Study of ABBV-383 Assessing Adverse Events and Clinical Activity With Subcutaneous (SC) Injection in Adult Participants With Relapsed or Refractory Multiple Myeloma

Overall Response Rate (ORR)

The ORR is defined as the percentage of participants who achieve a best overall response of confirmed PR or better determined by international myeloma working group (IMWG) criteria, prior to the initiation of subsequent myeloma therapy.

Time frame: Up to 24 months

Percentage of Participants Achieving Stringent Complete Response (sCR),

sCR is defined as participants achieving negative immunofixation on the serum and urine, disappearance of any soft tissue plasmacytomas, \< 5% plasma cells in bone marrow, normal free light chain (FLC) ratio, and Absence of clonal cells in bone marrow by immunohistochemistry.

Time frame: Up to 24 months

Percentage of Participants Achieving Complete Response (CR)

CR is defined as participants achieving negative immunofixation on the serum and urine, disappearance of any soft tissue plasmacytomas, \< 5% plasma cells in bone marrow, and for participants in whom the only measurable disease is by serum FLC levels, a normal FLC ratio.

Time frame: Up to 24 months

Percentage of Participants Achieving Very Good Partial Response (VGPR)

VGPR is defined as participants achieving serum and urine M-protein detectable by immunofixation but not on electrophoresis, \>= 90% reduction in serum M-protein plus urine, and for participants in whom the only measurable disease is by serum FLC levels, \>= 90% decrease in the difference between involved and uninvolved FLC levels.

Time frame: Up to 24 months

Percentage of Participants Achieving Partial Response (PR)

PR is defined as participants achieving \>= 50% reduction of serum M-protein, reduction in 24-hour urinary M-protein by \>= 90% as noted in the protocol, \>= 50% reduction in the size of soft tissue plasmacytomas is also required, if present at baseline.

Time frame: Up to 24 months

Duration of Response (DoR)

DoR will be defined as the time from the date of first response \[partial response (PR) + VGPR + complete response (CR) + stringent complete response (sCR)\] to the earliest occurrence of progressive disease, or death, whatever occurs first.

Time frame: Up to 24 months

Progression Free Survival (PFS)

PFS is defined as the duration from the date of randomization to the date of confirmed disease progression (PD) per international myeloma working group (IMWG) criteria, or death, whichever occurs first.

Time frame: Up to 24 months

Time to Response (TTR)

TTR is defined as the number of months from the date of first dose to the date of best overall response of CR or PR ('responders') determined by IMWG criteria.

Time frame: Up to 24 months

Immunogenicity of ABBV-383 as Determined by Anti-Drug Antibodies (ADAs)

Incidence and concentration of ADAs.

Time frame: Up to 27 months

Immunogenicity of ABBV-383 as Determined by Neutralizing Anti-Drug Antibodies (NAbs)

Incidence and concentration of NAbs.

Time frame: Up to 27 months