Pacritinib in Vacuoles, E1 Ubiqutin-activating Enzyme, X-linked, Autoinflammatory, Somatic (VEXAS) Syndrome

Inclusion Criteria: * Patients must have UBA1 mutation with a variant allele frequency (VAF) of ≥ 2% detected on a next generation sequencing panel and have at least one of the following current or past clinical manifestation of VEXAS syndrome, as determined by the attending physician: * skin rash * vasculitis * chondritis * ocular/orbital inflammation (e.g., uveitis/iritis, episcleritis) * genitourinary inflammation (e.g., epididymitis/orchitis) * arthritis/arthralgias * pulmonary inflammation (e.g., alveolitis/pleural effusion,) * fever * thrombosis * splenomegaly * hepatomegaly * myocarditis or pericarditis * cytopenias (defined as hemoglobin \<11 g/dL, platelets \< 100 X 10\^9 /L, OR absolute neutrophil count \<1.0 X 10\^9 /L). * Patients with VEXAS syndrome who have never been treated with a JAK-I will be eligible to enroll on study. A stable corticosteroid dose must be maintained for at least 14 days prior to start of pacritinib. * Patients who have previously been treated with a JAK-I other than pacritinib, or who are currently being treated with a JAK-I other than pacritinib, may be eligible after a 28 day washout if either (i) their symptoms are not adequately controlled, as determined by the treating physician, or (ii) they have been unable to taper corticosteroids to an equivalent of \<10 mg prednisone/day, and in the opinion of the treating physician, may benefit from a change in JAK-I. A stable corticosteroid dose must be maintained for at least 14 days prior to start of pacritinib. * At least 18 years of age. * ECOG performance status ≤ 3. * Organ function as defined below: * Absolute neutrophil count ≥ 0.5 K/cumm * Platelets ≥ 25 K/cumm * PT/PTT \<2.5 X upper limit of normal (ULN) * Total bilirubin ≤ 1.5 x IULN * AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN * Creatinine clearance ≥ 30 mL/min by Cockcroft-Gault * QTcF \< 480 msec. * The effects of pacritinib on the developing human fetus are unknown. For this reason and because pacritinib was shown to be teratogenic in animal studies, women of childbearing potential and men must agree to use highly effective contraception prior to study entry, for the duration of study participation, and for 30 days after completion of study treatment. Hormonal contraception is no longer considered highly effective alone as pacritinib is a CYP3A4 inducer and accelerated progesterone metabolism. The contraceptive methods considered highly effective for WOCBP who receive pacritinib are intrauterine devices, bilateral tubal occlusion, vasectomized partner, or total sexual abstinence. Hormonal contraceptives (e.g., Depo-Provera) alone are not considered highly effective methods of contraception on their own when in treatment with pacritinib; such hormonal contraceptives must be combined with an additional barrier method (condom, diaphragm with spermicidal gel, or condoms with spermicides to be considered highly effective. Highly effective contraceptive methods in males include vasectomy, sexual abstinence, and condoms when combined with their partner using a highly effective method (including oral contraceptives). * Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. * Ability to understand and willingness to sign an IRB approved written informed consent document or that of legally authorized representative, if applicable. * Patients with myelodysplastic neoplasms (MDS) or plasma cell dyscrasias are eligible if they are not undergoing active treatment. Supportive care is permitted. Exclusion Criteria: * Prior use of pacritinib. * Use of another JAK inhibitor within 28 days of C1D1 of pacritinib. * Currently receiving any other investigational agents. Patients may be eligible after 28 day washout. * Thrombotic events (arterial or venous) within 60 days prior to enrollment. * Any recent clinically significant bleeding within at least 7 days prior to enrollment. * Any active or acute infection. * History of malignancy within the prior 2 years, with the exception of MDS and plasma cell dyscrasias, or non-melanoma skin cancers that have been treated. * History of clinically significant cardiovascular disease or clinically significant abnormalities in rhythm or conduction during screening EKG, including severe cardiac events, symptomatic congestive heart failure, unstable angina pectoris, uncontrolled cardiac arrhythmia, or heart failure. * Currently receiving immunosuppressants (other than corticosteroids), disease-modifying antirheumatic drugs (DMARDs), or biologic cytokine inhibitors. Patients may be eligible after 28 day washout. * A history of allergic reactions attributed to compounds of similar chemical or biologic composition to pacritinib. * Concurrent use of strong CYP3A4 inhibitors or inducers. Patients may be eligible after washout period of 28 days (or 5 half-lives, whichever is shorter). * Diagnosis or history of moderate (Child-Pugh B) and severe hepatic impairment (Child-Pugh C). * Pregnant and/or breastfeeding. Women of childbearing potential must have a negative serum pregnancy test within 7 days of C1D1 or negative urine pregnancy test within 3 days of C1D1. * Known active viral infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C virus (HCV). * Patients with latent tuberculosis. Patients must have a negative T-Spot during screening to be eligible.