Belimumab in Patients with Chronic Lymphocytic Leukemia

Inclusion Criteria: * Male or female ≥18 years of age. * Diagnosis of CLL/SLL established according to iwCLL criteria * Refractory or relapsed CLL that warrants treatment (according to modified criteria for initiation of therapy (Hallek et al., 2018)): 1. Massive (ie, lower edge of spleen ≥6 cm below the left costal margin), progressive, or symptomatic splenomegaly, or 2. Massive (ie, ≥10 cm in the longest diameter), progressive, or symptomatic lymphadenopathy, or 3. Progressive lymphocytosis in the absence of infection, with an increase in blood ALC≥50% over a 2-month period or lymphocyte doubling time of \<6 months (as long as initial ALC was ≥30,000/L), or 4. Autoimmune anemia and/or thrombocytopenia that is poorly responsive to corticosteroids or other standard therapy, or 5. Constitutional symptoms, defined as any one or more of the following disease-related symptoms or signs occurring in the absence of evidence of infection: * Unintentional weight loss of ≥10% within the previous 6 months, or * Significant fatigue (≥Grade 2), or * Fevers \>38.0°C for ≥2 weeks, or * Night sweats for \>1 month. * CLL relapsing after any line of treatment that included radiotherapy, chemotherapy, immunotherapy, or small molecules. Patients who relapse after a previous therapy with venetoclax can be included in the study in case of a late relapse (i.e. \>18 months after venetoclax was discontinued. * Discontinuation of all therapy (including radiotherapy, chemotherapy, immunotherapy, or small molecules) for the treatment of CLL ≥2 weeks before study treatment excluding systemic corticosteroids for symptomatic control. * All acute toxic effects of any prior antitumor therapy resolved to Grade ≤1 before treatment (with the exception of alopecia \[Grade 1 or 2 permitted\], neurotoxicity \[Grade 1 or 2 permitted\], or bone marrow parameters \[any of Grade 1, 2, 3, or 4 permitted). * Eastern Cooperative Oncology Group \[ECOG\] \< 3. * Required baseline laboratory data (within 4 weeks prior to treatment): * Serum total bilirubin ≤1.5 x ULN (unless directly attributable to CLL disease or to Gilbert's Syndrome) * ALT/AST ≤2.5 x ULN * Renal creatinine clearance \>30 ml/min * Neutrophile count \>1.000/μl (unless directly attributable to CLL disease) * Negative serological Hepatitis B and C test or negative PCR in case of positive serological test without evidence of an active infection, negative HIV test within 6 weeks prior to treatment. * Written informed consent of the subject after clarification Exclusion Criteria: * (Suspicion of) transformation of CLL (i.e. Richter's transformation, pro-lymphocytic leukemia) or central nervous system (CNS) involvement * IgG \< 4 g/L under substitution of immunoglobulins * Early relapse (i.e \<18 months) after any line of treatment that included venetoclax. * Malignancies other than CLL currently requiring systemic therapies * Evidence of active systemic bacterial (e.g. tuberculosis), fungal, or viral infection (e.g., CMV) at the time of initiation of therapy * Confirmed progressive multifocal leukencephalopathy (PML) * Known history of drug-induced liver injury (DILI), chronic/active hepatitis C (HCV), chronic/active hepatitis B (HBV) * Requirement of therapy with strong CYP3A4 inhibitors/ inducers or anticoagulant with phenprocoumon or other vitamin K-antagonists * Active inflammatory bowel disease * History of prior allogeneic bone marrow or organ transplantation * Ongoing immunosuppressive therapy. Subjects may use topical, enteric, or inhaled corticosteroids as therapy for comorbidities and systemic steroids for autoimmune anemia and/or thrombocytopenia. Ongoing use of low-dose systemic corticosteroids (≤5 mg/day of methylprednisolone or equivalent) for rheumatologic conditions is permitted * History of primary immunodeficiency * Concurrent participation in another therapeutic clinical trial * History of serious suicide risk including any suicidal behaviour in the last 6 months * Live vaccination 30 days prior to treatment * Hypersensitivity known from medical history to one of the drugs used or their ingredients or to drugs with a similar chemical structure * Simultaneous participation in another interventional clinical trial (including within the last 4 weeks before inclusion) * Addictions or other illnesses that do not allow the person concerned to assess the nature and extent of the clinical trial and its possible consequences * Pregnant or breastfeeding women * Women of childbearing potential, except women who meet the following criteria: * post-menopausal (12 months natural amenorrhoea or 6 months amenorrhoea with serum FSH \> 40 U/ml) * postoperative (6 weeks after bilateral ovarectomy with or without hysterectomy) * regular and correct use of a contraceptive method with an Pearl Index \< 1% per year, which will have to be continued for up to four months after the discontinuation of the study drug * sexual abstinence * Vasectomy of the partner * Male subjects who are able to father a child, except men who meet the following criteria: * willingness to abstain from heterosexual intercourse or use a protocolrecommended method contraception from the screening visit throughout the study treatment period and for 90 days following the last dose of study drug * refrain from sperm donation from screening visit throughout the study treatment period and for four months following the last dose of study drug * Indications that the subject is unlikely to adhere to the protocol (e.g., lack of compliance)