Abstract Background: Published preclinical findings provided new insights into the functional 'cross-talk' between the oestrogen receptor (ER) and the progesterone receptor (PR) in breast cancer (BC) (Mohammed et al., Nature, 2015). Addition of a PR agonist to anti-oestrogens directly modifies ERa chromatin binding and the transcriptional response in breast cancer cells, and is anti-proliferative in vitro and in vivo. Megestrol Acetate (MA), an off-patent semi-synthetic derivative of progesterone, has been licensed for many years as treatment for ER+ metastatic BC. There is also good evidence for the effectiveness of MA as a supportive therapy to ameliorate endocrine therapy-related hot flushes. Trial design: PIONEER is a three-arm, open label, multi-centre randomized phase II pre-surgical window trial evaluating effects of 15 days of preoperative therapy with Letrozole (LET), or LET plus MA 40mg, or LET plus MA 160mg in postmenopausal women with ER+ HER2- invasive primary BC. Eligibility criteria: Inclusion CriteriaPostmenopausal women with histologically confirmed invasive BC, ≥T1c, clinical NX or N0-N3, ER+ (Allred≥3), and HER2-2 groups of patients are potentially eligible: Cohort A: Patients whose cancers have been deemed to be operable by the Multi-Disciplinary Team (MDT) with surgery planned for the next 2-6 weeks Cohort B: Patients with early or locoregionally advanced BC planned for primary endocrine therapy either in lieu of surgery or as neoadjuvant therapy before surgery ECOG performance status of 0-2Adequate Liver, Renal, Bone marrow function Exclusion CriteriaHormone replacement therapy in the last 6 monthsTreatment with tamoxifen or an aromatase inhibitor (AI) in the last 6 monthsA progestogen-containing intrauterine system in situ, unless removed prior to randomisationMetastatic disease on presentationRecurrent BC (patients with a new primary invasive BC are eligible) Specific aims: The primary endpoint is % change in proliferation between baseline and day 15 tumor biopsies, measured by Ki67 IHC assessment. Secondary endpoints include: expression of Aurora Kinase A, Caspase 3 and AR/PR/EMT markers by IHC; and safety endpoints. Exploratory endpoints: transcription factor mapping (ChIP-seq) and identification of differential ERa-associated proteins (RIME) on paired fresh-frozen tumor samples. PIONEER will help determine whether there is value in conducting a Phase III adjuvant study to investigate the longer term benefit of combining an AI with MA, and if so, at what dose (40mg vs. 160mg). Statistical methods: Patients are randomized in a 1:1.5:1.5 ratio into arms A:B:C. Based on results from previous clinical trials, a mean 66% reduction in Ki67 is anticipated for LET alone (arm A), and a 77.5% reduction for the combination arms B and C, based on preclinical data. Present and target accrual: Patients are being recruited in Cambridge, Cornwall, Belfast & London with 6 other UK sites due to open q3/4 2018. At the time of submission, 29 patients had been recruited. A recruitment total of 189 patients is required. Citation Format: Kumar SS, Benson J, McIntosh S, King P, Dougall G, Kateb A, Vallier A-L, Jones L, Qian W, Provenzano E, Caldas C, Pantziarka P, Carroll J, Baird RD. PIONEER- Pre-operative wIndOw study of letrozole plus PR agonist megestrol acetate versus letrozole aloNE in post-menopausal patients with ER-positive breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr OT1-01-07.