Abstract

Ropivacaine is a new amide-type local anesthetic agent. Unlike bupivacaine and mepivacaine, two structurally similar local anesthetic compounds, ropivacaine is exclusively the S-(-)-enantiomer. Ropivacaine is predominantly eliminated by extensive metabolism in the liver, with only 1% of the dose being excreted unchanged in the urine of humans. Four of the metabolites formed in human liver microsomes were identified as 3-OH-ropivacaine, 4-OH-ropivacaine, 2-OH-methyl-ropivacaine, and 2',6'-pipecoloxylidide (PPX). The enzymes involved in the human metabolism of ropivacaine have not been identified. To ascertain which forms of cytochrome P450 are involved, ropivacaine was incubated with human microsomes from 10 different livers having different cytochrome P450 activities. A strong correlation was found between the formation of 3-OH-ropivacaine and CYP1A (r = 0.87-0.89) and between the formation of 4-OH-ropivacaine, 2-OH-ropivacaine, and PPX and CYP3A (r = 0.97-1). Incubation of ropivacaine and human liver microsomes in the presence of alpha-naphthoflavone or furafylline, inhibitors of CYP1A, decreased the formation of 3-OH-ropivacaine by about 85%, without affecting the formation of the other metabolites. The formation of 4-OH-ropivacaine, 2-OH-methyl-ropivacaine, and PPX was markedly inhibited in the presence of troleandomycin, an inhibitor of CYP3A. Microsomes from cells expressing CYP1A2 formed 3-OH-ropivacaine, whereas 4-OH-ropivacaine, 2-OH-methyl-ropivacaine, and PPX were formed in microsomes from cells expressing CYP3A4. Inhibitors of CYP2C (sulfaphenazole), CYP2D6 (quinidine), and 2E1 (diethyldithiocarbamate) did not inhibit the formation of any metabolite from ropivacaine. In conclusion, CYP1A catalyzes the formation of 3-OH-ropivacaine, the main metabolite formed in vivo, whereas the formation of 4-OH-ropivacaine, 2-OH-methyl-ropivacaine, and PPX was catalyzed by CYP3A.