Human CYP2B6 catalyzes metabolism of many clinically important drugs, such as the antitumor alkylating agent cyclophosphamide, which is used in antitumor therapy of malignancies including neuroblastoma. Drug metabolism substantially varies between individuals, in which genetic polymorphisms of CYP enzymes play an important role; however, CYP phenotype may transiently change due to phenoconverting non-genetic factors (e.g. drug therapy, age, gender, co-morbidities). In human liver tissues, we investigated potential association between the hepatic CYP2B6 phenotype and CYP2B6 genetic variants most frequent in Caucasian population, as well as non-genetic factors recorded in donor history. We have developed a two-step genotyping assay for identification of one of the most common CYP2B6 polymorphisms (g.18053A>G). CYP2B6 genetic variability was responsible for variations in S-mephenytoin N- demethylase activity and mRNA expression in approximately one third of the liver tissues, while in further one third of the tissue samples, phenoconverting non-genetic factors (CYP2B6 inhibitors and inducers, amoxicillin+clavulanic acid treatment and chronic alcohol consumption) caused altered phenotype. In conclusion, CYP2B6 genotype is appropriate for phenotype estimation with some limitations in clinical practice. In neuroblastoma patients, cyclophosphamide treatment induced severe hematotoxicity (lymphopenia, thrombocytopenia, and monocytopenia) and mild liver injury were significantly associated with CYP2B6 genotype based phenotype. However, the occurence of kidney and urinary bladder (hemorrhagic cystitis) toxicities was not related to CYP2B6 genotype. The patients’ age and gender, but not the CYP2B6 or CYP2C19 metabolizer phenotypes were associated with the cyclophosphamide treatment outcomes. Furthermore, our results suggest that on the basis of patients’ CYP2B6 genotype, interventions to alleviate cyclophospahmide-induced adverse effects can be prepared during cyclophopshamide treatment. CYP2B6 genotype based phenotype estimation considering phenoconverting factors may contribute to tailored cyclophosphamide therapy and to reduction of the risk of toxic side effects.
