Low Toxicity and Mortality with Reversed-Order Conditioning (Cyclophosphamide Followed by Targeted Intravenous Busulfan) in Allogeneic Hematopoietic Cell Transplantation: Preliminary Results of a Prospective Clinical Trial.
Authors: Rezvani Andrew R. , McCune Jeannine S. , Batchelder Ami , Storer Barry E. , McDonald George B. , Deeg H. Joachim 
Published: October 2019

Abstract
Abstract 1175
Poster Board I-197

BACKGROUND:
Regimen-related toxicity is a significant cause of morbidity and mortality after myeloablative allogeneic hematopoietic cell transplantation (HCT). The widely-used conditioning regimen of busulfan followed by cyclophosphamide (BU/CY) has been associated with mortality rates of 12–13% at 100 days after HCT (BBMT 2007 13:355–65, Blood 2002 100:1201–7). Additionally, regimen-related liver injury in the form of moderate or severe hepatic sinusoidal obstruction syndrome (SOS) occurs in 9% of allogeneic HCT recipients conditioned with targeted BU/CY. Preclinical evidence suggests that BU, when given first in order, may deplete hepatic glutathione and potentiate the toxicity of CY, and that reversing the sequence of conditioning agents might reduce toxicity (BBMT 2007 13:853–62). We report the preliminary clinical and pharmacokinetic findings of an ongoing prospective Phase I/II clinical trial designed to test this hypothesis by reversing the standard order of conditioning agents, using CY followed by targeted IV BU.

PATIENTS AND METHODS:
As of June 17, 2009, 25 patients with myelofibrosis (n=14) or acute myeloid leukemia (AML)/myelodysplasia (MDS) (n=11) were enrolled and observed through day +100 after HCT. Median age was 55 (range, 34–62) years. Twelve patients (48%) had related donors, while 13 had unrelated donors. Among the 13 unrelated donors, 12 were HLA-matched while 1 was mismatched at one HLA allele. Patients received IV CY 60 mg/kg/day on days -7 and -6 before HCT, followed by IV BU (Busulfex, Otsuka Pharmaceutical) 4 mg/kg/day on days -5 through -2. After the first BU dose, subsequent doses were adjusted based on pharmacokinetic monitoring to achieve plasma BU concentrations at steady state (Css) of 800–900 ng/mL. Prophylactic phenytoin was administered on days -6 through -1, and prophylactic ursodiol was administered starting 2 weeks before HCT and continuing through day +90. All patients received G-CSF-mobilized peripheral blood stem cell grafts. Graft-vs.-host disease (GVHD) prophylaxis consisted of tacrolimus combined with methotrexate 10 mg/m2 on days +1, +3, +6, and +11. Endpoints included engraftment, liver and renal toxicity, day +100 mortality, overall survival, and pharmacokinetics of CY and its metabolites 4-hydroxycyclophosphamide (HCY) and O-carboxyethyl-phosphoramide mustard (CEPM).

RESULTS:
All patients initially engrafted, with neutrophil engraftment at a median of 17 (range, 11–26) days after HCT. One patient (4%) with an HLA-allele-mismatched unrelated-donor allograft had late graft failure approximately 3 months after HCT. Bilirubin values in the first 20 days after HCT peaked at a median of 1.9 (range, 0.5–12.4) mg/dL. SOS occurred in 1 patient (4%), who had a 10 kg weight gain, peak bilirubin of 2.3, and tender hepatomegaly with eventual resolution. Median peak serum creatinine in the first 20 days after HCT was 1.0 (range, 0.6–3.7) mg/dL. One patient (4%) experienced acute renal insufficiency, defined as a doubling of serum creatinine from pre-transplant baseline, within the first 20 days after HCT in the setting of septic shock. Day +100 survival was 96%; one patient died of sepsis at day +42. Kaplan-Meier estimated survival at the median follow-up of 8 months was 91% (myelofibrosis, 100% and AML/MDS, 80%). In the 6 patients who had died at last follow-up, causes of death included graft-vs.-host disease (n=2), sepsis (n=2), relapsed AML (n=1), and metastatic prostate cancer (n=1). Peak plasma concentrations of CY, HCY, and CEPM were 365.3 ± 58.2 μM, 9.5 ± 5.0 μM, and 12.7 ± 5.9 μM respectively after dose 1 of CY. After dose 2 of CY, peak plasma concentrations of CY, HCY, and CEPM were 312.9 ± 54.7 μM, 21.7 ± 8.6 μM, and 27.7 ± 11.0 μM respectively. The areas under the curve from time 0 to 48 hours for CY, HCY, and CEPM were 4737.0 ± 1361.3 μM • h, 182.4 ± 51.3 μM • h, and 508.5 ± 185.6 μM • h respectively.

CONCLUSIONS:
Reversing the sequence of conditioning agents in allogeneic HCT, from BU/CY to CY/BU, produced engraftment with low rates of regimen-related toxicity and 96% survival at day +100 after HCT. Our preliminary findings lend clinical support to the hypothesis that reversing the sequence of these agents may lessen exposure to toxic CY metabolites and improve clinical outcomes.

Disclosures:
Rezvani: Otsuka: Research Funding. Off Label Use: Describes intravenous busulfan in combination with cyclophosphamide as conditioning for allogeneic hematopoietic cell transplantation in patients with acute myeloid leukemia, myelodysplasia, and myelofibrosis.. McCune:Otsuka: Consultancy, Research Funding. Deeg:Otsuka: Research Funding.