A Phase II Study of Allogeneic Hematopoietic Stem Cell Transplant for Subjects With VEXAS (Vacuoles, E1 Enzyme, X-linked, Autoinflammatory, Somatic) Syndrome

stem cell transplant on day 0

0.8 mg/kg IV over 2 hours. May be skipped if real-time PKs are done during conditioning.

Mycophenolate mofetil (MMF): 15 mg/kg IV over 2 hours BID starting on day +5 until approximately day +35 (+/-2 days)

Starting on day +5, start at 0.02 mg/kg IV continuous infusion over 24 hours until day +180 and titrated to trough levels of 5-15 mg/ml.

AUC Targeted Dose based on busulfan test dose PKs, IV infusion over 3 hours once daily (3.2 mg/kg IV per day will be the default dose) per the below time frame: For 8/8 Matched Related or Unrelated Donor Busulfan dose will be on days -6, -5, and -4 For 7/8 Matched Related or Unrelated or Haploidentical Donor Busulfan dose will be on days -4 and -3

For 7/8 Matched Related or Unrelated or Haploidentical Donor, 200cGy on day -1

40 mg/m2 IV over 30 mins daily For 8/8 Matched Related or Unrelated Donor Fludarabine dose will be on days -6, -5, -4, and -3 For 7/8 Matched Related or Unrelated or Haploidentical Donor Fludarabine dose will be on days -6, -5, -4, -3, and -2

For 7/8 Matched Related or Unrelated or Haploidentical Donor, prior to transplant 14.5 mg/kg IV daily on days -6 and -5

Post-Transplant Cyclophosphamide: 50 mg/kg IV daily over 2 hours on days +3 and +4, dosed according to ideal body weight