Mechanism of Action (Pathogenesis)
How exactly do statins work? The entire process unfolds in hepatocytes and consists of a precise sequence of biochemical reactions:
- Enzyme Blockade. The drug competitively and reversibly inhibits HMG-CoA reductase.
- Synthesis Arrest. Normally, cholesterol is formed via the chain: acetyl-CoA $\rightarrow$ mevalonic acid $\rightarrow$ cholesterol. Statins interrupt this pathway at the stage of mevalonate formation. Intracellular cholesterol stores in the liver drop sharply.
- Compensatory Response. The liver attempts to compensate for the deficit by increasing the number (expression) of low-density lipoprotein (LDL) receptors on its cell membranes.
- Blood Clearance. Upregulation of receptors enhances receptor-mediated endocytosis: hepatocytes actively take up atherogenic LDL from the bloodstream. Additionally, the liver decreases the release of VLDL.
Outcome: A potent lipid-lowering effect (reduction of atherogenic fractions) and a concomitant increase in 'good' HDL levels.
Classification and Chemical Structure
Statins are classified by their source of origin and structural features, which directly determine their pharmacological properties.
- Natural: Lovastatin. It is isolated from the culture of the fungus Aspergillus terreus.
- Semisynthetic: Simvastatin and pravastatin (chemical derivatives of lovastatin). Their structure features a hydronaphthalene ring (responsible for binding to the enzyme) and a side chain in the form of a hydroxy acid (imitating mevalonate).
- Synthetic: Fluvastatin (a mevalonolactone derivative). It contains a fluorophenylindole moiety that binds to the coenzyme A site.
It is critically important to distinguish drugs by their pharmacological status:
- Prodrugs: Lovastatin and simvastatin. These are inactive lactones. To become active, they must undergo hydrolysis in the body and transform into active $\beta$-hydroxy acids.
- Active substances: Pravastatin and fluvastatin. They initially contain a hydroxy acid in their structure and are pharmacologically active immediately after administration.
Pharmacokinetic Features
When administered orally, statins are characterized by low bioavailability (ranging from less than 5% for lovastatin and simvastatin to 24% for fluvastatin). This is due to active hepatic uptake during the first pass (presystemic metabolism).
Food intake affects absorption differently:
- Improves: Lovastatin.
- No effect: Simvastatin.
- Worsens: Pravastatin and fluvastatin.
Penetration through histochemic barriers (blood-brain barrier, placenta) depends on the lipophilicity of the molecule. Lovastatin and simvastatin cross barriers well, whereas fluvastatin, pravastatin, and atorvastatin practically do not cross them.
Pleiotropic Effects
The anti-atherosclerotic effect of statins is not solely due to lipid reduction. They possess a number of additional (pleiotropic) properties that protect the cardiovascular system:
- Endothelial protection and vasodilation: Protection of the vascular wall from oxidized LDL and stimulation of NO synthase expression (increased nitric oxide production).
- Antithrombotic action: Reduction of thrombin levels, suppression of platelet aggregation, and activation of fibrinolysis.
- Antiproliferative effect: Inhibition of vascular smooth muscle cell proliferation (due to enzyme inhibition in angiomyocytes).
- Anti-inflammatory and anti-ischemic action (increased exercise tolerance).
Pharmacological Profile of Atorvastatin
Atorvastatin stands out as the most effective drug in the group. Despite high absorption (80%), its bioavailability is only 14% due to presystemic metabolism.
It is metabolized in the liver with the participation of the CYP3A4 isoenzyme. This results in active metabolites ($\beta$-oxidation products, ortho- and parahydroxylated derivatives) that continue to inhibit HMG-CoA reductase for 20–30 hours. The drug is excreted primarily via bile (elimination half-life is 14 hours).
Specific effects of atorvastatin:
- Stabilization and regression of atherosclerotic plaques.
- Reduction of C-reactive protein levels.
- Antiarrhythmic action and increased cardiac output (improved coronary blood flow).
- Improvement of psychological well-being through normalization of cerebral circulation.