Atherosclerosis is the abnormal accumulation of lipids and products
resulting from an inflammatory response in the walls of arteries,
and is the leading cause of death in the Western world. Heart attacks,
angina pectoris, peripheral arterial disease, and strokes are common
sequelae of atherosclerosis. In some cases, lowering serum lipid
concentrations has been shown to prevent the sequelae of atherosclerosis
and decrease mortality in patients with a history of cardiovascular
disease and hyperlipidemia. The five drug classes discussed in this
chapter (Figure 26–1) are used to decrease serum concentrations
of lipids in the blood (hyperlipidemia) and to prevent or reverse associated
atherosclerosis, or, in the case of hypertriglyceridemia, prevent
pancreatitis. Although the drugs are generally safe and effective,
adverse effects include drug–drug interactions and rare
toxic reactions in skeletal muscle and the liver.
The five classes of lipid-lowering drugs. These classes
are based on the mechanisms of action of these drugs.
Lipids, mainly cholesterol and triglycerides, are transported
in human plasma by macromolecular complexes termed lipoproteins.
Lipoproteins are composed of a lipid core surrounded by apolipoproteins
that regulate the uptake and off-loading of lipids and interactions
with cell membrane receptors. The lipoproteins that are primarily
responsible for delivering cholesterol and triglycerides to peripheral
tissues originate in the liver and contain a key apoprotein called
B-100. These B-100–containing lipoproteins include very low-density lipoprotein (VLDL),low-density lipoprotein (LDL), and intermediate-density lipoprotein (IDL) (Figure
26–2). The uptake by cells of B-100–containing
lipoproteins can occur by receptor-mediated endocytosis or by scavenger
receptors. Receptor-mediated uptake is a carefully regulated process
that protects cells from being overloaded with lipids. In contrast,
uptake by scavenger receptors is an unregulated process that can
overwhelm the ability of a cell to sequester potentially toxic lipids
safely. Macrophages in arterial walls use scavenger receptors to
take up circulating lipoproteins, especially particles with apolipoproteins
that have been modified by free radicals. When these macrophages
become overloaded with lipids, they are transformed into distressed
foam cells that initiate a local inflammatory response. Engorged
foam cells, foam cells that have burst, and the products of the
inflammatory responses form the core of an atherosclerotic plaque.
Whereas plaques can slowly occlude coronary and cerebral vessels,
clinical symptoms are more frequently precipitated by rupture of
unstable plaques, leading to occlusive thrombi.
Schematic diagram of lipoprotein handling by hepatocytes.
The sites of action of several antihyperlipidemic drugs are shown.
LDL receptors (R) are increased by treatment with resins and HMG-CoA
reductase inhibitors. For identification of abbreviations of the
lipoproteins, some drug classes, and additional discussion, see
Another lipoprotein, high-density lipoprotein
(HDL), exerts several antiatherogenic effects. HDL participates
in pathways that retrieve cholesterol from the artery wall and inhibit
the oxidation of atherogenic lipoproteins. Low levels of HDL are ...