A tipping-point for apoptosis following dual inhibition of HER2 signaling network by T-DM1 plus GDC-0980 maximizes anti-tumor efficacy.

HER2 signaling network and its complex relationship with the PI3K-AKT-mTOR pathway explain the acquired resistance to anti-HER2 therapy observed in clinics. Such complexity has been clinically evident from the limited efficacy of data in the BOLERO-1 and BOLERO-3 trials, which tested combinations of trastuzumab (T), everolimus, and chemotherapy in women with HER2+ advanced BC. In the following MARIANNE trial also, a combination of T-DM1 plus pertuzumab delivered a non-inferior but yet not superior PFS compared to trastuzumab plus a taxane. Algorithmic inhibition of PI3K/mTOR along with T or T-DM1 is, therefore, an attractive drug combination, and we tested the combination(s) in HER2+ BC, especially in T-resistant and PIK3CA mutated conditions. GDC-0980, a dual pan-PI3K/mTOR inhibitor alone or in combination with T or T-DM1, was examined in a panel of HER2+ T-sensitive (BT474, SKBR3), HER2+ T-resistant (BT474HerR), HER2+/PIK3CA mutant (HCC1954, MDA-MB453), and HER2+/PTEN mutant (HCC1569) BC cell lines. GDC-0980 re-sensitized trastuzumab-resistant, PIK3CA mutant, or PTEN mutant cells to T and acted additively with T. Importantly, this activity was more when GDC-0980 is combined with T-DM1. The combination (with T or with T-DM1) was then tested in the HER2+/T-sensitive, HER2+/T-resistant, and HER2+/PIK3CA mutated BC xenograft models for the anti-tumor effect. Along with its anti-tumor effect, GDC-0980 effectively decreased tumor angiogenesis (CD31 staining). Maximum anti-tumor (from tumor growth inhibition to tumor regression) efficiency was observed in all three xenograft models when T-DM1 was combined with GDC-0980. The anti-proliferative effects of GDC-0980 as evidenced by a decreased p-AKT (Ser473, The308), p-P70S6K, p-S6RP, and p-4EBP1, along with blockade of clonogenic 3D growth was accompanied by the initiation of apoptotic activity (annexin V, CASPASE3, cleaved PARP1 and mitochondrial depolarization); and was significantly superior when GDC-0980 combined with T-DM1. Interestingly, both trastuzumab and T-DM1 induce PD-L1 expression in HER2 amplified BC cells. Our data provide evidence that an oncogenic mutation of PIK3CA and HER2-amplification may represent biomarkers to identify patients who may benefit most from the use of GDC-0980 and an opportunity to include immunotherapy in the combination of anti-HER2 therapy.