Lipid-Lowering Drugs

What are lipoproteins?

• Lipoproteins are protein-lipid complexes .

The Players – Lipids

Triacylglycerol Phospholipids Cholesterol Cholesteryl 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 Helices

Lipoprotein Surface

Lipoprotein Classes

Chylomicrons, VLDL, and their catabolic remnants

> 30 nm

D<1.006 g/ml LDL

20–22 nm

D=1.019-1.063g/ml HDL

9–15 nm

D=1.063-1.21 g/ml 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.

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

TG Rich: VLDL

Surface Monolayer Phospholipids (12%) Free Cholesterol (14%) Protein (4%) Hydrophobic Core Triglyceride (65%) Cholesteryl Esters (8%) Cholesterol and Atherosclerosis, Grundy)

VLDL Metabolism

Apo C’s and apoE and cholesteryl ester are acquired from HDL in circulation Cholesterol and Atherosclerosis, Grundy)

Fatty Acid Transport

ApoC-II activates lipoprotein lipase which catalyses the hydrolysis of TG Cholesterol and Atherosclerosis, Grundy)

VLDL Conversion to LDL

Further action on IDL by hepatic lipase loses additional apolipoproteins (apoE) becomes and is converted to LDL Cholesterol and Atherosclerosis, Grundy)

Cholesterol and Atherosclerosis, Grundy)

CE Rich: LDL

Surface Monolayer Phospholipids (25%) Free Cholesterol (15%) Protein (22%) Hydrophobic Core Triglyceride (5%) Cholesteryl Esters (35%)

LDL is removed by apoB100 receptors which are mainly expressed in the liver

LDL Metabolism

Hepatic Lipase Cholesteryl ester transfer protein Cholesterol and Atherosclerosis, Grundy)

LDL Uptake by Tissues

X X

Defects in the LDL receptor leads to familial hypercholesterolemia Cholesterol and Atherosclerosis, Grundy)

Cholesterol and Atherosclerosis, Grundy)

CE Rich: HDL

Surface Monolayer Phospholipids (25%) Free Cholesterol (7%) Protein (45%) Hydrophobic Core Triglyceride (5%) Cholesteryl Esters (18%)

HDL Metabolism

Nascent HDL (lipid-poor apoA-I) is produced by the liver and intestine

Hepatic Cholesterol Metabolism

Hepatic Cholesterol Synthesis

Cholesterol and Atherosaclerosis, Grundy) Rate Limiting Only pathway for cholesterol degradation Energetically expensive; prefer to conserve what is already made/acquired – LDL receptor pathway

LDL Cellular Metabolism

LDL are taken up by the LDL Receptor into clathrin-coated pits Cholesterol and Atherosaclerosis, Grundy)

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

LDL Readily Enter the Artery Wall Where They May be Modified Vessel Lumen

LDL Oxidation of Lipids and ApoB Aggregation

LDL

Endothelium

Hydrolysis of Phosphatidylcholine to Lysophosphatidylcholine Other Chemical Modifications Modified LDL Modified LDL are Proinflammatory Steinberg D et al. N Engl J Med 1989;320:915-924.

Intima

Lipids Online

Role of LDL in Atherosclerosis

Monocyte

Vessel Lumen

Remnant Lipoproteins Adhesion Molecules Cytokines Macrophage Doi H et al. Circulation 2000;102:670-676.

MCP-1

Endothelium

Remnants

Modified Remnants

Intima

Growth Factors Metalloproteinases Foam Cell Cell Proliferation Matrix Degradation Lipids Online

HDL Prevent Foam Cell Formation

Monocyte LDL Adhesion Molecules MCP-1

LDL

Cytokines Modified LDL Foam Cell Macrophage HDL Promote Cholesterol Efflux Miyazaki A et al. Biochim Biophys Acta 1992;1126:73-80.

Vessel Lumen

Endothelium

Intima

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 industrial countries Hypertension 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 PGI 2 monocyte attachment and NO synthesis

-

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 CHO

ENDOGENOUS PATHWAY

may be stored oxidised to bile acids secreted in the bile unaltered

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

Dyslipidemia

The 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: Statins

HMG-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 DRUG Statins -

Pharmacokinetics well absorbed when given orally extracted by the liver (target tissue), undergo extensive presystemic biotransformation Simvastatin is an inactive pro-drug

LIPID-LOWERING DRUG Statins

• • •

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 DRUG Statins -

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 colestipol anion 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).