High throughput screening
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high throughput screening drug discovery overview 2025
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Here is a thorough overview of High-Throughput Screening (HTS):
High-Throughput Screening (HTS)
Definition
HTS is a systematic, automation-driven method for identifying biologically active compounds ("hits") by testing very large chemical or biological libraries against a defined target or cellular phenotype. It combines robotics, miniaturized assays, sensitive detection, and data analytics to compress months of manual testing into days to weeks.
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, p. 27
- Harper's Illustrated Biochemistry, 32nd Ed., p. 77
Core Components
| Component | Role |
|---|---|
| Compound library | Thousands to millions of small molecules stored in multi-well plates (96-, 384-, 1536-well format) |
| Liquid handling robots | Automated dispensing of tiny compound volumes into assay wells |
| Optical plate readers | Measure luminescence, fluorescence, or absorbance as the assay readout |
| Data informatics | Software pipelines to flag hits, calculate Z-factors (assay quality), and filter false positives |
The HTS Workflow
- Target selection and validation - identify a protein (enzyme, receptor, ion channel) relevant to disease
- Assay development - design a miniaturized, reproducible readout (fluorescence, HTRF, AlphaScreen, luciferase, etc.)
- Library screening (primary screen) - test the full compound library; a typical hit rate is 0.1% or less
- Hit confirmation - retest all apparent hits to filter false positives; many initial "actives" are artifacts
- Hit-to-lead - purchase analogues (analogue by catalog) or synthesize similar compounds, build SAR (structure-activity relationships)
- Lead optimization - iterate medicinal chemistry to improve potency, selectivity, and DMPK/safety properties
- Candidate nomination - a clinical candidate (and backups) emerge for preclinical and clinical development
Assay Types Used in HTS
- Biochemical assays - purified enzyme or receptor, faster and simpler but less biologically relevant
- Cell-based assays - whole-cell readouts (reporter genes, calcium flux, viability); more physiologically relevant, dominated market in 2025 with ~44% share
- Phenotypic assays - screen for a disease-relevant cellular phenotype without knowing the exact target upfront
- Binding assays - measure direct compound-target interaction (e.g. TR-FRET, fluorescence polarization, SPR)
Library Design
Libraries are often tailored to the application:
- Diversity-based libraries - maximize chemical space coverage
- Focused/targeted libraries - enriched for compounds active against a class (e.g., kinase-focused, GPCR-focused)
- Covalent libraries - contain reactive warheads to form covalent bonds with the target
- Natural product libraries - microbial or plant-derived compounds with unusual scaffolds
A random compound from a typical library has a hit probability of ~0.1% or lower against any given target.
Beyond Classic HTS: Emerging Approaches
Fragment-Based Drug Discovery (FBDD)
Even screening millions of compounds barely samples the ~10^60 possible drug-sized organic molecules. FBDD solves this by testing small "fragments" (~150 Da), then building up from productive fragments using X-ray crystallography or NMR to guide "fragment growing" or "fragment linking." Vemurafenib (B-Raf inhibitor) is the canonical FBDD success story.
- Goodman & Gilman's, p. 27
DNA-Encoded Libraries (DELs)
DELs attach a unique DNA barcode to every compound and mix them all together in a single tube - libraries can reach billions to trillions of compounds. After incubation with an immobilized target, non-binders are washed away and hit compounds are identified by sequencing their DNA tags. This vastly exceeds what conventional HTS can achieve in throughput.
- Goodman & Gilman's, p. 28
Ultra-HTS (uHTS)
Running at >100,000 wells/day (AstraZeneca's DISC facility can screen >1 million compounds/day). Uses 1536-well or even higher-density plates.
Key Limitations
- Confirmed HTS hits are far from drugs: typical hits have weak affinity (μM range), poor selectivity, and fail ADME/safety criteria - they are starting points only
- High false-positive rates from compound autofluorescence, aggregation, reactivity, or assay interference
- Chemical space is so vast that even 10^6 compound screens sample a tiny fraction
- Assay miniaturization requires extensive validation to ensure reliability without user intervention
- Cost and infrastructure requirements are substantial (automation, compound storage, cryogenic facilities)
Current Landscape (2025-2026)
- The global HTS market was led by cell-based assays (44.6% share) and primary/secondary screening applications (53% share)
- Pharmaceutical and biotech companies held ~48% of end-user market share
- CDMOs are the fastest-growing segment, at ~11.8% CAGR projected to 2031
- Lab-on-a-chip and microfluidic platforms are growing rapidly (~10.5% CAGR to 2031), enabling even smaller volumes and lower reagent costs
AstraZeneca's HTS facility at The Discovery Centre (DISC) in Cambridge is one of the world's largest:
HTS in Context: The Drug Discovery Pipeline
Target ID → Target Validation → Assay Development → HTS → Hit Confirmation
→ Hit-to-Lead → Lead Optimization → Preclinical → Clinical Trials → Approval
HTS sits at the very beginning of this pipeline. Most discovered hits never become drugs - the overall probability of an HTS hit reaching the market is extremely low, but the scale and speed of HTS compensates by generating large numbers of starting points from which medicinal chemists can work.
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