Predictive, preventive, and personalized medicine in breast cancer: targeting the PI3K pathway

The PIK3CA gene, which encodes the p110α catalytic subunit of PI3K [36, 37], is frequently mutated in BC [38, 39]. Studies have shown that patients with PIK3CA mutations tend to have increased activation of PI3K pathway and may have increased sensitivity to PI3K inhibitors [38]. Identifying PIK3CA mutations through testing can assist in determining the potential effectiveness of PI3K inhibitor therapy for patients.

Several biomarkers tested in tumor tissue samples provide additional information on PI3K pathway activation and response to PI3K inhibitors. These include markers such as phosphorylated AKT (pAKT) [40], and phosphorylated S6 (pS6) [41] that indicate activation of the PI3K pathway.

PI3K inhibitors can target different isoforms of the PI3K enzyme [42]. A biomarker that indicates the presence or activation of a particular isoform will help select the most appropriate PI3K inhibitor for a particular patient. For example, PIK3CA mutations are more commonly linked with sensitivity to PI3Kα inhibitors.

Liquid biopsy, which analyzes circulating tumor DNA (ctDNA) or circulating tumor cells (CTCs) in blood [43, 44], provides a non-invasive approach to assess PIK3CA mutations and monitor treatment response [45, 46]. Detection of PIK3CA mutations in ctDNA or CTCs may help select patients for PI3K inhibitor therapy.

Next-generation sequencing, such as genomic profiling, enables comprehensive analysis of the genetic landscape of breast cancer [47]. This approach allows us to identify additional genetic alterations that may affect response to PI3K inhibitors, including alterations in other genes involved in PI3K signaling and genes associated with therapy resistance.

It is important to note that although these biomarkers are associated with response to PI3K inhibitors, they do not conclusively predict individual patient outcome. Further validation with clinical trials and real-world data is needed to optimize patient selection for PI3K inhibitor therapy. Ongoing research aims to identify additional biomarkers and develop more accurate predictive models for personalized BC treatment.

The PTEN gene, short for phosphatase and tensin homolog, is pivotal in suppressing tumors. It aids in regulating the PI3K/AKT pathway, which is crucial for maintaining cellular functionality and preventing abnormal cell proliferation [48]. This pathway is complicated in the control of some cellular processes such as cell growth [49, 50], survival [51] and metabolism [52]. The PTEN gene acts as a negative PI3K/AKT pathway regulator, inhibiting its activation and maintaining cellular homeostasis [53, 54].

Loss of PTEN expression or PTEN gene mutation, another biomarker, is linked with enhanced activation of the PI3K pathway and potential susceptibility to PI3K inhibitors [55]. Moreover, as PIK3CA mutations are more common in ER+ BC [56], hormone receptor status, particularly estrogen receptor (ER) positivity, is associated with the potential efficacy of PI3K inhibitors in this subtype.

Loss of PTEN function due to genetic mutation or epigenetic silencing disrupts the PI3K/AKT pathway, leading to increased activation and tumor formation [57]. The dysregulation of PTEN can result in uncontrolled cell proliferation and survival, key factors in the development and progression of cancer. Specifically, BC patients who have experienced loss of PTEN function tend to exhibit a greater potential for therapeutic benefits from PI3K inhibitors [58]. By specifically targeting the overactive HER PI3K/AKT pathway, these inhibitors could restore the balance of cellular processes and potentially inhibit cancer cell proliferation and metastasis.

AKT1 (v-AKT Murine Thymoma Viral Oncogene Homolog 1) has emerged as a prospective predictive biomarker in breast cancer, offering valuable insights into disease prognosis and treatment response. Beyond its role as a signaling molecule, AKT1 functions as a genomic marker, facilitating the tailoring of treatment strategies for individual patients [31]. The identification of specific genetic alterations within the AKT1 gene, particularly within the pleckstrin homology domain (PH), has been substantiated in breast cancer cases. These somatic mutations lead to the constitutive activation of AKT1 and disrupt the normal regulatory mechanisms governing its activation [59, 60].

Notably, the prevalence of AKT1 mutations is notably higher in specific breast cancer subtypes, particularly HR+ and HER2− tumors [60]. Serving as a predictive biomarker, AKT1 mutations exert an influence on disease prognosis. Studies indicate that breast cancer patients harboring AKT1 mutations may exhibit distinct clinical features and outcomes. These mutations correlate with specific clinicopathological features such as tumor size, grade, and lymph node metastasis, thereby providing additional information for stratifying patients based on their risk profiles [61, 62].

Moreover, investigations into AKT1 mutations aim to predict responses to targeted therapies within the PI3K/AKT/mTOR signaling pathway, where AKT1 assumes a central role [31, 59]. Understanding AKT1 mutation status enables the identification of patients who may benefit from targeted inhibitors of the PI3K pathway [63, 64]. This personalized treatment approach aligns with the principles of precision medicine, allowing the selection of potentially effective treatments based on specific genetic changes in a patient's tumor.

Clinical studies evaluating the efficacy of AKT inhibitors in breast cancer patients with AKT1 mutations underscore the role of AKT1 as a predictive biomarker. These studies seek to assess the safety and efficacy of targeted therapies that directly inhibit the aberrant signaling caused by AKT1 mutations. If successful, these inhibitors hold promise as a treatment option for certain breast cancer patients with AKT1-mutated tumors.

Phosphoinositide 3-kinase regulatory subunit 1 (PIK3R1) serves as the regulatory subunit of the PI3K, a pivotal player in cell signaling pathways governing growth, survival, and metabolism. PIK3R1 plays a crucial role in modulating the activity of the catalytic subunit of PI3K, thereby exerting influence over downstream signaling events [65].

In the realm of breast cancer, PIK3R1 has emerged as a prospective predictive biomarker, offering valuable insights into disease prognosis and treatment responsiveness. Genetic alterations in PIK3R1, encompassing both somatic mutations and copy number variations, have been discerned in breast cancer. These alterations have the potential to induce dysregulation within the PI3K signaling pathway, contributing to unbridled cellular proliferation and enhanced survival capabilities [65].

Of significance is the association of PIK3R1 mutations with specific breast cancer subtypes, suggesting potential implications for disease stratification [66]. As a predictive biomarker, PIK3R1 mutations have been linked to diverse clinical outcomes in breast cancer patients [65]. Investigations have delved into the correlation between PIK3R1 status and various clinicopathological features, including tumor grade, hormone receptor status, and HER2 amplification.

An understanding of the specific molecular characteristics of tumors, particularly the presence of PIK3R1 mutations, holds promise for refining prognostic assessments and tailoring treatment strategies. Furthermore, PIK3R1 status assumes importance in predicting responses to targeted therapy [67]. The PI3K signaling pathway, in which PIK3R1 is a critical component, stands out as a crucial therapeutic target in breast cancer [68]. Clinical trials involving PI3K inhibitors have been conducted, and the presence of PIK3R1 mutations may exert an influence on treatment responsiveness.

Identification of breast cancer patients with PIK3R1 mutations presents an opportunity to select individuals poised to derive greater benefit from PI3K pathway inhibitors, aligning with the principles of precision medicine. The study of PIK3R1 as a predictive biomarker extends beyond its individual role. Through a comprehensive analysis encompassing multiple genomic changes within the PI3K pathway, including not only PIK3R1 mutations but also mutations in PIK3CA and PTEN, a more nuanced understanding of the molecular landscape of breast cancer emerges. This integrated approach enhances the ability to predict patient outcomes and make well-informed therapeutic decisions.

Inositol polyphosphate 4-phosphatase type II (INPP4B) is a phosphoinositide phosphatase that plays a pivotal role in the regulation of intracellular signaling pathways, particularly the phosphoinositide 3-kinase (PI3K) pathway. Functionally, INPP4B acts as a tumor suppressor by exerting negative regulation on PI3K signaling, a signaling cascade crucial for controlling cell proliferation, survival, and metabolism. In breast cancer, INPP4B has emerged as a potential predictive biomarker, offering valuable insights into disease prognosis and treatment response [69, 70].

Genetic alterations in INPP4B, including mutations and changes in expression levels, have been identified in breast cancer. Particularly noteworthy is the common occurrence of the loss of INPP4B expression in breast tumors, resulting in heightened activity of the PI3K signaling pathway. This dysregulation contributes significantly to the uncontrolled cellular proliferation and survival characteristic of cancer [69].

As a predictive biomarker, the status of INPP4B is associated with clinical outcomes in breast cancer patients. Rigorous studies have explored the correlation between INPP4B expression levels and various clinicopathological features, such as tumor grade, hormone receptor status, and HER2 amplification. The loss of INPP4B expression has been consistently linked to a more aggressive tumor phenotype and poor prognosis, underscoring its potential as a robust prognostic indicator [71].

Moreover, the predictive utility of INPP4B extends to its role in determining the response to specific therapeutic interventions. Preclinical investigations have suggested that breast cancer cells exhibiting reduced INPP4B expression may display heightened sensitivity to selective inhibitors of the PI3K pathway, presenting a promising avenue for targeted therapeutic strategies [72].

The comprehensive research on INPP4B as a predictive biomarker also encompasses understanding its role within the broader molecular landscape of breast cancer [71]. Integrative analyses, including the evaluation of INPP4B alongside other genetic alterations within the PI3K pathway, are poised to contribute to a more nuanced and holistic understanding of the disease's molecular underpinnings. This integrated approach holds significant potential for refining prognostic assessments and tailoring treatment strategies in breast cancer patients.

Cell lines have emerged as essential preclinical models for evaluating the efficacy of PI3K inhibitors in BC research (Fig. 2A) [73, 74]. These models have several advantages (Table 1), including ease of establishment, controlled culture conditions, and high-throughput screening capabilities. The researchers used cell lines to gain valuable insight into the molecular mechanisms of changes in PI3K signaling that commonly occur in BC.

The establishment and characterization of BC cell lines is a critical step in preclinical research [75]. Methods to establish these cell lines from patient tumor samples or cell line repositories have been refined over the years [76, 77]. In addition, extensive molecular characterization, including identification of alterations in the PI3K signaling pathway, such as mutations in the PIK3CA gene [78, 79], ensures that the cell lines used in our experiments accurately represent the genetic landscape of BC.

Cell line studies can gain insight into the mechanism of response to PI3K inhibitors [80]. Researchers can manipulate gene expression in these models using techniques such as RNA interference (RNAi) [81], and CRISPR-Cas9 [82, 83], enabling targeted studies of the role of specific genes and signaling pathways. Conducting these studies will enhance our understanding of the molecular mechanisms underlying drug response and resistance, ultimately facilitating the advancement of novel therapeutic approaches and driving progress in the field of medicine.

Cell lines also serve as valuable tools for studying combinations of PI3K inhibitors and other agents [84, 85]. Evaluating drug combinations, such as chemotherapeutic agents and targeted therapies, allows researchers to identify potential synergies and optimize therapeutic approaches. These studies provide important insights into potential combination strategies to improve the efficacy of PI3K inhibitors in BC treatment.

Despite their advantages, it is important to recognize the limitations of cell line models (Table 1). They lack the in vivo tumor microenvironment, interaction with the immune system, and three-dimensional structural complexity observed in patient tumors. Therefore, it is important to evaluate the results of cell line studies using more clinically relevant models. Patient-derived xenografts or transgenic mouse models prior to being placed in clinical trials.

Patient-derived xenografts (PDX) have proven to be a valuable preclinical model for evaluating the efficacy of PI3K inhibitors in BC research (Fig. 2B) [86]. In these models, the patient's own BC tissue is directly transplanted into immunodeficient mice, preserving the heterogeneity and complexity of the original tumor microenvironment.

PDX models provide a more clinically relevant representation of human breast tumors compared to traditional cell line models [87]. PDX models have several advantages (Table 2), such as using the patient's own tissue ensures that the molecular and genetic features of the tumor are preserved, including the presence of alterations in the PI3K signaling pathway that are common in BC patients. This will allow researchers to assess the effects of PI3K inhibitors on tumors that closely resemble the patient's original tumors, increasing the translatability of the results. One of the key advantages of PDX models is their ability to recapitulate the tumor microenvironment [88]. Breast tumors reside in a complex ecosystem composed of stromal cells [89], immune cells [90], and blood vessels [91]. The PDX model retains these elements, allowing the study of tumor-stromal interactions and the effects of PI3K inhibitors on the tumor microenvironment. Assessing the effectiveness of PI3K inhibitors holds significant relevance, given the crucial involvement of the PI3K signaling pathway in mediating communication between tumor cells and their microenvironment. This understanding helps shed light on the interplay between these factors and enables informed evaluation of PI3K inhibitors' efficacy.

The PDX model also provides a platform to study the mechanisms of tumor response and resistance to PI3K inhibitors [92, 93]. Longitudinal studies can be performed by treating PDX mice with PI3K inhibitor and monitoring tumor growth and molecular changes over time. This enables the discovery of potential predictive biomarkers for response and resistance, which can inform patient stratification and create personalized therapeutic strategies.

It is important to recognize the limitations of PDX models (Table 2), such as the reliance on immunodeficient mice and the time required to establish and disseminate these models. Furthermore, transplant success rates vary with tumor type and individual patient characteristics. Nevertheless, the PDX model remains a valuable tool for the preclinical evaluation of PI3K inhibitors in BC and provides a more clinically relevant predictive model system.

The genetically engineered mouse model (GEMM) has proven to be a valuable preclinical model for evaluating the efficacy of PI3K inhibitors in BC research. These models involve genetically engineering mice to introduce specific mutations in genes associated with BC, such as the PIK3CA gene, which encodes the PI3K catalytic subunit (Fig. 3) [94].

GEMM offers several advantages for the study of PI3K inhibitors in BC (Table 3). By introducing specific genetic alterations, GEMM can closely mimic the molecular properties of human BC, such as activation of the PI3K pathway [94]. This will allow researchers to study the effects of PI3K inhibitors on tumorigenesis, progression and response to therapy in vivo. One of the main strengths of GEMM is its ability to represent the complex nature of BC development and progression. These models mimic the stepwise progression from preinvasive lesions to invasive breast tumors and provide insight into the earliest events of tumorigenesis and the effects of PI3K inhibitors at various stages. GEMM can also be used to assess tumor growth dynamics, metastasis, and interactions between tumors and the surrounding microenvironment [95, 96].

GEMM allows assessment of both the therapeutic efficacy and potential toxicity of PI3K inhibitors [97]. Researchers can monitor tumor response to treatment by measuring tumor volume, assessing histopathological changes, and analyzing molecular changes in tumor tissue. In addition, GEMM provides an opportunity to study possible side effects of PI3K inhibitors on normal tissues and organs and helps identify potential off-target side effects.

Furthermore, GEMM enables mechanistic studies to understand the molecular mechanisms underlying response and resistance to PI3K inhibitors [98]. By manipulating gene expression within specific cell types or specific tumor subtypes, researchers can uncover the role of individual genes or pathways in mediating drug sensitivity or resistance [99, 100]. This information can guide the development of combination therapies and the identification of predictive biomarkers for patient stratification.

Despite its advantages, GEMM also has limitations (Table 3). Creating and maintaining them can be time-consuming and expensive. The genetic alterations introduced into GEMM may not fully reflect the mutational landscape found in human BC. Therefore, it is critical to integrate insights from GEMM with other preclinical models and clinical data to understand the effects of PI3K inhibitors in BC fully.

3D organoids have emerged as promising preclinical models for evaluating the efficacy of PI3K inhibitors in BC research [101]. These models are derived from patient-derived cells and self-assemble into three-dimensional structures that mimic the architecture and cellular heterogeneity of the original tumor [102].

There are various advantages of the 3D organoids model (Table 4). One of the major advantages of 3D organoids is their ability to recreate complex tumor microenvironments in a controlled laboratory environment [103]. 3D organoids provide a more physiologically relevant model for assessing the effects of PI3K inhibitors. 3D organoids also preserve patient tumours' genetic and molecular characteristics, such as altered PI3K signaling. This will allow researchers to study the effects of PI3K inhibitors on tumor growth, survival and response to therapy in settings that mimic patient tumors. By using the patient's own cells, 3D organoids offer a personalized approach to drug screening, allowing assessment of individual patient response to PI3K inhibitors.

Furthermore, 3D organoids show structural and functional similarities to the original tumor, enabling the study of important aspects such as invasion and metastasis [104]. These models allow researchers to assess the effects of PI3K inhibitors on the development of tumor invasion and metastatic potential. By incorporating features such as extracellular matrix components, blood vessels, and lymphatics, 3D organoids enable a more comprehensive understanding of the effects of PI3K inhibitors on tumor behavior.

Another advantage of 3D organoids is the potential for high-throughput drug screening [105, 106]. The ability to generate multiple organoids simultaneously allows researchers to quickly and inexpensively test a wide range of PI3K inhibitors and combination therapies. This screening approach provides valuable insight into the efficacy of PI3K inhibitors and helps identify potential drug candidates for further development.

Additionally, 3D organoids offer the potential for long-term culture and monitoring of therapeutic response [107]. Researchers can track changes in organoid growth, morphology, and molecular profile over time, allowing them to assess both short-term and long-term effects of PI3K inhibitors. This longitudinal approach helps identify potential resistance mechanisms and paves the way for developing strategies to overcome drug resistance.

Although 3D organoids have many advantages, they also have some limitations (Table 4). Compared to in vivo models, they may not fully capture the complexity of the tumor microenvironment. The lack of immune cells within the organoids and the limited vasculature may limit the assessment of immune responses and drug penetration. However, efforts are being made to integrate immune cells and vasculature into organoid models to address these limitations.

Co-culture models have emerged as valuable preclinical models for evaluating the efficacy of PI3K inhibitors in BC research. These models utilize co-cultivation of diverse cell types, such as tumor cells and stromal cells, in close proximity to replicate the intricate interactions observed within the tumor microenvironment [108].

Co-culture models have several advantages (Table 5). One of the major advantages of co-culture models is their ability to capture the complexity and heterogeneity of the tumor microenvironment [109]. Breast tumors consist of malignant cells and diverse stromal cell types, including fibroblasts, endothelial cells, and immune cells [110]. The utilization of co-culture models enables the investigation of interactions between tumor and stromal cells, which hold significant importance in both tumor progression and response to therapeutic interventions [111]. These models provide a more physiologically relevant environment for evaluating the effects of PI3K inhibitors.

Co-culture models also facilitate the study of drug resistance mechanisms with PI3K inhibitors [112]. It is known that the tumor microenvironment may contribute to drug resistance by providing cancer cells with a protective niche. By co-culturing tumor cells with stromal cells, researchers can study the effects of PI3K inhibitors on the development of drug resistance and identify possible molecular mechanisms involved. This information is important for developing strategies to overcome resistance and improve the efficacy of PI3K inhibitors in BC therapy.

Despite its advantages, the co-culture model has limitations (Table 5). They lack the spatial organization and three-dimensional structure observed in patient tumors and may not fully capture the complexity of the tumor microenvironment in vivo. Furthermore, choosing the appropriate stromal cell types and their ratios in the co-culture system is critical to ensure the relevance and accuracy of the model.

PI3K inhibitors exert their prophylactic effects by potently blocking PI3K activity, thereby inhibiting the conversion of phosphatidylinositol lipids into PIP3 [143]. This inhibition is vital in decreasing the activation of downstream effectors such as AKT and mTOR [115].

The PI3K/AKT/mTOR signaling pathway helps promote cell survival, growth and angiogenesis [11]. By suppressing this signaling cascade, PI3K inhibitors interfere with key cellular processes such as proliferation and survival that are required for the development and progression of BC.

By inhibiting PI3K, the formation of PIP3, a crucial activator of AKT, is prevented. As a result, reduced AKT activity restricts the activation of downstream targets involved in cell proliferation and survival [144]. Furthermore, inhibition of mTOR, another downstream effector of PI3K, interferes with regulation of protein synthesis and cell cycle progression [145]. By targeting both AKT and mTOR, PI3K inhibitors comprehensively block multiple signaling pathways that drive tumor growth and BC progression.

Overall, inhibition of the PI3K/AKT/mTOR signaling pathway by PI3K inhibitors disrupts a complex network of cellular processes that contribute to BC development. This markedly reduces cell proliferation, promotes apoptosis, inhibits angiogenesis, and ultimately prevents BC tumor formation and progression.

An essential mechanism contributing to the protective impact of PI3K inhibitors in BC involves their capability to induce cell cycle arrest [146], and promote apoptosis [147], in BC cells. Dysregulation of the PI3K signaling pathway often disrupts the fine-tuned control of cell cycle progression, allowing cancer cells to evade cell cycle checkpoints and proliferate uncontrollably.

PI3K inhibitors play a central role in restoring proper cell cycle control by targeting key proteins involved in cell cycle progression. By inhibiting the activity of CDKs, central cell cycle regulators, these inhibitors halt cell cycle progression and prevent uncontrolled proliferation of BC cells [148]. PI3K inhibitors suppress tumor growth and prevent BC development by restoring cell cycle checkpoints [149]. In addition, PI3K inhibitors exert a preventive effect by promoting apoptosis, a natural mechanism that eliminates damaged or abnormal cells. Dysregulation of the PI3K signaling pathway often confers anti-apoptotic properties on cancer cells, enabling cancer cell survival and resistance to cell death signals. PI3K inhibitors counter this effect by interfering with anti-apoptotic proteins and promoting the activation of pro-apoptotic signaling pathways [150]. These inhibitors can directly inhibit proteins such as Bcl-2 that inhibit apoptosis, thus shifting the balance towards apoptosis induction in BC cells.

Angiogenesis, the process of forming new blood vessels, is crucial in tumor growth and metastasis. The PI3K signaling pathway assumes a central role in promoting angiogenesis by stimulating the production of pro-angiogenic factors, particularly vascular endothelial growth factor (VEGF) (Fig. 4) [50, 151].

PI3K inhibitors offer a promising approach to prevent angiogenesis in BC [152]. These inhibitors target the dysregulated PI3K signaling pathway and effectively attenuate angiogenesis by inhibiting the expression and secretion of pro-angiogenic factors. In particular, inhibition of PI3K signaling reduces the production of VEGF, a potent inducer of angiogenesis [151]. By inhibiting VEGF and other pro-angiogenic factors, PI3K inhibitors disrupt the delicate balance between stimulators and inhibitors of angiogenesis, thereby inhibiting the formation of new blood vessels required for tumor growth and progression. This decreased angiogenesis limits the blood supply to tumors, deprives them of vital nutrients and oxygen, and ultimately inhibits tumor growth.

Moreover, inhibition of PI3K signaling in endothelial cells lining the inner walls of blood vessels directly affects the cells' ability to form new blood vessels in response to pro-angiogenic signals [153]. PI3K inhibitors disrupt key signaling pathways involved in endothelial cell migration, proliferation, and tubule formation, thereby contributing to the suppression of angiogenesis.

Targeting the PI3K signaling pathway and inhibiting angiogenesis, PI3K inhibitors provide a valuable strategy for preventing the development and progression of BC. By inhibiting pro-angiogenic factors like VEGF, these inhibitors effectively disrupt the formation of new blood vessels, compromising tumor nutrition and impeding tumor growth and metastatic capabilities.

Hormone receptor-positive BC is characterized by the presence of estrogen or progesterone receptors on cancer cells, which respond to hormone stimulation and promote tumor growth [154]. The PI3K signaling pathway has been shown to interact with hormone receptor signaling pathways, leading to hormone resistance and contributing to tumor progression in hormone receptor-positive BC [155].

A key mechanism underlying the preventive effects of PI3K inhibitors is disruption of the crosstalk between PI3K signaling and hormone receptor signaling. By targeting the dysregulated PI3K pathway, these inhibitors effectively interfere with communication between the PI3K pathway and hormone receptor signaling, allowing HR+ BC cells to respond to antiestrogenic drugs such as tamoxifen [155].

The PI3K pathway activates downstream effectors such as AKT that directly phosphorylate hormone receptors and alter their activity [143]. This phosphorylation increases hormone receptor activity and may promote resistance to endocrine therapy. PI3K inhibitors counteract this effect by inhibiting AKT activation and downstream signaling, thereby attenuating hormone receptor phosphorylation and activation.

In addition, PI3K inhibitors may modulate other components of hormone receptor signaling pathways. For example, these inhibitors may affect the expression and function of co-regulatory proteins that interact with hormone receptors, thereby affecting hormone receptor activity and response to endocrine therapy.

Combining PI3K inhibitors with endocrine therapy synergistically prevents HR+ BC by targeting multiple signaling pathways, addressing complexity, and improving patient outcomes.

In addition to their direct effects on cancer cells, PI3K inhibitors have shown remarkable immunomodulatory properties by influencing the tumor microenvironment (TME) and enhancing anti-tumor immune responses [156]. The PI3K signaling pathway is vital in suppressing immune cell function and promoting an immunosuppressive environment within tumors [157].

PI3K inhibitors counteract these immunosuppressive effects by promoting immune cell activity and promoting immunostimulatory TME. One of the key mechanisms by which PI3K inhibitors exert immunomodulation is by enhancing the function of immune cells such as T cells and natural killer (NK) cells [158]. Activation of the PI3K signaling pathway in immune cells can impair the cytotoxic capacity of immune cells and weaken anti-tumor responses. By inhibiting PI3K, these inhibitors restore immune cell function and enhance the ability to recognize and eliminate cancer cells.

Furthermore, PI3K inhibitors modulate the TME and promote an immunostimulatory environment by hindering the recruitment and activation of immunosuppressive cells like regulatory T cells (Treg) [159]. Furthermore, PI3K inhibitors can reduce the production of immunosuppressive factors such as cytokines and chemokines while promoting the secretion of immunostimulatory factors [160].

The immunomodulation exerted by PI3K inhibitors helps prevent tumor formation and progression. By enhancing immune cell activity and promoting an immune-stimulating TME, these inhibitors facilitate the immune system's recognition and elimination of cancer cells, thereby hindering tumor growth and metastasis.

The intricate interplay between metabolism and PI3K inhibitors in breast cancer underscores the nuanced nature of targeted therapy. Metabolically, breast cancer cells prominently exhibit the Warburg effect, marked by heightened glucose uptake and glycolysis to sustain rapid cellular proliferation [161, 162]. Altered lipid metabolism and increased reliance on specific amino acids further accentuate the metabolic demands associated with abnormal growth.

PI3K inhibitors specifically intervene in this metabolic milieu by targeting the dysregulated PI3K signaling pathway, a common feature in breast cancer progression [163]. Mutations in the PIK3CA gene contribute to the hyperactivation of PI3K signaling, underscoring the therapeutic significance of PI3K inhibitors [164, 165]. Designed to selectively target isoforms such as p110α, PI3K inhibitors disrupt downstream signaling events involving molecules like AKT and mTOR. This targeted intervention disrupts the orchestrated processes that maintain cancer cell viability and uncontrolled growth [163].

This precision in targeting aligns with the metabolic vulnerabilities of breast cancer cells, presenting a strategic approach to disrupt signaling cascades intricately linked to cellular metabolism [166]. Clinical trials evaluating PI3K inhibitors, such as alpelisib and taselisib, have demonstrated their potential in breast cancer treatment. These studies explore the synergistic effects of PI3K inhibitors in combination therapy, often integrating them with endocrine therapy, HER2-targeted therapy, or chemotherapy to maximize therapeutic efficacy [34, 167].

Patient stratification based on molecular profiling becomes paramount to optimize the prophylactic efficacy of PI3K inhibitors [168]. Identifying PIK3CA mutations enables a personalized approach, ensuring that individuals most likely to benefit from PI3K inhibitor therapy are precisely targeted [169, 170]. This tailored strategy not only optimizes treatment outcomes but also minimizes potential side effects [171, 172].

Despite these advancements, challenges persist, including the emergence of resistance to PI3K inhibitors and the intricate regulatory mechanisms within the PI3K signaling pathway [173]. Ongoing research efforts aim to unravel this complexity, refining preventive strategies and providing insights to further advance breast cancer treatments [6, 174]. The dynamic interactions between metabolism and PI3K inhibitors offer a promising avenue for the development of more effective and personalized therapies in the context of breast cancer [175].

Cancer cells show remarkable adaptability to PI3K inhibition, which can activate alternative signaling pathways and circumvent the silencing effects of PI3K inhibitors. A key mechanism in these cells is the activation of parallel signaling pathways such as the MAPK pathway (mitogen-activated protein kinase) and the mTOR pathway. Cancer cells can evade the blocking of PI3K signaling by using these signalling pathways, thus ensuring cancer cell survival and uncontrolled proliferation (Fig. 6) [9].

The MAPK signaling pathway is pivotal in governing fundamental cellular processes, including cell proliferation and division [239, 240]. In response to PI3K inhibition, cancer cells may activate the MAPK pathway as an alternative way to sustain proliferation. Through this activation, cancer cells can evade the effects of PI3K inhibitors, facilitating their survival and proliferation despite the presence of these targeted therapies.

The mTOR pathway, a critical regulator of cell proliferation and protein synthesis, acts as a compensatory pathway when PI3K is inhibited. Cancer cells can upregulate mTOR pathway activity, even in the presence of PI3K inhibitors. This allows them to sustain growth and proliferation despite the blockade of PI3K signaling [241].

By activating these parallel signaling pathways, cancer cells evade the target's inhibition of PI3K, thereby conferring resistance to PI3K inhibitors. This adaptive response highlights the complex nature of cancer and the challenges in developing effective therapeutic strategies against dysregulated PI3K signaling.

Mutations in the PI3K signaling pathway pose a significant challenge to the efficacy of PI3K inhibitors in cancer therapy, particularly secondary to key components such as the PIK3CA gene and other essential components of the signaling pathway [194]. Mutations can impair the efficacy of PI3K inhibitors. These mutations can cause structural or functional alterations in PI3K or its downstream targets, thereby reducing the efficacy of inhibitors that block signaling pathways. As a result, cancer cells harboring such mutations retain the ability to maintain dysregulated activation of the PI3K pathway, ultimately resulting in resistance to PI3K-targeted therapy [242].

Specifically, mutations in the PIK3CA gene, responsible for encoding the PI3K catalytic subunit, can lead to the continuous activation of PI3K signaling, rendering it unaffected by inhibitors [243]. These mutations often involve changes in key domains of the protein, such as the kinase domain, resulting in increased enzymatic activity and maintenance of downstream signaling. Furthermore, mutations in other components of the PI3K signaling pathway, including PTEN, impair negative feedback regulation and enhance activation of signaling pathways, leading to resistance [244].

The adaptability of cancer cells to PI3K inhibition is demonstrated by their ability to upregulate compensatory pathways such as the MAPK and mTOR pathways [126]. These alternative pathways act as countermeasures against PI3K blockade, allowing cancer cells to maintain proliferation despite inhibitory effects.

Compensatory upregulation of the MAPK signaling pathway is common in cancer cells in the presence of PI3K inhibitors [245]. The MAPK signaling pathway plays a pivotal role in governing cellular processes including cell growth, proliferation, and survival. Cancer cells can exploit this pathway by amplifying its activity, bypassing the inhibitory effects of PI3K inhibitors and promoting unchecked cancer cell proliferation. This activation ensures that critical cellular functions required for survival and proliferation are maintained even in the presence of PI3K inhibitors.

Similarly, cancer cells use upregulation strategies involving the mTOR signaling pathway as compensatory mechanisms. The mTOR pathway is a key regulator of protein synthesis, cell growth and proliferation [246]. PI3K inhibition causes cancer cells to increase the activity of mTOR pathway to compensate for loss of PI3K signaling. This upregulation provides an alternative way to sustain PI3K proliferation and overcome the inhibitory effects of PI3K inhibitors. By activating these compensatory pathways, cancer cells establish mechanisms to overcome blockade of PI3K signaling and ensure survival and proliferation. Upregulation of the MAPK and mTOR signaling pathways allows cancer cells to compensate for PI3K loss of function and maintain proliferative potential despite treatment with PI3K inhibitors.

Loss of PTEN, a key negative regulator of PI3K signaling [247], plays an important role in conferring resistance to PI3K inhibitors [248]. PTEN normally functions as a tumor suppressor by inhibiting PI3K signaling [249]. However, when PTEN is lost or inactivated by genetic alterations or other mechanisms, the PI3K pathway is overactivated. This loss of PTEN function results in dysregulation of the PI3K signaling pathway, resulting in increased cell proliferation and survival signals.

When cancer cells lose functional PTEN, they become less susceptible to the inhibitory effects of PI3K inhibitors [58]. In the absence of PTEN, the PI3K pathway is persistently misactivated, evading the intended therapeutic effects of PI3K inhibitors. As a result, cancer cells lacking functional PTEN may continue to proliferate uncontrolled and evade the inhibitory effects of PI3K inhibitors.

Loss or inactivation of PTEN can occur through various mechanisms, including genetic mutations [250], epigenetic modifications [251], and altered protein stability [252]. These alterations disrupt normal PTEN function and cause imbalances within the PI3K signaling pathway.

Understanding the importance of PTEN loss in the emergence of resistance to PI3K inhibitors is critical for the development of effective therapeutic strategies. Therapeutic approaches targeting restoration of PTEN function and alternative vulnerabilities in PTEN-deficient cancer cells may overcome resistance and improve outcomes of PI3K-targeted therapies.

A dual inhibition strategy has emerged as a powerful approach to improve the efficacy of PI3K-targeted therapy by blocking compensatory signals and overcoming resistance [253]. These strategies simultaneously target multiple components of the PI3K pathway to achieve comprehensive and synergistic blockade, ultimately improving therapeutic outcomes.

The PI3K signaling pathway consists of several key components, including PI3K itself, downstream effectors such as AKT and mTOR [145]. Cancer cells often evolve compensatory mechanisms by activating alternative signaling pathways when a single component of the PI3K signaling pathway is attacked. To circumvent this resistance, dual-inhibition approaches aim to simultaneously target multiple components of the PI3K signaling pathway. One strategy involves combined inhibition of PI3K and its downstream effector mTOR. This dual-inhibition approach effectively suppresses two key nodes in the signaling pathway, resulting in broader blockade. Simultaneous targeting of PI3K and mTOR prevents compensatory signaling through mTOR activation, enhances therapeutic efficacy, and overcomes resistance to PI3K inhibitors [33, 115].

A dual inhibition strategy targeting both PI3K and AKT maybe effective in disrupting the signaling cascade and preventing compensatory activation of AKT, leading to potent inhibition of downstream events. Combining PI3K inhibitors with agents targeting alternative pathways, such as the MAPK pathway, may further enhances therapeutic response by preventing compensatory activation of MAPK signaling and suppressing pro-survival signals. This comprehensive approach will hold promise for overcoming resistance mechanisms and improving outcomes in PI3K-associated cancers.

The development of new drugs that specifically target and overcome known resistance mechanisms is an important tool for improving cancer outcomes [254]. Identifying and targeting specific resistance mechanisms that emerge during therapy. By doing so, these new drugs have the potential to be transformative in order to increase the efficacy of targeted therapies [255].

Resistance mechanisms occur through various processes such as genetic alterations, epigenetic modifications, and activation of compensatory pathways. Understanding the specific mechanisms of resistance allows researchers to design drugs that directly target and counteract these mechanisms [255].

For example, in the context of therapies targeting the PI3K signaling pathway, new drugs may be developed that selectively block secondary mutations that impair the efficacy of PI3K inhibitors. These agents can be designed to specifically target and inhibit variants of PIK3CA and other key components of the PI3K signaling pathway, thereby increasing the sensitivity of cancer cells to PI3K inhibitors.

Additionally, drugs can be designed to restore the function of, or mimic the action of, negative regulators that are lost or inactivated during therapy. For example, if PTEN is lost, new drugs can be developed to restore PTEN expression or enhance its tumor suppressor activity. These agents can sensitize cancer cells to PI3K inhibitors and overcome resistance by restoring balance in the PI3K signaling pathway.

Another strategy involves developing drugs that specifically target and inhibit compensatory pathways that are activated in response to treatment. For example, drugs can be designed to selectively block the MAPK [256], or mTOR [257, 258] signaling pathways, which are often upregulated in response to PI3K inhibition. By inhibiting these compensatory pathways, these novel agents may prevent their activation and enhance the efficacy of therapies targeting the PI3K pathway.

Additionally, novel drugs can be developed that target specific signaling nodes or molecular targets important for resistance development. A thorough understanding of the mechanisms underlying resistance will allow researchers to identify points of weakness within signaling pathways and design drugs that selectively inhibit these targets, thereby reducing resistance.

Biomarker-based therapy has shown promise to optimise outcomes for cancer patients receiving PI3K inhibitors [259]. By identifying predictive biomarkers that can stratify patients based on their likelihood of responding to PI3K inhibitors, individualized treatment strategies can be tailored to individual profiles, resulting in improved treatment efficacy.

An important aspect of biomarker-based therapy is the identification of specific biomarkers that can reliably predict a patient's response to PI3K inhibitors. These biomarkers can be genetic, molecular, or cellular traits associated with sensitivity or resistance to PI3K inhibition. Researchers can identify and validate these biomarkers by analyzing tumor samples or performing non-invasive tests such as liquid biopsies. For example, genetic alterations in the PIK3CA gene [260, 261], which encodes the PI3K catalytic subunit, have been identified as potential predictive biomarkers. Patients with certain PIK3CA mutations may show an increased response to PI3K inhibitors. Therefore, biomarker-based therapy could identify these mutations through genetic testing and select patients with PIK3CA mutations as good candidates for PI3K inhibitor therapy.

In addition to genetic biomarkers, other molecular markers or changes in signaling pathways may also serve as predictive biomarkers. For example, altered expression or activity of PTEN, which negatively regulates PI3K signaling [262], could influence responses to PI3K inhibitors. Biomarker-based therapy can assess PTEN status through immunohistochemistry or molecular testing to identify patients who may benefit from PI3K inhibitor therapy.

Biomarker-based therapies play a crucial role in tailoring PI3K inhibitor treatments. Identifying biomarkers associated with compensatory pathway activation, such as MAPK and mTOR, can guide combination therapies. Positive biomarker profiles indicate patients likely to respond to PI3K inhibitors, while negative profiles suggest alternative treatment options or combination therapies targeting alternative pathways. This individualized approach optimizes therapeutic response and minimizes side effects, advancing precision oncology in PI3K-associated cancers.

Adaptive therapeutic approaches have emerged as dynamic strategies to optimize therapeutic efficacy and overcome resistance in cancer patients [263]. By continuously monitoring treatment response and adjusting treatment based on real-time patient-specific data, these approaches can enable personalized and dynamic adjustments to improve outcomes [264].

Central to the adaptive treatment approach is regular monitoring of treatment response using various techniques such as imaging [265], and liquid biopsy [266]. These monitoring methods provide real-time insight into a patient's tumor dynamics, allowing physicians to assess the efficacy of ongoing therapy.

Treatment strategies can be adjusted to optimize treatment efficacy based on the data collected. For example, if initial treatment with a PI3K inhibitor produces a suboptimal response or demonstrates the emergence of resistance, an adaptive approach allows for timely changes. This may include switching to an alternative PI3K inhibitor with a different binding profile or choosing a combination therapy targeting compensatory pathways identified through real-time monitoring.

Adaptive approaches also include changing treatment schedules and dosages [267]. By fine-tuning treatment regimens based on patient response and tolerability, efficacy can be maximized while minimizing toxicity [268]. For example, doses may be adjusted or treatment interrupted to manage side effects or improve drug delivery. Additionally, adaptive treatment approaches integrate the use of predictive models and algorithms to guide decision making. Combining patient-specific data such as molecular profiling, treatment history and clinical parameters with computational analysis, these models can generate individualized treatment response predictions and identify potential resistance mechanisms. Such predictive modeling enables proactive decision-making, allowing physicians to anticipate and counteract resistance before it becomes clinically apparent.

Adopting adaptive therapeutic approaches in PI3K-associated cancers requires collaboration among clinicians, researchers, and data analysts. Establishing a robust data infrastructure and clinical guidelines enables real-time collection and analysis of patient-specific data, facilitating treatment adjustments based on individual needs. This dynamic approach improves outcomes by allowing timely intervention, early resistance detection, and optimized treatment strategies.

In HER2 BC, PI3K signaling is often co-activated with HER2 signaling, which contributes to resistance and poor response to HER2-targeted therapy [269]. To meet this challenge, combinations of PI3K inhibitors and HER2-targeting agents, such as trastuzumab [270, 271], and lapatinib [272, 273], have emerged as promising therapies.

The combination of HER2-targeted therapy and PI3K inhibition in HER2+ BC aims to suppress proliferative signals from HER2 and inhibit the PI3K signaling pathway. This approach enhances anti-tumor activity and improves patient outcomes by overcoming resistance mechanisms and interfering with compensatory signaling.

In addition, this combined strategy offers the potential for synergistic effects, as the PI3K signaling pathway can regulate multiple cellular processes such as cell cycle progression, survival and metabolism that influence tumor growth and therapeutic response [274]. By blocking both pathways, combination therapy aims to achieve a more comprehensive inhibition of tumor cell signaling and proliferation. Pre-clinical trials have shown encouraging results that the combination of PI3K inhibitors and HER2-targeted agents improves objective response rates and progression-free survival in patients with HER2+ BC [275]. These results support the concept of dual pathway inhibition as a powerful strategy to overcome resistance and improve patient outcomes.

Overall, the combination of PI3K inhibitors and HER2-targeted therapy represents a rational and targeted therapeutic approach for HER2+ BC, addressing co-activation of PI3K signaling pathways, overcoming resistance mechanisms, and exerting antitumor efficacy offers the possibility of strengthening. This ultimately provides new opportunities to improve therapeutic efficacy in this specific subtype of breast cancer.

CDK4/6 inhibitors, including Palbociclib [276, 277], ribociclib [278, 279], and abemaciclib [280, 281], have demonstrated notable effectiveness in HR+ BC. These inhibitors work by targeting CDK4/6, inducing cell cycle arrest and suppressing tumor cell proliferation.

A combination of CDK4/6 inhibitors and PI3K inhibitors has proven to be a promising therapeutic strategy. Preclinical and clinical studies have provided evidence of synergy when these two inhibitors are used together [282]. The combination of PI3K inhibitors and CDK4/6 inhibitors in HR+ breast cancer is driven by their complementary mechanisms. PI3K inhibitors suppress cell survival and proliferation by targeting the PI3K signaling pathway, while CDK4/6 inhibitors directly block cell cycle progression at the G1/S phase transition by inhibiting Rb phosphorylation.

Combining these inhibitors may enhance inhibition of tumor growth. This combination synergistically disrupts both the cell cycle and PI3K signaling pathways, resulting in a more comprehensive inhibition of tumor cell proliferation. This approach addresses potential resistance mechanisms associated with single-pathway targeting and has broad-spectrum effects on tumor growth control [283].

Clinical trials investigating the combination of CDK4/6 inhibitors and PI3K inhibitors in HR+ BC have demonstrated promising results, including longer progression-free survival, improved objective response rate, and longer duration of response compared to monotherapy. This combination therapy offers potential advantages in terms of minimizing treatment resistance and optimizing patient outcomes by targeting multiple key pathways involved in tumor growth and survival, overcoming resistance mechanisms that may arise during monotherapy.

Endocrine therapies such as aromatase inhibitors [284, 285], and selective estrogen receptor modulators are important treatments for HR+ BC. However, the interplay between PI3K signaling and estrogen receptor signaling may lead to resistance to endocrine therapy. In order to overcome resistance mechanisms and improve treatment response in HR+ BC, the combination of PI3K inhibitors and endocrine therapy has been explored as a potential approach [117].

The PI3K signaling pathway interacts with estrogen receptor signaling to promote cell proliferation and survival [286]. Dysregulation of PI3K signaling may contribute to endocrine resistance by evading or interfering with the effects of endocrine therapy [17]. Combining PI3K inhibitors with endocrine therapy can simultaneously target both signalling pathways, resulting in more comprehensive tumour growth inhibition. PI3K inhibitors block the PI3K signaling pathway, whereas endocrine therapy interferes with estrogen receptor signaling and globally impedes cancer cell survival and proliferation [17].

Preclinical and clinical studies have demonstrated promising results with the combination of PI3K inhibitors and endocrine therapy in HR+ BC. This approach leads to increased tumor growth suppression, improved response rates, and longer progression-free survival compared to endocrine therapy alone [287]. By targeting multiple signaling pathways, this combination strategy overcomes acquired resistance and prevents the emergence of resistance, addressing the complexity of HR+ BC and offering potential for optimized treatment outcomes.

The mTOR is closely related to the PI3K signaling pathway and is central in regulating cell growth and proliferation [288]. The PI3K and mTOR pathways share some points of interaction and co-regulation. Dysregulation of these pathways can lead to increased cell survival, increased cell proliferation, and resistance to therapy [114]. Targeting both pathways simultaneously can achieve a more comprehensive inhibition of tumor growth. The combination of PI3K and mTOR inhibitors represents a dual blockade approach that effectively suppresses signaling cascades in promoting cancer cell growth. This combination inhibits PI3K-mediated activation of mTOR and downstream effectors, thereby synergizing tumor cells [289].

The combination of PI3K inhibitors and mTOR inhibitors has demonstrated encouraging outcomes in cancer treatment. Compared to monotherapy, this synergistic approach enhances progression-free survival, objective response rate, and overall survival. This strategy effectively tackles resistance mechanisms by concurrently targeting the PI3K and mTOR pathways, offering a compelling therapeutic option for patients with this specific type of BC.

Activation of the MAPK, particularly by the downstream kinase MEK, may serve as a compensatory mechanism in response to PI3K inhibition [290]. The PI3K and MAPK signaling pathways are interrelated and often activated in cancer cells [291]. When PI3K signaling is inhibited, cancer cells activate MAPK signaling as an alternative survival mechanism, promoting cell proliferation and therapy resistance.

The combination of PI3K and MEK inhibitors represents a dual inhibitory strategy that simultaneously targets the PI3K and MAPK pathways [292, 293]. By blocking both pathways, this combined approach aims to achieve broader inhibition of tumor cell proliferation and survival.

Preclinical studies have shown that the combination of PI3K inhibitors and MEK inhibitors may provide synergistic effects and increase anti-tumor activity compared to monotherapy. This synergistic effect results from the complementary actions of these inhibitors, jointly repressing key signaling nodes in both signaling pathways and impeding compensatory signaling [294].

This combination strategy may overcome resistance mechanisms that may arise during treatment with PI3K inhibitors by simultaneously targeting the PI3K and MAPK signaling pathways. This interferes with compensatory activation of the MAPK pathway, which may contribute to treatment resistance, and improves overall therapeutic response.