A Study to Investigate the Safety, Pharmacokinetics, and Preliminary Effectiveness of GSK4418959 Alone or in Combination With Other Anti-cancer Agents in Participants With Solid Tumors

Inclusion Criteria: Parts 1, 2, and 3 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 GSK4418959 (alone or in combination with PD-1 inhibitor, as determined between Investigator and sponsor) as next line of 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 Parts 1 and 3 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 le…