Received for publication April 23, 2008.
Revised July 17, 2008.
Accepted for publication July 18, 2008.

Apparent high CYP3A5 expression is required for significant metabolism of vincristine by cryopreserved human hepatocytes

Abstract

Vincristine is metabolized to one primary metabolite M1 by cDNA-expressed CYP3A4 and CYP3A5 and by CYP3A enzymes in human liver microsomes. For both systems, CYP3A5 is predicted to mediate approximately 80% of the CYP3A metabolism for individuals with high CYP3A5 expression (at least one CYP3A5*1 allele). In the current study, the role of CYP3A5 was quantified in the metabolism of vincristine with human cryopreserved hepatocytes. The hepatocytes were genotyped for common CYP3A5 allelic variants (CYP3A5*3, CYP3A5*6, and CYP3A5*7) to predict CYP3A5 expression. For each hepatocyte preparation, the rates of vincristine depletion and metabolite formation were quantified. While human hepatocytes with predicted low CYP3A5 expression did not detectably metabolize vincristine, human hepatocytes with predicted high CYP3A5 expression metabolized vincristine to one primary metabolite M1. In paired experiments using cryopreserved hepatocytes from the same donor, vincristine was incubated with intact cells and cell lysates supplemented with NADPH. The rates of M1 formation were 4 to 69-fold higher for the cell lysates as compared to the intact cells. For one representative donor, the intact cells had a 3-fold higher K_m value and a 3-fold lower V_max value for M1 formation as compared to the cell lysates. Thus, the rate of M1 formation in the hepatocytes may be influenced by the rate of vincristine translocation across the plasma membrane. We conclude that genetically determined CYP3A5 expression in human cryopreserved hepatocytes plays a major role in vincristine metabolism.

Key words: CYP3A5, cryopreserved hepatocytes, metabolism, pharmacogenetics, vinca alkaloids, vincristine