How Well Does the Clinical Pipeline Cover Approved Drug Space? A Reproducible Chemical Diversity Audit of ChEMBL Phase 1–4 Small Molecules
We quantify the structural overlap between FDA-approved small molecule drugs and clinical-stage candidates using a fully executable cheminformatics pipeline. Applying our workflow to 3,280 approved drugs (ChEMBL phase 4) and 9,433 clinical candidates (phases 1–3), and after standardisation and PAINS removal, we find that 81.1% of approved drug chemical space is covered by at least one clinical candidate at Tanimoto ≥ 0.4 (Morgan fingerprints, radius=2). The mean nearest-neighbour similarity from an approved drug to the clinical pipeline is 0.580, suggesting broad but imperfect overlap. Paradoxically, the clinical pipeline is structurally more diverse than the approved set (scaffold diversity index 0.605 vs. 0.419), yet 18.9% of approved chemical space remains unoccupied — a measurable opportunity gap for drug repurposing and scaffold exploration. Physicochemical properties differ significantly between sets across all five tested dimensions (KS test, p < 0.05), with clinical candidates being more lipophilic (mean LogP 2.84 vs. 1.92) and less polar (TPSA 84.8 vs. 98.8 Ų) than approved drugs. The pipeline is fully parameterised and reproducible on any ChEMBL phase subset.