One of the leading causes of tumor growth and medication resistance in breast cancer is cell cycle disruption. CDK4/6 is a sensor that links multiple signaling pathways to cell cycle initiation and progression (9, 10). By encouraging the phosphorylation of retinoblastoma protein (Rb), which releases the transcription factor E2F and stimulates gene transcription from the G to S phase, CDK4/6 stimulates cell proliferation. In HR+ breast cancer, the Cyclin D-CDK4/6 signaling pathway activated by the estrogen pathway is an important cause of tumor proliferation and endocrine therapy resistance (11). Anti-tumor effects can be achieved by inhibiting CDK4/6, which stops cells in the G1 phase and prevents cell mitosis (12) (Figure 1).

Palbociclib, ribociclib, and abemaciclib are the three FDA-approved CDK4/6is for breast cancer, which are currently the first- and second-line systemic treatment choices for HR+/human epidermal growth factor-2 negetive (HER2-) advanced/metastatic BC. These drugs can be used alone or in conjunction with endocrine therapy (13). Additionally, CDK4/6is has demonstrated remarkable clinical effects in neoadjuvant endocrine therapy and early breast cancer, as evidence continues to grow and the therapeutic range of CDK4/6 inhibitors continues to broaden. Clinical trials are underway for new CDK4/6is, like dalpiciclib.

The phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway (PAM pathway) plays a pivotal role in breast carcinogenesis, with approximately 70% of breast tumors exhibiting hyperactivation of this pathway (43, 44). Activation of the pathway is initiated when ligands bind to receptor tyrosine kinases (RTKs) or G protein-coupled receptors (GPCRs) on the cell membrane, leading to the recruitment and activation of PI3K proteins. PI3K converts phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidyl inositol 3,4,5-trisphosphate (PIP3), a second messenger that recruits and activates AKT. AKT, in turn, phosphorylates and activates downstream effectors such as pyruvate dehydrogenase kinase isozyme 1 (PDK1) and mammalian target of rapamycin complex 2 (mTORC2) at the plasma membrane. PDK1 and mTORC2 further activate AKT, which then phosphorylates and activates the mammalian target of rapamycin complex 1 (mTORC1). Activation of mTORC1 promotes protein and lipid synthesis via induction of ribosomal protein S6 (S6) and eukaryotic Translation Initiation Factor 4E-Binding Protein1 (4E-BP1) while reducing autophagy, ultimately driving cell growth and proliferation. The phosphatase and tensin homolog (PTEN) negatively regulates this pathway by converting PIP3 to PIP2. AKT phosphorylates and inactivates the negative regulator of mTORC1, TSC1/2, further activating this pathway (45–48).

In HR+ BC, activation of the PI3K/AKT/mTOR pathway induces non-estrogen-dependent transcriptional activity through phosphorylation of estrogen receptor α (ERα) by AKT or mTOR. Similarly, ER activates the PI3K/AKT/mTOR pathway through activation of PI3K, which may be the reason for the ineffectiveness of endocrine therapy alone in some patients. This may be the reason for the poor outcome of endocrine therapy alone in some patients. In addition, it is worth noting that activation of the mutant PI3K/AKT/mTOR pathway in PI3K also promotes the progression of the cell cycle from G1 to S phase through downstream target activation of CDK4/6, which may attenuate the effect of CDK4/6is (Figure 2) (49, 50). Therefore, targeting the PAM pathway is an effective means to inhibit tumor proliferation.

Alpelisib is currently the only PI3K inhibitor approved for marketing by the FDA, and as an oral-specific PI3K kinase inhibitor, it inhibits PI3K better and has fewer side effects compared to pan-PI3K inhibitors (51). When treating PIK3CA-mutated HR+/HER2- ABC patients in the SOLAR-1 phase III clinical trial, apelalis plus fulvestrant considerably extended their mOS and mPFS in comparison to the control group (52). Based on this result, in 2019, the FDA approved the combination of apelalisb with fulvestrant for patients with advanced or metastatic breast cancer that progressed to HR+/HER2-, PIK3CA-mutated after endocrine therapy. The subsequent NEO-ORB phase II clinical study failed to demonstrate that apelalisib, in combination with letrozole, enhances neoadjuvant therapy in HR+/HER2- patients (53). The BELLE-2 and BELLE-3 studies demonstrated that buparlisib, the first pan-PI3K inhibitor to enroll in a worldwide phase III trial for HR+ MBC, extended mPFS in patients with ABC who had already advanced on treatment (54, 55). Inavolisib, as a selective inhibition of selective PI3Kα, was shown in the INAVO120 study to significantly improve mPFS in patients with locally advanced or metastatic breast cancer with PIK3CA mutation and HR+/HER2- (15.0 months vs. 7.3 months; HR, 0.43; 95% CI, 0.32–0.59; p < 0.001) (56). Other PI3K inhibitors, such as pictilisib and taselisib, have not delivered satisfactory therapeutic outcomes in clinical trials.

Capivasertib is a novel ATP-competitive pan-AKT kinase inhibitor that ultimately inhibits cell growth and proliferation by preventing substrate phosphorylation of AKT and thereby blocking the activation of PAM downstream targets (9). In the FAKTION phase II clinical trial, fulvestrant combined with capasertinib significantly improved mPFS in the group of HR+/HER2- ABC patients who progressed after failure of endocrine therapy compared to placebo combined with fulvestrant (10.3 months vs. 4.8 months; HR = 0.56; 95% CI 0.38–0.81; p = 0.0023) (57). Similarly, in the Phase III CAPItello-291 trial involving HR+/HER2- ABC patients experiencing relapse or progression during or after endocrine therapy (with or without prior CDK4/6 inhibitor therapy), the mPFS was significantly longer for those receiving capivasertib in combination with fulvestrant compared to those receiving a placebo with fulvestrant (7.2 months vs. 3.6 months; HR = 0.60; 95% CI 0.38–0.81; p = 0.0023; HR = 0.60; 95% CI 0.51–0.71; p < 0.001) (58). Capivasertib and fulvestrant have been approved by the FDA to treat patients with HR+/HER2-metastasized or locally advanced breast cancer. However, another AKT inhibitor, ipatasertib, is presently undergoing additional clinical research after failing to show a therapeutic benefit in the IPATunity130 clinical trial.

The mTOR-selective inhibitor everolimus reduces tumor proliferation by irreversibly inhibiting the phosphorylation of S6K1. In the GINECO Phase III trial, adding an mTOR inhibitor to tamoxifen alone increased the median time to progression (TTP) from 4.5 to 8.6 months in patients with HR+ ABC who had previously progressed on endocrine therapy. Similarly, in the BOLERO-2 Phase III trial, the addition of everolimus resulted in an increase in mPF from 3.2 to 7.8 months in patients with HR + ABC that had previously progressed on endocrine therapy, compared to exemestane alone (59, 60). Dual inhibition of mTORC1/2, which is currently under development, can block both S6K1, which is dependent on mTORC1 phosphorylation, and AKT, which is dependent on mTORC2 phosphorylation, thereby blocking the PAM pathway more completely (61). However, it has not yet been demonstrated that mTORC1/2 dual inhibition provides a better clinical survival benefit in breast cancer treatment compared with mTORC1 inhibitors alone (Table 2).

HER2-positive (HER2+) breast cancers represent about 10% of all breast cancer cases. This subtype is also referred to as luminal B2, characterized not only by estrogen activation but also by the overexpression of HER2 and the interaction between these two signaling pathways (62, 63). Estrogen receptors (ER) activate non-nuclear and non-genomic pathways through their interaction with receptor tyrosine kinases, such as HER2, and their downstream signaling intermediates, including p42/44 mitogen-activated protein kinase (MAPK) and Akt. This interaction leads to increased cell proliferation. At the same time, overactive HER2 signaling activates downstream kinases, including Akt and MAPK, which can reduce the expression of ER at both the mRNA and protein levels. Nonetheless, these kinases also phosphorylate ER and its co-regulators, thereby enhancing and regulating ER’s transcriptional activity. This phenomenon can counteract the effects of endocrine therapy and contribute to endocrine resistance (64) (Figure 3). According to the guidelines from the American Society of Clinical Oncology (ASCO), adding anti-HER2 targeted therapy is more effective for this specific group of HER2+ patients (Figure 3).

In recent years, differences observed in HER2- mid-tests have led to new definitions of HER2-. Specifically, cases exhibiting immunohistochemistry (IHC) scores of 1+ or 2+, but which are FISH-negative, are now categorized as HER2 hyperexpression. Meanwhile, the presence of faint or barely detectable incomplete staining in less than 10% of tumor cells, which is not amplified by FISH examination, is classified as HER2 ultralow expression (65, 66). Although HER2 underexpression and ultralow expression do not significantly contribute to the proliferation and differentiation of breast cancer cells, there is increasing evidence that targeting HER2 therapy can still provide clinical benefits in cases of underexpression and ultralow expression.

Trastuzumab and patuzumab are recombinant humanized monoclonal antibodies that target HER2 to prevent cancer growth, and the TANDEM, EGF30008, and eLEcTRA studies have demonstrated the value of single-targeted endocrine therapies (67–69). However, there remains a risk of recurrence with single-agent targeted therapy. As a result, clinical research is now shifting its focus toward dual-targeted chemotherapy (70). The APHINITY and PERTAIN studies have demonstrated that a combination of dual-targeted endocrine or chemotherapy provides greater clinical benefits than single-targeted therapies alone (71, 72).

Pyrotinib is an irreversible pan-HER2 tyrosine kinase inhibitor (TKI) targeting HER1, HER2, and HER4, and a multicenter, single-arm, phase II clinical trial initially demonstrated that treatment with pyrotinib in combination with letrozole could be used as first-line therapy in patients with HR+/HER2- MBC (73). In another multicenter, single-arm, phase II clinical trial, pyrotinib in combination with fulvestrant increased mPFS in HR+/HER2- MBC patients who had failed prior trastuzumab therapy, suggesting that pyrotinib in combination with fulvestrant may be a practical approach (74). Neratinib binds primarily to EGFR (HER1) and HER4, thereby blocking signaling. The ExteNET study demonstrated a significant reduction in the risk of disease recurrence when neratinib was initiated within 1 year of trastuzumab-based therapy in a population of patients with treated HER2+/HR+ BC (75). Lapatinib, a selective oral tyrosine kinase inhibitor (TKI), was evaluated in the NeoALTTO study, which found that combining lapatinib with trastuzumab increased the complete remission rate in patients with HER2+ eBC. However, no long-term benefits were observed in subsequent follow-ups (76). Tucatinib, in combination with trastuzumab and capecitabine, significantly reduced the risk of intracranial progression and the risk of death in patients with brain metastases from HER2 + BC who were already receiving treatment in the HER2CLIMB trial (77).

Antibody-drug conjugates (ADCs) are monoclonal antibodies that are covalently linked to biologically active cytotoxic drugs through chemical linkers. This design allows them to specifically target tumor tissues and release cytotoxic agents, enhancing therapeutic efficacy while minimizing adverse effects (78–80). Enmetrastuzumab (T-DM1) was the first FDA-approved drug for the treatment of HER2+ MBC and ADCs that did not achieve pathologic complete remission (pCR) after neoadjuvant therapy. In the EMILIA study, T-DM1 was shown to prolong mPFS and OS compared to capecitabine combined with lapatinib in patients with HER2+ MBC (81). In the KATHERINE study, patients with eBC who did not achieve a pCR after neoadjuvant therapy for HER2+ achieved longer PFS with enmetrastuzumab compared to receiving trastuzumab monotherapy (82). Trastuzumab deruxtecan (T-Dxd) has exhibited durable antitumor activity in the DESTINY-Breast01 clinical trial involving previously treated HER2-positive MBC patients. However, in the DESTINY-Breast03 trial, T-Dxd did not show a significant improvement in PFS compared to T-DM1 for previously treated HER2-positive MBC patients receiving trastuzumab and paclitaxel analogs, although it did result in a notable increase in mPFS for patients with HER2-positive breast cancer (83).

Trastuzumab has been validated as the first HER2-targeted agent that improves outcomes for patients with HER2-low or metastatic breast cancer. In the DESTINY-Breast04 trial, it significantly prolonged the survival of patients with HER2-overexpressing metastatic breast cancer who had previously received chemotherapy, regardless of HR status, when compared to chemotherapy alone (84). DESTINY-Breast06, on the other hand, further demonstrated the role of T-Dxd in patients with HER2 low or HER2 -ultralow /HR+ ABC (85). In addition, the PILHLE-001 trial demonstrated a lower residual cancer burden with neoadjuvant pyrotinib in combination with chemotherapy in HER2- high-risk EBC (86) (Table 3).

Tumor cells, immune cells, fibroblasts, cytokines released by stromal and related cells, and non-cellular elements like microvessels make up the complex ecosystem known as the tumor microenvironment. These elements are essential for controlling the fundamental survival and sustaining the function of tumor cells (87). Different solid tumors have distinct tumor microenvironment compositions, and breast cancer usually has large concentrations of immunosuppressive cells and cancer-associated fibroblasts (CAF) (88). Furthermore, different breast cancer subtypes have different tumor microenvironments; HR+ breast cancers, in contrast to triple-negative and HER2+ breast cancers, show lesser immune cell infiltration and a worse response to immune checkpoint blockade (76). Potential targets for tumor therapy have been investigated in recent attempts to target the tumor microenvironment. Vascular endothelial growth factor and fibroblast growth factor are the two targets of this review.

Angiogenesis plays a critical role in the formation and dissemination of tumors by supplying oxygen and nutrients to the tumor, which promotes tumor growth. Vascular endothelial growth factor (VEGF) is produced in response to the tumor’s high pressure, low oxygen content, and low pH, which encourages the development of neovascularization. In order to facilitate tumor growth and metastasis, these new blood vessels typically lose their natural structure, take on a convoluted shape, and have increased permeability. Additionally, VEGF suppresses dendritic cell maturation and antigen presentation by recruiting and inhibiting anti-tumorigenic regulatory T cells by chemotactic action (89) (Figure 4). Interestingly, a separate study found that the size of big arteries in the tumor vasculature was likewise linked to a lower survival rate in HR+ breast cancer (90). Thus, inhibition of vascular growth is considered to have antitumor effects in HR+ BC.

Bevacizumab is an anti-VEGF monoclonal antibody that the FDA currently approves for the treatment of advanced metastatic breast cancer. In the phase III clinical trial (NCT00333775), the combination of bevacizumab and docetaxel as a first-line treatment for HER2-negative, locally recurrent, or metastatic breast cancer demonstrated a slight improvement in mPFS compared to the chemotherapy-only group however, no benefit in overall survival OS was observed (91). In the RIBBON-2 clinical trial, bevacizumab was used as a second-line treatment for patients with HER2-positive metastatic breast cancer. This treatment increased the mPFS from 5.1 months to 7.2 months (HR, 0.78; 95% CI, 0.64 to 0.93; p = 0.0072) compared to chemotherapy alone. However, no OS benefit was observed (92). Bevacizumab, in combination with NCT, increased the rate of histological remission in HER2-negative breast cancer in the NCT00567554 clinical study (93). Additionally, the ROSE/TRIO-12 clinical study investigated the use of docetaxel in combination with ramucirumab, another anti-vascular growth factor inhibitor, instead of using docetaxel alone. The study found that ramucirumab did not improve the mPFS in the first-line therapy of metastatic breast cancer (94). Although several phase II clinical trials have been carried out, neither monotherapy nor combination therapy has been shown to improve survival thus far (95, 96).

The FGF/FGFR signaling pathway, which consists of fibroblast growth factor receptors (FGFR) and fibroblast growth factors (FGF), plays a crucial role in regulating cell proliferation, survival, migration, and differentiation (97, 98). Abnormalities in this pathway are frequently linked to the development and progression of cancer, as well as drug resistance. There are four subtypes of FGFRs: FGFR1, FGFR2, FGFR3, and FGFR4. Research has shown that FGFR1 amplification occurs in approximately 15% of patients with estrogen receptor-positive breast cancer. Consequently, targeting and inhibiting FGFR could be an effective treatment strategy for this type of cancer (99, 100).

Dovitinib is a multi-targeted oral tyrosine kinase inhibitor that effectively inhibits the proliferation of breast cancer cell lines that have amplified FGFR1/2. However, it does not impact cell lines with normal FGFR. In a phase II clinical study (NCT01528345), dovitinib, when combined with fulvestrant, demonstrated promising clinical efficacy for the treatment of postmenopausal breast cancer patients who were HR+ and HER2- and who had progressed despite prior ET. Unfortunately, the clinical trial has been suspended due to issues with the selection of the inclusion group (101). To better target the FGF/FGFR signaling pathway, selective FGFR inhibitors (infigratinib, erdafitinib, AZD4547, Debio1347, TAS-120) have been developed. In a phase II trial (NCT01795768), the FGFR1/2/3 inhibitor AZD4547 demonstrated potential therapeutic effects in patients with FGFR1-amplified breast tumors (102) (Table 4).

Cancer is a genetic and epigenetic disease. Epigenetic inheritance refers to heritable changes in gene expression or cellular expression caused by various modifications to DNA, such as methylation, acetylation, and chromatin remodeling, without altering the DNA sequence. Histone acetyltransferases (HAT) acetylate histones, which results in more open or loose chromatin structure causing gene expression. Histone deacetylases remove acetyl groups from histones, favoring chromatin compaction. Usually associated with gene silencing (110, 111). By inhibiting HDAC, histones remain acetylated, which allows for the reactivation of silenced anti-oncogenes and promotes tumor apoptosis (Figure 6). Additionally, HDAC is a key regulator of various tumor-related physiological processes, including angiogenesis, cell cycle regulation, immune response, DNA repair, and apoptosis. Therefore, blocking HDAC can inhibit tumor growth by targeting multiple pathways.

Histone deacetylases (HDACs) are categorized into four main classes: I, II, III, and IV. Cell nuclei are where Classes I and II mostly operate. In contrast, class III HDACs, also known as sirtuins, require NAD+ and play a role in a broader range of cellular processes, including metabolism and cell aging (senescence). Class IV HDACs are predominantly expressed in the brain, heart, testis, and kidneys.

Tucidinostat is a subtype-selective histone deacetylase (HDAC) inhibitor that explicitly inhibits Class I and II (112). In the ACE clinical trial, treatment with tucidinostat in combination with exemestane significantly improved patient prognosis and slowed disease progression compared with exemestane alone in postmenopausal endocrine-resistant advanced HR+ breast cancer (113).

Entinostat, a selective inhibitor of HDAC I and IV, showed a significant improvement in mPFS and mOS in the phase II (ENCORE301) trial of exemestane plus entinostat in HR+ ABC patients who had progressed on prior endocrine therapy alone, compared with the exemestane alone group (114). Entinostat in combination with exemestane did not result in a survival benefit for breast cancer patients in the subsequent phase III clinical study of E2112 (115) (Table 5).