**8. Cholesterol-lowering drugs**

Mainly cholesterol lowering drugs have been developed and used clinically, which includes 1) HMG-CoA reductase inhibitors, e.g., atorvastatin, fluvastatin, lovastatin, pravastatin, rosuvastatin and simvastatin, 2) bile acid sequestrants — colesevelam, cholestyramine and colestipol — and nicotinic acid (niacin), and 3) cholesterol absorption inhibitor - ezetimibe. Other available drugs are gemfibrozil, fenofibrate and clofibrate which are fibric acid deriva‐ tives primarily used for lowering high triglyceride levels

#### **8.1. Statins – Benefits versus side effects**

Statins or inhibitors of 3-hydroxy-3-methylglutaryl–coenzyme A (HMG-CoA) reductase are widely used to reduce the risk of cardiovascular events and death. Statins includes lovastatin, which is a fungal metabolite, and many synthetic derivatives, pravastatin, atorvastatin and simvastatin etc. Statins target predominantly hepatocytes and inhibit HMG-CoA reductase, the enzyme that converts HMG-CoA into mevalonic acid, a cholesterol precursor. Statins alter the conformation of the enzyme when they bind to its active site. This prevents HMG-CoA reductase from attaining a functional structure. The change in conformation at the active site makes these drugs very effective and specific [53]. Binding of statins to HMG-CoA reductase is reversible, and their affinity for the enzyme is in the nanomolar range, as compared to the natural substrate, which has micromolar affinity [54]. The inhibition of HMG-CoA reductase determines the reduction of intracellular cholesterol, inducing the activation of a protease which slices SREBPs from the endoplasmic reticulum. SREBPs are translocated into nucleus, where they increase the gene expression for LDL receptor. The reduction of cholesterol in hepatocytes leads to the increase of hepatic LDL receptors that leads to the reduction of circulating LDL and of its precursors (intermediate density - IDL and very low density- VLDL lipoproteins). All statins reduce LDL cholesterol non-linearly, dose-dependent, and after administration of a single daily dose [53]. Statins inhibit hepatic synthesis of apolipoprotein B-100, determining a reduction of the synthesis, secretion of triacylglycerols-rich lipoproteins, and an increase of receptors for apolipoproteins B/E. Statins have a modest effect on HDL increase, and no influence on lipoprotein(s) concentration. Statins can also prevent LDL oxidation by preserving the activity of the endogenous antioxidant system, like superoxide dismutase [53].

In general, the currently used statins are well tolerated and have a good safety profile [55]. Statins acting as inhibitor of HMG-CoA reductase, inhibits all the pathway in which mevalo‐ nate is a precursor such as synthesis of nonsteroid isoprenoids such as farnesylpyrophosphate (FPP), geranylgeranylpyrophosphate (GGPP), dolichol and side chain of coenzyme Q which play an essential role in cellular physiology. Coenzyme Q is a component of mitochondrial respiratory chain and a lipid-soluble antioxidant. Farnesyl- and geranylgeranyl groups are needed for protein isoprenylation and formation of small GTP-binding proteins including Ras, Rho and Rab, which are involved in signal transduction pathways. Inhibition of isoprenoid synthesis by statins decreases the activity of these proteins and modifies the respective signalling pathways; the mechanism responsible for cholesterol-independent pleiotropic effects of statins. The best recognized and most commonly reported adverse effect of statins are muscle adverse effect with muscle pain, fatigue and weakness as well as rhabdomyolysis. Rhabdomyolysis is the most severe form of statin induced myopathy, characterized by marked increase in CK activity, myoglobinemia, myoglobinuria, and myoglobin induced acute renal failure. These symptoms arising on statins are shown to be reversed with discontinuation. Coenzyme Q10 deficiency produces mitochondrial encephalomyopathy, resulting in fatigue, muscle symptoms, and cognitive problems. Gastrointestinal and neurological symptoms, psychiatric symptoms, sleep problems, glucose elevations, and a range of other symptoms are also reported on statins [56].

#### **8.2. Ezetimibe – Benefits and side effects**

Ezetimibe is the first of a new class of highly selective cholesterol absorption inhibitors. It does not inhibit cholesterol synthesis in the liver or increase bile acid excretion. It belongs to a class of lipid-lowering compounds that selectively inhibits the intestinal absorption of cholesterol and sterols. Ezetimibe's pharmacological effect is complementary to that of the statins [57]. Mechanism of action of ezetimibe involves inhibiting the absorption of cholesterol in the small intestine. Through a mechanism that is not yet fully elucidated, ezetimibe appears to block a protein transporter called Niemann-Pick C1-like 1 protein (NPC1L1) that is located at the apical membrane of the small intestine enterocytes [58]. Unlike other cholesterol-lowering agents, ezetimibe localizes and appears to act at the brush border of the small intestine and inhibits the absorption of dietary and biliary cholesterol in the small intestine, leading to a decrease in the delivery of intestinal cholesterol to the liver. This leads to a reduction of hepatic cholesterol stores and an increase in clearance of cholesterol from the blood. Ezetimibe has been demon‐ strated to have no significant effect on the plasma concentrations of the fat-soluble vitamins A, D, and E [57]. No published reports could be identified that assessed the potential impact of ezetimibe therapy on other traditional CVD risk factors, including blood pressure and obesity. Recent reports have raised concerns about an association between ezetimibe and an increased incidence of cancer [58]. Ezetimibe may rarely cause hepatotoxicity, severe choles‐ tatic hepatitis, or acute autoimmune hepatitis [59].

Ezetimibe is proved to be more effective in combination with statin drugs in lowering LDL-c than monotherapy. Ezetimibe has an additive and at times synergistic effect on the reduction of LDL-C and total cholesterol (TC) concentrations when combined with statin therapy. Ezetimibe has not been associated with increased rates of myopathy or rhabdomyolysis, whether used alone or in combination with statins, although there have been some case reports of myopathy attributed to this agent. Moreover, ezetimibe has been associated with mild elevations of liver transaminases, mainly in combination with a statin. Other side effects are extremely rare. It should be noted, however, there are no long-term safety data or outcome studies for ezetimibe yet [60].

#### **8.3. Niacin (Nicotinic acid)**

the enzyme that converts HMG-CoA into mevalonic acid, a cholesterol precursor. Statins alter the conformation of the enzyme when they bind to its active site. This prevents HMG-CoA reductase from attaining a functional structure. The change in conformation at the active site makes these drugs very effective and specific [53]. Binding of statins to HMG-CoA reductase is reversible, and their affinity for the enzyme is in the nanomolar range, as compared to the natural substrate, which has micromolar affinity [54]. The inhibition of HMG-CoA reductase determines the reduction of intracellular cholesterol, inducing the activation of a protease which slices SREBPs from the endoplasmic reticulum. SREBPs are translocated into nucleus, where they increase the gene expression for LDL receptor. The reduction of cholesterol in hepatocytes leads to the increase of hepatic LDL receptors that leads to the reduction of circulating LDL and of its precursors (intermediate density - IDL and very low density- VLDL lipoproteins). All statins reduce LDL cholesterol non-linearly, dose-dependent, and after administration of a single daily dose [53]. Statins inhibit hepatic synthesis of apolipoprotein B-100, determining a reduction of the synthesis, secretion of triacylglycerols-rich lipoproteins, and an increase of receptors for apolipoproteins B/E. Statins have a modest effect on HDL increase, and no influence on lipoprotein(s) concentration. Statins can also prevent LDL oxidation by preserving the activity of the endogenous antioxidant system, like superoxide

194 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

In general, the currently used statins are well tolerated and have a good safety profile [55]. Statins acting as inhibitor of HMG-CoA reductase, inhibits all the pathway in which mevalo‐ nate is a precursor such as synthesis of nonsteroid isoprenoids such as farnesylpyrophosphate (FPP), geranylgeranylpyrophosphate (GGPP), dolichol and side chain of coenzyme Q which play an essential role in cellular physiology. Coenzyme Q is a component of mitochondrial respiratory chain and a lipid-soluble antioxidant. Farnesyl- and geranylgeranyl groups are needed for protein isoprenylation and formation of small GTP-binding proteins including Ras, Rho and Rab, which are involved in signal transduction pathways. Inhibition of isoprenoid synthesis by statins decreases the activity of these proteins and modifies the respective signalling pathways; the mechanism responsible for cholesterol-independent pleiotropic effects of statins. The best recognized and most commonly reported adverse effect of statins are muscle adverse effect with muscle pain, fatigue and weakness as well as rhabdomyolysis. Rhabdomyolysis is the most severe form of statin induced myopathy, characterized by marked increase in CK activity, myoglobinemia, myoglobinuria, and myoglobin induced acute renal failure. These symptoms arising on statins are shown to be reversed with discontinuation. Coenzyme Q10 deficiency produces mitochondrial encephalomyopathy, resulting in fatigue, muscle symptoms, and cognitive problems. Gastrointestinal and neurological symptoms, psychiatric symptoms, sleep problems, glucose elevations, and a range of other symptoms are

Ezetimibe is the first of a new class of highly selective cholesterol absorption inhibitors. It does not inhibit cholesterol synthesis in the liver or increase bile acid excretion. It belongs to a class of lipid-lowering compounds that selectively inhibits the intestinal absorption of cholesterol

dismutase [53].

also reported on statins [56].

**8.2. Ezetimibe – Benefits and side effects**

Niacin is the most potent HDL increasing drug currently available and it also can efficaciously lowers triglycerides and LDL cholesterol. It is the only lipid-lowering drug that considerably lowers lipoprotein(a). Beneficial effect of niacin to reduce triglycerides and apolipoprotein-B containing lipoproteins (e.g., VLDL and LDL) are mainly through: a) decreasing fatty acid mobilization from adipose tissue triglyceride stores, and b) inhibiting hepatocyte diacylgly‐ cerol acyltransferase and triglyceride synthesis leading to increased intracellular apo B degradation and subsequent decreased secretion of VLDL and LDL particles. Niacin raises HDL by decreasing the fractional catabolic rate of HDL-apoAI without affecting the synthetic rates. Additionally, niacin selectively increases the plasma levels of lipoprotein-AI (HDL subfraction without apoAII), a cardioprotective subfraction of HDL in patients with low HDL. Recent studies indicate that niacin selectively inhibits the uptake/removal of HDL-apoAI (but not HDL-cholesterol ester) by hepatocytes, thereby increasing the capacity of retained HDLapoAI to augment cholesterol efflux through reverse cholesterol transport pathway [61]. Niacin treatment is associated with a number of side effects, including headache, itching and gastrointestinal disturbances, but these are generally mild. The most severe of the side effects is flushing, and this is sufficiently severe to negatively affect compliance [62].

#### **8.4. Bile acid sequestrant**

Bile acid sequestrant or bile binding anion (Chloride) exchange resins that is effective in reducing total cholesterol and LDL cholesterol levels. The primary and direct action of the bile acid sequestrants is to bind to bile acids in the gut and thus interrupt the enterohepatic recirculation of bile acids [63]. Three key enzyme are affected by bile acid sequestrant, which are phosphatidic acid phosphatase, cholesterol 7-alpha-hydroxylase and HMG-CoA reduc‐ tase. Activation of phosphatidic acid phosphatase promotes hepatic triglyceride synthesis, induces secretion of triglyceride-rich VLDL particles and consequently increases plasma triglyceride levels. The activation of hepatic cholesterol 7-alpha-hydroxylase promotes the conversion of intracellular cholesterol to bile acids. The decrease in intracellular cholesterol stores, in turn, increases LDL receptor expression on hepatocyte membranes and consequently, increases receptor-mediated fractional catabolism of LDL or LDL uptake by liver cells. Reduction of intracellular cholesterol may also increase the synthesis of cholesterol through activation of HMG-CoA reductase. The potential loss of the bile acid sequestrant's cholesterollowering efficacy can be overcome by adding HMG-CoA reductase inhibitor (statins). Finally, bile acid sequestrants promote apoprotein AI synthesis and tend to raise high-HDL cholesterol levels, primarily by increasing plasma HDL-2 concentrations. Three drugs in this class are synthetic cholestyramine, colestipol, colesevelam. The side effect profile of the bile acid sequestrants is tolerable, with most complaints related to effects on the gastrointestinal tract and the bulkiness of resins [64].

#### **8.5. Fibrates**

Fibrates are primarily effective for the treatment of hypertriglyceridemia or mixed hyper‐ lipidemia by stimulating the peroxisomal β-oxidation pathway. Their main action is to lower plasma triglyceride levels, but they also reduce total and LDL cholesterol concen‐ trations and induce a moderate increase in HDL cholesterol. Fibrates act by stimulating the activity of peroxisome proliferator-activated receptor (PPAR)-α, a member of the PPAR subfamily of nuclear receptors [65]. It controls the transcription of regulatory genes of fatty acids and cholesterol metabolism. It inhibits the synthesis and secretion of triglycerides by the liver and a stimulation of the degradation of triglyceride-rich lipopro‐ teins. This increased clearance of triglycerides results from a stimulation of the expres‐ sion of lipoprotein lipase and a decreased expression and concentration of apolipoprotein-CIII, an inhibitor of lipoprotein lipase activity. PPAR-α activation modifies the expression of several key genes controlling HDL cholesterol metabolism and reverse transport of cholesterol [65]. Several fibrate drugs such as ciprofibrate, bezafibrate, fenofi‐ brate, and gemfibrozil has revolutionized lipid-lowering but research has shown the pro‐ longed use of some of these drugs like clofibrate and ciprofibrate causes peroxisome proliferation leading to hepatomegaly and tumor formation in the liver of rodents [66].

A recent report demonstrated that between the periods 1988-1994 and 1999-2002, mean total cholesterol and mean LDL cholesterol declined in American adults. Coincidently, during this time there also was an increase in the percentage of adults receiving lipid-lowering medica‐ tions. However, among adults not receiving lipid-lowering medications, trends in lipids were similar to those reported for adults overall. Among obese adults, mean total cholesterol, non– HDL cholesterol, LDL cholesterol, and geometric mean triglycerides declined between 1988 and 2010[67]. These data suggest that in addition to the increased use of lipid-lowering medications, something else must have been involved in the overall reduction of blood total cholesterol and LDL cholesterol. Although those factors, the application of natural lipidlowering products plays a critical role and is thus described below.
