Company operational decision. Decision to stop study is not due to safety or efficacy concerns

This is a multicenter, open-label, phase I/II clinical study, to assess the safety, tolerability, antitumor activity, pharmacokinetics and pharmacodynamics of EOS884448 (also known as EOS-448 or GSK4428859A), alone or in combination with iberdomide with and without dexamethasone in participants with relapsed/refractory multiple myeloma (RRMM).

Inclusion Criteria:

• All participants must have a documented diagnosis of relapse/refractory Multiple Myeloma and have measurable disease as defined per IMWG criteria.
• All participants must have received at least 3 prior lines of MM therapy with IMiD, PI and Anti-CD-38, and progressed on their last therapy (prior BCMA targeted therapy allowed).
• All participants must have Eastern Cooperative Oncology Group (ECOG) Performance Status of 0,1or 2.
• All participants must have adequate organ function.
• Females of childbearing potential (FCBP) or male participant must agree to follow contraception requirements.

Exclusion Criteria:

• All participants with non-secretory MM
• All participants with known auto-immune disease
• All participants with history of life-threatening toxicity related to prior immune therapy.
• All participants with active graft versus host disease after allogeneic stem cell transplantation.
• All participants with active, unstable cardiovascular function.
• All participants with active infection requiring systemic therapy.
• All participants with hypersensitivity to any of the treatments.
• All participants with any active gastrointestinal dysfunction that prevents the patient from swallowing tablets or interferes with absorption of study treatment.