Multicenter, phase II trial with safety run-in to evaluate the efficacy and safety of momelotinib in patients with VEXAS (Vacuoles, E1 enzyme, X-linked, Autoinflammatory and Somatic) syndrome with or without associated myelodysplastic syndrome (MDS).

The study will consist of two consecutive steps, a dose-finding safety run-in and a single-arm prospective phase II.

During safety run-in phase, three fixed dose levels will be tested according to a 3+3 design, using cohorts of size 3 in order to establish the maximum tolerated dose.

After this safety run-in phase, patients included in phase II will be treated with momelotinib at the maximum tolerated dose preliminary fixed.

Patients included in the phase II will receive momelotinib continuously until disease progression or loss of response, at physician's discretion.

All patients included in the study will receive glucocorticoids (prednisone/prednisolone equivalent) at baseline (at least > 10mg/day).

Response assessment regarding VEXAS related symptoms will be evaluated after 4, 12, 24 and 48 weeks. Response assessment regarding MDS features will be evaluated at 12 and 24 weeks.

During safety run-in phase, the three fixed dose levels tested are :

• Dose level (DL) -1: 150 mg once daily (QD)
• DL1: 200 mg QD
• DL2: 300 mg QD. Between 6 and 18 patients will be enrolled during the safety run-in phase. Up to 39 patients could be included in the phase II study and will be treated with momelotinib at the maximum tolerated dose preliminary fixed during the safety run-in phase.

Baseline steroids daily dose required for VEXAS inflammatory manifestations will be defined during screening period (28 days period) for each patient. It is defined as the minimal daily dose of steroids used in the last 14 days prior momelotinib onset (according to physician disposition) that allow disease control. In case of related VEXAS inflammatory manifestation during screening period with a first fixed dose, an increased dose of steroids should be evaluated during at least an extra 14 days prior momelotinib onset. This baseline dose defined during screening period will be used for response criteria during follow-up.

Momelotinib treatment will be discontinued after 24 weeks at optimal dosing regimen (up to 300 mg/day), in case of absence of response.

Treatment might also be discontinued during follow-up in case of loss of response/hematological progression or non-tolerable adverse event.

Inclusion Criteria:

• ECOG (Eastern Cooperative Oncology Group) performance status 0-2 at the time of screening

• Age ≥ 18 years

• Written informed consent

• Diagnosis of VEXAS (Vacuoles, E1 enzyme, X-linked, Autoinflammatory and Somatic) syndrome with UBA1 (Ubiquitin Like Modifier Activating Enzyme 1) mutation and clinically symptomatic disease requiring immunosuppressive treatment and at least 10mg/d of glucocorticoids

• Patients with uncontrolled symptoms related to VEXAS with prior treatment line(s) (including steroids)

• Patients refractory/dependent to steroids

• Single concomitant steroids therapy (e.g., prednisone or equivalent) at the time of inclusion is allowed

• For patients treated with other immunosuppressive/immunomodulatory therapy than glucocorticoids, a wash out period of 28 days is required prior momelotinib onset

• Erythropoietin/luspatercept used as a growth factor treatment is not allowed 28 days prior enrollment

• Adequate liver function (serum transaminases ≤ 3 x ULN (Upper Limits of Normal), Bilirubin ≤ 1.5 x ULN (isolated bilirubin > 1.5 x ULN is acceptable if bilirubin fractionated and direct bilirubin < 35%)

• Adequate renal function (creatinine clearance with MDRD (Modification of Diet in Renal Disease) formula > 30 ml/min)

• Women of child-bearing potential (i.e., women who are pre-menopausal or not surgically sterile) must:

  1. Have a negative serum or urine pregnancy test within 24 hours prior to beginning treatment on this study. Lactating patients are excluded.
  2. Agree to use, and to be able to comply with, effective contraception without interruption, 4 weeks before starting study drug throughout the entire duration study drug therapy (including doses interruptions) and for 12 weeks after the end of the study drug therapy.
  3. Agree to learn about the procedures for preservation of egg before starting treatment.

• Male patients must:

  1. Agree the need for the use of a condom if engaged in sexual activity with a woman of childbearing potential during the entire period of treatment, even if disruption of treatment and during 12 weeks after end of treatment.
  2. Agree to learn about the procedures for preservation of sperm before starting treatment.

Exclusion Criteria:

• Patients with MDS (Myelodysplastic syndrome) scheduled for allogeneic stem cell transplant or high risk MDS according to IWG (International Working Group) 2023
• Patients who are or have been already treated with Janus Kinase (JAK) inhibitors for VEXAS syndrome or another indication
• Patients who are unable to receive a starting daily dose of momelotinib of at least 100 mg
• Subjects with any other active malignancies are not eligible, except for the following: adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or other cancer from which subject has been disease-free for at least 3 years
• Uncontrolled congestive heart failure (New York Heart Association Classification 3 or 4), angina, myocardial infarction, cerebrovascular accident, coronary/peripheral artery bypass graft surgery, transient ischemic attack, or pulmonary embolism within 12 weeks prior to initiation of momelotinib
• Known infection with acute and chronic active Human Immunodeficiency Virus, Hepatitis B Virus, Hepatitis C Virus
• Any medical or psychiatric condition not allowing the informed consent of the subject
• Presence of clinically meaningful active bacterial, fungal, parasitic or viral infection which requires therapy
• Previous history of Progressive Multifocal Leuko-encephalopathy
• Active gastrointestinal conditions that may affect absorption
• No affiliation to a health insurance system
• Known hypersensitivity to the study investigational medicinal product, the metabolites, or formulation excipients