Mycoplasma pulmonis and lymphoma

Pathologists' perspective on the study design, analysis, and interpretation of proliferative lesions in a lifetime rodent carcinogenicity bioassay of sucralose

Sucralose, a sugar substitute first approved for use in 1991, is a non-caloric sweetener regulated globally as a food additive. Based on numerous experimental animal studies (dating to the 1980s) and human epidemiology studies, international health agencies have determined that sucralose is safe when consumed as intended. A single lifetime rodent carcinogenicity bioassay conducted by the Ramazzini Institute (RI) reported that mice fed diets containing sucralose develop hematopoietic neoplasia, but controversy continues regarding the validity and relevance of these data for predicting health effects in humans. The present paper addresses the controversy by providing the perspective of experienced pathologists on sucralose-related animal toxicity and carcinogenicity data generally, and the RI carcinogenicity bioassay findings specifically, using results from publicly available papers and international regulatory authority decisions. In the authors' view, flaws in the design, methodology, data evaluation, and reporting of the RI carcinogenicity bioassay for sucralose diminish the value of the data as evidence that this agent represents a carcinogenic hazard to humans. This limitation will remain until the RI bioassay is repeated under Good Laboratory Practices and the design, data, and accuracy of the pathology diagnoses and interpretations are reviewed by qualified pathologists with experience in evaluating potential chemically-induced carcinogenic hazards.

Lung, Pleura, and Mediastinum

The lung is constantly exposed to a large volume of inhaled air that may contain toxicant xenobiotics. With the possibility of exposure to a variety of respiratory toxicants from airborne pollutants in our environment during the course of daily activities, in occupational settings, the use of aerosol sprays for household products, and the development of inhalant bronchial therapies, pulmonary toxicology has become an important subspecialty of toxicology. The lung is susceptible to injury following hematogenous exposure to toxicants. Susceptibility to injury and the type of response following exposure to air- or blood-borne toxicants is largely dependent on the physiochemical characteristics and concentration of the toxicant, duration of exposure, site/tissue specific sensitivity, and the integrity of the defense mechanisms of the lung. In this chapter, nonneoplastic and neoplastic spontaneous lesions and those that develop in the lungs of rats following exposure to toxicants by various routes, but primarily by inhalation, are discussed in detail which provides insight into our understanding of how human lungs respond to toxic chemicals. In addition, the gross and microscopic anatomy of the rat lung is also discussed some detail. Although inhalation is the primary route of exposure in experimental studies, in the past, many studies used intratracheal instillation or direct injection of known carcinogens into the lung. These experiments often resulted in the development of squamous cell carcinomas even though they are very rare as a naturally occurring neoplasm. Instillation of chemicals or particles into the trachea or pleura or direct injection into the lung results in lesions or responses that may not be as relevant to understanding the mechanism of pulmonary carcinogenesis as inhalation of materials under more normal conditions. There remain, however, many areas where our understanding of the response of the lung to toxic chemicals is incomplete.

Formation of hydroxymethyl DNA adducts in rats orally exposed to stable isotope labeled methanol

Methanol is a large volume industrial chemical and widely used solvent and fuel additive. Methanol's well known toxicity and use in a wide spectrum of applications has raised long-standing environmental issues over its safety, including its carcinogenicity. Methanol has not been listed as a carcinogen by any regulatory agency; however, there are debates about its carcinogenic potential. Formaldehyde, a metabolite of methanol, has been proposed to be responsible for the carcinogenesis of methanol. Formaldehyde is a known carcinogen and actively targets DNA and protein, causing diverse DNA and protein damage. However, formaldehyde-induced DNA adducts arising from the metabolism of methanol have not been reported previously, largely due to the absence of suitable DNA biomarkers and the inability to differentiate what was due to methanol compared with the substantial background of endogenous formaldehyde. Recently, we developed a unique approach combining highly sensitive liquid chromatography-mass spectrometry methods and exposure to stable isotope labeled chemicals to simultaneously quantify formaldehyde-specific endogenous and exogenous DNA adducts. In this study, rats were exposed daily to 500 or 2000 mg/kg [¹³CD₄]-methanol by gavage for 5 days. Our data demonstrate that labeled formaldehyde arising from [¹³CD₄]-methanol induced hydroxymethyl DNA adducts in multiple tissues in a dose-dependent manner. The results also demonstrated that the number of exogenous DNA adducts was lower than the number of endogenous hydroxymethyl DNA adducts in all tissues of rats administered 500 mg/kg per day for 5 days, a lethal dose to humans, even after incorporating an average factor of 4 for reduced metabolism due to isotope effects of deuterium-labeled methanol into account.

Hypothesis-Based Weight-of-Evidence evaluation of methanol as a human carcinogen

Recent scientific debate has focused on the potential for exposure to methanol to cause lymphomas in humans. The concern stems from a few animal studies reporting an association, although evidence suggests the studies may have been confounded by chronic respiratory infection. Although the toxicological evidence for methanol carcinogenesis is weak, two modes of action have been put forth, one involving metabolism of methanol to formaldehyde, followed by formaldehyde induction of lymphoma, and another involving oxidative stress caused by hydrogen peroxide release during catalase-induced metabolism of methanol to formaldehyde. In this article, we apply our Hypothesis-Based Weight-of-Evidence (HBWoE) approach to evaluate the evidence regarding methanol exposure and lymphoma, attending to how human, animal, and mode-of-action results inform one another, tracing the logic of inference within and across all studies, and articulating how one could account for the suite of available observations. Upon comparison of alternative proposals regarding what causal processes may have led to the array of observations as we see them, we conclude that the apparent association between methanol exposure and lymphoma in some animal studies is weak and strains biological plausibility, and is better interpreted as due to confounding or to a mechanism not relevant in humans.

Mycoplasma pulmonisand Lymphoma in Bioassays in Rats

Lymphomas were reported to be induced in rats in bioassays of aspartame, methyl- tertiary-butyl ether (MTBE), and other chemicals conducted by a nonprofit cancer research organization. European regulatory authorities concluded that lymphomas in the aspartame study were caused by Mycoplasma pulmonis and suggested that this also was the case for the MTBE bioassay. To assess the role of M. pulmonis in these bioassays, we reviewed the tumor data for the aspartame and MTBE bioassays and, additionally, the organization's bioassay of methanol. For all 3 studies, the most frequently reported hematopoietic neoplasm was lympho-immunoblastic lymphoma, the most frequently affected organ was the lung, and, in almost half of the rats with this diagnosis, the lung was the only affected organ. Lesions diagnosed as lymphoma in published illustrations had pleomorphic cellular morphology and appeared to contain neutrophils. Information from these reports and other sources indicated that lesions typical of M. pulmonis disease were prevalent among the aspartame and MTBE study rats and that the rats were not specific-pathogen-free. Because the lymphoma type, cellular morphology, and organ distribution reported in these studies are atypical of lymphoma in rats, because lymphocyte and plasma cell accumulation in the lung is characteristic of M. pulmonis disease, and because M. pulmonis disease can be exacerbated by experimental manipulations, including chemical treatment, we suggest that a plausible alternative explanation for the reported results of these bioassays is that the studies were confounded by M. pulmonis disease and that lesions of the disease were interpreted as lymphoma.