Skin sensitizers are substances with the potential to cause allergic contact dermatitis, and skin sensitization is the process by which a sensitizer induces the development of allergic contact dermatitis. The biological pathway leading to skin sensitization is well‑characterized and thus is a promising area for the near‑term development of testing strategies that do not require the use of animals.
ICCVAM is developing a U.S. plan for the evaluation of alternative skin sensitization test methods and testing strategies. ICCVAM has requested public comment as it develops a plan for moving forward with alternative skin sensitization test methods and testing strategies. Specifically, ICCVAM is interested in receiving information on the state of the science regarding alternative test methods and testing strategies for skin sensitization, as well as comments on what role ICCVAM, regulated industries, nongovernmental organizations, and other interested parties should play in the development and evaluation of these methods.
NICEATM is collaborating with industry scientists to develop an integrated decision strategy that will allow consideration of all available information, including results from multiple test methods and a test substance’s physical or chemical properties, to support hazard classification decisions for skin sensitizers.
ICCVAM has evaluated several versions of the murine local lymph node assay (LLNA), an alternative test method used to identify substances that could cause allergic contact dermatitis in humans. The LLNA uses fewer animals than the traditional guinea pig test methods used for this purpose, requires less time to perform, provides dose–response information, and eliminates the pain and distress associated with an allergic reaction.
In 2007, the U.S. Consumer Product Safety Commission (CPSC) requested that NICEATM and ICCVAM assess the validation status of new versions and applications of the LLNA. One activity conducted by ICCVAM in response to this request was an evaluation of the use of the LLNA for potency categorization of chemicals causing allergic contact dermatitis in humans. ICCVAM concluded that the LLNA can be used to categorize substances as strong skin sensitizers (Globally Harmonized System of Classification and Labelling of Chemicals Subcategory 1A). However, substances that are not identified as strong sensitizers using the LLNA require additional information to categorize them as other than strong sensitizers (Globally Harmonized System of Classification and Labelling of Chemicals Subcategory 1B). Acceptance of the LLNA for categorization of substances as strong skin sensitizers could broaden the use of the LLNA in place of guinea pig tests, reducing and refining animal use for this purpose.
U.S. Federal agencies have endorsed several ICCVAM‑recommended alternative testing methods for assessing potential allergic contact dermatitis hazards presented by chemicals and products. The recommended test methods were the topic of a January 2011 workshop organized by NICEATM and ICCVAM. The workshop provided information on current best practices for safety testing to determine whether chemicals and products may cause allergic contact dermatitis in humans using methods and approaches that reduce, refine, and replace animal use.
ICCVAM has made recommendations to Federal agencies on the use of two nonradioactive versions of the LLNA. The availability of LLNA methods that do not use radioactivity is expected to allow more institutions to take advantage of the animal welfare benefits of the LLNA, and provides environmental benefits as well.
Following its original evaluation of the LLNA in 1999 (see last paragraph below), ICCVAM recommended the LLNA as a valid alternative to traditionally accepted guinea pig test methods for assessing the allergic contact dermatitis hazard potential of substances for most testing situations. The CPSC nomination noted above requested that NICEATM and ICCVAM evaluate the applicability of the LLNA for testing formulations, metals, substances in aqueous solutions, and other products with the expectation that a wider applicability domain for the LLNA would enable wider use. ICCVAM has recommended that the LLNA may be used to test any chemical or product for allergic contact dermatitis hazard potential unless the chemical or product to be tested has properties that may interfere with the ability of the LLNA to detect sensitizing substances.
ICCVAM has prepared performance standards for the LLNA, which can be used to evaluate the accuracy and reliability of new versions of the LLNA that are considered functionally and mechanistically similar to the accepted LLNA test method. These performance standards reflect extensive international discussions and represent a set of internationally harmonized standards for the conduct of the LLNA (essential test method components, a minimum list of reference substances, and accuracy and reliability criteria). The ICCVAM‑recommended LLNA performance standards also include a updated LLNA test method protocol that reduces animal use by 20% compared to the original ICCVAM‑recommended LLNA test method protocol by decreasing the minimum number of animals per dose group from five to four.
One of the modifications of the LLNA evaluated by NICEATM and ICCVAM in response to the CPSC nomination was the “reduced” LLNA (rLLNA). When used to test a substance for the potential to cause allergic contact dermatitis, the rLLNA uses fewer animals than the LLNA to provide a “yes-no” result. ICCVAM has recommended that the rLLNA be used routinely to determine the allergic contact dermatitis hazard potential of chemicals and products before conducting the multidose LLNA in testing situations in which a negative result is expected or dose–response information is not required. Since the rLLNA uses only a single high‑dose group along with concurrent negative and positive control groups, use of the rLLNA can reduce the number of animals needed for each test by 40% compared to the multidose LLNA.
In September 1998, NICEATM hosted a meeting of an independent peer review panel to review the validity of the LLNA as an alternative test method to guinea pig tests for assessing the allergic contact dermatitis potential of chemicals. The panel concluded that the LLNA is a valid alternative to currently accepted guinea pig test methods, and that the LLNA reduces the number of animals required for testing, eliminates the pain and distress associated with an allergic response, requires less time to perform, and provides dose–response information.