Statins and Their Mechanism of Action

Statins are a group of cholesterol-lowering drugs that function as enzyme inhibitors.
They dominate the cholesterol-lowering drug market, with significant profits for producers.
Examples include atorvastatin ($7 billion revenue) and simvastatin ($5.3 billion revenue) in 2002.

Cholesterol is essential for cell membranes and steroid hormone production.
Excess dietary cholesterol can cause cardiovascular disease.
Cholesterol is transported in blood via lipoproteins:
- LDL (Low-Density Lipoprotein): Carry cholesterol from liver to tissues; associated with plaque formation in arteries.
- Particle size: ~22 nm diameter; mass of ~3 million Daltons.
- HDL (High-Density Lipoprotein): Carry cholesterol from tissues back to the liver.
- Size: 8–11 nm diameter; higher protein content than LDL.
High LDL or low HDL levels increase risk of atherosclerosis, heart attacks, and strokes.

HMGR (3-hydroxy-3-methylglutaryl-coenzyme A reductase) is the key target enzyme in cholesterol biosynthesis.
Inhibited by statins to lower cholesterol synthesis.
The HMGR reaction:
- Converts HMG-CoA to mevalonate using NADPH.

HMGR consists of four subunits; regulated by cholesterol levels in three ways:
- Inhibition via phosphorylation when cholesterol is high.
- Transcription/translation of HMGR is regulated by cholesterol levels.
- Degradation rate influenced by cholesterol.

Active site interactions involve:
- Lys-735 interacts with carboxylate of HMG-CoA.
- Ser-684 and Asp-690 interact with alcohol group via hydrogen bonds.
His-866 acts as an acid catalyst for substrate reaction.
Glu-559 provides a proton for final reduction stage.

Initial statin discovery focused on microbial compounds as potential HMGR inhibitors.

Compactin (mevastatin): First potent statin, isolated in the 1970s but not marketed due to toxicity concerns.
Lovastatin: Developed by Merck, became the first marketed statin in 1987.
Other Type I statins include simvastatin and pravastatin.
Common structures include polar head group and hydrophobic decalin ring.
Type I statins can cause significant side effects and are complex to synthesize.

Synthetic statins, such as fluvastatin and atorvastatin, have larger hydrophobic moieties that are easier to synthesize.
Atorvastatin became the highest-selling drug, generating high revenue for Pfizer.
Lower hydrophobicity in statins (like pravastatin and rosuvastatin) leads to increased liver selectivity and decreased side effects.

Statins function as competitive inhibitors, mimicking natural substrates like HMG-SCoA.
They bind more strongly due to additional hydrophobic interactions and resist enzyme reactions due to structural differences.
Statins resemble mevaldyl-CoA, acting as transition-state analogues.

Binding studies show flexibility in HMGR allows statins to alter the binding site, increasing their inhibition effectiveness.
Type I versus Type II Binding Differences:
- Comparing statins reveals differences in binding moieties affecting their interaction with HMGR.
- Atorvastatin and rosuvastatin form unique hydrogen-bonding interactions with HMGR, enhancing their binding.

Statins not only inhibit HMGR but also increase the synthesis of hepatic LDL receptors, enhancing LDL cholesterol clearance.

Inhibiting late-stage enzymes in cholesterol biosynthesis can lead to toxic substrate accumulation, whereas inhibiting HMGR prevents toxic build-up, making it a safer target.