Inclusion Criteria

          -  Subject must be >=18 years of age at the time of signing the informed consent.

          -  Subjects with advanced/recurrent solid tumors, who have progressed on, be intolerant
             of, or ineligible for, all available therapies for which clinical benefit has been
             established.

          -  Histological or cytological documentation of an advanced solid tumor.

          -  A biopsy of the tumor tissue obtained at any time from the initial diagnosis to study
             entry. Although a fresh biopsy obtained during screening is preferred, archival tumor
             specimen is acceptable if it is not feasible to obtain a fresh biopsy. Subjects
             enrolled in the PK/Pharmacodynamic Cohorts must provide a fresh biopsy of a tumor
             lesion not previously irradiated during the screening period and must agree to provide
             at least one additional on-treatment biopsy.

          -  Measurable disease per RECIST version 1.1. Palpable lesions that are measurable by
             radiologic or photographic evaluations may be utilized as the only measurable lesion.

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

          -  Life expectancy of at least 12 weeks.

          -  In France, a subject will be eligible for inclusion in this study only if either
             affiliated to or a beneficiary of a social security category.

          -  Adequate organ function (hematologic system: Absolute neutrophil count [ANC]
             >=1.5x10^9/Liter, Hemoglobin >=9 grams per deciliter [g/dL], Platelets
             >=100x10^9/Liter, prothrombin time [PT]/ INR and partial thromboplastin time [PTT]
             [unless subject is receiving anticoagulant] <1.5 times ULN; Hepatic system: Total
             bilirubin <=1.5 times ULN and for subjects with Gilbert's Syndrome [only if direct
             bilirubin <=35%] total bilirubin <=3.0 times ULN; ALT <=2.5 times ULN, for subjects
             with liver metastases/tumor infiltration <=5 times ULN; renal system: Estimated
             glomerular filtration rate [eGFR] by Chronic Kidney Disease Epidemiology Collaboration
             [CKD-EPI] >60 milliliter per minute [mL/min]; Endocrine system: thyroid stimulating
             hormone (TSH) within normal limits; Cardiac system: Ejection fraction >= 50% by
             echocardiogram.

          -  Male or female: Female subjects are eligible to participate if they are not either
             pregnant or breastfeeding, and at least one of the following conditions applies: Is
             not a woman of childbearing potential (WOCBP) OR Is a WOCBP and using a contraceptive
             method that is highly effective (with a failure rate of <1% per year), with low user
             dependency, during the study treatment period and for at least 7 days (subjects
             receiving monotherapy) or 120 days (subjects receiving pembrolizumab) after the last
             dose of study treatment and agrees not to donate eggs (ova, oocytes) for the purpose
             of reproduction during this period. The investigator should evaluate the effectiveness
             of the contraceptive method in relationship to the first dose of study treatment. A
             WOCBP must have a negative highly sensitive pregnancy test (urine or serum) as
             required by local regulations) within 7 days before the first dose of study
             intervention. If a urine test cannot be confirmed as negative (e.g., an ambiguous
             result), a serum pregnancy test is required. In such cases, the subject must be
             excluded from participation if the serum pregnancy result is positive. Additional
             requirements for pregnancy testing during and after study treatment. The investigator
             is responsible for review of medical history, menstrual history, and recent sexual
             activity to decrease the risk for inclusion of a woman with an early undetected
             pregnancy.

          -  Capable of giving signed informed consent.

        Exclusion Criteria

          -  Malignancy other than disease under study with the exception of those from which the
             subject has been disease-free for more than 2 years and not expected to affect the
             safety of the subject or the endpoints of the trial. Curatively treated non-melanoma
             skin cancer is permitted.

          -  Symptomatic central nervous system (CNS) metastases or asymptomatic CNS metastases
             that have required steroids within 2 weeks prior to first dose of study treatment.
             Subjects with carcinomatous meningitis or leptomeningeal spread are excluded
             regardless of clinical stability.

          -  Active autoimmune disease that has required systemic disease modifying or
             immunosuppressive treatment within the last 2 years. Replacement therapy (e.g.,
             thyroxine or physiologic corticosteroid replacement therapy for adrenal or pituitary
             insufficiency, etc.) is permitted.

          -  Concurrent medical condition requiring the use of systemic immunosuppressive treatment
             within 28 days before the first dose of study treatment. Physiologic doses of
             corticosteroids for treatment of endocrinopathies or steroids with minimal systemic
             absorption, including topical, inhaled, or intranasal corticosteroids may be continued
             if the subject is on a stable dose.

          -  Current unstable liver or biliary disease per investigator assessment defined by the
             presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia, esophageal or
             gastric varices, persistent jaundice, or cirrhosis. Stable chronic liver disease
             (including Gilbert's syndrome or asymptomatic gallstones) or hepatobiliary involvement
             of malignancy is acceptable if subject otherwise meets entry criteria.

          -  History of vasculitis at any time prior to study treatment.

          -  Evidence or history of significant active bleeding or coagulation disorder.

          -  Active infection requiring systemic treatment, known human immunodeficiency virus
             infection, or positive test for hepatitis B surface antigen or hepatitis C.

          -  QTcF >450 milliseconds (msec) or QTcF >480 msec for subjects with bundle branch block.
             QTcF is the QT interval corrected for heart rate according to Fridericia's formula,
             machine-read or manually over-read.

          -  Recent history (within the past 6 months) of acute diverticulitis, inflammatory bowel
             disease, intra-abdominal abscess, or gastrointestinal obstruction.

          -  Recent history of allergen desensitization therapy within 4 weeks of starting study
             treatment.

          -  History or evidence of cardiovascular (CV) risk including any of the following: Recent
             (within the past 6 months) history of serious uncontrolled cardiac arrhythmia or
             clinically significant ECG abnormalities including second degree (Type II) or third
             degree atrioventricular block; Cardiomyopathy, myocardial infarction, acute coronary
             syndromes (including unstable angina pectoris), coronary angioplasty, stenting, or
             bypass grafting within the past 6 months before enrolment; Congestive heart failure
             (Class II, III, or IV) as defined by the New York Heart Association functional
             classification system (NYHA).

          -  Recent (within the past 6 months) history of symptomatic pericarditis.

          -  History of idiopathic pulmonary fibrosis, interstitial lung disease, or organizing
             pneumonia, or evidence of active, non-infectious pneumonitis. Note: post-radiation
             changes in the lung related to prior radiotherapy and/or asymptomatic
             radiation-induced pneumonitis not requiring treatment may be permitted if agreed by
             the investigator and Sponsor.

          -  History of (non-infectious) pneumonitis that required steroids or current pneumonitis.

          -  Recent history (within 6 months) of uncontrolled symptomatic ascites or pleural
             effusions.

          -  Any serious and/or unstable pre-existing medical, psychiatric disorder, or other
             condition that could interfere with the subject's safety, obtaining informed consent,
             or compliance to the study procedures.

          -  Is or has an immediate family member (e.g., spouse, parent/legal guardian, sibling or
             child) who is investigational site or sponsor staff directly involved with this trial,
             unless prospective Institutional Review Board (IRB) approval (by chair or designee) is
             given allowing exception to this criterion for a specific subject.

          -  Prior treatment with the following agents: Stimulator of Interferon Genes (STING)
             agonist at any time. Subjects treated in Part 1/monotherapy with GSK3745417 may be
             enrolled into Part 2/combination with pembrolizumab upon disease progression and upon
             discussion and approval from the GSK Medical Monitor; Anticancer therapy or
             investigational therapy within 28 days or 5 half-lives of the drug, whichever is
             shorter; Checkpoint inhibitors, including Programmed death receptor-1 (PD-1), PD-L1
             and Cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) inhibitors within 28 days;
             Prior radiation therapy: permissible if at least 1 non-irradiated measurable lesion is
             available for assessment according to RECIST version 1.1 or if a solitary measurable
             lesion was irradiated, objective progression is documented. A wash out of at least 28
             days before start of study treatment for radiation of any intended use to the
             extremities for bone metastases and 28 days for radiation to the chest, brain, or
             visceral organs is required. Palliative radiation is permissible at any time before or
             during the study.

          -  Receipt of any live vaccine within 30 days of the start of study treatment.

          -  Prior allogeneic or autologous bone marrow transplantation or other solid organ
             transplantation.

          -  Toxicity from previous treatment including: Toxicity Grade >=3 related to prior
             immunotherapy and that led to study treatment discontinuation; Toxicity related to
             prior treatment that has not resolved to Grade <=1 (except alopecia, hearing loss or
             grade <=2 neuropathy or endocrinopathy managed with replacement therapy).

          -  Received transfusion of blood products (including platelets or red blood cells) or
             administration of colony stimulating factors (including granulocyte colony stimulating
             factor (G-CSF), granulocyte-macrophage colony-stimulating factor, and recombinant
             erythropoietin) within 14 days before the first dose of study treatment.

          -  Concomitant administration of drugs that are sensitive substrates or narrow
             therapeutic range substrates for cytochrome p450 (CYP) 3A4, 1A2, 2C19 enzymes and
             OATP1B1 transporter, and moderate to strong inducers and inhibitors of CYP 3A4 should
             be excluded during the study and for 7 days prior to and following treatment with
             GSK3745417 (14 days for itraconazole).

          -  Major surgery <=28 days before the first dose of study treatment. Subjects must have
             also fully recovered from any surgery (major or minor) and/or its complications before
             initiating study treatment.

          -  Known drug or alcohol abuse.