---
title: "From Hit to Lead: Structure-Activity Relationships"
description: "A screening hit is a starting point, not a drug. Making analogues one change at a time shows which parts of the molecule the target actually needs, and the pattern that emerges is what guides the next"
canonical: https://lightmysky.com/learn/science/from-hit-to-lead-structure-activity-relationships-mt_p53KgFKAUg
source: https://lightmysky.com/learn/science/from-hit-to-lead-structure-activity-relationships-mt_p53KgFKAUg.md
retrieved: 2026-09-12
---

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# From Hit to Lead: Structure-Activity Relationships

A screening hit is a starting point, not a drug. Making analogues one change at a time shows which parts of the molecule the target actually needs, and the pattern that emerges is what guides the next round of synthesis.

Subject: Science · Area: Chemistry · Ages 23 to 24
Page: https://lightmysky.com/learn/science/from-hit-to-lead-structure-activity-relationships-mt_p53KgFKAUg

## Ready when they can

- Designs a set of analogues that isolates the contribution of one substituent
- Reads a potency table and states which structural feature the target requires
- Distinguishes a real potency gain from an artefact of assay conditions or compound aggregation
- Explains what ligand efficiency measures and why potency alone is a poor guide

## Lesson: From screening hit to lead compound

A screening hit is a starting point, not a drug. To learn what one substituent does, you make analogues that differ at that spot alone. Then any shift in potency belongs to that single change.

Lay the analogues out in a potency table and hunt for a pattern. A feature found in every strong compound and missing in the weak ones is one the target requires. If a weak compound carries the feature too, it helps but is not the whole story.

A better number is not always better binding. Assay conditions can flatter a compound, and some compounds clump together in solution and fake activity. Change the conditions and retest before you trust a gain.

**Tip.** Potency alone is a poor guide, because bigger molecules often score higher by bulk. Ligand efficiency measures potency per unit of size, so it rewards compact strength. Pick the efficient compound as your lead, then grow it with care.

**Recap.** Change one spot at a time, read the table for the pattern, distrust flattering numbers, and let efficiency guide the next compounds.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Enzyme Inhibition and Metabolic Control](https://lightmysky.com/learn/science/enzyme-inhibition-and-metabolic-control-mt_8VYolkJ_wR)
- [Ligand Binding: Dissociation Constants and Receptor Occupancy](https://lightmysky.com/learn/science/ligand-binding-dissociation-constants-and-receptor-occupancy-mt_i-v79OaM0n)

## Opens up

- [Drug-Like Properties: Solubility, Permeability and Metabolic Fate](https://lightmysky.com/learn/science/drug-like-properties-solubility-permeability-and-metabolic-fate-mt_HGnGjBCLM6)