Potency of bupivacaine stereoisomers tested in vitro and in vivo - Biochemical, electrophysiological, and neurobehavioral studies

Background: Chiral local anesthetics, such as ropivacaine and levobupivacai ne, have the potential advantage over racemic mixtures in showing reduced t oxic side effects. However, these S-(levo, or "-") isomers also have report edly lower potency than their optical antipode, possibly resulting in no ad vantage in therapeutic index. Potency for local anesthetics inhibiting Nachannels or action potentials depends on the pattern of membrane potential and so also does the stereopotency ratio. Here the authors have quantitated the stereopotencies of R-, S-, and racemic bupivacaine, comparing several in vitro assays of neuronal Na+ channels with those from in vivo functional nerve block, to establish relative potencies and to understand better the role of different modes of channel inhibition in overall functional anesthe sia. Methods: The binding of bupivacaine to Na+ channels was assessed indirectly by its antagonism of [H-3]-batrachotoxin binding to rat brain synaptosomes . Inhibition of Na+ currents by bupivacaine was directly assayed In voltage -clamped GH-3 neuroendocrine cells. Neurobehavioral functions were disrupte d by bupivacaine percutaneously injected (0.1 ml; 0.0625-1.0%) at the rat s ciatic nerve and semiquantitatively assayed. Concentration-dependent action s of R-, S-, and racemic bupivacaine were compared for their magnitude and duration of action. Results: Competitive batrachotoxin displacement has a stereopotency ratio o f R:S = 3:1, Inhibition of Na+ currents with different prepulse potentials shows that S > R potency when the membrane is hyperpolarized, and R > S pot ency when it is depolarized from normal resting values. Functional deficits assayed in vivo usually demonstrate no consistent enantioselectivity and o nly a modest stereopotency (R:S = 1.2-1.3) for peak analgesia achieved at t he lowest doses. Other functions display no significant stereopotency In ei ther the degree, the duration, or their product (area under the curve) at a ny dose. Conclusion: Although the in vitro actions of bupivacaine showed stereoselec tivity ratios of 1.3-3:1 (R:S), in vivo nerve block at clinically used conc entrations showed much smaller ratios for peak effect and no significant en antioselectivity for duration. A primary role for the blockade of resting r ather than open or inactivated Na+ channels may explain the modest stereose lectivity in vivo, although stereoselective factors controlling local dispo sition cannot be ruled out Levo-(S-)bupivacaine is effectively equipotent t o R- or racemic bupivacaine in vitro for rat sciatic nerve block.