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Clinical trials have demonstrated that inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA)reductase (statins) greatly reduce cardiovascular-relatedmorbidity and mortality in patients with and withoutcoronary artery disease.1,2 These drugs were designed toinhibit the rate-limiting enzyme of cholesterol synthesis inthe liver, thereby decreasing hepatic production of low-density lipoprotein (LDL), and upregulating expression ofhepatic LDL receptors, thus lowering concentrations ofcirculating LDL. With lower plasma LDL concentrations,plaque development should be retarded or should regressbecause of lipid loss, which would render the plaque lessocclusive and less likely to disrupt or cause thrombosis.

The use of simvastatin in the Scandinavian (4S)secondary intervention study1 and the use of pravastatin inthe West of Scotland Coronary Prevention Study(WOSCOPS) primary intervention trial2 supported thehypothesis that drugs that lower plasma cholesterolconcentration are of benefit to patients with coronaryartery disease. However, the clinical benefit of the drugsused in these studies is manifest early in the course of lipid-lowering therapy before plaque regression could occur.

Quantitative angiographic assessments of the impact ofstatin therapy on coronary atherosclerosis havedemonstrated that improvement in arterial topographicalmorphology occurs slowly and only to a small extent.3 Inthe Multicentre Anti-Atheroma Study (MAAS),3

statistically significant improvement in arterialmorphology occurred with simvastatin after 4 years andwas not evident after 2 years.3 However, it is difficult toattribute the time scale of less than 2 years, in whichclinical benefit appeared in the 4S and WOSCOPS trials,solely to a decrease in LDL-cholesterol concentration.About 9 years elapsed before any real clinical benefit wasfound in the Program on the Surgical Control of theHyperlipidaemias (POSCH) trial.4 In the POSCH trial,plasma cholesterol was decreased by partial ilial bypass toa degree similar to that achieved in the statin trials and itmay be that its clinical effects are attributable purely tochanges in circulating lipoproteins.

The activity of HMG-CoA reductase limits the rate ofsynthesis not only of cholesterol but also of a range ofother molecules involved in functions such as cellularrespiration and cell-cell recognition. Therefore it shouldnot be surprising that the statins, as inhibitors of thisenzyme, might modify constituents of the vascular milieuother than LDL cholesterol.

Rudolph Virchow (1821–1902) originally proposed thatvascular thrombosis was caused by a triad of changes: in

the blood vessel wall, in blood flow, and in theconstituents of the blood. We suggest that the clinicalbenefits of simvastatin and pravastatin therapy are bestexplained by their direct effects, in each component of thetriad, on atherosclerotic and thrombotic mechanismswithin arteries, as well as through the more conventionallyaccepted means of decreasing plasma LDLconcentrations.

Changes in the vessel wallThe process of plaque fissuring, which results inthrombosis, triggers most acute coronary events. Mostlesions prone to fissuring and rupture have a large core oflipid-laden macrophages and a thin fibrous cap underlyingthe endothelium. Although these vulnerable lesionsaccount for 10–20% of all lesions, they are responsible for80–90% of acute clinical events.5 The reduction in clinicalevents secondary to lipid lowering has been conventionallyattributed to the selective depletion of both the lipid andfoam-cell content of this vulnerable subset of plaques byaltering the balance between LDL accumulation andefflux in the plaque. This alteration of composition makesthe plaque less likely to fissure, disrupt, or cause acutethrombosis. Whereas changes in plaque composition anddevelopment are likely to contribute significantly to thereduction in clinical endpoints in the long term,concurrent changes in other constituents or actions of thevessel wall, or both, may contribute to, and explain, theearly benefit seen with statin treatment.

Atherosclerosis is considered to be a chronicinflammatory disorder characterised by the presence ofmonocytes or macrophages and T lymphocytes inatherosclerotic lesions as well as the proliferation ofsmooth muscle cells, elaboration of extracellular matrix,and neovascularisation. Macrophages participate in theuptake and metabolism of lipids in the early stages ofatherogenesis6 and may accelerate atherogenesis by othermechanisms (figure 1) including secretion of mitogenicfactors similar to platelet-derived growth factor (PDGF),which stimulate smooth-muscle proliferation and plaqueneovascularisation.7 Macrophages have been implicated inthe pathophysiology of acute coronary syndromes as theyproduce enzymes which include members of themetalloproteinase family (interstitial collagenase,gelatinase, and stromelysin) that digest and weaken theplaque cap, making disruption more likely.8 The site ofplaque disruption, in addition to containing a largepopulation of inflammatory cells, also expresses, inabundance, HLA-DR antigens, which indicates an activeinflammatory response.

Pravastatin has been shown to influence cholesterolmetabolism in macrophages directly in vivo and in vitro, ina manner analogous to its effect in hepatocytes.9 Singledose administration to normocholesterolaemic andhypercholesterolaemic individuals decreases cholesterol

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Department of Pharmacology and Therapeutics,Clinical Sciences Unit, University College Cork, Ireland(C J Vaughan MB, M B Murphy MD, B M Buckley DPhil)

Correspondence to: Dr Brendan M Buckley, The Cork Clinic,Bon Secours Hospital, College Road, Cork, Ireland

Statins do more than just lower cholesterol

Carl J Vaughan, Michael B Murphy, Brendan M Buckley

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smooth-muscle-cell proliferation. A modulation ofsmooth-muscle-cell proliferation by non-sterol productssynthesised from mevalonate has also been demonstratedin other studies.14,15 These findings indicate that thebeneficial adjunctive effects of statin therapy may also bemediated by a decrease in cholesterol precursors and arenot solely because of the effects on LDL levels.

Blood flow, vasomotor tone, and endothelialfunctionThe endothelium is involved in the regulation ofvasomotor tone, inhibition of platelet activity, inhibition ofthrombosis, and promotion of fibrinolysis. Endothelium-derived relaxing factor (EDRF) is a prominent modulatorof normal endothelial function (figure 2). EDRFproduction is stimulated by various factors includingplatelet activation, shear stress, and concentrations ofthrombin, serotonin, and catecholamines. The importanceof EDRF and changes in EDRF production inatherosclerosis have been clearly established. Normalatheroma-free coronary arteries dilate when exposed toacetylcholine, an EDRF agonist, whereas atheromatousarteries constrict paradoxically when so exposed. Thisconstriction may contribute to episodic myocardialischaemia in patients with coronary artery disease.16,17

Improvements in endothelial function and vasomotionhave been demonstrated in hypercholesterolaemic patientstreated with statins. In one study, therapy with pravastatinled to an 80% reduction in the epicardial coronary arteryconstrictor response to acetylcholine in conjunction with a60% increase in coronary blood flow after 6 months oftherapy.18 In another study lovastatin produced similarimprovements in endothelial vasomotor function.19

Improvements in myocardial perfusion as demonstratedby thallium-201 single-photon emission computed

synthesis in macrophages by 62% and 47%, respectively.Pravastatin also causes a dose-dependent inhibition ofmacrophage cholesterol synthesis in vitro and increasesthe cellular degradation of LDL. Inhibition of endogenouscholesterol synthesis by macrophages has the potential toreduce macrophage activation, foam-cell formation, andthe thrombogenicity of plaques, and alters the lipid to cellratio of the atherosclerotic lesion, which may make theplaque less prone to rupture.

In usual dose, statins also modulate immune function invitro. Statins have been shown to alter regulation of DNAtranscription, regulate natural-killer-cell cytotoxicity, andinhibit antibody-dependent cellular cytotoxicity.10 Theseobservations in vitro are substantiated in clinical practiceby the observation that cardiac transplant recipients have areduced incidence of cardiac rejection, better survival, andreduced cardiac transplant vasculopathy when treatedwith pravastatin.11 In that study a correlation betweencholesterol levels and the development of vasculopathy1 year after transplantation was apparent.

Evidence that statins alter other biological processes inthe vessel wall is also accumulating. A study examining theeffects of simvastatin on PDGF-induced DNA synthesisand PDGF-� chain gene expression in human glomerularmesangial cells showed that exposure of cells tosimvastatin completely inhibited PDGF-induced DNAsynthesis.12 PDGF is known to contribute to macrophage,platelet, smooth muscle cell, and fibroblast migration andproliferation within blood vessels as well as inatherosclerotic lesions.7 Potentially, statins may attenuatePDGF-induced mitogenesis in plaques and limitdeleterious mitogenic responses that lead to migration andactivation of smooth-muscle cells and macrophages. Thisinhibition of smooth-muscle-cell proliferation by statinscan be overcome by mevalonate.13 This finding suggests apro-proliferative role for other cholesterol precursors in

1080 Vol 348 • October 19, 1996

PlateletFibrinogen

Fibrinolysis

Thrombosis

TxB2

TxA2PDGF Macrophage

Smooth-muscle cell

FPA TAT-III

PAI-1 TM

EDRF PGI2 PAI

Figure 2: Thrombosis, fibrinolysis, and the endotheliumPlatelet activation leads to the production of thromboxanes whichpromote platelet aggregation and vasoconstriction. Platelets alsosecrete PDGF which is a potent mitogen. The endothelium releases anumber of factors that inhibit platelet aggregation and vasodilation.EDRF=endothelium derived relaxing factor; FPA=fibrinopeptide A;PDGF=platelet derived growth factor; PGI2=prostacyclin; PAI-1=plasminogen activator inhibitor-1; TAT-III=thrombin anti-thrombin-IIIcomplex; TM=thrombomodulin; TxA2=thromboxane A2;TxB2=thromboxane B2.

Metalloproteinases

Neovascularisation

PDGFFoam cell

Macrophage

Cytokines

T-lymphocyte

Smooth-muscle cell

Figure 1: Role of macrophages in plaque formationActivated macrophages produce various substances that contribute toatherosclerosis and acute coronary syndromes: uptake and endogenoussynthesis of cholesterol by macrophages leads to foam-cell formation;release of metalloproteinases weakens the plaque cap; secretion ofplatelet-derived growth factor (PDGF) induces mitogenesis andpromotes plaque neovascularisation; macrophages also elaboratecytokines which stimulate smooth-muscle cell and lymphocyteproliferation.

tomography have been documented in patients with coronary artery disease after short-term statin therapy.'O In this study the time scale during which improvement occurred was 12 weeks, an interval too short for anatomic regression to occur. In addition to abnormal coronary vasomotion, forearm blood flow abnormalities have also been demonstrated in hypercholesterolaemic patients and normalisation has been achieved by statin therapy.21 T h e endothelial dysfunction that accompanies hypercholesterolaemia may be because of an abnormality in the nimc oxide Larginine pathway which is corrected when LDL concentrations are decreased by statins.

Changes in cardiovascular reactivity and blood pressure can provoke plaque rupture and conmbute to coronary vasospasm and ischaemia. In a randomised, double-blind, crossover study Straznicky and colleaguesZz evaluated the effects of short-term cholesterol reduction on cardiovascular reactivity in mildly hypertensive patients given pravastatin or placebo. Cardiovascular reactivity was assessed by measurement of blood-pressure responses to incremental infusions of angiotensin I1 and noradrenaline, by cold pressor testing and isomeaic exercise. Compared with placebo, pravastatin caused a significant fall in diastolic blood pressure responses (expressed as the infusion rate required to raise blood pressure by 20 mm Hg) to both angiotensin I1 and noradrenaline. Improvements in endothelial function are probably at the basis of these improvements in cardiovascular reactivity. Attenuation of cardiovascular reactivity would be expected to lower the haemodynamic stress on plaques and lessen the probability of acute disruption and thrombosis.

Changes In the conrtltuentr of Mood Platelets contribute to both atherosclerosis and thrombosis and play a central role in acute coronary syndromes (figure 2). Hypercholesterolaemia is associated with hypercoagulability as well as enhanced platelet reactivity at sites of acute vascular damage." Patients with hypercholesterolaemia have higher resting levels of malondialdehyde, thromboxane B,, and P-thromboglobulin, which suggests that high blood cholesterol induces lipid peroxidation and platelet activation.z' The following studies indicate that statins may modulate this prothrombotic state. Therapy with lovastatin in patients with type-IIa hypercholesterolaemia caused a significant reduction in serum fibrinogen levels and in adenosine-diphosphate-induced aggregation." Baseline platelet thrombus formation in an ex-vivo system was significantly higher in hypercholesterolaemic patients than in normo- cholesterolaemic patients and platelet aggregation decreased at both low and high shear rates in

' hypercholesterolaemic patients with documented coronary artery disease after 2-3 months treatment with pravastatin." The precise mechanism responsible for t h i s effect on platelets is unclear but may be through decreased platelet thromboxane production. High LDL levels appear to cause platelet hyper-reactivity in association with enhanced thromboxane A, biosynthesis. Enhanced thromboxane A, production has also been demonstrated in the majority of patients with type IIa hypercholesterolaemia, and simvastatin has been shown to reduce ex-vivo platelet aggregation and the production of thromboxane B, and thromboxane A, in patients with this dy~lipidaemia.~'

The calcium content of plasma membrane and cytosol of platelets is altered by hypercholesterolaemia, and platelet reactivity is increased. The finding that the cholesterol content of the membranes of both erythrocytes and platelets is reduced in patients treated with pravastatin2' indicates that the properties of these membranes may be altered in a manner that renders them less prone to participation in thrombosis. Additionally, since increased fibrinogen binding to platelets has also been documented in familial hypercholesterolaemia,U and statin therapy has been shown to reduce serum fibrinogen levels, it is plausible that statins also decrease thrombus formation by decreasing platelet interactions with fibrinogen.

In conjunction with alterations in platelet reactivity, humoral thrombogenic and hypofibrinolytic factors have also been found in hypercholesterolaemia. Significantly elevated mean plasma levels of thrombin-antithrombin 111 complex, fibrinopepetide A, thrombomodulin, and plasminogen activator inhibitor 1 (PAI-1) have been described in patients with hypercholesterolaemia,ZP and all of these are significantly reduced after pravastatin treatment. The effects of statins in tending to normalise this state point to the possibility that statins may in part act as anti-thrombotic agents.

conclurlon Although the components of Vichow's t i ad are far less discrete than originally envisaged, the mad is still a conceptually useful way to explore the vascular milieu with respect to both thrombosis and atherosclerosis. The complex interactions between endothelium, platelets, and macrophages cannot be viewed separately. The production of a myriad of factors by platelets and macrophages on one hand, and by the endothelium on the other, makes any evaluation of the in-vivo effects of statin therapy difficult. Attention has been almost entirely focused on the extent to which therapy with these drugs lowers plasma LDL concentrations and the established clinical benefits of statin therapy have been directly, and virtually exclusively, attributed to this effect. However, it is increasingly likely that we cannot attribute the relatively early reduction in mortality seen in c l i c a l uials of statin therapy solely to LDL-dependent reduction in plaque volume, plaque lipid, and regression of coronary atherosclerosis. There is experimental evidence that statins have effects on immune function, macrophage metabolism, and cell proliferation independent of changes in plasma LDL concentrations and that their modulation of the pathophysiological determinants of acute coronary syndromes accounts for the early c l i c a l benefit observed. We suggest that statins improve haemorheological characteristics involved in coagulation and vasomotion early in the course of therapy (figure 3). The time c o m e

c 0

actions

Plaque geometry

2

!

U e! .- B -

5

Time Figure 3 Time courses of putative beneffcial effecta of M t i ~

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of improvement in endothelial function as well asamelioration of aberrant components of the coagulationcascade is very likely to yield clinical benefit relativelyquickly. However, alterations in plaque geometry, lipidcontent, and thrombogenicity are likely to require therapyfor longer before clinical benefit becomes evident.

References1 Scandinavian Simvastatin Survival Study Group. Randomised trial of

cholesterol lowering in 4444 patients with coronary heart disease: theScandinavian Simvastatin Survival Study (4S). Lancet 1994; 344:1383–89.

2 Shepherd J, Cobbe SM, Ford I, et al. Prevention of coronary heartdisease with pravastatin in men with hypercholesterolaemia. N Engl JMed 1995; 333: 1301–07.

3 MAAS investigators. Effect of simvastatin on coronary atheroma: theMulticentre Anti-Atheroma Study (MAAS). Lancet 1994; 344:633–38.

4 Buchwald H, Campos CT, Boen JR, et al. Disease-free intervals afterpartial ilial bypass in patients with coronary heart disease andhypercholesterolaemia: report from the program on the surgical controlof the hyperlipidemias (POSCH). J Am Coll Cardiol 1995; 26: 351–57.

5 Brown BG, Zhao XQ, Sacco DE, Albers JJ. Lipid lowering and plaqueregression: new insights into prevention of plaque disruption andclinical events in coronary disease. Circulation 1993; 87: 1781–91.

6 Gerrity RG. The role of the monocyte in atherogenesis: I. Transitionof blood-borne monocytes into foam cells in fatty lesions. Am J Pathol1981; 103: 181–90.

7 Ross R, Raines EW, Bowen-Pope DF. The biology of platelet-derivedgrowth factor. Cell 1986; 46: 155–69.

8 Henney AM, Wakeley PR, Davies MJ, et al. Location of stromelysingene in atherosclerotic plaques using in situ hybridisation. Proc NatlAcad Sci 1991; 88: 8154–58.

9 Keidar S, Aviram M, Maor I, Oiknine J, Brook JG. Pravastatin inhibitscellular cholesterol synthesis and increases low density lipoproteinreceptor activity in macrophages: in vitro and in vivo studies. Br J ClinPharmacol 1994; 38: 513–19.

10 McPherson R, Tsoukas C, Baines MG, et al. Effects of lovastatin onnatural killer cell function and other immunological parameters inman. J Clin Immunol 1993; 13: 439–44.

11 Kobashigawa JA, Katznelson S, Laks H, et al. Effect of pravastatin onoutcomes after cardiac transplantation. N Engl J Med 1995; 333:621–27.

12 Grandaliano G, Biswas P, Choudhury GG, Abboud HE. Simvastatininhibits PDGF-induced DNA synthesis in human glomerularmesangial cells. Kidney Int 1993; 44: 503–08.

13 Rogler G, Lackner KJ, Schmitz G. Effects of fluvastatin on growth ofporcine and human vascular smooth muscle cells in vitro. Am J Cardiol1995; 76: 114A–16A.

14 Munro E, Patel M, Chan P, et al. Inhibition of human vascularsmooth muscle cell proliferation by lovastatin: the role of isoprenoidintermediates of cholesterol synthesis. Eur J Clin Invest 1994; 24:766–72.

15 Hidaka Y, Eda T, Yonemoto M, Kamei T. Inhibition of culturedvascular smooth muscle cell migration by simvastatin (MK-733).Atherosclerosis 1992; 95: 87–94.

16 Yeung AC, Vekshtein VI, Krantz DS, et al. The effect ofatherosclerosis on the vasomotor response of coronary arteries tomental stress. N Engl J Med 1991; 325: 1551–56.

17 Vita JA, Treasure CB, Yeung AC, et al. Patients with evidence ofcoronary endothelial dysfunction as assessed by acetylcholine infusiondemonstrate marked increase in sensitivity to constrictor effects ofcatecholamines. Circulation 1992; 85: 1390–97.

18 Egashira K, Hirooka Y, Kai H, et al. Reduction in serum cholesterolwith pravastatin improves endothelium-dependent coronaryvasomotion in patients with hypercholesterolemia. Circulation 1994;89: 2519–24.

19 Treasure CB, Klein JL, Weintraub WS, et al. Beneficial effects ofcholesterol-lowering therapy on the coronary endothelium in patientswith coronary artery disease. N Engl J Med 1995; 332: 481–87.

20 Eichstadt HW, Eskotter H, Hoffman I, Amthauer HW, Weidinger G.Improvement of myocardial perfusion by short-term fluvastatin therapyin coronary artery disease. Am J Cardiol 1995; 76: 122A–25A.

21 Hayoz D, Weber R, Rutschmann B, et al. Postischemic blood flowresponse in hypercholesterolemic patients. Hypertension 1993; 26:497–502.

22 Straznicky NE, Howes LG, Lam W, Louis WJ. Effects of pravastatinon cardiovascular reactivity to norepinephrine and angiotensin II inpatients with hypercholesterolemia and systemic hypertension. Am JCardiol 1995; 75: 582–86.

23 Badimon JJ, Badimon L, Turitto VT, Fuster V. Platelet deposition athigh shear rates is enhanced by high plasma cholesterol levels: in vivostudy in the rabbit model. Arterioscler Thromb 1991; 11: 395–402.

24 DiMinno G, Silver MJ, Cerbone AM, Rainone A, Postiglione A,Mancini M. Increased fibrinogen binding to platelets from patientswith familial hypercholesterolemia. Arteriosclerosis 1986; 6: 203–11.

25 Mayer J, Eller T, Brauer P, et al. Effects of long-term treatment withlovastatin on the clotting system and blood platelets. Ann Hematol1992; 64: 196–201.

26 Lacoste L, Lam JYT, Hung J, Letchacovski G, Solymoss CB,Waters D. Hyperlipidaemia and coronary disease: correction of theincreased thrombogenic potential with cholesterol reduction.Circulation 1995; 92: 3172–77.

27 Notarbartolo A, Davi G, Averna M, et al. Inhibition of thromboxanebiosynthesis and platelet function by simvastatin in type IIahypercholesterolemia. Arterioscler Thromb Vasc Biol 1995; 15: 247–51.

28 Le Quan Sang KH, Levenson J, Megnien JL, Simon A, Devynck MA.Platelet cytosolic Ca2+ and membrane dynamics in patients withhypercholesterolemia. Effects of pravastatin. Arterioscler Thromb VascBiol 1995; 15: 759–64.

29 Wada H, Mori Y, Kaneko T, et al. Elevated plasma levels of vascularendothelial cell markers in patients with hypercholesterolemia. Am JHematol 1993; 44: 112–16.

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