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NTP Report on the Assessment of Contact Hypersensitivity to o-Cresol in Female BALB/c Mice (CASRN: 95-48-7)


The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program (NTP). The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by the NTP on March 2009 (see The findings and conclusions for this study should not be construed to represent the views of the NTP or the US Government.

NTP Report Number IMM20710


2-Methylphenol (ortho-cresol or o-cresol) is a cresol (a monomethyl derivative of phenol) found in coal tar, some resins, pesticides and industrial solvents (Dietz, 1991). It can also be formed during the metabolism of toluene, menthofuran and some essential oils (Madyastha and Raj, 1992; Sequeira et al., 1992). Ortho-cresol (OCS) can penetrate the skin without being metabolized (Boogaard et al., 2000) and has been detected at a concentration of 76.9 mg/L in the urine of workers employed in the distillation of the phenolic fraction of tar (Bieniek, 1994). Bruze and Zimerson (2002) have reported that OCS may be the contact sensitizer present in phenol formaldehyde resin. Using the local lymph node assay and the guinea pig maximization test, Yamano et al. (2003) have demonstrated that p-chloro-m-cresol is a more potent sensitizer than p-chloro-m-xylenol. Several government agencies including the American Conference of Governmental Industrial Hygienists (ACGIH), Environmental Protection Agency (EPA) and National Institute for Occupational Safety and Health (NIOSH) have listed cresols as potential contact sensitizers; however, information on the contact sensitizing effects of OCS is lacking.

The objective of this study was to determine the sensitization potential of OCS when applied dermally to female BALB/c mice. The Local Lymph Node Assay (LLNA) was initially performed to measure the sensitization potential of OCS at the concentrations of 0.5%, 1%, 2.5%, 5%, 10%, 25% and 50% in vehicle acetone: olive oil (AOO). However, OCS was found to cause overt toxicity at concentrations of 25% and 50%, which manifested as either morbidity or moribundity. Therefore, evaluation of the contact hypersensitivity potential of OCS could only be performed at lower concentrations. Although OCS at a concentration of 10% increased the draining lymph node cell proliferation in the first LLNA study, the increase did not reach the level of 3-fold enhancement over the vehicle control, one of the criteria used by the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) to classify a compound as a sensitizer. Additionally, this increase was not reproducible because similar treatment had no effect on the draining lymph node cell proliferation in the second LLNA study conducted.

The irritant potential of OCS was examined using the irritancy (IRR) assay. Significant increases in percent ear swelling at 24 hr following the last exposure were observed in animals treated with OCS at a concentration of 10%.

The Mouse Ear Swelling Test (MEST) was performed to further investigate the potential contact hypersensitivity effects of OCS. In the sensitization phase, three concentrations of OCS (2.5%, 5%, and 10%) were used, and a concentration of 10% was applied during the challenge phase. OCS had no effect on the ear thickness at either 24 hr post challenge or at 48 hr post-challenge when compared to the Vehicle Irritancy Control group. However, there were significant differences in percent ear swelling between the Vehicle Irritancy Control group and the Vehicle Control group at 48 hr post-challenge, which was consistent with the irritant effect of OCS.

Overall, the results from these studies have demonstrated that OCS at concentrations of 0.5%-10% had minimal effect on the lymph node cell proliferation in the LLNA assay, which was consistent with the negative findings in the MEST. However, OCS was found to be an irritant at 10% in the irritancy assay.

Chart 1
Summary of Contact Hypersensitivity Results of O-Cresol


Type of Response Method Used Significant Response
(p < 0.05)
Trend Analysisb
(p < 0.05)
Contact Hypersensitivity LLNA 1st Study Yesa Yes
  LLNA 2nd Study No Yes
  IRR Yes Yes
  MEST 24 Hrs No Yes
  MEST 48 Hrs No Yes

Abbreviations: LLNA = Local Lymph Node Assay; IRR = Irritancy Assay; MEST = Mouse Ear Swelling Test

aWhile statistically significant, the increase did not reach the level of 3-fold enhancement over the vehicle control, and it was not reproduced in the second study.
bThe statistical significance of the exposure-related trend analysis in the LLNA study, the IRR study, and the MEST study was obtained from the Jonckheere's Test.

Report Date: December 2010


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