Drug Repurposing

Definition, terminology & conceptual placement

• Definition — Drug repurposing (= drug repositioning, re-profiling, re-tasking, therapeutic switching, drug rescue) is the strategy of identifying a new therapeutic indication for a drug that is already approved, was investigational/shelved, or was discontinued/withdrawn — deploying an existing molecule against a different disease from the one it was originally developed for.
• Contrast with de-novo discovery — Modern new drugs are "more invented than discovered" — optimised against a validated target through medicinal chemistry, HTS and CADD. Repurposing bypasses this front end by starting from a molecule whose chemistry, formulation and human safety are already characterised.
• Not me-too — A me-too / follow-on drug is a structurally similar new molecule against an already-validated target (e.g. atorvastatin as the 7th statin; esomeprazole) — a new chemical entity, not a new use of an old one. Do not conflate me-too with repurposing.
• Not off-label use — Off-label use = prescribing an approved drug for an unapproved indication without a new regulatory submission; legal for a physician but often the clinical signal that precedes formal repurposing (which adds a new approved label).
• Not chiral switch / NDDS — Single-enantiomer/chiral switching (esomeprazole, escitalopram, levosalbutamol) and new delivery systems are reformulations of the same indication — adjacent to, but distinct from, repurposing. NDDS is explicitly out of scope here.
• Why it matters — The conventional pipeline takes "at least 10 years and costs US$ 500–1000 million" (KDT); G&G puts a new molecular entity at US$ 1–4 billion. Repurposing is the principal pharmacological strategy attacking this cost/time/attrition problem.