Why start from something that barely binds
Fragment-based drug discovery screens molecules far smaller than typical drugs — often under 250 daltons, a dozen or so heavy atoms. A fragment hit usually binds weakly, with affinity in the high-micromolar to millimolar range. By HTS standards that is pathetic. The insight is that a small molecule makes few contacts, so any binding it does achieve must come from a genuinely good fit at a real hotspot — there is no room to win by accident.
The right yardstick is not raw potency but ligand efficiency — binding energy per heavy atom. A fragment with weak affinity but only ten atoms can be far more efficient than a bloated HTS hit, and efficiency is what you can build on. Starting small also samples chemical space vastly more thoroughly: a few thousand well-chosen fragments cover the space of small fragments better than a million drug-sized compounds cover theirs.
Seeing the invisible: biophysical detection
Binding this weak is invisible to an ordinary activity assay, so fragment screening leans on biophysical methods that detect binding directly. Surface plasmon resonance watches mass accumulate on a chip as fragments stick to immobilized protein. NMR sees fragments perturb protein signals. Thermal-shift assays read how a fragment stabilizes the folded protein. X-ray crystallography is the gold standard: it not only confirms binding but shows exactly where and how, inside the pocket.
Because the signals are faint, false positives and false negatives both abound. The standard answer is orthogonal confirmation: a fragment is only believed when two independent methods agree — say SPR plus a crystal structure. This pairing with structure is why fragments and structure-based design are natural partners; you rarely run one without the other.
Growing a fragment into a lead
Once you know where a fragment sits, you make it bigger on purpose. The main move is fragment growing — extending the fragment toward nearby unfilled space in the pocket to pick up new contacts and affinity. Two related moves are linking two fragments that bind adjacent sub-pockets, and merging features of overlapping fragments into one molecule.
- Confirm the fragment by two orthogonal methods and obtain its bound structure.
- Map the unfilled space and the polar/hydrophobic features around it in the pocket.
- Grow toward a new contact — a hydrogen bond or hydrophobic surface — one small step at a time.
- Re-measure affinity AND ligand efficiency after each step; keep only growth that earns its atoms.