Evaluation of Local Anaesthetics

Evaluation of Local Anaesthetics

1. Definition, rationale & what is being evaluated

• Local anaesthetics (LAs) are drugs that, on topical application or local injection, cause reversible loss of sensory perception (especially pain) in a restricted area by blocking generation and conduction of the nerve impulse, without structural nerve damage (KDT 8e Ch.26, p.386).
• Evaluation (screening) of LAs = the set of validated in vitro and in vivo preclinical methods used to detect, quantify and rank a candidate molecule's local-anaesthetic activity relative to a reference (standard) LA, and to characterise its safety. It is a special application of comparative bioassay — "comparative assessment of the relative potency of a test compound (T) against a standard compound (S) on a living animal or biological tissue" (Medhi ch.2, p.45).
• Why a battery of models is needed: clinically an LA is used in several distinct ways — surface (topical), infiltration, conduction (nerve) block, spinal and epidural — and a molecule may be excellent in one mode yet useless in another (e.g. procaine is a good injectable but is not a surface anaesthetic; benzocaine/oxethazaine are topical-only) (KDT 8e Ch.26, pp.391–4). No single assay captures all clinical uses, so screening uses a tiered panel mapped to the clinical routes.
• Core questions every LA-evaluation answers (the standard endpoints): does the candidate produce anaesthesia (efficacy / yes-no), how potent is it versus the standard, how fast is the onset (latency), how long is the duration, is the block reversible, and what is its toxicity (local + systemic) and vasoactivity (Medhi ch.1 Suggested Reading — Defalque & Stoelting, Anesth Analg 1967; KDT 8e Ch.26, pp.392–3).
• Reference (standard) drugs against which a candidate is benchmarked are chosen by intended route (KDT 8e Ch.26, Tables 26.2 & 26.4, pp.392, 396):
  • Surface/topical: lidocaine, tetracaine, cocaine (historical positive control), proparacaine (ophthalmic).
  • Infiltration / nerve block: lidocaine (the workhorse standard, relative potency 1), procaine (classic low-potency standard), bupivacaine (long-acting, high-potency standard, relative potency 4–5), ropivacaine (relative potency 3–4).
  • Spinal: lidocaine, bupivacaine (heavy/hyperbaric), ropivacaine.
• ⚠ The granular procedural detail of each named LA-screening protocol (exact stimulus counts, scoring scales) belongs to the dedicated animal-model methods literature (e.g. Medhi's animal-models chapter on local anaesthetic / NMB screening). The models here are named and clinically anchored from KDT Ch.26 and route-/animal-/bioassay-anchored from Medhi Ch.1–2; the step-by-step procedural fine-detail is noted as a limitation rather than reproduced.