Abstract 5388: Triple-threat: Inavolisib and giredestrant combine with palbociclib to achieve sustained cell cycle arrest in ER-positive breast cancer models
Authors: Marohl Taylor , Tan Jenille , Zhou Wei , Guan Jane , Carlos Sandra Melo , Xie Shiqi , Hafner Marc , Metcalfe Ciara 
Published: June 2022

Abstract
Targeted therapies have dramatically improved progression-free survival (PFS) in hormone-receptor positive (HR+) breast cancers, however, achieving durable responses remains a challenge. In particular, mutations in PIK3CA and ESR1 are common in advanced settings and can potentially drive resistance to aromatase inhibitors, including when combined with CDK4/6 inhibitors. With the goal of improving patient outcomes, next-generation endocrine agents and PIK3a-specific inhibitors are being developed and are in active clinical studies. Importantly however, the molecular underpinnings of how such agents combine with one another, and with CDK4/6 inhibitors, have not yet been fully explored.
Here we profiled the latest generation molecules, inavolisib (GDC-0077), a PIK3a-specific inhibitor, and giredestrant (GDC-9545), a full estrogen receptor (ER) antagonist and selective ER degrader, in combination with palbociclib, a CDK4/6 inhibitor. Specifically, we evaluated the transcriptional and phenotypic effects of these molecules on PIK3CA mutant MCF7 cells expressing either ESR1 wild-type (WT) or ESR1-Y537S. Cells were treated with single-, double-, or triple-agent inavolisib, giredestrant, and palbociclib and were harvested at 24 and 96 hours for transcriptional profiling by both bulk and multiplexed scRNA-sequencing, or assayed for viability.
In both WT and mutant (MUT) cells, expression of cell cycle-related genes was reduced at 24h upon treatment with single-agent palbociclib, but rebounded to baseline by 96h. Combination with giredestrant induced the expected downregulation of ER activity at both 24 and 96h. Notably, the combination further reduced the expression of cell cycle-related genes compared to palbociclib alone. Surprisingly, the repression of cell cycle-associated genes driven by the combination of giredestrant and palbociclib was sustained through 96h in WT cells but not in MUT cells, despite sustained repression of ER activity. Addition of PIK3a-inhibitor inavolisib to the palbociclib/giredestrant combination further reduced expression of cell cycle-related genes in both WT and MUT cells to an extent not achieved by any other treatment variation in this study. Most importantly, the triple-combination also prevented the rebound of cell cycle genes in the ESR1-MUT, consistent with a sustained response. Analysis of the scRNA-seq data revealed a substantial heterogeneity in the vehicle-treated population. While giredestrant treatment results in a particularly profound impact on transcriptional programs, heterogeneity is maintained. Together, our data provide mechanistic support for ongoing triple-combination studies in HR+ breast cancers.
Citation Format: Taylor Marohl, Jenille Tan, Wei Zhou, Jane Guan, Sandra Melo Carlos, Shiqi Xie, Marc Hafner, Ciara Metcalfe. Triple-threat: Inavolisib and giredestrant combine with palbociclib to achieve sustained cell cycle arrest in ER-positive breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5388.