Inclusion Criteria:

          -  Male or female patients, ≥18 years.

          -  Documented (histologically or cytologically proven), locally advanced or metastatic
             solid tumor malignancy (must be one of the following):

               1. Squamous cell carcinoma of the head and neck

               2. Non-small-cell lung carcinoma-adenocarcinoma histology subtype

               3. Pancreatic ductal adenocarcinoma

               4. Cholangiocarcinoma

               5. Colorectal carcinoma (microsatellite stable [MSS] and microsatellite
                  instability-high [MSI-H] phenotypes)

               6. Gastric carcinoma (includes gastroesophageal carcinoma)

               7. Esophageal carcinoma (includes squamous cell and adenocarcinoma)

               8. Mesothelioma (pleural and peritoneal)

          -  Malignancy that is not currently amenable to surgical intervention due to either
             medical contraindications or non-resectability of the tumor.

          -  Measurable disease according to RECIST v1.1.

          -  Refractory to or intolerant of existing therapy(ies) known to provide clinical
             benefit.

          -  Agreeing to mandatory tumor tissue biopsies (2 total).

          -  ECOG PS of 0 or 1.

          -  Adequate organ function as indicated by the following laboratory values.

          -  Adequate contraception required as appropriate.

        Exclusion Criteria:

          -  Central nervous system (CNS) malignancies.

          -  Clinically significant cardiovascular disease or condition.

          -  Active thrombosis, or a history of deep vein thrombosis or pulmonary embolism within 4
             weeks prior to the first dose of study drug(s).

          -  Active uncontrolled bleeding or a known bleeding diathesis.

          -  Significant ocular disease or condition.

          -  Significant pulmonary disease or condition.

          -  Current or recent (within 6 months) significant gastrointestinal disease or condition.

          -  Active, known or suspected autoimmune disease.

          -  History of organ transplantation (i.e., stem cell or solid organ transplant).

          -  Known history of human immunodeficiency virus (HIV) or known active infection with
             hepatitis B virus (HBV) or hepatitis C virus (HCV).

          -  Any other serious/active/uncontrolled infection.

          -  History of significant toxicities associated with previous administration of immune
             checkpoint inhibitors.

          -  Known or suspected hypersensitivity to any of the excipients of formulated study drug.

          -  Unresolved >Grade 1 toxicity associated with any prior antineoplastic therapy.

          -  Inadequate recovery from any prior surgical procedure, or patients having undergone
             any major surgical procedure within 4 weeks prior to the first dose of study drug(s).

          -  Any other serious, life-threatening, or unstable preexisting medical condition (aside
             from the underlying malignancy).

        Therapeutic Exclusions

          -  Prior therapy with Sym024 or other inhibitors of CD73, CD39 or adenosine receptors
             ADORA2A, ADORA2B.

          -  Part II and Part III, prior anti-PD-(L)1 therapy, except for indications where it is
             approved.

          -  Any antineoplastic agent for the primary malignancy (standard or investigational)
             within 4 weeks or 5 elimination half-lives.

          -  Any other investigational treatments within 2 weeks prior to the first dose of study
             drug(s).

          -  Radiotherapy, with exceptions.

          -  Live vaccines against infectious diseases 4 weeks prior to the first dose of study
             drug(s).

          -  Immunosuppressive or systemic glucocorticoids therapy (>10 mg daily prednisone or
             equivalent) within 2 weeks prior to the first dose of study drug(s), with exceptions.

          -  Prophylactic use of hematopoietic growth factors within 1 week prior to the first dose
             of study drug(s).