Phase 2 — Target Identification & Compound Profiling

Phase 2 — Target Identification & Compound Profiling contains 19 topic pages in Pharmacology.

Once the regulatory and conceptual landscape of pharmacological research has been established, the practical research pathway begins with a deceptively simple question: which biological molecule should the drug act upon? Phase 2 addresses target identification, the classification of druggable target families, the technological armamentarium used to identify and validate a target, and the chemical profiling techniques — structure-activity relationship analysis and ADMET prediction — used to characterise candidate compounds before they proceed to biological testing. A drug target is the specific biological macromolecule — most commonly a protein, but occasionally a nucleic acid or lipid — with which a drug molecule interacts to bring about its characteristic pharmacological effect. The concept of a discrete molecular target, now central to rational pharmacology, replaced the earlier, largely empirical view of drug action as an unexplained whole-organism response, and it is this conceptual shift that allows modern drug discovery to be approached as a rational engineering problem rather than a matter of chance observation. A well-chosen target satisfies several criteria: it must be causally linked to the disease process (rather than merely correlated with it), it must be experimentally 'druggable' — that is, physically and structurally amenable to modulation by a small molecule, peptide, or biologic — and it must be sufficiently selectively expressed or engaged that modulating it does not produce unacceptable effects on unrelated physiological processes. Druggable targets fall into a small number of structural and functional families, and recognising which family a candidate target belongs to immediately suggests the appropriate assay technology, the likely mode of pharmacological action, and relevant precedent drugs. Modern target identification typically follows a structured, multi-step workflow that moves from population-level genetic evidence to molecular-structural confirmation.

Step 1 — Disease Genetics (GWAS)

Genome-Wide Association Studies compare the genomes of affected and unaffected individuals to identify genetic variants statistically associated with disease, nominating the genes harbouring or nearest to those variants as candidate targets.

Step 2 — Omics and Proteomics Approaches

Transcriptomic profiling by RNA sequencing compares gene expression between diseased and healthy tissue, with consistently upregulated genes flagged as candidate targets. Proteomic techniques such as two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry identify differentially expressed proteins and map protein–protein interaction networks that may reveal druggable nodes.

Step 3 — Phenotypic Screening

In parallel or as an alternative to target-based approaches, compounds may be screened directly for a desired cellular or organismal phenotype without prior knowledge of the molecular target, with target identity deconvoluted afterwards.

Step 4 — Affinity Chromatography

A compound of interest, immobilised on a solid support, is used to 'fish out' the proteins that bind it from a cell lysate, directly identifying its molecular binding partner(s).

Step 5 — Structural Biology

X-ray crystallography and cryo-electron microscopy (Cryo-EM) resolve the three-dimensional structure of the candidate target, often in complex with a ligand, enabling structure-based drug design.

Step 6 — Bioinformatic Validation

Candidate targets and their interactions are cross-referenced against curated databases: UniProt for protein function and sequence annotation, STRING for protein–protein interaction networks, ChEMBL for bioactivity data, and DrugBank for known drug–target relationships. Once a hit compound has been identified, medicinal chemists undertake structure-activity relationship (SAR) analysis: the systematic modification of chemical structure to understand how specific structural features govern biological activity. SAR analysis identifies the pharmacophore — the minimum constellation of structural features essential for activity, such as a hydrogen-bond donor or acceptor, a hydrophobic region, or a charged group — and guides bioisosteric replacement, in which a functional group is substituted with one of similar electronic or steric character to improve selectivity, potency, or ADMET properties without abolishing activity.

Lipinski's Rule of Five

A widely used heuristic for predicting oral bioavailability, Lipinski's Rule of Five states that a drug-like molecule generally has a molecular weight below 500 Da, a calculated logP (lipophilicity) below 5, fewer than 5 hydrogen-bond donors, and fewer than 10 hydrogen-bond acceptors. Compounds violating two or more of these criteria are statistically less likely to be orally bioavailable, although numerous clinically important exceptions exist (particularly among natural products and biologics), so the rule should be applied as a guide rather than an absolute filter. ADMET — Absorption, Distribution, Metabolism, Excretion, and Toxicity — prediction is undertaken early in compound profiling to eliminate candidates likely to fail for pharmacokinetic or safety reasons, long before the expense of animal testing is incurred. Each ADMET parameter has established in-silico and in-vitro surrogate measures. Phase 2 has traced the path from an unvalidated biological hypothesis to a chemically and pharmacokinetically profiled lead compound: the classification of druggable target families, the genomic-to-structural workflow of target identification, and the SAR and ADMET techniques used to refine a hit into a viable candidate. With a validated target and a profiled compound in hand, the next stage of the pipeline — described in Phase 3 — is to test that compound's actual biological activity using in-vitro pharmacological screening.