in TNBC remains controversial. FGFR2 expression was associated with poorer OS. FGFR inhibitors are one of the hottest small molecule drugs in recent years. If triple-negative breast cancer patients can also benefit from FGFR inhibitors, the survival rate of patients will be greatly improved. However, the resistance of breast cancer patients to FGFR inhibitor therapy is currently the biggest obstacle to clinical approval. There are also preclinical data investigating the efficacy of antibodies against FGFR isoforms and fibroblast growth factor ligand inhibitors, showing encouraging results in phase I clinical trials in solid tumors, including BC.

NOTCH signaling activates several genes related to cell differentiation, proliferation, and death. Four heterodimeric transmembrane Notch receptors (1-4) that react with the transmembrane ligands Delta-like (Dll) 1,2,4 and Jagged (JAG) 1 and 2 are identified in humans. Studies confirm that the Notch pathway has major participation in breast cancer progression through overexpression and abnormal genetic type expression of the notch receptors and ligands that determine angiogenesis. In breast cancer, functionally mutated NOTCH accounts for about 10% of TNBC. Studies have shown a correlation between NOTCH-1 and positive lymph node status and Jagged-1 and larger tumor status. Studies have also shown that increased expression of NOTCH-1, NOTCH-4, or Jagged-1 is associated with adverse prognostic factors such as decreased survival, so targeting the inhibition of the NOTCH signaling pathway may be beneficial for breast cancer treatment. Several inhibitors have been developed for this pathway development.

The Prolactin Receptor (PRLR) is a type 1 cytokine receptor expressed in a subset of breast cancers and may contribute to its pathogenesis. The Jak/Stat, PI3-kinase/AKT, and MAPK signaling pathways are activated when PRLR is combined with PRL, GH, or PL, resulting in tumor cell proliferation. PRLR is overexpressed in malignant breast epithelium relative to normal breast tissue and expressed across molecular subtypes of human breast cancers. The neutralizing antibody LFA-102 antagonizes PRLR signaling and proliferation in breast cancer cells and regresses PRL-dependent tumor xenografts. While LFA-102 was well tolerated as an unconjugated antibody in Phase 1 clinical trials in patients with metastatic breast and prostate cancer, efficacy was limited.

Despite the remarkable clinical efficacy of immune checkpoint blockade (ICB) in cancer treatment, ICB benefits for triple-negative breast cancer (TNBC) remain limited. The immunosuppressive tumor microenvironment of TNBC reduces the effective response of tumors to those drugs, so if new targets can be found to reprogram the tumor microenvironment, it will help improve the efficacy of immunotherapy in TNBC. A study published in cells showed that deletion of the E3 ubiquitin ligase Cop1 in cancer cells decreases secretion of macrophage-associated chemokines, reduces tumor macrophage infiltration, enhances anti-tumor immunity, and strengthens ICB response. Therefore, Cop1 is a target for improving cancer immunotherapy efficacy in TNBC.

TROP-2 is a type I transmembrane glycoprotein. TROP-2 plays an important role in cell migration, proliferation, cell cycle progression and metastasis, and high expression in breast cancer, lung cancer, etc., making it a great target in tumor therapy.

On April 7, 2021, Gilead announced the FDA approval to sacituzumab govitecan (Trodelvy, Immunomedics Inc.) for unresectable locally advanced or metastatic triple-negative cancer patients breast cancer (mTNBC) who have received two or more prior systemic therapies, at least one of them for metastatic disease. Trodelvy is an antibody-drug conjugate (ADC) composed of a humanized RS7 antibody targeting Trop-2 linked with SN38 at a DAR of 7.6:1.

GPNMB is involved in cell migration, invasion, angiogenesis, or epithelial-mesenchymal transition, highly overexpressed in TNBC. GPNMB is a poor prognosis biomarker in BC.

LIV-1 is a multi-span transmembrane protein of the solute carrier family 39 with putative zinc transporter and metalloproteinase activity. As a downstream target of STAT3, it promotes the epithelial-to-mesenchymal transition that is important in the malignant progression to metastasis. LIV-1 is expressed in several cancers with the highest prevalence and expression level in breast, prostate, and melanoma. Additionally, LIV-1 has been linked with malignant progression to metastasis and associated with lymph node involvement in breast cancer.

Ladiratuzumab vedotin is an ADC composed of a humanized anti-LIV-1 monoclonal antibody conjugated to the microtubule-disrupting agent MMAE via a protease-cleavable linker with Seattle Genetics' proprietary technology. In a 1/2 phase clinical trial, the combination of Ladiratuzumab vedotin and Keytruda, a PD-1 inhibitor, achieved an objective response rate of 54% in patients with unresectable and metastatic TNBC.

CA-6 is an ideal ADC target selectively expressed in solid tumors. SAR566658 consists of DM4 and an antibody targeting CA6. In a phase, I clinical trial involving 114 patients, 60% of patients in the 190 mg/m2 and 90 mg/m2 dose groups experienced tumor regression on days one and eight.

Figure 3 ADCs that have been approved or are in the clinical stage for the treatment of breast cancer (data from literature)

As a leading manufacturer of recombinant proteins, ACROBiosystems has developed a series of breast cancer-related target proteins with highly bioactivity, purity, comprehensive species, and label, such as VEGF/EGFR/TROP-2/LIV-1/GPNMB, which can fully support the development of breast cancer-related drugs.