Abstract 242: Deciphering the role of PTEN as a predictive biomarker to next generation isoform-selective PI3K inhibitors in PIK3CA mutant breast cancer cells
Authors: Moore Heather M. , Savage Heidi M. , Lackner Mark R. , Wilson Timothy R. 
Published: July 2016

Abstract
The phosphoinositide-3 kinase (PI3K) pathway is one of the most frequently activated signaling pathways in human cancer and represents a promising therapeutic target. Taselisib (GDC-0032), a potent and selective inhibitor of the PI3K alpha, delta and gamma isoforms, but less potent in the beta isoform, displays increased sensitivity in models that are driven by mutant PIK3CA (Ndubaku CO et al, J Med Chem, 2013). Selective pressure caused by, small molecules such as PI3K inhibitors will ultimately lead to clinical resistance and disease progression. Recently, a published mechanism of acquired resistance to the PI3K inhibitor alpelisib (BYL-719, a PI3K alpha-selective inhibitor) reported that genomic loss of the tumor suppressor PTEN led to clinical progression in a metastatic breast cancer patient harboring an activating PIK3CA mutation (Juric D et al, Nature, 2014). As taselisib and alpelisib are next generation PI3K inhibitors, we sought to determine if there are any differences to drug sensitivity in PIK3CA mutant cells that have concomitant loss of PTEN. To investigate this hypothesis, we first measured viability in breast cancer cells, each with differing genetic backgrounds treated with two PI3K inhibitors, taselisib and alpelisib. We found that PIK3CA mutant and PIK3CA mutant/PTEN null cells were sensitive to treatment when compared to wild-type and PTEN null cells. However, PIK3CA mutant/PTEN null cells displayed higher IC50 values for both inhibitors when compared to PIK3CA mutant cells. To control for differences in genetic backgrounds, we next tested whether knockdown (KD) of PTEN expression in PIK3CA mutant breast cancer cells would promote resistance and reactivate the PI3K signaling cascade even in the presence of a PI3K inhibitor. Transient, siRNA-targeted KD of PTEN in the PIK3CA mutant breast cancer cell lines EFM19 and T47D conferred resistance to alpelisib after 6 days of drug treatment, an observation that was less pronounced in taselisib treated cells. Additionally, PTEN KD in DMSO-treated cells led to an increase in cell proliferation and enhanced downstream PI3K signaling as measured by phosphorylated AKT and S6 proteins when compared to non-targeting siRNA controls. The reactivated, pro-survival signaling with PTEN KD was maintained upon treatment with both inhibitors when compared to control cells, an observation that was more pronounced in alpelisib treated cells. These data suggest that PIK3CB can compensate for PIK3CA inhibition and that PTEN loss could lead to clinical resistance to alpelisib, but may not constitute a resistance mechanism to taselisib. As both molecules have entered phase 3 studies in estrogen receptor positive breast cancer, understanding how biomarker-defined genotypes respond to next generation PI3K inhibitors is critical for identifying patients most likely to gain therapeutic benefit.
Citation Format: Heather M. Moore, Heidi M. Savage, Mark R. Lackner, Timothy R. Wilson. Deciphering the role of PTEN as a predictive biomarker to next generation isoform-selective PI3K inhibitors in PIK3CA mutant breast cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 242.