TITLE: HER2 mediates PSMA/mGluR1-driven resistance to the DS-7423 dual 1 PI3K/mTOR inhibitor in PTEN wild-type prostate cancer models

45 Prostate cancer remains a major cause of male mortality. Genetic alteration of the 46 PI3K/AKT/mTOR pathway is one of the key events in tumor development and 47 progression in prostate cancer, with inactivation of the PTEN tumor suppressor being 48 very common in this cancer type. Extensive evaluation has been performed on the 49 therapeutic potential of PI3K/AKT/mTOR inhibitors and the resistance mechanisms 50 arising in patients with PTEN mutant background. However, in patients with a PTEN 51 wild type phenotype, PI3K/AKT/mTOR inhibitors have not demonstrated efficacy and 52 this remains an area of clinical unmet need. In this study, we have investigated the 53 response of PTEN wild-type prostate cancer cell lines to the dual PI3K/mTOR 54 inhibitor DS-7423 alone or in combination with HER2 inhibitors or mGluR1 inhibitors. 55 Upon treatment with the dual PI3K/mTOR inhibitor DS-7423, PTEN wild-type 56 prostate cancer CWR22/22RV1 cells upregulate expression of the proteins PSMA, 57 mGluR1 and the tyrosine kinase receptor HER2, while PTEN mutant LNCaP cells 58 upregulate androgen receptor and HER3. PSMA, mGluR1 and HER2 exert control 59 over one another in a positive feedback loop that allows cells to overcome treatment 60 with DS-7423. Concomitant targeting of PI3K/mTOR with either HER2 or mGluR1 61 inhibitors results in decreased cell survival and tumor growth in xenograft studies. 62 Our results suggest a novel therapeutic possibility for PTEN wild-type-PI3K/AKT 63 mutant prostate cancer patients based in the combination of PI3K/mTOR blockade 64 with HER2 or mGluR1 inhibitors.