To gain a better understanding of the metabolic properties between the open acid and lactone form of HMG-CoA reductase inhibitors (statins), the paper focused primarily on characterizing the metabolic properties of statins. We compared the metabolism of the acid and lactone forms of several statins, including atrovastatin, simvastatin, cerivastatin fluvastatin, pitavastatin and rosuvastatin with respect to metabolic clearance, CYP enzymes involved and drug-drug interactions. A remarkable increase in metabolic clearance was noted for all lactones compared with all acids except for pitavastatin lactone. The metabolic clearances of the atrovastatin, simvastatin, cerivastatin, fluvastatin and rosuvastatin lactones were 73-, 70-, 30-, 7- and 64-fold higher, respectively, than those of the corresponding acids. CYP2Cs were critically involved in the metabolism of cerivastatin, fluvastatin and pitavastatin acids. In contrast, CYP2Cs were not involved in the metabolism of the corresponding lactones and CYP3A4 was mainly involved. Moreover, a substantial difference in the metabolic inhibition of statins was found between acids and lactones. Overall, the study demonstrates that CYP-mediated metabolism of lactones is also a common metabolic pathway for statins and that the CYP3A4-mediated metabolism of the lactone forms clearly will need to be taken into account in assessing mechanistic aspects of drug-drug interaction involving statins.