This study will evaluate the safety and tolerability profile of belantamab mafodotin when administered in combination with approved regimens of either Lenalidomide Plus Dexamethasone [Len/Dex (Arm A)] or Bortezomib Plus Dexamethasone [Bor/Dex (Arm B)] in participants with RRMM, i.e., those who have relapsed or who are refractory to at least 1 line of approved therapy. Part 1 of the study will be a dose escalation phase to evaluate the safety and tolerability of up to 3 dose levels and up to 2 dosing schedules of belantamab mafodotin in combination with the two standard of care (SoC) regimens. Part 2 will further evaluate the safety and preliminary clinical activity of belantamab mafodotin at selected dose levels and dosing schedules in combination with Len/Dex or Bor/Dex.
A total of 152 evaluable participants will be enrolled in the study with up to 27 in Part 1 and up to 125 in Part 2. Participants receiving treatment Arm A, may continue combination treatment until the occurrence of progressive disease (PD), intolerable adverse events (AEs ), consent withdrawal, death or end of study. The participants receiving treatment Arm B, may continue combination treatment for a total of up to 8 cycles. After 8 cycles of combination therapy, the participants will continue treatment with belantamab mafodotin, as a monotherapy until the occurrence of PD, intolerable AEs, consent withdrawal, death or end of study.