Background: Neoadjuvant endocrine therapy is standard care for women with hormone receptor-positive breast cancer. However, both primary and acquired endocrine resistance is not uncommon, thereby limiting efficacy. [1] The PI3K-Akt-mTOR pathway is a major mediator of endocrine resistance. [2,3] Therefore, we determined the efficacy and safety of the mTORC1/2 inhibitor TAK-228 in combination with tamoxifen in neoadjuvant setting. Methods: In this single-arm, open-label phase II trial, newly diagnosed patients with stage I−III ER-positive, HER2-negative breast cancer received TAK-228 (30 mg weekly) and tamoxifen (20 mg daily) for 16 weeks until 2-4 weeks prior to surgery. The primary endpoint was the change in Ki67 after 6 weeks. Secondary endpoints included pathological complete response rate (pCR), preoperative endocrine prognostic index (PEPI) score, and safety. Results: Of the 28 patients enrolled in the study, 3 were excluded due to non-compliance. Mean patient age was 51.7 years. Most patients had stage I or II disease (12 [43%] each); 4 (14%) had stage III disease. Mean Ki67 was significantly lowered from baseline to Week 6 (17.2% vs. 15.2%, p = 0.0023). Interestingly, mean Ki67 increased to 20.1% from baseline to the time of surgery. This may have been due to a rebound effect, as TAK-228 was discontinued 2-4 weeks prior to surgery. Tumor size also significantly decreased from baseline to surgery, with a median decrease of 0.75 centimeters (p < 0.0001). PEPI score was intermediate risk (score 1−3) in 6 patients and high risk group (score ≥4) in 15 patients. No patients achieved a PEPI score of 0 and no pCR was achieved. Overall, the combination was well tolerated, the most common side effects were nausea (72%), vomiting (72%), fatigue (72%), mucositis (45%), and headache (45%). The any Grade 3 AE rate was 7.7%. Conclusions: The TAK-228 and tamoxifen combination was found to be an effective neoadjuvant strategy with a favorable safety profile in newly diagnosed patients with hormone receptor-positive breast cancer. Further molecular analysis (PI3K-Akt-mTOR pathway) are pending and will be presented. Clinical trial information: NCT02988986