Beneficial effects of cholesterol-lowering therapy on the coronary endothelium in patients with coronary artery disease

Pravastatin limitation of atherosclerosis in the coronary arteries (PLAC I): reduction in atherosclerosis progression and clinical events. PLAC I investigation

OBJECTIVES

This study was designed to evaluate the effect of pravastatin on progression of coronary atherosclerosis and ischemic events in patients with coronary artery disease and mild to moderate hyperlipidemia.

BACKGROUND

Few clinical trial data support the use of lipid-lowering therapy in patients with coronary artery disease and mild to moderate elevations in cholesterol levels.

METHODS

Four hundred eight patients (mean age 57 years) with coronary artery disease and low density lipoprotein (LDL) cholesterol > or = 130 mg/dl (3.36 mmol/liter) but < 190 mg/dl ([4.91 mmol/liter]) despite diet were randomized in a 3-year study to receive pravastatin or placebo. Atherosclerosis progression was evaluated by quantitative coronary arteriography.

RESULTS

Baseline mean LDL cholesterol was 164 mg/dl (4.24 mmol/liter). Pravastatin decreased total and LDL cholesterol and triglyceride levels by 19%, 28% and 8%, respectively, and increased high density lipoprotein cholesterol by 7% (p < or = 0.001 vs. placebo for all lipid variables). Progression of atherosclerosis was reduced by 40% for minimal vessel diameter (p = 0.04), particularly in lesions < 50% stenosis at baseline. There was a consistent although not statistically significant effect on mean diameter and percent diameter stenosis. There were also fewer new lesions in those assigned pravastatin (p < or = 0.03). Myocardial infarction was reduced during active treatment (8 in the pravastatin group, 17 in the placebo group; log-rank test, p < or = 0.05; 60% risk reduction), with the benefit beginning to emerge after 1 year.

CONCLUSIONS

In patients with coronary artery disease and mild to moderate cholesterol elevations, pravastatin reduces progression of coronary atherosclerosis and myocardial infarction. The time course of event reduction increases the potential for a relatively rapid decrease in the clinical manifestations of coronary artery disease with lipid lowering.

Atherosclerosis, oxidative stress, and antioxidant protection in endothelium-derived relaxing factor action

The vascular endothelium plays a central role in the regulation of vascular function. In particular, the local release of endothelium-derived relaxing factor (EDRF) regulates vascular tone and prevents platelet adhesion to the vascular wall. Impairment of EDRF action develops early in atherosclerosis and, in part, contributes to platelet deposition and vasospasm involved in the clinical expression of coronary artery disease. Recent evidence suggests that an imbalance between vascular oxidative stress and antioxidant protection is involved in the development of this vascular dysfunction. In this report, the relation between oxidative stress, atherosclerosis, and abnormal EDRF action is reviewed with particular attention to the effects of antioxidant supplementation in animal models of atherosclerosis and hypercholesterolemia.

Vascular function in the forearm of hypercholesterolaemic patients off and on lipid-lowering medication

To study whether vascular dysfunction in hypercholesterolaemia is reversible, we investigated patients without overt arterial disease who were taking maintenance treatment for hypercholesterolaemia. Medication was stopped for 2 weeks, reinstituted for 12 weeks, and again stopped for 6 weeks. During both maintenance treatment and the 12 weeks of step-up medication the lipid profile was improved but did not return to normal. Dose-response curves for serotonin-induced vasodilatation, an index of nitric oxide-dependent vasodilatation, showed a comparable and significant rightward shift after a medication-free period of 2 and 6 weeks compared with control subjects, indicating endothelial dysfunction, which was already maximum after 2 weeks. After 12 weeks of lipid-lowering medication, the difference in endothelial function between controls and patients had disappeared. Co-infusion of L-arginine, the substrate for nitric oxide synthase, returned the impaired serotonin response during hypercholesterolaemia to normal, but had no effect on this response in controls or in patients while on lipid-lowering medication. Neither endothelium-independent vasorelaxation, assessed by sodium nitroprusside infusion, nor vasoconstriction induced by the nitric oxide blocker L-NMMA, were different between controls and patients, whether the latter were on or off lipid-lowering medication. Our results show an L-arginine-sensitive, impaired nitric-oxide-mediated vascular relaxation of forearm resistance vessels in hypercholesterolaemia which is reproducible, and reversible after short-term lipid-lowering therapy. Demonstration of such changes in this readily accessible vascular bed will allow larger trials assessing vascular function during lipid-lowering therapy to be done.

Is endothelial dysfunction reversible?

To date, the published database concerning the impact of therapy on endothelial function in essential hypertension and hypercholesterolemia is small and incomplete. Chronic antihypertensive therapy had not yet been proven to restore endothelial function in patients with essential hypertension. In contrast, chronic lipid-lowering therapy is effective in restoring endothelial function of the coronary and peripheral circulation in patients with hypercholesterolemia. A beneficial effect was already documented after 3 months of therapy with fluvastatin but full restoration may last more than 1 year.

Modulation of coronary autoregulatory responses by endothelium-derived nitric oxide

There is increasing evidence that endothelium-derived nitric oxide production is an important mechanism contributing to the regulation of myocardial perfusion during ischemia distal to a coronary stenosis. Studies in conscious chronically instrumented animals have extended observations in isolated arterioles to demonstrate that inhibiting nitric oxide synthase with L-arginine analogs increases the vulnerability of the myocardium to ischemia. The variable extent to which endothelium-dependent function is impaired in human atherosclerosis raises the possibility that abnormalities in resistance vessel control contribute to the functional significance of a fixed epicardial coronary stenosis. This may explain the wide variability between the physiological effects of a given coronary stenosis and its angiographic severity. Aggressive intervention to normalize endothelium-dependent vasodilation and local nitric oxide release may have beneficial effects on the functional significance of a coronary stenosis.

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.