Toxicologic significance of histologic change in the larynx of the rat following inhalation exposure: a critical review.

The larynx is a site in the respiratory tract of animals that often shows a response to inhaled substances. In many cases, the most sensitive endpoint in repeated dose inhalation studies is squamous metaplasia (often of minimal severity) of the larynx. The U.S. Environmental Protection Agency has speculated that squamous metaplasia in the rodent larynx might be a pre-neoplastic lesion or a precursor to other serious effects and has proposed to use the effect of squamous metaplasia occurring in subchronic inhalation toxicology studies as a toxicologic endpoint for use in quantitative risk assessment [U.S. Environmental Protection Agency, 2006a. Reregistration Eligibility Decision for MGK-264, U.S. Environmental Protection Agency, 2006b, Reregistration Eligibility Decision for Piperonyl Butoxide, U.S. Environmental Protection Agency, 2006c. Reregistration Eligibility Decision for Pyrethrins]. To reach a conclusion as to its significance, we sought to establish the nature of this effect in the relevant context of rodent inhalation studies. A comprehensive review of the literature shows that laryngeal metaplasia can be produced by a wide range of chemically dissimilar substances, and even by "non-chemical" means such as irritation by aerosols and particles, and dehydration by alcohols or low humidity air. There is no published evidence that this effect is pre-neoplastic and it is clearly and repeatedly characterized as an adaptive response. Moreover, the well-differentiated character of laryngeal squamous metaplasia, the reversibility of incidence and severity of it during recovery periods, combined with no significant clinical observations and the lack of progression over time indicates that this response is adaptive and should not be considered to be indicative of significant human risk. We therefore conclude that squamous metaplasia of the rodent larynx is not a relevant toxicologic endpoint.