Abstract PD10-06: PD10-06 Clinical outcomes and exploratory gene expression analysis of OPPORTUNE: a phase II window-of-opportunity study to evaluate pictilisib+anastrozole versus anastrozole alone in ER-positive breast cancer
Authors: Chibly Alejandro Martinez , Shia Alice , Johnson Radia , Hwang Michael S. , Hafner Marc , Metcalfe Ciara , Shah Kalpit , Slyper Michal , Bolen Chris , Pinder Sarah E. , Thompson Alastair M. , Gendreau Steven , Schmid Peter 
Published: March 2023

Background: Endocrine therapy (ET) is the mainstay of ER+ BC treatment. However; up to 20% of ER+ BC tumors progress into metastatic disease and develop ET resistance, underscoring the need for combination therapies. Preclinical data suggest that combining phosphatidylinositol 3-kinase (PI3K) inhibitors with ET may overcome resistance. OPPORTUNE, a preoperative phase II window trial, evaluated whether the combination of the PI3K inhibitor pictilisib with anastrozole (PIC+ANA) can increase the antitumor effect of ANA in newly diagnosed operable ER+ BC. Early results showed greater suppression of tumor Ki67 in patients treated with PIC+ANA versus ANA alone. Here, we present gene expression analysis from tumors collected pre- and post-treatment, and their associations with Ki67 outcomes. Methods: Postmenopausal women with newly diagnosed operable ER+/HER2-negative BC were randomly allocated (2:1, favoring the combination) to 2 weeks of preoperative treatment with ANA 1 mg once per day (n = 47) or the combination of ANA 1 mg with PIC 260 mg once per day (n = 89). The primary end point was inhibition of tumor cell proliferation measured by change in Ki67 protein expression via IHC between tumor samples taken pre- and post-treatment. Samples were analyzed by RNA-sequencing—ER pathway activity, PAM50 intrinsic subtypes, and pathway analyses were assessed by Ki67 outcomes. Elastic net regression analysis and transcription factor activity inference with Dorothea were performed to identify features strongly associated with Ki67 outcomes. Results: 124 patients (ANA, n=43; PIC+ANA, n=81) had paired tumor samples at baseline/week 2 that were evaluable for both Ki67 and RNA-seq. PIC+ANA showed improved suppression of Ki67 compared to ANA alone (-83.78% vs -73.85%, p=0.012) and a greater proportion of patients achieved complete cell cycle arrest (CCCA, as defined by Ki67&lt; 2.7%) in the PIC+ANA arm (45.68% vs 36.59%). ER pathway activity suppression was comparable between treatments. PAM50 classification based on RNA-seq showed that 83.0% of tumors were luminal (Lum) A at baseline and 12.9% were LumB, with the remainder being classified as Normal-like. PIC+ANA showed greater suppression of Ki67 in LumB tumors compared to LumA (-92.29% vs -81.62%). This effect was much greater in the PIC+ANA arm compared to LumB tumors treated with ANA alone (-92.29% vs -37.87%); however, given the low number of LumB tumors in the ANA arm (n=3), we could not determine statistical significance. Bioinformatic analysis of RNA-seq from baseline specimens showed that MYBL2 activity was associated with resistance to ANA (as defined by Ki67≥7.5% at week2). Tumors in the top quartile of MYBL2 activity at baseline showed improved Ki67 outcomes in the PIC+ANA arm compared to ANA alone (-91.71% vs -51.44%), while no differences were observed between treatments for tumors with lower MYBL2 activity (-79.16% vs -76.72%). Single-nucleus (sn)-RNA-seq from untreated ER+ BC tumors showed that MYBL2 was enriched in a single cluster of tumor cells, which also had the highest expression of a subset of actionable targets (including CDK1, CDK2, CDK4, EZH2, and AKT1), as compared to other tumor cells. Expression data from DepMap and drug-sensitivity data from ER+ BC cell lines show a positive trend between high MYBL2 expression and sensitivity to the ER degrader, giredestrant. Conclusions: PIC exhibited greater antiproliferative effects in combination with ANA in ER+/HER2- early BC compared to ANA alone, particularly in LumB and MYBL2-high tumors. Furthermore, the transcriptional profile and in vitro response of tumor cells with high MYBL2 expression suggest potential sensitivity to other combination therapies.

Citation Format: Alejandro Martinez Chibly, Alice Shia, Radia Johnson, Michael S. Hwang, Marc Hafner, Ciara Metcalfe, Kalpit Shah, Michal Slyper, Chris Bolen, Sarah E. Pinder, Alastair M. Thompson, Steven Gendreau, Peter Schmid. PD10-06 Clinical outcomes and exploratory gene expression analysis of OPPORTUNE: a phase II window-of-opportunity study to evaluate pictilisib+anastrozole versus anastrozole alone in ER-positive breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD10-06.