Drug Repurposing

Drug Repurposing

1. Definition, terminology & conceptual placement

• Drug repurposing (also drug repositioning, re-profiling, re-tasking, therapeutic switching, drug rescue) = the strategy of identifying a new therapeutic indication for a drug that is already approved, was investigational/shelved, or was discontinued/withdrawn — i.e. deploying an existing molecule against a different disease from the one for which it was originally developed (concept grounded in G&G 14e Ch.1's account of new indications emerging post-approval, pp.13–14; KDT 8e Ch.5, p.91).
• Contrast with de-novo drug discovery/invention — G&G stresses that most modern new drugs are "more invented than discovered," painstakingly optimised against a validated target through medicinal chemistry, HTS and CADD (G&G 14e Ch.1, p.4). Repurposing bypasses this front end by starting from a molecule whose chemistry, formulation and human safety are already characterised.
• Repurposing is distinct from the related but narrower concepts the textbooks DO name:
  • "Me-too" / follow-on drugs — a structurally similar new molecule developed against an already-validated target to improve potency, dosing or side-effect profile (e.g. atorvastatin as the 7th statin; esomeprazole as the single-isomer follow-on to omeprazole). This is a new chemical entity, not a new use of an old one (G&G 14e Ch.1, pp.5, 18, Box 1–4). ⚠ Do not conflate "me-too" with "repurposing."
  • Off-label use — prescribing an approved drug for an unapproved indication WITHOUT a new regulatory submission; a US physician "may legally prescribe a drug for any purpose that he or she deems reasonable," though third-party payers may not reimburse off-label use unless backed by a statutory compendium (e.g. AHFS-DI) (G&G 14e Ch.1, p.14). Off-label use is often the clinical signal that precedes formal repurposing.
  • Single-enantiomer switch / chiral switching — marketing the active enantiomer of an existing racemate (e.g. esomeprazole, escitalopram, levosalbutamol, dexketoprofen) (KDT 8e Ch.5, p.89, "Drugs marketed as single enantiomers" box). This creates new market exclusivity but is a reformulation, adjacent to repurposing.
• Why "repurposing" matters as an MD-Pharmacology concept: the conventional new-drug pipeline takes "at least 10 years and costs US$500–1000 million" (KDT 8e Ch.5, p.88) — G&G puts the cost of a new molecular entity (NME) from lab to FDA approval at $1 billion to $4 billion (G&G 14e Ch.1, Table 1–1). Repurposing is the principal pharmacological strategy that attacks this cost/time/attrition problem.