This study is an open-label, multi-center, Phase 1 study of ISB 1342 in subjects with relapsed/refractory multiple myeloma refractory to proteasome inhibitors (PIs), immunomodulators (IMiDs), and daratumumab. There will be a dose escalation phase (Part 1) and dose expansion phase (Part 2). In Part 1 of the study, subjects will be treated at escalating dose levels. Once the recommended part 2 dose (RP2D) of ISB 1342 is declared in Part 1, the expansion phase (Part 2) will be initiated at the RP2D.