reast cancer is one of the most prevalent malignant tumors in women, accounting for 23% of all malignant tumors. Approximately 1.3 million people are diagnosed with breast cancer every year in the world, and approximately 400,000 people die of breast cancer. The overall median survival time of patients with advanced breast cancer is only 2-3 years, and the 5-year survival rate is only about 20%. At present, a considerable number of patients are diagnosed as LABC at the time of initial diagnosis. Because of the wide range of lesions or poor lymph node status, surgical treatment is very difficult, and even some patients cannot be operated on.

With the development of neoadjuvant therapy for tumors, NAC has become one of the most common and effective methods for preoperative systemic treatment of LABC. The 2018 V2 version of the National Comprehensive Cancer Network guidelines for breast cancer diagnosis clearly states that preoperative chemotherapy for LABC should be based on an anthracyclines and can be combined with taxanes, but about 20% of LABC patients are not sensitive to this protocol. Heller et al. performed NAC with 5-fluorouracil + epirubicin + cyclophosphamide for 6 weeks in 88 patients with LABC, and the total effective rate was 78%. After 4 weeks of docetaxel treatment in patients with poor response, approximately 10% of patients are still not sensitive. In addition, many studies in and outside China have pointed out that the NAC effect in some estrogen receptor-positive LABC patients, especially postmenopausal luminal A patients, is worse than that of estrogen receptor negative or Ki67 overexpression patients. Ring et al. treated 435 cases of breast cancer with NAC of doxorubicin/cyclophosphamide or cyclophosphamide/methotrexate/fluorouracil. Pathological complete remission rate was 8.1% in estrogen receptor-positive patients and 21.6% in estrogen receptor negative patients. Similar results were obtained in NSABP B-27 test, with the ratio of 8.3% and 16.7% respectively. Simultaneously, Fashing et al. found that the pathological complete remission of patients with relatively high Ki67 after NAC with anthracyclines combined with taxanes was higher than that of patients with low Ki67. Many clinical studies in China have also obtained similar results; that is, the low expression of Ki67 indicates that the effect of NAC in LABC is not good.

Considering the above situation, some scholars gradually introduce endocrine therapy into the NAC of postmenopausal estrogen receptor-positive LABC, namely neoadjuvant endocrine therapy. At present, neoadjuvant endocrine therapy is practical and feasible for breast cancer with large tumors and positive hormone receptor. Simultaneously, the efficacy of the third-generation aromatase inhibitors is better than that of tamoxifen. Z1031 clinical trial of 381 patients with stage II or III postmenopausal estrogen receptor-positive breast cancer in the United States preliminarily confirmed that the third-generation aromatase inhibitors are currently the first choice of effective neoadjuvant endocrine therapy for postmenopausal estrogen receptor-positive breast cancer. However, 30% to 40% of breast cancer patients receiving endocrine therapy still have disease progression due to drug resistance, which is very disadvantageous for LABC patients. The emergence of cyclin dependent kinase 4/6 inhibitors has given hope to such patients. In the guidelines for advanced breast cancer, Cardoso et al. suggested that a first-line treatment with aromatase inhibitors combined with cyclin dependent kinase 4/6 inhibitors should be preferred. In a phase II randomized controlled trial to explore the treatment of estrogen receptor-positive and epidermal growth factor receptor-2-negative advanced breast cancer by combination of palbociclib, a cyclin dependent kinase 4/6 inhibitor, and letrozole, an aromatase inhibitor, the results found that the former could significantly prolong progression-free survival of hormone receptor-positive advanced breast cancer. Therefore, in February 2015, the US Food and Drug Administration approved the combination of palbociclib and letrozole for first-line treatment of hormone receptor-positive advanced breast cancer. However, for inoperable LABC patients, especially those who are not sensitive to chemotherapy, whether the combination of palbociclib and letrozole can be used as a neoadjuvant endocrine therapy instead of NAC is not clear. Although there are relatively few controlled studies on neoadjuvant endocrine therapy and NAC, the existing evidence suggests that the high level of Ki67 in hormone receptor-positive cases indicates that the effect of chemotherapy is better than that of endocrine therapy, but the adverse reaction rate of chemotherapy is high, especially for LABC or older and infirm patients, while neoadjuvant endocrine therapy can achieve similar effect to NAC. Therefore, the efficacy of palbociclib combined with letrozole as neoadjuvant endocrine therapy is still worth looking forward to. Because the principle of endocrine therapy is mainly to induce tumor cell cycle arrest, leading to apoptosis of cancer cells, so the effect is slower than that of chemotherapy. In addition, whether the efficacy can replace chemotherapy as NAC in inoperable LABC patients to improve the operability rate has not yet been fully evidenced.

Therefore, this trial aims to conduct the prospective randomized controlled phase IV clinical trial using palbociclib combined with letrozole versus epirubicin combined with cyclophosphamide and sequential docetaxel as NAC to prove the efficacy of palbociclib combined with letrozole in postmenopausal estrogen receptor-positive LABC patients with low Ki67 expression.