Inclusion Criteria

          1. Male or female at least 18 years of age and able to provide informed consent.

          2. Histologically-confirmed diagnosis of CD20-positive DLBCL based on the WHO
             classification (2016); the diagnosis must be confirmed at the enrolling site. Subjects
             with high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements and
             high-grade B-cell lymphoma, NOS are eligible.

          3. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2.

          4. International Prognostic Index (IPI) score of at least 3.

          5. Estimated life expectancy of at least 12 weeks.

          6. Adequate organ function as follows (within 14 days prior to randomization):

               1. Hepatic: total bilirubin ≤1.5 times upper limit of normal (ULN); alanine
                  transaminase (ALT) and aspartate transaminase (AST) ≤1.5 times ULN (<5 times ULN
                  if liver involvement)

               2. Renal: creatinine clearance of >50 mL/min by Cockcroft- Gault equation

               3. Bone marrow: platelets ≥75 x 109/L, absolute neutrophil count (ANC) ≥1.5 x 109/L,
                  hemoglobin ≥10 g/dL. (Platelets ≥50 x 109/L, ANC ≥1.0 x 109/L, hemoglobin ≥8 g/dL
                  permitted if documented bone marrow involvement)

          7. Male or female with reproductive potential, must be willing to use an approved
             contraceptive method (for example, intrauterine device (IUD), birth control pills, or
             barrier device) during and for 3 months after discontinuation of study treatment.
             Women of childbearing potential must have a negative serum pregnancy test within 7
             days prior to randomization.

               1. Men are considered of reproductive potential unless they have undergone a
                  vasectomy and confirmed sterile by a post-vasectomy semen analysis.

               2. Women are considered of reproductive potential unless they have undergone
                  hysterectomy and/or surgical sterilization (at least 6 weeks following a
                  bilateral oophorectomy, bilateral tubal ligation, or bilateral tubal occlusive
                  procedure that has been confirmed in accordance with the device's label) or
                  achieved postmenopausal status (defined as cessation of regular menses for
                  greater than 12 consecutive months in women at least 45 years of age).

          8. Left ventricular ejection fraction ≥50% by echocardiography or nuclear medicine
             multi-gated scan.

          9. Must be able to swallow tablets.

         10. Must be able to comply with study protocol procedures.

         11. Willing to consent to have blood stored for possible future biomarker and disease
             analysis.

         12. Must have available and willing to submit pre-systemic treatment DLBCL tumor biopsy
             tissue/slides for central pathology review.

        Exclusion Criteria

          1. Received treatment with an investigational drug within the last 30 days.

          2. Receiving or has received radiation or any other systemic anticancer treatment for
             lymphoma (Up to 7 days of corticosteroids are permitted but must be administered after
             eligibility IPI determination and imaging scans).

          3. History of indolent lymphoma or follicular Grade 3b lymphoma.

          4. Primary mediastinal (thymic) large B-cell lymphoma.

          5. B-cell lymphoma, unclassifiable, with features. intermediate between DLBCL and
             classical Hodgkin lymphoma.

          6. Burkitt lymphoma.

          7. Pregnancy or breastfeeding.

          8. Known central nervous system (CNS) involvement.

          9. Any significant concomitant disorder based on the discretion of the investigator,
             including but not limited to active bacterial, fungal, or viral infection,
             incompatible with participation in the study.

         10. A second primary malignancy (except adequately treated non-melanoma skin cancer);
             subjects who have had another malignancy in the past, but have been disease-free for
             more than 5 years, and subjects who have had a localized malignancy treated with
             curative intent and disease free for more than 2 years are eligible.

         11. Use of a strong inducer or moderate or strong inhibitor of CYP3A4 within 7 days prior
             to start of study therapy or expected requirement for use on study therapy.

         12. Personal or immediate family history of long QT syndrome, QTc interval >450 msec
             (males) or >470 msec (females) at screening (recommended that QTc be calculated using
             Fridericia correction formula, QTcF: see Section 6.2.1), or a history of unexplained
             syncope.

         13. Use of any medication that can prolong the QT/QTc interval within 7 days prior to
             start of study therapy or expected requirement for use on study therapy.

         14. History of severe allergic or anaphylactic reaction to monoclonal antibody therapy.

         15. Confirmed diagnosis of progressive multifocal leukoencephalopathy.

         16. Ongoing grade 2 or higher peripheral neuropathy.

         17. Have any of the following cardiac disorders: uncontrolled hypertension, unstable
             angina, myocardial infarction within 8 weeks of Day1, NYHA Grade 2 or higher
             congestive heart failure, ventricular arrhythmia requiring medication within 1 year of
             Day 1, NYHA Grade 2 or higher peripheral vascular disease.

         18. Received a live vaccine within 28 days of study Day 1.

         19. HIV positive.

         20. Evidence of chronic hepatitis C infection as indicated by antibody to HCV with
             positive HCV-RNA.

         21. Evidence of chronic hepatitis B infection as indicated by either:

               1. HBsAg+ or

               2. HBcAb+ with HBV-DNA+ (any detectable amount is considered positive)