Malathion and malaoxon: histopathology reexamination of the National Cancer Institute's carcinogenesis studies

In the early 1970s the National Cancer Institute (NCI) studied malathion and the oxygen analog, malaoxon, for possible carcinogenicity in rats and mice. The results from these long-term studies were reported in three NCI Technical Reports with the conclusions that neither chemical was shown to be carcinogenic in rodents. In response to the renewed public health interest and concern about the increasing use of malathion in agriculture and especially its use to eliminate Mediterranean fruit fly infestations in California and Florida during the 1980s, the National Toxicology Program (NTP) in consultation and agreement with the NCI reevaluated the histopathology of the NCI studies of malathion in Osborne-Mendel and Fischer 344 rats and of malaoxon in Fischer 344 rats. The NTP histopathology reexamination confirmed the original NCI interpretative conclusions that malathion was not carcinogenic. For the malaoxon study, the only difference between the original and subsequent interpretations was for C-cell neoplasms of the thyroid gland, in that the NTP concluded there was equivocal evidence of carcinogenicity for male and female F344 rats.     

Carcinogenicity of dimethoate

Studies on the carcinogenicity of the insecticide dimethoate in animals were reviewed. Examination of histological sections showed that dimethoate is highly carcinogenic in Osborne-Mendel rats. Neoplasms at all sites, as well as malignant neoplasms, were increased in both low and high doses of dimethoate-treated male rats in the National Cancer Institute study. The malignant neoplasms were both carcinomas and sarcomas. Neoplasms of the endocrine organs, particularly carcinomas, were increased in male and female rats given dimethoate. These carcinomas were observed in the adrenal, thyroid, and pituitary glands. Neoplasms were also increased in the liver of male and female rats and in the reproductive organs of female rats given dimethoate. Male and female rats treated with dimethoate developed monocytic leukemia. There also were toxic changes in rats. Male rats had atrophy of the testes, chronic renal disease, parathyroid hyperplasia, and polyarteritis. Wistar male and female rats given dimethoate by gavage or intramuscularly developed a significant increase in malignant neoplasms, mainly sarcomas, and granulocytic leukemia. AB male and female mice also had an increased incidence of malignant neoplasms and granulocytic leukemia after dermal applications of dimethoate.     

Carcinogenicity of lindane.

Lindane is highly carcinogenic in rats and mice. Lindane induced benign and malignant neoplasms at all sites in male and female rats. Benign and malignant neoplasms of the endocrine organs were also increased. Carcinomas of the adrenal and pituitary were increased markedly in male and female rats. Females were more susceptible than males. Carcinomas of the ovary were increased in lindane-treated female rats. There were increased incidences of neoplasms of the liver. Hepatic carcinomas were induced in four strains of mice. The incidence of neoplasms of the liver was markedly increased in CF1 strain mice. Rats given lindane also developed toxic changes, particularly male rats, as a result of treatment. These lesions included interstitial fibrosis of the kidney and atrophy of the testes. These toxic changes may have interfered with the development of neoplasms in male rats.     

Pathologic effects of chronic administration of hydrochlorothiazide, with and without sodium nitrite, to F344 rats

The diuretic drug hydrochlorothiazide was administered to 24 male and 24 female F344 rats as a mixture of 0.1% in powdered food. A parallel group of the same size was given 0.1% hydrochlorothiazide plus 0.2% sodium nitrite in the food. A third group received 0.2% sodium nitrite in the food and there was a similar group of untreated controls. The treatments were well tolerated and there was no significant life shortening. A majority of the rats given hydrochlorothiazide, with or without nitrite, developed chronic progressive nephropathy, which was more severe in males than in females. Associated with this were diffuse parathyroid hyperplasia in both groups receiving the drug, also more severe in males than in females, and parallel increases in lesions of the blood vessels (mural thrombosis of the heart and polyarteritis). The few adenomas of the parathyroid and tubular cell adenomas of the kidney in rats ingesting hydrochlorothiazide were not statistically significant.     

Hyalin droplet nephrosis in male Fischer-344 rats following inhalation of diisobutyl ketone

Four groups, each consisting of 10 male and 10 female Fischer-344 rats, were exposed 6 h/day, 5 days/week, for 9 days to diisobutyl ketone (DIBK) vapor at concentrations of 905, 300, 98, or 0 (control) ppm. An additional 10 rats/sex were assigned to the 905 and 0 ppm groups and allowed two weeks recovery prior to sacrifice. Rats exposed to 905 ppm had mild ocular irritation (lacrimation) and evidence of kidney toxicity, manifested as: 1) an increase in absolute and relative (as a percentage of body weight) kidney weights, 2) an increase in urine volume (and water intake) with a concomitant decrease in urine osmolality (males only), and 3) an increase in severity of hyalin droplet nephrosis in the proximal tubules (males only). Absolute and relative liver weights were also increased in both male and female rats of the 905 and 300 ppm groups. These effects either disappeared or lessened in severity following the 2-week recovery period. Male rats exposed to 300 ppm had similar renal alterations to the males of the 905 ppm group, although the alterations were fewer in number and smaller in magnitude. Kidney weights and renal histology of the males of the 98 ppm group were similar to the control male rats, although an increase in urine volume with a decrease in urine osmolality was observed. The kidney findings in this study were not surprising because of the chemical relationship of DIBK with other aliphatic ketones (e.g., methyl isobutyl ketone, methyl isoamyl ketone) which, after repeated inhalation exposure, cause hyalin droplet nephropathy in male rats. The significance of this male rat nephrosis with regard to human exposure is unknown.     

The determination of the repeated oral toxicity of halocarbon oil, series 27-S

Halocarbon 27-S (HC 27-S), a polymer of chlorotrifluoroethylene (CTFE), is used as a lubricating oil for pumps in hyperbaric chambers. Although monomeric CTFE has been shown to produce renal lesions in rats, the toxicity of CTFE polymers have not been investigated. To assess the toxicity of repeated exposure to HC 27-S, three groups (N = 6/group) of male and female Fischer-344 rats were dosed with 2.5 g HC 27-S/kg for 7 or 21 consecutive days. Groups were sacrificed at 7, 21, and 35 days (14 days after the 21-day dosing). Corresponding control groups (N = 6) were dosed with deionized water. Decreased water consumption and urine output were apparent in all test groups. Statistically significant increases in fluoride excretion were noted in 24-hr urine samples assessed periodically during the study. Neurotoxic signs were observed in female rats but not in male rats. Significant increases in liver and kidney weights were seen in all rats, regardless of number of dosing days. The increased fluoride burden in treated animals appeared sufficient to alter bone calcium/phosphate ratios in male rats but not female rats. Gross liver enlargement and hepatocellular cytomegaly indicated that the liver was probably the primary target organ following repeated administration of HC 27-S.     

Thirteen-week oral toxicity study of 1,4-dichloro-2-nitrobenzene in rats and mice

Subchronic toxicity of 1,4-dichloro-2-nitrobenzene (DCNB) was examined by feeding F344 rats and BDF mice of both sexes a diet containing 1,481, 2,222, 3,333, 5,000 or 7,500 ppm DCNB (w/w) for 13 wk. Oral administration of DCNB in feed to rats and mice retarded growth rates and induced subchronic toxicity affecting the liver, kidney, testes and blood. Liver and kidney were most responsive to DCNB. BMDL10 values for relative liver weight were 12.0 and 22.6 mg/kg/d for male and female rats, respectively, and 88.7 and 94.4 mg/kg/d for male and female mice, respectively. Increased liver weights and centrilobular hypertrophy of hepatocytes were observed in the DCNB-fed rats and mice of both sexes. Both increased serum activities of AST and ALT and liver necrosis occurred in the DCNB-fed mice. Increased incidences of hyaline droplets and granular casts in the proximal renal tubules were observed only in the DCNB-fed male rats, indicating alpha2v-globulin nephropathy. Eosinophilic droplets in the renal tubular cells and increased BUN concentrations occurred in the DCNB-fed female rats. DCNB-induced testicular and hematologic changes were noted in rats and mice. On the basis of these results, the highest dose level for the 2-yr bioassay study of rodent carcinogenicity was determined to be 2,000 ppm.     

Acute Toxicity and Tissue Distributions of Malathion in <Emphasis Type="Italic">Ambystoma tigrinum</Emphasis>

The kinetics of the bioaccumulation of malathion (O,O-dimethyl phosphorodithioate of diethyl mercaptosuccinate) and the biological impact of exposure for tiger salamanders, Ambystoma tigrinum, were assessed through exposure to soil surface contaminated with 50 μg/cm² or 100 μg/cm² malathion and ingestion of an earthworm exposed to soil contaminated with 200 μg/cm² malathion. Malathion and malaoxon burdens in salamanders sampled at different times after exposure(s) were measured by gas chromatography in four tissue/organ subgroups: liver, epaxial muscle, pooled viscera (except the liver and brain), and pooled avisceral carcass (muscle, skin, and bone). The total tiger salamander xenobiotic burdens were calculated from these data. The malathion/malaoxon burden 1 day after exposure was greatest in the avisceral carcass and 2 days after exposure was greatest in the viscera. Bioconcentration and bioaccumulation factors remained less than unity throughout the experiment and did not support the hypothesis of bioaccumulation of malathion in the tiger salamander. Biological impact was assessed with a colorimetric brain cholinesterase microassay. Brain cholinesterase activities in salamanders exposed to malathion-contaminated soil (50 μg/cm² or 100 μg/cm² malathion) were suppressed ∼50–65% and 90%, respectively, compared to unexposed controls. The exposed animals did not exhibit overt clinical signs of malathion toxicosis.     

Compensatory regeneration as a mechanism for renal tubule carcinogenesis of fumonisin B1 in the F344/N/Nctr BR rat

Fumonisin B1(FB1) is a fungal metabolite of Fusarium verticillioides (= F. moniliforme), a fungus that grows on many crops worldwide. Previous studies demonstrated that male BD IX rats consuming diets containing 50 ppm fumonisin B1 developed hepatocellular carcinomas. In our recent studies, diets containing FB1 at 50 ppm or higher concentrations induced renal tubule carcinomas in male F344/N/Nctr BR rats and hepatocellular carcinomas in female B6C3F1/Nctr BR mice. The carcinogenicity of FB1 in rats and mice is not due to DNA damage, as several laboratories have demonstrated that FB1 is not a genotoxin. FB1 induces apoptosis in cells in vitro. Including FB1 in the diets of rats results in increased hepatocellular and renal tubule epithelial cell apoptosis. In studies with F344/N/Nctr BR rats consuming diets containing up to 484 ppm FB1 for 28 days, female rats demonstrated more sensitivity than male rats in the induction of hepatocellular apoptosis and mitosis. Conversely, induction of renal tubule apoptosis and regeneration were more pronounced in male than in female rats. Induction of renal tubule apoptosis and hyperplasia correlated with the incidence of renal tubule carcinomas that developed in the 2-year feeding study with FB1 in the F344/N/Nctr BR rats. The data are consistent with the hypothesis that the induction of renal tubule carcinomas in male rats could be partly due to the continuous compensatory regeneration of renal tubule epithelial cells in response to the induction of apoptosis by fumonisin B1.     

A Comparison of Multiple Esterases as Biomarkers of Organophosphate Exposure and Effect in Two Earthworm Species

Two different earthworm species, Eisenia fetida and Lumbricus terrestris, were exposed to 5 μg/cm² of malathion to evaluate their usefulness as sentinels of organophosphate exposure and to assess three different esterases, as biomarkers of malathion exposure and effect. Tissue xenobiotic burdens and esterase activity were determined for each species and each esterase in order to assess variability. E. fetida exhibited 4-fold less variability in tissue burdens than did L. terrestris and had less variable basal esterase activities. An attempt was made to correlate malathion and malaoxon tissue burdens with esterase activity post-exposure. There was no malaoxon present in the earthworm tissues. No significant correlations were determined by comparing acetylcholinesterase, butyrylcholinesterase, nor carboxylesterase activities with malathion burdens.     

Fumonisin b1 carcinogenicity in a two-year feeding study using F344 rats and B6C3F1 mice

Fumonisin B1 (FB1) is a mycotoxin isolated from Fusarium fungi that contaminate crops worldwide. A previous study demonstrated that FB1 promoted preneoplastic foci in initiated rats and induced hepatocellular carcinomas in BD IX rats at 50 parts per million (ppm), but fundamental dose-response data were not available to assist in setting regulatory guidelines for this mycotoxin. To provide this information, female and male F344/N/Nctr BR rats and B6C3F1 Nctr BR mice were fed for two years a powdered NIH-31 diet containing the following concentrations of FB1: female rats, 0, 5, 15, 50, and 100 ppm; male rats, 0, 5, 15, 50, and 150 ppm; female mice, 0, 5, 15, 50, and 80 ppm; male mice, 0, 5, 15, 80, and 150 ppm. FB1 was not tumorigenic in female F344 rats with doses as high as 100 ppm. Including FB1 in the diets of male rats induced renal tubule adenomas and carcinomas in 0/48, 0/40, 9/48, and 15/48 rats at 0, 5, 15, 50, and 150 ppm, respectively. Including up to 150 ppm FB1 in the diet of male mice did not affect tumor incidence. Hepatocellular adenomas and carcinomas were induced by FB1 in the female mice, occurring in 5/47, 3/48, 1/48, 19/47, and 39/45 female mice that consumed diets containing 0, 5, 15, 50, and 80 ppm FB1, respectively. This study demonstrates that FB1 is a rodent carcinogen that induces renal tubule tumors in male F344 rats and hepatic tumors in female B6C3F1 mice.     

Cumulative effects of 1,2-dibromo-3-chloropropane (DBCP) on kidney and testis

Male rats were repeatedly given 1,2-dibromo-3-chloropropane (DBCP) at 10, 30, or 100 mg/kg twice a week for 12 weeks and were examined pathologically at the 12th, 24th and 36th week. DBCP produced primary lesions in kidneys and testes dose-dependently. The renal lesions were the lining of the proximal tubules in the outer medulla by enlarged, occasionally giant, epithelial cells. Testicular lesions were the atrophy of the seminiferous tubules. These lesions were qualitatively and quantitatively similar throughout the experiment. The results indicated that repeated exposure of relatively low doses of DBCP produced irreversible lesions in the kidney and testis, and that the effects on these tissues may be cumulative.     

Biokinetics of inhaled 244Cm oxide in the rat: effect of heat treatment at 1150 degrees C

A study was conducted in rats to determine solubility and subsequent metabolism of an inhaled aerosol of curium treated at high temperatures. Young adult Fischer-344 rats received a single inhalation exposure to one of three monodisperse aerosols of 244Cm2O3 (0.70, 1.3, or 2.6 micron activity median aerodynamic diameter) heat-treated at 1150 degrees C. Animals were maintained individually in metabolism cages for excreta collection and serially sacrificed in groups of two male and two female rats from 2 to 33 days after inhalation exposure. Additionally an injection study with curium citrate was done to define the systemic behavior of Cm in this rat model. The in vivo solubility was inversely related to the aerosol particle size. The relationship of the results of this study to results from other experimental inhalation studies with curium oxide aerosols is discussed, as is the relevance to bioassay interpretation and risk assessment in man.     

Reversibility of testicular atrophy induced by Di(2-ethylhexyl) phthalate in rats

Previous studies have shown that di(2-ethylhexyl) phthalate (DEHP) results in atrophy of testes and accessory sex organs accompanied by a decreased testicular concentration of zinc in rats. An experiment was performed to determine whether those changes in rat testes are reversible changes. Young Wistar male rats were administered 2.0 g/kg of DEHP for 14 days. At that time, one-half of the rats were killed for determination of zinc and testosterone concentrations in the testis, liver, and serum, and organ weights. The remaining rats were maintained for additional 45 days without further DEHP administration. Testicular weight of rats administered DEHP over the 14-day period was significantly less than that of the control animals, and testicular testosterone content and zinc concentrations were less than those of the control animals. At the end of the 45-day recovery phase, serum testosterone concentration returned to the control level, but testicular zinc concentration and testosterone content were still lower than those of the control animals. In addition, testicular weight of DEHP-treated rats was lower than that of the control animals and histologically, only a small number of seminiferous tubules showed spermatogenesis. These results indicate that, morphologically, DEHP-induced testicular atrophy appears to be of limited reversibility.     

Toxicity of malathion to mammals,aquatic organisms and tissue culture cells

The effect of malathion on rats (75 and 38 mg/kg bwt), aquatic organisms (100 to 0.001 mg/L), and cells in tissue culture (1000 to 1 ppm) was studied. The conventional toxicological tests conducted for 90 days on rats yielded negative results. ChE activity was determined in plasma, liver, brain and erythrocyte samples. It was significantly reduced in the erythrocytes of animals treated with the larger dose for 21 days and in the cerebral cortex of rats fed either of the doses. ChE activity of rats consuming malathion for 90 days did not differ significantly from that of the controls. In contrast, the psychophysiological examinations utilized in the experiments indicated abnormalities within 21 days. Alterations were observed in the EEG and EMG records after 90 days of feeding.Malathion had a definitely harmful effect on phylogenetically and ontogenetically young aquatic organisms, as well as on the cells of monkey kidney culture. The latter finding suggests that the preparation has a direct destructive effect on cells.Although it is not suggested that malathion should be regarded a toxic agent thus requiring limitation of application, attention is directed to the fact that inconsiderate use of the preparation may involve potential dangers for man and his environment.     

Damage to spermatogenesis in juvenile rat treated with DDVP and malathion

Summary Juvenile male rats received either 20 mg DDVP or 40 mg malathion/kg on the 4th and 5th day of life, and either 10 mg DDVP or 20 mg/kg malathion daily from the 4th to the 23rd day of life. The histological examination of the testes showed slight reductions of the spermatogenetic cells and Leydig cells. Therefore, it is assumed that testosterone synthesis is reduced, followed by damage to spermatogenetic cells. All disturbances return to normal by the 50th day of life.     

Effect of casein diets on the toxicity of malathion and parathion and their oxygen analogues

Summary The toxicities of malathion and parathion and their oxygen analogues were investigated in rats fed different levels of dietary protein. Male weanling rats were fed either a 5 or 20% casein diet for 10 days.Rats fed 5% casein diets were more susceptible to malathion and malaoxon acute toxicity when compared with rats pair-fed 20% casein diet or with rats fed 20% casein dietad libitum. Parathion and paraoxon were likewise more toxic to rats fed the 5% casein diet than to those fed the 20% casein diet (pair-fed or fedad libitum). The urinary excretion of p-nitrophenol as affected by dietary protein was not indicative of the toxicity of parathion and paraoxon.Supported in part by Public Health Service Grant No. ES00257 of the National Institutes of Health.Conducted as partial fulfillment of the requirements for the M. S. degree, Department of Biochemistry and Nutrition, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.     

Effect of manganese treatment on the levels of neurotransmitters, hormones, and neuropeptides: modulation by stress

Six weeks of daily intraperitoneal injection with manganese chloride (15 mg/kg body wt) reduced the normal weight gain of male Fischer-344 rats. This treatment depressed plasma testosterone and corticosterone levels, but prolactin levels were unaffected. The only significant changes in the levels of a variety of neuropeptides assayed in several regions were increases in the levels of hypothalamic substance P and pituitary neurotensin. Striatal serotonin, dopamine, and their metabolites were unchanged in manganese-exposed rats relative to saline-injected controls. However, the stress of injection combined with the effect of manganese appeared to significantly increase concentrations of striatal monoamines relative to uninjected controls.     

三羟异黄酮对断乳后大鼠生殖发育的影响

目的研究三羟异黄酮对断乳大鼠生殖发育的影响。方法将3周龄幼鼠48只按体重随机分为3组,雌雄各8只,口饲给予含不同剂量三羟异黄酮的基础饲料[对照组0mg/(kg.d),中剂量组20mg/(kg.d),高剂量组100mg/(kg.d)],待各组大鼠发育至9周龄时,测雌鼠血清雌二醇水平,雄鼠血清睾酮水平;待各组大鼠发育至13周龄时,分离雌鼠子宫、卵巢,雄鼠睾丸进行病理检查,用Western blotting方法测定雌激素受体(ER)的表达水平。结果与对照组相比,9周龄时雌鼠血清雌二醇水平降低,雄鼠血清睾酮水平升高;13周龄时,雌鼠体重和摄食量下降,阴道开放提前,生殖器与直肠间距(AGD)减小,卵巢称重减少。病理切片显示雌鼠子宫腔扩大,子宫扩张,血清雌二醇水平有所降低,子宫雌激素受体ER表达量降低;雄鼠摄食量10周龄左右有所升高,血清睾酮水平升高,但体重无明显升高,睾丸下降时间未见提前,睾丸称重无差别,病理切片未见异常。结论三羟异黄酮对断乳后大鼠的生殖指标影响较大,其中对雌性大鼠的影响大于雄性大鼠,但是对大鼠体重和摄食量的影响主要在其成年后,并且此种影响与其激素水平和受体表达有关。     

Louis Schwartz,MD

The effects of parathion, dichlorvos, diazinon, malathion, apholate, and tepa on the rat fetus were studied when the material was given intraperitoneally. At levels producing toxic symptoms in the dams, parathion, dichlorvos, diazinon, apholate, and tepa were found to affect the fetus either by causing malformations, resorptions, or reducing the weight of the fetus and placenta. Malathion at toxic dosage levels neither affected the weight of the fetus nor produced malformations. At dosage levels which were tolerated by the dams, parathion, diazinon, and tepa caused a great number of resorptions. The incidence of resorptions was not higher than that of the controls in rats that had been given dichlorvos, malathion, or apholate.