A Study of Melphalan Flufenamide (Melflufen)-Dex or Pomalidomide-dex for RRMM Patients Refractory to Lenalidomide

Inclusion Criteria: 1. Male or female, age 18 years or older 2. A prior diagnosis of multiple myeloma with documented disease progression requiring further treatment at time of screening 3. Measurable disease defined as any of the following: * Serum monoclonal protein ≥ 0.5 g/dL by protein electrophoresis. * ≥ 200 mg/24 hours of monoclonal protein in the urine on 24-hour electrophoresis * Serum free light chain ≥ 10 mg/dL AND abnormal serum kappa to lambda free light chain ratio 4. Received 2-4 prior lines of therapy, including lenalidomide and a PI, either sequential or in the same line, and is refractory (relapsed and refractory or refractory) to both the last line of therapy and to lenalidomide (≥ 10 mg) administered within 18 months prior to randomization. Refractory to lenalidomide is defined as progression while on lenalidomide therapy or within 60 days of last dose, following at least 2 cycles of lenalidomide with at least 14 doses of lenalidomide per cycle. 5. Life expectancy of ≥ 6 months 6. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2. 7. Females of child bearing potential (FCBP) must have a negative serum or urine pregnancy test prior to start of treatment. Participants must agree to ongoing pregnancy testing. All patients must be willing to comply with all requirements of the USA pomalidomide Risk Evaluation and Mitigation Strategy (REMS) program or the pomalidomide Pregnancy Prevention Plan (PPP). 8. Ability to understand the purpose and risks of the study and provide signed and dated informed consent. 9. 12-lead Electrocardiogram (ECG) with QT interval calculated by Fridericia Formula (QTcF) interval of ≤ 470 msec Fridericia Formula. 10. The following laboratory results must be met during screening and also immediately before study drug administration on Cycle 1 Day 1: * Absolute neutrophil count (ANC) ≥ 1,000 cells/mm\^3 (1.0 x 10\^9/L) * Platelet count ≥ 75,000 cells/mm\^3 (75 x 10\^9/L) * Hemoglobin ≥ 8.0 g/dl * Total Bilirubin ≤ 1.5 x upper limit of normal (ULN), or patients diagnosed with Gilberts syndrome, that have been reviewed and approved by the medical monitor. * Aspartate transaminase (AST /SGOT) and alanine transaminase (ALT/SGPT) ≤ 3.0 x ULN. * Renal function: Estimated creatinine clearance by Cockcroft-Gault formula ≥ 45 mL/min. 11. Must be able to take antithrombotic prophylaxis. 12. Must have, or be willing to have an acceptable central catheter. (Port a cath, peripherally inserted central catheter \[PICC-line\], or central venous catheter) (Insertion only required if randomized to Arm A). Exclusion Criteria: 1. Primary refractory disease (i.e. never responded (≥ MR) to any prior therapy) 2. Evidence of mucosal or internal bleeding or platelet transfusion refractory 3. 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. 4. Prior exposure to pomalidomide 5. Known intolerance to IMiDs. 6. Known active infection requiring parenteral or oral anti-infective treatment within 14 days of randomization. 7. 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. 8. Pregnant or breast-feeding females 9. Serious psychiatric illness, active alcoholism, or drug addiction that may hinder or confuse compliance or follow-up evaluation 10. Known human immunodeficiency virus or active hepatitis C viral infection 11. Active hepatitis B viral infection (defined as HBsAg+). * Patients with prior hepatitis B vaccine are permitted (defined as HBsAg-, Anti-HBs+, Anti-HBc-). * Non-active hepatitis B (HBsAg-, Anti-HBs+, Anti-HBc+) may be enrolled at the discretion of the investigator after consideration of risk of reactivation. 12. Concurrent symptomatic amyloidosis or plasma cell leukemia 13. POEMS syndrome 14. Previous cytotoxic therapies, including cytotoxic investigational agents, for multiple myeloma within 3 weeks (6 weeks for nitrosoureas) prior to randomization. IMiDs, PIs and or corticosteroids within 2 weeks prior to randomization. Other investigational therapies and monoclonal antibodies within 4 weeks of randomization. Prednisone up to but no more than 10 mg orally q.d. or its equivalent for symptom management of comorbid conditions is permitted but dose should be stable for at least 7 days prior to randomization 15. Residual side effects to previous therapy \> grade 1 prior to randomization (Alopecia any grade and/or neuropathy grade 2 without pain are permitted) 16. Prior peripheral stem cell transplant within 12 weeks of randomization 17. Prior allogeneic stem cell transplantation with active graft-versus-host-disease. 18. Prior major surgical procedure or radiation therapy within 4 weeks of the randomization 19. Known intolerance to steroid therapy