Cholesterol is an essential component of cell membranes and is the immediate precursor of steroid hormones and bile acids (11
). However, in excessive amounts, cholesterol becomes an important risk factor for cardiovascular disease, as demonstrated in clinical trials from the Framingham Heart Study (3
) and the Multiple Risk Factor Intervention Trial (13
). Although dietary cholesterol can contribute to changes in serum cholesterol levels, more than two thirds of the body’s cholesterol is synthesized in the liver. Therefore, inhibition of hepatic cholesterol biosynthesis has emerged as the target of choice for reducing serum cholesterol levels (15
The rate-limiting enzyme in cholesterol biosynthesis in the liver is HMG-CoA reductase (11
), which catalyzes the conversion of HMG-CoA to mevalonic acid (16
). Inhibitors of HMG-CoA reductase, or statins, were originally identified as secondary metabolites of fungi (17
). HMG-CoA reductase catalyses the rate-limiting step of cholesterol biosynthesis, a four-electron reductive deacylation of HMG-CoA to CoA and mevalonate. One of the first natural inhibitors of HMG-CoA reductase was mevastatin (compactin, ML-236B), which was isolated from Penicillium citrinium
by A. Endo et al. in 1976 (18
). In its active form, mevastatin resembles the cholesterol precursor, HMG-CoA. When mevastatin was initially administered to rats, it inhibited cholesterol biosynthesis with a Ki
of 1.4 nM. Unfortunately, it also caused unacceptable hepatocellular toxicity and further clinical development was discontinued. Subsequently, a more active fungal metabolite, mevinolin or lovastatin, was isolated from Aspergillus terreus
by Hoffman and colleagues in 1979 (19
). Lovastatin differs from mevastatin in having a substituted methyl group. Compared to mevastatin, lovastatin was a more potent inhibitor of HMG-CoA reductase, with a Ki
of 0.6 nM, but did not cause hepatocellular toxicity when given to rats. Lovastatin, therefore, became the first of this class of cholesterol-lowering agents to be approved for clinical use in humans. Since then, several new statins, both natural and chemically modified, have become commercially available, including pravastatin, simvastatin, fluvastatin, atorvastatin, cerivastatin, and most recently, pitavastatin and rosuvastatin (21
). Indeed, statins have emerged as one of the most effective class of agents for reducing serum cholesterol levels.
Statins work by reversibly inhibiting HMG-CoA reductase through side chains that bind to the enzyme’s active site and block the substrate-product transition state of the enzyme (22
). Thus, all statins share an HMG-like moiety and inhibit the reductase by similar mechanism (). Recently, the structure of the catalytic portion of human HMG-CoA reductase complexed with different statins was determined (22
). The bulky, hydrophobic compounds of statins occupy the HMG-binding pocket and block access of the substrate HMG. The tight binding of statins is due to the large number of van der Waals interactions between statins and HMG-CoA reductase. The structurally diverse, rigid, hydrophobic groups of the different statins are accommodated in a shallow nonpolar groove that is present only when COOH-terminal residues of HMG-CoA reductase are disordered. There are subtle differences in the modes of binding between the various statins, with the synthetic compounds atorvastatin and rosuvastatin having the greatest number of bonding interactions with HMG-CoA reductase (22
). Statins bind to mammalian HMG-CoA reductase at nanomolar concentrations, leading to effective displacement of the natural substrate, HMG-CoA, which binds at micromolar concentrations (23
Figure 1 Structural basis of HMG-CoA reductase inhibition by statins. The active forms of statins resemble the cholesterol precursor, HMG-CoA (right panels). All statins share the HMG-like moiety and competitively inhibit the reductase by the similar mechanism (more ...)
Oral administration of statins to rodents and dogs showed that these drugs are predominantly extracted by the liver and resulted in > 30%–50% reduction in plasma total cholesterol levels and substantial decrease in urinary and plasma levels of mevalonic acid, the end product of the HMG-CoA reductase reaction. Similar reduction in cholesterol synthesis and decrease in circulating total and low-density lipoprotein (LDL)-containing cholesterol (LDL-C) by these agents have been subsequently confirmed in humans. Because hepatic LDL-C receptors are the major mechanism of LDL-C clearance from the circulation, the substantial declines in serum cholesterol levels are accompanied by an increase in hepatic LDL-C receptor activity. Statins, therefore, effectively reduce serum cholesterol levels by two separate mechanisms. They not only inhibit endogenous cholesterol biosynthesis via HMG-CoA reductase inhibition but also increase cholesterol clearance from the bloodstream via increases in LDL-C receptor.
The rank order of potency for HMG-CoA reductase inhibition among the second-generation statins is simvastatin > pravastatin > lovastatin
mevastatin, with tissue IC50
values of simvastatin and mevastatin being approximately 4 nM and 20 nM, respectively (24
). The IC50
values for these statins correspond to their relative potency for lowering serum cholesterol levels in vivo (i.e., simvastatin > lovastatin) (25
). The newer third-generation synthetic statins, which include fluvastatin, cerivastatin, the penta-substituted pyrrole atorvastatin, pitavastatin (NK-104), and rosuvastatin, are much more potent than the mevastatin derivatives. These newer statins are active compounds, which share some physico-chemical properties with pravastatin, but have greater lipophilicity and half-life (26
). Consequently, these statins, especially atorvastatin, pitavastatin, and rosuvastatin, appear to be quite effective in lowering serum cholesterol levels, perhaps, in part, owing to their ability to bind hepatic HMG-CoA reductase at higher affinity and inhibit the enzyme for a longer duration.
Because statins differ in their tissue permeability and metabolism, they possess different potencies for extrahepatic HMG-CoA reductase inhibition. These differences in tissue permeability and metabolism may account for some of the observed differences in their peripheral side effects (27
). Lipophilic statins, such as simvastatin, are considered more likely to enter endothelial cells by passive diffusion than hydrophilic statins, such as pravastatin and rosuvastatin, which are primarily targeted to the liver. However, lipophilicity does not entirely predict the ability of statins to exert extrahepatic effects in animal and human studies, and so other unidentified factors may play a role. It may be that there are specific mechanisms for hydrophilic statins to enter extrahapetic cells, such as endothelial cells. Such a mechanism is present in the liver, where the organic anion transporter (OATP-C) enables hydrophilic statins to enter hepatocytes (28
Until recently, all cholesterol-independent or “pleiotropic” effects of statins were believed to be mediated by inhibition of mevalonate synthesis. However, statins can reportedly bind to a novel allosteric site within the β2 integrin function-associated antigen-1 (LFA-1), independent of mevalonate production (29
). LFA-1 belongs to the integrin family and plays an important role in leukocyte trafficking and in T cell activation. Random screening of chemical libraries identified the HMG-CoA reductase inhibitor, lovastatin, as an inhibitor of the LFA-1/intercellular adhesion molecule (ICAM)-1 interaction.