Chiral Drug Design
History & Importance of Chiral
Drug Design
We’ve been talking about Stereoisomers….
• Isomers
– Constitutional Isomers
– Stereoisomers
•
•
•
•
Cis vs trans (EZ)
Enantiomers
Diastereomers
Meso Compounds (later next week)
Molecules with Asymmetric
Centers…
An sp3 atom, usually carbon, with 4
UNIQUE groups attached!
Asymmetric center = chiral center
CH3
Cl
H
OH
If a molecule has at least one
chiral center…
It can be an optically active
molecule.
-Enantiomers
-Diastereomers
*Meso compounds are
optically INACTIVE.
Optically Active…
Rotates the plane of polarized light clockwise (+) or
counterclockwise (-)
(+/- have no bearing on the type of
asymmetric center; we will explain types
of chiral centers in a minute!)
Chiral centers have…
“handedness”
Enantiomers are non-superimposable
mirror images!
Enantiomers
- Chiral Centers are Complete Opposites (R/S, S/R, etc)
- R/S determination after slide show!
- Identical chemical and physical properties
- Except rotation of plane-polarized light
(S)-Alanine (left) and (R)-alanine
(right)
How are Diastereomers
Different?
- Have at least 2 chiral centers – partially
opposite chirality.
- Chiral centers are PARTIAL opposites
- R,S : R, R
- R, S : S, S
- Have DIFFERENT chemical & physical
properties.
(-)Threose
(-) Erythrose
Importance For Drug Design
• The body is chiral
• Enantiomers of compounds can react
differently in the body, with greatly helpful
or harmful outcomes
“Nature has a way of knowing how to make things work.
Reactions often run in a catalytic mode, and material use,
energy, and waste are minimized. Many molecules are chiral,
and their unique handedness has both intricate and dramatic
influences on how they interact with biological systems.”
1957: Thalidomide
(Racemic Mixture)
The two enantiomers of thalidomide:
Left: (S)-thalidomide
Right: (R)-thalidomide
Thalidomide
• German drugmaker Chemie Grünenthal introduced thalidomide,
under the name Contergan, to the German market on Oct. 1, 1957
• It was a sedative to treat insomnia as well as to reduce nausea
associated with pregnancy.
• By 1960, the drug was in more than 20 countries in Europe and
Africa.
• On Nov. 18, 1961, the German paper Welt am Sonntag reported on
a study finding that pregnant women who had been taking
thalidomide were giving birth to babies with gross deformities.
• "By November 27, Grünenthal had pulled the drug off the market,
blaming the sensationalism of the press"
Frances Oldham Kelsey
• Joined the FDA in 1960
• Richardson-Merrell Inc. submitted a
New Drug Application to market
thalidomide in the U.S. under the
brand name Kevadon.
• At the time, the prevailing law was the
1938 Federal Food, Drug & Cosmetic
Act
• On March 8, 1962, Richardson-Merrell
withdrew its drug application, after
the effects were widely publicized.
Frances helped uncover…
The S-enantiomer of thalidomide,
caused the teratogenic effects (body
mutations etc).
The once believed "safe" R-isomer can
be converted to the teratogenic isomer
once metabolized human body.
1938 Federal Food,
Drug & Cosmetic Act
• required proof of safety to be submitted
to FDA before a drug could be approved
for marketing
• did not require demonstration of efficacy
• allowed "experimental" use of drugs
while approval was being sought, which
meant that a drug could be distributed
widely before it was approved
Thalidomide sparked major changes for the
FDA
“the drug approval process was under considerable
criticism, particularly the quality of the scientific data
submitted in New Drug Applications and the lack of an
efficacy requirement,’ according to Kelsey, writing in the
1996 annual report of FDA's Office of Compliance. ‘The
nature and magnitude of the thalidomide disaster,’ she
wrote, spurred the swift passage of legislation
addressing the shortcomings of the 1938 law that had
been sitting in Congress before the disaster. Known as
Kefauver-Harris Drug Amendments, they were signed
into law by President John F. Kennedy in October 1962.”
Impact on Today
• Old racemic mixture drugs are often
reassessed as single enantiomers
• Synthesis, separation and analysis of chiral
compounds has advanced greatly, and is
highly sought (chromatography)
Some Statistics…
• 2006, 80% of small-molecule drugs approved by
the FDA were chiral
• 75% were single enantiomers
• About 200 chiral compounds should enter the
market each year
• The field of asymmetric synthesis is expanding
rapidly, 50% of chiral drugs use chiral technologies
• Due to FDA policy as of 1994, decision to create
racemic mixture must be justified in quality,
safety, and efficacy, may be case-by-case
Sepracor
•
•
•
•
Located in Marlborough, MA
Founded in 1984, first major drug in 2000
Lunesta, Xopenex, Xopenex HFA, Brovana
Schering-Plough for CLARINEX; Sanofi-Aventis for ALLEGRA;
and UCB Pharma for XYZAL /XUSAL™.
• Focus on treatment of respiratory and central nervous system
disorders
• Insomnia, asthma, chronic obstructive pulmonary disease,
depression
Levalbuterol
• XOPENEX is (R)-enantiomer of albuterol
• short-acting bronchodilator for reversible
obstructive airway disease, such as asthma.
Levalbuterol caused fewer reported side effects
than albuterol.
Effects: Levalbuterol vs albuterol
Tremor, 22% vs 78%.
Nervousness, 0% vs 56%.
Palpitations, 0% vs 56%.
Tachycardia, 0% vs 44%.
Conclusion
Chiral drugs are very important, in terms of
efficacy and applications in the human
body, but they are not a universal solution,
and there are still many risks in their
development.
“Nature has a way of knowing how to make things work.
Reactions often run in a catalytic mode, and material use, energy,
and waste are minimized. Many molecules are chiral, and their
unique handedness has both intricate and dramatic influences on
how they interact with biological systems.”
• So let’s continue practicing!!!!
– Drawing our Perspective & Fischer (bow-tie on a
skelton) Projections
– Identifying type
of Chirality