Methylene chloride: an inhalation study to investigate toxicity in the mouse lung using morphological, biochemical and Clara cell culture techniques

Evaluation of the carcinogenicity of dichloromethane in rats, mice, hamsters and humans

Dichloromethane (DCM) is a high production volume chemical (>1000 t/a) mainly used as an industrial solvent. Carcinogenicity studies in rats, mice and hamsters have demonstrated a malignant tumor inducing potential of DCM only in the mouse (lung and liver) at 1000-4000 ppm whereas human data do not support a conclusion of cancer risk. Based on this, DCM has been classified as a cat. 2 carcinogen. Dose-dependent toxicokinetics of DCM suggest that DCM is a threshold carcinogen in mice, initiating carcinogenicity via the low affinity/high capacity GSTT1 pathway; a biotransformation pathway that becomes relevant only at high exposure concentrations. Rats and hamsters have very low activities of this DCM-metabolizing GST and humans have even lower activities of this enzyme. Based on the induction of specific tumors selectively in the mouse, the dose- and species-specific toxicokinetics in this species, and the absence of a malignant tumor response by DCM in rats and hamsters having a closer relationship to DCM toxicokinetics in humans and thus being a more relevant animal model, the current classification of DCM as human carcinogen cat. 2 remains appropriate.

Relevance of mouse lung tumors to human risk assessment

Mouse lung is a common site for chemical tumorigenicity, but the relevance to human risk remains debated. Long-term bioassays need to be assessed for appropriateness of the dose, neither exceeding Maximum Tolerated Dose (MTD) nor Kinetically based Maximum Dose (KMD). An example of the KMD issue is 1,3-dichloropropene (1,3-D), which only produced an increased incidence of lung tumors at a dose exceeding the KMD. In addition, since mouse lung tumors are common (>1% incidence), the appropriate statistical significance is p < .01. Numerous differences exist for mouse lung and tumors compared to humans, including anatomy, respiratory rate, metabolism, tumor histogenesis, and metastatic frequency. The recent demonstration of the critical role of mouse lung specific Cyp2 F2 metabolism in mouse lung carcinogenicity including styrene or fluensulfone indicates that this tumor response is not qualitatively or quantitatively relevant to humans. For non-DNA reactive and non-mutagenic carcinogens, the mode of action involves direct mitogenicity such as for isoniazid, styrene, fluensulfone, permethrin or cytotoxicity with regeneration such as for naphthalene. However, the possibility of mixed mitogenic and cytotoxic modes of action cannot always be excluded. The numerous differences between mouse and human, combined with epidemiologic evidence of no increased cancer risk for several of these chemicals make the relevance of mouse lung tumors for human cancer risk dubious.

Preparation of large porous deslorelin-PLGA microparticles with reduced residual solvent and cellular uptake using a supercritical carbon dioxide process

PURPOSE

The purpose of this study was to prepare large-porous peptide-encapsulating polymeric particles with low residual solvent that retain deslorelin integrity, sustain drug release, and exhibit reduced epithelial and macrophage uptake. We hypothesized that supercritical carbon dioxide (SC CO2) pressure-quench treatment of microparticles prepared using conventional approach expands these particles and extracts the residual organic solvent.

METHODS

Initial studies with crystalline L-lactide (L-PLA) and amorphous copolymers of lactide-co-glycolide (PLGA) 50:50, 65:35, and 75:25 indicated that PLGA 50:50 was the most amenable to morphological changes upon SC CO2 treatment. Therefore, we prepared deslorelin-PLGA (50:50) microparticles using the conventional emulsion-solvent evaporation method, and in a second step equilibrated with SC CO2 at various temperatures (33-37 degrees C) and pressures (1200-2000 psi) for discrete intervals followed by rapid isothermal depressurization. The particles were then characterized for morphology, polymer thermal properties, particle size, porosity, bulk density, and residual solvent content. Also, deslorelin integrity, conformation, release, and cellular uptake before and after SC CO2 treatment was determined.

RESULTS

Upon SC CO2 treatment (1200 psi, 33 degrees C for 30 min), the mean particle size of the deslorelin PLGA microparticles increased from 2.2 to 13.8 microm, the mean porosity increased from 39 to 92.38% the mean pore diameter increased from 90 to 190 nm, the mean bulk density reduced from 0.7 to 0.082 g/cc, mass spectrometry indicated structural integrity of released deslorelin, the circular dichroism spectrum indicated stabilization of beta-turn conformation, and the scanning electron microscopy confirmed increased particle size and pore formation. The deslorelin release was sustained during the 7-day study period. Also, the peak Tg of PLGA decreased from 51 to 45 degrees C, and the residual solvent content was reduced from 4500 ppm to below detection limit (< 25 ppm). The accumulation of drug from SC CO2 treated particles in cell layers of Calu-3, A549, and rat alveolar macrophages was reduced by 87, 91 and 50%, respectively, compared to untreated particles.

CONCLUSION

An SCF-derived process could be successfully applied to prepare large porous deslorelin-PLGA particles with reduced residual solvent content, which retained deslorelin integrity, sustained deslorelin release, and reduced cellular uptake.

Increased vulnerability of neonatal rats and mice to 1-nitronaphthalene-induced pulmonary injury

The postnatal period of lung development is a critical window of susceptibility to environmental toxicants, including polyaromatic hydrocarbons (PAHs) and furans. To determine whether the increased susceptibility of neonatal lung injury due to environmental toxicants is a universal response across species and also applies to nitrated compounds, adult and 7-day-old male mice and rats were given a single intraperitoneal dose (0, 12.5, 25, 50, or 100 mg/kg) of 1-nitronaphthalene and killed 24 h later. Exposure to 1-nitronaphthalene, a nitro-polyaromatic hydrocarbon, results in pulmonary lesions in both adult rats and mice, although the severity of the injury is species-specific (greater in rats than in mice). Pulmonary lesions, as assessed by quantitative histopathology, included dose-dependent vacuolization and exfoliation of both ciliated and nonciliated airway epithelial cells throughout the airway tree in both rats and mice. In both species, the 7-day-old animals were more susceptible to injury by 1-nitronaphthalene than adult animals. In contrast to adult response, neonatal mice were more susceptible to 1-nitronaphthalene-induced pulmonary injury than neonatal rats. This indicates that neonatal susceptibility to environmental pollutant-induced lung injury cannot be reliably predicted based on adult susceptibility.

A COMPARATIVE STUDY ON THE RELIABILITY OF TOXICOKINETIC PARAMETERS FOR PREDICTING HEPATOTOXICITY OF DDT IN RATS RECEIVING A SINGLE OR REPEATED ADMINISTRATION

A comparative study on the reliability of toxicokinetic parameters for predicting hepatotoxicity was conducted in male F344 rats receiving a single (106 mg/kg by gavage) or 7-day repeated (1000 ppm in feed, 97 mg/kg/day) administration of p,p'-DDT. DDT was selected as the test substance because it is known as a hepatotoxic agent and its metabolic pathway is well documented. Concentrations of DDT and its metabolites (DDE and DDD) in the plasma, brain, and liver were measured at various time intervals during the study and the results were compared with the area under the concentration-time curve (AUC) in relation to hepatotoxic response. Increases in the absolute and relative (ratio to body weight) liver weights were observed as a typical toxic response after a single or repeated exposure to DDT. The coefficient (R2) of correlation between the increases in the relative liver weight and the concentrations or AUC of DDT and its metabolites in the plasma and liver was estimated. The values of R2 between the relative liver weight and AUC of DDT or the total DDT (T-DDT) in the plasma and liver were found to be more consistent and higher than those with their concentrations in the repeated dose study. In addition, the R2 values in correlation with their AUCs after a single exposure were lower than those in the repeated dose study. These results indicate that the AUC of DDT or T-DDT in the plasma and liver would be more reliable than their concentrations for predicting hepatotoxicity caused by DDT, especially in the repeated dose study.

The role of cytochromes P-450 in styrene induced pulmonary toxicity and carcinogenicity

Exposure of CD-1 mice to atmospheres of 40 and 160 ppm styrene, daily for up to 10 days, caused pulmonary toxicity characterised by focal loss of cytoplasm and focal crowding of non-ciliated Clara cells, particularly in the terminal bronchiolar region. The toxicity was accompanied by an increase in cell replication rates in terminal and large bronchioles of mice exposed for 3 days or longer. The toxicity and increased cell replication were no longer apparent after a 2-day break in exposure, but re-occurred when exposure was resumed. Similar effects were seen in mice given oral doses of 10, 100 or 200 mg/kg styrene, daily for 5 days. Increases in cell replication rates were seen in the terminal bronchioles in mice dosed with 100 and 200 mg/kg styrene, but not 10 mg/kg. Toxicity was limited to 3 to 10 animals in the 200 mg/kg group. Neither morphological nor cell proliferation effects were seen in the alveolar region of the mouse lung in any of these studies, nor were any effects observed in the lungs of CD rats exposed to 500 ppm styrene for up to 10 days. The pulmonary toxicity and increased cell division seen in mice, but not rats, correlates with the known species differences in pulmonary carcinogenicity of styrene, suggesting that the acute and chronic responses are causally related. 5-Phenyl-1-pentyne was shown to inhibit the pulmonary cytochrome P-450 metabolism of styrene in vivo. Cell replication rates in the lungs of mice treated with this inhibitor and exposed to styrene were comparable with controls demonstrating that the pulmonary effects of styrene on the mouse lung are caused by a metabolite of styrene, probably styrene oxide. The risks associated with exposure to styrene appear to correlate well with the metabolic capacity of the lung.

Inhaled naphthalene causes dose dependent Clara cell cytotoxicity in mice but not in rats

Current OSHA standards for naphthalene exposure are set at 10 ppm (time-weighted average) with a standard threshold exposure concentration of 15 ppm. While several studies have thoroughly delineated the time course and dose response of injury by naphthalene administered ip, the pattern and severity of injury by inhalation exposure are unknown. These studies compare the regiospecific and dose-dependent cytotoxicity of naphthalene after inhalation exposure. Mice and rats were exposed for 4 h to naphthalene vapor at concentrations of 0-110 ppm. In rats, no injury was observed in the lung epithelium at exposure concentrations up to 100 ppm. Exposures as low as 2 ppm produced proximal airway injury in mice, with increased severity in a concentration-dependent fashion up to 75 ppm. Terminal airways of exposed mice exhibited little or no injury at low concentrations (1-3 ppm). Exposures of 8.5 ppm or higher were required to produce injury to Clara cells in the terminal airways. In contrast, administration of naphthalene (</=200 mg/kg ip) caused Clara cell cytotoxicity, which was limited to distal airways in mice. Higher doses (>300 mg/kg) extended the injury pattern toward the lobar bronchus. We conclude (1) the pattern of injury to naphthalene is highly dependent on the route of exposure, (2) lung injury to inhaled naphthalene is species dependent, and (3) Clara cells of mouse airways are exquisitely sensitive to inhaled naphthalene at concentrations well below the current OSHA standard for human exposure.

Cell-specific loss of cytochrome P450 2B1 in rat lung following treatment with pneumotoxic and non-pneumotoxic trialkylphosphorothioates

This study was designed to test the hypothesis that the reduction in cytochrome P450 (CYP) 2B1 content and activity of rat lung microsomes, following dosing with pneumotoxic trimethylphosphorothioates, results from damage to specific cell types. Of the lung cells exhibiting immunolabelling for CYP2B1, only type I cells showed signs of susceptibility to the pneumotoxins O,O.S-trimethylphosphorothioate and O,S,S-trimethylphosphorodithioate. While most type I cells became necrotic, type II and Clara cells showed no signs of injury, despite their gradual loss of CYP2B1, as detected by immunogold labelling. This loss of labelling was accompanied by a 75% reduction in the immunoreactive CYP2B1 content and an 85% reduction in pentoxyresorufin O-dealkylase activity in lung microsomes. In contrast, the non-pneumotoxic analogue O,O,S-trimethylphosphorodithioate, differing from O,O,S-trimethylphosphorothioate by only the presence of a P = S rather than a P = O moiety, caused an even more rapid fall in pulmonary pentoxyresorufin O-dealkylase activity, but only a slight reduction in the microsomal content of CYP2B1. The recovery of this activity began within 12 hr of dosing. O,O,S-Trimethylphosphorodithioate, which acts as a suicidal inhibitor of pulmonary CYP2B1, did not cause any detectable lung injury or increase in cell division. These results are consistent with the initial reduction in both enzyme content and activity caused by the P = O - containing pneumotoxins resulting, almost entirely, from death of type I cells. Subsequent reductions that occur long after clearance of the toxin may be exacerbated by the onset of mitosis in Clara and type II cells.

Selective Clara cell injury in mouse lung following acute administration of coumarin

Coumarin is a known hepatotoxicant in laboratory animals, particularly rats. However, the mouse lung was identified as a major target organ in a chronic bioassay, with an oral gavage dosage of 200 mg/kg coumarin increasing the incidence of alveolar/bronchiolar adenomas and carcinomas. The purpose of the present work was to determine whether coumarin was acutely toxic in the mouse and rat lung. Male and female B6C3F1 mice were dosed orally by gavage with coumarin at 0, 10, 20, 50, 100, 150, and 200 mg/kg and lung toxicity was determined 24 h later by histological evaluation. The results indicated that coumarin dosages >/= 150 mg/kg caused selective injury to Clara cells in the distal bronchiolar epithelium. The time course of this injury was studied from 6 h to 7 days after a single dosage of coumarin (200 mg/kg). At 12 h after dosing, Clara cell swelling was apparent along with the onset of necrosis and bronchiolar epithelial disorganization. At 24-48 h, necrotic Clara cells were observed sloughed into the lumens of the terminal bronchioles, with concomitant thinning of the epithelium and flattening of the remaining ciliated cells. By 72-96 h, there was epithelial hypertrophy and hyperplasia, and by 7 days after dosing, the Clara cells had regenerated and the bronchiolar epithelial architecture appeared nearly normal. Unlike the mouse, oral administration of coumarin (200 mg/kg) caused severe hepatotoxicity in male F344 rats, seen histologically as centrilobular necrosis and associated with increases, up to 140-fold, in serum ALT, AST, and SDH levels. Clara cell toxicity was not observed in the distal bronchioles of treated rats. However, in the upper airways, coumarin treatment produced generalized epithelial necrosis involving both ciliated and nonciliated cells. 3,4-Dihydrocoumarin (DHC), which is not a mouse lung carcinogen, did not cause Clara cell injury when dosed to mice at 800 mg/kg. This finding suggests, because DHC lacks a 3,4-double bond, that bioactivation of coumarin to a 3,4-epoxide intermediate may contribute to mouse lung Clara cell toxicity. Collectively, the results indicate that coumarin is a Clara cell toxicant and establish the mouse lung as a target organ for coumarin toxicity. These new findings lay the foundation for studies to determine the mechanisms of coumarin-induced toxicity and carcincogenicity and to define the relevance of these effects to humans.

Methylene chloride induced mouse liver and lung tumours: an overview of the role of mechanistic studies in human safety assessment

B6C3F1 mice exposed to high dose levels of methylene chloride by inhalation for 2 years had an elevated incidence of liver and lung tumours. These tumours were not increased in rats or hamsters exposed under the same or similar conditions. This paper gives an overview of research conducted over the last 10 years into the mechanism of action of methylene chloride as a mouse carcinogen and into the relevance of the mouse data to humans exposed to this chemical. Data are presented on the comparative metabolism and pharmacokinetics of methylene chloride in mice, rats, hamsters and humans, on the toxicity of methylene chloride to the target organs in the mouse, and on the genotoxicity of methylene chloride in vitro and in vivo. The enzyme which activates methylene chloride to its carcinogenic form has been isolated, sequenced, and cloned, and its distribution studied within cells, organs and between species. Evidence has been obtained to show the methylene chloride caused cancer in mice as a result of interactions between metabolites of the glutathione S-transferase pathway and DNA. Damage to mouse lung Clara cells and increased cell division are believed to have influenced the development of the lung tumours. The species specificity was a direct consequence of the very high activity and specific cellular and nuclear localisation of a theta class glutathione S-transferase enzyme which was unique to the mouse. Consequently, DNA damage was not detectable in rats in vivo, or in hamster and human hepatocytes exposed to cytotoxic dose levels of methylene chloride in vitro. These results provide evidence that the mouse is unique in its response to methylene chloride and that it is an inappropriate model for human health assessment.