Micheal P. Jokinen

Bronchial and Bronchiolar Fibrosis in Rats Exposed to 2,3-Pentanedione Vapors: Implications for Bronchiolitis Obliterans in Humans

2,3-Pentanedione (PD) is a component of artificial butter flavorings. The use of PD is increasing since diacetyl, a major butter flavorant, was associated with bronchiolitis obliterans (BO) in workers and has been removed from many products. Because the toxicity of inhaled PD is unknown, these studies were conducted to characterize the toxicity of inhaled PD across a range of concentrations in rodents. Male and female Wistar-Han rats and B6C3F1 mice were exposed to 0, 50, 100, or 200 ppm PD 6 h/d, 5 d/wk for up to 2 wk. Bronchoalveolar lavage fluid (BALF) was collected after 1, 3, 5, and 10 exposures, and histopathology was evaluated after 12 exposures. MCP-1, MCP-3, CRP, FGF-9, fibrinogen, and OSM were increased 2- to 9-fold in BALF of rats exposed for 5 and 10 days to 200 ppm. In mice, only fibrinogen was increased after 5 exposures to 200 ppm. The epithelium lining the respiratory tract was the site of toxicity in all mice and rats exposed to 200 ppm. Significantly, PD also caused both intraluminal and intramural fibrotic airway lesions in rats. The histopathological and biological changes observed in rats raise concerns that PD inhalation may cause BO in exposed humans.

Incidences of Selected Lesions in Control Female Harlan Sprague–Dawley Rats from Two-Year Studies Performed by the National Toxicology Program:

The NTP has a long history of using Fischer rats and has compiled a large database of incidences of lesions seen in control animals. Such a database is lacking for Harlan Sprague–Dawley (SD) rats. The intention of this paper is to report spontaneous lesions observed in female vehicle control Harlan SD rats, and to compare the incidence in 2 strains of rats (Fischer and Harlan SD) used in NTP studies. Female Harlan SD rats served as the test animals for a special series of 2-year studies. Male rats were not used in these studies. Complete histopathology was performed on all animals, and the pathology results underwent comprehensive NTP pathology peer review. The most commonly observed neoplasms in these female control Harlan SD rats were mammary gland fibroadenoma (71%), tumors of the pars distalis of the pituitary (41%) and thyroid gland C-cell tumors (30%). Female Fischer rats had incidences of 44% for mammary gland fibroadenomas, 34% for tumors of the pars distalis, and 16% for thyroid gland C-cell tumors. Fischer rats had a 15% incidence of clitoral gland tumors, while the Harlan SD rats had an incidence of < 1%. In contrast to Fischer F344 rats, the Harlan SD rats had a high incidence of squamous metaplasia of the uterus (44.2%). Squamous metaplasia is not a lesion commonly observed in NTP control Fischer rats. The Harlan SD rats had a very low incidence of mononuclear cell leukemia (0.5%), compared with an incidence of 24% in female Fischer rats.

Morphologic Aspects of Rodent Cardiotoxicity in a Retrospective Evaluation of National Toxicology Program Studies

The heart is increasingly recognized as a target for toxicity. As studies in laboratory rodents are commonly used to investigate the potential toxicity of various agents, the identification and characterization of lesions of cardiotoxicity is of utmost importance. Although morphologic criteria have been established for degenerative myocardial lesions in rats and mice, differentiation of spontaneously occurring lesions from toxin-induced or toxin-related lesions remains difficult. A retrospective light microscopic evaluation was performed on the hearts of F344 rats and B6C3F1 mice from National Toxicology Program (NTP) studies of six chemicals identified in the NTP database in which treatment-induced myocardial toxicity was present. Two previously defined myocardial lesions were observed: “cardiomyopathy” that occurred spontaneously or as a treatment-related effect and “myocardial degeneration” that occurred as a treatment-related effect. Both lesions consisted of the same basic elements, beginning with myofiber degeneration and necrosis, with varying amounts of inflammation, interstitial cell proliferation, and eventual fibrosis. This observation is indicative of the heart’s limited repertoire of responses to myocardial injury, regardless of the nature of the inciting agent. A prominent differentiating factor between spontaneous and treatment-induced lesions was distribution and lesion onset. Once the respective lesions had undergone fibrosis, however, they generally appeared morphologically indistinguishable.

Respiratory Tract Lesions in Noninhalation Studies

This paper reviews respiratory tract lesions observed in rodents administered various chemicals by noninhalation routes. Chemicals administered by inhalation caused lesions in the respiratory tract and were well described; however, when chemicals were administered by noninhalation routes the effort to evaluate tissues for lesions may have been less or not considered, especially in the upper respiratory tract, and some lesions may have gone undetected. Lesions described in this review mostly occurred in rodent chronic noninhalation studies conducted by the National Toxicology Program; however, some were noted in studies of shorter duration. The nasal cavity was vulnerable to damage when chemicals were administered by noninhalation routes. Changes included respiratory epithelial hyperplasia, degeneration and necrosis of olfactory epithelium, olfactory epithelial metaplasia, adenoma, adenocarcinoma, squamous cell carcinoma, and neuroblastoma. In the lung, compound-related lesions included alveolar histiocytosis, alveolar epithelial hyperplasia, bronchiolar metaplasia of the alveolar epithelium, squamous metaplasia, alveolar/bronchial adenoma and carcinoma, and squamous tumors. Pathogenesis of these lesions included regurgitation of volatiles, metabolites arriving from the blood stream, and additional metabolism by olfactory epithelium or Clara cells. The presence of respiratory tract lesions in noninhalation studies emphasizes the need for a thorough examination of the respiratory tract including nasal passages, regardless of the route of administration.

Reproductive Lesions in Female Harlan Sprague-Dawley Rats Following Two-Year Oral Treatment with Dioxin and Dioxin-like Compounds:

Results from previously published animal studies suggest that prenatal and postnatal exposure to dioxin and dioxin-like compounds (DLCs) may profoundly affect the reproductive system of both sexes via endocrine disruption. In the present work, we evaluate the toxicity and carcinogenicity of various DLCs, with an emphasis on their effect on the reproductive organs, induced by chronic exposure of female adult Harlan Sprague-Dawley rats. This investigation represents part of an initiative of the National Toxicology Program to determine the relative potency of chronic toxicity and carcinogenicity of polychlorinated dioxins, furans, and biphenyls. For fourteen, thirty-one, or fifty-three weeks or for two years, animals were administered by gavage 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); 3,3′,4,4′,5-pentachlorobiphenyl (PCB126); 2,3,4,7,8-pentachlorodibenzofuran (PeCDF); 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153); 2,3′,4,4′,5-pentachlorobiphenyl (PCB118); a tertiary mixture of TCDD, PCB126, and PeCDF; a binary mixture of PCB126 and 153; or a binary mixture of PCB126 and PCB118. The ranges of treatment-related changes in the reproductive system included chronic active inflammation in the ovary that occurred in the 1,000 and 3,000 μg/kg core groups (two-year exposure) of PCB153 and in the 300 ng/3,000 μg/kg core group of binary mixture of PCB126 and PCB153. Increases in the incidence of acute and/or chronic active inflammation of the uterus were observed in all dosed groups, including the stop-exposure group (withdrawal after thirty-week exposure) of PeCDF and the 1,000 μg/kg and/or higher group dosed with PCB153. The incidence of cystic endometrial hyperplasia was marginally increased in the 92 PeCDF ng/kg group at two years. The incidence of squamous metaplasia was significantly increased in the 44 ng/kg and higher dose group, including the stop-exposure group. The incidence of uterine squamous cell carcinoma was significantly or marginally increased in the 6 ng/kg core and 100 ng/kg stop-exposure groups of TCDD and in the 300 ng/300 μg/kg core group that received the binary mixture of PCB126 and 153. The incidence of uterine carcinoma was marginally increased in the 92 ng/kg PeCDF group at two years and clearly increased in the 1,000 and 4,600 μg/kg PCB118 core group and the 4,600 μg/kg stop group. In the studies of PCB 126, the tertiary mixture, and the binary mixture of PCB126 and PCB118, no increased incidence of any change occurred in the reproductive systems. The range of changes seen with the different compounds suggests that more than one mechanism may have been involved in promoting the female reproductive pathology.

Chemical Reactivity and Respiratory Toxicity of the α-Diketone Flavoring Agents 2,3-Butanedione, 2,3-Pentanedione, and 2,3-Hexanedione

Occupational exposure to 2,3-butanedione (BD) vapors has been associated with severe respiratory disease leading to the use of potentially toxic substitutes. We compared the reactivity and respiratory toxicity of BD with that of two structurally related substitutes, 2,3-pentanedione (PD) and 2,3-hexanedione (HD). Chemical reactivity of the diketones with an arginine substrate decreased with increasing chain length (BD > PD > HD). Animals were evaluated the morning after a 2-week exposure to 0, 100, 150, or 200 ppm BD, PD, or HD (postexposure) or 2 weeks later (recovery). Bronchial fibrosis was observed in 5/5 BD and 5/5 PD rats at 200 ppm and in 4/6 BD and 6/6 PD rats at 150 ppm in the postexposure groups. Following recovery, bronchial fibrosis was observed in all surviving rats exposed to 200 ppm BD (5/5) or PD (3/3) and in 2/10 BD and 7/9 PD rats exposed to 150 ppm. Bronchial fibrosis was observed only in 2/12 HD-exposed rats in the 200 ppm postexposure group. Patchy interstitial fibrosis affected lungs of recovery groups exposed to 200 ppm PD (3/3) or BD (1/5) and to 150 ppm PD (4/9) or BD (7/10) and correlated with pulmonary function deficits. BD and PD were more reactive and produced more bronchial fibrosis than HD.

Olfactory Epithelial Metaplasia and Hyperplasia in Female Harlan Sprague-Dawley Rats Following Chronic Treatment with Polychlorinated Biphenyls

The National Toxicology Program recently completed a series of studies to evaluate the relative potency for toxicity and carcinogenicity of several polyhalogenated aromatic hydrocarbons including dioxin-like compounds (DLCs) and polychlorinated biphenyls. Female Sprague–Dawley rats were administered by gavage for up to 2 years with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); 3,3′,4,4′,5-pentachlorobiphenyl (PCB126); 2,3,4,7,8-pentachlorodibenzofuran (PeCDF); 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153); a tertiary mixture of TCDD, PCB126, and PeCDF; a binary mixture of PCB126 and 153; or a binary mixture of PCB126 and 2,3′,4,4′,5-pentachlorobiphenyl (PCB118); control animals received corn oil-acetone vehicle (99:1) alone. Nasal epithelial changes were observed only in animals exposed for 2 years to the higher doses of the binary mixtures of PCB126 + PCB153 (1000 ng/kg and 1000 ug/kg) and PCB126 + PCB118 (216 and 360 ng TCDD equivalents/kg). In both studies, the changes were of the same nonneoplastic nature, localized to nasal sections II and III located, respectively, at the level of the incisive papilla anterior to the first palatial ridge (section II) and through the middle of the second molar teeth (section III). The changes consisted of hyperplasia of the respiratory epithelium (level II) and metaplasia of olfactory epithelium to respiratory epithelium with further hyperplasia of the metaplastic respiratory epithelium (levels II and III). Variable amounts of acute inflammatory exudate appeared within the lumen of the nasal cavity, overlying the affected epithelium. Occasionally, the inflammation eroded through the skull and into the adjacent olfactory bulbs.

Pulmonary Lesions in Female Harlan Sprague-Dawley Rats Following Two-Year Oral Treatment with Dioxin-Like Compounds

Dioxin and dioxin-related compounds have been associated with high incidences of pulmonary dysfunctions and/or cancers in humans. To evaluate the relative potencies of effects of these compounds, the National Toxicology Program completed a series of two-year bioassays which were conducted using female Harlan Sprague-Dawley rats. The rats were treated orally for up to 2 years with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3′,4,4′,5-pentachlorobiphenyl (PCB126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), and a ternary mixture of TCDD, PCB126 and PeCDF. In addition to treatment-related effects reported in other organs, a variety of pulmonary lesions were observed that were related to exposure. Pulmonary CYP1A1-associated 7-ethoxyresorufin-O-deethylase (EROD) activity was increased in all dosed groups. The most common non-neoplastic lesions, which occurred in all studies, were bronchiolar metaplasia and squamous metaplasia of the alveolar epithelium. Cystic keratinizing epithelioma was the most commonly observed neoplasm which occurred in all studies. A low incidence of squamous cell carcinoma was associated only with PCB126 treatment. Potential mechanisms leading to altered differentiation and/or proliferation of bronchiolar and alveolar epithelia may be through CYP1A1 induction or disruption of retinoid metabolism.

Dermal Toxicity and Carcinogenicity of 4-Vinyl-1 -cyclohexene Diepoxide in Fischer Rats and B6C3F1 Mice

4-Vinyl-1-cyclohexene diepoxide (VCHD) is used as a chemical intermediate and as a reactive diluent for diepoxides and epoxy resins. Studies were conducted by administering VCHD in acetone by dermal application, 5 days per week for 105 weeks, to groups of 60 rats of each sex at 0, 15, or 30 mg/animal. Groups of 60 mice of each sex were administered 0, 2.5, 5, or 10 mg/animal on the same schedule for up to 103 weeks. Ten animals from each group were humanely killed, necropsied, and examined histopathologically during Month 15. At the 15-month evaluation, 2 of 10 male rats that received 30 mg had a squamous cell carcinoma of the skin at or adjacent to the site of application. Squamous cell papillomas and carcinomas were seen in all mice that received 5 or 10 mg. Two of nine female mice given 10 mg had granulosa cell tumors of the ovary, and one of nine female mice given 10 mg had an ovarian papillary cystadenoma. In the 2-year studies, body weight and survival were lower in high-dose rats and mid- and high-dose mice than in vehicle controls. All high-dose male mice died by Week 83; remaining high-dose female mice were killed during Week 84 for humane reasons. Squamous cell papillomas of the skin in dermally exposed male rats and squamous cell carcinomas and basal cell adenomas or carcinomas of the skin in exposed male and female rats were increased. The incidence of squamous cell carcinomas of the skin was increased in male and female mice at all dose levels. Mid- and high-dose female mice had an increased incidence of benign or malignant granulosa cell tumors and of benign mixed tumors of the ovary.

Thyroid Follicular Lesions Induced by Oral Treatment for 2 Years with 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Dioxin-like Compounds in Female Harlan Sprague-Dawley Rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and structurally-similar dioxin-like compounds affect thyroid function and morphology and thyroid hormone metabolism in animals and humans. The National Toxicology Program conducted eight 2-year gavage studies in female Harlan Sprague-Dawley rats to determine the relative potency of chronic toxicity and carcinogenicity of TCDD, 3,3′,4,4′,5-pentachlorobiphenyl (PCB126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 2,3′,4,4′,5-pentachlorobiphenyl (PCB118), 2,2′,4,4′,5,5′-hexachloro-biphenyl (PCB153), a tertiary mixture of TCDD/PCB126/PeCDF, and two binary mixtures (PCB126/PCB153 and PCB126/PCB118). Administration of these compounds was associated with increased incidences of thyroid follicular cell hypertrophy, variably observed in the 14-, 31-, and 53-week interim and 2-year sacrifice groups. In all studies, the incidences of follicular cell adenoma and carcinoma were not increased. Decreased levels of serum thyroxine were primarily noted in the 14-or-later -week interim groups of all chemicals. Serum triiodothyronine (T3) levels were increased in the TCDD, PCB126, PeCDF, TCDD/PCB126/PeCDF, and PCB126/PCB153 studies, while decreased levels were noted in the PCB153 and PCB126/PCB118 studies. TCDD, PCB126, PCB126/PCB153, and PCB126/PCB118 increased levels of serum thyroid-stimulating hormone almost in a dose-dependent manner in the 14-week groups. These data suggest that although dioxin-like compounds alter thyroid hormones and increase follicular cell hyperplasia, there is not an increase in thyroid adenoma or carcinoma in female Sprague-Dawley rats.