Inclusion Criteria:  Male or female, between the ages of 18 years and 70 years at the planned time of study treatment; patients greater than 70 years of age may qualify on a case by case basis Diagnosis of multiple myeloma Received a previous Autologous Stem Cell Transplantation ( ASCT) (single or tandem) that resulted in disease progression within 24 months Received at least 2 prior lines of therapy Refractory to previous treatment with a Proteasome Inhibitor (PI), an immunomodulatory drug (IMiD) and an anti-Cluster of Differentiation 38 monoclonal antibody (anti-CD38 mAb) Male and women of childbearing potential agrees to use contraception during the treatment period and during a specified time period after the last dose  Exclusion Criteria:  Prior treatment with melphalan flufenamide (melflufen) or OPD5 Any medical condition that may interfere with safety or participation in this study Other malignancy diagnosed or requiring treatment within the past 3 years with the exception of adequately treated basal cell carcinoma, squamous cell skin cancer, carcinoma in-situ of the cervix or breast or very low and low risk prostate cancer in active surveillance Prior allogeneic stem cell transplantation or prior salvage ASCT