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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
2,4,5-Trichlorophenoxyacetic acid (TPA) was selected for evaluation as a sensitizing agent for contact hypersensitivity in mice. TPA is a broad-leaf herbicide and defoliant. It was a major component of the defoliant Agent Orange which was used extensively in Vietnam. It was used extensively in this country until reports from studies indicated it to be teratogenic. TPA has been shown to be nonmutagenic and carcinogenesis data is equivocal. It is irritating to the eyes, skin, mucous membranes and upper respiratory tract. Exposure to this compound occurs primarily by inhalation or dermal exposure.
TPA was obtained from Aldrich Chemical Company (Milwaukee Wis.). Dilutions were made in 100% ethanol which served as the vehicle. Three assays were used in this assessment, a primary Irritancy study, to screen for toxicity and determine the Minimal Irritating Concentration (MIC) and the Maximal Non-Irritating Concentration (MNC) and two assays, the Mouse Ear Swelling Test (MEST) and the Local Lymph Node Assay (LLNA), to test the dermal sensitizing potential of TPA.
For irritancy testing four mice per group were used to test six concentrations of test article in vehicle, vehicle alone and untreated controls. The lowest concentration of test article demonstrating a statistically significant -increase in percent ear swelling when compared to vehicle controls was considered the MIC. The highest concentration not demonstrating a significant increase in percent ear swelling over the vehicle controls was considered the MNC. Doses chosen for the hypersensitivity assays were based on the MIC and MNC. The design for the MEST is shown in the table below (Table 1). Pre-, 24 and 48 hr post-treatment ear measurements were taken and used to calculate the percent ear swelling. Measurements from animals dosed with varying concentrations of test article were compared to the TPA background control group for significance. The chemical concentrations and experimental groups used for the LLNA are shown in the table below (Table 2). After 3 consecutive days of dosing, animals were rested for 1 day and then injected with 3H-thymidine. 5 hours later the draining lymph nodes were dissected out and radloassayed. Chemical exposed groups were compared to vehicle controls for statistical significance.
No signs of toxicity were seen in TPA exposed animals The Minimal Irritating Concentration was determined to be 10% and the Maximal Non-Irritating Concentration to be 3%. A contact hypersensitivity response was not demonstrated by the MEST in TPA exposed animals. The positive control group in the MEST produced a statistically significant response at a sensitizing concentration of .25% and challenge concentration of .25% DNFB as compared to the .25% DNFB challenge only group. In the LLNA the highest concentration of TPA tested (10%) and .25% DNFB (the positive control) produced a significant response as compared to the vehicle treated group.
2,4,5-Trichlorophenoxyacetic acid was found to produce irritation at concentrations as low as 10%. Testing for sensitizing potential using the Mouse Ear Swelling test failed to indicate TPA as a contact sensitizer. At the highest dose tested, 10%, the minimal irritating concentration, a positive response was seen in the Local Lymph Node Assay. These data indicate the potential of TPA as a weak contact sensitizer or the possibility of lymph node proliferation associated with irritation or a type 1, IgE mediated response. Further study is needed to interpret the results of these tests.