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142 Background: Napabucasin is an NQO1-bioactivatable investigational agent hypothesized to affect multiple oncogenic cellular pathways including pSTAT3 through the generation of reactive oxygen species. This phase 1 open-label study evaluated the DDI potential of napabucasin and its major metabolite (M1) with respect to 7 major human drug cytochrome P450 (CYP) enzymes and the breast cancer resistance protein (BCRP) transporter. Methods: Healthy adult subjects who initially demonstrated they could tolerate administration of 240 or 480 mg twice daily (BID) napabucasin over 2-days (D) received single doses of the CYP and transporter substrates, followed by ≥7-day washout. In the DDI portion, subjects received napabucasin 240 mg BID on D1–11 with the phenotyping cocktail administered on D6 (omeprazole [CYP2C19] 20 mg, caffeine [CYP1A2] 100 mg, flurbiprofen [CYP2C9] 50 mg, bupropion [CYP2B6] 150 mg, dextromethorphan [CYP2D6] 30 mg, and oral midazolam [CYP3A] 2 mg), intravenous (IV) midazolam 2 mg on D7, repaglinide (CYP2C8) 0.25 mg on D8, and rosuvastatin (BRCP transporter) 10 mg on D9. Results: DDI potential was evaluated in 17 subjects. Exposure to omeprazole, flurbiprofen, and oral midazolam with (test) or without (reference) napabucasin 240 mg BID were similar. Napabucasin increased exposure (area under the curve to infinity) to caffeine (124%), IV midazolam (118%), repaglinide (127%), and rosuvastatin (213%); and decreased exposure to bupropion (79%) and dextromethorphan (71%). None of these changes were expected to be clinically meaningful. The exposure of the major metabolites of the probe drugs with or without napabucasin or M1 were similar. Of the 17 subjects, 12 (70.6%) reported adverse events (AEs); 58.8% reported gastrointestinal disorders. One patient had a grade 3 AE (neutrophil count low); no serious AEs were observed. Conclusions: The data suggest minimal in vivo DDI potential for napabucasin with respect to 7 major human drug CYP enzymes and the BCRP transporter. Napabucasin 240 mg BID was generally tolerable in healthy subjects. Co-administration of napabucasin with CYP and transporter substrates was safe and tolerable. Clinical trial information: NCT03411122.