Local anesthetic: biotransformation and excretion

How are amide-linked local anesthetics metabolized?

A.Hofmann elimination
B. Cytochrome p450 enzymes of the liver
C. Plasma cholinesterases
D. Tissues esterases

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Correct Answer:  B

Explanation:

Local anesthetics consist of three components: a hydrophobic aromatic ring, an amide group, and an intermediary ester or amide chain. They are categorized as either ester-linked or amide-linked according to the type of the intermediary chain in its structure.

Amide local anesthetics undergo biotransformation primarily in the liver through the cytochrome P450 enzymatic system, while ester-linked local anesthetics are hydrolyzed by plasma cholinesterases and tissue esterases. The kidneys are the primary excretory organ for local anesthetics and their metabolites.

An alteration in local anesthetic metabolism can potentially increase systemic concentrations and toxicity. Examples include impaired liver function and hepatic perfusion for amide local anesthetics, and plasma cholinesterase deficiency for ester local anesthetics. Significant reductions in renal function can also impair local anesthetic elimination.

References:
Heavner, J.E. (2008). Pharmacology of local anesthetics. In D.E. Longnecker et al (eds) Anesthesiology. New York: McGraw-Hill Medical.

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