MT2023-20: Hematopoietic Cell Transplant With Reduced Intensity Conditioning and Post-transplant Cyclophosphamide for Severe Aplastic Anemia and Other Forms of Acquired Bone Marrow Failure.

For patients with EBV IgG seropositivity or EBV PCR positivity on pre-transplant evaluations, Rituximab 375 mg/m2 is given IV once on day -14 (+/-2 day) in the outpatient setting. Pre-medicate 30 minutes prior to rituximab with methylprednisolone (1 mg/kg) IV, acetaminophen 15 mg/kg (maximum 650mg) IV or PO and diphenhydramine 1 mg/kg (maximum 50mg) IV or PO.

Rabbit ATG will be administered at doses and days indicated above, infused through a 0.22 micrometer filter over 4-6 hours. Pre-medicate 30 minutes prior to ATG infusion with methylprednisolone 1 mg/kg IV, (max dose = 125 mg), acetaminophen 15 mg/kg dose (max dose = 650 mg) enterally and diphenhydramine 1 mg/kg/dose (max dose = 50 mg) enterally or IV.

Mycophenolate mofetil (MMF) therapy will begin on day +5. For pediatric service patients dosing of MMF will be 15 mg/kg/dose (max = 1000 mg) three times daily. For adult service patients dosing of MMF will be 15 mg/kg/dose (max = 1500 mg) twice daily. The same dosage is used orally or intravenously. Consider dose modification and/or pharmacokinetic measurements if renal and/or hepatic impairment (GFR\<25 mL/minute corrected). Stop MMF at Day +35 or 7 days after engraftment achieved (ANC\>500 x 106 neutrophils/L x 3 days) if later than day +35. If sufficient acute GvHD is observed to require systemic therapy, MMF should be continued for 7 days after initiation of systemic therapy. Afterward, use of MMF is at the discretion of the treating physician.

Cyclophosphamide 14.5 mg/kg is be given as a 2-hour infusion on day -6. If the patient is obese (actual body weight (ABW) \>/= 125% of the ideal body weight (IBW)), cyclophosphamide should be dosed using the adjusted body weight (AdjBW): 0.5(ABW-IBW) + IBW. Uroprotection with MESNA (14.5 mg/kg/day) in IV continuous infusion will be provided per institutional guidelines. Hyperhydration is not required for 14.5 mg/kg cyclophosphamide doses. Cyclophosphamide will be administered at 50 mg/kg using ABW over 2 hours on days +3 and +4. If the patient is obese (ABW \>/= 125% of the ideal body weight (IBW)), cyclophosphamide should be dosed using the adjusted body weight (AdjBW): 0.5(ABW-IBW) + IBW. Uroprotection with MESNA (50 mg/kg/day) in IV continuous infusion as well as hyperhydration will be provided per institutional guidelines.

For all patients, fludarabine dosing will be model-based using Bayesian methodology IV every 24 hours on days -6 to -3 with a cumulative area under the curve (cAUC) of 20 mg\*hr/L.

For patients age \>/= 25 years, with myelodysplasia, or clonal hematopoiesis, total body irradiation will be 4 Gy, provided in two fractions on day -1. For all other patients, total body irradiation will be 2 Gy provided in a single fraction on day -1. Each dose of 2 Gy will be given at a dose rate between 1 and 1.9 Gy/minute prescribed to the midplane of the patient at the level of the umbilicus.

On day 0 the cells will be infused per cell source specific institutional guidelines.

Beginning on day +5, patients will receive G-CSF SQ or IV 5 micrograms/kg once daily until post-nadir ANC \> 1500/μL for 3 consecutive days or \>3000/μL for 1 day.

Tacrolimus will begin on day +5 at an initial dose of 0.03 mg/kg/day IV via continuous infusion. Goal trough levels will be 10-15 ug/mL until day +14 posttransplant, then decreased to a goal of 5-10 ng/mL thereafter. In the absence of GvHD, tacrolimus will discontinue at day +180 without a taper.