A Study to Evaluate Mezigdomide, Bortezomib and Dexamethasone (MEZIVd) Versus Pomalidomide, Bortezomib and Dexamethasone (PVd) in Participants With Relapsed or Refractory Multiple Myeloma (RRMM)

Inclusion Criteria

\- Participant has documented diagnosis of MM and measurable disease, defined as any of the following:.

i) M-protein ≥ 0.5 grams per deciliter (g/dL) by serum protein electrophoresis (sPEP) or.

ii) M-protein ≥ 200 milligrams (mg) per 24-hour urine collection by urine protein electrophoresis (uPEP).

iii) For participants without measurable disease in sPEP or uPEP: serum free light chain (sFLC) levels \> 100 mg/L (10 mg/dL) involved light chain and an abnormal kappa/lambda FLC ratio.

* Participants received 1 to 3 prior lines of antimyeloma therapy.
* Participants achieved minimal response \[MR\] or better to at least 1 prior antimyeloma therapy.

Exclusion Criteria

\- Participant has had progression during treatment or within 60 days of the last dose of a proteasome inhibitor, except as noted below:.

i) Subjects who progressed while being treated with, or within 60 days of last dose of bortezomib maintenance given once every 2 weeks (or less frequently) are not excluded.

ii) Participants who progressed while being treated with ixazomib monotherapy maintenance ≥ 6 months prior to the time of starting study treatment are not excluded.

* For participants with prior treatment of a bortezomib containing regimen, the best response achieved was not a minimal response (MR) or better, or participant discontinued bortezomib due to toxicity.
* Participant has had prior treatment with mezigdomide or pomalidomide.
* Other protocol-defined Inclusion/Exclusion criteria apply.