Abstract OT-09-07: Winpro: A window of opportunity study of endocrine therapy with and without prometrium in postmenopausal women with early stage hormone receptor-positive breast cancer
Authors: Lau Brandon , Segara Davendra , Ong Andrew , Bingham Janne , Kiely Belinda , Carson Emma-Kate , Chen Julia , Mak Cindy , Warrier Sanjay , Hui Mun , Middleton Kate , Parker Andrew , Mann Bruce , Lindeman Geoffrey , Tilley Wayne , Lim Elgene 
Published: February 2021

Background: There is bidirectional interplay between PR and ER in human breast cancers (Lim, Endo Rel Can 2016). Evidence in breast cancer cell lines suggests that dual treatment with estrogen and progesterone compared to estrogen alone leads to reprogramming of ER chromatin binding sites via regulation of 470 genes (Mohammed, Nature 2015). Importantly, there was an additive anticancer effect in preclinical breast cancer models when natural progesterone was combined with standard endocrine therapy; we hypothesize that this combination has activity in women with breast cancer.Trial design: We are conducting a phase II multi-center, randomised, open-label, three-arm study in which 200 postmenopausal women with early-stage ER-positive (ER ≥10%), PR-positive (PR ≥10%), HER2-negative breast cancer will be randomised 1:1:1 to letrozole 2.5mg daily (arm 1); letrozole 2.5mg and prometrium 300mg PO daily (arm 2); or tamoxifen 20mg and prometrium 300mg PO daily (arm 3). Surgery will occur on day 14 after treatment initiation. Eligible subjects must have tumor size ≥10mm on imaging, no history of uterine cancer or venous thromboembolism, and no receipt of other preoperative therapies. The primary objective is to assess the reduction in proliferative marker Ki67 following treatment in either combination arm compared to letrozole alone.Methods: Blood will be collected at baseline and at end of treatment. Tissue samples will be collected from the diagnostic biopsy and at the time of surgery. The primary endpoint is geometric mean reduction of centrally assessed Ki67 expression after two weeks of treatment compared with baseline. Given the expected geometric mean reduction of 76% for aromatase inhibitor alone (Dowsett, J Natl Can Inst 2007) and allowing 4% dropouts, 200 patients provides 80% power to detect an improvement in Ki67 suppression to 92% in either experimental arm with p-value 0.025 for each comparison.The secondary endpoint of safety and tolerability will be assessed (NCI-CTCAE v4.0). Translational endpoints include definition of a predictive gene set biomarker for Ki67 reduction; tumor biomarkers after treatment, including apoptotic markers Bcl-2 and cleaved-caspase3, as well as protein and mRNA expression of ER, PR, AR, FoxA1, and CyclinD1; and levels of estrone, estradiol, E2, progesterone, testosterone, DHT, and DHEAS in serum and finger prick dried blood spot after treatment.Accrual: Enrolment commenced in February 2018 and 70 patients have been randomized from 7 sites to date (1 July 2020). Target accrual is 200 patients enrolled from 8 sites.Contact information: This study is led at The Kinghorn Cancer Centre, St Vincent’s Sydney Hospital Sydney, Australia, and funded by the Cancer Council of NSW and the NHMRC Translational Breast Cancer Project grant. Contact Elgene Lim MBBS FRACP PhD at e.lim@garvan.org.au.

Citation Format: Brandon Lau, Davendra Segara, Andrew Ong, Janne Bingham, Belinda Kiely, Emma-Kate Carson, Julia Chen, Cindy Mak, Sanjay Warrier, Mun Hui, Kate Middleton, Andrew Parker, Bruce Mann, Geoffrey Lindeman, Wayne Tilley, Elgene Lim. Winpro: A window of opportunity study of endocrine therapy with and without prometrium in postmenopausal women with early stage hormone receptor-positive breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr OT-09-07.