• Documented relapsed or refractory CLL or SLL (SLL in phase II part only) following at least one systemic 1st-line treatment

• Requiring treatment according to IWCLL criteria (appendix A);

• Age at least 18 years;

• ECOG/WHO performance status 0-2;

• In case of prior venetoclax treatment, enrollment can only occur at least 24 months after end of treatment and patients must not have progressed during venetoclax treatment;

• Adequate BM function defined as:

‣ Hemoglobin \>5.6 mmol/l or Hb \> 9 g/dL, unless low Hb is directly attributable to CLL infiltration of the BM, proven by BM biopsy;

⁃ Absolute neutrophil count (ANC) \>1.0 x 109/L (1,000/μL), unless low ANC is directly attributable to CLL infiltration of the BM, proven by BM biopsy;

⁃ Platelet count \>30 x 109/L (30,000/μL), unless low platelets is directly attributable to CLL infiltration in the BM;

• Estimated Glomerular Filtration Rate (eGFR) (MDRD) or estimated creatinine clearance (CrCl) ≥ 50ml/min (Cockcroft-Gault appendix F);

• Adequate liver function as indicated:

‣ Serum aspartate transaminase (ASAT) and alanine transaminase (ALAT) ≤ 3.0 x upper limit of normal (ULN);

⁃ Bilirubin ≤1.5 x ULN (unless bilirubin rise is due to Gilbert's syndrome or controlled autoimmune hemolytic anemia);

• Prothrombin time (PT)/International normal ratio (INR) \<1.5x ULN and activated partial thromboplastin time (aPTT) \<1.5 x ULN; unless receiving anticoagulation;

• Negative serological testing for hepatitis B virus (HBV) (Hepatitis B surface antigen (HBsAg) negative and hepatitis B core antibody (anti-HBc) negative) and hepatitis C virus (hepatitis C antibody). Patients who are positive for anti-HBc or hepatitis C antibody may be included if they have a negative PCR within 6 weeks before enrollment. Those who are PCR positive will be excluded; Please note: For patients positive for anti-HBc, HBV-DNA PCR has to be repeated every month until 12 months after last dose of study treatment.

• Patient is able and willing to adhere to the study visit schedule and other protocol requirements;

• Patient is capable of giving informed consent;

• Written informed consent.