• Patients with biopsy-proven Richter transformation, diffuse large B-cell lymphoma subtype (RT-DLBCL) from an antecedent or concurrently diagnosed chronic lymphocytic leukemia (CLL) and/or small lymphocytic lymphoma (SLL).

• Be ineligible for frontline anthracycline-based chemoimmunotherapy (determined by treating investigator) OR have clinical evidence of disease progression after any prior treatment for RT-DLBCL.

• Participants who have adverse events (AEs) due to previous anti-cancer therapies must have recovered to ≤ grade 1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement or participants who have ≤ grade 2 neuropathy are eligible.

‣ Note: Participants who have lingering cytopenias from prior anti-cancer therapy or progressive disease may be eligible at the discretion of the study principal investigator (PI), provided they meet all other study criteria.

• Have measurable disease as determined by imaging (by positron-emission tomography \[PET\] and/or computed tomography \[CT\] scans), immunohistochemistry, and/or flow cytometry, as per the Cheson criteria.

• Have the ability to swallow and retain oral medication.

• Age 18 years and older on the day of signing informed consent.

• Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.

• Be free from other malignancy within 2 years prior to enrollment (with the exception of CLL/SLL, low-risk and early stage \[T1-T2a- Gleason score ≤ 6, and prostate-specific antigen \[PSA\] \< 10 ng/mL\] prostate cancer, or localized skin cancer that has undergone potentially curative therapy).

• Absolute neutrophil count: ANC ≥ 500 cells/µL (without G-CSF dose within the last 7 days prior to initiation of study treatment

• Platelets: ≥ 25,000/µL -not requiring transfusion within the last 3 days prior to initiation of study treatment). Patients on medications that increase bleeding risk (e.g. systemic anticoagulation, anti-platelet therapies, etc.) must have a platelet count ≥50,000 /µL and have no history of major bleeding.

• Hemoglobin: ≥ 7gm/dL (transfusion support allowed).

• Total bilirubin: ≤ 1.5 x upper limit of normal (ULN) OR direct bilirubin ≤ ULN for participants with total bilirubin levels \> 1.5 x ULN.

• Aspartate aminotransferase (AST)(serum glutamic-oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT)(serum glutamic-pyruvic transaminase \[SGPT\]): ≤ 2.5 x ULN (≤ 5 x ULN for participants with liver metastases).

• Creatinine clearance (CrCl): ≥ 30 mL/min (per Cockroft-Gault equation).

• International normalized ratio (INR) (prothrombin \[PT\]/activated partial thromboplastin time \[aPTT\]): ≤ 1.5 x ULN, unless participant is receiving anticoagulant therapy, as long as PT or aPTT is within therapeutic range of intended use of anticoagulants.

• Patients with history of human immunodeficiency virus (HIV) infection are potentially eligible (after conferring with the PI) if they meet ALL of the following criteria:

‣ Must have a CD4+ T-cell count ≥ 350 cells/mm\^3 AND an HIV viral load below the detectable level as per locally available testing at the time of screening

⁃ It is advised that participants must not have had any AIDS-defining opportunistic infections within the past 12 months.

⁃ Participants on anti-retroviral therapy (ART) must have been on a stable regimen, without changes in drugs or dose modification, for at least 4 weeks before study entry (day 1) and agree to continue ART throughout the study.

⁃ The combination ART regimen must not contain any antiretroviral medications that interact with strong CYP3A4 inhibitors/inducers/substrates (\<https://www.fda.gov/drugs/drug-interactions-labeling/drug-development-and-drug-interactions-table-substrates-inhibitors-and-inducers\>). Participants receiving ART that are strong CYP3A4 inducers are not eligible to be included in the study.

• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks and have undetectable HBV viral load prior to randomization.

∙ Note: Participants should remain on anti-viral therapy throughout study intervention and follow local guidelines for HBV anti-viral therapy post completion of study intervention. Hepatitis B screening tests, including HBsAg and hepatitis B core antibodies (anti-HBc), are required for all participants.

• Participants with history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable at screening (Participants must have completed curative anti-viral therapy at least 4 weeks prior to the first administration of the study treatment).

• A person of childbearing potential must have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.

• Participants of child-bearing potential must agree to use 2 methods of birth control or be surgically sterile or abstain from heterosexual activity for the course of the study through 6 months after the last dose of study medication. Should a person of child-bearing potential become pregnant or suspect they are pregnant while they or their partner is participating in this study, they should inform their treating physician immediately.

• Participant must understand the investigational nature of this study and sign an Independent Ethics Committee/Institutional Review Board approved written informed consent form prior to receiving any study related procedure.