4-Chloro-m-cresol triggers malignant hyperthermia in susceptible swine at doses greatly exceeding those found in drug preparations

Background: Chlorocresols are used as preservatives in numerous commercial drugs that have been shown to induce myoplasmic Ca²⁺ release; the most potent isoform is 4-chloro-m-cresol. The aims of this study were to (1) examine the in vivo effects of 4-chloro-m-cresol on swine susceptible to malignant hyperthermia and (2) contrast in vivo versus in vitro dose-response curves. Methods: Susceptible swine (weight: 38.5 kg ± 3.55 kg) were anesthetized and monitored for variations in physiological responses, including end-tidal CO₂, heart rate, blood pressure, blood chemistry, and temperatures. In the first animals studied, 4-chloro-m-cresol, at equivalent cumulative doses of 0.14, 0.28, 0.57, 1.14, 2.27, 4.54, and 9.08 mg/kg (n = 3; 12.5, 25, 50, 100, 200, 400, and 800 μmol) were administered, and in a second group, larger doses were used: 1.14, 3.41, 7.95, 17.04 (n = 4), and/or 35.22 (n = 1) mg/kg (100, 300, 700, 1,500, and/or 3,100 μmol). For comparison, in vitro rectus abdominis muscle preparations obtained from normal and susceptible swine were exposed to 4-chloro-m-cresol, at cumulative concentrations of 6.25, 12.5, 25, 50, 100, 200, 400, 800, and 1,600 μmol; standard caffeine and halothane contracture testing was also performed. Results: Episodes of malignant hyperthermia were not triggered in response to administration of low doses of 4-chloro-m-cresol, but transient cardiovascular reactions (e.g., tachycardia, arrhythmias, and hypotension) were observed. Subsequently, episodes in these animals were triggered when halothane (0.87; 1 MAC) and succinylcholine (2 mg/kg) were given. Animals administered the higher doses of 4-chloro-m-cresol all had fulminant episodes of malignant hyperthermia that were fatal, when equivalent cumulative concentrations were greater than 1,500 μmol. The levels of 4-chloro-m- cresol in the plasma rapidly decreased: e.g., 5 min postadministration of the 1,500-μmol dose, the mean plasma level was only 52 ± 18 μmol (n = 4). Hemolysis was detected following 4-chloro-m-cresol administration at concentrations > 200 μmol. In vitro, muscle from susceptible animals elicited contractures > 200 mg at 50-μmol bath concentrations of 4-chloro- m-cresol (n = 29), whereas normal muscle did not elicit such contractures until bath concentrations were > 800 μmol (n = 10). Conclusions: 4-chloro- m-cresol is a trigger of malignant hyperthermia in susceptible swine, but only when serum concentrations are far above those likely to be encountered in humans. A relatively low concentration of 4-chloro-m-cresol, 50 μmol, is sufficient to activate sarcoplasmic [Ca⁺²] release in vitro (e.g., contractures); this same bolus dose administered in vivo (0.57 mg/kg) has minimal effects due to the rapid decrease in its plasma levels.