Background: To explore the safety and synergistic anti-tumor effect of fruquintinib (a VEGFR inhibitor) in combination with sintilimab (an anti-PD-1 Ab) in patients (pts) with advanced colorectal cancer (CRC) and other solid tumors. Methods: This is an ongoing phase Ib/II, multicenter, two-stage study. Pts with variety cancer types, including CRC, were enrolled and is continuously enrolling in the study. For this interim analysis, all pts were analyzed for safety whereas only CRC pts were analyzed for efficacy. MMR status were analyzed for all enrolled CRC pts. Stage 1 was classical “3+3” dose escalation with pts assigned to one of the following 4 cohorts, fruquintinib taken orally at 3mg (Cohort A, 3 weeks on/ 1 week off), 4mg (Cohort B, 3 weeks on/ 1 week off), 5mg-intermittent (Cohort C, 2 weeks on/ 1 week off) or 3mg-continuous (Cohort E, once daily), while sintilimab was given at 200mg intravenously with Q4W in Cohort A and Cohort B whereas Q3W in Cohort C and Cohort E. DLT was observed for 28 days. Stage 2 was dose expansion with pts receiving 5mg-intermittent or 3mg-continuous fruquintinib plus sintilimab (200mg, Q3W). The primary endpoints were safety and tolerability and secondary endpoint was objective response rate (ORR). Results: As of Jan 5, 2021, 44 CRC pts which failed to at least 2 previous lines of therapy containing fluoropyrimidine, oxaliplatin or irinotecan were enrolled. They received either 5mg-intermittent or 3mg-continous dosage (n = 22, each), the ORR was 22.7% (10/44, 95% CI: 11.5-37.8%) with 27.3% (6/22, 95% CI: 10.7-50.2%) in 5mg-intermittent group and 18.2% (4/22, 95% CI: 5.2-40.3%) in 3mg-continuous group. With a median follow-up time of 8.3 (range: 0-9.6) months, the K-M estimated median PFS was 6.8 (95% CI:5.6-NA) months and 4.3 (95% CI:3.5-NA) months for 5mg-intermittent group and 3mg-continuous group, respectively. Overall, 60 pts were enrolled for safety analysis, including 23 in stage1 and 37 (only CRC) in stage 2. In stage 1, all pts experienced TEAEs, 52.2% of which were ≥ grade 3. The most frequently reported TEAEs were TSH increasing (73.9%), fecal occult blood positive (56.5%), and Palmar-plantar erythrodysaesthesia syndrome (PPES) (56.5%). SAEs occurred in 8 (34.8%) pts and no treatment-related death was reported. One patient in Cohort B reported manageable DLT. In stage 2, all pts experienced TEAEs, 18 (48.6%) pts experienced ≥ grade 3 TEAEs with 6 (31.6%) in 5mg-intermittent group and 12 (66.7%) in 3mg-continuous group. The most common TEAEs were proteinuria (45.9%) and TSH increasing (37.8%). TEAEs leading to either fruquintinib or sintilimab discontinuation occurred in 3 (5%) pts each. Conclusions: Fruquintinib plus sintilimab showed promising efficacy and favorable safety profile in advanced CRC. Clinical trial information: NCT03903705