Statins: Powerful Inhibitors of Cholesterol Biosynthesis

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Statins Powerful Inhibitors of Cholesterol
Biosynthesis
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Cholesterol What is it?1
Cholesterol is a fatty steroid made primarily in
the liver of most animals and humans. It is an
integral component in the synthesis of hormones,
can also be found in cell walls of animals and
humans. Isolated cholesterol is a white, flaky
solid that is insoluble in aqueous environments.
Cholesterol
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Two types of transportation for cholesterol

• In order to transport the steroid through
  blood, cholesterol is attached to a set of
  proteins called lipoproteins. There are two types
  of lipoproteins high density and low density
  lipoproteins.
• HDL High-density lipoproteins collect
  cholesterol particles as they travel through
  blood vessels and deposits them in the liver
  where they are transferred to bile acids and
  disposed off.
• LDL Low-density lipoproteins deposits on the
  walls of blood vessels, and over time, builds up
  into cholesterol plaque and blocks blood vessels,
  especially arteries that feed blood to the heart.
  
• 1. The liver manufactures, secretes and
  removes LDL cholesterol from the body. To remove
  LDL cholesterol from the blood, there are special
  LDL receptors on the surface of liver cells.
• 2. LDL receptors remove LDL
  cholesterol particles from the blood and
  transport them inside the liver. A high number of
  active LDL receptors on the liver surfaces is
  necessary for the rapid removal of LDL
  cholesterol from the blood and low blood LDL
  cholesterol levels.
• A deficiency of LDL receptors is associated
  with high LDL cholesterol blood levels.
• Diets that are high in cholesterol diminish
  the activity of LDL receptors!!!!

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Biological Role1

• It is an important component of cell linings
• It helps in the digestion of lipids
• It is a key component in the building of hormones

• Hypercholestraemia High blood cholesterol
• Usually a result of high LDL/low HDL cholesterol
  levels
• Leads to
• narrowing of artery walls (atherosclerosis)
• decreased blood and oxygen supply to heart
• heart attack
• death
• Coronary heart disease1 Leading cause of death
  in western
• countries.

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Initial treatment of hypercholesteraemia was
directed toward limiting LDL-cholesterol levels
through
Low-cholesterol diet and regular
exercise. Exercise burns fat so it is not
coverted to cholesterol which the Body will
have to dispose off.

• This approach was not very successful because
  high blood
• cholesterol is also hereditary (Familial
  Hypercholestraemia (FH))1 and a chronic
  condition. People with FH have defective or
  nonexistent LDL receptors and need rigorous,
  long-term treatment.
• Scientific Approach
• Know and understand how the body makes
  cholesterol
• Find a way to effectively control cholesterol
  levels with
• minimum adverse effects

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The Mevanolate Pathway2

• The biosynthesis of cholesterol and
  isoprenoids (a group of compounds responsible for
  cell fluidity and cell proliferation)

5-pyrophosphomevalonate
isopentenyl pyrophosphate
geranyl pyrophosphate
farnesyl pyrophosphate
squalene
2,3-oxidosqualene
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In 1976..

• ML-236A, ML-236B, ML-236C metabolites isolated
  from a fungus (Penicillium citrinum) were found
  to reduce serum cholesterol levels in rats.
• This work was done by Akira Endo, Masao Kuroda
  and Yoshio Tsujita at the Fermentation Research
  Laboratories, Tokyo, Japan.3

Preliminary experiments showed that these fungal
metabolites had no effect on mevanolate or other
steps in the biosynthetic pathway. This led to
the speculation that their action was somewhere
between the mevanolate and the HMG-CoA
ß
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Target HMG-CoA Reductase (HMGR)

• The enzyme that catalyzes the conversion of
  HMG-CoA to mevanolate.
• This reaction is the rate-determining step in the
  synthetic pathway.

3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA)
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RESULTS

• Rats received oral dose of test compounds (5
  mg/kg suspended in 0.5 mL of saline)
• Control group received 0.5 mL of saline
• Of the 3 substances tested, ML-236B had the
  highest level of hypocholesterolemic activity.
• Amounts required for 50 inhibition
• ML-236A 0.18 µg/mL
• ML-236B 0.01 µg/mL
• ML-236C 0.08 µg/mL

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Statins

• ML-236B was later called compactin(6-demethylmevin
  olin or mevastatin). A related fungal metabolite
  called lovastatin (mevinolin) was also found to
  be another good inhibitor of HMG-CoA reductase.
  Lovastatin was isolated from Aspergillus terreus.

Today, there are two classes of statins Natural
Statins Lovastatin(mevacor), Compactin,
Pravastatin (pravachol), Simvastatin
(Zocor). Synthetic Statins Atorvastatin
(Lipitor), Fluvastatin (Lescol). Statins are
competitive inhibitors of HMG-CoA reductase.
They are bulky and literally get stuck in the
active site. This prevents the enzyme from
binding with its substrate, HMG-CoA.
Ester side-chain
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Making the synthetic statins

• Lovastatin and compactin can be made in the lab
  in multistep syntheses.
• This allowed scientists to study the
  structural-activity relationship of statins. The
  lactone was found to be the business end of the
  drugs.4

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Modification of Lovastatin

• Since statins are competitive inhibitors, an
  increase in the amount of HMG-CoA will reduce the
  effectiveness of the drugs.
• New drug design approaches are geared towards
  making lovastatin analogs that will have longer
  interaction with the enzyme increase duration of
  drug occupancy of active site.
• Structural modification i. making ether
  side-chain analogs
• (Lee, et. al. 1982) ii. homologation
  of the lactone ring
• iii. converting lovastatin to
  
  mevanolate analog (changing
  stereochemistry at the hydroxy-
  bearing carbon in the lactone)

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i. making ether side-chain analogs5
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1. homologation of the lactone ring6

• Purpose is to develop a lactone homolog that is
  compatible with the complex and sensitive
  structural features of lovastatin.
• As in the case of making the ether analogs, the
  hydroxy-bearing carbon had to be protected

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• iii. converting lovastatin to mevanolate analog
  (placing a methyl group at the hydroxy-bearing
  carbon in the lactone)6

• The hydroxy-bearing carbon in HMG-CoA and
  mevanolate have a methyl group. This substituent
  is lacking in lovastatin
• Purpose is to investigate the biological
  consequence of this methyl group
• 16 and 11 are epimers diastereomers that differ
  in configuration at only one stereogenic center.

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Results

• Mevanolate and lactone modifications no
  biological test and results have been report.
• Results from ether analogs (Lee, et. al. in
  1991)5
• i. The ethers were tested against their ester
  analogs
• ii. Compactin was used as standard and assigned
  a relative potency of 100

In vitro HMG-CoA reductase inhibitory
activity showed that absence of the carbonyl has
detrimental effect on the inhibitory
strength. General conclusion side-chain ether
analogs are weaker inhibitors of HMGR than their
Corresponding ether analog. The role of the
ester group in the synthetic pathway is still
under investigation.
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Conclusions

• Coronary heart disease, a condition caused by
  hypercholestraemia is a major leading cause of
  death in most western countries.
• The discovery of natural statins (lovastatin and
  compactin) lead to innovative approaches to
  treatment of high cholesterol.
• These natural statins have also served as
  templates for making synthetic statins, most of
  which are on the market today.
• With understanding of the SAR of statins and
  their interactions with HMGR (bonding nature,
  etc), we can improve the effectiveness of these
  drugs and limit side-effects.

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References

• Lee, D. Cholesterol and the heart.
  http//www.medicinenet.com/cholesterol/ (Sept
  2004).
• Diwan, J. J. Cholesterol Synthesis.
  http//www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/
  part1/cholesterol.htm (Sept 2004).
• 3. Endo, A. Kuroda, M. Tsujita, Y. J. Antibio.
  (Tokyo) 1976, 29, 1346-1348.
• 4. Istvan, E. S. American Heart Journal 2002,
  144, S27-32.
• 5. Lee, T. J. Holtz, W. J. Smith, R. L.
  Alberts, A. W. Gilfillan, J. L. J Med Chem 1991,
  34, (8), 2474-7.
• 6. Lee, T. J. H., W. J. Smith, R. L. Journal of
  Organic Chemistry 1982, 47, (24), 4750.