The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of the NTP or the U.S. Government.
1-Butyl-3-methylimidazolium chloride is a hydroscopic solid and classified as an "ionic liquid" (Landry et al 2005). Ionic liquid are salts of organic cations with melting points generally below 100°C. ILs are being widely investigated in industrial and laboratory processes as "green" replacements for volatile organic solvents in catalysis, synthesis, and separation processes (Landry et al. 2005, Sipes et al. 2008). Three ILs, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium chloride, and N-butylpyridinium chloride, were nominated to the National Toxicology Program by the Center for Green Manufacturing, University of Alabama for toxicological testing based on their potential use as possible alternatives to organic solvents. According to the Material Safety Data Sheet from Solvent Innovation GmbH, the LD50 of 1-butyl-3-methylimidazolium chloride in rats is >2000 mg/kg for dermal application (OECD directive 402). However, according to Landry et al. (2005), lethality was observed in 2 out of 6 female BALB/c mice exposed dermally to 1900 mg/kg, 75% (w/v) 1-butyl-3-methylimidazolium chloride in N,N dimethylformamide vehicle. In addition, Landry et al. (2005) demonstrated that 1-butyl-3-methylimidazolium chloride at concentrations of 12.5% and 25% (w/v) increased ear swelling when compared to the DMF vehicle in female BALB/c after 3 consecutive days of dermal application. In the same study, mice treated with 2% or 10% (w/v) 1-butyl-3-methylimidazolium chloride also produced a 3-fold increase in lymph node cell proliferation over DMF vehicle control in the local lymph node assay (Landry et al. 2005).
The objective of this study was to determine the sensitizing potential of 1-butyl-3-methylimidazolium chloride when applied dermally to female BALB/c mice. Measurement of the contact hypersensitivity response was initially accomplished using the combined LLNA and irritancy assay. Based upon the potential toxicity of the chloride moiety and on solubility tests completed by the sponsor, the highest achievable concentration of 1-butyl-3-methylimidazolium chloride in acetone: olive oil (4:1; AOO) was 25% (w/v). Accordingly, the concentrations of 1-butyl-3-methylimidazolium chloride used for the combined LLNA and IRR assays were 25%, 12.5%, 6.25%, and 3.12%. In the LLNA, 1-butyl-3-methylimidazolium chloride at concentrations of 3.12%-25% induced significant increases in lymph node cell proliferation (disintegrations per minutemouse) when compared to the vehicle control. In addition, 1-butyl-3-methylimidazolium chloride was not an irritant at concentrations up to 25%, as demonstrated by a lack of effect on ear thickness at 24 hr following the last dermal exposure.
To further confirm the positive LLNA response, the Mouse Ear Swelling Test was performed following exposure to 1-butyl-3-methylimidazolium chloride. In the sensitization phase, two concentrations of 1-butyl-3-methylimidazolium chloride (3.12% and 6.25%) were used, and a concentration of 6.25% was applied during the challenge phase. 1-Butyl-3-methylimidazolium chloride at the sensitization concentration of 3.12% significantly increased mouse ear swelling at 24 hr post-challenge. There was also a slight increase in percent ear swelling at 24 hr post-challenge in mice sensitized with 6.25% 1-butyl-3-methylimidazolium chloride, however the increase did not reach the level of statistical significance. There was no significant difference in the percent ear swelling at 48 hr following challenge in mice treated with 1-butyl-3-methylimidazolium chloride when compared to the vehicle irritancy control.
Overall, the results from these studies have demonstrated that dermal exposure to 1-butyl-3-methylimidazolium chloride at concentrations of 3.12%-25% produced significant increases in lymph node cell proliferation in the LLNA and negative in the irritancy assay at dose levels up to and including 25%. In addition, 1-butyl-3-methylimidazolium chloride at a concentration of 3.12% induced a statistically significant increase in ear swelling as evaluated using the MEST.
Landry T.D., Brooks K., Poche D., Woolhiser M. (2005). Acute toxicity profile of 1-Butyl-3-Methylimidazolium Chloride. Bull. Environ. Conm. Toxicol. 74: 559-565.
Sipes I.G., Knudsen G.A., Kuester R.K. (2008). The effects of dose and route on the toxicokinetics and disposition of 1-Butyl-3-Methylimidazolium Chloride in male F-344 and Female B6C3F1 mice. Drug metabolism and disposition 36: 284-293.