PFS is defined as the duration from the date of randomization to the date of confirmed disease progression (PD) determined by independent review committee (IRC) per international myeloma working group (IMWG) (2016) response criteria, or death, whichever occurs first.

ORR is defined as the percentage of participants who achieve confirmed partial response (PR) + VGPR + complete response (CR) + stringent complete response (sCR) or per IRC assessment.

An AE is defined as any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have a causal relationship with this treatment.

Clinical activity is defined as change in response rates \[Overall Response Rate (ORR), Complete Response (CR) or Better, Very Good Partial Response (VGPR), Partial Response (PR)\] as determined International Myeloma Working Group (IMWG (2016).

MRDnegCR rate, is defined as the percentage of participants who have achieved stringent complete response (sCR) or CR as assessed by independent review committee (IRC) and have negative MRD defined at 10\^-5 threshold as assessed by next generation sequencing (NGS).

PFS is defined as the duration from the date of randomization to the date of confirmed disease progression (PD) determined by IRC per IMWG (2016) response criteria, or death, whichever occurs first.

DLT events are defined as clinically significant adverse events or abnormal laboratory values assessed as unrelated to disease progression, underlying disease, intercurrent illness, or concomitant medications.

An adverse event is defined as any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have a causal relationship with this treatment.

DLT events are defined as clinically significant adverse events or abnormal laboratory values assessed as unrelated to disease progression, underlying disease, intercurrent illness, or concomitant medications.

DLT events are defined as clinically significant adverse events or abnormal laboratory values assessed as unrelated to disease progression, underlying disease, intercurrent illness, or concomitant medications.

DLT events are defined as clinically significant adverse events or abnormal laboratory values assessed as unrelated to disease progression, underlying disease, intercurrent illness, or concomitant medications.

DLT events are defined as clinically significant adverse events or abnormal laboratory values assessed as unrelated to disease progression, underlying disease, intercurrent illness, or concomitant medications.

Cytokine Release Syndrome events will be graded using American Society for Transplantation and Cellular Therapy (ASTCT), with a higher grade indicating higher severity.

ICANS events will be graded using ASTCT, with a higher grade indicating higher severity.

Cmax of ABBV-383.

Tmax of ABBV-383.

Ctrough of ABBV-383.

AUC of ABBV-383.

CRS is defined by fever, hypoxia, and hypotension and graded according to the American Society for Transplantation and Cellular Therapy (ASTCT) guidelines

AEs of special interest will be graded according to American Society for Transplantation and Cellular Therapy (ASTCT) 2019 guidelines. All other AEs will be graded according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0.