Permanent Alopecia in Children Following Busulfan Based Conditioning Is Associated with Glutathione M1 Null Genotype.

Permanent alopecia (PA) is an important complication of allogeneic bone marrow transplantation (BMT). In an analysis of 30 consecutive Omani children who underwent BMT in this center, we have previously reported a strong association between high systemic exposure to busulfan (bu) and PA. In our current study, we investigated the role of the highly polymorphic glutathione S transferase M₁ genotype (GST M₁) in children with PA, since GST M₁ is an important isoenzyme which metabolizes bu. We analyzed twenty consecutive children transplanted in this center between May 1999 and October 2003 with bu based conditioning regimens, for whom bu pharmacokinetics and pre-transplant DNA were available. Bu pharmacokinetics was performed retrospectively on frozen plasma samples by liquid chromatography mass spectrometry and GST M₁ and T₁ polymorphisms analyzed by a multiplex PCR technique previously described. The frequency of the GST M₁ null genotype was found to be 38% in the normal Omani population. All patients had a minimum follow up of one year. The median age was 9 yrs (3–16 yrs), 10 were male and 10 female. Indications for transplant were hereditary disorders 18 (90%) and hematologic malignancy 2 (10%). Nineteen patients (95%) received bu and cyclophosphamide-based myeloablative conditioning regimens with or without anti thymocyte globulin (ATG). One patient received a non-myeloablative regimen consisting of fludarabine, bu (6.4mg/kg) and ATG. The average dose of busulfan for the entire group was 489mg/m². Varying degrees of PA were seen in 9 (45%) of patients. The GST M₁ null genotype was present in 78% of patients with PA as compared to 18% in those with normal hair re-growth (p=0.012). Patients with PA were younger, received higher doses of bu and had significantly higher minimum and steady state busulfan concentrations (C_min and C_ss).

The clearance of bu in patients with GST M₁ genotype has been previously reported to be increased resulting in lower bu levels. This was shown to be due to upregulation of GST A₁, the major GST isoenzyme responsible for the metabolism of bu. Our study shows permanent alopecia post BMT following a bu based conditioning regimen to be significantly associated with the GST M₁ null genotype and higher bu systemic exposure. It is possible that in these patients, the level of up-regulation of GST A₁, if any, might not have been sufficient to optimally metabolize the doses of bu that they received. Although more patients need to be studied, we conclude that patients with the GST M₁ null genotype are particularly at risk for developing PA, and that targeted dosing of bu may be a strategy to minimize this complication.