A Single-arm, Phase 2, Open-label, Multicenter Study to Evaluate NX-5948 in Adults With Relapsed/Refractory (R/R) Chronic Lymphocytic Leukemia (CLL) or Small Lymphocytic Lymphoma (SLL) Previously Exposed to a Bruton's Tyrosine Kinase Inhibitor (BTKi) and a B-cell Lymphoma-2 Inhibitor (BCL-2i)

Inclusion Criteria: * Age: ≥ 18 years * Confirmed relapsed/refractory CLL/SLL that meets iwCLL criteria for diagnosis and systemic treatment * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Must have received a covalent BTK inhibitor (BTKi), a non-covalent BTKi, and a BCL-2 inhibitor either in separate lines of therapy or in combination; a line of therapy is considered 2 or more consecutive cycles of a systemic anti-CLL/SLL regimen * Participants with SLL must have measurable disease by radiographic assessment * Adequate organ and bone marrow function * Must sign an informed consent form indicating that he or she understands the purpose of the procedures required for the study and is willing to participate Exclusion Criteria: * Known or suspected prolymphocytic leukemia or Richter's transformation before entering study * Investigational agent or anticancer therapy within 5 half-lives or 14 days (whichever is shorter) before planned start of study drug * Antibody therapy must stop at least 4 weeks before the first dose of study drug * No other systemic anticancer therapy is allowed at the same time as this study; exception: continuation of hormonal therapy for breast and prostate cancer is allowed, if they are not on the list of prohibited concomitant medications in this study * Palliative limited-field radiotherapy within 7 days of the first dose of study or broad field radiotherapy within 28 days of first dose of study drug * Use of systemic corticosteroids \>20 mg/day prednisone or equivalent within the 7 days before start of study drug except for those used as premedication for radio diagnostic contrast * Use of systemic immunosuppressive drugs other than systemic corticosteroids within 60 days before the first dose of study drug * Previously treated with a BTK degrader * Previous chimeric antigen receptor (CAR) T-cell therapy or allogeneic or autologous hematopoietic cell transplant within the past 90 days prior to enrollment * Thromboembolic events (eg, deep vein thrombosis, pulmonary embolism, or symptomatic cerebrovascular events), stroke, or intracranial hemorrhage within 6 months of planned start of study drug Note: Other Inclusion/Exclusion criteria may apply as defined in the protocol.