Involvement of nucleophiles in the inactivation of atracurium

Response to pancuronium after loss of atracurium-induced neuromuscular blockade

OBJECTIVE

To describe a previously unreported event in which a patient became refractory to atracurium-induced neuromuscular blockade, but subsequently was adequately paralyzed with a standard dosage of pancuronium.

CASE SUMMARY

A previously healthy 17-year-old woman who sustained multiple trauma developed tolerance to an atracurium infusion she was receiving while undergoing mechanical ventilation. On day 3 of neuromuscular blockade, she became unresponsive to atracurium as evidenced by excessive physical movement, increased peak airway pressures, and overbreathing assist control ventilation. Repeat boluses and increases in the atracurium infusion rate to a maximum of 1.27 mg/kg/h failed to provide a desired clinical response. A bolus dose of pancuronium 0.15 mg/kg was administered and the constant infusion was then changed to pancuronium 0.078 mg/kg/h. Within minutes, decreased respirations, peak airway pressures, and agitation were noted. The pancuronium infusion rate was then tapered to 0.045 mg/kg/h over 72 hours and continued to maintain adequate neuromuscular blockade.

DISCUSSION

Potential pharmacokinetic and pharmacodynamic causes of loss of neuromuscular blockade in this patient are postulated. Possible explanations for loss of neuromuscular blockade include increased degradation of atracurium and/or a change in acetylcholine receptor physiology.

CONCLUSIONS

The development of resistance to a specific neuromuscular blocking agent in the intensive care setting does not necessarily imply cross-tolerance or resistance to alternative agents. Also, loss of respiratory control by one neuromuscular blocking agent may be overcome by changing agents.

Carrier-mediated transport in the hepatic distribution and elimination of drugs, with special reference to the category of organic cations

Carrier-mediated transport of drugs occurs in various tissues in the body and may largely affect the rate of distribution and elimination. Saturable translocation mechanisms allowing competitive interactions have been identified in the kidneys (tubular secretion), mucosal cells in the gut (intestinal absorption and secretion), choroid plexus (removal of drug from the cerebrospinal fluid), and liver (hepatobiliary excretion). Drugs with quaternary and tertiary amine groups represent the large category of organic cations that can be transported via such mechanisms. The hepatic and to a lesser extent the intestinal cation carrier systems preferentially recognize relatively large molecular weight amphipathic compounds. In the case of multivalent cationic drugs, efficient transport only occurs if large hydrophobic ring structures provide a sufficient lipophilicity-hydrophilicity balance within the drug molecule. At least two separate carrier systems for hepatic uptake of organic cations have been identified through kinetic and photoaffinity labeling studies. In addition absorptive endocytosis may play a role that along with proton-antiport systems and membrane potential driven transport may lead to intracellular sequestration in lysosomes and mitochondria. Concentration gradients of inorganic ions may represent the driving forces for hepatic uptake and biliary excretion of drugs. Recent studies that aim to the identification of potential membrane carrier proteins indicate multiple carriers for organic anions, cations, and uncharged compounds with molecular weights around 50,000 Da. They may represent a family of closely related proteins exhibiting overlapping substrate specificity or, alternatively, an aspecific transport system that mediates translocation of various forms of drugs coupled with inorganic ions. Consequently, extensive pharmacokinetic interactions can be anticipated at the level of uptake and secretion of drugs regardless of their charge.

Reactivity and toxicity of atracurium and its metabolites in vitro

Cytotoxicity of atracurium and of its metabolites was tested in vitro. Exposure of isolated rat hepatocytes to atracurium produced cellular damage evidenced by extrusion of an intracellular enzyme, lactate dehydrogenase (LDH), into the incubation medium. Leakage of LDH was directly related to the concentration of atracurium in the medium (250 to 800 microM). If the spontaneous degradation of atracurium (presumably via Hofmann elimination) was first carried out in vitro and the degradation products subsequently added to the isolated hepatocytes, the leakage of LDH was also dose-dependent but larger than that observed after the addition of the parent drug. When l-cysteine was admixed to the products of the spontaneous degradation of atracurium prior to their addition to the liver cells, no leakage of LDH was observed. The results are compatible with the working hypothesis that atracurium itself and, even more so, acrylates formed in Hofmann elimination of atracurium, are reactive toward nucleophiles and damage the cells by alkylating nucleophiles present in cellular membranes. Antecedent covalent binding of acrylates to the nucleophile cysteine, i.e., the formation of acrylate-cysteine adducts, saturated the reactive capacity of acrylates for nucleophiles and thus prevented the reactive metabolites from alkylating the endogenous nucleophiles. Possible clinical consequences resulting from in vivo generation of reactive metabolites are not clear at the present time but are projected to be related to (a) the dose of atracurium administered, (b) the amount of acrylates generated, (c) the functional importance of the endogenous nucleophiles alkylated, and (d) the pathway and the speed of detoxification of atracurium and its metabolites.

Generation of reactive metabolites during spontaneous degradation of atracurium in vitro

The hypothesis was tested in vitro that the spontaneous degradation of atracurium leads to formation of electrophilic metabolites. Variable amounts of atracurium were incubated in saline (0.9% NaCl) for 120 minutes at pH 8.0 and 37 degrees C. Subsequently, cysteine was added to the incubation solutions and the incubation was continued at pH 7.4 and 37 degrees C. Frequent determination of the mercapto groups of cysteine revealed a progressive diminution of the mercapto groups remaining in the incubation solutions. The consumption of sulfhydryl groups was maximal at 20 minutes after the addition of cysteine and amounted to approximately twice the molar amount of atracurium. Kinetic analysis indicated that one mercapto group was consumed almost instantly, whereas the consumption of the other proceeded with a half-life of 4 minutes. No consumption of mercapto groups was observed when laudanosine was incubated with cysteine. Incubation of atracurium or of its degradation products with carboxylesterase markedly reduced the amount of reactive metabolites present in the incubation solutions. The results are compatible with the working hypothesis that spontaneous degradation of atracurium via Hofmann elimination results in generation of two equivalents of reactive electrophilic esters, probably acrylates. We propose that in vivo the portion of the aliphatic chain in the atracurium molecule that is converted to acrylates by Hofmann elimination may be eliminated in part in urine as a conjugate of mercapturic acid.