1. Part A:

             • No previous therapy for cancer. Patients may have received prior neoadjuvant or
             adjuvant therapy as long it was completed without disease recurrence for at least 6
             months since last treatment.

          2. Part B:

               -  Disease progression during first-line therapy or within 4 months after the last
                  dose of first-line therapy.

               -  Documentation of elevated DKK1 mRNA expression from a fresh tumor biopsy or a
                  biopsy obtained within the 6 months of screening.

          3. Able to provide written informed consent prior to any study-specific procedures.

          4. Confirmed diagnosis of gastric adenocarcinoma or GEJ adenocarcinoma.

          5. One or more tumors measurable on radiographic imaging as defined by RECIST 1.1.

          6. ECOG performance status ≤ 1 within 7 days of first dose of study drug

          7. Acceptable liver, renal, hematologic, and coagulation function

          8. Females of childbearing potential and male partners of female patients must agree to
             use adequate contraception during the study and for 6 months after their last dose of
             study drug.

        Exclusion:

          1. Part A:

               1. Diagnosis of HER2-positive G/GEJ adenocarcinoma.

               2. Unable to swallow capsules or disease significantly affected gastrointestinal
                  function (add diseases as examples).

          2. Part B:

             a. Major surgery or chemotherapy within 21 days of first dose of study drug.

          3. Patients with active autoimmune diseases or history of autoimmune diseases that may
             relapse.

          4. Any condition that required treatment with steroids or any other immune suppressive
             drugs within 14 days prior to first dose of study drug.

          5. Active leptomeningeal disease or uncontrolled brain metastases.

          6. Any active cancer ≤ 2 years before first dose of study drug with the exception of
             cancer for this study.

          7. New York Heart Association Class III or IV cardiac disease, myocardial infarction
             within the past 6 months, or unstable arrhythmia.

          8. Fridericia-corrected QT interval (QTcF) > 470 msec (female) or history of congenital
             long QT syndrome.

          9. Active, uncontrolled bacterial, viral, or fungal infections, within 14 days of study
             entry requiring systemic therapy.

         10. Serious nonmalignant disease

         11. Pregnant or nursing.

         12. History of osteonecrosis of the hip or have evidence of structural bone abnormalities
             in the proximal femur on MRI scan that are symptomatic and clinically significant.

         13. Known osteoblastic bony metastasis.

         14. Major surgery 28 days prior to study entry.

         15. Prior radiation therapy within 14 days prior to study entry.

         16. Previously treated with an anti-DKK1 therapy, PD-1, anti-PD-L1, anti-PD-L-2

         17. Significant allergy to a pharmaceutical therapy that, in the opinion of the
             Investigator, poses an increased risk to the patient.

         18. Active substance abuse.

         19. Known dihydropyrimidine dehydrogenase deficiency.

         20. Administration of a live vaccine within 28 days before first dose of study drug.