Abstract
760 Background: The anti–programmed death 1 (PD-1) antibody, pembrolizumab provided an objective response rate (ORR) of 40% in patients (pts) with Mismatch Repair–Deficient (dMMR) or Microsatellite Instability–High (MSI-H) metastatic colorectal cancer (mCRC) vs 0% in pts with MMR-proficient/Microsatellite Stable (MSS) mCRC. The WNT/β-catenin signaling has been reported to prevent anti-tumor immunity and promote resistance of anti-PD-1/PD-L1 antibodies. This study investigates efficacy and safety of the combination of BBI608, which blocks phosphorylated STAT3 and downregulates WNT/β-catenin signaling, with pembrolizumab in pts with mCRC (both MSS and MSI-H). Here, we present the results of the phase I part. Methods: Phase I part was designed to determine the recommended phase II dose (RP2D) in a “3+3” cohort-based dose escalation design of BBI608 (240mg BID every day on level 1 and 480mg BID every day on level 2) with pembrolizumab (200mg/body q3w). We also conduct biomarker research using paired samples from repeated tumor biopsies and blood collections. Results: Five pts were enrolled in level 1, and 3 pts in level 2. All pts were MSS mCRC. Seven pts were included in the safety analyses. Two pts in level 1 were excluded from dose-limiting toxicity (DLT) evaluation because of disease progression during DLT evaluation period. No DLTs were observed at either level. Grade 3 or worse treatment-related adverse events (TRAEs) were not observed. Grade 1 or 2 TRAEs included diarrhea related to BBI608 (57%), hyperthyroidism (14%), hypothyroidism (14%), and fever (14%) without unexpected safety signals. One patient in level 2 showed a tumor shrinkage lasting more than 12 weeks for lung and lymph node metastases with remarkable decline of CEA level. Analysis of repeated tumor samples from this case demonstrated that CD8+ T-cells infiltration inside the tumor on treatment was observed by immunohistochemistry and flow-cytometry. Conclusions: BBI608 480mg BID with pembrolizumab was tolerable and determined as RP2D. This combination showed a hint of activity and might evoke immunity in MSS mCRC, which will be confirmed by ongoing phase II part. Clinical trial information: NCT02851004..
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