Multicenter Study of Pacritinib Combined With Ibrutinib in Relapsed/Refractory Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (CLL/SLL)

Inclusion Criteria: * Diagnosis of CLL/SLL * Relapsed or refractory CLL or SLL following at least 1 prior line of systemic therapy with active disease meeting criteria for treatment * Age ≥18 and \<80 * ECOG (Eastern Cooperative Oncology Group) ≤2 (This is a performance status that attempts to quantify a patients daily activities where 0 represents normal activity and 5 represents death) * Adequate organ function defined as AST and ALT ≤ 2 times upper limit of normal (ULN), Total Bilirubin ≤ 1.5 times ULN (exception of Gilbert disease), Renal function: CrCl ≥30 mL/min, Bazett-corrected Q-T interval ≤ 0.45 seconds * Peripheral blood counts of ANC \>500 cells/μL, platelets ≥ 50,000 cells/ μL, Hemoglobin≥ 8 g/dL * Prior treatment allowed if: at least 30 days have elapsed since last chemotherapy and/or radiation and patient has recovered from all clinically significant treatment-related toxicity, or at least 90 days have passed since date of autologous stem cell transplant and patient has recovered to ≤grade 1 toxicity related to this procedure. * Ability to provide written informed consent * Ability to take oral medications. Exclusion Criteria: * Pregnant or breast feeding women * Primary or metastatic CNS (Central Nervous System) disease prior to study enrollment * Uncontrolled current illness including, but not limited to, ongoing or active infections requiring intravenous antimicrobials, symptomatic congestive heart failure, unstable angina pectoris, unstable cardiac arrhythmia and/or psychiatric illness or social situations that would limit compliance with study requirements * Known HIV infection * Active infection with Hepatitis B or C virus * Concomitant therapy in last 30 days of any of the following: cytotoxic chemotherapy, immunosuppressive agents, other investigational therapies or chronic use of systemic corticosteroids * Prior treatment with ibrutinib * Uncontrolled autoimmune hemolytic anemia (AIHA) or autoimmune thrombocytopenia (ITP). * Requires anticoagulation with warfarin or equivalent Vit K antagonist * Allergy to either ibrutinib or pacritinib or components within medication * Treatment with strong CYP3A4 inducer or inhibitor, for which no alternative is available. * Unwilling or unable to use a medically acceptable form of contraception. * Any gastrointestinal or metabolic condition that could interfere with the absorption of oral medication.