Pharmacology  ·  Local Anesthetics

Chemistry, Pharmacokinetics, and Individual Agents

Amides vs. esters, pharmacokinetic determinants, and high-yield agent profiles

• The amide/ester classification is the organizing axis for everything in this module. The linkage bond determines where metabolism happens: esters are cleaved by plasma pseudocholinesterase; amides are metabolized in the liver. That metabolic difference determines how long the drugs last, which patients face toxicity risk with impaired elimination, and — importantly for allergy — what breakdown product causes hypersensitivity reactions.
• The "i before -caine" mnemonic for amides is an examination shortcut, not pharmacology. The pharmacologically important fact is that amides and esters do not cross-react. A patient allergic to an ester is allergic to para-aminobenzoic acid, the metabolite. That allergy is irrelevant to amide agents. The rule box after the class cards states this explicitly because misidentification of cross-reactivity is a prescribing error with real consequences.
• The pharmacokinetics table translates three physical properties into clinical behavior. Lipid solubility determines how well the drug penetrates membranes and how tightly it binds to neural tissue — both are captured in the potency and duration columns. Protein binding is what keeps the drug at the site; high protein binding means slow release and long duration. The pKa column revisits the ionization concept from Module 1: a pKa closer to 7.4 means more un-ionized drug at physiological pH and faster onset.
• Bupivacaine and lidocaine anchor the individual agent section because they represent the two most tested trade-offs in local anesthetic pharmacology. Lidocaine is moderate on everything — onset, duration, toxicity — which makes it the workhorse. Bupivacaine is long-acting and highly protein-bound, which makes it ideal for epidurals, but its cardiac sodium channel kinetics — fast block, very slow recovery — make resuscitation from bupivacaine-induced cardiac arrest extremely difficult.
• Cocaine stands alone as the only local anesthetic that causes vasoconstriction — all others vasodilate. That property is exploited for nasal and pharyngeal procedures where vasoconstriction reduces bleeding. Methemoglobinemia links prilocaine and benzocaine: both generate metabolites that oxidize hemoglobin iron, and both are tested in that context regardless of their other properties.