Plasma concentrations of the stereoisomers of prilocaine after administration of the racemate: implications for toxicity?

A chiral high pressure liquid chromatography method was developed to measure the separate isomers of prilocaine in plasma after administration of the racemate. The concentrations of the isomers in six patients were similar (S(+)/R(-) = 1.06 (SD 0.06)) after brachial plexus block with 1.5% (RS)-prilocaine hydrochloride 35 ml, suggesting that a higher systemic safety margin may not be achieved by substituting racemic prilocaine by one of its isomers. Much higher plasma concentrations of the S(+)- than the R(-)-form after oral administration of 300 mg of the racemate (n = 4) indicated a large difference in intrinsic metabolic clearance of the isomers on first pass through gut, liver or both organs.