Comprehensive Study Guide: Drugs for the Treatment of Hypercholesterolemia

Learning Objectives

• Explain the role of dyslipidemia in atherosclerosis and cardiovascular disease.

• Compare the mechanisms of action of major LDL-lowering therapies.

• Describe the mechanisms, indications, and adverse effects of triglyceride-lowering therapies.

• Discuss specialized therapies for familial hypercholesterolemia.

• Recognize important adverse effects, contraindications, and clinically relevant drug interactions associated with various lipid-lowering drugs.

Understanding Dyslipidemia and its Effects

• Definition of Dyslipidemia: An abnormal amount of lipids in the blood, characterized by any of the following:   - Elevated LDL cholesterol or triglycerides.   - Reduced HDL cholesterol.   - A combination of both.

• Scope of Dyslipidemias: Includes hyperlipidemia and low levels of high-density-lipoprotein cholesterol ($HDL-C$).

• Pathological Consequences: Dyslipidemia is a major cause of:   - Atherosclerosis.   - Coronary heart disease (CHD).   - Ischemic cerebrovascular disease.   - Peripheral vascular disease.

• Risk Factors: Both genetic disorders and lifestyle factors contribute to dyslipidemias, which are major causes of increased atherogenic risk.

Genetic Dyslipidemias (Primary Etiologies)

• Familial Hypercholesterolemia:   - Genetic Defect: LDL receptor mutation.   - Lipid Signature: Extreme LDL concentration ($>190\,mg/dL$).   - Clinical Hallmarks: Tendon xanthomas and premature coronary artery disease (CAD).

• Familial Combined Hyperlipidemia:   - Genetic Defect: $ApoB/VLDL$ overproduction.   - Lipid Signature: Mixed elevation of LDL and TG.   - Clinical Hallmarks: Variable presentation and high CAD risk.

• Dysbetalipoproteinemia:   - Genetic Defect: $ApoE$ mutation.   - Lipid Signature: Remnant accumulation, specifically IDL and Chylomicron remnants.   - Clinical Hallmarks: Palmar xanthomas and premature vascular disease.

• LPL Deficiency:   - Genetic Defect: Lipoprotein Lipase (LPL) defect.   - Lipid Signature: Extreme Triglycerides.   - Clinical Hallmarks: Acute pancreatitis and eruptive xanthomas.

Atherosclerotic Plaque Formation Phases

• Phase 1: Initiation: LDL enters the arterial wall and becomes oxidized.

• Phase 2: Foam Cell Formation: Macrophages consume the oxidized LDL and transform into foam cells.

• Phase 3: Plaque Growth: Growth of the plaque occurs, characterized by a lipid core and a fibrous cap.

• Phase 4: Complication: The fibrous cap ruptures, leading to blood clot formation.

Lipoprotein Composition and Metabolism

• Lipoprotein Structure: Macromolecular assemblies containing lipids and proteins.   - Lipid Constituents: Includes free and esterified cholesterol, triglycerides, and phospholipids.   - Protein Components (Apolipoproteins/Apoproteins): Provide structural stability. The most water-insoluble lipids (cholesteryl esters and triglycerides) form the core, while polar, water-soluble apoproteins are on the surface.

• Classes of Lipoproteins:   - Chylomicrons: Synthesized from dietary fatty acids; they are the largest and lowest density plasma lipoproteins.   - Very-low-density Lipoproteins (VLDL): Produced in the liver. The major component is triglycerides. Catabolized in plasma by lipoprotein lipase (LPL).   - Low-density Lipoproteins (LDL): Derived from VLDL. Most plasma cholesterol is found here. Hepatic clearance is mediated primarily by LDL receptors, which remove approximately $75\%$ of all LDL from plasma.   - High-density Lipoproteins (HDL): Protective lipoproteins involved in reverse cholesterol transport, acquiring excess cholesterol from cells and transferring it to the liver for excretion.

• Factors Influencing LDL Receptor Gene Expression:   - Enhancement: Thyroxine, estrogen, statins, and decreased consumption of saturated fat and cholesterol.

• Factors Influencing HDL Levels:   - Hepatic lipase (HL): Modulates $HDL-C$ levels; higher HL activity is associated with lower $HDL-C$ levels.   - Androgens: Increase HL gene expression and activity, accounting for lower $HDL-C$ values in men compared to women.

Classification of Lipid Levels ($mg/dL$)

• LDL Cholesterol:   - Optimal: $<100$   - Near or above optimal: $100-129$   - Borderline high: $130-159$   - High: $160-189$   - Very High: $\geq 190$

• Total Cholesterol:   - Desirable: $<200$   - Borderline high: $200-239$   - High: $\geq 240$

• HDL Cholesterol:   - Low: $<40$   - High: $\geq 60$

• Triglycerides:   - Normal: $<150$   - Borderline high: $150-199$   - High: $200-499$   - Very high: $\geq 500$

Statins (HMG-CoA Reductase Inhibitors)

• Drugs: Atorvastatin, Simvastatin, Rosuvastatin.

• Mechanism of Action:   - Inhibit HMG-CoA reductase, the enzyme catalyzing the early rate-limiting step in cholesterol biosynthesis (converting HMG-CoA to mevalonate).   - This inhibition leads to increased LDL receptor expression and reduced hepatic VLDL production due to reduced cholesterol availability.

• Efficacy: Most effective for treating dyslipidemia. High doses of potent statins (Atorvastatin, Rosuvastatin) can also reduce triglycerides.

• ADME (Absorption, Distribution, Metabolism, Excretion):   - Prodrugs: Simvastatin and Lovastatin are administered as inactive lactones and must be transformed in the liver to active $\beta$-hydroxy acids (Simvastatin acid and Lovastatin acid).   - Protein Binding: $>95\%$ protein bound, with the exception of Pravastatin.   - Peak Concentration: Achieved in $1-4\text{ hours}$.   - Half-life ($t_{1/2}$): Atorvastatin and Rosuvastatin have longer half-lives, contributing to greater efficacy.   - Elimination: Biotransformed by the liver and eliminated in feces.

• Adverse Effects:   - Hepatotoxicity: Requires baseline alanine aminotransferase (ALT) measurement.   - Myopathy: The major adverse effect; risk for myopathy and rhabdomyolysis increases with dose and plasma concentration.   - Risk Factors for Myopathy: Advanced age ($>80\text{ years}$), hepatic/renal dysfunction, multisystem disease (diabetes), and untreated hypothyroidism.

• Drug Interactions and Administration:   - Hepatic cholesterol synthesis peaks between midnight and $2:00\text{ a.m.}$ Statins with $t_{1/2} \leq 4\text{ h}$ should be taken in the evening. Atorvastatin and Rosuvastatin can be taken anytime due to long half-lives.   - Avoid concomitant use with drugs that diminish statin metabolism: Fibrates (specifically Gemfibrozil), Cyclosporine, Digoxin, Warfarin, Macrolide antibiotics, Azole Antifungals, HIV protease inhibitors, and Amiodarone.

Bile-Acid Sequestrants (Resins)

• Drugs: Cholestyramine, Colestipol, Colesevelam.

• Mechanism of Action:   - Positively charged resins bind negatively charged bile acids in the intestine.   - Large resins are not absorbed; bound bile acids are excreted in stool, depleting the pool of bile acids (normally $>95\%$ reabsorbed).   - This triggers increased hepatic bile-acid synthesis, declining cholesterol content, and stimulation of LDL receptors.

• Indications: Used as second agents if statins are insufficient; recommended for patients $11-20\text{ years}$ of age.

• Adverse Effects:   - Bloating, dyspepsia, and constipation (less likely with Colesevelam).   - Rare hyperchloremic acidosis (as they are chloride salts).   - Contraindication: Severe hypertriglyceridemia (baseline TG $>300\,mg/dL$) because resins increase hepatic TG synthesis.

• Interactions: Bind many drugs (Thiazides, Furosemide, Propranolol, Thyroxine, Digoxin, Warfarin). Administer other drugs $1\text{ h}$ before or $3-4\text{ h}$ after resins.

Ezetimibe (Cholesterol Absorption Inhibitor)

• Mechanism of Action: Inhibits cholesterol absorption by enterocytes in the small intestine by targeting the transport protein $NPC1L1$.

• Synergy: Triggers a compensatory increase in cholesterol synthesis, which is why it is often combined with statins.

• ADME: Water insoluble. Can be taken anytime. Excreted mainly in feces and $10\%$ in urine as a glucuronide conjugate.

• Contraindications: Bile-acid sequestrants inhibit ezetimibe absorption (do not co-administer). Combination with statins is contraindicated in pregnant/nursing women.

PCSK9 Targeted Therapies

• PCSK9 Monoclonal Antibodies (Alirocumab, Evolocumab):   - Mechanism: Bind PCSK9 to prevent it from degrading LDL receptors, increasing receptor recycling and hepatic LDL uptake.   - Adverse Effects: Injection site reactions, nasopharyngitis, flu-like symptoms. Evolocumab involves back pain; Alirocumab can increase liver enzymes.

• PCSK9 siRNA (Inclisiran):   - Mechanism: Small interfering RNA that inhibits hepatic PCSK9 synthesis at the mRNA level (gene silencing).   - Adverse Effects: Injection site reactions and mild flu-like symptoms; generally well-tolerated.

ATP-Citrate Lyase (ACLY) Inhibitor

• Drug: Bempedoic acid.

• Mechanism: Inhibits ACLY (upstream of HMG-CoA reductase), decreasing liver cholesterol synthesis and increasing LDL receptor expression.

• Distinguishing Feature: Activated only in the liver (not muscle), leading to a lower risk of myopathy compared to statins.

• Adverse Effects: Increases uric acid (risk for gout), rare tendon rupture, and mild elevation of liver enzymes.

Fibrates (Peroxisome Proliferator-Activated Receptor Alpha Activators)

• Drugs: Fenofibrate, Gemfibrozil, Bezafibrate, Clofibrate.

• Indications: Drugs of choice for severe hypertriglyceridemia to prevent pancreatitis.

• Mechanism of Action:   - Bind to $PPAR\alpha$ to alter gene transcription.   - Stimulate fatty acid oxidation and lipoprotein lipase (LPL) synthesis.   - Enhance clearance of VLDL and chylomicrons.   - Increase $HDL-C$ levels by stimulating $apoA-I$ and $apoA-II$ expression.

• Adverse Effects: GI side effects ($5\%$), myalgia, fatigue, headache, impotence, anemia.

• Interactions: Potentiate oral anticoagulants by displacing them from albumin. Gemfibrozil should never be given with Statins or Ezetimibe due to extreme myopathy/rhabdomyolysis risk.

• Contraindications: Renal failure, severe liver disease, children, and pregnant women. Fenofibrate is associated with gallstone formation.

Niacin (Vitamin B3)

• Mechanism of Action:   - Adipose Tissue: Inhibits lipolysis by hormone-sensitive lipase, reducing FFA transport to the liver.   - Liver: Inhibits synthesis and esterification of fatty acids, reducing VLDL production.   - HDL: Decreases hepatic uptake and catabolism of HDL, prolonging its half-life.

• Adverse Effects: flushing and pruritus (prostaglandin-mediated; alleviated by aspirin), dyspepsia, and serious hepatotoxicity.

Fish Oils (Omega-3 Fatty Acids)

• Examples: Icosapent ethyl and Omega-3-acid ethyl esters.

• Icosapent Ethyl (Vascepa): Ethyl ester of eicosapentaenoic acid (EPA). Used for severe hypertriglyceridemia ($\geq 500\,mg/dL$).

• Adverse Effects: Arthralgia. May prolong bleeding time; monitor patients on anticoagulants.

Therapies for Familial Hypercholesterolemia (HoFH)

• Lomitapide:   - Mechanism: Inhibits microsomal triglyceride transfer protein (MTP), preventing assembly of VLDL (liver) and chylomicrons (intestine).   - Adverse Effects: Diarrhea, vomiting, hepatotoxicity (steatosis/fatty liver), and reduced absorption of fat-soluble vitamins.

• Mipomersen:   - Mechanism: ApoB antisense oligonucleotide that binds $ApoB-100$ mRNA, preventing translation and hepatic VLDL production.   - Clinical Benefit: Lowers LDL even when LDL receptor function is severely impaired.

• Evinacumab:   - Mechanism: Monoclonal antibody against ANGPTL3. Blocking ANGPTL3 increases lipase activity (LPL and endothelial lipase) and enhances clearance of triglyceride-rich lipoproteins.   - Advantage: Mechanism is relatively independent of LDL receptor function.