Between a screening “hit” and a polished drug candidate lies the most chemistry-intensive part of discovery: finding a lead, proving it’s real, and refining it. Here is that journey, step by step.
Hit vs lead
- Hit — a compound that shows the desired, confirmed activity in a screening assay.
- Lead — a prototype with fundamental desired activity but still sub-optimal properties (potency, selectivity, ADME, toxicity) — the starting point for optimisation.
- Sequence: Hit → Hit-to-lead → Lead identification → Lead validation → Lead optimisation → Drug candidate.
Lead identification — methods
- Random / HTS screening of large compound libraries.
- Natural-product screening (plants, microbes, marine sources).
- Combinatorial chemistry — rapid synthesis of many analogues.
- Rational / structure-based design from the target’s 3D structure.
- Fragment-based lead discovery, virtual/in-silico screening (docking), and drug repurposing.
What makes a good lead?
- Potency and selectivity for the target.
- Novelty and patentability (intellectual property).
- Drug-likeness — satisfies Lipinski’s Rule of Five.
- Synthetic feasibility and amenability to SAR.
- Acceptable early ADME and toxicity profile.
Lead validation (qualification)
Confirms the lead is genuinely worth optimising:
- Confirm activity & reproducibility (re-test; rule out assay artefacts and false positives).
- Establish a dose–response relationship and the mechanism of action.
- Confirm selectivity against related targets (counter-screens).
- In-vivo proof of concept in a disease model.
- Early ADME/Tox screening (solubility, metabolic stability, hERG, cytotoxicity).
Lead optimisation
Iterative medicinal chemistry guided by structure–activity relationship (SAR) — improving potency, selectivity, ADME/PK and physicochemical properties while reducing toxicity. The output is a drug (development) candidate ready for preclinical studies.
Exam tip: a hit is active in an assay; a lead is a prototype with desired activity but sub-optimal properties. Validation = confirm activity, selectivity, mechanism, in-vivo proof of concept and early ADME/tox. Optimisation = SAR-driven chemistry → candidate. A good lead is potent, selective, novel/patentable and drug-like (Rule of 5).
Lead work is where biology hands over to chemistry. Master the hit-to-candidate sequence and the marks of a good lead, and you can answer any question on this stage of drug development.