Study to Determine Recommended Phase 2 Dose of Intravenous (IV) Eftozanermin Alfa in Combination With IV or Subcutaneous (SC) Bortezomib and Oral Dexamethasone Tablet and to Assess Change in Disease Symptoms in Adult Participants With Relapsed or Refractory Multiple Myeloma

Inclusion Criteria:

* Documented diagnosis of multiple myeloma (MM) based on standard International Myeloma Working Group (IMWG) criteria.
* Has measurable disease at screening, defined by at least 1 of the following:

  * Serum M-protein \>= 1.0 g/dL (\>= 10 g/L); OR
  * Urine M-protein \>= 200 mg/24 hours; OR
  * Serum free light chain (sFLC) \>= 10 mg/dL (100 mg/L), provided serum FLC ratio is abnormal.
* Relapsed or refractory MM after receiving at least 3, but no more than 6 prior lines of therapy, including an immunomodulatory agent (IMiD), proteasome inhibitor (PI), and an anti-CD38 antibody, and has documented disease progression that occurred during or after the most recent therapy.
* Has adequate hematologic, hepatic and renal function as defined in the protocol.
* Eastern Cooperative Oncology Group (ECOG) 0 or 1.
* Life expectancy \>= 12 weeks.

Exclusion Criteria:

* Received bortezomib as part of the most recent prior therapy.
* Has primary refractory disease defined as disease that is non-responsive.
* Has not achieved a minimal response or better per IMWG criteria with any therapy.
* Has discontinued bortezomib due to toxicity.
* History of chronic liver disease or significant unresolved liver disease; currently active (within the last 6 months) hepatic impairment according to Child-Pugh Classification B or C.
* History of cataract surgery within 6 months prior to study treatment and participant is not anticipated to have cataract surgery during the study treatment period (as assessed by ophthalmological exam at baseline).
* Evidence of (as assessed by ophthalmological exam at baseline) uveitis, neovascular age related macular degeneration, retinal vein or artery occlusion and/or macular edema; no evidence of moderate or worsening diabetic retinopathy, retinal vascular disease or glaucoma (including participants with history of developing increased intraocular pressure after corticosteroid treatment) per clinical discretion of the consulting eye specialist.
* Peripheral neuropathy Grade \>= 2 or Grade 1 with pain.
* Receipt of one of the following:

  * Corticosteroids at a dose equivalent to \> 4 mg daily of dexamethasone or a single dose of \> 40 mg of dexamethasone within 2 weeks prior to first dose.
  * Monoclonal antibodies used for multiple myeloma treatment within 4 weeks prior to first dose of study treatment.
  * Any other systemic therapies used for multiple myeloma treatment within 5 half-lives or 2 weeks prior to first dose, whichever is longer (or 2 weeks if half-life is unknown).