Lidocaine plasma protein binding

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Abstract
The percent of unbound lidocaine [LD] in the plasma of 24 healthy [human] subjects was measured by equilibrium dialysis after addition of 3 .mu.g/ml C14 LD hydrochloride. The percentage of unbound LD varied from 19.9-38.8 (30.2 .+-. 5, mean .+-. SD) and was inversely related to the concentration of .alpha.1-acid glycoprotein (AAG) in the plasma. The binding ratio (mol bound divided by mol unbound) of LD was directly related to the plasma AAG concentration. The binding ratio of LD in solutions containing AAG but no albumin, prepared from the plasma of subjects in the study, was also directly related to the concentration of this acute-phase protein. Human serum albumin solution (4 gm/100 ml) bound LD to the extent of 20% under the same conditions. There was no relationship between the binding ratio of LD and the albumin concentration in the plasma of the 24 subjects. In 7 normal subjects variation in AAG between 2 samples collected at least 2 mo. apart was associated with a concomitant change in plasma LD binding. There is considerable interindividual and intraindividual variation in LD binding, and measurements of AAG concentration in plasma may be a useful predictor of the extent of lidocaine plasma binding.