This is a phase 3, open-label, randomized, multi-center study assessing the efficacy and safety of DZD8586 versus investigator's choice in participants with chronic lymphocytic leukemia/small lymphocytic lymphoma who have progressed following prior therapy.

Primary objective of this study is to assess the efficacy using progression free survival assessed by independent review committee as primary endpoint. Approximately 250 participants are estimated to be randomized into the study.

Inclusion Criteria:

1. Male and female participants ≥ 18 years of age.
2. ECOG performance status 0-2.
3. Confirmed diagnosis of relapsed or refractory CLL/SLL with indication for treatment.
4. Adequate bone marrow reserve and organ system functions.
5. Participants willing to comply with contraceptive restrictions.

Exclusion Criteria:

1. Any unresolved > Grade 1 adverse event at the time of starting study treatment.
2. Known or suspected Richter transformation.
3. Known or suspected CNS involvement.
4. Previous or current therapy and comedications meet exclusion criteria.
5. Participants with major cardiovascular disease, active infection, malignancy or uncontrolled systemic disease.
6. Nausea and vomiting not controlled or chronic gastrointestinal diseases, unable to swallow the formulated product or previous bowel resection that would preclude adequate absorption.
7. Women who are breast feeding.
8. History of hypersensitivity to active or inactive excipients of DZD8586 or drugs with a similar chemical structure or class.