A First Time in Human (FTIH) Study of GSK3745417 Administered to Participants With Advanced Solid Tumors

This study aims to evaluate the safety, tolerability, and preliminary clinical activity and establish a recommended dose of GSK3745417 administered alone (Part 1A) or co-administered (Part 2A) with dostarlimab in participants with refractory/relapsed solid tumors. Both parts will consist of a dose escalation phase.

Inclusion Criteria: * Participant must be more than or equal to (\>=)18 years of age. * Participants 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. * Participants must provide a fresh biopsy. * Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-1. * Adequate organ function per protocol specifications. * Male or female participants. * Female participants are eligible to participate if they are not breastfeeding or pregnant (or intend to breastfeed or become pregnant). Women of childbearing potential must use a highly effective method of contraception. * Capable of giving signed informed consent. Exclusion Criteria: * Active autoimmune disease that has required systemic disease modifying or immunosuppressive treatment within the last 2 years. * Concurrent medical condition requiring the use of systemic immunosuppressive treatment within 28 days before the first dose of study treatment. * Current unstable liver or biliary disease. * 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. * QT duration corrected for heart rate by Fridericia's formula (QTcF) more than (\>)450 milliseconds (msec) or QTcF \>480 msec for participants with bundle branch block. * 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 * 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. * History of (non-infectious) pneumonitis that required steroids or current pneumonitis. * Recent history (within 6 months) of uncontrolled symptomatic ascites or pleural effusions. * Prior treatment with the following agents: 1. Stimulator of Interferon Genes (STING) agonist at any time. 2. Anticancer therapy or investigational therapy or used an investigational device within 28 days or 5 half-lives of the drug, whichever is shorter. 3. Checkpoint inhibitors, including Programmed death receptor-1 (PD-1), Programmed death Ligand-1 (PD-L1), PD-L2 and Cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) inhibitors within 28 days. 4. 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. * Pregnant and/or breast feeding participants or those who plan to become pregnant and/or breastfeed. * 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. * Major surgery less than or equal to (\