A Study to Investigate the Efficacy and Safety of MS-553 in CLL/SLL

Inclusion Criteria: * Age 18 years or older, male or female. * Diagnosis of CLL/SLL per IWCLL2018 criteria. * Patients with relapsed or refractory CLL/SLL who have failed at least 1 prior therapy (chemotherapy and/or targeted drug therapy, which must include BTK inhibitor therapy), are indicated for treatment per IWCLL2018. * Measurable lesions detected by contrast-enhanced computed tomography (CT): at least one lymph node with a maximum axis of more than 1.5 cm and one measurable vertical dimension. * WHO/ECOG performance status of 0 to 2. * Patients with an estimated survival of more than 3 months. Exclusion Criteria: * Biopsy-proven and pathologically confirmed current or past transformation to Richter's syndrome. * Patients with active and uncontrolled autoimmune cytopenia, including autoimmune hemolytic anemia and idiopathic thrombocytopenic purpura. * Patients who have received any of the following treatments within 14 days prior to the first dose: * Major surgery; * Glucocorticoids (at a dose equal to or greater than 20 mg/day prednisone or equivalent), unless used by inhalation, topical or intraarticular route, or unless necessary for premedication before and after iodinated contrast dye. After discussion with the Sponsor, steroid therapy at high doses for an extended period may be allowed under the following circumstances: 1. Treatment of autoimmune hemolysis or autoimmune thrombocytopenia associated with CLL/SLL; 2. Short-term (within 14 days) use to treat inactive infections of diseases unrelated to CLL/SLL (e.g. arthritis, asthma), which results in acute exacerbation, including steroid dose modifications required for adrenal insufficiency; * Cytotoxic chemotherapy or biologic therapy, excepting BCR pathway kinase inhibitors for which a washout of 24 hours prior to the first dose is required. * Toxicity from prior anti-tumor therapy (chemotherapy, radiotherapy, or biotherapy) did not restore to ≤ Grade 1 (except for alopecia); atrial fibrillation from prior treatment with BTK inhibitors did not restore to ≤ Grade 2. * Central nervous system (CNS) leukemia or lymphoma, including a history of asymptomatic, previously treated CNS disease.