Bioltest Active Compounds
Transcript of Bioltest Active Compounds
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PRE-CLINICALBIOLOGICAL TESTING OFACTIVE COMPOUNDS
Dr Charles FOKUNANG
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Strategy for HTS Screens Run HTS Decided which compounds are active
and which are inactive Cluster the actives to put them into
series Visualize clusters of actives (showing
2D structures) and pick series of
interest Identify scaffold for each series Use similarity or substructure search on inactives
to find inactives related to these series Use SAR techniques to discover differences
between actives and inactives in a series
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n orma on genera e a eren po n s n eDrug Design process
Gene chip experiments
Project selection decisionsAssay protocols
HTS resultsSeries selection decisions
SAR studies
Protein structures
Combinatorial Expts.
PharmacophoresADME studies
Toxicology studiesScaleup reactions
Lead cmpd decisions
Clinical Trials data
Doctor/patient studies
Marketing, surveys, etc
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Information storagebreakdowns Large amounts of information generated:
Some is not kept at all
Some is kept but loses its meaning
Often datais kept, but not semanticsor decisions
e.g. keep the HVX2 assay result for this compound was3.2, but not what the assay protocol was, whether thecompound was considered active, nor whether it wasfollowed up on.
Bigger picture or derivative information is usually not
stored E.g. all the compounds with a tri-methyl group seemed to
have much lower activity for this project
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Information access breakdowns
Some information is only available in one physical
location Some information is only available within one part
of the discovery process
Often information is not contextualized for use
outside a particular domain When someone is clear about a piece of
information they need; that piece of informationexists, but they dont know how to access it.
E.g. What system to use, what Oracle table itsin, or even the knowledge of whether that pieceof information does exist!
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Missed opportunities Not a specific breakdown, but if the right piece of
information had been available at the right time, betterdecisions could have been made
E.g.
A group of compounds is being followed up as potentialdrugs, but a rival company just applied for a patent onthe compounds
A large amount of money is being spent developing anHTS assay for a target, but marketing research showsany drug is unlikely to be a success
A group of compounds is selected from an HTS as goodcandidates for follow up, but 20 years ago they werefollowed up for a similar project and had severe solubilityproblems
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Information use breakdowns
The meaning of data is incorrectlyinterpreted
A single piece of information is used, whilst
using a wider range of information wouldlead to different conclusions
Lessons learned from one project areincorrectly applied to another
Fuzzy information is taken as concreteinformation
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Genomics & Proteomics Information Handling
Understanding the link between diseases,genetic makeup and expression of proteins
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Genomics Genomics is fast-forwarding our understanding of how DNA, genes,
proteins and protein function are related, in both normal and diseaseconditions
Human genome project has mapped the genes in human DNA
Hope is that this understanding will provide many more potential protein
targets Allows potential personalization of therapies
ATACGGAT
TATGCCTAfunctions
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Gene Chips Gene chips allow us to
look for changes inprotein expression fordifferent people with avariety of conditions, andto see if the presence of
drugs changes thatexpression
Makes possible the designof drugs to targetdifferent phenotypes
compounds administered
people / conditions
e.g. obese, cancer,caucasian
expression profile
(screen for 35,000
genes)
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DRUG DISCOVERY:FINDING A LEAD
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Drug discovery: Finding a lead
When a pharmaceutical company oruniversity research group initiates a new
medicinal chemistry project through to the
identification of a lead compound, they willconsider the following steps in order:
1-Choosing the disease
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Drug discovery: Finding a lead
2-Choosing a drug target Drug targets
Discovering drug targets
Target specificity and selectivity betweenspecies
Target specificity and selectivity within
the body Targeting drugs to specific organs andtissues
Pitfalls
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Drug discovery: Finding a lead
3-Identifying a bioassay Choice of bioassay In vitro test
In vivo tests Test validity High-through screening Screening by NMR Affinity screening Surface Plasmon resonance Scintillation proximity assay
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Drug discovery: Finding a lead 4-Finding a lead compound
Screening of natural products (the plantkingdom, the microbial world, the marineworld, animal sources, venoms andtoxins)
Medical folklore
Screening synthetic compound libraries
Existing drugs
Starting from natural ligand or modulator(natural ligands for receptors, naturalsubstrates for enzymes, enzymeproducts as lead compounds, natural
modulators as lead compounds)
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Drug discovery: Finding a lead
4-Finding a lead compound
Combinatorial synthesis
Computer aided design
Serendipity and prepared mind
Computerized searching ofstructural databases
Designing lead compounds by NMR
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Drug discovery: Finding a lead
5-Isolation and purification
6-Structural determination
7-Herbal medicine
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I- Choosing the disease
Pharmaceutical companies tend to concentrate ondeveloping drugs for diseases which are prevalent
in developed countries, and aim to produce
compounds with better properties than existing
drugs.
Pharmaceutical companies have to consider
economic factors as well as medical ones when
they decide which disease to target whendesigning a new drug.
A huge investment has to be made towards the
research and development of a new drug.
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I- Choosing the disease
Therefore, companies must ensure that they get a goodfinancial return for their investment.
As a result, research projects tend to focus on diseasesthat are important in the developed world, because it is thebest market for new drugs.
Thus, research is carried out on ailments such as migraine,depression, ulcers, obesity, flu, cancer and cardiovasculardisease.
Less is carried out on the tropical diseases of the developedworld. Only when such diseases start to make an impact inricher countries, the pharmaceutical companies sit up andtake notice.
Example: research in antimalarial drugs has increased due toincrease in tourism to more exotic countries and the spread
of malaria into southern states of US.
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II- Choosing a drug target
Choosing which disease to tackle is usually amatter for companys market strategists. The
science becomes important at the next stage.
A molecular target is chosen which is believedto influence a particular disease when
affected by a drug.
The greater the selectivity that can beachieved, the less chance of side effects.
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II- Choosing a drug target1- Drug targets
Once a therapeutic area has been identified the next stage is to identify
a suitable drug target (e.g. receptor, enzyme or nucleic acid)
Understanding which biomacromolecules are involved in a particular
disease state is very important.
This will allow the medicinal chemist whether agonist or antagonist to be
designed for a particular receptor or whether inhibitors should be
designed for a particular enzyme.
For example, tricyclic antidepressants such asDesipramineare known to
inhibit the uptake of NA from nerve synapses. However, these drugs also
inhibit uptake of serotonin, so the possibility arose that inhibiting
serotonin uptake might be beneficial. A search for selective serotonin
uptake inhibitors has led to the discovery ofFluoxetine, the best selling
antidepressant.
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II- Choosing a drug target2- Discovering drug targets
If a drug or a poison produces a biological effect, there must be a
molecular target for that agent in the body.
In the past, the discovery of drug targets depends on finding the drug
first. Then, natural chemical messengers started to be discovered.
But many targets still stay hidden (orphan receptors i.e,novel receptors
whose endogenous ligand is unknown) and their chemical messengers are
also unknown.
The challenge is to find a chemical that will interact with these targets in
order to find their function and whether they will be suitable as drug
targets. This is one of the main driving forces behind the rapidly expanding
area of Combinatorial synthesis (synthesis of a large number of compounds
in a short period of time using different reagents and starting material
and are tested for activity.)
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II- Choosing a drug target3-Target specificity and selectivity betweenspecies
Target specificity and selectivity is a crucial factor in modern
medicinal chemistry research
The more the selective a drug is for its target, the less chance thatit will interact with different targets and have less undesirable side
effects.
For example, penicillin target an enzyme involved in bacterial cell
wall biosynthesis. Mammalian cells does not have a cell wall, so this
enzyme is absent in human cells and penicillin has few side effects.
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II- Choosing a drug target4-Target specificity and selectivity
within the body Selectivity is also important for drug acting on targets within the body
Enzyme inhibitors should only inhibit the target enzyme and not some other
enzyme.
Receptors agonist/ antagonist should ideally interact with a specific kind of
receptor (adrenergic receptor) rather than a variety of different
receptors, or even a particular receptor type ( such as - receptor) or even
a particular receptor subtype 2- receptor.
Ideally, enzyme inhibitors should show selectivity between the various
isozymes of an enzyme.
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II- Choosing a drug target5-Targeting drugs to specific organs
and tissues Targeting drugs against specific receptor subtypes often allows
drugs to be targeted against specific organ or against specific
areas of brain.
This is because the various receptor subtypes are not uniformly
distributed around the body, but are often concentrated in
particular tissues. For example, adrenergic receptors in the heart
are predominantly1 while those in the lungs are 2. If a drug acts on
either, less side effects would be observed.
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II- Choosing a drug target6-Pitfalls
The body is a highly complex system. It is
possible to identify whether a particular enzyme
or receptor plays a role in a particular aliments.
For any given function, there are usually several
messengers, receptors, and enzymes involved in
the process
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II- Choosing a drug target6-Pitfalls
For example, there is no one simple cause forhypertension, there are variety of receptors and
enzymes which can be targeted in its treatment. These
include 1-adrenoceptors, calcium ion channels,
angiotessin-converting enzyme (ACE), and potassium ion
channels.
Sometimes, more than one target may need to be
addressed for a particular ailment. For example, most ofthe current therapies for asthma involve a combination
of bronchodilator (2 agonist) and an anti-inflammatory
agent such as a corticosteroid
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III-Identifying a bioassay1-Choice of bioassay
Choosing the right bioassay or test system is crucial tothe success of a drug research program.
The test should be simple, quick and relevant as there
are usually a large number of compounds to be analyzed. Human testing is not possible at such early stage, so
the test has to be done in vitro first. Because in vitrotests are cheaper, easier to carry out, less
controversial and can be automated than in vivo one. In vivo tests needed to check the drugs interaction
with specific target and to monitor theirpharmacokinetics properties.
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III-Identifying a bioassay2-In vitro tests
They do not involve live animals. Instead, specific
tissues, cells, or enzymes are isolated and used.
Enzyme inhibitors can be tested on pure enzyme in
solution.
Receptor agonist and antagonists can be tested on
isolated tissues or cells. Antibacterial drugs are tested in vitro by measuring how
effectively they inhibit or kill bacterial cells in culture
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III-Identifying a bioassay3-In vivo tests
In vivo tests on animals often involve inducing a clinical
condition in the animal to produce observable symptoms.
The animal is then treated to see whether the drugalleviates the problem by eliminating the observable
symptoms. For example, the development of non-
steroidal inflammatory drugs was carried out by inducing
inflammation on test animals.
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III-Identifying a bioassay3-In vivo tests
The animals used may be transgenic i.e,some mouse
genes are replaced by human genes so the mouse
produces the human receptor or enzyme. Or the mouses
gene may be altered to be susceptible for some disease
such as breast cancer.
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III-Identifying a bioassay3-In vivo tests
There are several problems associated with in vivo
testing. It is slow and it also causes animal suffering.
There are also many problems of pharmacokinetics and
the result obtained may be misleading. For example,
penicillin methyl ester is hydrolyzed in mice into active
penicillin, while it is not hydrolyzed in humans or rabbits.Also, thalidomide is teratogenic in rabbits and humans
while it is not in mice.
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III-Identifying a bioassay4-Test validity
Sometimes the validity of testing procedure is easy and
clear. For example, the antibacterial drug can be tested
by its effect on killing bacteria. Local anaesthetics are
tested by their effect on blocking action potential in
isolated nerve.
In other cases, the testing procedure is more difficult.For example, there is no animal model for antipsychotic
drug.
Thus, validity of the test should be carried out.
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III-Identifying a bioassay5-High throughput screening (HTS)
HTS involves the miniaturization and automation of in
vitro tests such that a large number of tests can be
carried out in a short period of time.
It involves testing of large number of compounds versus
a large number of targets. The test should produce
easily measurable effect. This effect may be cellgrowth, an enzyme catalyzed reaction which produces a
color change (may be a dye) or displacement of
radioactive labelled ligand from its receptors.
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III-Identifying a bioassay6-Screening by NMR
NMR was used as a tool for determining the molecular structures of
compounds
Recently, compounds can be tested or screened for their affinity to
a macromolecular target by NMR spectroscopy. The relaxationtimes of ligands bound to a macromolecule are shorter than when
they are unbound (cant be detected).
In NMR spectroscopy the compound is radiated with a short pulse
of energy which excites the nuclei of specific atoms (H,N,C)
afterwards, the excited nuclei slowly relax back to the ground state
giving off energy as they so.
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III-Identifying a bioassay6-Screening by NMR
There are, several advantages in using NMR as adetection system:
1-It is possible to screen 1000 small molecular weightcompounds a day with one machine.
2-The method can detect weak binding which would bemissed by conventional screening methods.
3-It can identify the binding of small molecules todifferent regions of binding site.
4-It is complementary to HTS. The later may give false-positive results, but these can be checked by NMR toensure that the compounds concerned are binding in thecorrect binding site.
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III-Identifying a bioassay6-Screening by NMR
5-The identification of weakly binding molecules allows
the possibility of using them as building blocks for the
construction of larger molecules that bind more strongly. 6-Screening can be done on a new protein without
needing to know its function.
NMR screening also has limitations, the main one being
that at least 200 mg of the protein required.
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III-Identifying a bioassay7-Surface Plasmon resonance (SPR) &
scintillation proximity assay (SPA)
SPR (change in refractive index)& SPR (reduction of
emission of light) are two visual methods of detecting
whether ligands bind to macromolecular targets .
IV Fi di l d d
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IV-Finding a lead compound
Once a target and a testing system have been chosen, the next
stage is to find a lead compound. A lead compound is a compound
which shows the desired pharmaceutical activity.
The level of the activity may not be very great and there may
be undesirable side effects.
The lead compound provides a start for the drug design and
development process.
There are various ways in which a lead compound might be
discovered. However, the following are the ways of discovering
the lead compound:
1-Screening of natural products (the plant kingdom, themicrobial world, the marine world, animal sources, venoms
and toxins)
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IV-Finding a lead compound
2-Medical folklore 3-Screening synthetic compound libraries
4-Existing drugs (Me too drugs & Enhancing the sideeffects)
5-Starting from natural ligand or modulator (naturalligands for receptors, natural substrates for enzymes,enzyme products as lead compounds, natural modulatorsas lead compounds)
6-Combinatorial synthesis 7-Computer aided design
8-Serendipity and prepared mind
9-Computerized searching of structural databases
10-Designing lead compounds by NMR
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IV-Finding a lead compound1-Screening of natural products
Natural products are a rich source of biologically activecompounds.
Many of todays medicines are either obtained directlyfrom a natural source or were developed from a leadcompound originally obtained from a natural source.
The compound responsible for that activity is known asthe active principle.
Most biologically active natural products are secondarymetabolites with quite complex structures. This hasadvantage in that they are extremely novel compounds.
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IV-Finding a lead compound1-Screening of natural products
But the disadvantage of their complexity makes their synthesisdifficult and the compound needs to be extracted from itsnatural source (i.e. costly & inefficient process).
As a result, there is a need to design simpler analogues of the
lead compounds . Natural products can be obtained from different sources such
as:
1-The plant kingdom: It is rich source of lead compounds(e.g. morphine, cocaine, digitalis, quinine, tubocurarine, nicotineand muscarine, paclitaxel (Taxol, recent anticancer), either usefuldrugs as morphine or basis for synthetic ).Plants continue toremain a promising source of new drugs.
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IV-Finding a lead compound1-Screening of natural products
2-The microbial world: microorganisms such as bacteria and fungiare rich for lead compounds (e.g. Antgimicrobial Drugs: pencillins,
cephalosporines, tetracyclines, aminoglycosides, chloramphenicol,
rifamycins).
3-The marine world: coral, sponges, fish and marine
microorganisms have biological potent chemicals, with interesting,
anti-inflammatory, antiviral, and anticancer activity. E.g Curacin A
(anti-tumour, from marine cyanobacterium) 4-Animal sources: antibiotic peptides were extracted from the
skin of African clawed frog.
Epibatidine (potent Analgasic) was also obtained from Ecuadorian
fro .
F d l d d
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IV-Finding a lead compound1-Screening of natural products
5-Venoms and toxins: from animals, plants,snakes, spiders, scorpions, insects andmicroorganisms. They are potent because theyhave specific interaction with a macromoleculartarget in the body. Thus, they provide importanttools in studying receptors, ion channels, and
enzymes. e.g. Teprotide (from venom of viper) was the lead
compound for the development of
antihypertensive agents Cilazapril & Captopril
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IV-Finding a lead compound2- Medical folklore
Berries, leaves and roots used by local healer or shaman
as medicines. Many are useless or dangerous and if they
work this may be due to Palcebo Effect.
Some of these extracts indeed have a real effect. (e.g.
quinine (cinchona), reserpine (Rauwolfia), atropine
(atropa beladona), morphine (opium poppy), digitalis
(foxglove), emetine (ipeca), cocaine (coca).
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IV-Finding a lead compound3-Screening synthetic compounds
(libraries) Thousands of compounds have been synthesized . The
majority of these compounds are not used or not been in
the market. They have been stored in compound
libraries, and are still available for testing.
Pharmaceutical companies screen their library to study
a new target and find a lead compound
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IV-Finding a lead compound
4-Existing drugs A) Me too drugs: Many companies use established drugs
from their competitors as a lead compound in order to
design a drug. By modifying the structure in such way that
avoids the patent restrictions, retain the activity, andimproved the therapeutic properties.
For example i) Captopril (Anti-hypertension) used as lead
compound by different companies to produce their ownanti-hypertension drugs.
ii) Modern penicillins are more selective, more potent and
more stable than original penicillins
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IV-Finding a lead compound
4-Existing drugs B) Enhancing a side effect: An existing drug may have a
minor or undesirable side effect, which might be used in
another area of medicine. And such compound could be a
lead compound on the basis of its side effects.
The aim is to enhance the desired side effect and to
eliminate the major biological activity.
e.g. Sulfonamides are Antibacterial agents but some
sulfonamides has convulsive side effect due tohypoglycaemia effect. This, undesirable side effect was
useful in the development sulfonamides drugs for
treatment of diabetes (e.g.antidiabetic sulfonyl urea,
Tolbutamine).
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IV-Finding a lead compound5- Starting from the natural ligands or
modulator A) Natural ligands for receptors:
Natural ligands of a target has been sometimeused as the lead compound.
E.g. Adrenaline and noradrenaline (naturalneurotransmitters) were used fordevelopement adrenergic -agonists such as
Salbutamol, dobutamine, xamoterol, H2antagonists as cimetidine, and morphine(led toopiate receptors, and endogenous
opiates:endorphins and enkephalins.
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IV-Finding a lead compound5- Starting from the natural ligands or modulator
B) Natural substrates forenzymes: The natural substrate for an enzyme
can be used as the lead compound inthe design of enzyme inhibitors.
e.g. enkephalines used as a lead
compound to design an inhibitor ofenkephalinases.
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IV-Finding a lead compound5- Starting from the natural ligands or
modulator C) Enzyme products as lead compounds: enzymes catalyze a
reaction in both directions ,so enzyme products can be used as a lead
compound for an enzyme inhibitor e.g. L-benzyl succinic acid inhibit
enzyme catalyzed carboxy peptidase hydrolysis of peptides.
D) Natural modulators as lead compounds:the natural or
endogenous chemicals that exert allosteric control of receptor or
enzymes called Modulators and can be also as lead compounds.
e.g. Benzodiazepines: were discovered to modulate the receptor -
aminobutyric acid (GABA) by binding to allosteric binding site then
endogenous endozepines were discovered.
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IV-Finding a lead compound
6-Combinatorial synthesis Combinatorial synthesis is automated solid-phase
procedure aimed at produce as many as different
structures as possible in short time as possible.
The reactions are carried out on very small scale, often
in a way that will produce mixtures of compounds.
Combinatorial synthesis aims to mimic what plants do, i.e.
produce a pool of chemicals.
One of these compounds may be prove to be a useful lead
compound.
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IV-Finding a lead compound7-Computer aided design
Knowledge of target binding site aids in design of novel
compounds intended to bind with that target.
The enzyme and receptors can be crystallized and it is
possible to determine their structure (structure of
protein & binding site) by X-ray crystallography.
Molecular modelling software programs can be used to
study the binding site and to design drugs.
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IV-Finding a lead compound7- Serendipity and the prepared mind
Lead compounds are found as a result of serendipity (i.e.chance)
e.g. i) Cisplatin (Anti-cancer) & peniciilins
ii) Development of propanolol (-blocking) haveunexpected give a benefit of discover Practolol.
Propanolol is a -blocker but it is a lipophilic drug and canenter CNS and cause side effect, by introducinghydrophilic amide group inhibit passage the blood-brainbarrier and Practolol produced more selectivecardioselective 1 inhibitor with fewer side effects onCNS.
Sulfonamides and tolbutamide
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IV-Finding a lead compound7- Serendipity and the prepared mind
Workers in TNT factories always complained fromheadache due to dilatation of brain blood vessels.TNT was the basis to prepare nitro derivatives
which were used in angina to dilate coronary bloodvessels and alleviate pain. Mustard gas tanks used in second world war
exploded in italian harbor. They discovered thatpersons who survived and inhaled this gas lost their
defense against microorganisms due to destructionof white blood cells.This led to the discovery of mustard like drugs
which were used in leukemia to inhibit excessiveproliferation of white blood cells.
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IV-Finding a lead compound9-Computerized searching of
structural databases
New lead compounds can be found by carrying out
computerized searches of structural databases.
In order to carry out such search, it is necessary to know
the desired pharmacophore.
Data base searching is known as database mining.
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IV-Finding a lead compound10-Designing lead compounds by NMR
Recently NMR spectroscopy has been used to design a
lead compound rather than to discover one.
The method sets out to find small molecules
(epitopes) which can bind to specific binding site.
Lead discovering by NMR can be applied to proteins
of known structure which are labeled with N15.
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V-Isolation and purification
If a lead compound is present in a mixture of other compounds it
has to be isolated and purified.
The isolation and purification depends upon structure, stability,
and quantity of the compound.
e.g. Fleming recognized penicillin, qualities & non-toxic to human
but could not use it clinically because he was unable to purify it.
He could isolate it in aqueous solution, but when he tried to remove
water the drug was destroyed.
Purification and isolation of penicillins were possible until
development of new experimental procedure such as freeze-drying
and chromatography.
6 St t d t i ti
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6-Structure determination
X-ray crystallography, NMR spectroscopy, mass,
and IR are important in structure
deterimination.
7 H b l di i
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7-Herbal medicines
Herbal medicines contain a large variety of different
compounds.
Several of these may have biological activity, but there
is a significant risk of side effects and toxicity. The
active principle present in small amount, so herbals areexpected to be less active than pure compound.
Herbal medicines may be interacting with prescribed
medicines and there is no regulations or control of thismatter and their uses.
But it is an important lead to discover and design new
drugs.
Summary
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A lead compound is a structure which shows a useful pharmacological
activity and can act as the starting point for drug design. Natural products are a rich source of lead compounds. The agent
responsible for biological activity of a natural extract is known as
the active principle.
Lead compound have been isolated from plants, trees,microorganisms, animals, venoms, and toxin. A study of medical
folklore indicates plants and herbs which may contain novel lead
compounds.
Lead compounds can be found by screening synthetic compounds
obtained from combinatorial syntheses and other sources.
Existing drugs can be used as a lead compounds for design of novel
structures in the same therapeutic area. Alternatively, the side
effects of an existing drug can be enhanced to design novel drugs in
a different therapeutic area.
Summary
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Preclinical StudiesADME/PK
Objective: To study the effects of test materialswith respect to absorption, distribution, metabolism,and excretion
Duration: hours to days
Animals Required: typically 2 species (rodent andnon-rodent)
Safety Pharmacology Objective: To investigate undesirable
pharmacological effects of the test material
Duration: Usually single dose
Animals Required: 2 species (rodent and non-rodent)
Core battery: Cardiovascular, Respiratory, CNS
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Preclinical StudiesAcute Toxicity
Objective: To determine Maximum Tolerated Dose(MTD) and No Observable Effect Level (NOEL)
Duration: Typically 14 days after single dose
Animals Required: 2 species (rodent and non-
rodent)
Parameters:
Mortality Clinical pathology Gross necropsy Weight change Clinical observations
Points to consider:
Dose selection for repeat dose studies
Choice of Species (Fialuridine)
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Preclinical StudiesSub Acute Toxicity
Objective: To determine toxicity after repeatedadministration of the test material
Duration: 14 28 days
Animals Required: 2 species (rodent and non-rodent)
Parameters: Mortality Clinical pathology Urinalysis Histology Weight change Clinical obs
Points to consider: Dosing regimen similar to clinical Recovery period Duration of clinical trials (Phase I, II, III) Toxicokinetics Immunotoxicity
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Preclinical Studies
Subchronic/Chronic Toxicity Objective: In support of products used to
treat chronic conditions
Duration: 30 days to 2 years
Animals Required: 2 species (rodent andnon-rodent)
Parameters: Mortality Clinical pathology
Clinical obs Behavioral Assessment
Histology Weight change
Points to consider: Clinical Trials (EU)
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Preclinical StudiesCarcinogenicity
Objective: To evaluate the tumorigenic potential inanimals and risk to humans
Duration: 12 months +
Species: Mouse or Rat
Parameters: Tumor development Clinical pathology Clinical observations and assessment
Points to consider: Considerations from:
Pharmacology, Pharmacokinetic or Toxicology(mechanistic in vitro and in vivo) data Structure-activity relationships Compound accumulation over long-term use Continuous use in humans for 6 months +
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Summary The natural ligand, substrate, product, or modulator for a
particular target can act as a lead compound. The ability to crystallize a molecular target allows the use
of X-ray crystallography and molecular modeling to designlead compounds which will fit the relevant binding site.
Serendipity has played a role in the discovery of new lead
compounds. Knowledge of an existing drugs pharmacophore allows the
computerized searching of structural databases toidentify possible new lead compounds which share thepharmacophore.
NMR spectroscopy can be used to identify whether smallmolecules (epitopes) bind to specific region of a bindingsite. Epitopes can be optimized then linked together togive a lead compound.
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Summary If a lead compound is present in a natural extract or a
combinatorial synthetic mixture, it has to be isolated andpurified such that its structure can be determined. X- raycrystallography and NMR spectroscopy are particularimportant in structure determination.