Inclusion Criteria:

          1. Age ≥18 years at time of informed consent.

          2. Histologically confirmed MM and evidence of disease progression while on one of the
             below MM regimens:

               -  Arm 1: Refractory to or disease progression while on a carfilzomib-containing
                  regimen

               -  Arm 2: Refractory to or disease progression while on a pomalidomide-containing
                  regimen

               -  Exploratory Arm: Refractory to or disease progression while on a
                  daratumumab-containing regimen

          3. Measurable disease as defined: Serum M-protein ≥ 0.5 g/dL; urine M-protein excretion
             at least 200 mg/24h; serum FLC ≥ 100 mg/L, provided that FLC is abnormal; if serum
             protein electrophoresis is felt to be unreliable for routine M-protein measurement
             (i.e. IgA MM) then quantitative Ig levels by nephelometry or turbidometry are
             acceptable; for non-secretory disease, bone marrow plasma cells ≥ 20% or a
             biopsy-proven target lesion by PET/CT or MRI is acceptable

          4. Documented evidence of disease progression (by IMWG criteria) or refractory disease on
             the current treatment as defined as:

               -  Achieving SD or less for ≥ 1 cycle during treatment with regimens stated in #2
                  (i.e. relapsed) OR

               -  <25% response (i.e. never achieved MR) or PD during or within 60 days from the
                  end-of most recent MM regimen as listed in #2 (i.e. refractory).

          5. Any non-hematological toxicities (except for peripheral neuropathy) that patients
             experienced from treatments in previous regimens have resolved to Grade 2 or less by
             Cycle 1 Day 1.

          6. ECOG 2 or less

          7. Adequate hepatic function within 28 days prior to C1D1:

               1. Total bilirubin < 1.5 × upper limit of normal (ULN) (except patients with
                  Gilbert's syndrome who must have a total bilirubin of < 3 × ULN), and

               2. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) normal to <2
                  × ULN.

          8. Adequate renal function within 28 days prior to C1D1 as determined by estimated
             creatinine clearance of ≥ 30 mL/min, calculated using the Cockcroft and Gault formula
             (140 - Age) • Mass (kg)/ (72 • creatinine mg/dL); multiply by 0.85 if female
             (Cockcroft 1976).

          9. Adequate hematopoietic function within 7 days prior to C1D1: total white blood cell
             (WBC) count ≥1500/mm3, absolute neutrophil count ≥1000/mm3, hemoglobin ≥8.5 g/dL and
             platelet count ≥75,000/mm3 (patients for whom <50% of bone marrow nucleated cells are
             plasma cells) or ≥50,000/mm3 (patients for whom ≥50% of bone marrow nucleated cells
             are plasma cells).

               1. Patients receiving hematopoietic growth factor support, including erythropoietin,
                  darbepoetin, granulocyte-colony stimulating factor (G-CSF), granulocyte
                  macrophage-colony stimulating factor (GM-CSF), and platelet stimulators (eg,
                  eltrombopag, romiplostim, or interleukin-11) must have a 2-week interval between
                  growth factor support and the Screening assessments, but they may receive growth
                  factor support during the study.

               2. Patients must have:

                    -  At least a 2-week interval from the last red blood cell (RBC) transfusion
                       prior to the Screening hemoglobin assessment, and

                    -  At least a 1-week interval from the last platelet transfusion prior to the
                       Screening platelet assessment.

             However, patients may receive RBC and/or platelet transfusions as clinically indicated
             per institutional guidelines during the study.

         10. Female patients of childbearing potential must have a negative serum pregnancy test at
             Screening. Female patients of childbearing potential and fertile male patients who are
             sexually active with a female of childbearing potential must use highly effective
             methods of contraception throughout the study and for 3 months following the last dose
             of study treatment.

        Exclusion Criteria:

          -  Patients meeting any of the following exclusion criteria are not eligible to enroll in
             this study:

               1. Smoldering MM

               2. Radiation therapy, chemotherapy or immunotherapy other than above stated regimens
                  in #2 ≤2 weeks prior to C1D1. Patients on long-term glucocorticosteroids during
                  screening do not require a washout period. Prior RT is permitted for treatment of
                  fractures or to prevent fractures as well as for pain management.

               3. Active graft versus host disease after allogeneic stem cell transplant.

               4. Life expectancy <3 months.

               5. Known active hepatitis A, B, or C infection; or known to be positive for
                  hepatitis C virus ribonucleic acid (RNA) or hepatitis B virus surface antigen.

               6. Has any concurrent medical condition or disease (eg, uncontrolled active
                  hypertension, uncontrolled active diabetes, active systemic infection, etc.) that
                  is likely to interfere with study procedures.

               7. Uncontrolled active infection requiring parenteral antibiotics, antivirals, or
                  antifungals within 1 week prior to Cycle 1 Day 1 (C1D1). Patients on prophylactic
                  antibiotics or with a controlled infection within 1 week prior to C1D1 are
                  acceptable.

               8. Known intolerance, hypersensitivity, or contraindication to glucocorticoids.

               9. Pregnant or breastfeeding females.

              10. Body surface area (BSA) <1.4 m2 at baseline, calculated by the Dubois (Dubois
                  1916) or Mosteller (Mosteller 1987) method.

              11. Any active gastrointestinal dysfunction interfering with the patient's ability to
                  swallow tablets, or any active gastrointestinal dysfunction that could interfere
                  with absorption of study treatment.

              12. Inability or unwillingness to take supportive medications such as anti-nausea and
                  anti-anorexia agents as recommended by the National Comprehensive Cancer Network®
                  (NCCN) Clinical Practice Guidelines in Oncology (CPGO) (NCCN CPGO) for antiemesis
                  and anorexia/cachexia (palliative care).

              13. Any active, serious psychiatric, medical, or other conditions/situations that, in
                  the opinion of the Investigator, could interfere with treatment, compliance, or
                  the ability to give informed consent.

              14. Contraindication to any of the required concomitant drugs or supportive
                  treatments.

              15. Patients unwilling or unable to comply with the protocol