Subject is ≥ 18 years of age at the time of signing the informed consent form (ICF).
2. Subject and/or subject’s legal representative must understand and voluntarily sign an ICF
according to local regulations prior to any study-related assessments/procedures being
conducted. For country-specific requirements, see APPENDIX I.
3. Subject is willing and able to adhere to the study visit schedule and other protocol
requirements.
4. Subject has documented diagnosis of MM and measurable disease, defined as any of the
following:
a. M-protein ≥ 0.5 g/dL by serum protein electrophoresis (sPEP) or
b. M-protein ≥ 200 mg/24-hour urine collection by urine protein electrophoresis (uPEP)
or,
c. For subjects 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.
5. Subject has received 1 to 3 prior lines of antimyeloma therapy. (Note: One line can contain
several phases [eg, induction, (with or without) hematopoietic stem cell transplant, (with or
without) consolidation, and/or (with or without) maintenance therapy) See APPENDIX H.
6. Subject must have received prior treatment with a lenalidomide-containing regimen. For
country-specific requirements, see APPENDIX I.
7. Subject achieved minimal response [MR] or better to at least 1 prior antimyeloma therapy.
8. Subject must have documented disease progression during or after their last antimyeloma
regimen.
9. Subject has an Eastern Cooperative Oncology Group (ECOG) performance status score of 0,
1 or 2.
10. Females of childbearing potential (FCBP) must agree and adhere to all testing and
contraception requirements in the respective Global Pregnancy Prevention Plan (PPP) for
mezigdomide or pomalidomide. Duration of contraception for FCBP must be in accordance
with the Global PPP for mezigdomide or pomalidomide, or seven months following the last
dose of bortezomib, whichever is later.
11. Male subjects must agree and adhere to all contraception requirements in the respective Global
PPP for mezigdomide or pomalidomide. Duration of contraception for male subjects must be
in accordance with the Global PPP for mezigdomide or pomalidomide, or four months
following the last dose of bortezomib, whichever is later.

Male subjects must agree to refrain from donating sperm in accordance with the Global PPP
for mezigdomide or pomalidomide, or for four months following the last dose of bortezomib,
whichever is later.
13. Females must agree to refrain from donating eggs in accordance with the Global PPP for
mezigdomide or pomalidomide.
14. Subjects must agree to refrain from donating blood while on study treatment, during dose
interruptions and for at least 28 days following the last dose of study treatment.
15. All male and female subjects must also follow all other requirements defined in the Global
PPP for mezigdomide or pomalidomide.

Subject who has had progression during treatment or within 60 days of the last dose of a
proteasome inhibitor, except as noted below:
a. Subjects who progressed while being treated with, or within 60 days of last dose of
bortezomib maintenance given once every 2 weeks or less are not excluded.
2. For subjects with prior treatment of a bortezomib containing regimen, the best response
achieved was not a minimal response (MR) or better, or subject discontinued bortezomib due
to toxicity.
3. Subject who has had prior treatment with mezigdomide or pomalidomide.
4. Subject who has had any investigational agents within 28 days or 5 half-lives (whichever is
shorter) of initiating study treatment.
a. Participation in any concurrent study where subjects will receive any drug or treatment
or any procedure that would interfere with study assessments is not permitted. Subjects
who have completed treatment with the prior investigational agent(s) and are currently
in Long-term Follow up are permitted.
5. Subject has received any of the following:
a. Plasmapheresis within the last 28 days of initiating study treatment
b. Major surgery (as defined by the investigator) within 28 days of initiating study
treatment.
c. Radiation therapy, other than local palliative therapy, for myeloma associated bone
lesions within 14 days of initiating study treatment.
d. Use of any systemic antimyeloma drug therapy within 14 days of initiating study
treatment.
6. Subject has previously received allogeneic stem cell transplantation at any time during prior
therapy or received autologous stem cell transplantation within 12 weeks of initiating study
treatment.
7. Subject has plasma cell leukemia, Waldenstrom’s macroglobulinemia, POEMS syndrome
(polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes) or
light-chain amyloidosis.
8. Subject with known central nervous system (CNS) involvement with myeloma.

Subject has any significant medical condition, including active or uncontrolled infection,
presence of laboratory abnormality, or psychiatric illness that places the subject at an
unacceptable risk for treatment-related complications, if he/she were to participate in the study.
10. Coronavirus disease 2019 (COVID-19) within 7 days for mild or asymptomatic infections or
14 days for moderate/severe infections prior to initiating study treatment. A longer duration
may be needed based on the investigator’s clinical judgment.
 Acute symptoms must have resolved and there are no sequelae that would place the subject at
a higher risk of receiving study treatment, based on investigator assessment in consultation
with the Sponsor Medical Monitor. No repeat/follow-up COVID-19 testing is required.
11. Subject has any condition that confounds the ability to interpret data from the study
12. Subject has any of the following laboratory abnormalities:
a. Absolute neutrophil count (ANC) < 1,000/μL. It is not permissible to administer GCSF
to achieve minimum ANC levels within 7 days prior to the complete blood count (CBC)
which will be used to determine eligibility (or within 14 days prior for pegfilgrastim).
b. Platelet count: < 75,000/μL for subjects in whom < 50% of bone marrow nucleated
cells are plasma cells; or a platelet count < 50,000/μL for subjects in whom ≥ 50% of
bone marrow nucleated cells are plasma cells (transfusions are not permitted within
7 days prior to the CBC which will be used to determine eligibility).
c. Hemoglobin < 8 g/dL (< 4.9 mmol/L). Not applicable for Protocol Amendment 4
and later.
d. Estimated glomerular filtration rate (eGFR) < 30 mL/min or requiring dialysis. eGFR
will be calculated using the Modification of Diet in Renal Disease (MDRD) formula

Corrected serum calcium > 13.5 mg/dL (> 3.4 mmol/L)
f. Serum aspartate aminotransferase (AST) or alanine aminotransferase (ALT) > 2.5 ×
upper limit of normal (ULN)
g. Serum total bilirubin > 1.5 × ULN, or for subjects with documented Gilbert’s syndrome
> 3.0 mg/dL