Mednotes

L08 Clinical Pharmacology Statins

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tagsclinicalpharmacology, lecture
aliasesL08 Clinical Pharmacology Statins

21 min read

Overview

This lecture covers the pharmacological management of dyslipidaemia, working from why plasma lipids matter to how each drug class intervenes. It first reviews cholesterol biosynthesis, lipoprotein transport and the regulation of the hepatic LDL receptor, then links high LDL cholesterol to atherosclerosis and to the spectrum of arteriovascular occlusive disease. The main drug studied is the HMG-CoA reductase inhibitor class, with Atorvastatin as the example: its mechanism, its pleiotropic (non-cholesterol) actions, its pharmacokinetics, and its adverse effects and drug interactions. Other agents are provided for noting: the cholesterol uptake inhibitor ezetimibe, the PCSK9 inhibitors evolocumab and inclisiran, and the bile acid binding resins colestipol and cholestyramine. Each of these acts at a different point of the same cholesterol economy, which the closing summary diagram ties together.

Dyslipidaemia and cardiovascular risk

  • There is a high association between dyslipidaemias and cardiovascular disease, particularly in diabetes. Hypercholesterolaemia is therefore treated as a major modifiable risk factor in the development of coronary heart disease, hypertension and related conditions.
  • Dyslipidaemia contributes to progression of vascular endothelial lesions, leading to thrombotic occlusion, stroke, myocardial infarction and renal injury.
  • Cardiovascular disease and kidney disease form a vicious circle. Risk factors for CVD in patients with chronic kidney disease include oxidative stress, inflammation, dyslipidaemia, endothelial dysfunction, vascular calcification, insulin resistance and renal dysfunction.
  • Risk factors converging on the lipid abnormality of low HDL with high LDL: genetics, hypertension, smoking, poor exercise, and diet/drugs. Hypertension, with confounding factors such as diabetes, forms the most important risk factor in dyslipidaemia.

Epidemiological evidence linking cholesterol to coronary heart disease:

  • MRFIT (n = 356,222): CHD mortality rate per 1,000 rises steadily and roughly curvilinearly with serum cholesterol across the range 100 to 300 mg/dL.
  • Framingham (n = 5,209): CHD incidence per 1,000 rises across serum cholesterol bands (<204, 205-234, 235-264, 265-294, >295), with a marked increase in the highest bands, especially >295.
  • Log-linear relationship between LDL-C and relative risk for CHD: for every 30 mg/dL change in LDL-C, relative risk for CHD changes in proportion by about 30%. Relative risk is set at 1.0 for LDL-C = 40 mg/dL; the plotted range runs 40 to 190 mg/dL with relative risk 1.0 to 3.7.

Cholesterol biosynthesis

  • Cholesterol is made from acetyl-CoA by a 26 step pathway.
  • Key early steps: ATP citrate lyase acts on acetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA); HMG-CoA reductase converts HMG-CoA to mevalonate; mevalonate is then converted through further steps to cholesterol.
  • Bempedoic acid inhibits ATP citrate lyase, upstream of the statin target.
  • Full pathway as drawn in the pleiotropy slide: acetyl-CoA + acetoacetyl-CoA → HMG-CoA → (HMG-CoA reductase, the statin target) → mevalonate → 5-pyrophosphomevalonate → isopentenyl pyrophosphate → 3,3-dimethylallyl pyrophosphate → geranyl pyrophosphate → farnesyl pyrophosphate (these four are the isoprenoids) → squalene → lanosterol → cholesterol.
  • Branches off the mevalonate pathway supply dolichol, haem, ubiquinone and Ras (cellular growth and proliferation), and via farnesyl-PP and geranylgeranyl-PP reach RhoA, Rac1 and Cdc42. Cholesterol itself goes on to lipoproteins, vitamin D, bile acids and steroid hormones.

Cholesterol transport and the lipoproteins

Cholesterol and triglyceride are not readily soluble in plasma and are transported in lipoproteins. There are five classes, all containing triglyceride, cholesterol, cholesterol esters, phospholipids and apoproteins. Particles can be ranked by size and density, from small/high density (HDL) to large/low density (chylomicrons).

Functional summary:

  • HDL: reverses cholesterol transport, protects against atherosclerosis, carries 20-30% of TG.
  • LDL: most atherogenic, carries 60-70% of TG.
  • IDL: [slide does not elaborate].
  • VLDL: not involved in atherosclerosis, but is the precursor to IDL and LDL.
  • Chylomicrons: transport fatty acids and cholesterol from the diet.

Table of classes by increasing density:

ClassDiameterSource and functionMajor apolipoproteins
Chylomicrons (CM)500 nmIntestine. Transport of dietary TGB48, A, C(I,II,III), E
VLDL43 nmLiver. Transport of endogenously synthesised TGB100, C(I,II,III), E
LDL22 nmFormed in circulation by partial breakdown of IDL. Delivers cholesterol to peripheral tissuesB100
HDL8 nmLiver. Removes “used” cholesterol from tissues and takes it to liver; donates apolipoproteins to CM and VLDLA(I,II), C(I,II,III), D, E

Lipoproteins can also be grouped by their defining apoprotein: ApoB lipoproteins (VLDL, IDL, LDL) and ApoA-I lipoproteins (HDL2 and HDL3), plus the chylomicron.

Reverse cholesterol transport (HDL):

  1. Apoproteins combine with HDL.
  2. “Used”/excess cholesterol is scavenged from peripheral tissues and transferred to HDL, where it is converted to cholesterol ester.
  3. Some cholesterol ester is transferred to circulating VLDL via CETP.
  4. HDL undergoes HDL receptor mediated endocytosis by the liver; VLDL undergoes LDL receptor mediated endocytosis, contributing LDL to peripheral tissues.
  5. Cholesterol in the liver is either converted to bile salts for excretion or repackaged into VLDL for redistribution.

The LDL receptor and cellular regulation of cholesterol

Receptor mediated endocytosis of LDL:

  1. Apolipoprotein B100 on LDL binds the LDL receptor (LDL-R).
  2. Binding promotes internalisation of LDL into endocytic vesicles.
  3. Vesicles fuse with lysosomes.
  4. The lipoprotein particle content is hydrolysed into amino acids and free cholesterol.
  5. The LDL-R is recycled to the cell surface. LDL-Rs are also subject to enzymatic degradation.

Raised intracellular cholesterol has three regulatory effects on the cell:

  1. Decreases the activity of HMG-CoA reductase.
  2. Activates acetyl-CoA:cholesterol acyltransferase (ACAT), which esterifies free cholesterol into cholesteryl ester (for example cholesteryl oleate) that can be stored in the cell.
  3. Inhibits transcription of the gene encoding the LDL receptor, decreasing further uptake of cholesterol by the cell.

Hepatic LDL receptor degradation by PCSK9:

  • Hepatic LDL receptors facilitate removal of LDL cholesterol from the blood.
  • Proprotein convertase subtilisin kexin type 9 (PCSK9), made in the endoplasmic reticulum, binds to the LDL receptor (with or without LDL-C bound) and degrades it.
  • The PCSK9-LDLR complex is internalised and completely degraded in the lysosome, rather than the receptor being recycled.
  • Loss of LDL receptors means less LDL cholesterol uptake by liver cells and higher blood LDL-C levels.

Atherosclerosis

  • Raised plasma cholesterol leads to atherosclerosis: deposition of fatty material in the lumen of arteries with eventual occlusion of the artery.
  • High LDL is a major risk factor. Oxidised LDL is taken up by macrophages, causing cholesterol loading and foam cell formation, with deposition in the intima of arteries.

Sequence of events at the vessel wall:

  1. A monocyte adheres to the endothelium via adhesion molecules and migrates into the intima, where it becomes a macrophage.
  2. LDL crosses the endothelium into the intima and becomes modified LDL (with MCP-1 signalling involved).
  3. Modified LDL is taken up by macrophages together with growth factors and metalloproteinases, releasing cytokines.
  4. The macrophage becomes a foam cell.
  5. Downstream effects are cell proliferation and matrix degradation.

Stages of the arterial lesion:

  1. Normal wall (intima, media, adventitia).
  2. Fatty streak (intercellular lipid).
  3. Fibrous plaque (foam cells, lipid core, smooth muscle fibre).
  4. Complicated lesion (fibrous cap with ground substance and collagen fibres, thrombosis, ulceration, calcification, haemorrhage).

Factors associated with atherosclerosis: high cholesterol diet, high LDL, more oxidised LDL, and loss of LDL-ApoB receptors (LDL receptors), for example deficiency in familial hypercholesterolaemia, producing high circulating LDL. Low LDL-ApoB receptor numbers reduce the rate of LDL clearance, so cholesterol is “trapped” in the circulation. The system is really “designed” to function at low cholesterol.

Warning

The transcript flags that the text extracted for the “atherosclerosis associated with” slide appears truncated or incomplete, and that because the page was text only and not rendered, any additional text or diagram on it could not be checked.

Arteriovascular occlusive disease

The common denominator is endothelial dysfunction. In each case ischaemia results from reduced flow, and infarction is associated with vessel occlusion through plaque formation and thrombosis.

OrganConditionIschaemiaInfarction
HeartCoronary heart disease (CHD)Angina pectorisMyocardial infarction
BrainCerebrovascular disease (CVD)Transient ischaemic attackStroke
KidneyRenovascular disease (RVD)Renovascular hypertensionRenal impairment, renal failure
Leg musclesPeripheral vascular disease (PVD)Intermittent claudicationGangrene

Treatment targets and non-drug management

  • How low should LDL-C be lowered? Trial data (4S, CARE, LIPID, WOS, AFCAPS, each with placebo and treatment arms) plot CHD event rate against LDL-C from about 1.3 to 5.4 mmol/L. The secondary prevention trend line rises steeply through the higher LDL-C and higher event points; the primary prevention line is much flatter at lower event rates. The very low LDL-C region (about 1.3 to 2.3 mmol/L) has no trial data, so how low to go remains an open question.
  • Moderate physical activity for at least 30-60 minutes on 5 days a week or longer helps to raise HDL-C, lower total and LDL cholesterol, lower triglycerides, and lower glucose, insulin and blood pressure levels.

Classes of antihyperlipidaemic drug

Endogenous cholesterol ester attenuators:

  • Statins.
  • PCSK9 inhibitors (alirocumab, used in patients maxed out on statins).
  • Fibrates (no longer commonly used in New Zealand).

Exogenous cholesterol ester uptake inhibitors:

  • Cholesterol re-uptake inhibitors (ezetimibe).
  • Bile acid binding resins (colestipol and cholestyramine).

The main class for study is the statins (HMG-CoA reductase inhibitors, example atorvastatin). Provided for noting: ezetimibe, evolocumab and colestipol.

Statins: indications and mechanism of action

  • Statins are structural analogues of 3-hydroxy-3-methylglutaric acid (HMG).
  • They are the most effective treatment for dyslipidaemia, and are indicated as adjunctive therapy to diet in adults with all forms of hyperlipidaemia except homozygous LDL receptor deficiency.
  • Indicated for patients with existing cardiovascular disease, high 5 year CV risk, high LDL to HDL or TG to HDL ratios, and familial hyperlipidaemia.
  • Atorvastatin is given orally.

Mechanism, in order:

  1. Statins are potent, reversible, competitive inhibitors of HMG-CoA reductase, acting mainly on hepatic de novo cholesterol biosynthesis.
  2. Inhibiting hepatic cholesterol production depletes sterol pools and lowers intracellular cholesterol.
  3. Falling hepatocyte cholesterol causes protease activation, which cleaves inactive sterol regulatory element binding protein (SREBP) to active SREBP.
  4. Active SREBP enters the nucleus and binds the sterol regulatory element on the LDL-R gene, upregulating LDL receptor expression (production of HMG-CoA reductase is also “activated”).
  5. Increased LDL receptor levels increase uptake of IDL and LDL (non-HDL particles) from the plasma, including LDL and VLDL remnants via Apo B and Apo E.
  6. Result: increased LDL clearance, so decreased plasma LDL-cholesterol and triglycerides, with decreased serum VLDL remnants and serum IDL.
  7. There is also a slight increase in plasma HDL-cholesterol, due to statin inhibition of plasma CETP activity.

Pleiotropic effects of statins

Statins are described as pleiotropic drugs. Mevalonic acid, the product of the HMG-CoA reductase reaction, is the precursor not only of cholesterol but also of nonsteroidal isoprenoid compounds with adverse effects, so inhibiting HMG-CoA reductase produces effects independent of the hypocholesterolaemic action.

Mechanisms given:

  • Isoprenoids promote post-translational modification of cell-signalling proteins. Statins decrease isoprenylation of signalling molecules such as the GTP-binding proteins Ras, Rho and Rac, which inhibits activation of specific MAPKs.
  • Statins inhibit transcription factor pathways such as NFκB, reducing binding of this inflammatory transcription factor to its DNA target, which produces anti-inflammatory effects.
  • Activation of peroxisome proliferator-activated receptors in hepatocytes, skeletal muscle, macrophages and myocardium: PPARα reduces triglyceride formation and is involved in regulation of energy homeostasis; PPARγ enhances insulin sensitisation and glucose metabolism.
  • Inhibition of LDL uptake by macrophages reduces C-reactive protein release by 15-47%, by decreasing plasma LDL availability for macrophage uptake.
  • Endogenous nitric oxide release is maintained or increased to promote systemic vasodilation.
  • Many other effects are associated with this pleiotropic drug class.

Categories of pleiotropic effect shown in the hub-and-spoke figure:

  • Excitotoxicity: decreased glutamate activity.
  • Inflammation: decreased CRP and CD40, decreased MMP activity.
  • Angiogenesis: increased NO, increased EPCs.
  • Immune response: decreased MHC II, altered TH1/TH2 balance.
  • Platelet aggregation: decreased FV, FVIIa, decreased PAR and tissue factor.
  • Apoptosis: decreased IGF-1 and PDGF, decreased p21-RhoB.

Downstream targets of the isoprenoid branch include RhoA, Rac1 and Cdc42 acting on NAD(P)H oxidase (increased oxidative stress), the actin cytoskeleton, and proliferation/migration; also eNOS, t-PA, PAI-1 and ET-1, and PI3-kinase/protein kinase Akt leading to eNOS, BMP-2 and bone formation, and angiogenesis.

Atorvastatin pharmacokinetics

  • Oral dosing, peak plasma concentration within 1-2 hours.
  • Poor bioavailability, 14%.
  • Highly protein bound, 98%.
  • Extensively metabolised in the liver to active ortho- and para-hydroxylated derivatives.
  • Active metabolites are eliminated in bile after extensive first pass hepatic metabolism and CYP3A4 metabolism.
  • Half-life: atorvastatin about 14 hours; active metabolites about 25 hours.
  • Hepatic impairment increases atorvastatin retention; renal impairment has no major impact.

Comparison across the class (lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, cerivastatin, rosuvastatin, pitavastatin) covers generation (1st, 2nd, 3rd; cerivastatin withdrawn), potency as IC50 in nM, solubility (most are lipophilic; pravastatin and rosuvastatin are hydrophilic), oral absorption, protein binding, bioavailability, hepatic extraction, half-life, renal excretion, CYP450 metabolism, and SLC and ABC transporter substrate status. Atorvastatin is highlighted in the table.

Warning

The transcript flags that the statin pharmacokinetic comparison table is small and dense in the source, so some individual cell values may be imprecisely mapped to their columns; the figures were transcribed as best legible.

Statins: adverse effects and interactions

Major adverse drug reactions:

  • Elevated liver enzymes: statins may elevate the markers of liver injury aspartate aminotransferase (AST) and alanine aminotransferase (ALT), in under 1% of patients.
  • Myopathy and rhabdomyolysis: rare but significant. Breakdown of muscle fibres releases myoglobin into plasma and eventually the kidney, with risk of acute kidney injury. Presents as intense spreading myalgia and fatigue, leading to myoglobinuria, renal failure and death. Sequence: drug causes muscle injury → release of myoglobin into the bloodstream → renal tubular obstruction.
  • Cognitive impairment: mild and reversible.
  • Avoid in pregnancy. Statins are a potential teratogen, inhibiting cholesterol formation and affecting developmental genes.

Important

Adverse effect risk is dependent on the plasma concentration of statin, so anything inhibiting statin metabolism or elimination increases risk: advanced age, hepatic or renal disease, and hyperthyroidism.

Drug interactions: drugs that interfere with statin uptake into the liver (for example fibrates) or that inhibit statin metabolism by hepatic glucuronidation or by CYP3A4 and CYP2C9 (for example erythromycin). There is a major risk of drug interactions.

Drugs with the potential to interact with statins and increase myopathy risk:

  1. CYP3A4 inhibitors: protease inhibitors, macrolide antibiotics, azole antifungals, non-dihydropyridine calcium channel blockers (diltiazem and verapamil), antidepressants (nefazodone), cyclosporine, amiodarone, and drinking large amounts of grapefruit juice or cranberry juice (>250 mL per day).
  2. CYP2C9 inhibitors: fluconazole, amiodarone, fenofibrate (mild-to-moderate effect).
  3. OATP1B1 inhibitors: cyclosporine, protease inhibitors.
  4. Glucuronidation inhibitors: gemfibrozil.

Ezetimibe (cholesterol uptake inhibitor)

  • Ezetimibe impairs dietary and biliary cholesterol absorption at the enterocyte brush border, blocking NPC1L1-mediated uptake of cholesterol and plant sterols.
  • Reduced hepatic cholesterol results in LDL receptor upregulation.
  • Related enterocyte handling shown: free cholesterol is esterified by ACAT to cholesteryl ester, MTP forms chylomicrons which enter the lymph, and ABCG5/G8 effluxes sterols back into the gut lumen. In the hepatocyte, ABCG5/G8 mediates biliary secretion and atorvastatin acts on synthesis.
  • Available in combination with atorvastatin as a combined tablet.

PCSK9 inhibitors

  • Used for patients not sufficiently managed with statins, and used with statins to prevent hepatic LDL receptor destruction.
  • Evolocumab (Repatha) is a monoclonal antibody given subcutaneously; it binds circulating PCSK9 and inhibits PCSK9 binding to the LDL receptor, so the receptor is not degraded and instead recycles to the surface. Alirocumab is another PCSK9 monoclonal antibody, used in patients maxed out on statins.
  • Inclisiran is an siRNA that acts within the hepatocyte to reduce PCSK9 production.
  • Net effect: LDL receptors are rescued or recovered, so hepatic LDL uptake increases and blood LDL-C falls.

Bile acid sequestrants

  • Cholestyramine and colestipol powders, taken orally; they sequester bile acids in the gastrointestinal tract and are not absorbed.
  • Positively charged resins bind negatively charged bile, preventing bile acid reabsorption and recirculation.
  • This decreases cholesterol absorption and increases cholesterol metabolism to bile acids in the liver.
  • Consequently LDL receptor synthesis increases, LDL clearance increases and plasma LDL-cholesterol falls.
  • Used alone or in combination with statins.
  • Adverse effects: resins are confined to the gut, and can cause bloating, diarrhoea or constipation.

Summary: strategy for controlling hyperlipidaemia

Serum cholesterol is supplied by diet and by biosynthesis, and each limb has a drug target:

  • Biosynthesis is inhibited by bempedoic acid (at ATP citrate lyase, upstream) and by statins (at HMG-CoA reductase).
  • Serum cholesterol exchanges reversibly with cellular cholesterol, which links to the LDL receptor (where PCSK9 monoclonal antibodies act) and to conversion into hormones or storage as granules within cells.
  • Serum cholesterol flows to bile acids → intestine → faeces; reabsorption from the intestine back to serum cholesterol is inhibited by ezetimibe.
  • Serum cholesterol also flows to lipoprotein catabolism, which is promoted by fibrates.

Self-test

  1. Define what statins are structurally, and state their licensed indication and the one form of hyperlipidaemia they are not indicated for.
  2. Describe the steps by which atorvastatin lowers plasma LDL-cholesterol, from enzyme inhibition to LDL clearance.
  3. Explain the role of SREBP in the statin mechanism.
  4. What effect do statins have on plasma HDL-cholesterol, and by what mechanism?
  5. List the five classes of lipoprotein with the function of each.
  6. State the diameter, source and major apolipoproteins of LDL and of HDL.
  7. Describe the steps of receptor mediated endocytosis of LDL.
  8. List the three regulatory effects of raised intracellular cholesterol on the cell.
  9. Explain how PCSK9 raises blood LDL-C, and predict what happens to hepatic LDL receptor numbers if PCSK9 is inhibited.
  10. Distinguish the mechanisms of evolocumab and inclisiran.
  11. Describe the steps of reverse cholesterol transport by HDL.
  12. Describe the sequence by which LDL leads to foam cell formation in the arterial intima.
  13. List the four stages of the arterial lesion in atherosclerosis, with one feature of each.
  14. For each of the heart, brain, kidney and leg muscles, name the occlusive disease and its ischaemic and infarction presentations.
  15. What is the quantitative relationship between LDL-C and relative risk for CHD given in the lecture?
  16. Explain what “pleiotropic” means in relation to statins, and give the biochemical reason these effects occur.
  17. List four pleiotropic mechanisms of statins.
  18. Distinguish the roles of PPARα and PPARγ activation by statins.
  19. State the pharmacokinetic profile of atorvastatin: peak concentration, bioavailability, protein binding, metabolism, half-lives, and the effect of organ impairment.
  20. Describe the major adverse reactions of statins, including the pathophysiology of the muscle-related one.
  21. What patient factors increase the risk of statin adverse effects, and why?
  22. List the four categories of drugs that interact with statins to increase myopathy risk, with an example of each.
  23. Explain the mechanism of ezetimibe and where it acts.
  24. Describe the steps by which bile acid sequestrants lower plasma LDL-cholesterol, and state their main adverse effects.
  25. A patient on atorvastatin is started on a macrolide antibiotic and develops intense spreading myalgia and dark urine. Explain the mechanism linking the new drug to this presentation and the risk to the kidney.
  26. Integrative: for a patient with familial hyperlipidaemia, explain how statins, ezetimibe and a PCSK9 inhibitor each raise hepatic LDL uptake by a different route.

Answers

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