Lipid-Lowering Drugs. What are lipoproteins? Lipoproteins are protein-lipid complexes.
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Transcript of Lipid-Lowering Drugs. What are lipoproteins? Lipoproteins are protein-lipid complexes.
Lipid-Lowering Drugs
What are lipoproteins?• Lipoproteins are protein-lipid complexes.
The Players – Lipids
Triacylglycerol
Phospholipids
CholesterolCholesteryl esters
The Players - Apolipoproteins
• Apo AI (liver, small intestine)– Structural; activator of lecithin:cholesterol acyltransferase (LCAT)
• Apo AII (liver)– Structural; inhibitor of hepatic lipase; component of ligand for HDL
binding
• Apo A-IV (small intestine)– Activator of LCAT; modulator of lipoprotein lipase (LPL)
• Apo A-V (liver)– Direct functional role is unknown; regulates TG levels.
Apolipoproteins
• Apo B-100 (liver)– Structural; synthesis of VLDL; ligand for LDL-receptor
• Apo B-48 (small intestine)– Structural; synthesis of chylomicrons; derived from apo B-
100 mRNA following specific mRNA editing
• Apo E (liver, macrophages, brain)– Ligand for apoE receptor; mobilization of cellular
cholesterol
Apolipoproteins
• Apo C-I (liver)– Activator of LCAT, inhibitor of hepatic TGRL uptake
• Apo C-II (liver)– Activator of LPL, inhibitor of hepatic TGRL uptake
• Apo C-III (liver)– Inhibitor of LPL, inhibitor of hepatic TGRL uptake
Amphipathic HelicesLipoprotein Surface
Lipoprotein Classes
Doi H et al. Circulation 2000;102:670-676; Colome C et al. Atherosclerosis 2000;149:295-302; Cockerill GW et al. Arterioscler Thromb Vasc Biol 1995;15:1987-1994.
HDLLDLChylomicrons,VLDL, and their catabolic remnants
> 30 nm 20–22 nm 9–15 nm
D<1.006 g/ml D=1.019-1.063g/ml D=1.063-1.21 g/ml
Lipids Online
Lipoprotein Metabolism
• Exogenous/chylomicron pathway (dietary fat)• Endogenous pathway (lipids synthesized by
the liver)• HDL metabolism (apolipoprotein transfer,
cholesteryl ester transfer, reverse cholesterol transport
Lipoprotein Metabolism
• Exogenous/chylomicron pathway (dietary fat)• Endogenous pathway (lipids synthesized by
the liver)• HDL metabolism (apolipoprotein transfer,
cholesteryl ester transfer, reverse cholesterol transport
Surface Monolayer Phospholipids (12%)Free Cholesterol (14%)Protein (4%)
Hydrophobic CoreTriglyceride (65%) Cholesteryl Esters (8%)
TG Rich: VLDL
Cholesterol and Atherosclerosis, Grundy)
VLDL Metabolism
Cholesterol and Atherosclerosis, Grundy)
Apo C’s and apoE and cholesteryl ester are acquired from HDL in circulation
Fatty Acid Transport
Cholesterol and Atherosclerosis, Grundy)
ApoC-II activates lipoprotein lipase which catalyses the hydrolysis of TG
VLDL Conversion to
LDL
Cholesterol and Atherosclerosis, Grundy)
Further action on IDL by hepatic lipase loses additional apolipoproteins (apoE) becomes and is converted to LDL
Surface Monolayer Phospholipids (25%)Free Cholesterol (15%)Protein (22%)
Hydrophobic CoreTriglyceride (5%) Cholesteryl Esters (35%)
CE Rich: LDL
Cholesterol and Atherosclerosis, Grundy)
LDL Metabolism
Cholesterol and Atherosclerosis, Grundy)
Hepatic LipaseCholesteryl ester transfer protein
LDL is removed by apoB100 receptors which are mainly expressed in the liver
LDL Uptake by Tissues
Cholesterol and Atherosclerosis, Grundy)
Defects in the LDL receptor leads to familial hypercholesterolemia
X X
Surface Monolayer Phospholipids (25%)Free Cholesterol (7%)Protein (45%)
Hydrophobic CoreTriglyceride (5%) Cholesteryl Esters (18%)
CE Rich: HDL
Cholesterol and Atherosclerosis, Grundy)
HDL MetabolismNascent HDL (lipid-poor apoA-I) is produced by the liver and intestine
Hepatic Cholesterol Metabolism
Hepatic Cholesterol Synthesis
Cholesterol and Atherosaclerosis, Grundy)
Rate LimitingOnly pathway for cholesterol degradation
Energetically expensive; prefer to conserve what is already made/acquired – LDL receptor pathway
LDL Cellular Metabolism
Cholesterol and Atherosaclerosis, Grundy)
LDL are taken up by the LDL Receptor into clathrin-coated pits
Endothelial Dysfunction
• Increased endothelial permeability to lipoproteins and plasma constituents mediated by NO, PDGF, AG-II, endothelin.
• Up-regulation of leukocyte adhesion molecules (L-selectin, integrins, etc).
• Up-regulation of endothelial adhesion molecules (E-selectin, P-selectin, ICAM-1, VCAM-1).
• Migration of leukocytes into artery wall mediated by oxLDL, MCP-1, IL-8, PDGF, M-CSF.
Ross, NEJM; 1999
Formation of Fatty Streak
• SMC migration stimulated by PDGF, FGF-2, TGF-B
• T-Cell activation mediated by TNF-a, IL-2, GM-CSF.
• Foam-cell formation mediated by oxLDL, TNF-a, IL-1,and M-CSF.
• Platelet adherence and aggregation stimulated by integrins, P-selectin, fibrin, TXA2, and TF.
Ross, NEJM; 1999
Formation of Advanced, Complicated Lesion
• Fibrous cap forms in response to injury to wall off lesion from lumen.
• Fibrous cap covers a mixture of leukocytes, lipid and debris which may form a necrotic core.
• Lesions expand at shoulders by means of continued leukocyte adhesion and entry.
• Necrotic core results from apoptosis and necrosis, increased proteolytic activity and lipid accumulation.
Ross, NEJM; 1999
Development of Unstable Fibrous Plaque
• Rupture or ulceration of fibrous cap rapidly leads to thrombosis.
• Occurs primarily at sites of thinning of the fibrous cap.
• Thinning is a result of continuing influx of and activation of macrophages which release metalloproteinases and other proteolytic enzymes.
• These enzymes degrade the matrix which can lead to hemorrhage and thrombus formation
Ross, NEJM; 1999
Role of LDL in Atherosclerosis
Steinberg D et al. N Engl J Med 1989;320:915-924.
Endothelium
Vessel LumenLDL
LDL Readily Enter the Artery Wall Where They May be Modified
LDL
Intima
Modified LDL
Modified LDL are Proinflammatory
Hydrolysis of Phosphatidylcholineto Lysophosphatidylcholine
Other Chemical Modifications
Oxidation of Lipidsand ApoB
Aggregation
Lipids Online
Role of LDL in Atherosclerosis
Endothelium
Vessel LumenMonocyte
Macrophage
MCP-1AdhesionMolecules
Foam Cell
IntimaModifiedRemnantsCytokines
Cell ProliferationMatrix Degradation
Doi H et al. Circulation 2000;102:670-676.
Growth FactorsMetalloproteinases
Remnant Lipoproteins
Remnants
Lipids Online
HDL Prevent Foam Cell Formation
LDL
LDL
Miyazaki A et al. Biochim Biophys Acta 1992;1126:73-80.
Endothelium
Vessel LumenMonocyte
Modified LDL
Macrophage
MCP-1AdhesionMolecules
Cytokines
IntimaHDL Promote Cholesterol Efflux
Foam Cell
Lipids Online
Atherosclerosis and lipoprotein metabolism
Atheromatous disease is ubiquitous and underlies the commonest causes of death (e.g. myocardial infarction) and disability (e.g. stroke) in industrialcountriesHypertension and dyslipidemia are ones of the most important risk factors, amenable to drug therapy
ATHEROMA is a focal disease of the intima of large and medium-sized arteries A t h e r o g e n e s i s involves several stages:- endothelial dysfunction with altered PGI2 and NO synthesis- monocyte attachment- endothelial cells bind LDL- oxidatively modified LDL is taken up by macrophages- having taken up oxidised LDL, these macrophages (now foam cells)
migrate subendothelially- atheromatous plaque formation- rupture of the plaque
Atherosclerosis and lipoprotein metabolism
LIPIDS, including CHOLESTEROL (CHO) and TRIGLYCERIDES (TG), are transported in the plasma as lipoproteins, of which there are four classes:
- chylomicrons transport TG and CHO from the GIT to the tissues, where
they are split by lipase, releasing free fatty acids.There are taken up in muscle and adipose tissue. Chylomicron remnants are taken up in the liver
- very low density lipoproteins (VLDL), which transport CHO and newly synthetised TG to the tissues, where TGs are removed as before, leaving:
- low density lipoproteins (LDL) with a large component of CHO, some of which is taken up by the tissues and some by the liver, by endocytosis via specific
LDL receptors
- high density lipoproteins (HDL).which absorb CHO derived from cell breakdown in tissues and transfer it to VLDL and LDL
Atherosclerosis and lipoprotein metabolism
There are two different pathways for exogenous and endogenous lipids:
THE EXOGENOUS PATHWAY: CHO + TG absorbed from the GIT are transported in the lymph and than in the plasma as CHYLOMICRONS to capillaries in muscle and adipose tissues. Here the core TRIGL are hydrolysed by lipoprotein lipase, and the tissues take up the resulting FREE FATTY ACIDS
CHO is liberated within the liver cells and may be stored, oxidised to bile aids or secreted in the bile unaltered
Alternatively it may enter the endogenous pathway of lipid transpor in VLDL
Atherosclerosis and lipoprotein metabolism
EXOGENOUS PATHWAY
Atherosclerosis and lipoprotein metabolism
THE ENDOGENOUS PATHWAY
CHO and newly synthetised TG are transported from the liver as VLDL to muscle and adipose tissue, there TG are hydrolysed and the resulting
FATTY ACIDS enter the tissues
The lipoprotein particles become smaller and ultimetaly become LDL ,
which provides the source of CHO for incorporation into cell membranes, for synthesis of steroids, and bile acids
Cells take up LDL by endocytosis via LDL receptors that recognise LDL apolipoproteins
CHO can return to plasma from the tissues in HDL particles and the resulting cholesteryl esters are subsequently transferred to VLDL or LDL
One species of LDL – lipoprotein - is associated with atherosclerosis (localised in atherosclerotic lesions). LDL can also activate platelets, constituting a further thrombogenic effect
DyslipidemiaThe normal range of plasma total CHO concentration < 6.5 mmol/L.
There are smooth gradations of increased risk with
elevated LDL CHO conc, and with reduced HDL CHO conc.
Dyslipidemia can be primary or secondary.
The primary forms are genetically determined
Secondary forms are a consequence of other conditions
such as diabetes mellitus, alcoholism, nephrotic sy,
chronic renal failure, administration of drug…
Lipid-lowering drugs• Several drugs are used to decrease plasma
LDL-CHO• Drug therapy to lower plasma lipids is only
one approach to treatment • and is used in addition to dietary
management and correction of other modifiable
cardiovascular risk factors
LIPID-LOWERING DRUGS: StatinsHMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase
inhibitors. The reductase catalyses the conversion of HMG-CoA to mevalonic acid; blocks the synthesis of CHO in the liver:
Simvastatin + pravastatin + atorvastatin
decrease hepatic CHO synthesis: lowers total and LDL
increase in synthesis of CHO receptors + increased clearance of LDL
Stimulates the exprssion of more enzyme restores CHO synthesis to normal.Several studies demonstrated positive effects on morbidity and mortality.Reltatively few side-effects...However, adverse effects: myopathy (incr in pts given combined therapy with nicotinic acid or fibrates. Should not be given during pregnancy.
LIPID-LOWERING DRUGS Statins
Promising pharmacodynamic actions: improved endothelial function reduced vascular inflammation and platelet aggregability antithrombotic action stabilisation of atherosclerotic plaques increased neovascularisation of ischaemic tissue enhanced fibrinolysis immune suppression osteoclast apoptosis and increased synthetic activity in osteoblasts
LIPID-LOWERING DRUGStatins
Pharmacokinetics- well absorbed when given orally- extracted by the liver (target tissue), undergo
extensive presystemic biotransformation
Simvastatin is an inactive pro-drug
LIPID-LOWERING DRUGStatins
C l i n i c a l u s e s• Secondary prevention of myocardial infarction and
stroke in patients who have symptomatic atherosclerotic disease (angina, transient ischemic attacks) following acute myocardial infarction or stroke
• Primary prevention of arterial disease in patients who are at high risk because of elevated serum CHO concentration, especially it there are other risk factors for atherosclerosis
• Atorvastatin lowers serum CHO in patients with homozygous familiar hypercholesterolemia
LIPID-LOWERING DRUGStatins
A d v e r s e e f f e c t s:- mild gastrointestinal disturbances
- increased plasma activities in liver enzymes
- severe myositis (rhabdomyolysis)
and angio-oedema (rare)
LIPID-LOWERING DRUGS: Fibrates
- stimulate the β-oxidative degradation of fatty acids - liberate free fatty acids for storage in fat or for metabolism in striated muscle- Are ligands for nuclear txn receptor, peroxisome proliferator-activated recptor-α (PARP-α)- increase the activity of lipoprotein lipase, hence increasing hydrolysis of triglyceride in chylomicrons and VLDL particles.
- reduce hepatic VLDL production and increase hepatic LDL uptake.- Produce a modest decrease in LDL (~ 10%) and increase in
HDL (~ 10%).- But, a marked decrease in TGs (~ 30%).
LIPID-LOWERING DRUGS Fibrates
O t h e r e f f e c t s : improve glucose tolerance inhibit vascular smooth muscle inflammation
fenofibrate clofibrate gemfibrozil ciprofibrate
LIPID-LOWERING DRUGS Fibrates
A d v e r s e e f f e c t s: in patients with renal impairment myositis (rhabdomyolysis)
myoglobulinuria, acute renal failure
Fibrates should be avoided in such patients and also in alcoholics)
mild GIT symptoms
LIPID-LOWERING DRUGS
Fibrates
1st-line defense for:
*mixed dyslipidemia (i.e. raised serum TG and CHO)
* patients with low HDL and high risk of atheromatous disease (often type 2 diabetic patients)
* patients with severe treatment- resistant dyslipidemia (combination with other lipid-lowering drugs).* Indicated in patients with VERY HIGH [TG]s who are at risk for pancreatitis
LIPID-LOWERING DRUGS
Bile acid binding resins (Anion-exchange resins)
sequester bile acids in the GIT prevent their reabsorption
and enterohepatic recirculation
The r e s u l t is:
decreased absorption of exogenous CHO and increased metabolism of endogenous CHO into bile acid acids
increased expression of LDL receptors on liver cells
increased removal of LDL from the blood
reduced concentration of LDL CHO in plasma (while an unwanted increase in TG)
Anion-exchange Resins
• Increase the excretion of bile acids, causing more CHO to be converted to BAs.
• The decr in hepatocyte [CHO] compenatory incr in HMG CoA reductase activity and the number of LDLRs.
• Because these resins don’t work in patients with homozygous familial hypercholesterolemia, increased expression of hepatic LDLRs is the main mechanism by which resins lower plasma CHO.
LIPID-LOWERING DRUGS
Bile acid binding resins
Colestyramin colestipolanion exchange resins
C l i n i c a l u s e s: heterozygous familiar hypercholesterolemia an addition to a statin if response has been inadequate
hypercholesterolemia when a statin is contraindicated
uses unrelated to atherosclerosis, including: pruritus in patients with partial biliary obstruction bile acid diarrhea (diabetic neuropathy)
LIPID-LOWERING DRUGS
Bile acid binding resins
A d v e r s e e f f e c t s:
GIT symptoms - nauzea, abdominal bloating, constipation or diarrhea, bec resins not absorbed. resins are unappetizing. This can be minimized by suspending them in fruit juice interfere with the absorption of fat-soluble vitamins and drugs (chlorothiazide, digoxin, warfarin)
These drugs should be given at last 1 hour before or 4-6 hours after a resin
LIPID-LOWERING DRUGS
Others
Nicotinic acid inhibits hepatic TG production and VLDL Secretion (by ~ 30-50%) modest reduction in LDL and increase in HDL.
Nicotinic acid was the 1st lipid-lowering drug to decr overall mortality in patients with CAD.But its use is limited by the desirable
A d v e r s e e f f e c t s:flushing, palpitations , GIT disturbances.
Currently, nicotinic acid is rarely used.
LIPID-LOWERING DRUGS
Others
Fish oil (rich in highly unsaturated fatty acids)
the omega-3 marine TG - reduce plasma TG but increase CHO (CHO is more strongly associated wih coronary artery disease)- the effects on cardiac morbidity or mortality is unproven( although there is epidemiological evidence that eating fish regularly does reduce ischemic heart disease)
LIPID-LOWERING DRUGS
Others
Inhibitors of Intestinal CHO Absorption: Ezetimibe:
Reduces CHO and phytosterol absorption and decreases LDL CHP by ~18%, but with little change in HDL CHO.
May be synergistic with statins: so good for combination therapy.
Drug Combinations• Severe hyperlipidemia often requires multiple LLDs
to get the job done.• As usual, combinations should involve drugs with
different mechanisms of action (e.g., statins with fibrates).
• Even though some combinations (foregoing) may increase risk of, say, myopathy, the benefits of lowering LDL CHO outweigh the small incr in adverse effects.
• Recent trial with gemfibrozil (fibrate) decr myocardial infarction, stroke, and overall mortality in men with CAD assoc with low HDL (this drug inc HDL CHO w/o decr LDL CHO).