Part A and Part B Abbreviated Inclusion Criteria:

          -  ≥ 18 years of age

          -  Locally advanced or metastatic (Stage IV) solid tumor that has progressed during or
             after standard therapy, and for whom no available therapies are appropriate (based on
             investigator judgment)

          -  Patients in Part B with advance or metastatic ccRCC or HCC must have at least 1
             measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1

          -  Patients with HCC in Part B must have at least 1 measurable target lesion according to
             modified RECIST (mRECIST)

          -  Patients with HCC must have unresectable disease, Barcelona Clinic Liver Cancer
             (BCLC1) Stage B (not eligible for transcatheter arterial chemoembolization [TACE]) or
             Stage C

          -  For patients in Part B with ccRCC, demonstrated progressive disease (PD) during or
             after the most recent treatment regimen. Prior treatment history must include
             progression during or after treatment with regimen(s) that have included a vascular
             endothelial growth growth factor (VEGF)-targeted agent and an immune checkpoint
             inhibitor. Patients who did not progress on but discontinued the VEGF-targeted agent
             for toxicity or intolerability are permitted.

          -  For patients in Part B with HCC, demonstrated PD during or after the most recent
             treatment regimen. Prior treatment history must include progression during or after
             treatment with a VEGF-targeted agent. Patients who did not progress on but
             discontinued the VEGF-targeted agent for toxicity or intolerability are permitted.

          -  For Part B patients in the tumor biopsy subsets only, must have tumor tissue that is
             accessible for pretreatment and on-treatment tumor biopsy in the opinion of the
             Investigator and be willing to undergo pretreatment and on-treatment biopsies per
             protocol

          -  Serum creatinine clearance ≥ 30 mL/min per Cockcroft-Gault formula or serum creatinine
             ≤ 2.0 x the upper limit of normal (ULN)

          -  Total bilirubin ≤ 1.5 x ULN (≤ 3 x ULN if elevated because of Gilbert's syndrome and ≤
             2 x ULN for patients with HCC or patients with known liver metastases)

          -  Aspartate aminotransferase (AST/SGOT) and alanine aminotransferase (ALT/SGPT) < 2.5 x
             ULN (< 5 x ULN if liver metastasis or for patients with HCC)

          -  For patients with HCC, Child-Pugh class A or B7 with a serum albumin ≥ 2.8 g/dL (≥ 28
             g/L)

          -  Adequate hematologic function, defined as absolute neutrophil count (ANC) ≥ 1.0 x
             109/L, hemoglobin ≥ 9.0 g/dL, and platelet count ≥ 100 x 109/L. For patients with HCC,
             platelet count ≥ 75 x 109/L without transfusion

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1

        Part C Abbreviated Inclusion Criteria:

          -  Advanced RCC of any histology or advanced HCC previously treated with at least one
             systemic anticancer therapy

          -  Patients with HCC must have unresectable disease, Barcelona Clinic Liver Cancer (BCLC)
             Stage B (not eligible for transcatheter arterial chemoembolization) or Stage C

          -  At least 1 measurable lesion per RECIST 1.1

          -  Patients with HCC must have at least 1 measurable target lesion according to modified
             RECIST (mRECIST)

          -  ANC ≥1000/µL (1.0 x 109/L)

          -  Platelets ≥100 000/µL (≥ 100 x 109/L)

          -  Hemoglobin for participants with RCC: ≥9.0 g/dL1; for participants with HCC: ≥8.5
             g/dL1

          -  Creatinine OR measured or calculated creatinine clearance (GFR can also be used in
             place of creatinine or CrCl) ≤1.5 × ULN OR ≥30 mL/min for participant with creatinine
             levels >1.5 × institutional ULN

          -  Total bilirubin ≤1.5 × ULN OR direct bilirubin ≤ULN for participants with total
             bilirubin levels >1.5 × ULN

          -  AST (SGOT) and ALT (SGPT) ≤2.5 × ULN (≤5 × ULN for participants with liver metastases)

          -  International normalized ratio (INR) OR prothrombin time (PT) Activated partial
             thromboplastin time (aPTT) ≤1.5 × ULN unless participant is receiving anticoagulant
             therapy as long as PT or aPTT is within therapeutic range of intended use of
             anticoagulants

          -  For patients with HCC, Child-Pugh class A or B7 with a serum albumin ≥ 2.8 g/dL (≥ 28
             g/L)

          -  Willingness of male and female patients who are not surgically sterile or
             postmenopausal to use medically acceptable methods of birth control for the duration
             of the study drug period (or beginning 14 days before the initiation of pembrolizumab
             for oral contraception), including 75 days after the last dose of SRF388 or 120 days
             after the last dose of pembrolizumab; male patients must refrain from donating sperm
             during this period. Sexually active men, and women using oral contraceptive pills,
             should also use barrier contraception with spermicide. Azoospermic male patients and
             WCBP who are continuously not heterosexually active are exempt from contraceptive
             requirements. However, female patients must still undergo pregnancy testing as
             described in this section.

        Part C Abbreviated Inclusion Criteria Specific to Patients with RCC or HCC from Part A or
        Part B:

          -  Progressed on SRF388 by RECIST 1.1

          -  Did not experience prior Grade ≥ 3 toxicity related to SRF388

          -  Willingness to undergo pretreatment core or excisional biopsy if deemed safe and tumor
             is accessible, in the opinion of the Investigator

          -  Has received no systemic anticancer therapies between SRF388 doses

        Part A and Part B Abbreviated Exclusion Criteria:

          -  Previously received an anti-IL-27 antibody or anti-IL-27 targeted therapy

          -  For patients in Part B with renal cell carcinoma (RCC), non-clear cell RCC histology

          -  For patients with HCC, fibrolamellar or mixed hepatocellular cholangiocarcinoma

          -  History of Grade 4 allergic or anaphylactic reaction to any monoclonal antibody
             therapy or any excipient in the study drugs

          -  Major surgery within 4 weeks prior to Screening

          -  Unstable or severe uncontrolled medical condition (eg, unstable cardiac function,
             unstable pulmonary condition including pneumonitis and/or interstitial lung disease,
             uncontrolled diabetes) or any important medical illness or abnormal laboratory finding
             that would, in the Investigator's judgment, increase the risk to the patient
             associated with his or her participation in the study

        Part C Abbreviated Exclusion Criteria:

          -  Is currently participating in or has participated in a study of an investigational
             agent or has used an investigational device within 4 weeks prior to the first dose of
             study drug

          -  Previously received an anti-IL 27 antibody or anti-IL 27 targeted therapy (exception
             to patients who received SRF388 in Part A or Part B)

          -  No prior systemic therapy for unresectable or metastatic disease

          -  Received > 5 prior systemic regimens for unresectible or metastatic disease (prior
             PD-(L)1 inhibitors are allowed if the patient did not discontinue therapy due to ≥
             Grade 3 drug-related toxicity)

          -  For patients with HCC, fibrolamellar histology or mixed hepatocellular
             cholangiocarcinoma

          -  For patients with HCC, moderate or severe ascites

          -  For patients with HCC, inability to undergo disease evaluation with triphasic computed
             tomography or magnetic resonance imaging because of contrast allergy or other
             contraindication

          -  For patients with HCC, imaging findings consistent with ≥ 50% liver occupation by HCC
             tumors

          -  History of Grade 4 allergic or anaphylactic reaction to any monoclonal antibody
             therapy or any excipient in the study drugs

          -  Surgeries that required general anesthesia must be completed at least 2 weeks before
             first study drug administration

          -  Prior autologous stem cell transplant ≤ 3 months before the first dose

          -  Prior allogeneic hematopoietic cell transplant within 6 months of the first dose or
             with a history of or current clinical Graft-Versus-Host Disease

          -  Has had an allogenic tissue/solid organ transplant

          -  Other unstable or severe uncontrolled medical condition (eg, unstable cardiac
             function, unstable pulmonary condition, uncontrolled diabetes) or any important
             medical illness or abnormal laboratory finding that would, in the Investigator's
             judgment, increase the risk to the patient associated with his or her participation in
             the study