Cholybar

Although deaths due to coronary heart disease have declined significantly over the past 25 years, coronary heart disease remains a leading cause of death in American adults. It is generally accepted that an elevated serum cholesterol level or, more precisely, an elevated low-density lipoprotein (LDL) cholesterol level, is a major risk factor for the development of coronary heart disease. Recent primary prevention trials such as the Lipid Research Clinics Trial[1] and the Helsinki Heart Study[2] have shown that pharmacologic intervention can significantly reduce this risk by lowering total and LDL serum cholesterol levels.

Although diet should be the mainstay of lipid-lowering therapy, some patients are unable to maintain a diet that is suffi ciently low in saturated fat and cholesterol, and sometimes diet alone is not sufficient to achieve treatment goals. In these cases, drug therapy may be necessary.

Table 1 lists the drugs used in the treatment of the three causes of hyperlipidemia. The relative efficacy of diet and drug therapies is shown in Table 2.[3] Many of the lipid-lowering drugs have unpleasant side effects. At tolerable doses, these drugs may not sufficiently lower cholesterol levels.

Newer agents such as the 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors have been shown to significantly lower total and LDL cholesterol levels while being relatively free of significant or unpleasant side effects.[4-6] However, no study has yet shown that the HMG CoA reductase inhibitors reduce the risk of coronary heart disease.

Mechanism of Action

The mechanism of action of these drugs is competitive inhibition of HMG CoA reductase, the enzyme that catalyzes the reduction of HMG CoA to mevalortic acid, the rate-limiting step in cholesterol biosynthesis.[7] The resulting decrease in cholesterol synthesis causes a fall in the serum cholesterol level and, in particular, a fall in the serum LDL cholesterol level. In liver cells (the target organ for HMG CoA reductase inhibitor action), cholesterol is obtained directly by synthesis using the pathway depicted in Figure 1 or indirectly via the LDL receptor on the cell surface. When intrahepatic cholesterol concentrations fall, LDL receptor activity on the cell surface increases. This increase in LDL receptor activity causes increased clearance of LDL cholesterol from the circulation, resulting in a drop in the LDL cholesterol level.

This "up-regulation" of the LDL receptor is thought to be the major mechanism in the reduction of serum LDL and total cholesterol levels by HMG CoA reductase inhibitors. Other possible mechanisms include decreased hepatic cellular cholesterol synthesis and decreased LDL cholesterol secretion.

Clinical Pharmacology

The chemical structures of the currently available HMG CoA reductase inhibitors are shown in Figure 2.[8] Although an three agents are structurally related, pravastatin (Pravachol) has a hydroxy group on the hexahydronaphthalene nucleus, while lovastatin (Mevacor) and simvastatin (Zocor) differ from each other in the number of methyl groups in the ester side chain. Lovastatin and simvastatin are administered as the prodrug lactones and are converted to their respective hydroxy acid active forms in vivo. Pravastatin is administered in its active form as the sodium salt of its hydroxy acid.

Table 3 lists the usual dosage and cost of HMG CoA reductase inhibitors. Reductions in total cholesterol by 16 to 35 percent and in LDL cholesterol by 17 to 39 percent have been reported with these agents. Reductions in triglyceride levels and increases in high-density lipoprotein (HDL) cholesterol levels have not been consistently reported.[5,9-11]

[TABULAR DATA OMITTED]

The HMG CoA reductase inhibitors may be given initially as a single dose with the evening meal or at bedtime, since cholesterol biosynthesis is greatest at night and the up-regulated LDL receptors have a long half-life. Lovastatin may need to be taken twice a day for maximum effectiveness.

Response to therapy can be observed with lowering of total and LDL cholesterol levels after two weeks of therapy. The maximum effect is achieved after approximately four weeks of treatment. Dosage should be adjusted on the basis of fasting LDL cholesterol levels after four to six weeks of therapy.

When HMG CoA reductase inhibitors are prescribed, the lowest available dose should be used. Many patients, particularly those with less severe hypercholesterolemia, may respond to very low doses of these agents.[12] Using the lowest effective dose also helps minimize the cost of treatment with these expensive drugs.

Although few direct comparisons have been made of the effectiveness of these drugs, all three appear to be equally effective.[13] 13 At least one large multicenter study comparing the effectiveness of pravastatin and simvastatin is presently under way.

Adverse Reactions

HMG CoA reductase inhibitors are generally better tolerated than other cholesterol-lowering drugs. Mild, transient gastrointestinal disturbances, rash and headache are the most commonly reported side effects (Table 4).[14] Some patients report sleep disturbances with lovastatin and simvastatin; pravastatin seems less likely to cause this adverse effect. (Pravastatin may be less likely to cause sleep disturbances because it is less lipophilic and thus may be less likely to cross the blood-brain barrier.)[15] Severe myalgia and myopathy with significant elevation of serum creatine kinase are rare adverse reacfions.

An increase in plasma liver enzymes, particularly the aminotransferases, to more than three times normal levels has been reported in approximately 2 percent of patients.[16,17] Symptomatic hepatitis is rare. Nevertheless, frequent monitoring of liver enzymes is recommended.

Patients receiving HMG Coa reductase inhibitors should undergo liver function tests before treatment is begun, every six weeks for the first three months of therapy, every eight weeks for the next nine months of therapy and every six months thereafter.

Therapy should be discontinued if transaminase levels increase to three times the upper limit of normal and remain at that level. HMG CoA reductase inhibitors should be used with particular caution in patients with a history of liver disease or alcohol abuse.

Use with Other Lipid-Lowering Drugs

Concomitant use of an HMG CoA reductase inhibitor with a second lipid-lowering drug is necessary in some patients, particularly those with high cholesterol levels (above 300 mg per dL [7.75 mmol per L]) before treatment. Combination therapy with a bile acid resin such as cholestyramine (Cholybar, Questran) or colestipol (Colestid) is particularly effective, often resulting in reductions of 50 percent or more in the LDL cholesterol level. When used in combination with a resin, the HMG CoA reductase inhibitor should be taken either one hour before or at least four hours after the resin, because the resin will significantly decrease the bioavailability of the reductase inhibitor if the two are taken together.

Use of an HMG Coa reductase inhibitor with fibrate-type drugs such as gemfibrozil (Lopid) is not recommended because of the possible development of myopathy and rhabdomyolysis. Combination therapy with nicotinic acids may also increase the risk of myopathy and rhabdomyolysis and should generally be avoided.

REFERENCES

[1.] The Lipid Research Clinics Coronary Primary Prevention Trial results. I. Reduction in incidence of coronary heart disease. JAMA 1984;251:351-64. [2.] Frick MH, Elo O, Haapa K, et al. Helsinki Heart Study: primary-prevention trial with gemfibrozil in middle-aged men with dyslipidemia. Safety of treatment, changes in risk factors, and incidence of coronary heart disease. N Engl J Med 1987,317:1237-45. [3.] Snyder S. Comparison of cholesterol-lowering regimens. Am Fam Physician 1990; 42:761-8. [4.] Bradford RH, Shear CL, Chremos AN, et al. Expanded Clinical Evaluation of Lovastatin (EXCEL) study results. I. Efficacy in modifying plasma lipoproteins and adverse event profile in 8245 patients with moderate hypercholesterolemia. Arch Intern Med 1991;151: 43-9. [5.] Shear CL, Franklin FA, Stinnett S, et al. Expanded Clinical Evaluation of Lovastatin (EXCEL) study results. Effect of patient characteristics on lovastatin-induced changes in plasma concentrations of lipids and lipoproteins. Circulation 1992;85:1293-303. [6.] Jones PH, Farmer JA, Cressman MD, et al. Once-daily pravastatin in patients with primary hypercholesterolemia: a dose-response study. Clin Cardiol 1991;14:146-51. [7.] Endo A. Chemistry, biochemistry, and pharmacology of HMG-CoA reductase inhibitors. Klin Wochenschr 1988;66:421-7. [18.] Serajuddin AT, Ranadive SA, Mahoney EM. Relative lipophilicities, solubilities, and structure-pharmacological considerations of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors pravastatin, lovastatin, mevastatin, and simvastatin. J Pharm Sci 1991;80:830-4. [9.] Havel RJ, Hunninghake DB, Illingworth DR, et al. Lovastatin (mevinolin) in the treatment of heterozygous familial hypercholesterolemia. A multicenter study. Ann Intern Med 1987;107:609-15. [10.] Betteridge DJ, Bhatnager D, Bing RF, et al. Treatment of familial hypercholesterolaemia. United Kingdom lipid clinics study of pravastatin and cholestyramine. BMJ 1992; 304:1335-8. [11.] Jones PH, Farmer JA, Cressman MD, et al. Once-daily pravastatin in patients with primary hypercholesterolemia: a dose-response study. Clin Cardiol 1991;14:146-51. [12.] Rubinstein A, Lurie Y, Groskop I, Weintrob M. Cholesterol-lowering effects of a 10 mg daily dose of lovastatin in patients with initial total cholesterol levels 200 to 240 mg/dL (5.18 to 6.21 mmol/liter). Am j Cardiol 1991; 68:1123-6. [13.] McPherson R, Bedard J, Connelly P, et al. Comparison of the short-term efficacy and tolerability of lovastatin and pravastatin in the management of primary hypercholesterolemia. Clin Ther 1992;14:276-91. [14.] Bays HE, Dujovne CA, Lansing AM. Drug treatment of dyslipidemias: practical guidelines for the primary care physician. Heart Dis Stroke 1992;1:357-65. [15.] Vgontzas AN, Kales A, Bixler EO, Manfredi RL, Tyson KL. Effects of lovastatin and pravastatin on sleep efficiency and sleep stages. Clin Pharmacol Ther 1991;50:730-7. [16.] Tobert JA, Shear CL, Chremos AN, Mantell GE. Clinical experience with lovastatin. Am J Cardiol 1990;65:23F-6F. [17.] Oberman A, Kreisberg R, Henkin Y. Principles and management of lipid disorders: a primary care approach. Baltimore: Williams & Wilkins, 1992:238.

THEODORE R. KANTNER, M.D. is professor and chairman of the Department of Family Medicine at Texas Tech University Health Sciences Center School of Medicine in Lubbock, Tex. He received his medical degree from Pennsylvania State University College of Medicine, Hershey, Pa., and completed a family practice residency at Lancaster (Pa.) General Hospital.

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