fragment-based drug discovery

Most screening looks for whole, ready-made keys. Fragment-based drug discovery (FBDD) instead looks for tiny key-bits — very small molecules that latch weakly onto one good spot of the target — and then builds the rest of the key around them. The bet is that finding a small piece that fits perfectly in one corner is easier than finding a large molecule that happens to fit everywhere.

Fragments are small (often under ~300 daltons) and bind weakly, with affinities in the high-micromolar to millimolar range. Because they are small, they sample chemical space far more efficiently — a few thousand fragments can cover the diversity of millions of larger compounds. But weak binding means it cannot be detected by ordinary activity assays; FBDD relies on sensitive biophysical methods such as surface plasmon resonance, NMR, X-ray crystallography, or thermal shift to see the binding event directly.

Once a fragment hit is found and its binding pose is known (usually by crystallography), chemists grow it, link it to a second fragment, or merge it with another scaffold to build affinity while keeping the molecule efficient and small. Because you start lean, the resulting leads tend to have good ligand efficiency and room to optimize properties — though the early signals are subtle and demand rigorous, artifact-aware validation.