IARC MONOGRAPHS ON THE EVALUATION OF CARCINOGENIC ...

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hyperplasia, atrophy, chronic inflammation and hyaline degeneration of the olfactory
epithelium; hyperplasia, squamous metaplasia, hyaline degeneration and goblet cell
hyperplasia of the respiratory epithelium; and glandular hyperplasia and squamous
metaplasia. The incidence and severity of these lesions increased with increasing
exposure concentration.
(c) In-vitro studies
IARC MONOGRAPHS VOLUME 82
Naphthalene and its metabolite 1-naphthol (0–100 μM; 2 h, 37°C) were cytotoxic to
mononuclear leukocytes after metabolic activation with human liver microsomes. Other
metabolites of naphthalene, including 1,2-naphthoquinone and 1,4-naphthoquinone
(0–100 μM; 2 h, 37 °C), were directly toxic to mononuclear leukocytes and depleted
glutathione to 1.0% of control levels. The primary metabolite of naphthalene, the
1,2-epoxide, was not cytotoxic at concentrations up to 100 μM and did not deplete glutathione,
suggesting that the quinones are responsible for the cytotoxicity of naphthalene
in human mononuclear leukocytes (Wilson et al., 1996).
Perfusion of the lungs of male Swiss Webster mice (4–5 weeks of age) with naphthalene
(0.02–2 mM in Waymouth’s medium for a one-hour period followed by 4 h in
medium) resulted in swelling and vacuolation of Clara cells. This was followed by
concentration-dependent losses of Clara cells from the bronchiolar epithelium. Pulmonary
glutathione levels decreased over a range of 60% (at the 0.2-mM dose) to less than
10% (at the 2-mM dose) of the corresponding control level. Following perfusion with
[ 14 C]naphthalene (1.67–167 μM for 30 min followed by 4.5 h in medium), reactive
metabolites were covalently bound to protein in the lung and perfusate. Total binding
(nanomoles bound) and specific activity (nanomoles per milligram protein) increased in
lung tissue with increasing concentrations of naphthalene (Kanekal et al., 1990). A subsequent
study with this isolated perfused mouse lung system demonstrated that the
circulating epoxides of naphthalene play a significant role in naphthalene-induced lung
injury. Injury to Clara cells in lungs perfused with naphthalene or secondary metabolites
such as naphthoquinones, 1-naphthol and 1,2-dihydro-1,2-dihydroxynaphthalene was
less dramatic than the effects observed following exposure to naphthalene 1,2-oxide
(Kanekal et al., 1991).
The metabolism and cytotoxicity of naphthalene and its metabolites were investigated
in vitro in Clara cells isolated from male Swiss Webster mice (4–5 weeks of age).
The cells were incubated for 2 or 4 h with 0.1, 0.5 and 1 mM naphthalene, 1,4-naphthoquinone,
1,2-naphthoquinone, 1-naphthol, naphthalene 1,2-oxide and 1,2-dihydroxy-
1,2-dihydronaphthalene. The only metabolites that were more toxic to the Clara cells
than the parent compound were 1,4-naphthoquinone and naphthalene 1,2-oxide, the latter
being the most potent. Piperonyl butoxide, a CYP monooxygenase inhibitor, blocked the
toxic effect of naphthalene but not that of naphthalene 1,2-oxide, which suggested that
the epoxide is a key participant in the process leading to the loss of cell viability in
isolated Clara cells (Chichester et al., 1994).