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3110 Background: Preclinical studies have shown that activation of the glucocorticoid receptor (GR) leads to resistance to chemotherapeutics (eg taxanes) and antiandrogens across multiple tumor types, while GR inhibition enhances therapeutic efficacy. ORIC-101 is a potent, selective, and orally bioavailable small molecule GR antagonist undergoing clinical development in combination with nab-paclitaxel in patients with advanced solid tumors and in combination with enzalutamide in patients with metastatic prostate cancer. Methods: 21 patients were enrolled in the dose escalation portion of the phase 1b study, which evaluated both intermittent (5 days on, 2 days off for 21 days) and continuous dosing regimens of ORIC-101 across 5 cohorts (NCT03928314). Tumor tissue was obtained pre-treatment for 19 out of 21 patients, and on study or at the end of treatment for 11 patients. GR protein status was retrospectively evaluated using a proprietary IHC assay optimized for staining nuclear GR in the epithelial compartment of major tumor type tissues. Biopsies were also profiled with RNA-seq to evaluate a proprietary GR activation signature as a potentially predictive and pharmacodynamic (PD) biomarker. Blood-derived peripheral blood mononuclear cells (PBMCs) were collected for 20 patients along with the pre-treatment biopsy, in the morning of days 1, 5 and/or 8 of Cycle 1, and 2.5 or 6 hours after ORIC-101 administration. Blood cortisol levels were also simultaneously measured. PD modulation in PBMCs was assessed by RT-qPCR for biomarkers FKBP5, GILZ and PER1, selected for their consistent stimulation by GR and reversal with ORIC-101 in preclinical studies and observed PD modulation in healthy volunteers administered ORIC-101. Results: Nuclear GR protein was detected in most pre-treatment biopsies regardless of tumor type, and on treatment reduction of GR protein was observed across dose levels. At physiological systemic cortisol levels, ORIC-101 demonstrated PD suppression in PBMCs on days 1, 5 and 8 in the majority of patients. Cortisol levels increased post-dose in these patients due to negative feedback between cortisol and GR. Steady-state target engagement was not consistently demonstrated with the intermittent regimen. In healthy volunteer studies of ORIC-101, steady-state target suppression was consistently achieved after 7 consecutive daily doses of 200 or 350 mg of ORIC-101. Thus, continuous ORIC-101 administration was selected as the recommended phase 2 dose (RP2D) regimen, aimed at achieving sustained GR suppression for optimal chemotherapy re-sensitization. Conclusions: Biomarker data from patients enrolled in the phase 1b study provide evidence of on-target tumor cell eradication and PD modulation and support the RP2D and the tumor types selected for the ongoing dose expansion portion of the study. Clinical trial information: NCT03928314.