A Study of an MMSET Inhibitor in Patients With Relapsed and Refractory Multiple Myeloma

Key Inclusion Criteria for Dose-Expansion: * ≥ 18 years of age * ECOG score ≤ 1 * Multiple myeloma (as per IMWG) * Prior therapy for MM: Participants must have received at least 1 and up to 3 prior lines of therapy as defined by IMWG, and the following drug classes: PI, IMiD, and anti-CD38 antibody. For mezigdomide combination Cohorts B1 and B2, participants must have received at least 2 prior lines of therapy * Participants must have a confirmed diagnosis of progressive MM (per IMWG), t(4;14) confirmed by fluorescence in situ hybridization (FISH) testing performed in a centralized Clinical Laboratory Improvement Amendments (CLIA) accredited laboratory via fresh tumor biopsy. * Measurable disease, including at least 1 of the following criteria: * Serum M protein ≥ 0.50 g/dL (by SPEP) * Serum IgA ≥ 0.50 g/dL (IgA myeloma patients) * Urine M protein ≥ 200 mg/24 h (by UPEP) * sFLC involved light chain ≥ 10 mg/dL (100 mg/L) (patients with abnormal sFLC ratio) * Bone marrow plasma cells ≥ 30% (if only criterion for measurability) * Agreement to enroll into the REMS program (Cohort D- pomalidomide cohort only) Key Exclusion Criteria for Dose-Expansion: * Treatment with the following therapies in the specified time period prior to first dose: * Patients in Cohorts B1 and B2 must not have received prior mezigdomide treatment * Carfilzomib in the immediate last prior line of therapy for patients enrolled in Cohorts C1 and C2 * Pomalidomide in the immediate last prior line of therapy for patients enrolled in cohort D * Radiation, chemotherapy, immunotherapy, or any other anticancer therapy ≤ 2 weeks * Cellular therapies ≤ 8 weeks * Autologous transplant \< 100 days * Allogenic transplant ≤ 6 months, or \> 6 months with active GVHD * Major surgery ≤ 4 weeks * Current plasma cell leukemia, POEMS (polyneuropathy, organomegaly, endocrinopathy, and skin changes) syndrome, solitary bone lesion or bone lesions as the only evidence for plasma cell dyscrasia, myelodysplastic syndrome or a myeloproliferative neoplasm or light chain amyloidosis * Active CNS disease: participants with previously treated stable CNS disease are eligible, except for Cohorts B1 and B2 for which known CNS myeloma involvement is completely excluded. * Inadequate bone marrow function * Inadequate renal, hepatic, pulmonary, and cardiac function * Active, ongoing, or uncontrolled systemic viral, bacterial, or fungal infection. Permitted prophylactic medications, antimicrobials or antiretroviral therapies defined in protocol. * Use of acid reducing agents and strong inhibitors or inducers of CYP3A4 within 7 days or 5 half-lives (whichever is longer) prior to first dose * Strong CYP1A2 inhibitors for patients receiving pomalidomide (Cohort D) * Active malignancy not related to myeloma requiring therapy within \< 2 years prior to enrollment, or not in complete remission, with exceptions defined in protocol.