DECREASED SENSITIVITY TO METOCURINE DURING LONG-TERM PHENYTOIN THERAPY MAY BE ATTRIBUTABLE TO PROTEIN-BINDING AND ACETYLCHOLINE-RECEPTOR CHANGES

Long-term phenytoin therapy induces resistance to the neuromuscular blocking effects of metocurine. The hypothesis that this is attributable to increased plasma protein binding of the drug (decreased free fraction) related to increased concentrations of alpha-1-acid glycoprotein (AAG) or attributable to the proliferation of acetylcholine receptors (AChR) at the muscle membrane was tested in the rat. After 14 days of phenytoin 40 mg.kg-1, administered intraperitoneally twice daily (n = 12), the neuromuscular pharmacodynamics were evaluated and compared with those of time-matched controls (n - 10). Protein binding was measured by equilibrium dialysis, AAG concentrations by radial immunodiffusion assay, and AChR by I-125-alpha-bungarotoxin binding. The effective dose for 50% inhibition of baseline twitch height (ED50) was significantly greater in the phenytoin group than in the control group (15.03 +/- 1.65-mu-g.kg-1 vs. 9.98 +/- 0.69-mu-g.kg-1, respectively). The concentrations of AAG increased gradually from 133.8 +/- 7.8-mu-g.ml-1 at day 0, to 343.1 +/- 58.0-mu-g.ml-1 at day 7, to 1,729.5 +/- 422.3-mu-g.ml-1 at day 14 in the phenytoin group. The induction of AAG concentrations in plasma was dependent on plasma phenytoin concentrations and was most prominent after 14 days of phenytoin (r = 0.77; P < 0.01; n = 22). The free fraction of metocurine was significantly decreased in the phenytoin group compared to the control group (67.2 +/- 0.18% vs. 74.5 +/- 2.5%). There was a significant negative correlation between increased AAG concentrations and decreased free fraction (r = 0.65). The spontaneous recovery of twitch to 75% of full recovery was significantly faster (22.7 +/- 3.3 min vs. 36.0 +/- 4.3 min) and was seen at greater total (42.6 +/- 2.5 ng.ml-1 vs. 32.3 +/- 2.8 ng.ml-1) and greater unbound (29.9 ng.ml-1 +/- 2.9 vs. 22.2 +/- 1.9 ng.ml-1) plasma metocurine concentrations in the phenytoin group. Concurrently, the AChR number on the muscle membrane was significantly increased in the phenytoin group (2.67 +/- 0.23 fmole/mg protein vs. 1.84 +/- 0.26 fmole/mg protein). A higher free fraction of metocurine required for a given twitch inhibition, together with an increased plasma protein binding of metocurine in the phenytoin group, suggests a pharmacodynamic and pharmacokinetic mechanism for the resistance that may include increased AChR numbers at the muscle membrane and increased plasma protein binding of metocurine related to augmented AAG concentrations.