A Study to Investigate the Safety, Tolerability, Pharmacokinetics (PK), and Preliminary Anticancer Activity of GSK4524101 Alone or With Niraparib in Participants With Solid Tumors

The primary purpose of this study is to determine the maximum tolerated dose of GSK4524101 monotherapy (MTD) and GSK4524101 in combination with niraparib (MTDc). The study consists of two parts - Part 1 (Dose Escalation) and Part 2 (Dose Expansion).

Inclusion Criteria: * More than or equal to (≥)18 years of age * Eastern cooperative oncology group (ECOG) class 0-2 * Life expectancy of a minimum of 3 month * Participant has histologically diagnosed advanced or metastatic solid tumor and has exhausted all standard of care treatment options (Part 1). * Participant has metastatic, gBRCAmut, HER2-negative or HER2-low breast cancer who has completed at most 3 or more prior lines of therapy (Part 2). Exclusion Criteria: * Participant has not recovered (i.e., to Grade less than or equal to \[≤1\] or to baseline) from prior chemotherapy-induced AEs. * Participant is currently participating in a treatment study or has participated in a study of any investigational agent within 4 weeks of the first dose of treatment. * Participant has symptomatic uncontrolled brain or leptomeningeal metastases. * Participant has a known additional malignancy that progressed or required active treatment within the last 2 years * Participant has a known history of Myelodysplastic syndrome (MDS) or Acute myeloid leukemia (AML). * Participant has uncontrolled hypertension with sustained systolic blood pressure (BP) \>140 millimetres of mercury (mmHg) or diastolic BP \>90 mmHg.