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 priortherapy (chemotherapy and/or targeted drug therapy, which must include BTK inhibitortherapy), are indicated for treatment per IWCLL2018.

• Measurable lesions detected by contrast-enhanced computed tomography (CT): at leastone lymph node with a maximum axis of more than 1.5 cm and one measurable verticaldimension.

• 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 toRichter's syndrome.

• Patients with active and uncontrolled autoimmune cytopenia, including autoimmunehemolytic anemia and idiopathic thrombocytopenic purpura.

• Patients who have received any of the following treatments within 14 days prior tothe first dose:

• Major surgery;

• Glucocorticoids (at a dose equal to or greater than 20 mg/day prednisone orequivalent), unless used by inhalation, topical or intraarticular route, orunless necessary for premedication before and after iodinated contrast dye.After discussion with the Sponsor, steroid therapy at high doses for anextended period may be allowed under the following circumstances:

1. Treatment of autoimmune hemolysis or autoimmune thrombocytopeniaassociated with CLL/SLL;
2. Short-term (within 14 days) use to treat inactive infections of diseasesunrelated to CLL/SLL (e.g. arthritis, asthma), which results in acuteexacerbation, including steroid dose modifications required for adrenalinsufficiency;

• Cytotoxic chemotherapy or biologic therapy, excepting BCR pathway kinaseinhibitors 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 priortreatment with BTK inhibitors did not restore to ≤ Grade 2.

• Central nervous system (CNS) leukemia or lymphoma, including a history ofasymptomatic, previously treated CNS disease.