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Background PTEN loss of function is frequent in GBM correlating with poor prognosis, impaired antitumor responses and reduced efficacy of Immune Checkpoint Inhibitors (ICI). Ipat is a potent, selective, small-molecule inhibitor of Akt. Ipat efficiently depletes FOXP3+ regulatory T cells from the tumor microenvironment (TME) resulting in increased infiltration of effector T cells in solid tumors (Lopez 2020, AACR). We hypothesize that Akt inhibition in PTEN loss glioblastomas may deplete the TME of suppressive immune cells, and render malignant brain tumors more responsive to ICIs. Methods Relapsed WHO grade IV GBM pts with stable neurological symptoms ≥5 days prior to enrollment, requiring Results 23 recurrent GBM pts (median age 55 yrs (25-71 yrs; 74% male) were enrolled. Median ECOG PS 1. All pts had surgery followed by radical-chemoradiotherapy. Median prior lines of therapy 1 (range 1-4). 15 pts had PTEN loss by IHC (H 6 cycles) in 19 efficacy evaluable pts was 32% (6/19 overall) and 28.6% (4/14) for pts with PTEN loss. Multiplex IHC of archival samples (n=19) showed PTEN loss tumors had significantly greater numbers of CD3+ infiltrating T-lymphocytes within the TME compared to PTEN wild type tumors (median 53.64/mm2 vs 1.94/mm2; p=0.0021). Within the PTEN loss cohort (n=13), pts with clinical benefit had significantly higher baseline number of CD8+ effector T cells as compared to non-responders (median 46.50/mm2 vs 8.21/mm2; p=0.0336). Two pts with PTEN loss proceeded to re-resection on trial. One exceptional responder, a 58yr male refractory to radical chemo-radiotherapy and Bevacizumab had a resection of enhancing disease after 5 cycles that showed >70% depletion of CD4+ T regulatory cells with an increase in CD8+ lymphocyte infiltration and no residual evidence of tumor (pathological CR). In contrast, a non-responding patient who proceeded to debulking surgery had no change in infiltration of CD8+ lymphocytes, but a marked increase (>27-fold) in CD4+ regulatory T cells. Conclusion The RP2D of 400mg Ipa OD + 1200mg A Q3W was well tolerated in GBM pts. Early efficacy signals were detected with PTEN loss being a promising predictive biomarker for response to combination. Citation Format: Crescens Tiu, Wing Hing Yau, Liam C. Welsh, Timothy L. Jones, Anna Zachariou, Toby Prout, Mona Parmar, Alison J. Turner, Robert W. Daly, Christina Yap, Mateus Crespo, Bora Gurel, Ruth M. Riisnaes, Shaun A. DeCordova, Karen E. Swales, Udai Banerji, Anna R. Minchom, Adam Sharp, Johann S. de Bono, Juanita S. Lopez. Preliminary evidence of antitumor activity of Ipatasertib (Ipat) and Atezolizumab (A) in glioblastoma (GBM) patients (pts) with PTEN loss in the Phase 1 Ice-CAP trial (NCT03673787) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT093.