Aim: The HMG-CoA reductase inhibitor, simvastatin (SV), is known to be extensively metabolised by CYP3A4 in humans. However, clinical studies have suggested a possible involvement of the polymorphic enzyme CYP2D6 in the metabolism of one or more of the active metabolites of the SV. The present in vitro study was conducted to investigate the role of CYP2D6 in SV metabolism, and in particular clarify whether any of the SV metabolites are possible substrates for CYP2D6.
Methods: Two different strategies were used. First, SV was incubated with human liver microsomes (HLM) in absence and presence of potent CYP3A4 or CYP2D6 inhibitors. Then, incubation of SV was performed with microsomes of recombinant CYP3A4 and CYP2D6 (T5-3A4 and T5-2D6). SV was first incubated with T5-3A4 microsomes, then, the remaining SV and metabolites produced by T5-3A4 were subjected to metabolism with T5-2D6 microsomes. SV and metabolites were analysed by a high-performance liquid chromatography-mass spectrometry (HPLC-MS) method, developed in the present work.
Results: Upon incubation of SV with HLM, five possible SV related metabolites were detected. All of the deduced metabolites were found to increase with incubation time. The metabolites together represented four different phase I metabolic modifications of SV, giving rise to the metabolites 6´-CH2OH-SV (I), 3´-OH-SV (II), 6´-exomethylene-SV (III), 3´-OH-6´-CH2OH-SV (IV) and 6´-COOH-SV (V). Inhibition studies and studies with recombinant CYP microsomes showed that CYP3A4 was involved in all these reactions. CYP2D6 was apparently not catalysing any of the reactions, but a clear trend of unspecific accumulation of SV and all measured metabolites in presence of the CYP2D6 inhibitor quinidine indicated that CYP2D6 might be responsible for a metabolic reaction not detected in this work. This was supported by an unspecific decline of SV and the metabolites produced by T5-3A4 following T5-2D6 incubation, which could not be reflected as new chromatographic peaks after T5-D6 incubations.
Conclusion: The present in vitro study suggests that CYP2D6 is involved in metabolism of SV and several phase I metabolites with pharmacological activity (in their acid form). However, the potential in vivo importance of CYP2D6 involvement needs to be evaluated in a study where pharmacokinetic measurements of SV and metabolites are investigated in relation to CYP2D6 phenotype. Attempts should also be made to identify the metabolitesapparently produced via CYP2D6.