Background: Overexpression of exportin 1 (XPO1) in malignant cells increases the nuclear export/inactivation of tumor suppressor proteins (eg. p53), and promotes the translation of eIF4E-bound oncoprotein mRNAs (eg. c-MYC). XPO1 inhibition reduces total androgen receptor levels, including ARv7, and may re-sensitize prostate cancer (PC) cells to androgen deprivation therapy. Selinexor, the first-in-human SINE compound, showed anticancer activity in patients (pts) with mCRPC. Eltanexor (ELTA), a second-generation SINE compound, showed promising anticancer activity in preclinical models of PC, including in abiraterone (ABI) resistant cell lines. Therefore, ELTA ± ABI was evaluated in mCRPC. Methods: This was part of a phase 1/2 study to determine the safety, preliminary efficacy, and recommended phase 2 doseof ELTA in pts with advanced cancers. Pts with mCRPC received oral ELTA once daily for 5 days per week over a 28-day cycle in 4 cohorts: single-agent (20 mg; n=7 or 30 mg; n=6) or in combination with ABI (20 mg; n=13 or 30 mg; n=4). Pts may have prior exposure to chemotherapy. Pts receiving ELTA + ABI had prior response to ABI then progressed. Response was evaluated by PCWG3 / RECIST v1.1. Results: As of 17 Sept 2018, 30 pts were treated with ELTA ± ABI with a median age of 71 and a median of 4 prior regimens; (87% ABI, 60% enzalutamide, and 57% chemotherapy). Twenty-one pts with mCRPC were evaluable for efficacy: 2 partial response (10%), 15 stable disease (71%), and 4 progressive disease (19%). The median treatment duration is 75.5 days; 4 pts still on treatment. Treatment-related adverse events (TRAEs) occurring in ≥30% of the pts: fatigue (67%; 13% Gr≥3), nausea (63.3%; 0% Gr≥3), decreased appetite (57%; 0% Gr≥3), diarrhea (47%; 3% Gr≥3), weight decreased (43%; 3% Gr≥3), vomiting (37%; 7% Gr≥3), anemia (33%; 7% Gr≥3), and dysgeusia (33%; 0% Gr≥3). The 2 Gr4 TRAEs were neutropenia and elevated AST. Conclusions: ELTA± ABI is well-tolerated with AEs mostly limited to Gr 1/2 and demonstrates preliminary anti-tumor activity in patients with mCRPC. With enrollment complete, these results warrant further investigation of ELTA ± ABIin mCPRC. Clinical trial information: NCT02649790