Inclusion Criteria:

• Subject must be ≥ 18 years of age.
• Subject has an ECOG performance score of ≤ 1.  
  • For subjects in the Phase 2 portion: ECOG performance score of ≤ 2.
• Diagnosis of multiple myeloma which requires treatment and has been previously treated with:
  • For subjects in the Dose Escalation cohort of the study:
    • ≥ 1 prior line of therapy. Induction therapy followed by stem cell transplant and maintenance therapy will be considered as a single line of therapy.
  • For subjects in the Safety Expansion cohort of the study:
    • Have received treatment with a proteasome inhibitor and an immunomodulatory (IMiD®) agent (e.g., thalidomide, lenalidomide, pomalidomide). Induction therapy followed by stem cell transplant and maintenance therapy will be considered as a single line of therapy.
  • For subjects in the Venetoclax-Dexamethasone (VenDex) Combination cohort of the study:
    • Have received treatment with a proteasome inhibitor and an immunomodulatory (IMiD®) agent (e.g., thalidomide, lenalidomide, pomalidomide). Induction therapy followed by stem cell transplant and maintenance therapy will be considered as a single line of therapy; AND
    • Have MM positive for the t(11;14) translocation, as determined by an analytically validated fluorescence in situ hybridization (FISH) assay per the central laboratory testing.
  • For subjects in the Phase 2 cohort of the study:
    • Have MM positive for the t(11;14) translocation, as determined by an analytically validated fluorescence in situ hybridization (FISH) assay per central laboratory testing (enrollment with local t(11;14)-positive FISH results will be considered at the discretion of the TA MD); AND
    • Subject must have evidence of disease progression on or within 60 days of the last dose of the most recent previous treatment regimen based on the IMWG criteria; AND
    • Subject must have previously received at least 2 lines of therapy, including an immunomodulatory drug (lenalidomide or pomalidomide), a proteasome inhibitor (bortezomib, carfilzomib or ixazomib), daratumumab, and glucocorticoids.
  • For United States (US) Subjects: Daratumumab combination regimen must be one of the prior lines of therapy (for this study, daratumumab plus corticosteroids will not be considered a combination regimen).
  • For Non-US Subjects: Either daratumumab monotherapy or daratumumab combination therapy is acceptable. Daratumumab monotherapy will be limited to approximately 20 percent of the total number of Phase 2 subjects.
• Subject must have measurable disease at Screening, defined as any of the following:
  • Serum monoclonal protein ≥ 1.0 g/dL (≥ 10 g/L) by protein electrophoresis;
  •  ≥ 200 mg of monoclonal protein in the urine on 24-hour electrophoresis; or
  • Serum immunoglobulin free light chain (FLC) ≥ 10 mg/dL provided serum FLC ratio is abnormal.
• Subjects with a history of autologous or allogenic stem cell transplantation must have adequate peripheral blood counts independent of any growth factor support, and have recovered from any transplant-related toxicity(s) and be:
  • 100 days post-autologous transplant (prior to first dose of study drug); or
  •  ≥ 6 months post-allogenic transplant (prior to first dose of study drug) and not have active graft-versus-host disease (GVHD); i.e., requiring treatment.
• Subjects must meet the following laboratory parameters, per laboratory reference range, at least once during screening period:
  • ANC ≥ 1000/μL;
  • Subjects may use growth factor support to achieve ANC eligibility criteria;
  • AST and ALT ≤3 × upper limit of normal range (ULN);
  • Calculated creatinine clearance ≥30 mL/min using a modified Cockcroft-Gault calculation or a 24-hour urine collection for Creatinine Clearance:
    • eCCr = (140 –Age) ● weight (kg) ● [0.85 if Female]
                    72 ● Serum Creatinine (mg/dL)
  • OR, if serum creatinine is in μmol/L:
    • eCCr = (140 –Age) ● weight (kg) ● [1.23 if Male, 1.04 if Female]
                            Serum Creatinine (μmol/L)
  • Platelet count ≥30,000 mm3, independent of transfusion for 2 weeks.
  • Hemoglobin ≥8.0 g/dL, subjects may receive blood transfusion to achieve hemoglobin eligibility criteria per investigator discretion.
  • Total bilirubin ≤1.5 × ULN.
  • Subjects with Gilbert's Syndrome may have bilirubin > 1.5 × ULN.

Exclusion Criteria:

• Subject exhibits evidence of other clinically significant uncontrolled condition(s), including, but not limited to:
  • Acute infection within 14 days prior to first dose of study drug requiring antibiotic, antifungal, or antiviral therapy;
  • Diagnosis of fever and neutropenia within 1 week prior to first dose of study drug.
• Subject has a cardiovascular disability status of New York Heart Association Class ≥ 3.
• Subject has a significant history of renal, neurologic, psychiatric, endocrinologic, metabolic, immunologic, cardiovascular or hepatic disease within the last 6 months that, in the opinion of the investigator, would adversely affect his/her participation in the study.
• Subject has a history of other active malignancies other than multiple myeloma within the past 3 years prior to study entry, with the following exceptions:
  • Adequately treated in situ carcinoma of the cervix uteri;
  • Basal cell carcinoma of the skin or localized squamous cell carcinoma of the skin;
  • Localized prostate cancer Gleason grade 6 or lower AND with stable Prostate Specific Antigen (PSA) levels off treatment;
  • Previous malignancy confined and surgically resected (or treated with other modalities) with curative intent.
• Known Human Immunodeficiency Viral (HIV) infection.
• Active hepatitis B or C infection based on screening blood testing.
• Subject is receiving other ongoing anti-myeloma therapy.
• Subject has received any of the following within 7 days prior to the first dose of study drug:
  • Strong or moderate CYP3A inhibitors; or
  • Strong or moderate CYP3A inducers.