Vactosertib in Combination with Pembrolizumab in Metastatic Colorectal or Gastric Cancer

This is phase 1b/2a, open label, multi-center study to assess safety, tolerability, pharmacokinetics and anti-tumor activity of vactosertib in combination with pembrolizumab in patients with mCRC including CMS4 or diffuse GC/GEJC with two phases (Dose Finding Phase and Dose Expansion Phase). At screening, CMS4 will be classified by an experienced pathologist in the central lab that will examine the histology of primary surgical tissues. Approximately, 67 total patients are expected to be enrolled in this study. The first phase of the study, the Dose Finding Phase, will determine the MTD of the combination regimen. The second phase, the Dose Expansion Phase, will further evaluate the combination regimen to confirm RP2D.