Breast cancer is among the most prevalent types of malignant tumors in women worldwide. Consequently, the development of treatment strategies for breast cancer has been a primary focus of medical research [1]. Patients suffering from breast cancer with stages I to II are usually cured by breast-conserving surgery followed by chemotherapy. The aim is to minimize the risk of cancer recurrence through systematic treatment while preserving breast tissue. In this treatment mode, the sequential chemotherapy regimen epirubicin, cyclophosphamide, and paclitaxel taxane (EC-T) is widely used due to its good efficacy. However, with the further study of the EC-T chemotherapy regimen, its potential cardiotoxicity has gradually attracted clinical attention. Cardiotoxicity, especially disturbance of myocardial energy metabolism, is an important side effect of EC-T chemotherapy [2, 3, 4, 5]. Myocardial energy metabolism disorders involve abnormalities in the production and use of energy by cardiomyocytes, which are essential for the normal function of the heart. As an organ with high energy requirements, the heart has strict requirements for the efficiency and stability of energy metabolism [6, 7]. Cardiomyocytes are responsible for the production of a significant quantity of adenosine triphosphate (ATP), primarily through the process of mitochondrial oxidative phosphorylation. This process is essential for maintaining sustained cardiac contraction [8, 9]. Kuang Z et al. [10] reviewed the current molecular pathways leading to cardiac toxicity in cancer treatment and systematically discussed prevention and treatment strategies for cardiac toxicity, providing a new perspective on the management of cardiac toxicity. Giffoni de Mello Morais Mata D et al. [11] reviewed randomized controlled trials conducted before March 11, 2024, to investigate the effects of docetaxel and cyclophosphamide on cardiac toxicity. The results showed that compared with anthracycline taxanes, docetaxel and cyclophosphamide had cardiotoxicity. In a retrospective review of patients with stages I–III cancer, Tsai JH et al. [12] compared the effectiveness of combination therapy involving epirubicin and PEGylated liposomal doxorubicin in treating cancer. The results indicated that the cardiac toxicity of patients under the proposed treatment plan was lower than that of EC-T.

In the EC-T chemotherapy regimen, epirubicin and paclitaxel are considered to be the main sources of cardiotoxicity. Epirubicin blocks the replication of cancer cells by cross-linking with DNA, but may also cause damage to heart muscle cells. Paclitaxel affect cell division by blocking microtubule dynamics, and their effects on cardiomyocytes may involve interference with the microtubule network and subsequent metabolic effects [13, 14]. Myocardial cell damage may lead to mitochondrial dysfunction, affecting the balance of ATP synthesis and energy metabolism. The mechanism of myocardial energy metabolism disorder may include many aspects, such as direct cytotoxic effects, oxidative stress, impaired mitochondrial function, and changes in energy metabolic pathways [15, 16, 17]. Oxidative stress refers to the inequality of intracellular reactive oxygen species (ROS) and antioxidant mechanisms, which can lead to damage to myocardial cell membranes, changes in protein function, and DNA damage. Mitochondria are energy factories in cardiomyocytes, and their dysfunction directly affects the energy supply of the myocardium [18, 19]. In addition, cardiomyocytes may adapt to chemotherapy-induced stress by altering energy metabolic pathways, such as increasing glycolysis as an energy source. At present, the treatment strategy for breast cancer is constantly being optimized, and it is urgent to comprehensively understand the cardiotoxicity caused by EC-T chemotherapy. A deep understanding can help develop effective prevention strategies, thereby improving treatment safety. This study aims to investigate how EC-T sequential chemotherapy regimens induce myocardial energy metabolism disorders and the potential mechanisms involved in this process.

In early breast cancer (stages I to II), breast-conserving surgery and adjuvant treatment are considered good approaches [20, 21]. With the development of medicine, the treatment plan for breast cancer is increasingly personalized, focusing on the balance of comprehensive treatment effect and patient quality of life. Breast cancer with stages I to II refers to the early stage of breast cancer in which the tumor is confined to the breast and may have slight metastases to local lymph nodes. At this stage, breast-conserving surgery (only the tumors and a small number of normal tissue are taken out to leave most of the breast tissue intact) is a common treatment option because it can maximize the appearance and function of the breast while removing the cancer. After surgery, adjuvant chemotherapy is often recommended to reduce the risk of cancer recurrence. EC-T sequential chemotherapy regimen is a common therapy. This regimen is first cured by an “EC” combination and then switched to “T” drug therapy. Epirubicin is an anthracycline antibiotic with anti-tumor activity, while cyclophosphamide is an alkylating agent that can interfere with DNA replication and repair [22]. Paclitaxel stop cancer cells from dividing by inhibiting the normal function of microtubules. This sequential chemotherapy regimen is designed to maximize the killing of cancer cells while minimizing damage to normal cells through a drug combination with different mechanisms. Although EC-T has shown good results in treating early-stage breast cancer, it can also be associated with side effects, including cardiotoxicity, hair loss, nausea, and vomiting. There is a close correlation between cardiotoxicity and myocardial energy metabolism. Cardiotoxicity refers to the negative effects of certain drugs or treatments on cardiac function, which may be manifested as myocardial damage, arrhythmia, heart failure, etc. [23, 24]. Myocardial energy metabolism disorders refer to the decline in the ability of cardiomyocytes to produce and use energy (primarily adenosine triphosphate). This study finds that the EC-T sequential chemotherapy regimen may impair cardiac function, especially in terms of myocardial energy metabolism.

By biochemical analysis, it is found that indicators of troponin, creatine kinase, CK-MB isoenzyme, and lactate dehydrogenase in the observation group are greatly higher than those in the control group, which has important clinical significance. Troponin is a specific marker of myocardial injury and occupies a vital place in the early diagnosis of acute myocardial infarction [25]. It is quickly released into the bloodstream after heart muscle cell damage. Therefore, the increase in levels usually indicates damage or necrosis of heart muscle cells. Similarly, creatine kinase and its MB isoenzyme are another sensitive indicator of myocardial injury. CK-MB is in high amounts in cardiac muscle tissue and is released into the blood when the heart muscle is damaged. An increase in CK-MB levels is frequently associated with myocardial damage [26]. In addition, as an enzyme that is widely present in various tissues of the body, an increase in lactate dehydrogenase may also reflect the overall situation of myocardial cell injury [27]. The elevation of these biochemical markers indicates that EC-T chemotherapy may cause some degree of toxic effects on the heart. Epirubicin and paclitaxel are known to have direct or indirect toxic effects on the heart, which can lead to damage and even necrosis of heart muscle cells. Specifically, epirubicin, an anthracycline chemotherapy agent, has gained significant recognition for its cardiotoxic effects, which may result in damage to cardiomyocytes by increasing oxidative stress, leading to mitochondrial dysfunction, and ultimately, apoptosis. The activity of complex I, complex III, and complex V in the observation group is significantly reduced compared to the control group, indicating that the EC-T chemotherapy regimen may have a negative impact on cardiac mitochondrial function. As an important place for energy metabolism in cardiomyocytes, the reduction of mitochondrial function may have a significant impact on the normal physiological activities of the myocardium [28]. Complex I, complex III, and complex V are key enzymes in the respiratory chain within the mitochondria, and are responsible for electron transport during oxidative phosphorylation and adenosine triphosphate synthesis [29]. The main function of complexes I and III is to transfer electrons in the electron transport chain, while complex V uses the proton gradient produced by electron transport to synthesize adenosine triphosphate. The observed reduction in the activity of the complexes indicates that mitochondria are engaged in the process of electron transfer and energy conversion efficiency. Chemotherapy drugs, especially anthracyclines such as epirubicin, may inhibit the activity of these complexes by causing mitochondrial DNA damage, increasing oxidative stress, or directly affecting the structure and function of mitochondria [30]. In addition, paclitaxel may have further indirect effects on mitochondrial function by affecting microtubule dynamics. The impairment of mitochondrial function has been demonstrated to result in a reduction of energy output by cardiomyocytes. In addition, this impairment may lead to a process of apoptosis and necrosis of cardiomyocytes, which can adversely affect the contractile function and overall performance of the heart. The changes in metabolic pathways are directly related to changes in mitochondrial function and energy production. Mitochondria are the main organelles responsible for energy conversion and supply within cells. In myocardial energy metabolism, changes in mitochondrial function directly affect the energy supply and cardiac function of the heart. In the state of heart failure, there may be significant changes in energy metabolism in the myocardium, leading to a decrease in mitochondrial adenosine triphosphate production capacity. In addition, myocardial metabolic disorders may lead to substrate utilization changes, mitochondrial structural alterations and dysfunction, adenosine triphosphate synthesis and transport disorders, etc., further exacerbating cardiac structure and dysfunction.

Cardiac MRS showed that the indices of adenosine triphosphate and phosphocreatine in the observation group were not greatly higher than those in the control group, while the indices of pyruvate and $\beta$-hydroxybutyric acid were on the contrary. It revealed the profound effect of EC-T sequential chemotherapy on myocardial energy metabolism. Adenosine triphosphate was the main energy currency of cardiomyocytes and creatine phosphocreatine. As part of the energy buffer system, it was essential for maintaining stable levels of adenosine triphosphate [31]. The reduction of these indicators might indicate that heart muscle cells were experiencing energy metabolism disorders during chemotherapy, resulting in insufficient energy supply. At the same time, the increase of pyruvate and $\beta$-hydroxybutyric acid might reflect the change in the energy metabolism pathway of cardiomyocytes [32]. Under normal circumstances, cardiomyocytes produce energy primarily through oxidative phosphorylation. However, under certain stressful conditions, such as chemotherapy-induced mitochondrial function impairment, cardiomyocytes might rely more on glycolysis and fatty acid oxidation. Pyruvate was an intermediate product of glycolysis, and $\beta$-hydroxybutyric acid was a byproduct of fatty acid oxidation. The accumulation of these substances might be a sign of changes in the metabolic pathways of cardiomyocytes under the stress of chemotherapy. In the observation group of patients receiving EC-T sequential chemotherapy, ROS and MDA were not at a high level, while SOD and GPx increased. These results indicate changes in the oxidative stress state of the heart during chemotherapy. In general, the elevation of ROS is associated with an increase in oxidative stress, which can lead to damage to cell structure and function [33]. However, in this study, ROS and MDA levels decreased in the observation group, which may reflect the effect of chemotherapy drugs on cardiac oxidative metabolic pathways, resulting in a weakened oxidative stress response of cardiomyocytes. This reduction may be due to mitochondrial dysfunction caused by chemotherapy drugs, which reduces ROS production. In addition, elevation of SOD and GPx suggests that the heart may activate an adaptive defense mechanism to neutralize increased free radicals and reduce oxidative damage [34]. The decrease in cardiac blood flow, myocardial glucose uptake, myocardial fatty acid uptake, and myocardial metabolic efficiency in the observation group may be related to cardiac toxicity. This indicates that heart muscle cells may suffer from inadequate blood supply after chemotherapy, leading to a decrease in the efficiency of energy metabolism. The reduction uptake of myocardial glucose and fatty acids may be connected with impairing mitochondrial function and changes in overall metabolic status, which may result in reduction in the energy production capacity of cardiomyocytes.

However, there are still issues with the small sample size and reliance on a single medical center in this study, which may affect the generalizability and external validity of the results. Therefore, future research should further conduct multicenter studies, including more patients to improve statistical efficacy and enhance the generalizability of results. And conduct long-term follow-up to evaluate the long-term effects of cardiac toxicity.

In summary, the results showed that in breast cancer patients with stages I to II who received an EC-T sequential chemotherapy regimen, myocardial energy metabolism disorders were associated with several factors. These factors included the increasing myocardial damage markers, the decreasing mitochondrial respiratory enzyme activity, the changes in energy metabolic pathways, and the reduced cardiac blood flow and metabolic efficiency. These results explained that EC-T chemotherapy might cause direct damage to cardiomyocytes and affect the normal metabolic function of the heart. Therefore, attention should be paid to monitoring cardiac function and preventing cardiotoxicity during therapy. This study is helpful for medical professionals to better understand the cardiac toxicity risk that breast cancer patients may face during chemotherapy. By monitoring myocardial energy metabolism markers, cardiac toxicity can be identified early, allowing for timely adjustment of treatment plans, reduction of serious cardiac events, and provision of valuable information for future research and clinical practice. However, the study only included 200 patients from a single hospital, with a relatively small sample size, which may lead to insufficient statistical power of the results and limit their generalizability. Consequently, subsequent research endeavors should prioritize the augmentation of the sample size and the incorporation of multicenter data to enhance the generalizability and reliability of the findings.

The dataset analyzed during the current study are not publicly available due to the involvement of sensitive patient information but are available from the corresponding author on reasonable request.

CL designed the research study. ML and WX conducted experiments. CW recorded data. HZ analyzed the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Ethics Committee of Baoding No.2 Central Hospital (Approval Number: 23). All participants signed informed consent forms before being included in the study, and the informed consent forms were archived by the Ethics Committee of Baoding No.2 Central Hospital, ensuring the confidentiality and security of the documents.

The research is supported by: the assignment of Baoding Science and Technology Plan Project, clinical research on heart injury caused by EC-T sequential chemotherapy after breast conserving surgery for patients with stage I–II breast cancer (No. 2241ZF019).

The authors declare no conflict of interest.