STATINS ANTI-CHOLESTEROL AGENTS

1. Patrick: An Introduction toMedicinalChemistry 4e Case Study 1 STATINS ANTI-CHOLESTEROL AGENTS

2. 1. Cholesterol • Notes • Important in biosynthesis and cell membrane structure • Excess cholesterol leads to cardiovascular disease • Fatty molecule transported round blood supply by low-density and high-density lipoproteins (LDLs and HDLs) • LDLs carry cholesterol to cells • HDLs carry cholesterol from cells to liver • Mortality is associated with high levels of LDLs or low levels of HDLs • Cholesterol can cause fatty plaques in arteries leading to a risk of artherosclerosis, clot formation, stroke and heart attack

3. Target enzyme 2. Target for statins • Notes • Inhibit biosynthetic pathway to cholesterol • Prevent synthesis of cholesterol within cells but not from diet • Target the enzyme catalysing the rate limiting step in the biosynthetic pathway

4. 3. Catalytic mechanism • Notes • Involves two hydride transfers • Two molecules of cofactor required (NADPH)

5. 3. Catalytic mechanism Substrate binding

6. 3. Catalytic mechanism • Notes • Lys, His, Glu and Asp are involved in reaction mechanism • Histidine acts as acid catalyst • Lysine stabilises negatively charged oxygen of mevaldyl-CoA and transition state leading to it • Lowers activation energy for first step

7. 3. Catalytic mechanism • Notes • Glutamic acid acts as an acid catalyst • Aspartate residue stabilises uncharged Glu-559 and charged Lys-691

8. Compactin (Mevastatin) IC50 = 23 nM 4. Identification of a Lead Compound • Notes • Screening of compounds produced by microorganisms • Microbes lacking HMGR might produce HMGR inhibitors to inhibit microbes having HMGR - chemical warfare • Compactin (Mevastatin) isolated from Penicillium citrinum • 10,000 higher affinity for enzyme than substrate • Never reached the market

9. Lovastatin IC50 = 24 nM Simvastatin IC50 = 24 nM Pravastatin IC50 = 1900 nM 5. Type I Statins • Notes • Lovastatin isolated from Aspergillus terreus • First statin to be marketed (Merck; 1987) • Revolutionised treatment of hypercholesterolaemia • Simvastatin introduced in 1988 as semi-synthetic analogue of lovastatin • Pravastatin derived from compactin by biological transformation (1991)

10. 5. Type I Statins • Notes • General structure of type I statins contains a polar head and a hydrophobic moiety including a decalin ring • Lovastatin and simvastatin are prodrugs where lactone ring is hydrolysed to give the polar head

11. * * * * * * * * = asymmetric centres * 5. Type I Statins • Disadvantages of Type I statins • Various side effects • Difficult to synthesise • Large number of asymmetric centres

12. Fluvastatin IC50 = 28 nM Atorvastatin IC50 = 8 nM Cerivastatin IC50 = 10 nM 6. Type II Statins • Notes • Synthetic agents • Contain larger hydrophobic moiety with no asymmetric centres • Easier to synthesise • Fluvostatin (1994), atorvastatin (1997), cerivastatin (1998), rosuvastatin (2003)

13. Rosuvastatin IC50 = 5 nM Pitavastatin IC50 = 6.8 nM 6. Type II Statins • Notes • Structures share a number of similar features (‘me too drugs’) • Rosuvastatin is the most potent - related to sulfonamide group • Cerivastatin is the most hydrophobic • Pravastatin and rosuvastatin are the least hydrophobic

14. 6. Type II Statins • Notes • Statins with lower hydrophobic character target liver cells and have lower side effects • Less hydrophobic statins do not cross cell membranes easily • Liver cells have transport proteins for statins whereas other cells do not • Majority of cholesterol synthesis takes place in liver cells • Side effects thought to be due to inhibition of HMGR in other cells such as muscle cells • Common side effect is myalgia (muscle pain) • Rhabdomyolysis = severe muscle toxicity which can be fatal • Cerivastatin withdrawn in 2001 due to rhabdomyolysis and 50 fatalaties

15. = 7. Statins - Mechanism of action • Notes • Competitive inhibitors of HMGR • Polar head group mimics the natural substrate (HMG-SCoA) • Same binding interactions for polar head group as natural substrate • Hydrophobic moiety forms additional binding interactions • Binds more strongly than natural substrate, but does not undergo reaction - no leaving group

16. 7. Statins - Mechanism of action • Notes • Statins are closer mimics of the first reaction intermediate mevaldylCoA than the substrate • Statins likely to bear a resemblance to the transition state for the first stage of the reaction mechanism • Can be viewed as transition-state analogues

17. 8. Statins - Binding interactions • Notes • Polar head group binds in similar manner to substrate • Hydrophobic moiety does not bind to the pocket for SCoA • Enzyme is flexible and alters shape to accommodate statins • Hydrophobic pocket is created to bind the hydrophobic moiety

18. methylethyl substituent 8. Statins - Binding interactions Methylethyl substituent of Type II statins binds to same region as decalin ring of type I statins

19. fluorophenyl substituent 8. Statins - Binding interactions • Arg-590 forms important polar interaction with fluorophenyl substituent • Planar guanidium group is also stacked over the phenyl ring

20. amide group 8. Statins - Binding interactions • Amide group forms an additional hydrogen bonding interaction with Ser-565 • Additional interaction not formed with other statins other than rosuvastatin

21. sulfone group 8. Statins - Binding interactions • Notes • Rosuvastatin forms additional H-bonding interactions • Sulfone oxygen forms a hydrogen bonding interaction with Ser 565 • Sulfone group also interacts uniquely with Arg-568 • Explains why rosuvastatin is most potent statin • Sulfone group important for binding as well as selectivity

22. 9. Other mechanisms of action for statins • Notes • Statins inhibit HMGR in liver cells • Lowers the levels of cholesterol in liver cells • Causes an increase in the synthesis of hepatic LDL receptors • Increases the number of LDL receptors in the cell membrane of liver cells • Increases the amount of LDL-cholesterol cleared from the plasma • Crucial to the effectiveness of statins