Describe the chemistry of local anesthetics.
Local anesthetics are drugs that produce
transient sensory, motor, and autonomic nervous system blockade following
regional anesthesia. Local anesthetics are weak bases that consist of a
lipophilic group (usually a benzene ring) and a hydrophilic group (usually a
tertiary amine) separated by a connecting hydrocarbon chain. Most local
anesthetics have an ester or an amide bond that links the hydrocarbon chain to
the lipophilic group. The nature of this bond is the basis for classifying them
as ester or amide local anesthetics. Examples of the ester group are cocaine,
procaine (Novocain), chloroprocaine (Nesacaine), benzocaine (Americaine), and
tetracaine (Pontocaine). Lidocaine (Xylocaine), mepiva-caine (Carbocaine),
bupivacaine (Marcaine, Sensorcaine), etidocaine (Duranest), prilocaine
(Citanest), and ropivacaine (Naropin) are examples of the amide local
anesthetics. Mepivacaine, bupivacaine, and ropivacaine have an asymmet-ric
carbon atom resulting in levoisomers and dextroisomers. Commercial preparations
of mepivacaine and bupivacaine consist of racemic mixtures, whereas the
recently developed ropivacaine and levo-bupivacaine consist of the pure
levoisomers.
Local anesthetics in aqueous solution exist in
dynamic equilibrium between the nonionized lipid-soluble form and the ionized
water-soluble form. The pH at which there are equal amounts of nonionized and
ionized molecules is known as the pKa of that drug. Local anesthetics have pKa
values somewhat above physiologic pH and, as a result, less than 50% of the
drug will exist in the lipid-soluble nonionized form.
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