Contributors

Accuracy
Closeness of the agreement between the result of a measurement and a true value of the property being measured.

Active
A sample that produces a response or signal above a defined threshold at the tested concentration in a single assay or screen and that has not yet been confirmed by subsequent experiment. 
See: INACTIVE
Note:  When the properties and identity of an ACTIVE sample are confirmed by subsequent experiment it becomes a HIT.  Use of ACTIVE and HIT as synonyms is inaccurate.

Activity
The response to a test sample measured in an assay.

Activity distribution
A plot or graphical representation of the number of samples present in each activity range. Often shown as a population bar chart, it is used to provide an overview of the screening results and typically allows the determination of the overall BACKGROUND signal and threshold for selection of ACTIVE samples.

Affinity
A descriptive non-quantitative term indicating the relative tendency of one molecule to associate with another.
Note: Often misused to refer to the dissociation equilibrium constant, Kd measured for a LIGAND, or Ki for an INHIBITOR.

Agonist
An agonist is an endogenous substance or a drug that can interact with a receptor and initiate a physiological or a pharmacological response characteristic of that receptor (contraction, relaxation, secretion, enzyme activation, etc.).
A‘full’ agonist induces maximal RECEPTOR response for the biological system in question, while ‘partial’ agonist does not.
See also PARTIAL AGONIST, INVERSE AGONIST, ANTAGONIST.

Alphascreen
A HOMOGENEOUS luminescence assay in which the extent of interaction between two biomolecules is measured.  The biomolecules (eg receptor/ligand or antibody/antigen pair) are conjugated to donor and acceptor beads.  Laser excitation of a photosensitiser on the donor bead produces singlet oxygen, which emits luminescence upon migration to the acceptor bead.
Note: proprietary to Perkin Elmer.

Antagonist
A drug or a compound that opposes the physiological effects of another. At the receptor level, it is a chemical entity that opposes the receptor-associated responses normally induced by another bioactive agent.

Artifact
An experimental result which is not a manifestation of the phenomenon under investigation, but is brought about erroneously by the particular arrangement of instrument and method.
See FALSE POSITIVE and FALSE NEGATIVE.

Assay
1. An experimentally controlled biochemical or biological system used for the quantitative analysis of perturbations imposed by a test sample.
2. A set of operations having the object of determining the value of a quantity.
In analytical chemistry, this term is synonymous with measurement.

Assay control, positive
Experimental conditions designed to produce the maximum signal in an assay.
Typically determined with a reference test compound.
Note: synonymous with “high” control.

Assay control, negative
Experimental conditions designed to produce the minimum signal in an assay.
It is typically the signal measured in the absence of a test compound and is relevant to the determination of the BACKGROUND signal.
Note: synonymous with “low control.

Assay format
Description of an assay in which the plate type (96-well, 384-well, etc.) and assay type (fluorescence, luminescence, etc.) are defined.

Assay type, absorbance
An assay in which response is determined by the detection of light absorption by an assay component. The concentration of the absorbing species can be quantified using the Beer-Lambert Law (or Beer-Lambert-Bouguer Law) where e is the molar absorption coefficient of the absorbing species, l is the absorption pathlength, and c the concentration.2 A sample is irradiated with a specific wavelength of light, typically of the ultraviolet or visible spectrum; the spectral radiant power of incident light on the sample (Po) and transmitted radiation after passing through the sample (P), are used to calculate absorbance (A):
(1)
Absorbance is also referred to as optical density, although its use is discouraged.7

Assay type, binding
An assay in which the specific interaction between two binding partners (e.g. ligand-receptor, antibody-antigen, protein-protein, ligand-transport protein) is measured.
The assay can be homogeneous or heterogeneous, competitive or non-competitive and may be run at equilibrium or in kinetic mode. The assay measures specific binding in contrast to non-specific adsorption processes.

Assay type, cell based
An assay in which the response of live cells to a test sample is measured. The type of cells, parameters measured, and detection systems used vary widely.
See also CYTOTOXICITY and CHEMOTAXIS assay.

Assay type, chemotaxis
A cell based assay in which the rate and/or direction of cell migration is measured. The response to a test sample is typically detected by means of a fluorescent PROBE.

Assay type, competitive binding
A molecular assay based on the competition between a ligand and a reference ligand for the same binding site on a receptor (e.g. antibody, transport protein).
Depending on the technology used to monitor the interaction, the reference ligand and/or the receptor can be labeled with a variety of technologies. Very rarely, and mostly out of the field of screening, neither is labeled and the interaction is assessed, for instance, by mass determination of the complex.
Former definitions of competition between labeled and non-labeled ligands are out of date.

Assay type, cytotoxicity
A cell based assay in which response to a test sample is determined by measuring cell death.

Assay type, electrochemiluminescence (ECL)
An assay in which light emitted by an electrochemical reaction is detected and used to quantify a PROBE, ruthenium (II) tris-(bipyridine), on one of the two assay binding components.
Coated magnetic beads bind the protein target and are then captured in a flow cell of the detector where the energy state of the Ru is chemically converted to release a photon measured at 620 nm with a photomultiplier tube. There is little or no compound interference during detection because of the wash step within the flow cell. Generally not used for HTS but for secondary assays.

Assay type, flow cytometry
A CELL BASED assay in which response is dependant on the detection of the phenotype of cells as determined through the light scattering properties of each cell, and, typically, the fluorescence intensity of a relevant PROBE.
Single cells are rapidly passed through a channel where they are optically analyzed and, in the case of fluorescence activated cell sorting (FACS), are separated based on response for further study.

Assay type, fluorescence anisotropy
See Assay type, fluorescence polarization.

Assay type, fluorescence correlation
An assay in which fluctuations in fluorescence intensity are detected and used to characterize a species that is labeled with a fluorescent PROBE. The fluctuations are measured for a small volume of the total assay sample and are attributed to local diffusion of the PROBE in the detected volume or interactions that change the quantum yield of fluorescence.

Assay type, fluorescence polarization
An assay in which the association between a LIGAND linked to a fluorescent PROBE and a larger macromolecule is detected. The degree to which the labeled LIGAND depolarizes plane-polarized light is directly proportional to the rate at which it tumbles in solution. Tumbling will be slower when the LIGAND is associated with a larger macromolecule as compared to free in solution, consequently resulting in a lower degree of depolarization. As a consequence, FLUORESCENCE POLARIZATION is a typical HOMOGENOUS BINDING ASSAY.

FLUORESCENCE POLARIZATION (FP) is determined from the fluorescence intensities measured parallel () and perpendicular () to the excitation plane:

Note: Fluorescence anisotropy (r) differs only in normalization and is determined from
r = F- - F^ / F- + 2F^

Assay type, fluorescence resonance energy transfer (FRET)
An assay based on the non-radiative energy transfer that occurs when the emission spectrum of a fluorescent donor molecule overlaps with the absorbtion spectrum of an energy acceptor molecule. The donor and the acceptor fluorophores are linked directly or indirectly to two binding partners and energy transfer occurs when the fluorophores are in close proximity, i.e. when the binding partners interact.
The fluorophores can be either of chemical or protein nature. When a europium (HTRF, CisBio) or a lanthanide (DELPHIA and LANCE, PerkinElmer) derivative is used as donor, its long fluorescence lifetime allows time-delayed (or time-resolved) fluorescence measurement (TR-FRET), providing conditions where other fluorophores in the assay medium are not detected.

Assay type, functional
An assay in which target activity is determined by measuring its native biological or physiological activity (e.g. rate of catalysis for an enzyme).

Assay type, heterogeneous
Assay in which the response is detected only after the physical separation of one or more assay components.
See ASSAY TYPE, HOMOGENEOUS.

Assay type, heterogeneous binding
An assay in which one binding component is immobilized to a surface (e. g. cellular membrane, glass, gold or biopolymer layer such as polyethylene glycol or dextrane) while its binding partner is freely diffusible. The extent of interaction between the binding partners is determined after their physical separation, which is usually achieved by filtration, centrifugation, magnetic field or chromatography.
Used to qualitatively discriminate specific and non-specific interaction or quantitatively determine the concentration of a component part of a mixture.

Assay type, homogeneous
An assay in which all reagents and reactants are of a uniform phase and assay response is detected without the need for physical separation of assay components. See ASSAY TYPE, HETEROGENEOUS.

Assay type, homogeneous binding
A single-phase system used to qualitatively discriminate between specific and non-specific interactions or quantitatively determine the concentration of a component part of a mixture.

Assay type, HPLC
An assay in which response is determined by the physical separation of assay components using high performance liquid chromatography (HPLC) with concurrent detection, usually by monitoring absorbance at an appropriate wavelength.
Changes in the concentration of assay components at one or more times over the course of the assay can be determined by the relative area under the detection peaks as compared to assay controls.

Assay type, IMAP
An assay in which the interaction between nanoparticles coated with trivalent metal ions and phosphate groups on a substrate that carries a fluorescent tag is detected. A phosphorylated fluorescent substrate bound to the nanoparticle will exhibit high fluorescence polarization relative to the unbound (unphosphorylated) peptide substrate.
Used for kinase assays where a phosphate group is added to a peptide substrate or for phosphatase assays where the phosphate is removed from a peptide substrate.
See Assay Type, Fluorescence Polarization.

Assay type, in vitro
An assay in which the component(s) under study is/are removed from their complete biological systems. Literally “in glass”.
See ASSAY TYPE, IN VIVO.

Assay type, in vivo
An assay that involves a complete living organism.12
See ASSAY TYPE, IN VITRO.

Assay type, luminescence
An assay in which response is measured by the detection of light of a non-radiative origin, such as bioluminescence or chemiluminescence.
A luciferase reporter system for the measurement of gene transcription regulation is a commonly used example. In this assay variation luciferase, transcribed under the regulation of a promoter, oxidizes the substrate luciferin to oxyluciferin, in the process emitting light at 560 nm which is detected as the signal.

Assay type, multiplex
An assay in which a single sample is used to measure the activity of more than one target.

Assay type, non-competitive binding (direct)
An assay in which the interaction of a ligand with a receptor is measured without a requirement for added competing agents. It can be homogeneous or heterogeneous.

Assay type, nuclease
A method in which fluorescently labeled 20 to 30 bp oligo probes are used in combination with the 5'→3' exonuclease activity of Taq polymerase to determine the presence or quantify the amount of specific target nucleotide sequences.
The 5' nuclease assay has now become a staple of real-time PCR quantification techniques. In the 5' nuclease assay, the current standard for PCR quantification, fluorescent signals are generated from the exponential range of the reaction, where component concentrations are not limiting. As a result, initial template levels can be determined with high accuracy.

Assay type, optical density
A term synonymous with absorbance assay, although its use is discouraged1
See ASSAY TYPE, ABSORBANCE.

Assay type, scintillation proximity (SPA)
A homogeneous assay in which beads incorporated with scintillant emit light in the presence of radiolabeled molecules within the proximity of the bead’s scintillant detection.
NOTE: An alternative form of the assay incorporates the scintillant into the base of a microplate instead of the bead – this version is called a Flashplate assay (Perkin Elmer).

Assay type, surface plasmon resonance (SPR)
An assay in which the binding of a ligand to a protein target immobilized on the platinum surface of a chip is detected by changes in the angle of the light reflected from the surface.
This resonance shift assay is not usually used for HTS but has been expanded recently to four simultaneous detection channels and has improved sensitivity to detect low molecular weight binding molecules.

Assay validation
Experiments conducted to verify that the output measurement of the assay is reflective of the target activity. Results are compared (where possible) to existing literature parameters such as Kd, Ki, Km, or EC50.

Automation
Mechanization with process control, where process means a sequence of manipulations. One or several functions in an instrument may be automated.

Automated device or peripheral
A device that performs one or more functions without human intervention by means of direct control or by programmed operations. Often referred to as a peripheral or peripheral device when it is an instrument incorporated into a module, a workstation or a fully automated system. See LIQUID HANDLER, MODULE AND WORKSTATION.

Average
Arithmetic mean of a set of values. A non-technical term.

Background
1. The amount of a signal produced in an assay or screen in the absence of a test substance.
2. The signal detected from an assay in the absence of TARGET activity; often equivalent to negative control (see ASSAY CONTROLS).

Batch
A homogeneous preparation of a reagent/reactant (small molecule, enzyme, CLONE, etc.) produced (synthesized, purified, or otherwise) at one time.

Bmax
The total number of receptors or binding sites in a system, [R]T, as determined by Scatchard analysis of a binding isotherm.
When the system is a simple one containing a single binding site and lacking cooperative interactions, binding is described by the following equation:

B = (Bmax * L) / (Kd + L)

Where B is the amount of ligand bound, L is the free concentration of ligand, and Kd is the equilibrium dissociation constant of the ligand.
The equation describes a hyperbola with Bmax as the asymptote. Units of Bmax are amount of ligand bound per amount of binding material e.g. μmol mg-1, fmol cell-1, nmol mol-1.

Cell membrane preparation
A preparation, from tissue or cells, in which the membranous elements (generally the cell membrane) are enriched as opposed to other cellular constituents; these are obtained by disruption of the cells and differential centrifugation or other techniques to isolate the desired components. Cell membrane preparations are the standard biological material for RECEPTOR binding assays.

Cell mortality
The ratio of counted dead cells to the total number of observed cells. Dead cells are often counted using apoptosis or nuclear markers.

Cell viability
The ratio of counted alive cells to the total number of observed cells. Live cells are often counted using nuclear or membrane markers.

Channel blocker
A compound that reduces or eliminates the conductance of an ion channel by impeding the movement of ions through that channel.
Note: Verapamil, which blocks the conductance of some L-type calcium channels, is an example of a channel blocker. Dihydropyridine antagonists such as nifedipine which also block the conductance of some L-type calcium channels, are not classified as channel blockers since their action is to inhibit channel gating.

Channel opener
A compound that increases the conductance of an ion channel by a mechanism other than alteration of membrane potential or activation of the cognate receptor.
Retigabine and cromakalim are examples of channel openers. The term channel activator is sometimes used.

Chemometrics
The use of mathematical and statistical methods to extract information from chemical data. For screening purposes, CHEMOMETRICS software packages and fundamental CHEMOMETRICS approaches are used in order to design compound libraries for general screening or for screening against a specific target class.

Clone
1. A population of cells derived from a single ancestor by asexual reproduction;
2. An exact copy of a DNA fragment, produced on a plasmid by recombinant DNA technology.

Coefficient of Variation (CV)
The standard deviation expressed as a percent of the mean.
Standard deviation / mean x 100%

Combinatorial library
A set of compounds prepared by combinatorial chemistry. May consist of a collection of pools, or sub-libraries. Its composition may be described by the chemset notation.

Compound collection
A set of chemicals that has been labeled or annotated for easy storage and retrieval and that is available for screening.
Consists of compounds synthesized by combinatorial or standard synthetic methods, purchased from commercial sources or of samples of natural products either as pure samples or as mixtures.

Compound library
See compound collection.

Concentration response
Whenever a figure quantifying the affinity or efficacy of a compound in an assay is required, the compound is evaluated at increasing concentrations in order to determine its concentration-dependent effect. The classical thermodynamics usually result in a hyperbolic increase of the assay signal upon linear increase in compound concentration. Thus, compound concentration responses are usually determined using logarithmic serial dilutions, e.g. 10 µM to 1 nM (See also Ki, KD, IC50, EC50).

Counter-screen
A screen in which test samples are assessed against a TARGET for unwanted activity. This target may or may not be structurally or functionally related to the intended target.

DNA
Deoxyribonucleic acids (DNA). High molecular weight, linear polymers, composed of nucleotides containing deoxyribose and linked by phosphodiester bonds; DNA contain the genetic information of organisms. The double-stranded form consists of a double helix of two complementary chains that run in opposite directions and are held together by hydrogen bonds between pairs of the complementary nucleotides and Hoogsteen (stacking) forces.

EC50 ((Effective concentration 50):
The concentration of an effector that produces one half of the maximal response for that system. Usually refers to an agonist in a receptor system, compared to a reference agonist that produces a maximal response in the system.
See also IC50

Efficacy
The extent to which a compound produces a response in an assay, relative to the high and low assay controls.
When the compound increases the signal up to 100% or decreases the signal down to 0%, it has a 100% efficacy. When the plateau reaches another intermediary efficacy value, the compound is said to have a partial effect (see partial agonist).

End-point assay
A KINETIC ASSAY run for a set constant incubation time, followed by the addition of a reagent that stops the reaction and allows postponed measurement of the signal.
See also equilibrium assay, kinetic assay.

Enzymes
Macromolecules, mostly of protein nature, that function as (bio)catalysts by increasing the reaction rates. In general, an enzyme catalyses only one reaction type (reaction specificity) and operates on only one type of substrate (substrate specificity). Substrate molecules are attacked at the same site (regiospecificity) and only one or preferentially one of the enantiomers of chiral substrates or of racemic mixtures is attacked (stereospecificity).

Enzyme-linked immunosorbent assay (ELISA or EIA)
A heterogeneous assay in which two antibodies are used detect the level of antigen in a sample. One antibody is specific to the antigen and the other is coupled to an enzyme that will cause a chromogenic or fluorogenic substrate to produce an amplified signal.

Epitope mapping
The identification and localization of the specific regions of protein molecules that are recognized by the immune system. An epitope (antigenic determinant) is the minimum molecular structure that will react with an antibody and may be only a portion of an antigen.

The epitope-mapping assay is a novel test developed at Genencor to identify T cell epitopes present in proteins. The epitope mapping assay developed by Genencor can pinpoint T cell epitopes within any protein of interest that have the potential to initiate deleterious immune responses.

Equilibrium assay
An assay in which the time-course of the signal intensity is not measured. Such an assay has an incubation time sufficient for the plateau phase of the signal to be reached, corresponding to an equilibrium between the reactants.
See also KINETIC ASSAY, END-POINT ASSAY.

False negative
An assay result in which a sample known to be active does not produce either the expected signal or a signal above the activity threshold.
FALSE NEGATIVES can occur when an assay lacks appropriate discriminatory power, when the threshold is inappropriately set, or as a result of mistaken identity of the test sample.
See also FALSE POSITIVE.

False positive
An assay result in which a sample known to be inactive produces a signal or response above the activity threshold.
FALSE POSITIVES can occur when an assay lacks appropriate discriminatory power, when the threshold is inappropriately set, as a result of certain physical properties of the substance (e.g. a fluorescent compound in a fluorescence intensity assay, aggregation), or as a result of mistaken identity of the substance
See also FALSE NEGATIVE.

Flow injection (flow analysis)
Analytical methods that are based on the introduction and processing of test samples in flowing media.
A primary classification can be based on (a) the way the test portion is introduced, i.e. continuously or intermittently/­discretely and (b) the basic character of the flowing media, i.e. either segmented, unsegmented or monosegmented, where segmentation is applied to prevent intermixing of successive analyte zones. This leads to the following classification tree (as in analytic compendium:

Fig. Classification scheme of flow methods of analysis.

GPCR (GTP-binding Protein Coupled Receptor)
A family of transmembrane receptors that directly connects to a family of GTP binding protein/GTPases for signal transduction.

High content screening assay
An assay that produces multiple biological readouts.
Most commonly used in relation to the mathematical analysis of an image acquired using an automated microscope whereby analysis algorithms quantify cellular parameters (e. g. number, motility, neurite outgrowth, size, shape) and subcellular events (e. g. receptor internalization, protein translocation, protein expression nuclei shape).

High throughput
A relative term, applied to the generation of a large number of results (eg 100,000) in a short timeframe (week or month). Usually achieved by employing a substantial degree of automation.

High throughput screening (HTS)
A method in which a large number of assays (from thousands to millions) are performed and assessed in a relatively short time period. Typically, these assays are carried out in microplates of at least 96 wells using automated or robotic technologies.
Note: The rate of at least 100,000 assays per day has been termed 'Ultra High Throughput Screening' (UHTS)

Hit
A sample that produces confirmed activity above the hit threshold in an assay and whose structural identity has been confirmed.
A substance becomes a HIT when the properties of an ACTIVE are confirmed by elimination of FALSE POSITIVE results and ARTIFACTS,
Note:  In the past, the terms confirmed hit, true hit, and confirmed active were used with this meaning.

Hit rate
The portion of hits that displays confirmed activity in a screen beyond a minimum defined level, the hit threshold. Expressed as a percentage.

Hit threshold
The minimum activity that defines ACTIVES in a PRIMARY SCREEN. It is usually expressed as percentage of inhibiton or stimulation. For example, a widely used hit threshold is 50%.

IC50 (Inhibition concentration 50)
The concentration of an effector that reduces a response by 50%.
Note: IC50 values of are influenced by experimental conditions (eg: substrate concentration).
See also: EC50.

Image-based assay
An assay in which image analysis algorithms, that determine the sub-cellular compartmentalization of fluorescent probes, give quantitative as well as topological information. The use of such automated cellular imaging for screening has been enabled by the development of digital imaging technology.
See also HIGH CONTENT SCREENING

Inactive
A substance that does not produce a response above the hit threshold in an assay at the tested concentration.
Note:  A substance may also be designated as inactive when attempts to confirm an ACTIVE fail
See ARTIFACT, FALSE POSITIVE, HIT.

Inhibitor
A compound that decreases the rate of a catalyzed reaction (enzyme) or transport process (transporter).
Characterized by an IC50 at the cellular or at the molecular level.
At the molecular level, the mechanism of inhibition can be assigned by experimentation. The two main mechanisms involve either competition with one of the substrates to occupy the active site (competitive INHIBITOR), or binding at an alternative site inducing conformational changes in the active site (allosteric INHIBITOR). Independently from its site of action, INHIBITOR binding can be reversible (thermodynamic equilibrium) or irreversible (most often covalent, as in the case of ‘suicide substrates’ which bind covalently to the active site upon activation by an enzyme.

Inverse agonist
A ligand that decreases signaling through a receptor below the level of constitutive activity. The effect of an INVERSE AGONIST is dependent upon the level of constitutive activity of the particular RECEPTOR and is manifested in the absence of a conventional AGONIST (see also AGONIST, PARTIAL AGONIST, ANTAGONIST).

Isotope
An atom of any particular element that has the same number of protons but differing numbers of neutrons in the nucleus. Some ISOTOPES may be unstable and undergo decay, emitting energy in the form of radiation: radioisotopes (see also ISOTOPIC LABELLING, RADIOLIGAND).

Isotopic labelling
The incorporation of an atomic ISOTOPE [radioactive or stable] into a molecule, by replacement of, or substitution at, one of its constituent atoms
See also ISOTOPE, RADIOLIGAND, probe.

k1
In enzyme kinetics, the rate of formation of the enzyme-substrate (ES) complex, expressed in units of inverse time ( s-1).

k-1
In enzyme kinetics, the rate of dissociation of the enzyme-substrate (ES) complex to enzyme (E) and substrate (S). Units of M-1s-1

Ka (Equilibrium association constant) 
The ratio, at equilibrium, of [AB]/[A][B] for the reversible binding interaction of A and B to yield the complex AB. Units are M-1.
Note: Equal to 1/Kd, or kon/koff,  synonym to kass.

Kass
See Ka.

Kd (Equilibrium dissociation constant) 
The ratio, at equilibrium, of [A][B]/[AB], for the reversible binding interaction of A and B to yield the complex AB. Units are concentration (molar).
Note: Reciprocal of Ka, (Kd = 1/Ka), equal to koff/kon and synonym to kdiss.

kdiss
See Kd.

Ki
The equilibrium dissociation constant of an enzyme-inhibitor complex:  Ki = [E][I]/[EI].
This value is usually obtained through competition binding experiments, where the inhibitor dissociation constant is determined after its IC50 obtained in a competition assay performed in the presence of a known concentration of labeled reference ligand (Lr) which has its own dissociation constant Kd[Lr] for the target : Ki = IC50/(1+(Lr/Kd[Lr]))

Km
The Michaelis constant. The concentration of substrate at which v=1/2Vmax. Km = (k-1 + k2)/k1

Kinetic assay
An assay in which the time-course of the signal intensity is measured. Such an assay typically requires that signal values are acquired at several time intervals in order to calculating kinetic parameters.
Note:  KINETIC ASSAYS should be designed in such a way that the change in signal is linear throughout the experiment.
See also equilibrium assay, END-POINT ASSAY.

Label free detection
The direct detection of compound activity on a target without use of preliminary ISOTOPIC LABELLING or FLUORESCENT LABELLING.
Most common technologies include mass spectrometry, SPR, NMR, microbial growth, CELL MORTALITY, CELL VIABILITY
See Assay type, SPR, NMR.

Laboratory information management systems (LIMS)
A computerized system designed to provide on-line information about the samples analyzed in a laboratory. Information provided may include the current location of each sample in the laboratory, the method and status of each analysis, and experimental data and calculated results.18
Typically, LIMS connect analytical instruments to one or more personal computers and manage processes from sample log-in to reporting test results. With interfaces to enterprise resource planning or manufacturing execution systems a LIMS can be embedded in a complex IT-infrastructure.

Lead
A compound (or compound series) that satisfies predefined minimum criteria for further structure and activity optimization. Typically, a lead will demonstrate appropriate activity, selectivity, tractable SAR and the potential to be patentable.

Lead identification
A process that is targeted toward the generation of at least one compound series that meets the requirements for progression to lead optimization. It typically encompasses the steps from the detection of initial activity (via high throughput screening and other lead finding activities) through hit confirmation and hit-to-lead activities.

Lead optimization
A process in which the drug-like properties of an initial lead or lead series are improved. Typically, biological activity will be enhanced, in vivo efficacy will be demonstrated and compounds with a physicochemical, pharmacological and toxicological profile consistent with progression to the clinic will be identified.

Library
1. A collection of samples (e.g. chemical compounds, natural products, over-expression library of a microbe) available for screening.
2. A set of compounds produced through combinatorial chemistry.

Ligand
Any small molecule that binds to a larger molecule or macromolecular structure. Examples frequently encountered in biomolecular screening include AGONISTS, ALLOSTERIC MODULATORS, ANTAGONISTS, CHANNEL BLOCKERS, CHANNEL OPENERS, and ENZYME INHIBITORS.

Ligand activated channel
Any ion channel that is gated (i.e., opened or closed) by a chemical messenger.
Note: Distinct from voltage-gated or stretch-activated ion channels.
See also VOLTAGE ACTIVATED CHANNELS).

Liquid handler or liquid handling machine
A programmable device that accurately and precisely delivers pre-defined quantities of liquid to a MICROPLATE. It may be free-standing; incorporated into a WORKSTATION or part of a fully automated system.
See: AUTOMATED DEVICE

Microarray
A planar surface where assay reagents and samples are distributed as sub-microliter drops. This screening format is a direct offshoot of genomic microarray technologies and makes use of ultra-low volume MINIATURAIZATION, using nanodispensing technologies,

Microplate
Any of a number of plates containing a series of wells in which to store reagents, clones, etc., or perform individual assays. Typically, these plates are constructed of a variety of clear and opaque plastics, and contain 96, 384, or 1,536 individual wells, although 24-well and 3,456-well plates are also available.
See also MICROPLATE STANDARDS and ASSAY or PLATE FORMAT.

Microplate standards
Standards that define the footprint, the height, the flanges and the well positions of 96, 384 and 1536 well MICROPLATES. These have been established by the SBS along with ANSI (American National Standards Institute) and accredited by ANSI. They are numbered as ANSI-SBS-1 (2004) – Microplate footprint; ANSI-SBS-2(2004) – Microplate Height; ANSI-SBS-3 (2004) – Microplate Flange and ANSI-SBS-4 (2004) – Microplate Well Positions.

Miniaturization
Experimental design aimed at decreasing the reaction volume of an assay. Examples include high-density microplates (384 wells, 1536 wells), chip-based microarrays and microfluidic devices. So-called NANODISPENSING systems are required for liquid handling of submicroliter volumes.

Module
An individual automated device within a fully automated assay system that usually performs a complete single assay step or procedure. A fully enclosed MODULE may allow for the control of temperature, humidity and the gaseous environment. A MODULE would be essentially a WORKSTATION within a system.

Molecular imprinted polymers (MIPS)
Synthetic polymers in which well-defined volume cavities (imprints) created by template molecules mimic a biomolecular specific ligand-receptor interaction. These artificially generated recognition sites have shapes, sizes and functionalities complementary to the template molecule, and are capable of rebinding the template molecule in preference to other closely related structures.
MIPs have found uses as stationary phases in chromatography, as recognition elements in chemosensors, and as enzyme mimics in catalysis.

1. Number of independent experiments.
2. Number of replicates, may or may not indicate experimental independence of the repeated observations.
Note: statistical treatment and interpretation of the data will depend on whether or not the experiments are independent.

Natural Products
1. Complex mixtures derived from natural (biological) sources used in screening as a resource for identification of lead compounds.
2. Pure compounds of natural (biological) origin (whether obtained by purification of natural mixtures or by laboratory synthesis), as opposed to compounds originating from synthetic chemistry.
Typical origins include microbes and higher organisms from terrestrial or marine environments.

Noise
The random fluctuations occurring in a signal that are inherent in the combination of instrument and method.

pA2
A measure of antagonist potency. The negative logarithm of the antagonist concentration that shifts the midpoint of the agonist concentration-response curve to the right by a factor of 2.

Partial agonist
An agonist which is unable to induce maximal activation of a receptor population, regardless of the amount of drug applied.
See EFFICACY.

Pipettor
A device that aspirates and dispenses liquid (nL – mL range) for the purposes of transferring from a source to destination. The device may be fully manual, electronic, single-channel, multi-channel (typically 8- or 12-channel), or part of a LIQUID HANDLER.

Plate format
The number and configuration of wells on a microplate. The most widely used formats are arrays of 96 wells (8 by 12) or 384 wells (16 by 24) or 1536 wells (32 by 48).
See Microplate, Microplate standards.

Plate gripper
A handling device that positions MICROPLATES on a workstation. A plate gripper may also be used to move microplates between independent modules of an automated platform. Often used in combination with plate stackers, washers and readers.

Plate map
The layout of samples and controls configured on a plate during an assay. For example, for a primary screen in 384-well plates, columns 1, 2, 23 and 24 are controls, and columns 3-22 are for individual test compounds, whereas for secondary screening, each row will contain a single compound at varying concentrations.

Plate reader
A laboratory instrument that uses optical and/or computer vision techniques to detect biological, chemical or physical events in samples stored in microtiter plates wells. Use of these plate readers reduces or eliminates human subjectivity in the evaluation of plate contents.
An ELISA plate reader, for example, measures color intensity in each well.
ELISPOT plate readers are used to count the colored spots that are formed in the course of ELISPOT assays.
Plate readers may employ the use of PLATE STACKERS.

Plate stacker
A device that provides loading, unloading and restacking of microplates. It is usually part of an integrated system connected to at least a reagent dispensor or a plate reader, and is designed to minimize microplate handling.

Plate washer
A device for the automated washing of microplates. It may be equipped with additional functions and features such as precise height adjustment for minimized residual volume, digitallycontrolled aspiration- or dispensing pumps to provide high accuracy.

Precision
The closeness of agreement between independent test results obtained by applying the experimental procedure under stipulated conditions. The smaller the random part of the experimental errors which affect the results, the more precise the procedure. A measure of precision (or imprecision) is the standard deviation.

Primary screen
The initial screen applied to assess the activity of a collection of compounds against a biological target of interest.
This SCREEN identifies ACTIVES from a LIBRARY.
See: SECONDARY SCREEN.

Probe
A molecule, chemical or protein, which is covalently linked to the molecule, chemical or protein to be assayed and which, is detected by an appropriate technology.  A probe is often required since most assay techniques are indirect and require the use of a marker which is detected by the appropriate technology.
Most frequently used probes are radioactive (ISOTOPIC LABELLING) or FLUORESCENT, but alternative technologies are emerging (spin probes, heavy atoms).

Quality control
An operation or series of operations that contributes to the validation of screening results. Such operations include, validation of liquid handling devices and plate readers, experiment controls, such as determination of the Z’ factor and use of assay controls, and post-experiment controls, such as data analysis validation and database administration. Results of a screen are validated only after a set of quality controls have been performed

Radioisotope
An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.
See ISOTOPE.

Radioligand
A LIGAND into which a radioisotopic label has been incorporated.
See also ISOTOPE, ISOTOPIC LABELLING.

Receptor
A cellular macromolecule [or complex of macromolecules] involved in chemical signaling within or between cells; the RECEPTOR recognizes a signal by binding a LIGAND [exogenous (e.g., a drug) or endogenous (e.g., a hormone or neurotransmitter)] with high affinity and chemical specificity, and then transduces a subsequent cellular response.

Reproducibility
The closeness of agreement between independent results obtained with the same method on identical test material but under different conditions (different operators, different apparatus, different laboratories and/or after different intervals of time).
The measure of reproducibility is the standard deviation qualified with the term ‘reproducibility’ as reproducibility standard deviation. In some contexts reproducibility may be defined as the value below which the absolute difference between two single test results on identical material obtained under the above conditions may be expected to lie with a specified probability. Note that a complete statement of reproducibility requires specification of the experimental conditions which differ.

Robot
An automated device that performs tasks i.e. SCREEN functions that would normally be performed by a human. A ROBOT in laboratory automation is usually used to move MICROPLATES in an automated assay procedure and usually consists of either a robotic arm with at least three degrees of freedom or a PLATE GRIPPER.

Robustness
An assay or screen is said to exhibit ROBUSTNESS when it has a high discriminatory power and produces a low number of FALSE NEGATIVE and FALSE POSITIVE results.

Sample
A portion of material selected from a larger quantity of material.
Typically a chemical compound or mixture of compounds submitted to an assay or a screen.

Scheduling software
A computer program (most often proprietary) used to organize the chronological sequence of experimental methods to be followed to execute an automated high throughput screening campaign.
Based on the experimental needs of the campaign and constraints imposed by the programmer, this software uses an iterative process to determine the optimal order of the use of ROBOTS, LIQUID HANDLERS, and MODULES to minimize screening time.
See AUTOMATED DEVICE, LIQUID HANDLER, MODULE, ROBOT and WORKSTATION.

Screen
The execution, analysis and interpretation of a large number of assays to evaluate the activity of a collection of samples against a target.
A screen will often employ automation.

Screen validation
Assay conditions as determined by ASSAY VALIDATION are performed in the chosen PLATE FORMAT with an acceptable signal to background ratio as described by the Z’ factor.

Secondary screen
A screen applied to independently confirm actives from the primary screen. A secondary screen may employ an assay that differs in type from the primary screen, e.g. biochemical assay vs. cell based assay, or it may be of the same type with different readout.

Selectivity assay
An assay used to determine the relative potency of active or lead compounds towards an alternative target. A selectivity assay (or panel of assays) may include targets of the same family or unrelated targets.

SEM (Standard Error Mean)
The standard deviation divided by the square root of the sample size. It is the standard deviation of a sample of means.

Stably transfected cells
Eukaryotic cells into which recombinant DNA has been introduced and incorporated into the genome, so that the cells replicate the new DNA in a stable fashion.

Structure-activity relationship
Association between specific aspects of molecular structure and defined biological action.

Sublibrary
A portion of a LIBRARY grouped or selected based on similarity of structure or biological effect.

Target
A biological molecule, such as an enzyme or receptor, whose activity and function is the focus of a screen.

Targeted library
Library designed, on the basis of preexisting information, to generate enhanced activity or hit rate against a particular biological target or target class.

Throughput
The number of results that can be generated in a given timeframe. In HTS, throughput is often defined by the number of assay samples that can be processed in a day (e.g. 50,000 sample per day)

Total internal reflection fluorescence (TIRF)
A spectroscopic technique that discriminates between labeled molecules in the homogeneous phase and those interacting with binding partners immobilized at a surface. TIRF measurements are restricted to a thin layer and are achieved by exciting the fluorophore with an evanescent wave, created by total internal reflection at the glass/ aqueous interface of the wave guide. The goal of using TIRF in biological applications is to study events close to the interface of two different media.

Toxic effect
A deleterious effect on cell or animal viability in a functional assay.

Transiently transfected cells
Eukaryotic cells into which recombinant DNA has been introduced and expressed by cellular transcription, but without incorporation into the stable genome.

Ultra-high throughput
A relative term, currently applied to the screening of 100,000 test samples in a 24 hour period

VMAX
The maximum velocity that an enzyme catalyzed reaction can achieve for a particular substrate under a given set of conditions.

Voltage activated channel
An ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse).

Workstation
A programmable device used to automate liquid handling operations on a MICROPLATE supplied to the device by a ROBOT (automated stacker system or a robotic manipulator).

Z factor
A dimensionless statistical parameter that provides a practical assessment of assay performance.  

Z = 1 – ((3σs + 3σc) / (| μs – μc |)

Where μs denotes the mean of the library sample signal, μc denotes the mean of the control signal and σs and σc denote the corresponding standard deviations.
1>Z³0.5 generally categorizes an excellent assay.

Z’ factor
A dimensionless statistical parameter that is used extensively in biomolecular screening. The Z' factor is a characteristic of an assay without the intervention of test compounds. It provides a practical index of the quality and reliability of the assay, and can be used as a guide to assay development and optimization.

Z’ = 1 – ((3σc+ + 3σc-) / (| μc+ - μc- |)

where σc+ and σc- are the standard deviations of the high and low controls and μc+ and μc- are the means of the high and low controls, respectively.

The useful range of Z’ values is from 0 (very poor) to +1 (excellent). In most cases, a Z' factor greater than 0.5 is required for an assay to be accepted for HTS.