MANY drug molecules exist as one or more enantiomers, and recent evidence indicates that different enantiomers of the same drug may have different biologic properties. 1–8These differences are presumed to exist because enantiomers differ in their interaction with chiral centers at their target site. 3,4,8,9
There is an alternative explanation that must also be given consideration: enantiomers may differ in their ability to reach the target site because of differential interaction with the chiral proteins, lipids, and carbohydrates in both the intracellular and the extracellular spaces. Multiple studies show that human skin 10,11and the blood–brain barrier 12,13differ in degree of permeability for enantiomers of the same drug.
The purpose of this study was to determine whether the two enantiomers of bupivacaine (R (+)-dextrobupivacaine and S (−)-levobupivacaine) differ in ability to move through tissue. To address this question, we used a previously described in vitro diffusion cell model 14–17to quantitate the permeability of both bupivacaine isomers in the spinal meninges of the monkey (Macaque nemestrina ).
Materials and Methods
All studies were approved by the University of Washington Animal Care and Use Committee, Seattle, Washington. All monkey (M. nemestrina ) meningeal tissue was obtained from animals killed as part of the Tissue Distribution Program of the University of Washington Regional Primate Research Center.
Details of the method have been previously described. 14–17Briefly, postage stamp–sized pieces of intact meningeal tissue—dura, arachnoid, and pia mater—were excised from the animal and placed over the port connecting two reservoirs of a temperature-controlled (37°C) diffusion cell. At time 0, 0.5 mg S - or R -bupivacaine (Celltech Chiroscience, Ltd., Cambridge, UK) was added to the donor reservoir, along with approximately 37 kBq of the corresponding 3H-labeled enantiomer (Amersham Life Science, Arlington Heights, IL):S -bupivacaine–specific activity, 2,072 GBq/mm; radiochemical purity, 98%;R -bupivacaine–specific activity, 3,145 GBq/mm; radiochemical purity, 98%. Thereafter, at 10 min-intervals for 100 min, 100 μl was removed from both reservoirs and placed in separate scintillation vials for later scintillation counting.
As previously described, bupivacaine concentration was determined by use of radiotracer methods using an Ecoscint scintillation cocktail (National Diagnostics, Atlanta, GA) and a liquid scintillation counter (Tri-Carb 2000; Packard Instruments, Downer’s Grove, IL). 14–17
Bupivacaine flux was determined from linear regression of the bupivacaine-versus -time data. These data were plotted and the permeability coefficient was calculated from the equation
where P = permeability coefficient (cm/min), Q = bupivacaine flux (μg/min), C = bupivacaine concentration in the donor reservoir (μg/ml), and A = tissue area (cm2).
Statistical Analysis
The permeability coefficients for R - and S -bupivacaine were compared using the unpaired Student t test. Differences were considered to be statistically significant if P < 0.05. Data are reported as mean ± SD.
Results
Each enantiomer was studied in 10 tissue specimens. The r2for regression lines used to determine bupivacaine flux averaged 0.973 ± 0.031 (range, 0.887–0.998), indicating excellent fit of the data to a linear model. The permeability coefficient averaged 1.6 ± 1.1 × 10−3cm/min for S -bupivacaine and 1.5 ± 0.7 × 10−3cm/min for R -bupivacaine (P = 0.786).
Discussion
To our knowledge, this is the first study to investigate whether there are differences between local anesthetic enantiomers regarding ability to move through tissue. This is a particularly important question to be addressed with local anesthetics because their use in anesthesiology relies on their ability to diffuse through tissue to reach their target sites. The data clearly indicate that there is no difference in the meningeal permeability of the R - and S -enantiomers of bupivacaine. In addition, the permeability coefficients measured in this study are nearly identical to those previously reported by our laboratory for the racemic bupivacaine mixture (1.6 ± 0.4 × 10−3cm/min). 14This finding shows the reproducibility and reliability of this model.
Several studies have shown pharmacodynamic differences between R - and S -bupivacaine. For example, Kopacz et al. 18demonstrated in humans that levobupivacaine produced significantly longer sensory block than did the racemic mixture of R - and S -bupivacaine. Our data suggest that this finding is the result of differential actions of the R - and S -enantiomers at the target site and is not the result of differences in ability to reach the target from the epidural space.
The authors thank Gary Strichartz, Ph.D., Professor of Anesthesia/ Pharmacology, Vice Chairman for Research, Anesthesia Research Laboratories, Brigham and Women’s Hospitals, Boston, Massachusetts, for the gift of tritiated R - and S -bupivacaine.
