The reason for analyzing ondansetron as a possible trigger of MH was a case report about a 5-year-old boy with suspected MH disposition who, after receiving 2 mg ondansetron for treatment of gastroenteritis, developed muscular rigidity, then hyperthermia, and finally died. A previously performed muscle biopsy had shown multiminicore disease, and screening of the RYR1 gene had detected a novel Arg3983His (c.11948G>A) mutation.2 However, an IVCT to diagnose MH susceptibilty had not been performed.5,6 There is reasonable doubt whether this was a true case of MH and whether ondansetron contributed to its onset, because in addition to the above-mentioned facts, the results of the postmortem examination revealed bronchopneumonia, mesenteric lymphadenopathy, a bacterial colonized intrahepatic thrombus, and an enterovirus infection. Further developments of the presented case are important to mention: the monozygotic twin brother of the deceased boy who carried the same genetic mutation in the ryanodine receptor gene died after developing muscular rigidity and hyperthermia without any triggering circumstances at the age of 7 years.7 A spontaneous uncontrolled calcium release in patients carrying the Arg3983His mutation seems to be a probable pathology in this case.
A mutation in the same position in exon 87 (Arg3983Cys) has recently been related to cases of awake MH-like episodes in the absence of pharmacological triggering agents. In vitro studies in myotubes of this RYR1 variant showed increased caffeine sensitivity of calcium release in comparison with wild-type RYR1.8 Although the mutation differs from the one in the above-presented case (histidine versus cysteine), both reports suggest that an alteration in this position of RYR1 causes transformation, which is functionally relevant for myoplasmic calcium release.4 In 2 of 6 patients in our study, mutations in the RYR1 gene have been detected. Thr2206Met (c.6617C>T) is a known causative mutation that has been published in 2 studies and is listed in the EMHG database.9,10 The second variant Val4234Leu (c.12700G>C) has recently been mentioned when a novel exome sequencing method for MH-relevant genes was evaluated.11 None of these were the same or similar to the mutations reported in the described case. Muscle of patients with RYR1 mutation did not show a different response to ondansetron in comparison with MHS muscle of patients without mutation in our study. Yet the possibility that the specific Arg3983His mutation would induce a more accentuated reaction after contact with ondansetron cannot be excluded on the basis of our results. However, the objective of our study was the evaluation of ondansetron's effects on muscle of MHS patients, not exclusively of a single RYR1 variant.
Besides volatile anesthetics and deporalizing muscle relaxants administered in the course of general anesthesia, other pharmacological agents are suspected of initiating the life-threatening metabolic disorder of MH. The 5-HT2-receptor agonist 1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) led to significant contractures in muscle specimens of MHS patients in vitro12 and induced typical symptoms of MH (generalized muscle contraction, hyperthermia, and metabolic failure) in MHS pigs in vivo.13 On the other hand the 5-HT2-receptor antagonists ketanserin and ritanserin successfully prevented halothane or 5-HT-induced MH-like episodes in susceptible pigs,13,14 and pretreatment with ritanserin significantly reduced muscular contractures in MHS muscle strips after DOI administration in vitro.15 Because these effects were present in the animal model and in isolated muscle bundles, the findings are suggestive of a direct effect of serotonin on skeletal muscle metabolism via activation or inhibition of G-protein-coupled 5-HT2-receptors.16
In contrast, the pharmacological mode of action of ondansetron is blockade of 5-HT3-receptors, which are ligand gated ion channels mainly located in the area postrema and in the peripheral vagal nerve terminals.17 There is evidence that ondansetron dose-dependently inhibited muscular nicotinic acetylcholine receptors, suggesting an additional impact on other types of ion channels that might contribute to unexpected side effects.18 At this time, however, a direct correlation between ondansetron administration and increased muscle rigidity or muscular calcium release is not known. Therefore, the development of contractures in MHS muscle bundles in our investigation was surprising.
In adults with normal hepatic function, maximum ondansetron plasma concentrations are reached between 0.8 and 2.0 hours after a single dose of 8 mg ondansetron, with peak levels varying between 31.2 ng/mL and 95.6 ng/mL after oral or IV administration, respectively.19 In our study, muscle bundles of MHS patients developed significantly stronger contractures than did those of MHN individuals at ondansetron concentrations >50 μg/mL. Obviously, these concentrations exceeded the therapeutic plasma levels by a minimum of 500 times.
Pharmaceutical agents are usually combined with solvents to ensure chemical stability of the drug. When in vitro investigations detected significant contracture development in muscle bundles of MHS patients after application of 4-chloro-m-cresol, an additive, e.g., used in injectable insulin,20 the question was raised as to whether therapeutic doses of insulin could cause MH. However, this risk had been denied, because plasma concentrations of 4-chloro-m-cresol in vivo were far lower than were the concentrations needed to cause a muscular response in vitro.21 For the same reason we doubt that the administration of clinically relevant doses of ondansetron poses a threat to MHS patients.
The stock solution of the IV ondansetron preparation used for our investigation contained sodium chloride, sodium citrate, citric acid-monohydrate, and ondansetronhydrochloride-dihydrate. On the basis of a recent review of literature, a correlation between these additives and MH has not been reported. Therefore, we assume that the effects demonstrated in our investigation are attributable to ondansetron rather than to any of the solvents.
With regard to the experimental setup of our study, it is not possible to clarify the underlying metabolic mechanism of the observed effects. However, because we found dose-dependent contractures in both investigated groups but different threshold concentrations for MHS and MHN muscle, we doubt that the reason is a nonspecific toxic effect of ondansetron on muscle tissue.
In conclusion, the results of the present study show that ondansetron dose-dependently induces significant muscular contractures in MHS and at higher concentrations also in MHN muscle in vitro. Because the concentration of ondansetron required to produce muscle contracture exceeded the therapeutic plasma levels by at least 500 times, it is very unlikely that ondansetron contributes to the development of MH in humans.
Name: Stephan Johannsen, MD.
Contribution: This author helped conduct the study, analyze the data, and write the manuscript.
Attestation: Stephan Johannsen has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Norbert Roewer, MD.
Contribution: This author helped design the study.
Attestation: Norbert Roewer has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Frank Schuster, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Frank Schuster has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
This manuscript was handled by: Peter J. Davis, MD.
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© 2012 International Anesthesia Research Society
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