Role of polymorphic human CYP2B6 in cyclophosphamide bioactivation

Area of Science:

• Pharmacogenetics
• Drug Metabolism
• Enzyme Kinetics

Background:

• Cyclophosphamide (CPA) is a vital chemotherapeutic agent.
• Its efficacy relies on bioactivation by cytochrome P450 enzymes.
• CYP2B6 is a key enzyme implicated in CPA metabolism.

Purpose of the Study:

• To investigate the role of polymorphic CYP2B6 in cyclophosphamide (CPA) bioactivation.
• To determine the impact of specific CYP2B6 variants, particularly CYP2B6*6, on CPA metabolism.
• To quantify CPA 4-hydroxylation activity in relation to CYP2B6 expression levels.

Main Methods:

• Genotyping of CYP2B6*5 and CYP2B6*6 alleles in 67 human liver specimens.
• Quantification of CYP2B6 apoprotein levels via immunoblotting in 55 preparations.
• Measurement of 4-hydroxy-CPA and hydroxy-bupropion using HPLC and LC-MS.
• Assay of CYP2B6-specific enzyme activity (7-ethoxy-4-trifluoromethyl coumarin O-deethylase).

Main Results:

• CYP2B6*5 and CYP2B6*6 allele frequencies were 9.0% and 16.4%.
• CYP2B6 protein was detected in 80% of samples, with significant variability.
• A strong correlation (r=0.81) was observed between CYP2B6 apoprotein levels and CPA 4-hydroxylation.
• CYP2B6*6 carriers exhibited significantly higher CPA 4-hydroxylation rates.
• Recombinant CYP2B6 demonstrated high Vmax and Km for CPA 4-hydroxylation.

Conclusions:

• Polymorphic CYP2B6 is a major enzyme responsible for cyclophosphamide (CPA) bioactivation.
• The CYP2B6*6 variant significantly enhances CPA 4-hydroxylation, influencing drug response.
• Understanding CYP2B6 polymorphisms is critical for optimizing CPA therapy.