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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
Heptachlor, also known as Aahepta, Drinox, Heptachlorane, Heptagran, and Velsicol 104, is a chlorinated cyclodiene pesticide used primarily as an agricultural and domestic insecticide from the mid-1960s to the early 1980s (Rought et al., 1999; Smialowicz et al., 2001) While agricultural and pest control use of heptachlor has been banned for two decades, residues of this pesticide and its major toxic metabolite, heptachlor epoxide, persist in the environment (Fendick et al., 1990). Heptachlor or its metabolites have been identified in food, human milk, plants, marine animals, and drainage systems (Fendick et al., 1990; Chuang and Chuang, 1998). Heptachlor can be absorbed through the skin following topical application, and the LD50 in rats is 195-250 mg/kg in a xylene solution (Gaines, 1969). Heptachlor has been reported to be toxic in various organ systems, including the immune system (Smialowicz et al., 2001; IPSC, 1995; Chuang and Chuang, 1991).
The objective of this study was to determine the sensitizing potential of heptachlor when applied dermally to female BALB/c mice. The local lymph node assay was initially performed to measure the sensitization potential of heptachlor at the concentrations of 0.2%, 0.5%, 1%, 2%, 2.5%, 5%, 10%, 25%, and 50% in vehicle acetone: olive oil. However, heptachlor at concentrations of 2.5% and above was found to cause overt toxicity. Therefore, evaluations of contact hypersensitivity to heptachlor could only completed at lower concentrations in the initial study. Heptachlor at concentrations of 0.5-2% increased the draining lymph node cell proliferation in the LLNA; however, the increase did not reach the level of 3-fold stimulation over the vehicle control.
In the irritancy assays, no significant increases in the percent ear swelling were observed at 24 hours following the last exposure, in animals treated with heptachlor at any concentrations.
To further confirm the positive LLNA response, the Mouse Ear Swelling Test was performed following exposure to Heptachlor. In the sensitization phase, three concentrations of Heptachlor (0.5%, 1%, and 2%) were used, and a concentration of 2% was applied during the challenge phase. Heptachlor at concentrations of 1% and 2% significantly increased the ear thickness at 24 hours post challenge but not at 48 hours post-challenge. In addition, there were no significant differences in percent ear swelling between the vehicle irritancy control group and the vehicle control group at either 24 or 48 hours post-challenge.
Overall, the results from these studies have demonstrated that HPT at a concentration of 2% produced a significant increase in lymph node cell proliferation in the LLNA in BALB/c mice. Furthermore, HPT also induced a significant increase in ear thickness as evaluated using the MEST, which is consistent with the LLNA results. However, HPT did not produce any significant changes in the irritancy responses.
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