Morphology of Nasal Lesions in F344/N Rats Following Chronic Inhalation Exposure to Naphthalene Vapors

Abstract

Naphthalene administered by inhalation at concentrations of 10, 30, or 60 ppm for 6 hours per day, 5 days per week for 105 weeks caused nonneoplastic and neoplastic effects in nasal respiratory and olfactory regions of male and female F344/N rats. Non-neoplastic nasal effects were characterized by an increase in the incidence and severity of a complex group of lesions, including atypical hyperplasia, atrophy, chronic inflammation, and hyaline degeneration of olfactory epithelium; hyperplasia, squamous metaplasia, hyaline degeneration, and goblet cell hyperplasia of the respiratory epithelium; and hyperplasia and squamous metaplasia of mucosal glands. Neoplastic effects were characterized by the induction of two types of rare primary nasal tumors, olfactory neuroblastomas and respiratory epithelial adenomas. The incidences of olfactory neuroblastomas in males at 0 ppm, 10 ppm, 30 ppm, and 60 ppm were, respectively, 0%, 0%, 8%, and 6%, whereas in females they were 0%, 4%, 6%, and 24%. The incidences of respiratory epithelial adenomas in males at 0 ppm, 10 ppm, 30 ppm, and 60 ppm were, respectively, 0%, 12%, 17%, and 31% and in females 0%, 0%, 8%, and 4%. The olfactory neuroblastomas and respiratory epithelial adenomas were considered carcinogenic effects related to naphthalene exposure based on their relatively high incidence in exposed rats, their absence in concurrent control rats and NTP historical controls, and their rare spontaneous occurrence in rats of any strain.

Naphthalene metabolism in relation to target tissue anatomy, physiology, cytotoxicity and tumorigenic mechanism of action

“…A partly genotoxic mode of action cannot be ruled out, as may be indicated by absence of nasal tumors in mice chronically exposed to 10 or 30 ppm naphthalene, despite evidence of nasal irritation, nasal respiratory epithelium hyperplasia, and nasal olfactory-epithelium metaplasia in these mice (NTP, 1992;Abdo et al, 1992). However, in contrast to the spectrum of non-neoplastic nasal lesions including atypical, strongly basophilic, multi-focal and/or "unusual" types-some involving moderate to severe atrophy-detected in naphthaleneexposed rats (Long et al, 2003) discussed above, average severity scores reported for nonneoplastic nasal lesions that occurred in similarly exposed male and female mice all fell within a "minimal" to "mild" range, below scores ≥3 associated with "moderate" or "severe" toxicity (Abdo et al, 1992). The combined rodent bioassay evidence therefore indicates a likely predominant cytotoxic mode of action contributing to the nasal tumors observed in naphthalene-exposed rats.…”

“…NTP rat nasal tumor occurrence was greatly enhanced, if not enabled, by adjacent, histologically related focal cellular proliferation Histopathology findings obtained from the NTP (2000) bioassay strongly suggest that nasal tumor formation in rats chronically exposed to naphthalene by inhalation was associated with, if not enabled solely, by chronic tissue damage and associated regenerative and focal hyperplasia. The extent of chronic nasal cytotoxicity and hyperplasia detected in nearly 100% of all exposed animals (regardless of dose group) was described by Long et al (2003) as follows (bold added):…”

“…As discussed by North et al (2008), these bioassays found increased incidences of nasal respiratory epithelial adenomas and of rare nasal olfactory epithelial neuroblastomas in female rats, and of nasal respiratory epithelial adenomas in male rats exposed to naphthalene vapor concentrations of 10, 30, or 60 ppm for 2 years (NTP, 2000;Abdo et al, 2001;Long et al, 2003). There were increased incidences of alveolar/bronchiolar adenomas or carcinoma in female (but not male) B6C3F1 mice exposed to 30 (but not to 10) ppm naphthalene for 2 years (NTP, 1992;Abdo et al, 1992).…”

Strain-Specific Effects of Alachlor on Murine Olfactory Mucosal Responses

“…Olfactory mucosal toxicity and carcinogenicity occur in rats treated with the phosphodiesterase inhibitor RP 73401, but not mice or dogs (Pino et al, 1999). Similarly, naphthalene is carcinogenic in the rat olfactory mucosa, but not the mouse (National Toxicology Program, 1992;Long et al, 2003). Coumarin caused olfactory mucosal degeneration in both rats and mice; however, rats were more sensitive than mice.…”

Hypothesis-based weight of evidence: A tool for evaluating and communicating uncertainties and inconsistencies in the large body of evidence in proposing a carcinogenic mode of action—naphthalene as an example

“…"Atypical" hyperplasia and olfactory neuroblastomas were observed in rat but not mouse olfactory epithelium (nearly 100% at all doses in rat)� As shown in Table 3, the mouse nasal data are categorized as "ad hoc assumptions required"; that is, if chronic inflammation and regenerative hyperplasia are key events in the naphthalene MoA, and these key events occur in mouse nasal tissue but do not lead to tumor formation, ad hoc assumptions are necessary to explain this observation� Hyperplasia is defined as an increase in the number of normal cells in a normal arrangement within a tissue; it exhibits a normal growth pattern and maturation sequence, and is not generally considered "preneoplastic" (Eustis, 1989)� Atypical hyperplasia (dysplasia), on the other hand, is defined as a proliferation of cells characterized by cellular atypia, alteration in the maturation sequence, or abnormal differentiation of cells within a tissue, and frequently indicates the emergence of a population of cells that may become cancerous (Eustis, 1989)� As discussed in Bogen et al� (2008) and Long et al� (2003), atypical hyperplasia as observed in the rat tumor assay was considered "an unusual proliferative lesion… [and] may represent a precursor for nasal olfactory carcinogenesis� " As shown in Table 3, the rat nasal data provide "some evidence" that CYP2F-induced chronic inflammation and regenerative hyperplasia are key events in the naphthalene MoA, again with the question as to the potential involvement of other CYPs or other metabolic enzymes� However, the olfactory epithelial data provide strong evidence that, regardless of the CYP involved in the primary metabolism of naphthalene, "atypical" hyperplasia may be the key event necessary for carcinoma formation�…”