Statins: Powerful
Inhibitors
of
Cholesterol Biosynthesis
Cholesterol: What is
1
it?
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
H O
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.
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.
1.
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!!!!
Biological



1
Role:
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.
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
The Mevanolate
2
Pathway
The biosynthesis of cholesterol and isoprenoids (a group of
compounds responsible for cell fluidity and cell proliferation)
HO
C
H2C
CH2
C

O
C
H2C
O
SCoA
O
geranyl pyrophosphate
farnesyl pyrophosphate
squalene
CH3
CH2
C
isopentenyl pyrophosphate
HMG-CoA
HMG-CoA
Reductase
2NADP+
+ HSCoA
HO

C
O
O
2NADPH
5-pyrophosphomevalonate
CH3
H2
C OH
2,3-oxidosqualene
mevalonate
19 steps
HO
HO
lanosterol
cholesterol
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
β
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
Target: HMG-CoA Reductase
(HMGR)

The enzyme that catalyzes
the conversion of HMG-CoA
to mevanolate.
HO
C
H2C

This reaction is the ratedetermining step in the
synthetic pathway.
3-hydroxy-3-methylglutaryl-coenzyme A
(HMG-CoA)
CH3
CH2
C

O
C
H2C
O
HMG-CoA
HMG-CoA
Reductase
2NADP+
+ HSCoA
HO

SCoA
O
O
2NADPH
C
CH3
CH2
C
O
H2
C OH
mevalonate
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, ML236B 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
Statins

ML-236B was later called compactin(6-demethylmevinolin 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).
Ester side-chain
lovastatin
compactin
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.
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
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 hydroxybearing carbon in the lactone)
i. making ether side-chain
analogs5
ii.


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
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.
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.
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.
References
1.
2.
3.
4.
5.
6.
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).
Endo, A.; Kuroda, M.; Tsujita, Y. J. Antibio. (Tokyo) 1976, 29, 1346-1348.
Istvan, E. S. American Heart Journal 2002, 144, S27-32.
Lee, T. J.; Holtz, W. J.; Smith, R. L.; Alberts, A. W.; Gilfillan, J. L. J Med Chem
1991, 34, (8), 2474-7.
Lee, T. J. H., W. J.; Smith, R. L. Journal of Organic Chemistry 1982, 47, (24), 4750.