Background: BBI-608 is an oral first-in-class cancer stemness inhibitor that blocks STAT3-mediated transcription of cancer stemness genes. Potent anti-CSC activity was observed in vitro and in vivo,in mono and combination therapy. In phase I studies, BBI-608 was generally well tolerated with encouraging signs of anti-tumor activity. Methods: A phase Ib open label, multi-center study in pts with advanced CRC was undertaken to confirm the RP2D of BBI-608 in combination with FOLFIRI with or without BEV. BBI-608 was administered at 240 mg BID in combination with FOLFIRI (5-FU 400 mg/m² bolus with 2400 mg/m², irinotecan 180 mg/m², and leucovorin 400 mg/m²infusion) with or without BEV 5 mg/kg, administered bi-weekly until progression of disease, unacceptable toxicity, or other discontinuation criterion was met. Results: 18 heavily pretreated pts with an average of > 3 prior lines of therapy of which 10 pts (56%) previously progressed on FOLFIRI were enrolled. Of the 17 pts evaluable for response, 8 pts received FOLFIRI and 9 pts received FOLFIRI with BEV in combination with BBI-608. Most common adverse events included grade 1 and 2 diarrhea, abdominal pain, nausea, vomiting and anorexia. No dose limiting toxicity or new adverse events were seen, and the safety profile was similar to that of each regimen as monotherapy. Grade 3 events related to protocol therapy included diarrhea occurring in 3 pts, fatigue in 2 pts and dehydration in 1 pt. All events resolved after dose reduction and/or start of anti-diarrheal medications. No significant pharmacokinetic interactions were observed. Disease control (PR+SD) was observed in 16 of 17 evaluable pts (94%) with 2 PR (per RECIST 1.1 criteria: 44% and 33% regression) and 14 SD (of which 13 (93%) had tumor regression < 25%). In the evaluable pts, median progression free survival was 5.54 months. Of 17 pts, 7 (41%) had prolonged SD of > 6 months. Conclusions: This phase Ib study confirmed that BBI-608 at 240 mg bid can be safely combined with FOLFIRI with and without BEV, and shows encouraging anti-tumor activity in heavily pretreated CRC pts, even in pts with prior progression on FOLFIRI-based therapy. Clinical trial information: NCT02024607