Suboptimal Busulphan Exposure Is Associated with Mixed Hematopoietic Chimerism, a Risk Factor for Rejection in Bone Marrow Transplantation for Homozygous beta Thalassemia.

Bone marrow transplantation is the only curative treatment modality available for patients with homozygous beta thalassemia. Although the transplant outcome for good-risk patients is uniformly excellent, rejection continues to remain a serious complication with reported incidences ranging from 10–40%. Since mixed hematopoietic chimerism (MC) has been shown to be a risk factor for rejection in homozygous beta thalassemia, we investigated the relationship between MC and busulfan pharmacokinetics (Bu PK). Between May 1999 and April 2004, we retrospectively analyzed 24 consecutive patients with homozygous beta thalassemia transplanted in this center, for whom data on Bu PK was available. The median age was 9.5yr (3–15 yr). Sixteen (67%) patients were Class II, six (25%) Class III, one patient (4%) Class I and one patient (4%) Class II or III as per the Lucarelli classification for transplant outcome. The conditioning regimen consisted of busulfan/cyclophosphamide in 21/24 (88%) and busulfan/cyclophosphamide/ATG in 3/24 (12%) patients. All patients received a standard dose of cyclophosphamide (200mg/kg) but had varying doses of busulfan, ranging from 15 to 29mg/kg (mean 21mg/kg). Nine out of the 24 patients (38%) received the intravenous preparation of busulfan. Cyclosporine and short-course methotrexate were used as graft versus host disease (GVHD) prophylaxis. Bu PK was performed by liquid chromatography mass-spectrometry on frozen plasma samples obtained after doses 1, 5 and 13. Chimerism was assessed after BMT by DNA fingerprinting using a panel of informative markers. Acute GVHD was seen in 13/23 (56%) evaluable patients, 9/13 were grade I & II and 3/13 grade III & IV. Veno-occlusive disease of the liver (VOD) as per the Seattle and Baltimore criteria occurred in 14/24 (58%) and 3/24 (12%) respectively. MC was documented in 5/24 (21%) patients while graft rejection occurred in 2/23 (9%) evaluable patients. Non-rejection mortality was 4% (1/24). Thalassemia-free survival was 88% (21/24) with a median follow up of 31 months. MC significantly predicted for rejection (p=0.04). Analysis of Bu PK showed that patients with MC had significantly lower minimum and steady state busulfan concentrations (Cmin and Css) and faster clearance of the drug (CL/F) when compared to patients with complete chimerism.

Although the number of patients is small, this study confirms MC to be a risk factor for rejection, and further demonstrates the occurrence of MC post BMT to be associated with low busulfan systemic exposure during conditioning. Individualizing busulfan dosage based on Bu PK may therefore be one strategy to minimize rejection in bone marrow transplantation for homozygous thalassemia.