In the Spotlight
Problems and progress with replacing the use of animals for skin corrosion testing
Published: November 16, 2013
Currently available skin corrosion test methods
Chemical skin corrosion data are required in some form by five US agencies, and, historically, generation of the data has required the use of the Draize rabbit skin test. Although the Draize nomenclature had fallen out of fashion by the time the method was standardized as Organisation for Economic Cooperation and Development (OECD) test guideline (TG) 404, the procedure remained fundamentally unchanged: injuries caused by applying a test substance directly to a shaved patch of skin on a live rabbit are subjectively scored at specified time intervals for up to 2 weeks following the exposure.
Fortunately, the use of rabbits for skin corrosion assessment was an early target for replacement, and three in vitro replacements have been validated and published as OECD TGs. The first of these methods, Corrositex, was approved for use by the US Department of Transportation (DOT) in 1994, reviewed and endorsed as valid by ICCVAM in 1999 and ECVAM in 2000, and published as OECD TG 435 in 2006. Additional in vitro replacement methods that can distinguish corrosive and noncorrosive chemicals have been validated and published as OECD TG 430 (Transcutaneous Electrical Resistance Test Method) and 431 (Reconstructed Human Epidermis (RHE) Test Method).
Incomplete coverage for all skin corrosion regulations
Availability of these replacement methods has dramatically reduced the number of rabbits used in skin corrosion testing across most agencies, but regulatory requirements of the DOT have kept the in vivo approach in use. As outlined in DOT’s Hazardous Materials Regulations (HMR), all chemicals in shipment must be accompanied by labeling that determines, among other things, maximum volumes and appropriate packaging for safe transport.
To satisfy DOT labeling requirements, skin corrosion test data are used to assign a chemical to one of three packing groups (PGs). The PG for most chemicals has been established and published in public dangerous goods lists or hazardous materials tables. For chemicals that have not yet been assigned a PG, new testing is required. TG 430 and TG 431 are unable to fully distinguish all three PGs. While TG 435 has this ability, it is only compatible with a limited range of chemical classes. When TG 435 cannot be used, rabbits are used.
A legal mandate in the European Union requiring preferential use of nonanimal alternatives in place of animal methods has led to policies that largely enable companies to avoid the use of rabbits for skin corrosion labeling, but DOT has not yet adopted similar practices. In fact, prior to 2012, shippers contacting DOT regarding the use of nonanimal methods were in many instances told to use non-animal approaches in conjunction with, rather than in place of, using rabbits. People for the Ethical Treatment of Animals (PETA) first petitioned DOT to end its recommendation of this practice in 2009, with the Department finally revising its HMR in 2011 and its recommendations to shippers in 2012. However, the practice likely continues in the absence of any prohibition against it and a lack of non-animal methods for identifying corrosion subclasses for most types of chemicals. In addition, certainty regarding the ongoing use of rabbits to satisfy DOT labeling requirements is impossible because the Department does not collect information from shippers on methods used to determine PGs.
The discrepancy between approaches in the US and the EU are being discussed at the United Nations (UN) and may ultimately impact the way in which in vivo and in vitro methods are used to satisfy transportation labeling requirements globally. The next meeting of the UN Sub-Committees of Experts on the Transport of Dangerous Goods (TDG) and on the Globally Harmonized System of Classification and Labeling of Chemicals (GHS), scheduled for December 4–6, 2013, will mark the third anniversary of this informal working group formed to harmonize skin corrosion classification criteria in the UN Model Regulations on the Transport of Dangerous Goods (Model Regulations) with those in GHS.
Under GHS, skin corrosion classification is not expressed in the PG terms used by DOT or the Model Regulations but rather in a similar but not identical system of three subcategories. A simple one-to-one relation of GHS skin corrosion sub-categories 1A, 1B, and 1C, and the Model Regulations Class 8 PGs I, II, and III is imperfect and causes unexpected problems. For instance, by the working group’s estimate, applying this one-to-one relationship as a harmonization strategy would cause the proportion of substances classified in PG I—”very dangerous substances and preparations”—to jump from the current 5% to as much as 90%. Because PG I designation carries a number of regulatory restrictions, such as banning transport by air or use in aerosols, this approach is considered excessive and economically unfeasible by regulators and industry. As DOT revises its regulations based almost exclusively on revisions to the UN Model Regulations, the recommendations of this working group will largely determine whether there is increased use of nonanimal skin corrosion test methods for transportation purposes in the US. and internationally.
Ongoing corrosion testing harmonization activities and retrofitting existing alternatives
While the harmonization discussion continues at the UN, researchers have reevaluated nonanimal methods for opportunities to broaden their applicability in corrosion subcategory classification. MatTek Corporation, developers of the EpiDerm™ human skin model covered by TG 431, noted that the initial EpiDerm™ validation data suggested the test system was highly sensitive at distinguishing GHS skin corrosion subcategory 1A from the less corrosive subcategories 1B and 1C. After a successful follow-up validation study financed in part by PETA, the OECD adopted revisions to TG 431 in July 2013 that permit its use (along with the other RHE models covered by the TG) for subcategorization. An OECD representative presented this information at the most recent meeting of the UN working group and encouraged the group to consider the revision in its continuing deliberations. The reason for this suggestion is clear, as the revised TG 431 observes that “[t]he regulatory framework in member countries will decide how this Test Guideline will be used.”
The history of moving toward full implementation of nonanimal skin corrosion tests is an exercise in diligent promotion and refinement of available replacements as well as strategies to limit the use of rabbits to that of an absolute last resort and eventually eliminate their use altogether. It is also an unfortunate reminder of the absence of mandate, and therefore urgency, for US agencies to implement validated alternative test methods or close gaps in the applicability of alternative methods. Testing of corrosive substances on rabbits would likely continue indefinitely in absence of outside intervention. PETA and the PETA International Science Consortium, Ltd. (PISC) continue to raise this issue with DOT and the US and UK representatives of the UN working group, and have brought the issue to the attention of the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). PETA has successfully petitioned DOT to modify its policies and to better promote the use of alternatives, but current policy nevertheless means that the practice of using animals for this assessment continues and will require additional follow-up.
The continued use of animals for corrosion labeling in the U.S. must remain a focal point in the discussion surrounding the difficulty in achieving full implementation of validated alternative methods. The discussion within the UN will hopefully provide the added impetus needed to close this loophole in U.S. policy for skin corrosion testing. Until the regulatory community is mandated and/or motivated to implement validated alternative methods, the burden of ensuring that new test methods are taken up in the regulatory and industrial sectors will continue to rest with interested parties who are not necessarily involved in regulation or manufacturing.