A Study to Assess the Effectiveness and Safety of Pacritinib in Patients With VEXAS Syndrome (PAXIS)

Key Inclusion Criteria: * Documented evidence of a pathogenic mutation at methionine-41 (M41) or neighboring splice site mutation (c.118-1, c.118-2) position in UBA1 mutation based on myeloid next-generation sequencing (NGS) droplet digital polymerase chain reaction (ddPCR), or Sanger sequencing in peripheral blood or bone marrow samples. * Current or documented evidence of past inflammatory involvement within 6 months prior to enrollment of at least one of the following organ systems by VEXAS syndrome: cutaneous (e.g., neutrophilic dermatosis, cutaneous vasculitis), vasculature (e.g., vasculitis), musculoskeletal (e.g., chondritis, arthritis), ocular (e.g., uveitis, scleritis), periorbital (e.g. periorbital edema), genitourinary (e.g., epididymitis), or pulmonary (e.g., alveolitis). * Receiving ongoing GC therapy (stable prednisone or prednisolone dose of 15-45 mg/day) leading up to enrollment. * Karnofsky Performance Status ≥50% * Adequate organ function, meeting all the following criteria within 30 days prior to enrollment: 1. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤3 × upper limit of normal (ULN) 2. Total bilirubin ≤4 × ULN (≤8 × ULN in the setting of Gilbert's syndrome) 3. Creatinine clearance (CrCl) ≥30 mL/min based on the Cockcroft-Gault formula 4. Absolute neutrophil count ≥500/μL 5. Prothrombin time (PT) or international normalized ratio (INR) ≤1.5 × ULN (unless prolonged due to therapeutic anticoagulation) 6. Partial thromboplastic time (PTT) or activated PTT ≤1.5 × ULN (unless prolonged due to therapeutic anticoagulation) 7. Platelet count ≥25 × 10\^9/L (value must be obtained in the absence of platelet transfusion in the prior 7 days) 8. Peripheral blasts \<5% * QT corrected by the Fridericia method (QTcF) ≤450 msec in males or ≤470 msec in females. Participants with QRS prolongation \>100 msec may enroll if their QTcF is ≤480 msec. Participants with ventricular paced rhythms may enroll if their QTcF is ≤500 msec. If QTcF is thought to be prolonged due to a modifiable factor (e.g., medication / electrolyte abnormality), QTcF may be reevaluated. * Women of child-bearing potential (WOCBP) must have a negative serum pregnancy test within 30 days prior to enrollment and a negative urine pregnancy test on Day 1 prior to randomization and dosing. * WOCBP and male participants must agree to use a highly effective method of contraception starting at the first dose of trial therapy through 30 days after the last dose of trial therapy. Key Exclusion Criteria * Prior allogenic hematopoietic stem cell transplant (allo-HSCT) or solid organ transplant (other than corneal). * Current use of systemic GCs for conditions other than VEXAS syndrome, which, in the opinion of the Investigator, would interfere with adherence to a GC taper regimen and/or assessment of efficacy. * More than one prior admission to an intensive care unit due to a VEXAS Syndrome flare within the prior 6 months. * Received ≥9 units of intensive red blood cell (RBC) transfusions in the 90 days prior to enrollment. * Known concurrent myelodysplastic syndrome (MDS) requiring antineoplastic treatment, or allo-HSCT, or known high-risk or very high-risk MDS based on the Revised International Prognostic Scoring System (IPSS-R). Participants with MDS who do not meet these criteria may enroll. * Malignancy within 1 year prior to enrollment with the exception of MDS (per exclusion criterion), curatively treated non-melanoma skin cancer, or curatively treated carcinoma in situ. Participants with pre-malignant hematologic conditions (e.g., monoclonal gammopathy of unknown significance \[MGUS\], clonal cytopenia of unknown significance) may enroll. * Exposure to hypomethylating agents (HMA) within 6 months prior to enrollment, or exposure to more than 6 cycles of HMAs at any time. * Exposure to non-GC anti-inflammatory therapy or hematologic support therapy within protocol defined timeframes prior to enrollment * Exposure to anti-platelet therapy with the exception of low-dose aspirin (≤100 mg daily) within 28 days prior to enrollment. * Known concomitant multiple myeloma, or serum M-protein ≥3 g/dL, involved-to uninvolved free light chain (FLC) ratio ≥100, or involved FLC level ≥100 mg/dL. Participants with MGUS may enroll. * Systemic treatment with a strong cytochrome P450 3A4 (CYP3A4) inhibitor or inducer within 5 half-lives prior to enrollment. * Significant recent bleeding history defined as National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE) grade ≥2 within 3 months prior to enrollment, unless precipitated by an inciting event. * History of clinically significant cardiovascular disease, or clinically significant abnormalities in rhythm or conduction during Screening ECG, including: 1. Severe cardiac event (CTCAE grade ≥3) within 3 months prior to enrollment 2. Heart failure resulting in limitations during ordinary activity. * Arterial or venous thrombotic or embolic events, including deep vein thrombosis, pulmonary embolism, and cerebrovascular accident (including transient ischemic attacks), within 60 days prior to enrollment. * Moderate or severe hepatic impairment that meets criteria for Child-Pugh Class B or C, or active viral hepatitis. * Uncontrolled human immunodeficiency virus (HIV) off antiretrovirals, or on antiretrovirals with detectable viral load. * Positive Quantiferon (or other interferon gamma release assay) during Screening. * Known history of disseminated mycobacterial infection. * Concurrent or prior enrollment in another prospective interventional trial of VEXAS-directed therapy, or concurrent enrollment in another interventional trial of non-VEXAS-directed therapy, or treatment with a non-VEXAS-directed experimental therapy within 28 days or five half-lives prior to enrollment, whichever is longer. * Pregnant, intending to become pregnant during the trial, or currently breastfeeding/lactating. * Participants with any acute, active infection requiring systemic antimicrobial treatment at the time of enrollment. Exceptions are made for prophylactic antibiotics or chronic antibiotic therapy for non-acute conditions. * Known hypersensitivity to pacritinib or any of the following inactive ingredients: microcrystalline cellulose, polyethylene glycol, and magnesium stearate.