The goal of this clinical trial is to learn if BMT-3566 can safety be given to adult patients with relapsed or refractory mature b cell lymphomas. It will also learn how well BTM-3566 works to treat relapsed or refractory mature b cell lymphomas. The main questions it aims to answer are:

What are the side effects of BTM-3566 at different doses? What are the levels of BTM-3566 in the blood at different timepoints around dosing? What is the clinical benefit of BTM-3566 in treating cancer (i.e. how well does it slow or stop disease progression)?

Participants will:

Take BTM-3566 in 14-day periods with 7 days of dosing followed by 7 days of no dosing Visit the clinic regularly for checkups and tests Keep a diary of their dosing and weight

This is an open label, phase 1 dose escalation trial of BTM-3566 in relapsed/refractory mature b cell lymphomas. Dose escalation will utilize traditional 3+3 methodology to determine the maximum tolerated dose of BTM-3566. Treatment will be given as two-week cycles, with BTM-3566 taken daily during the first week of the cycle, and the second week 'off therapy'. DLT window will be the first two cycles of therapy (days 1 to 28). All enrolled patients will undergo a baseline FDG-PET/CT scan within 3 weeks prior to starting therapy. FDG-PET/CT will be repeated at the start of Cycles 3 and 5 or more frequently as clinically indicated. Beginning at Cycle 5, imaging will be repeated every 3 months (or as clinically indicated) for up to 12 months, and every 6 months thereafter, until disease progression or start of subsequent anti-cancer therapy.

Inclusion Criteria:

• must be age ≥18 years
• must have a diagnosis of relapsed or refractory mature B cell lymphoma
• must have measurable disease per response evaluation criteria in lymphoma (Lugano classification)
• must have an Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-2
• must have a predicted life expectancy of ≥3 months
• must agree to use adequate birth control throughout their participation and for 90 days following the last dose of BTM-3566

Exclusion Criteria:

• has primary CNS lymphoma
• has ongoing toxicities from prior anti-cancer treatment > Grade 1
• has symptomatic or uncontrolled neurologic disease (brain metastases, leptomeningeal disease, or spinal cord compression) not definitively treated with surgery or radiation
• has received any anti-cancer therapy (including radiation of curative intent) <28 days prior to administration of BTM-3566
• has current second malignancy at other sites (exceptions: non-melanomatous skin cancer, adequately treated in situ carcinoma, or indolent prostate cancer under observation).
• is pregnant or breastfeeding