Breast cancer exhibits marked molecular heterogeneity. Different subtypes may develop distinct molecular features, including sustained HER2 receptor activation, enhanced PI3K-AKT signaling, uncontrolled cell-cycle progression, defects in DNA damage repair, glutamine metabolic dependence, and resistance to apoptosis. Small-molecule inhibitors can intervene in these abnormal processes at multiple levels, including receptor kinases, metabolic enzymes, membrane-protein homeostasis, and apoptosis regulation.

Keywords: breast cancer; small-molecule inhibitors; ErbB2; GAC; MYOF; Mcl-1; triple-negative breast cancer; targeted research

1 Molecular Dependencies in Breast Cancer Targeted Research

1.1 Molecular Heterogeneity of Breast Cancer

Different breast cancer subtypes show marked differences in their dependence on cellular signaling and metabolic processes. Hormone receptor-positive breast cancer mainly involves ER-mediated transcriptional programs, the Cyclin D-CDK4/6-RB cell-cycle axis, and PI3K-AKT-mTOR signaling. HER2-positive breast cancer usually exhibits ERBB2 amplification or HER2 protein overexpression, which can continuously activate proliferative pathways such as PI3K-AKT and RAS-ERK. Triple-negative breast cancer (TNBC) lacks the three classical molecular markers ER, PR, and HER2, and its research targets further extend to DNA damage repair, glutamine metabolism, membrane-protein trafficking, adhesion and invasion, and resistance to apoptosis.

1.2 Research Value of Small-Molecule Inhibitors

Small-molecule inhibitors can interfere with key molecular processes in breast cancer through mechanisms such as competition for ATP-binding sites, allosteric regulation, blockade of protein function, or alteration of protein stability. Their research value is reflected not only in inhibition of cell proliferation but also in validation of target activity, analysis of downstream signaling, determination of metabolic dependence, investigation of metastatic mechanisms, and evaluation of combination interventions. Different compounds should be evaluated using experimental endpoints selected according to their direct targets and mechanistic levels, rather than inferring specific mechanisms solely from changes in cell viability.

2 AG-825: Inhibition of ErbB2/HER2 Signaling

2.1 Molecular Characteristics of AG-825

AG-825 is an ATP-competitive ErbB2 tyrosine kinase inhibitor that can inhibit ErbB2-related tyrosine phosphorylation, with an IC50 of approximately 0.35 μM. It is mainly used to study ErbB2 kinase activity, HER2-related signal transduction, and changes in receptor protein complexes and is suitable as a small-molecule tool for investigating signaling mechanisms in HER2-dependent breast cancer.

2.2 ErbB2 and HER2-Positive Breast Cancer

ErbB2, also known as HER2, is an important member of the ErbB receptor tyrosine kinase family. HER2 has a strong dimerization capacity and can form dimers with EGFR, HER3, or HER4, thereby activating downstream pathways such as PI3K-AKT and RAS-RAF-MEK-ERK. ERBB2 amplification or HER2 overexpression can promote breast cancer cell proliferation, survival, and invasion. Therefore, HER2 receptor phosphorylation and downstream AKT and ERK activation are important indicators for evaluating ErbB2 signaling.

2.3 Mechanism of Action of AG-825

AG-825 acts on the intracellular kinase domain of ErbB2 and competes for ATP binding, thereby reducing tyrosine phosphorylation of the receptor itself and related substrates. When ErbB2 activation is restricted, proliferative signals transmitted to pathways such as PI3K-AKT and ERK are reduced, while protein complexes dependent on receptor activation status may also be affected. Therefore, experimental results with AG-825 should be comprehensively interpreted together with p-ErbB2, p-AKT, p-ERK, cell proliferation, and colony formation.

2.4 ErbB2-Nucleolin-Related Regulation

In HER2-related breast cancer models such as SKBR3, Nucleolin can participate in ErbB2-related protein complexes and signal regulation. AG-825 can interfere with ErbB2-related phosphorylation and the status of the ErbB2-Nucleolin complex. When combined with the Nucleolin-targeting oligonucleotide AS1411, it can further reduce cell survival and colony-forming capacity and inhibit anchorage-independent growth and basement-membrane invasion. This combination reflects an additive effect between receptor kinase inhibition and intervention in receptor-associated regulatory proteins.

2.5 Other HER2-Related Small Molecules

(1) Lapatinib

Lapatinib can simultaneously inhibit the intracellular kinase activity of EGFR and HER2 and is suitable for evaluating the effects of dual ErbB-node inhibition on AKT, ERK, and cell proliferation. Compared with AG-825, it has a broader range of action within the ErbB family.

(2) Neratinib

Neratinib is an irreversible ErbB-family inhibitor that can act on EGFR, HER2, and HER4. Its sustained inhibitory characteristics make it suitable for studying ErbB receptor reactivation, bypass compensation, and tolerance mechanisms.

(3) Tucatinib

Tucatinib has relatively high HER2 selectivity and weaker activity against EGFR and can be used to analyze HER2-specific dependence. Parallel application with Lapatinib and Neratinib is useful for comparing signaling differences produced by different ErbB inhibition modes.

3 UPGL00004: Inhibition of GAC-Mediated Glutamine Metabolism

3.1 Molecular Characteristics of UPGL00004

UPGL00004 is a potent allosteric inhibitor of Glutaminase C (GAC), with an IC50 of approximately 29 nM against GAC and a binding Kd of approximately 27 nM. This compound can inhibit GAC activity at nanomolar concentrations and produces marked antiproliferative effects in invasive TNBC cells with strong glutamine metabolic dependence, such as MDA-MB-231 and HS578T.

3.2 GAC and Metabolic Reprogramming in Breast Cancer

The Warburg effect is characterized by increased glucose uptake and glycolysis in tumor cells, with substantial lactate production even in the presence of oxygen. However, enhanced glycolysis does not mean that mitochondrial metabolism has stopped. Rapidly proliferating tumor cells still require continuous replenishment of tricarboxylic acid cycle intermediates to support energy production, lipid and nucleotide synthesis, and redox homeostasis. Glutamine therefore serves as an important supplementary source of carbon and nitrogen.

GAC is not an amino acid but an important splice isoform of GLS1. GAC catalyzes the hydrolysis of glutamine to form glutamate, which can be further converted into α-ketoglutarate and enter the TCA cycle, while also participating in glutathione synthesis and redox regulation. Some highly invasive TNBC cells show strong dependence on GLS1/GAC. Therefore, restricting GAC activity can simultaneously affect anaplerotic metabolism, biosynthesis, and cellular antioxidant capacity.

3.3 Mechanism of Action of UPGL00004

UPGL00004 does not directly compete with glutamine at the catalytic site but binds to an allosteric regulatory region of GAC. GAC catalytic activity is closely related to its oligomeric state and conformational changes, and the highly active state is associated with tetramer formation and stabilization of the active conformation. After binding to the allosteric region, UPGL00004 can restrict conformational changes that favor high catalytic activity, thereby reducing the metabolic flux from glutamine to glutamate.

3.4 Characteristics of Its Effects in TNBC

After GAC inhibition, production of glutamate and α-ketoglutarate decreases, which can further affect TCA-cycle replenishment, glutathione synthesis, and redox homeostasis. The inhibitory effect of UPGL00004 differs among breast cancer models. TNBC models with greater glutamine dependence are generally more sensitive. Therefore, this molecule is more suitable for studying metabolic dependence and should not be simply described as having the same cytotoxicity toward all breast cancer cells.

In combination studies, UPGL00004 combined with the anti-VEGF antibody bevacizumab can further suppress tumor growth in a patient-derived TNBC tumor graft model. One intervention limits glutamine utilization within tumor cells, whereas the other suppresses VEGF-associated angiogenesis, allowing analysis of the interdependence between tumor metabolism and the microenvironment.

3.5 Related GAC Inhibitors

(1) CB-839

CB-839 (Telaglenastat) is also a GLS1/GAC allosteric inhibitor and can reduce the conversion of glutamine to glutamate. Its region of action is similar to that of UPGL00004, and it can be used as a parallel compound to verify whether tumor cell phenotypes are consistent with GLS1/GAC inhibition.

(2) BPTES

BPTES is a classical GLS1 allosteric inhibitor and an important lead structure for the design of multiple subsequent GAC inhibitors. It is commonly used to study GAC conformational changes, allosteric regulation, and glutamine metabolism mechanisms.

4 WJ460: MYOF-Mediated Regulation of Invasion and Metastasis

4.1 Molecular Characteristics of WJ460

WJ460 is a small molecule that can directly interact with Myoferlin (MYOF) and markedly reduce the invasive capacity of highly invasive breast cancer cells at nanomolar concentrations. Unlike compounds that mainly inhibit cell proliferation or cell-cycle progression, WJ460 is more suitable for studying membrane-protein homeostasis, cell invasion, angiogenesis, and distant metastasis in breast cancer.

4.2 MYOF and Breast Cancer Metastasis

MYOF is a membrane-associated protein containing multiple C2 domains and participates in membrane fusion, membrane repair, vesicular trafficking, and membrane-receptor stability. MYOF expression is increased in some TNBC and highly invasive breast cancer cells and can affect the trafficking, stability, and degradation of membrane receptors such as EGFR and VEGFR, thereby further altering cell migration, metabolic adaptation, and angiogenesis. MYOF is therefore not a single proliferation-promoting node but an important regulatory protein linking membrane dynamics, receptor homeostasis, and invasive phenotypes.

4.3 Mechanism of Action of WJ460

After binding to MYOF, WJ460 can interfere with its normal membrane-trafficking and receptor-regulatory functions, thereby affecting the membrane-receptor homeostasis maintained by MYOF and further reducing invasion-related signaling output. Because MYOF itself is not a classical protein kinase, the effect of WJ460 should not be described as inhibition of MYOF phosphorylation. Its core action is interference with MYOF protein function and the membrane-dynamic processes maintained by MYOF.

4.4 Invasion, Angiogenesis, and Metastasis

WJ460 can inhibit highly invasive breast cancer cells such as MDA-MB-231 and BT549 from crossing collagen I or matrix barriers and can affect extracellular-matrix degradation and formation of three-dimensional invasive structures. In vivo studies have also observed reductions in primary tumor growth, angiogenesis, and lung metastasis, accompanied by changes in VEGFR-2-related signaling. Therefore, WJ460 should mainly be evaluated using invasion, metastasis, and membrane-receptor homeostasis as endpoints rather than solely using cell viability to assess its activity.

4.5 Related Invasion-Signaling Inhibitors

Dasatinib can inhibit multiple kinase nodes, including Src-family kinases, and is suitable for studying migration signaling associated with receptor tyrosine kinases, integrins, and focal adhesions. Defactinib is mainly used for FAK-related research and can affect integrin signaling, the cytoskeleton, and focal-adhesion remodeling. MYOF, Src, and FAK represent three distinct levels of invasion regulation: membrane trafficking, intracellular kinases, and focal adhesions, respectively.

5 BAY61-3606: Mcl-1 Regulation and TRAIL Apoptosis Sensitization

5.1 Molecular Characteristics of BAY61-3606

BAY61-3606 is an ATP-competitive, reversible Syk inhibitor with a Syk Ki of approximately 7.5 nM. In breast cancer cells, its effects are not limited to Syk signaling. It can also reduce the level of the anti-apoptotic protein Mcl-1 and enhance TRAIL-induced apoptosis. Therefore, BAY61-3606 can also be used to study Mcl-1 stability, transcriptional regulation, and death-receptor sensitivity.

5.2 Mcl-1 and Resistance to Apoptosis in Breast Cancer

Mcl-1 is an anti-apoptotic member of the BCL-2 family and can limit BAX/BAK-mediated mitochondrial outer-membrane permeabilization, increasing the threshold for cells to enter apoptosis. Mcl-1 has a relatively rapid protein turnover rate, and its level is jointly controlled by transcription, translation, phosphorylation, and ubiquitination. When some breast cancer cells maintain high Mcl-1 levels, the mitochondrial amplification phase of apoptosis may still be suppressed even when death receptors are activated.

5.3 Mechanism by Which BAY61-3606 Reduces Mcl-1

(1) Promotion of Mcl-1 Degradation

BAY61-3606 can promote Mcl-1 Ser159-related phosphorylation and enhance ubiquitin-proteasome-system-mediated Mcl-1 degradation. After Mcl-1 stability decreases, its restriction of pro-apoptotic proteins is weakened, making cells more likely to enter the mitochondrial apoptotic process.

(2) Inhibition of Mcl-1 Transcription

BAY61-3606 can also affect CDK9-mediated RNA polymerase II transcriptional regulation and reduce Mcl-1 expression at the transcriptional level. Therefore, its regulation of Mcl-1 includes both reduced protein production and enhanced protein clearance.

5.4 Sensitization to TRAIL-Induced Apoptosis

TRAIL initiates extrinsic apoptosis through death receptors, but anti-apoptotic proteins such as Mcl-1 can restrict the subsequent mitochondrial amplification process. After BAY61-3606 reduces Mcl-1, it can enhance Bak-related responses, Caspase activation, and DNA fragmentation, thereby increasing the sensitivity of breast cancer cells to TRAIL. This effect can show a certain degree of Syk independence, so the entire effect of BAY61-3606 in breast cancer models should not be directly attributed to Syk inhibition.

5.5 Direct Mcl-1 Inhibitors

S63845 can directly bind the BH3-binding region of Mcl-1 and block its functional interaction with pro-apoptotic proteins. Unlike BAY61-3606, which reduces Mcl-1 expression and stability, S63845 mainly directly inhibits Mcl-1 function. The two can be used to distinguish apoptotic phenotypes resulting from “reduced Mcl-1 protein” and “blocked Mcl-1 function.”

6 Other Breast Cancer-Related Small-Molecule Inhibitors

6.1 CDK4/6 Inhibitors

Palbociclib, Ribociclib, and Abemaciclib mainly act on CDK4/6, reduce RB phosphorylation, and restrict E2F-dependent transcription, causing cell-cycle arrest in G1 phase. These compounds are suitable for studying the Cyclin D-CDK4/6-RB axis in HR-positive breast cancer and can also be used to analyze resistance mechanisms such as RB loss and Cyclin E-CDK2 activation.

6.2 PI3K-AKT-mTOR Inhibitors

Alpelisib, Capivasertib, and Everolimus act on PI3Kα, AKT, and mTORC1, respectively, corresponding to different levels of the PI3K-AKT-mTOR pathway. PIK3CA mutations, reduced PTEN function, and excessive HER2 activation can all enhance this pathway. Therefore, these inhibitors can be used to study breast cancer growth signaling, feedback compensation, and targeted-therapy resistance.

6.3 PARP Inhibitors

Olaparib and Talazoparib mainly inhibit PARP1/2 and can increase DNA damage and replication stress in the context of BRCA1/2 or other homologous recombination repair defects. Different PARP inhibitors differ in their ability to stabilize PARP-DNA complexes and can be further used to analyze PARP trapping, replication-fork damage, and synthetic-lethality mechanisms.

7 Breast Cancer-Related Small-Molecule Inhibitor Products

7.1 HER2- and Growth-Signaling-Related Inhibitors

7.2 Metabolism-, Invasion-, and Apoptosis-Related Inhibitors

7.3 DNA Damage Repair-Related Inhibitors

AG-825, UPGL00004, WJ460, and BAY61-3606 intervene in key breast cancer processes at the levels of receptor tyrosine kinases, glutamine metabolism, membrane-protein homeostasis, and anti-apoptotic regulation, respectively. Defining the target background, mechanism of action, and applicable model for each specific inhibitor, together with validation using related inhibitors acting on the same pathway, helps establish a more complete research framework for molecular dependence and pharmacological intervention in breast cancer.

References

[1] Kim SY, et al. Bay 61-3606 Sensitizes TRAIL-Induced Apoptosis by Downregulating Mcl-1 in Breast Cancer Cells.

[2] Wolfson E, et al. Nucleolin and ErbB2 inhibition reduces tumorigenicity of ErbB2-positive breast cancer.

[3] Huang Q, et al. Characterization of the interactions of potent allosteric inhibitors with glutaminase C, a key enzyme in cancer cell glutamine metabolism.

[4] Zhang T, et al. A small molecule targeting myoferlin exerts promising anti-tumor effects on breast cancer.

For more related articles, please see below:

[1] FISH assay for detection of HER2 amplification in breast cancer

[2] Small Molecule Inhibitors Selection Guide

[3] Small-Molecule Inhibitors: A Practical Guide for Research Selection — A Minimal Evidence Chain and Scenario-Based Validation Framework (Including a Selection Matrix, Case Cards, and Product Tables 1–4)