Jane A. Myers

Alpha1‐acid glycoprotein and albumin in human serum bupivacaine binding

Bupivacaine protein binding was studied with the use of human serum, isolated human serum albumin, and isolated α1‐acid glycoprotein. The effect of lactic acid on bupivacaine binding was also studied. Bupivacaine protein binding in serum is best characterized by a model described by two classes of binding sites and that in α1‐acid glycoprotein or albumin is best characterized by a model described by one class of binding sites for each protein. Albumin binding closely correlated with the data obtained for the low‐affinity, high‐capacity binding site in serum, while α1‐acid glycoprotein data closely correlated with the data obtained for the high‐affinity, low‐capacity site in serum. A reduction in pH resulted in a significant reduction in the affinity for the high‐affinity, low‐capacity site in serum. No other binding parameters were affected. These data were in excellent agreement with results of the isolated protein studies. Our data demonstrate that acidosis results in significant increases in free bupivacaine concentrations only at relatively low total bupivacaine concentrations (<7 µg/ml) and that distribution characteristics for bupivacaine are essentially unchanged over a wide concentration range.

Bupivacaine protein binding in the term parturient: Effects of lactic acidosis

The effects of lactic acidosis on bupivacaine serum protein binding was studied in a group of term parturients and a group of nonpregnant female control subjects. Groups were matched in age and health. Distribution characteristics of bupivacaine in pregnancy were determined. Bupivacaine protein binding was best characterized by the model for two classes of binding sites in all studies. The parturients exhibited a lower capacity for the high‐affinity, low‐capacity (α1‐acid glycoprotein) site and higher affinity for the low‐affinity, high‐capacity (albumin) site. Lactic acidosis decreased the affinity constant for the high‐affinity, low‐capacity site in the control group but did not change binding characteristics in the parturients. Free concentration of bupivacaine (Cu) was elevated at low total bupivacaine concentrations (Ct) (<10 µg/ml). No differences in Cu were detected at concentrations in the cardiotoxic range (>20 µg/ml). The Cu values predicted by the estimated binding parameters from in vitro experiments were compared with actual Cu measured in nine parturients at delivery; they correlated significantly (r = 0.94). Distribution changes for bupivacaine in the parturients were consistent with known physiologic changes in body composition associated with pregnancy. Alterations that occur in serum protein binding during pregnancy should not result in increased risk of central nervous system or cardiovascular system toxicity since these alterations do not increase free tissue concentration.

The Influence of Lactic Acid on the Serum Protein Binding of Bupivacaine: Species Differences

: Various animal models have been used for studies of bupivacaine cardiovascular toxicity. These studies are difficult to relate to the clinical situation, since the disposition of bupivacaine in the various species is unknown. The serum protein binding of bupivacaine, therefore, was determined in human, sheep, monkey, dog, and rat at physiologic pH using ultrafiltration. Since a mixed acidosis results during a systemic toxicity reaction to bupivacaine, the influences of an acidic pH, resulting from the addition of lactic acid, also was examined. All sera exhibited two classes of binding sites, a high-affinity, low-capacity class (class 1) and a low-affinity, high-capacity class (class 2). When compared to human serum at physiologic pH, a significantly higher (P less than 0.05) affinity constant for the class 1 sites was observed for all species studied, with the exception of the rat. All species studied exhibited a significantly lower (P less than 0.05) capacity for the class 1 sites. The binding parameters of the class 2 sites displayed no significant difference. An acid pH resulted in a decrease in bupivacaine protein binding over the entire concentration range studied for all species, with the exception of the monkey. Monkey serum exhibited no change in bupivacaine binding with a decrease in pH. Since protein binding explains only a portion of the total disposition of bupivacaine, further delineation of each animal model under both acidotic and physiologic conditions needs to be accomplished before the animal studies currently under investigation can be extrapolated to the clinical situation.

The influence of diazepam on the serum protein binding of bupivacaine at normal and acidic pH.

Since both bupivacaine and diazepam are highly protein bound, it is possible that a drug displacement interaction could occur, resulting in an increase in free bupivacaine concentration that could exacerbate systemic toxicity. This study was undertaken to characterize the serum binding of diazepam and to evaluate any drug displacement interaction between diazepam and bupivacaine. Human serum obtained from venous blood of normal male and female volunteers was used for measurements of protein binding using an Amicon Micropartition System. Bupivacaine protein binding in the presence of 0, 0.5 and 1.5 μg/ml diazepam was best described by the model for two classes of binding sites. Neither concentration of diazepam significantly altered the capacity or affinity for either class of bupivacaine binding sites when compared to control. Free concentrations of bupivacaine were statistically identical in the presence of both diazepam concentrations. The complete diazepam binding profile in both serum and isolated human serum albumin was best described by a model describing two classes of binding sites. The effect of an acidic pH on bupivacaine was also independent of diazepam concentration. Diazepam protein binding was not affected by a reduction in pH from 7.4 to 7.0. The data reported here suggest no binding displacement interaction exists between bupivacaine and concomitantly administered diazepam. Thus, administration of diazepam during a toxic reaction associated with bupivacaine should not alter free bupivacaine concentration.

Diazepam and Free Plasma Bupivacaine Concentration

One can clearly see that for a constant amount of drug in the body, reducing the volume of distribution will automatically increase the concentration of drug in the blood. If applied to the case of bupivacaine, an increase in free plasma bupivacaine concentration (assuming that plasma protein binding of bupivacaine is unchanged) would be expected on displacement of bupivacaine from the tissues. This possible outcome seems quite incompatible with the authors’ statement, “thus, administration of diazepam during a toxic reaction would not be expected to alter free bupivacaine concentration.“ The authors may wish to comment on this apparent contradiction.

Bupivacaine Protein Binding in the Term Parturient: Effects of Lactic Acidosis

The effects of lactic acidosis on bupivacaine serum protein binding was studied in a group of term parturients and a group of nonpregnant female control subjects. Groups were matched in age and health. Distribution characteristics of bupivacaine in pregnancy were determined. Bupivacaine protein binding was best characterized by the model for two classes of binding sites in all studies. The parturients exhibited a lower capacity for the high-affinity, low-capacity (alpha 1-acid glycoprotein) site and higher affinity for the low-affinity, high-capacity (albumin) site. Lactic acidosis decreased the affinity constant for the high-affinity, low-capacity site in the control group but did not change binding characteristics in the parturients. Free concentration of bupivacaine (Cu) was elevated at low total bupivacaine concentrations (Ct) (less than 10 micrograms/ml). No differences in Cu were detected at concentrations in the cardiotoxic range (greater than 20 micrograms/ml). The Cu values predicted by the estimated binding parameters from in vitro experiments were compared with actual Cu measured in nine parturients at delivery; they correlated significantly (r = 0.94). Distribution changes for bupivacaine in the parturients were consistent with known physiologic changes in body composition associated with pregnancy. Alterations that occur in serum protein binding during pregnancy should not result in increased risk of central nervous system or cardiovascular system toxicity since these alterations do not increase free tissue concentration.

Influence of Cimetidine on Bupivacaine Disposition in Rat and Monkey

Recent reports have appeared describing a cimetidine-induced alteration in either the input or disposition function for many drugs. The total clearance component of the disposition function is the primary perturbation. In the present investigation, the influence of cimetidine on bupivacaine disposition was studied, using in vitro and in vivo models. Since the extraction ratio for bupivacaine is low, total clearance follows the capacity-limited theory. Hence, the influence of cimetidine on the intrinsic clearance of bupivacaine was assessed using microsome and hepatocyte models. Results for both systems are in excellent agreement and indicate a noncompetitive inhibition. Additionally, the influence of cimetidine on the protein-binding profile of bupivacaine was determined. In the presence of cimetidine, the these findings was assessed in vivo in rhesus monkeys. co-administration of cimetidine resulted in an increase in the volume of distribution at steady state and no change in the total clearance of bupivacaine.