A Phase 1/2 Open-label, Multicenter Study of Oral GSK5460025 Alone or in Combination With Other Anti-cancer Agents in Adult Participants With Mismatch Repair-deficient (dMMR) or Microsatellite Instability-High (MSI-H) Solid Tumors

Inclusion Criteria: * Has a histologically diagnosed advanced (unresectable, metastatic or recurrent) solid tumor * Has a known dMMR/MSI-H status as determined by a certified local laboratory at the time of Pre-screening or has an unknown Mismatch repair (MMR)/ Microsatellite Instability (MSI) status at the time of Pre-screening and MMR/MSI status will be determined by central reference laboratory * Provides an archival or fresh (preferred) formalin fixed, paraffin embedded (FFPE) sample * Intends to receive GSK5460025 as next treatment * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Is expected to have a minimum of 3 months life expectancy * Has adequate organ function, as defined in the protocol Part 1 inclusion criteria: • Has histologically diagnosed advanced (unresectable, metastatic or recurrent) solid tumor and has exhausted all standard of care treatment options Part 2 inclusion criteria: * Has histologically diagnosed advanced (unresectable, metastatic or recurrent) Colorectal cancer (CRC) or Endometrial cancer (EC) * Has received at least 1 but no more than 3 lines of systemic anticancer therapy for their advanced (unresectable, metastatic or recurrent) disease including at least one line of Immune checkpoint inhibitors (ICI) therapy * Has measurable disease (i.e., at least 1 target lesion) during the Screening period per RECIST 1.1, as determined by the investigator Exclusion Criteria: * Has not recovered (i.e., to Grade ≤1 or to baseline) from prior anticancer therapy-induced Adverse Events (AEs) * Has received prior treatment with a Werner (WRN) inhibitor or Nucleotide Excision Repair Targeting (NERT) agent. * Is unable to swallow and retain orally administered study treatment * Has untreated or progressed metastases in brain or CNS * Has a known additional malignancy that progressed or required active treatment within the last 2 years because reoccurrence of another malignancy would confound interpretation by RECIST 1.1 criteria. Exceptions include basal or squamous cell carcinomas of the skin or in situ carcinomas \[e.g., breast, cervix, bladder\] that have been resected with no evidence of metastatic disease. * Has any impairment of gastrointestinal function or gastrointestinal disease that may significantly alter the absorption of study drugs * Has cirrhosis or current unstable liver or biliary disease * Has known hypersensitivity to any of the study interventions or any of their excipients