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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
N-Butyl-pyridinium chloride belongs to a growing class of industrial compounds known as "ionic liquids" (Cheng et al., 2009). Ionic liquids are salts of organic cations with melting points generally below 100°C (Landry et al., 2005). They typically consist of nitrogen-containing organic cations and inorganic anions. ILs are being widely investigated in industrial and laboratory processes as replacements for volatile organic solvents in catalysis, synthesis, and separation processes (Landry et al., 2005; Sipes, et al., 2008). They have gained attention within the green chemistry community because they are distinguished by a range of useful properties such as negligible vapor pressure, thermal stability, nonflammability, high ionic conductivity, and remarkable solubility properties. Three ILs, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium chloride, and NBuPY, were nominated to the NTP by the Center for Green Manufacturing, University of Alabama, for toxicological testing based on the increasingly widespread interest in these compounds. Limited toxicity studies have evaluated the cytotoxicity of ILs in vitro in mammalian cell lines and in ecotoxicity studies using a variety of organisms. However, a recent NTP study (NTP, 2010) has indicated that BMIM, an ionic liquid with similar chemical structure to NBuPY, increased ear swelling at concentrations of 6.25%-25% (w/v) when compared to the 4:1 acetone:olive oil vehicle in female BALB/c mice after 3 consecutive days of dermal application. In the same study, BMIM demonstrated overt toxicity when applied to the ear at 25% (w/v) and when applied to the back at 12.5% (w/v).
The objective of this study was to determine the sensitization potential of NBuPY when applied dermally to female BALB/c mice. The Local Lymph Node Assay was performed to measure the sensitization potential of NBuPY at the concentrations of 1.65%, 3.12%, 6.25%, and 12.5% (w/v) in vehicle dimethylformamide. There were no significant differences in lymph node cell proliferation in the LLNA assay in mice treated with NBuPY when compared the vehicle control mice.
The irritant potential of NBuPY was examined using the irritancy assay. There were increases in percent ear swelling at 24 hr following the last exposure of mice with 1.65% - 12.5% NBuPY with statistically significant changes observed in animals treated with NBuPY at concentrations of 3.12% and 12.5%.
The Mouse Ear Swelling Test was performed to further investigate the potential contact hypersensitivity effects of NBuPY. In the sensitization phase, three concentrations of NBuPY (3.12%, 6.25%, and 12.5%) were used, and a concentration of 12.5% was applied during the challenge phase. Sensitization and subsequent challenge with NBuPY had no effect on ear thickness at either 24 hr post-challenge or 48 hr post-challenge when compared to the vehicle irritancy control group. However, there was a significant difference in percent ear swelling between the VHIC group and the vehicle control group at 24 hr post-challenge, which was consistent with the irritancy effect of NBuPY observed in the IRR assay.
In summary, the results from these studies have demonstrated that NBuPY at 1.65% - 12.5% (w/v) produced no effects in either the LLNA or the MEST. However, NBuPY produced an increase in percent ear swelling at 24 hr after the last exposure in the irritancy study.
Cheng Y., Wright S.H., Hooth M.J., & Sipes I.G. (2009). Characterization of the disposition and toxicokinetics of N-Butylpyridinium chloride in male F-344 Rats and Female B6C3F1 mice and its transport by organic cation transporter 2. Drug Metabolism and Disposition 37 (4): 909-916.
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. Drug Metab Dispos 36: 284-293.
NTP. (2010). Final Report: Assessment of contact hypersensitivity to 1-butyl-3-methylimidazolium chloride in female BALB/c mice.