Comparison of inhalation toxicity studies of gentamicin in rats and dogs.

Nebulized gentamicin solution was administered to rats (nose-only) and dogs (face mask) for 14 days with a 14-day recovery period. Control groups of each were exposed to saline aerosols. Mean estimated inhaled lung doses of gentamicin were 39, 123 and 245 mg/kg for rats (deposited doses 6, 17 and 34 mg/kg) over 30, 90 and 180 min, respectively. Since dogs do not tolerate exposures as long as rats, inhaled lung doses were limited to 7, 14 and 41 mg/kg (deposited doses of 1, 3 and 8 mg/kg) over 15, 30 and 60 min. Comparable doses were achieved at the low dose for rats and high dose for dogs. Serum AUCs (14 ± 2 µg/mL h (mean ± SD) at 6 mg/kg in rats and 11 ± 7 µg/mL h at 8 mg/kg in dogs) showed comparable exposure between the two, implying similar absorbed doses and confirming similar deposited lung doses. Rat exposures resulted in dose-related lung pathology (including low dose) manifested as upper respiratory tract irritant reactions with alveolar histiocytosis, inflammation, airway epithelial metaplasia and lymphomegaly in lung tissue. This was associated with high lung tissue gentamicin levels 24 h post-dose on Day 14 (375 ± 33 µg/g at deposited dose of 6 mg/kg). Dose-related kidney tubular necrosis (a well-known toxicity of parenteral gentamicin) was observed, but no test-article related effects on lung histopathology in dogs (even at highest deposited dose of 8 mg/kg) and low levels of lung tissue gentamicin (42 ± 11 µg/g) 24 h post-dose on Day 14.

Subchronic hepatotoxicity evaluation of hydrazobenzene in Fischer 344 rats.

Male F344 rats were exposed to hydrazobenzene (HZB) by dietary feed at concentrations of 0, 5, 20, 80, 200, or 300 ppm for 5 days, 2 weeks, 4 weeks, or 13 weeks duration. End points evaluated included clinical observations, body weights, liver weights, serum chemistry, blood HZB, gross pathology, and liver histopathology. There were no HZB exposure-related clinical signs of toxicity. During study weeks 8 through 13, body weight means in rats of the 300 ppm group were 6% lower compared to control rat means. Serum alkaline phosphatase concentrations were decreased in rats of the 300 ppm group at all time points. Relative (to body weight) liver weight increases were observed in rats of the 200 and 300 ppm groups following 5 days (300 ppm only), 2 weeks, 4 weeks, and 13 weeks of exposure. Following 13 weeks of exposure, microscopic findings in the liver were observed only in rats of the 200 and 300 ppm groups and consisted of hypertrophy, macrovesiculation, eosinophilic granular cytoplasm, and bile duct duplication. Blood HZB concentrations ranged from 0.002 to 0.006 µg/mL in rats of the 200 or 300 ppm groups. A no observed effect level of 80 ppm (4.80 mg/kg per d) was selected based on the observation of microscopic hepatocyte alterations at ≥200 ppm HZB.

Ultrastructural analysis in preclinical safety evaluation.

The first electron microscopic images of biological specimens were made in the 1940s, and the next 30 years comprised an era of descriptive ultrastructure during which transmission electron microscopy (TEM) was integral to an explosion in cellular and molecular biology. However, when questions could no longer be answered by ultrastructural information alone, the use of TEM in biological research declined. Innovative molecular techniques and newer imaging technologies such as confocal fluorescence microscopy filled the gap, providing faster answers with less rigorous training as a prerequisite to data collection. The use of TEM in toxicologic pathology has paralleled the rise and fall of its popularity in other disciplines. However, TEM remains an essential resource that provides direct and unequivocal data to explain and address safety concerns in preclinical toxicity studies. There is still an important place for TEM in preclinical safety evaluation and mechanistic studies, particularly when visualization of subcellular structures provides a link to other endpoints. This review reinforces the value of TEM in preclinical safety testing and model development and encourages best practices for ultrastructural evaluation.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after oral administration--part I.

This study was conducted as part of the International Life Sciences Institute (ILSI) program to evaluate the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following oral administration to Tg.AC (transgenic) and wild-type FVB (nontransgenic) mice for a minimum for 6 months. Clofibrate was well tolerated at doses up to 500 (males) and 650 (females) mg/kg/day. Oral administration of clofibrate to Tg.AC or FVB (wild-type) male and female mice for 6 months did not result in the increased formation of neoplastic lesions. Epithelial hyperplasia in the urinary bladder (Tg.AC and FVB) and prostate gland (Tg.AC only), and interstitial-cell hyperplasia in the testes (Tg.AC) were noted at 500 mg/kg/day. Non-neoplastic nonproliferative findings included hepatic hypertrophy and hematopoietic changes (myeloid hyperplasia, myelodysplasia, lymphoid depletion, and erythropoiesis) in Tg.AC and FVB mice of both sexes; reproductive (cystic degeneration and dilatation, hypospermia, spermatocele, dilated inspissated protein) and urogenital (tubular-cell hypertrophy, degenerative/regenerative nephropathy, necrosis/fibrosis) changes in Tg.AC and FVB male mice; congestion in the lung in male Tg.AC mice; gall bladder dilatation in female Tg.AC mice; and adrenal (intracellular lipofuscinosis and atrophy) and heart (eosinophillic myofibers) findings in Tg.AC mice of both sexes and in female FVB mice. The results of this study indicate that the clofibrate is not carcinogenic when administered to Tg.AC mice by oral gavage for 6 months at doses up to 500 (males) and 650 (females) mg/kg/day, which did produce liver hypertrophy.

Studies evaluating the utility of N-methyl-N-nitrosourea as a positive control in carcinogenicity studies in the p53+/- mouse.

Studies conducted under the auspices of International Life Sciences Institute (ILSI) have suggested that an alternative mouse carcinogenicity study may be substituted for the traditional 2-year mouse bioassay typically conducted to support the development of drug candidates. The purpose of this study was to characterize the carcinogenic potential of N-methyl-N-nitrosourea (MNU), a DNA alkylating agent, in p53+/- knockout mice to determine its suitability as a positive control agent in an alternative carcinogenicity model. p53+/- knockout mice were administered a single oral dose of 90 mg/kg and maintained for up to 13 weeks prior to evaluation of neoplasms. Treatment was generally well tolerated; however, 4 of 30 mice died between the days of 75 and 92 due to neoplasms. MNU-related macroscopic observations included enlargement of the thymus, spleen, mandibular and mesenteric lymph nodes; and pale liver, heart, kidney, and bone marrow, which correlated with the diagnosis of lymphoma of the hematopoietic system, noted in the thymus of all affected animals and in the spleen, liver, lungs, and kidneys of some animals. Other treatment-related single neoplasms included a squamous-cell carcinoma in the nonglandular stomach and leiomyosarcoma in the glandular stomach. Non-neoplastic proliferative lesions included acanthosis and hyperkeratosis in the nonglandular stomach, focal papillary hyperplasia of the nonglandular stomach, glandular hyperplasia of the stomach, and adenomatous hyperplasia of the duodenum or ileum. The increased incidence of neoplastic and proliferative changes in MNU-treated mice suggests MNU could serve as a positive control in alternative carcinogenicity studies conducted in p53+/- knockout mice.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after dermal application--part II.

This study was conducted as part of the International Life Sciences Institute (ILSI) Alternatives to Carcinogenicity Testing program and evaluated the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following dermal application to transgenic Tg.AC and nontransgenic FVB mice for a minimum of 26 weeks. Clofibrate doses of 12, 28, or 36 mg/200 microl/day were used. Positive controls for papilloma formation were benzene (174.8 mg/200 microl), and 12-o-tetradecanoylphorbol-13-acetate (TPA [0.00250 mg/200 microl]). Clofibrate was tolerated at doses up to 36 mg/200 microl. In Tg.AC mice, clofibrate produced a dose-related increase in the incidence of mice with cutaneous papillomas; and dose-related decreases in mean time to first tumor, mean multiplicity of tumors per mouse, and mean weeks to maximal yield, as well as numerous nonneoplastic microscopic lesions in the liver, kidney, spleen, and skin. Benzene and TPA induced both neoplastic and/or non-neoplastic proliferative lesions in Tg.AC mice. Clofibrate did not increase the incidence or multiplicity of papillomas, or any other tumors in FVB mice. These data show that the Tg.AC dermal model has increased sensitivity in detecting skin papillomas caused by the nongenotoxic rodent carcinogen, clofibrate, compared to wild type FVB mice, at systemic exposures that are 3x higher than the systemic exposure observed in humans taking clofibrate (AUC = 1100 microg.h/ml) at the recommended maximum therapeutic dose of 500 mg. In addition, this study supports the proposed concept that Tg.AC model may detect compounds with nongenotoxic carcinogenic potential in a shorter timeframe than conventional mouse carcinogenicity bioassays.

5-Hydroxytryptamine (5HT) receptors in the heart valves of cynomolgus monkeys and Sprague-Dawley rats.

5-Hydroxytryptamine-2B receptor (5HT2BR) stimulation is known to cause fibroblast mitogenesis, and the mitogenic effect has been proposed to trigger valvular heart disease in humans. In this study, we used real-time polymerase chain reaction (TaqMan) to quantify transcript levels of 5HT2B, 5HT2C, and 5HT1B receptors and immunohistochemistry (IHC) to detect the tissue localization of these receptors in the normal heart valves of cynomolgus (CM) monkeys and Sprague-Dawley (S-D) rats. In both species, positive immunostaining was noted for 5HT1B and 5HT2B receptors in mitral, tricuspid, aortic, and pulmonary valves, and the cell types showing positive staining were interstitial cells and endothelial cells lining the valve leaflet. In CM monkeys, 5HT2CR was expressed only in the endothelial cells lining the leaflet, whereas S-D valves were negative for this receptor. IHC results were correlated with 5HT2B and 5HT1B receptor transcripts for all four valves. However, 5HT2C receptor transcripts were lower than 5HT2B or 5HT1B in all CM monkey valves, whereas 5HT2C transcripts were below the level of detection in any of the S-D rat valves. Our data showed the expression of 5HT2B, 5HT1B, and 5HT2C receptors in the normal heart valves of CM monkeys and S-D rats, and IHC and TaqMan techniques may be used to study the potential mechanism of compounds with 5HT2BR agonist activity.

Vascular effects of GI262570X (PPAR-gamma agonist) in the brown adipose tissue of Han Wistar rats: a review of 1-month, 13-week, 27-week and 2-year oral toxicity studies.

We describe and discuss microscopic findings in the brown adipose tissue (BAT) blood vessels of Han Wistar rats treated with GI262570X, a peroxisome proliferator-activated receptor-gamma agonist (PPAR-gamma agonist) by oral gavage for 28 days, 13 weeks, 27 weeks, and 2 years. Review of these studies revealed a consistent vascular change, consisting of multifocal fatty infiltration in the BAT of treated rats. A similar vascular change was not seen in other vessels or organs. Microscopically, fatty infiltration was characterized primarily by round, clear vacuoles within the tunica media and/or tunica adventitia of small and medium-sized arteries and arterioles. Occasionally, these vacuoles had peripherally located nuclei and morphologically resembled adipocytes, suggesting a well-characterized PPAR effect (ie, differentiation of stem cells or preadipocytes into mature adipocytes). However, administration of GI262570X up to 2 years failed to induce more severe or progressive lesions in the blood vessels of rat BAT and, in particular, did not result in induction of any atherosclerotic-like lesions or foam cell infiltration. At the longer exposure, there was an apparent reduction of severity and/or incidence, indicating a possible adaptive response. These results suggest that the possibility of generating atherosclerotic-like lesions through prolonged treatment of GI262570X (PPAR-gamma agonist) is highly unlikely in rats.

Endocardial myxomatous change in Harlan Sprague-Dawley rats (Hsd:S-D) and CD-1 mice: its microscopic resemblance to drug-induced valvulopathy in humans.

A full assessment of all heart valves in rats and mice is often impractical and is usually not performed in routine toxicity studies, largely due to an inevitable inconsistency of histological sampling. The majority of reported heart valve changes involve the examination of a single, semirandom section through the heart and the valvulopathy occurring with age or induced by xenobiotics may have been generally underestimated in mice and rats. Here we describe the incidence and microscopic features of endocardial myxomatous change (EMC) in Hsd:S-D rats and CD-1 mice. EMC was common and widespread in both CD-1 mice and Hsd:S-D rats (188 of 220 rats and 96 of 215 mice were affected by EMC). Microscopically, EMC consisted of focal or segmental thickening of valves, primarily due to the presence of fibromyxoid tissue in the subendocardium. Occasionally, fibrin or thrombi deposits and collection of neutrophils or mononuclear cells were observed. These microscopic features were similar to those seen in valvular disease in humans induced by fenfluramine-phentermine (fen-phen), ergot alkaloids (ergotamine, methysergide), and carcinoid syndrome. The mitral valve in rats and pulmonary valve in mice were most frequently affected. An association between murine progressive cardiomyopathy (MPC) and EMC was noted only in rats, suggesting that there may be a possible relationship between MPC and EMC. However, additional research is needed to confirm a relationship between EMC and MPC in rats and/or mice.