Effect of chemical permeation enhancers on nerve blockade

Abstract

Chemical permeation enhancers (CPEs) have the potential to improve access of local anesthetics to the nerve, thereby improving nerve block performance. We assessed the effects of six CPEs on nerve blockade from tetrodotoxin (TTX) and from bupivacaine. Each of the six surfactants, representing three CPE subgroups (anionic, cationic, and nonionic surfactants) was coinjected with TTX or bupivacaine at the sciatic nerve of Sprague-Dawley rats. Myotoxicity of CPEs, alone and with TTX, was assessed in vitro in C2C12 myotubes and in vivo via histological analysis. All enhancers produced marked concentration-dependent improvements in the frequency and duration of block with TTX but not bupivacaine. An in vitro toxicity assay showed a wide range of CPE myotoxicity, but in vivo histological assessment showed no signs of muscle or nerve damage at concentrations of CPEs that produced a half-maximal increase in the duration of block of TTX (except in the case of the cationic surfactant DDAB). This study demonstrates that CPEs can provide marked prolongation of nerve blockade from TTX but not bupivacaine, without apparent local tissue toxicity. These results may enhance the clinical applicability of TTX for prolonged-duration local anesthesia.

Figure 1

Effect of CPE concentration on the duration of sensory block from 30 μm TTX. Durations are expressed as medians with 25^th and 75^th percentiles (n ≥ 4; see Methods). SOS = sodium octyl sulfate; SLS = sodium lauryl sulfate; OTAB = octyltrimethylammonium bromide; DDAB = dodecyltrimethylammonium bromide; T20 = polyoxyethylene (20) sorbitan monolaurate; T80 = polyoxyethylene (20) sorbitan monooleate.

Figure 2

Survival of C2C12 myotubes after a 2-hour exposure to the EC_50eff of each CPE alone and with 30 μM TTX, in vitro. Controls include cells exposed to PBS or TTX alone. Data are shown as means ± standard deviations (n = 4). SOS = sodium octyl sulfate; SLS = sodium lauryl sulfate; OTAB = octyltrimethylammonium bromide; DDAB = dodecyltrimethylammonium bromide; T20 = polyoxyethylene (20) sorbitan monolaurate; T80 = polyoxyethylene (20) sorbitan monooleate.

Figure 3

Histology of the site of injection CPEs at EC_50eff with or without TTX 4 days after injection. (A) Skeletal muscle and nerve from regions injected with T20 (shown), SOS, SLS, OTAB or T80 at their EC_50eff showed no evidence of injury to muscle or nerve. T20 at higher concentrations, of 21.4 mM (B) and 81.4 mM (C), led to increased inflammation with muscle fiber atrophy (arrows). DDAB at its EC_50eff (D) showed surface inflammation similar to that seen with the higher T20 concentrations. Injections of T20 (shown, E), T80, SOS, SLS, and OTAB with TTX showed no evidence of injury to muscle or nerve. Injection of DDAB with TTX (F) showed findings similar to those seen with DDAB in the absence of TTX. N = nerve. Magnification = 200X.

Figure 4

Maximum block duration (MBD) plotted against interpolated EC_100min values for each CPE indicate no correlation (R² = 0.08). Anionic, cationic, and nonionic surfactants are grouped by color (black, grey, and white, respectively). Block durations are expressed as medians with 25^th and 75^th percentiles (n ≥ 4; see Methods).

Figure 5

Maximum block duration (MBD) and EC_100min plotted against cell survival. The EC_50eff is the CPE concentration that caused a half-maximal increase in block duration. Cell survival data are means with standard deviations from C2C12 cells exposed to each CPE at its EC_50eff for 2 hours from Figure 2). Block durations are expressed as medians with 25^th and 75^th percentiles.