High throughput screening is a type of assay. By this assay we can identified the target or binding site of drugs. Its mainly performed during the drug discovery process.

1. HIGH THROUGHPUT SCREENING
MAHAMMAD SHAKHEEL B
PHARMACOLOGY DEPARTMENT

2. •HTS IS A METHOD FOR SCIENTIFIC EXPERIMENTATION
ESPECIALLY USED IN DRUG DISCOVERY AND RELEVANT TO
THE FIELDS OF BIOLOGY AND CHEMISTRY.
•DATA PROCESSING, AND CONTROL SOFTWARE, LIQUID
HANDLING DEVICES, SENSITIVE DETECTORS- HTS ALLOWS
A RESEARCHER TO QUICKLY CONDUCT CHEMICAL,
GENETIC AND PHARMACOLOGICAL TESTS.
•THE RESULTS OF THESE EXPERIMENTS PROVIDE FOR
DRUG DESIGN AND ROLE OF BIOCHEMICAL PROCESS IN
BIOLOGY.
High throughput screening

3.  Assays for HTS and validation play a key role in drug discovery and
development.
 Currently, most HTS assays are rely on the use of fluorescent and radioactive
labels to messure bichemical reactions or monitor cellular responses.
 Lables have adverse effect on binding interaction being investigated leading
to false conclusion about binding properties of analyte
 Lables also introduce additional complexity and assay develoment time to the
procees.

4. Commonly used terms in drug discovery
High throughput screen: an optimised, miniaturised assay format that
enables the testing of > 100,000 chemically diverse compounds per day.
Assay: a test system in which biological activity can be detected
Hit: a molecule with confirmed concentration-dependent activity in a screen,
and known chemical structure. The output of most screens

5.  Progressible hit: a representative of a compound series with activity via
acceptable mechanism of action and some limited structure-activity
relationship information
 Lead: a compound with potential (as measured by potency, selectivity,
physico-chemical properties, absence of toxicity or novelty) to progress to a
full drug development programme
 Pharmacophore: minimal structure with essential features for activity

6. Drug discovery

Initial characterisation
trials

Regulatory approval sought to start
trials in humans
Pre-clinical 
Clinical trials Phases I, II, III

Submission of marketing/manufacturing
authorisation application to regulatory authorities
or refuse) licences

Product goes on sale

Post-marketing surveillance
Regulatory authorities review
information and grant (
Library of compounds

In vitro screening: human/animal
receptor/enzyme assay; reporter system

Hits/lead

Biochemical, tissue or animal model of
function

lead

Animal model of therapeutic target

ADME, formulation, acute toxicology
The life history of a successful drug

7. High throughput screening for drug discovery
FACT 1: recent understanding of disease mechanisms has dramatically
increased no. of protein targets for new drug treatment
FACT 2: new technologies have increased the no. of drugs that can be tested
for activity at these targets.
high throughput screening (HTS) is 1° tool for early-stage drug discovery

8.  HTS is process by which large nos. of compounds are rapidly tested for
their ability to modify the properties of a selected biological target.
 Goal is to identify ‘hits’ or ‘leads’
- affect target in desired manner
- active at fairly low concs (∴ more likely to show specificity)
- new structure
 The greater the no. and diversity of compounds screened, the more
successful screen is likely to be.
 HTS = 50,000-100,000 cpds screened per day!!!

9. Goals and limitations of HTS
Aim of screening is to find progressible hits, not to discover
the lead molecule itself
The majority of drug targets are
a) G-protein coupled 7 TM receptors
b) nuclear receptors
c) ion channels
d) enzymes
progressible hit

targeted synthetic design

Lead

10.  Take top 100 drugs - 18 bind to GPCR
- 10 bind to nuclear receptors
- 16 bind to ion channels
- most of remainder inhibit enzymes
 Knowledge gained from one drug target can be transferred to related targets.
 e.g molecular technology required to work with 1 GPCR is useful for other GPCRs, including
cloning and expression systems and info on structure and ligands.

11. Implementation of HTS
Need 4 elements:
1) suitable libraries of compounds
Source of chemicals for screen:
- in-house collection (5x105
- 106
) of diverse samples.
- supplement by acquisitions from specialist companies
-combinatorial chemistry allows synthesis of large no of diverse molecules.
2) assay method configured for automation
Assay requirements:
a) pharmacology of the target should not be altered by the molecular
manipulations

12. b) cost of assay development and reagents low
c) easy to use and suitable for automation and miniaturisation. Use multi-well
plates: 96, 192, 384, 864, 1536 and assay requiring few manipulations, no
plate-o-plate transfers or washing steps
d) robust signal-to-noise ratio. Hit defined as activity above a certain threshold
e.g. Ki < 5 nM
Emax >30% increase over basal
e) ideally be non-radioactive
 Often express target genes in appropriate host systems
e.g. bacterial, yeast, viral, invertebrate and mammalian cells.

13. a) Radioligand binding assays
• Measures affinity of library compounds for target.
• Need high affinity radioligand that binds to site of
interest and cells transfected with target site.
• measure competitive displacement of radioligand from target site
• Specificity can be assessed by including other possible targets in screen
relative affinity for multiple sites
PROBLEM: hard to miniaturise radioactive assays (counting takes too long).
Alternative is to use fluorescence techniques.

14. b) Cell-based fluorescence and radiotracer assays
Useful for measuring ion-channel function
e.g.measure movement of Ca2+
in a fluorescent-imaging plate-reader (FLIPR)
• cells are loaded with the fluorescent Ca2+
indicator Fluo-3
• depolarisation with high KCl activates Ca2+
channels and allows Ca2+
entry
Ca2+
Ca2+
-60 mV
fluorescence
signal
488 nm
Ca2+
Ca2+
-30 mV
increased
fluorescence
488 nm
(a) rest (b) with KCl

15. c) melanophore assays
Melanophores = pigmented cells derived from neural crest. Prepare
immortalised melanophores from Xenopus laevis
Gi
melanophores
transfection
CMV Hß2-AR
Plasmid vector
AGGREGATED
MELANOSOMES

melatonin
DISPERSED
MELANOSOMES

light
Gs, Gq
ß agonist
ß antagonist

16. d) Reporter gene assays
Rather than measure the immediate cellular response, it may be easier to
measure the subsequent transcriptional change
isoprenaline binding to ß-adrenceptors

 cAMP

PKA activation and translocation to nucleus

phosphorylation of transcription factor CREB that recognises cAMP response
elements (CREs)

 expression of reporter gene whose transcription is driven by an enhancer
containing CREs

Measure reporter gene product in HTS format

17. • Examples of reporter genes: ß-galactosidase; luciferase; alkaline phosphatase;
green fluorescent protein.
• Useful for measuring responses from Gi, Gs or Gq-coupled receptors

18. All screens have danger of false negatives and false positives
Not such a problem waste time and resources
HTS is less useful for evaluating - bioavailability
- pharmackinetics
- toxicity
- absolute specificity
3) robotics workstation
• Robots handle assays in multi-well formats.
- sample dilutions
- sample dispensing
- plate washing

19.  Hard to automate cell lysis or permeabilisation steps (necessary for many 2nd
messenger responses).
• Full automation allows 24 h continuous operation without requiring shift work.
• More efficient and economical.
4) computerised data handling system
•A great deal of data is generated. Must be accurate and reproducible.
Need good computerised data handling systems.

20. Which strategy is best for hit identification?
When a target is identified, a decision has to be made about which chemicals to
screen, in order to identify potential lead compounds.
Random screening
All possible drug molecules screened against target.
Estimated no. of possible drug molecules is ± 1040
!!!
This is simply not possible.

21.  Focussed screening
 A limited number of compounds are pre-selected for screening.
 Has proved successful as a hit generation strategy.
 Useful when 3D structure of target is known (e.g. crystal structure of a
receptor)
 - use computer modelling to predict optimal structure to interact with target
 - use known ligand to construct 3D pharmacophore
 In either case, select compounds from library or design new compounds and
screen.
 Focussed screening will find novel hits BUT the required information may
not be available.

22. Choice of expression system for GPCRs
• Ligand binding and transduction properties are effected by cell type, receptor
density, and presence of signal transductional elements
ADVANTAGES
Yeast-based systems e.g. Saccharomyces cerevisiae
(also used to express protein tyrosine kinases, peptide hormones and functional ion channels)
- grow rapidly
- easy and cheap
- readily amenable to genetic manipulation
- tolerant to solvents used to dissolve drugs
e.g.DMSO, methanol
Bacterial cells e.g. E.coli
- grow rapidly
- easy and cheap
DISADVANTAGES
- May not get functional coupling of transfected GPCRs to
yeast G-proteins. Overcome this by coupling GPCRs to yeast
pheromone response pathway  agonists promote cell growth
or activity of reporter gene construct
- may not get proper post-tranlational modification e.g. many
GPCRs are not glycosylated in yeast, or cell surface
expression.
- Low expression levels
- Some post-translational modifications not performed
- no endogenous G-proteins

23. ADVANTAGES
Baculovirus/insect cell system e.g. Sf9 cells
- high levels of expression of functional protein
- ligand binding properties similar to native
receptor
- can co-express receptor and mammalian
G-proteins
- ideal for structural analysis
Mammalian cells e.g. CHO, HEK
- most authentic background for expression of GPCRs
• protein synthesis
• membrane insertion
• post-translational modification
• lipid composition of membranes
- receptors usually functional
DISADVANTAGES
- production of permanent cell lines may be a
difficult and lengthy process
- can be expensive
- receptor promiscuity (different G-proteins activated
by 1 receptor  multiple signals)
- no. of suitable G-proteins may be limited
- GPCR might not be glycosylated
- GPCR is occasionally inactive

24. Yeast-based screen and selection for identification of agonists for a human orphan
GPCR
Yeast
ligand
Yeast
receptor
Yeast G
protein
arrest


human
ligand
human
receptor
human G
protein
growth


(a) In yeast, normal GPCR-initiated pathway
results in cell cycle arrest in response to
agonist
(b) Substitution of human orphan GPCR and
human Gα protein yields a strain that only grows if
receptor is stimulated
Spot individual cpds onto lawn of modified
yeast. Detect agonists by growth of yeast on the
plate around the site of cpd application

25. orphan
receptor
peptide
growth
peptide
expression
library
peptide
ligand


(c) alternatively, introduce a random peptide expression library; cells will only grow if the peptide stimulates the
receptor

26. REFERENCE
 Short protocols in Pharmacology and Drug discovery edited by Enna S
J, et al.
 Drug discovery and evaluation Pharmacological assy by Vogel H G and
Vogel H G.
 Modern methods of Drug Discovery by Hillisch, A and Hilgenfeld.

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