Indium phosphide is used to make semiconductors, injection lasers, solar cells, photodiodes, and light-emitting diodes. Indium phosphide was nominated for study because of its widespread use in the microelectronics industry, the potential for worker exposure, and the absence of chronic toxicity data. Male and female F344/N rats and B6C3F1 mice were exposed to indium phosphide (greater than 99% pure) by inhalation for 14 weeks or 2 years. The frequency of micronuclei was determined in the peripheral blood of mice exposed to indium phosphide for 14 weeks.
Fourteen-week study in rats
Groups of 10 male and 10 female rats were exposed to particulate aerosols of indium phosphide with a mass median aerodynamic diameter of approximately 1.2 µm at concentrations of 0, 1, 3, 10, 30, or 100 mg/m3 by inhalation, 6 hours per day, 5 days per week (weeks 1 through 4 and weeks 10 through 14) or 7 days per week (weeks 5 through 9) to accommodate a concurrent teratology study. One male in the 100 mg/m3 group died before the end of the study. Body weight gains of all males and females exposed to 100 mg/m3 were less than those of the chamber controls.
As a result of indium phosphide exposure, the lungs of all exposed rats had a gray to black discoloration and were significantly enlarged, weighing 2.7- to 4.4-fold more than those of the chamber controls. Indium phosphide particles were observed throughout the respira-tory tract and in the lung-associated lymph nodes. A spectrum of inflammatory and proliferative lesions generally occurred in the lungs of all exposed groups of rats and consisted of alveolar proteinosis, chronic inflammation, interstitial fibrosis, and alveolar epithelial hyperplasia. Pulmonary inflammation was attended by increased leukocyte and neutrophil counts in the blood. The alveolar proteinosis was the principal apparent reason for the increase in lung weights. Indium phosphide caused inflammation at the base of the epiglottis of the larynx and hyperplasia of the bronchial and mediastinal lymph nodes. Exposure to indium phosphide affected the circulating erythroid mass. It induced a microcytic erythrocytosis consistent with bone marrow hyperplasia and hematopoietic cell proliferation of the spleen. Hepatocellular necrosis was suggested by increased serum activities of alanine aminotransferase and sorbitol dehydrogenase in all exposed groups of males and in 10 mg/m3 or greater females and was confirmed microscopically in 100 mg/m3 males and females.
Sixteen-day study in mice
Groups of 10 male and 10 female mice were exposed to particulate aerosols of indium phosphide with a mass median aerodynamic diameter of approximately 1.2 µm at concentrations of 0, 1, 3, 10, 30, or 100 mg/m3 by inhalation, 6 hours per day, 5 days per week (weeks 1 through 4 and weeks 10 through 14) or 7 days per week (weeks 5 through 9). Although the effects of indium phosphide exposure were similar in rats and mice, mice were more severely affected in that all males and females in the 100 mg/m3 groups either died or were removed moribund during the study. One male and three females in the 30 mg/m3 group were also removed before the end of the study. In general, body weight gains were significantly less in males and females exposed to 3 mg/m3 or greater compared to those of the chamber controls. Mice exposed to 30 or 100 mg/m3 were lethargic and experienced rapid, shallow breathing.
As in rats, lungs were discolored and enlarged 2.6- to 4.1-fold greater than those of chamber controls due to the exposure-induced alveolar proteinosis. Indium phosphide particles were observed in the nose, trachea, larynx, and lymph nodes of some exposed males and females. Alveolar proteinosis, chronic active inflam-mation, interstitial fibrosis, and alveolar epithelial hyperplasia were observed; these effects were more severe than in rats. Hyperplasia in the bronchial lymph nodes and squamous metaplasia, necrosis, and suppurative inflammation of the larynx were observed in some exposed males and females. Exposure to indium phosphide induced a microcytic erythrocytosis which was consistent with the observed hematopoietic cell proliferation of the spleen.
Two-year study in rats
Groups of 60 male and 60 female rats were exposed to particulate aerosols of indium phosphide at concentrations of 0, 0.03, 0.1, or 0.3 mg/m3, 6 hours per day, 5 days per week, for 22 weeks (0.1 and 0.3 mg/m3 groups) or 105 weeks (0 and 0.03 mg/m3 groups). Animals in the 0.1 and 0.3 mg/m3 group were maintained on filtered air from exposure termination at week 22 until the end of the studies. Ten males and 10 females per group were evaluated at 3 months.
Three-month interim evaluation
Exposure to indium phosphide for 3 months caused a microcytic erythrocytosis and also caused enlarged lungs and lesions in the respiratory tract and lung-associated lymph nodes. Although qualitatively similar to those observed in the 14-week studies, these effects were considerably less severe. However, the lesions in the lungs of rats exposed to 0.1 or 0.3 mg/m3 were considered sufficiently severe that exposure was discontinued in these groups, and the groups were allowed to continue unexposed for the remainder of the study.
Survival, body weights, and clinical findings
Exposure to indium phosphide had no effect on survival or body weight gain. During the last 6 months of the study, rats in the 0.03 and 0.3 mg/m3 groups became lethargic and males breathed abnormally.
Pathology findings
At 2 years, exposure to indium phosphide caused increased incidences of alveolar/bronchiolar adenomas and carcinomas in rats. Squamous cell carcinoma of the lung occurred in four male rats exposed to 0.3 mg/m3. As observed in the 14-week study and at the 3-month interim evaluation, a spectrum of inflammatory and proliferative lesions of the lung were observed in all exposed groups of males and females; however, the extent and severity of the lesions were generally greater and included atypical hyperplasia, chronic inflammation, alveolar epithelial hyperplasia and metaplasia, alveolar proteinosis, and interstitial fibrosis.
Exposure to indium phosphide also caused increased incidences of benign and malignant pheochromocytomas of the adrenal gland in males and females. Marginal increases in the incidences of mononuclear cell leukemia in males and females, fibroma of the skin in males, and carcinoma of the mammary gland in females may have been related to exposure to indium phosphide.
Two-year study in mice
Groups of 60 male and 60 female mice were exposed to particulate aerosols of indium phosphide at concentrations of 0, 0.03, 0.1, or 0.3 mg/m3, 6 hours per day, 5 days per week, for 21 weeks (0.1 and 0.3 mg/m3 groups) or 105 weeks (0 and 0.03 mg/m3 groups). Animals in the 0.1 and 0.3 mg/m3 groups were maintained on filtered air from exposure termination at week 21 until the end of the studies. Ten males and 10 females per group were evaluated at 3 months.
Three-month interim evaluation
Exposure to indium phosphide for 3 months affected the circulating erythroid mass and caused enlarged lungs and lesions in the respiratory tract and lung-associated lymph nodes. These effects, although qualitatively similar to those observed in the 14-week studies, were considerably less severe. However, the lesions in the lungs of mice exposed to 0.1 mg/m3 and greater were considered sufficiently severe that exposure was discontinued in these groups and the groups were allowed to continue unexposed for the remainder of the study.
Survival and body weights
In general, exposure to indium phosphide for 2 years reduced survival and body weight gain in exposed males and females.
Pathology findings
At 2 years, exposure to indium phosphide caused increased incidences of alveolar/bronchiolar carcinomas in males and alveolar/bronchiolar adenomas and carcinomas in females. In addition to the alveolar proteinosis and chronic active inflammation seen at earlier time points, serosa fibrosis and pleural mesothelial hyperplasia were also present.
The incidences of hepatocellular neoplasms were also significantly increased in exposed males and females. Exposed groups of males and females had increased incidences of eosinophilic foci of the liver at 2 years. Marginal increases in the incidences of neoplasms of the small intestines in male mice may have been related to exposure to indium phosphide. Exposure to indium phosphide also caused inflammation of the arteries of the heart, primarily the coronary arteries and the proximal aorta, and to a lesser extent the lung-associated lymph nodes in males and in females.
Tissue burden analyses
Deposition and clearance studies of indium following long term exposure of rats and mice to indium phosphide by inhalation were performed. Although there were quantitative differences in lung burden and kinetic parameters for rats and mice, qualitatively they were similar. Deposition of indium in the lungs appeared to follow a zero-order (constant rate) process.
Retained lung burdens throughout the studies were proportional to exposure concentration and duration. No differences in elimination rates of indium from the lungs were observed as a function of exposure concentration in either rats or mice. These studies indicated that elimination of indium was quite slow. Mice exhibited clearance half-times of 144 and 163 days for the 0.1 and 0.3 mg/m3 groups, respectively, as compared to 262 and 291 days for rats exposed to the same concentrations.
The lung deposition and clearance model was used to estimate the total amount of indium deposited in the lungs of rats and mice after exposure to 0.03 mg/m3 for 2 years or to 0.1 or 0.3 mg/m3 for 21 or 22 weeks, the lung burdens at the end of the 2-year study, and the area under lung burden curves (AUC). For both species, estimates at the end of 2 years indicated that the lung burdens in the continuously exposed 0.03 mg/m3 groups were greater than those in the 0.1 or 0.3 mg/m3 groups. The lung burdens were lowest in the 0.1 mg/m3 groups. Because of the slow clearance of indium, the lung burdens in the 0.1 and 0.3 mg/m3 groups were approximately 25% of the maximum levels in rats and 8% in mice approximately 83 weeks after exposure was stopped. The AUCs and the total amount of indium deposited per lung at the time exposure was stopped indicate that the 0.3 mg/m3 groups were exposed to a greater amount of indium phosphide than were the 0.03 or 0.1 mg/m3 groups, with the 0.1 mg/m3 group receiving the lowest exposure. In rats and mice, the second-year AUC for the 0.03 mg/m3 group was equivalent to that of the 0.3 mg/m3 group. Regardless of how the total dose of indium to the lung was estimated, total exposure to indium in the 0.1 mg/m3 groups was less than that in the other two groups implying that in these studies, 0.1 mg/m3 may be considered the low dose.
Genetic toxicology
No significant increases in the frequencies of micronucleated normochromatic erythrocytes were noted in peripheral blood samples of male or female mice exposed to indium phosphide for 14 weeks. Although there was a significant increase in micronucleated polychromatic erythrocytes in 30 mg/m3 male mice, there was no increase in female mice, and the percentage of polychromatic erythrocytes was not altered in males or females.
Conclusions
Under the conditions of these 2-year inhalation studies, there was clear evidence of carcinogenic activity of indium phosphide in male and female F344/N rats based on increased incidences of benign and malignant neoplasms of the lung. Increased incidences of pheo-chromocytoma of the adrenal medulla in males and females were also considered to be exposure related. Marginal increases in incidences of mononuclear cell leukemia in males and females, fibroma of the skin in males, and carcinoma of the mammary gland in females may have been related to exposure to indium phosphide. There was clear evidence of carcinogenic activity of indium phosphide in male B6C3F1 mice based on increased incidences of malignant neoplasms of the lung and benign and malignant neoplasms of the liver. Marginal increases in incidences of adenoma and carcinoma of the small intestine may have been related to exposure to indium phosphide. There was clear evidence of carcinogenic activity of indium phosphide in female B6C3F1 mice based on increased incidences of benign and malignant neoplasms of the lung. Increased incidences of liver neoplasms in females were also considered to be exposure related.
Exposure to indium phosphide by inhalation resulted in nonneoplastic lesions in the lung of male and female rats and mice, the adrenal medulla of female rats, and the liver and heart of male and female mice.