Busulfan is a chemotherapy drug that is primarily used as part of a conditioning/preparative regimen prior to bone marrow transplants to treat CML (chronic myeloid leukemia) and AML (acute myeloid leukemia) and other hematological malignancies. The preparative regiment serves to eradicate cancerous stem cells and residual hematopoiesis before bone marrow is transplanted. In addition, busulfan is FDA indicated for palliative treatment of CML. Busulfan is also used off-label to treat breast cancer, multiple myeloma, and ovarian cancer.
Busulfan can be given in both oral and IV dosage forms. The bioavailability of busulfan is highly variable, even with IV administration [Article:18690982]. Frequent monitoring of complete blood count (including white blood cells and platelets) should be performed due to severe myelosuppresion. Liver function (bilirubin, serum transaminases, and alkaline phosphatase) should also be monitored frequently [Article:18690982].
Busulfan is a bifunctional alkylating agent. It destroys cancerous cells by interfering with the DNA in cancer cells, via intrastrand crosslinks at 5'-GA-3' and 5'-GG-3', thereby preventing them from further dividing and ultimately leading to cell death through apoptosis [Article:15132775]. DNA adduct repair through dealkylation using the DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT), encoded by the gene MGMT, can lead to resistance for clinical alkylating agents [Articles:16428468, 11981013, 17112791]. Although MGMT expression has been associated with sensitivity of some alkylating agents (e.g. carmustine and temozolomide) in cell lines [Article:16428468], it does not seem to affect busulfan sensitivity [Article:16039682]. Other DNA repair mechanisms, such as mismatch repair (MMR) [Articles:9516945, 10235469] and base excision repair (BER) [Articles:19240165, 10878254, 8533150] may also lead to resistance to alkylating agents. DNA repair enzymes MLH1 and MSH2 involved in MMR have been implicated in resistance to alkylating agents such as temozolomide, cyclophosphamide and bulsulfan [Articles:9230204, 9312062, 11138465]. BER enzymes Apurinic/apyrimidinic endonuclease 1 (APEX1) and 3-methyladenine DNA glycosylase (MPG) are also associated with sensitivity busulfan [Articles:19470598, 9895121]. The expression of glutathione s-transferase has been hypothesized to play a role in resistance to alkylating agents. [Articles:8356908, 8770536] Overexpression of MGST2, a microsomal isotype of GST, has been found to confer resistance to busulfan [Article:15779864], although it is not known whether MGST2 plays a role in busulfan metabolism. Recent study using gene expression analysis to identify genes that were up- or down-regulated in busulfan resistance leukemia cell-lines and in leukemia patient samples (respond or not respond to bulsulfan-based stem cell transplantation) have been investigated [Articles:18339423, 19732952]. In these studies, cell-cyle regulating genes (e.g. CHK2 and CDC2), anti-apoptotic genes (BCL2, BAG3, XIAP) and pro-apoptotic genes (BNIP3, PUMA, and LTBR) were differentially expressed between busulfan sensitive and busulfan resistant cells [Articles:18339423, 19732952].
Busulfan is highly lipophilic and easily crosses the cell membrane. It is extensively metabolized and less than 2% is excreted renally unchanged [Articles:19611402, 10976660]. Membrane transporters for busulfan have not been elucidated. Busulfan is primarily metabolized in the liver by glutathione s-transferases (GSTs). GSTA1 is the predominant GST isoform catalyzing busulfan metabolism, whereas GSTM1 and GSTP1 also play a role [Articles:8886613, 18691123, 2753072, 2894960, 10437668]. The busulfan glutathione conjugate is then converted to a cysteine conjugate through the mercapturate pathway involving first -glutamyl transferase (GGT) and then dipeptidase/cysteinylglycinase (DPEP) [Article:2886318]. The cysteine conjugate can subsequently undergo enzymatic -elimination mediated by cystathionine gamma lyase (CTH) resulting in tetrahydrothiophene (THT), NH4+, and pyruvate [Article:18474673]. THT can then be oxidized either by the cytochrome P450 (CYP) or flavin containing monooxygenase (FMO) [Article:3078288], forming additional metabolites sulfolane and 3-hydroxysulfolane. Alternatively, the cysteine conjugate can be acetylated by N-acetyltransferase (NAT) [Article:18474673].
Pharmacogenomics of busulfan clearance have been investigated by various groups and the studies have focused on the genetic polymorphisms in various GSTs [Articles:18635758, 16448639, 19611402]. The result from in vitro data suggests that the GSTA1 SNPs or haplotypes are not associated with expression of GSTA1 protein and enzyme activity in hepatocytes [Article:12087351]. In a busulfan pharmacokinetic study involving 77 children, the genetic polymorphisms in the genes encoding GSTA1, GSTM1, GSTP1 and GSTT1 are not associated with the observed variability in the busulfan pharmacokinetics [Article:18641537]. However, in a smaller study involving 29 children, the GSTA1*B carrier (four variants in the promoter region of the GSTA1 gene which are in linkage disequilibrium,
-631T>G (rs4715333), -567 T>G (rs4715332), -69 C>T (rs3957357) and -52 G>A (rs3957356)] has reduced busulfan clearance by 30% [Article:18635758]. This result confirmed an even smaller study involving only 9 Japanese children (PMID: 16448639). In contrast, a busulfan pharmacokinetic study in 28 children found that there was no association of GSTA1 (C-1142G (rs58912740), G-631T (rs4715333), A-513G (rs11964968), C-69T (rs3957357) and GSTP1 A1578G (rs1695), C2293T (rs1138272)) polymorphisms with busulfan clearance and drug level [Article:19584821]. Whereas, in this study, the GSTM1-null genotype is associated with higher busulfan exposure and lower clearance [Article:19584821]. However, these findings remain to be validated in larger studies and also in adults.
There is a relationship between systemic exposure to busulfan, as measured by the area under the plasma concentration of busulfan versus time curve (AUC) or steady state concentration (Css) and busulfan-based transplantation outcomes [Articles:18690982, 10976660]. Treatment failure due to low systemic exposure and toxicity such as hepatic veno-occlusive disease (HVOD) due to high systemic level of busulfan are necessary to be avoided in patients [Articles:19611402, 8529285, 10976660]. In addition to busulfan disposition on treatment outcome and toxicity, the effect of genetic variants could play an important role and have been studied by various groups. A study involving 114 beta-thalassemia patients, who had busulfan-based bone marrow transplantation, has showed significant increased HVOD incidence in patients with the GSTM1-null genotype [Article:15142875]. As a result of bulsulfans narrow therapeutic window, monitoring of busulfan levels is regularly conducted in patients .
In summary, busulfan is used together with other chemotherapeutic agents, such as cyclophosphamide, etoposide and melphalan, in transplantation for various indications. Despite its wide indications and variability in response and toxicity, pharmacogenomics studies of busulfan remain a challenge due to limitation in patient samples with collected phenotype data.
M. Whirl-Carrillo, E.M. McDonagh, J. M. Hebert, L. Gong, K. Sangkuhl, C.F. Thorn, R.B. Altman and T.E. Klein. "Pharmacogenomics Knowledge for Personalized Medicine" Clinical Pharmacology & Therapeutics (2012) 92(4): 414-417. Full text
If you would like to reproduce this PharmGKB pathway diagram:
Entities in the Pathway
Drugs/Drug Classes (1)
Relationships in the Pathway
|Arrow From||Arrow To||Controllers||PMID|
|Busulfan glutathione conjugate||Busulfan glutathione conjugate||DPEP1, DPEP2, DPEP3, GGT1, GGT2||2886318|
|Busulfan glutathione conjugate||N-acetylated cysteine conjugate||NAT1, NAT2||18474673|
|Busulfan glutathione conjugate||NH4+, pyruvate, tetrahydrothiophene||CTH||18474673|
|busulfan||Busulfan glutathione conjugate||GSTA1, GSTM1, GSTP1||10437668, 2753072, 8886613|
|tetrahydrothiophene||3-hydroxysulfolane, sulfolane||CYP, FMO1, FMO2, FMO3, FMO4, FMO5||3078288|
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