The following criteria is provided for health care professionals.

Inclusion Criteria:

• Male or female, age 18 years or older
• A prior diagnosis of multiple myeloma with documented disease progression
• Measurable disease based on either of a) serum monoclonal protein by protein electrophoresis (SPEP), b) monoclonal protein in the urine on 24-hour urine electrophoresis (UPEP), and/or c) serum immunoglobulin free light chain combined with abnormal serum immunoglobulin kappa to lambda free light chain ratio
• A minimum of 2 prior lines of therapy including an IMiD and a PI and is refractory to pomalidomide and/or daratumumab
• Life expectancy of ≥ 6 months
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2
• Female of child bearing potential (FCBP) and non-vasectomized male agree to practice appropriate methods of birth control
• Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information
• 12-lead ECG with QTc interval within defined limit
• Acceptable laboratory results during screening and prior to first study drug administration of the following parameters: absolute neutrophil count (ANC), platelet count, hemoglobin, total bilirubin, aspartate transaminase (AST/SGOT) and alanine transaminase (ALT/SGPT), renal function based on estimated creatinine clearance
• Must have, or accept to have, an acceptable central catheter for infusion of melflufen

Exclusion Criteria:

• Evidence of mucosal or internal bleeding and/or is platelet transfusion refractory
• Any medical conditions that, in the Investigator's opinion, would impose excessive risk to the patient or would adversely affect his/her participating in this study
• Known active infection requiring parenteral or oral anti-infective treatment within defined period
• Primary refractory disease
• Other malignancy diagnosed or requiring treatment within the defined period with specific exceptions
• Pregnant or breast-feeding females
• Serious psychiatric illness, active alcoholism, or drug addiction that may hinder or confuse compliance or follow-up evaluation
• Known HIV or active hepatitis B or C viral infection
• Concurrent symptomatic amyloidosis or plasma cell leukemia
• POEMS syndrome [plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, monoclonal protein (M-protein) and skin changes]
• Previous cytotoxic therapies, including cytotoxic investigational agents, for multiple myeloma within defined values prior to start of study treatment
• Residual side effects to previous therapy over specific grade prior to initiation of therapy
• Prior autologous or allogeneic stem cell transplant within defined period of initiation of therapy
• Prior allogeneic stem cell transplant with active graft-versus-host- disease (GVHD).
• Prior major surgical procedure or radiation therapy within specified period of the first dose of study treatment (with defined exception).
• Known intolerance to steroid therapy