greatest number of bonding interactions with HGMR. Structure-activity relationship (SAR) All statins have the same pharmacophore so the difference in their pharmacodynamic effect is mostly based on the substituents. The activity of each statin is dependent on the binding affinity of the compound for the substrate site and the length of time it binds to the site. Type 2 statins have unique fluorophenyl group that causes additional polar interaction between the enzyme and the statins, which results in a tighter binding to the enzyme. The newest statin, rosuvastatin has a unique polar methane sulfonamide group, which is quite hydrophilic and confers low lipophilicity. The sulfonamide group forms a unique polar interaction with the enzyme. As a result, rosuvastatin has superior binding affinity to the HMGR enzyme compared to the other statins, which is directly related to its efficiency to lower LDL cholesterol. Lipophilicity Lipophilicity of the statins is considered to be quite important since the hepatoselectivity of the statins is related to their lipophilicity. The more lipophilic statins tend to achieve higher levels of exposure in non-hepatic tissues, while the hydrophilic statins tend to be more hepatoselective. The difference in selectivity is because lipophilic statins passively and non-selectively diffuse into both hepatocyte and non-heptatocyte, while the hydrophilic statins rely largely on active transport into hepatocyte to exert their effects. High hepatoselectivity is thought to translate into reduced risk of adverse effects. It has been reported that the organic anion transporting polypeptide (OATP) is important for the hepatic uptake of hydrophilic statins such as rosuvastatin and pravastatin. OATP-C is expressed in liver tissue on the basolateral membrane of hepatocytes and is considered to be a potential contributor for the low IC50 for rosuvastatin in hepatocytes. Of the marketed statins, cerivastatin was the most lipophilic and also had the largest percentage of serious adverse effects due to its ability to inhibit vascular smooth muscle proliferation and as a result was voluntarily removed from the market by the manufacturer. Metabolism All statins are metabolized by the liver, which causes their low systemic bioavailability. Lovastatin and simvastatin are administered in their lactone forms, which is more lipophilic than their free acid forms, and therefore they have to be activated by hydrolysis to the active anionic carboxylate form. Cytochrome P450(CYP) isoenzymes are involved in the oxidative metabolism of the statins, with CYP3A4 and CYP2C9 isoenzymes being the most dominant. CYP3A4 isoenzyme is the most predominant