The importance of correct assignment of CYP2B6 genetic variants with respect to cyclophosphamide metabolizer status
Authors: Nuala Helsby, Malcolm Tingle
Journal: American Journal of Hematology (2011)

We wish to comment on the assumptions made by Melanson et al. [1] that certain variants of CYP2B6 are associated with poor metabolism of cyclophosphamide. The importance of allelic variants of two cytochrome P450 enzymes, CYP2C19 and CYP2B6, in the bioactivation of cyclophosphamide to the 4-hydroxy metabolite (4OHCP) is becoming increasingly evident. An independent role for either variant CYP2B6 [2, 3] or variant CYP2C19 has been reported [4, 5]. Recently, we have demonstrated that low bioactivation of cyclophosphamide (in vitro and in lupus nephritis patients) is dependent on a combination of both CYP2C19 and CYP2B6 loss of function variants [6]. Melanson et al. [1] categorized the predicted cyclophosphamide metabolizer status (See Supporting Information Table 3 in Ref. [1]) of the patients based on CYP2B6 variants as: ultrarapid (*4); extensive (*5); and poor (*6 and/or *7). This is incorrect due to a number of mistaken assumptions and erroneous citations that has led to misleading analysis of the data. Individuals with CYP2B6 *6 (G516T and A785G) or *7 (G516T, A785G, and C1459T) are categorized by the authors as poor metabolizers. The functional effect of CYP2B6*6 is often contradictory and most importantly appears to be substrate dependent. For example, it is associated with decreased metabolism of bupropion [7, 8] but not cyclophosphamide [2, 3]. Indeed individuals with the G516T SNP have a significantly increased clearance of cyclophosphamide to 4-OHCP [2] and should be classified as extensive metabolizers. The authors have categorized *4 (A785G) as an ultrarapid phenotype; however, the references used to support this claim are erroneous. Xie et al. [2] only measured the G516T SNP and not A785G. This is also the case in another reference where only the G516T SNP was reported [9]. Two other references used to justify the categorization of the *4 variant as ultrarapid phenotype had no pharmacogenetic testing [10, 11],whereas [12] did find an association between carriers of the A785G variant and oral mucositis, no evidence for the effect of this variant on the metabolism of cyclophosphamide was presented. Moreover, the A785G SNP is associated with lower expression of the CYP2B6 protein [13], hence, may actually result in poor metabolizer status, although this has, yet, to be directly confirmed for cyclophosphamide. However, recategorization of patients with the A785G SNP (*4) as poor metabolizers may explain the significantly worse progression-free survival observed in these subjects. CYP2B6 *5 (C1459T) was classified by the authors as extensive metabolizer phenotype. There is no evidence for this. Indeed, we have recently shown that *5 (C1459T) is a loss of function variant for the bioactivation of cyclophosphamide [6]. Consistent with this, a lack of therapeutic response in individuals homozygous for CYP2B6*5 has been reported [14]. In the light of this erroneous categorization of CYP2B6 variants with respect to cyclophosphamide metabolizer phenotype, we urge the authors to reanalyse their data. Since we have recently demonstrated that individuals who have at least one loss of function allele (loss of function variants for cyclophosphamide are CYP2C19*2 and/or CYP2B6*5) at either CYP2C19 or CYP2B6 [6] have significantly decreased bioactivation of cyclophosphamide, we also suggest that the research group also consider reanalysis of their data using this combined genotype. Nuala Helsby*, Malcolm Tingle , * Molecular Medicine and Pathology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand, Pharmacology and Clinical Pharmacology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.