Myeloablative Conditioning with Pharmacokinetic-Targeted Intravenous Busulfan and Cyclophosphamide in Unrelated Cord Blood Transplantation for Myeloid Malignancies in Children

Abstract 1965

Unrelated cord blood transplantation (UCBT) has been an increasingly used stem cell source in hematopoietic stem cell transplantation for myeloid malignancies in children when a related HLA-identical donor is unavailable. Advantages of UCBT include a less stringent HLA compatibility and prompt availability.

Myeloablative regimens for UCBT use a combination of total body irradiation or Busulfan (Bu) with Cyclophosphamide (Cy) or other chemotherapy agent. Intravenous Bu has a more predictable bioavailability comparing with oral Bu. Nevertheless, there is still an important variation in Bu pharmacokinetics (PK) between patients. Dose modification of Bu based on its first-dose PK might decrease the risk of toxicity and graft rejection and increase its antitumoral effect.

In order to analyse the role of a myeloablative conditioning regimen of PK-adapted Bu for myeloid malignancies we analyzed 30 consecutive first UCBT performed in children between dec/2000 and aug/2010. Median age at transplant was 6.3(0.6-17.3) years, 18 (60%) were male and median weight was 26.2 (6.9-81.1) kg. There were 18 acute myeloid leukemia (AML) and 12 myelodysplastic syndromes (MDS). Seven, nine, and two patients with AML were transplanted in first remission (CR1), second remission (CR2), or in advanced phase of disease (>CR2 or in relapse), respectively. Twenty-eight patients received a single UCBT. Seven, thirteen, and ten patients received a 6/6, 5/6 and 3–4/6 HLA-matched graft. Median infused nucleated cells (NC) and CD34+ cells were 5.28 × 10⁷/kg and 2.07 × 10⁵/kg of recipient body weight, respectively. Cyclosporin A and steroids were used as graft versus host disease (GvHD) prophylaxis. All patients received anti tymoglobuline.

Conditioning regimen consisted of PK-adapted Bu (targeted steady-state concentration - Css - between 600–900 ng/ml) and Cy 200 mg/kg (n=27), Melphalan 135 mg/m² (n=2) or Cy 120 mg/kg and etoposide 30 mg/kg (n=1). PK data were available for all but one patient. For five patients the initial prescribed dose of Bu was not changed, for two patients initial prescribed dose of Bu was decreased and for 22 patients the initial prescribed dose of Bu was increased (median increase of 24.8% - range: 5.3–56,3 %).

Median follow-up was 53 months.

Cumulative incidence (CI) of neutrophil (>0.5×10⁹/L) at D+60 and platelet recovery (>50×10⁹/L) at D+150 was 83% and 83%, respectively. Median time to neutrophil and platelet recovery was 20 days and 69 days, respectively. There was a single case of graft failure. CI of acute GVHD grade II-IV at D+180 was 14% (with one case of grade III aGvHD). There were two cases of VOD (one mild and one moderate). Two-years CI of transplant-related mortality (TRM) was 17% (three patients died due to infections and one due to graft failure). CI of relapse at 2 years was 30% (with all relapses occurring within two years of UCBT). Event-free survival (EFS) at 5 years was 53%. EFS for patients with MDS (67%) and AML (45%) were not significantly different (P=0.45). Overall survival (OS) at 5 years was 61%. There was a tendency to a better OS for patients with MDS vs AML (83% vs 49% - P=0.19). For AML, disease status did not influence survival.

In conclusion, UCBT for myeloid malignancies is a viable option when a HLA-identical sibling donor is unavailable. PK study for ivBu is important as most patients needed Bu dose adjustment. PK-adapted BU seems to reduce acute toxicity and graft failure. However TRM due to infection and relapse remains a problem. New conditioning regimens associating fludarabine with PK-adapted Bu alongside with modification in GvHD prophylaxis to improve immunological recovery might contribute to further decrease TRM and relapse and improve UCBT results, especially for AML.