Age-dependent busulfan disposition and its relation to GSTA1-1 expressio

Busulfan is an alkylating agent used in the conditioning regimen prior to hematopoietic stem cell transplantation. The apparent oral clearance (CL/F) of busulfan is age-dependent, with young children (<5 years old) having a 1.3--2 fold greater CL/F compared with adults, when CL/F is expressed relative to body weight or body surface area. The mechanistic basis for enhanced busulfan CL/F in young children was the focus of this dissertation.;Busulfan is eliminated by conjugation with glutathione (GSH), and the product of the reaction is gamma-glutamyl-beta-(S-tetrahydrothiophenium) alanyl-glycine (THT+). We sought to identify the enzymes responsible for busulfan conjugation in humans. Glutathione S-transferases (GST) are a family of enzymes that catalyze the conjugation of electrophilic substrates with GSH. Purified GSTA1-1 was the predominant catalyst of busulfan conjugation, with a 2- and 5-fold greater intrinsic clearance than GSTM1-1 GSTP1-1, respectively.;The area under the plasma concentration-time curve (AUC) for busulfan and THT+ was measured in children and adults. The ratio of THT+ to busulfan AUC and busulfan CL/F were elevated to a similar extent. This finding suggests that children apparently eliminate busulfan through GSH conjugation more efficiently than adults. However, children and adults have comparable busulfan elimination half-lives, a parameter that reflects hepatic activity to a greater extent than the intestine. This suggests that children may differ from adults because they may conjugate a greater fraction of an oral dose of busulfan on first pass through the intestine.;To determine whether the small intestine contributes to the first-pass metabolism of busulfan, the intrinsic clearance of cytosol from the small intestine and liver of adult donors were compared. The intrinsic clearance of busulfan conjugation was comparable in cytosols prepared from the respective organs. Based on this finding, it was plausible to suggest that the small intestine contributes to first-pass busulfan metabolism.;Busulfan conjugation in incubations with biopsy specimens from young children and older children was compared. Intestinal biopsy specimens from young children had a 2-fold greater ability to conjugate busulfan relative to older children. This increase in busulfan conjugation was most likely due to enhanced GSTA1-1 expression because GSTA1-1 is the major isoform involved in busulfan conjugation and expressed in the adult small intestine. In conclusion, age-dependence in busulfan CL/F appears to result from enhanced intestinal GSTA1-1 expression in young children.