• Pathologic diagnosis of DLBCL, as defined by the 2016 World Health Organization (WHO) classification (including participants with DLBCL transformed from indolent lymphoma), or HGBCL, or Grade 3b FL.

• Measurable disease as defined by the 2014 Lugano Classification.

• Stages I-IV.

• ECOG PS 0-2; ECOG PS 3 allowed if decline in status is deemed related to lymphoma \& felt to be potentially reversible by the treating physician.

• Adequate organ function as defined by screening laboratory values within the following parameters:

‣ Absolute neutrophil count (ANC) ≥1.0 x 10\^3/µL (off growth factors at least 72 hours).

⁃ Platelet count ≥75 x 10\^3/µL without transfusion in the past 7 days.

⁃ Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and gamma glutamyl transferase (GGT) ≤2.5 x the upper limit of normal (ULN).

⁃ Total bilirubin ≤1.5 x ULN (participants with known Gilbert's syndrome may have a total bilirubin up to ≤3 x ULN).

⁃ Calculated creatinine clearance \>30 mL/min by the Cockcroft and Gault equation.

∙ Note: A laboratory assessment may be repeated a maximum of two times during the screening period to confirm eligibility.

• Women of childbearing potential (WOCBP) must agree to use a highly effective method of contraception from the time of giving informed consent until at least 12 months after the last dose of study treatment. Men with female partners who are of childbearing potential must agree to use a condom when sexually active or practice total abstinence from the time of the first dose until at least 7 months after the participant receives his last dose of study treatment.

• Unfit as defined by the simplified geriatric assessment (sGA). Includes all of the following:

‣ Aged ≥80 years

⁃ ADL score of 6

⁃ IADL score of 8

⁃ CIRS-G: no score of 3-4 and \<5 scores of 2

• Frail as defined by sGA:

‣ Aged ≥80 years

⁃ ADL score of \<6 and/or

⁃ IADL score of \<8 and/or

⁃ CIRS-G: ≥1 score of 3-4 and/or \>5 scores of 2 OR

• Aged ≥65 - \<80 with at least one of the following cardiac comorbidities that make anthracycline-containing regimens inadvisable as determined by the investigator.

‣ Left ventricular ejection fraction (LVEF) ≥30 to \<50%

⁃ History of myocardial infarction within 6 months prior to screening

⁃ Ischemic heart disease

⁃ History of stroke within 12 months prior to screening