Transcript Drug Discovery and Development

Drug Discovery and
Development
How are drugs discovered and
developed?
Basic Steps
• Choose a disease
• Choose a drug target
• Identify a “bioassay”
bioassay = A test used to determine biological
activity.
• Find a “lead compound”
“lead compound” = structure that has some activity
against the chosen target, but not yet good
enough to be the drug itself.
• If not known, determine the structure of the “lead
compound”
• Synthesize analogs of the lead
• Identify Structure-Activity-Relationships (SAR’s)
Basic Steps (cont.)
• Structure-Activity-Relationship (SAR) = How does
the activity change as structure is systematically
altered?
• Identify the “pharmacophore”
pharmacophore = the structural features directly
responsible for activity
• Vary structure to improve interactions with target
• Improve pharmacokinetic properties.
pharmacokinetic = The study of absorption,
distribution, metabolism and excretion of a drug
(ADME).
Basic steps (cont.)
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•
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Patent the drug
Study drug metabolism
Test for toxicity
Design a manufacturing process
Carry out clinical trials
Market the drug
Choosing a Disease
• Pharmaceutical companies must make a profit to
exist
• Pharmaceutical companies will, therefore, avoid
products with too small a market (i.e. a disease
which only affects a small subset of the
population)
• Pharmaceutical companies will also avoid
products that would be consumed by individuals
of lower economic status (i.e. a disease which
only affects third world countries)
Choosing a Disease (cont.)
• Most research is carried out on diseases
which afflict “first world” countries: (e.g.
cancer, cardiovascular diseases,
depression, diabetes, flu, migraine,
obesity).
Identifying a Drug Target
• Drug Target = specific macromolecule,
or biological system, which the drug will
interact with
• Sometimes this can happen through
incidental observation…
Identifying a Drug Target (cont.)
• Example: In addition to their being able to inhibit the uptake
of noradrenaline, the older tricyclic antidepressants were
observed to “incidentally” inhibit serotonin uptake. Thus, it was
decided to prepare molecules which could specifically inhibit
serotonin uptake. It wasn’t clear that this would work, but it
eventually resulted in the production of fluoxetine (Prozac).
HO
NH2
N
N
CH3
H3C
Imipramine
(a classical tricyclic antidepressant)
N
H
serotonin
F3C
HN
O
prozac
The mapping of the human
genome should help!
• In the past, many medicines (and lead compounds) were
isolated from plant sources.
• Since plants did not evolve with human beings in mind,
the fact that they posses chemicals which results in
effects on humans is incidental.
• Having the genetic code for the production of an
enzyme or a receptor may enable us to over-express that
protein and determine its structure and biological
function. If it is deemed important to the disease
process, inhibitors (of enzymes), or antagonists or
agonists of the receptors can be prepared through a
process called rational drug design.
Simultaneously, Chemistry is Improving!
• This is necessary, since, ultimately, plants
and natural sources are not likely to provide
the cures to all diseases.
• In a process called “combinatorial chemistry”
large numbers of compounds can be
prepared at one time.
• The efficiency of synthetic chemical
transformations is improving.
Selectivity is Important!
• e.g. targeting a bacterial enzyme, which
is not present in mammals, or which has
significant structural differences from
the corresponding enzyme in mammals
The Standards are Being Raised
• More is known about the biological
chemistry of living systems
• For example: Targeting one subtype of
receptor may enable the pharmaceutical
chemist to avoid potentially troublesome
side effects.
Problems can arise
• Example: The chosen target, may over
time, lose its sensitivity to the drug
• Example: The penicillin-binding-protein
(PBP) known to the the primary target of
penicillin in the bacterial species
Staphylococcus aureus has evolved a
mutant form that no longer recognizes
penicillin.
Choosing the Bioassay
• Definitions:
– In vitro: In an artificial environment, as in a test
tube or culture media
– In vivo: In the living body, referring to tests
conductedin living animals
– Ex vivo: Usually refers to doing the test on a
tissue taken from a living organism.
Choosing the Bioassay (cont.)
In vitro testing
• Has advantages in terms of speed and requires
relatively small amounts of compound
• Speed may be increased to the point where it is
possible to analyze several hundred compounds
in a single day (high throughput screening)
• Results may not translate to living animals
Choosing the Bioassay (cont.)
In vivo tests
• More expensive
• May cause suffering to animals
• Results may be clouded by interference
with other biological systems
Finding the Lead
Screening Natural Products
• Plants, microbes, the marine world, and
animals, all provide a rich source of
structurally complex natural products.
• It is necessary to have a quick assay for the
desired biological activity and to be able to
separate the bioactive compound from the
other inactive substances
• Lastly, a structural determination will need to
be made
Finding the Lead (cont.)
Screening synthetic banks
• Pharmaceutical companies have
prepared thousands of compounds
• These are stored (in the freezer!),
cataloged and screened on new targets
as these new targets are identified
Finding the Lead (cont.)
Using Someone Else’s Lead
• Design structure which is similar to existing
lead, but different enough to avoid patent
restrictions.
• Sometimes this can lead to dramatic
improvements in biological activity and
pharmacokinetic profile. (e.g. modern
penicillins are much better drugs than original
discovery).
Finding the Lead (cont.)
Enhance a side effect
O
H2N
S
NH2
O
sulphanilamide
(an antibacterial with the side effect of
lowering glucose levels in the blood and also
diuretic activity)
N
Cl
O
O
S
NH
O
NH
tolbutamide
(a compound which has been optimized to only
lower blood glucose levels. Useful in the treatment
of Type II diabetes.)
O
H2N
NH
S
S
O
O
O
Chlorothiazide
(a compound which has been optimized to only display diuretic
activity.)
Finding the Lead (cont.)
Use structural similarity to a natural ligand
NH2
N(CH3)2
H
N
HO
H3C
S
O
N
H
5-Hydroxytryptamine (5-HT)
Serotonin (a natural neurotransmitter
synthesized in certain neurons in the CNS)
O
N
H
Sumatriptan (Imitrex)
Used to treat migrain headaches
known to be a 5-HT1 agonist
Finding the Lead (cont.)
Computer-Assisted Drug Design
• If one knows the precise molecular structure of
the target (enzyme or receptor), then one can
use a computer to design a perfectly-fitting
ligand.
• Drawbacks: Most commercially available
programs do not allow conformational
movement in the target (as the ligand is being
designed and/or docked into the active site).
Thus, most programs are somewhat inaccurate
representations of reality.
Finding a Lead (cont.)
Serendipity: a chance occurrence
• Must be accompanied by an experimentalist
who understands the “big picture” (and is not
solely focused on his/her immediate research
goal), who has an open mind toward
unexpected results, and who has the ability to
use deductive logic in the explanation of such
results.
• Example: Penicillin discovery
• Example: development of Viagra to treat
erectile dysfunction
Finding a Lead (cont.)
Sildenafil (compound UK-92,480) was synthesized by a group of
pharmaceutical chemists working at Pfizer's Sandwich, Kent research
facility in England. It was initially studied for use in hypertension (high
blood pressure) and angina pectoris (a form of ischaemic
cardiovascular disease). Phase I clinical trials under the direction of Ian
Osterloh suggested that the drug had little effect on angina, but that it
could induce marked penile erections. Pfizer therefore decided to
market it for erectile dysfunction, rather than for angina. The drug was
patented in 1996, approved for use in erectile dysfunction by the Food
and Drug Administration on March 27, 1998, becoming the first pill
approved to treat erectile dysfunction in the United States, and offered
for sale in the United States later that year. It soon became a great
success: annual sales of Viagra in the period 1999–2001 exceeded $1
billion.
Wikipedia
Finding a Lead (cont.)
O
N
N
N
NH
O
O
S
O
N
N
viagra
(Sildenafil)
Structure-Activity-Relationships (SAR’s)
• Once a lead has been discovered, it is important to
understand precisely which structural features are
responsible for its biological activity (i.e. to identify the
“pharmacophore”)
• This may enable one to prepare a more active molecule
• This may allow the elimination of “excessive” functionality,
thus reducing the toxicity and cost of production of the
active material
• This can be done through synthetic modifications
• Example: R-OH can be converted to R-OCH3 to see if O-H
is involved in an important interaction
• Example: R-NH2 can be converted to R-NH-COR’ to see if
interaction with positive charge on protonated amine is an
important interaction
Metabolism of Drugs
• The body regards drugs as foreign substances,
not produced naturally.
• Sometimes such substances are referred to as
“xenobiotics”
• Body has “goal” of removing such xenobiotics
from system by excretion in the urine
• The kidney is set up to allow polar substances to
escape in the urine, so the body tries to
chemically transform the drugs into more polar
structures.
Metabolism of Drugs (cont.)
• Phase 1 Metabolism involves the
conversion of nonpolar bonds (eg C-H
bonds) to more polar bonds (eg C-OH
bonds).
• A key enzyme is the cytochrome P450
system, which catalyzes this reaction:
RH + O2 + 2H+ + 2e– → ROH + H2O
Metabolism of Drugs (cont.)
• Phase II metabolism links the drug to still
more polar molecules to render them even
more easy to excrete
UDP Glucuronic Acid
Glucuronic Acid
HO
O
HO
O
O
P
O
P
O
O
O
NH
O
glucuronosyltransferase
enzyme
N
HO
OH
OH
O
O
HO
R
O
HO
HO
O
O
HO
OH
OH
OH
More easily excreted than ROH itself
R
OH
Drug
Metabolism of Drugs (cont.)
• Another Phase II reaction is sulfation
(shown below)
NH2
N
O
O
R
OH
S
O-
N
O
O
P
O
N
O
O-
SO3-
N
R
Drug
O
O
P
OH
O-
O-
3'-Phosphoadenosine-5'-phosphosulfate
O
Sulfated Drug
(more easily excreted)
Manufacture of Drugs
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Pharmaceutical companies must make a profit to continue to exist
Therefore, drugs must be sold at a profit
One must have readily available, inexpensive starting materials
One must have an efficient synthetic route to the compound
– As few steps as possible
– Inexpensive reagents
• The route must be suitable to the “scale up” needed for the production
of at least tens of kilograms of final product
• This may limit the structural complexity and/or ultimate size (i.e. mw)
of the final product
• In some cases, it may be useful to design microbial processes which
produce highly functional, advanced intermediates. This type of
process usually is more efficient than trying to prepare the same
intermediate using synthetic methodology.
Toxicity
• Toxicity standards are continually becoming
tougher
• Must use in vivo (i.e. animal) testing to screen for
toxicity
– Each animal is slightly different, with different metabolic
systems, etc.
– Thus a drug may be toxic to one species and not to
another
Example: Thalidomide
Thalidomide was developed by German pharmaceutical
company Grünenthal. It was sold from 1957 to 1961 in almost
50 countries under at least 40 names. Thalidomide was
chiefly sold and prescribed during the late 1950s and early
1960s to pregnant women, as an antiemetic to combat
morning sickness and as an aid to help them sleep. Before its
release, inadequate tests were performed to assess the drug's
safety, with catastrophic results for the children of women who
had taken thalidomide during their pregnancies.
Antiemetic = a medication that helps prevent and control
nausea and vomiting
Example: Thalidomide
From 1956 to 1962, approximately 10,000 children were born with
severe malformities, including phocomelia, because their mothers had
taken thalidomide during pregnancy. In 1962, in reaction to the tragedy,
the United States Congress enacted laws requiring tests for safety
during pregnancy before a drug can receive approval for sale in the U.S.
O
N
O
NH
O
O
Thalidomide
Phocomelia presents at birth very short or absent long bones
and flipper-like appearance of hands and sometimes feet.
Example: Thalidomide
Researchers, however, continued to work with the drug. Soon after its
banishment, an Israeli doctor discovered anti-inflammatory effects of
thalidomide and began to look for uses of the medication despite its
teratogenic effects. He found that patients with erythema nodosum
leprosum, a painful skin condition associated with leprosy, experienced
relief of their pain by taking thalidomide. Further work conducted in 1991 by
Dr. Gilla Kaplan at Rockefeller University in New York City showed that
thalidomide worked in leprosy by inhibiting tumor necrosis factor alpha.
Kaplan partnered with Celgene Corporation to further develop the potential
for thalidomide. Subsequent research has shown that it is effective in
multiple myeloma, and it is now approved by the FDA for use in this
malignancy. There are studies underway to determine the drug's effects on
arachnoiditis, Crohn's disease, and several types of cancers.
Teratogenic = Causing malformations in a fetus
Clinical Trials
• Phase I: Drug is tested on healthy volunteers to
determine toxicity relative to dose and to screen for
unexpected side effects
• Phase II: Drug is tested on small group of patients
to see if drug has any beneficial effect and to
determine the dose level needed for this effect.
• Phase III: Drug is tested on much larger group of
patients and compared with existing treatments and
with a placebo
• Phase IV: Drug is placed on the market and patients
are monitored for side effects