Metam-Sodium

Chapter 107

Metam-Sodium Linda L. Carlock1 and Timothy A. Dotson2 1 2

Toxicology and Regulatory Consulting UCB Chemicals Corporation

107.1  Introduction Metam-sodium (C2H4NNaS2, CAS no. 137-42-8), also known as metham sodium, sodium metam, sodium-N-methyldithiocarbamate, methylcarbamodithioic acid sodium salt, methyldithiocarbamic acid sodium salt, carbam, and SMDC, is a white crystalline powder in the pure form but is normally found as a clear yellow liquid with a strong sulfurlike odor (Merck, 1989). Metam-sodium is prepared from methylamine, carbon disul-fide, and sodium hydroxide in an aqueous solution. Metam-sodium has a molecular weight of 129.18. Metam-sodium is stable in its dry, crystalline state, and in concentrated aqueous solution. In solution, metam-sodium has a vapor pressure of 21 mg Hg at 25°C (U.S. EPA, 1994a). Metam-sodium is very stable at a pH greater than 8.8, but at pH 7 and below it readily hydrolyzes. In soil or when diluted with water, metam-sodium is converted to methyl isothiocyanate (MITC). Other degradates of metam-sodium include carbon disulfide (CS2) and hydrogen sulfide (H2S). Metam-sodium is an agricultural general use pesticide used primarily as a broad spectrum preplant soil fumigant to control weeds, weed seeds, fungi, nematodes, and soil insects. End use products are formulated as 18–42% aqueous solutions sold under the trade names of Metam CLR, Vapam, and Sectagon. Metam-sodium has been registered since 1954. Registered uses of metam-sodium include agricultural soil fumigation, wood preservative, slimicide, tree-root killer, and aquatic weed control. Approximately 10 million pounds of metam-sodium were used in 1990, with 40–45% used for agricultural purposes (U.S. EPA, 1994a). As a soil fumigant, metam-sodium is applied after harvest and/or 14 to 21 days prior to planting by shank injection, disc, rotary tiller, drip irrigation, solid set sprinkler, or center pivot chemigation. In some parts of North America, fall applications are preferred because metam-sodium volatilizes over the winter and clears the soil, allowing planting to begin as soon as favorable springtime conditions arrive. By treating the soil with metam-sodium, fruit and vegetable Hayes’ Handbook of Pesticide Toxicology Copyright © 2001 Elsevier Inc. All rights reserved

growers can control weeds, reduce nematode populations, and control soil-borne pests. Metam-sodium may be used on all crops but is particularly important in the production of melons, peppers, tomatoes, potatoes, strawberries, citrus, grapes, almonds, artichokes, asparagus, carrots, lettuce, spinach, squash, forest tree seedlings, ornamentals, and cut flowers. By reducing competition from soil pests, metamsodium promotes healthier plants and increased yields. The U.S. EPA (1997) considers metam-sodium to be a commercially viable alternative to methyl bromide fumigation for fruit and vegetable production due to its low cost, wide range of control, and long record of safe use. It can be used to control weeds (e.g., bluegrass, Bermuda grass, chickweed, dandelion, ragweed, henbit, nutsedge, and wild morning glory), nematodes, and soil diseases caused by species of Rhizoctonia, Fusarium, Pythium, Phytophthora, Verticillium, and Sclerotinia (U.S. EPA, 1997). Metam-sodium has also been shown to be useful in integrated pest management systems as it can be used in conjunction with other treatment methods such as biological controls and soil pasteurization. Metam-sodium is a slightly to moderately toxic compound that when used according to label directions has been shown to be a safe and versatile product for over 45 years. For agricultural use, metam-sodium must be applied in a manner where there is no contact with workers or other persons, either directly or through drift. Only handlers equipped with the proper personal protection equipment may be in the area during application. In California, application must also be in compliance with the Technical Information Bulletin “Guidelines for All Application Methods for Metam-sodium in California.” The potential routes of human chemical exposures are oral (ingestion), dermal (direct skin contact), and inhalation, however, the chance for nonoccupational exposure to metam-sodium is minimal. Approved agricultural uses of metam-sodium do not leave residues on crops, thus eliminating diet as a source of exposure. The primary means of exposure to metam-sodium is through dermal occupational 2293

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exposure. Most of the potential for exposure to metamsodium itself comes from transloading and handling the liquid when preparing for application. The use of required protective gloves, boots, and clothing minimizes or eliminates dermal exposure to metam-sodium. The U.S. EPA’s Occupational and Residential Exposure Branch assumes that dermal exposure is minimal for handlers and nonexistent for nearby residents and bystanders (U.S. EPA, 1994a). There is little potential for inhalation exposure to metam-sodium, which has been proven through extensive monitoring and in a number of worker exposure studies. Proper protective equipment such as in-cab filtering systems and NIOSH-approved respirators that are used by workers provide protection in the unlikely situation that the liquid compound becomes aerosolized. The toxicology of “technical grade” or formulated metam-sodium (approximately 42% a.i.) is well established. Metam-sodium is synthesized in aqueous solution and then diluted with water, as necessary, to achieve the desired concentration and meet the label guarantee for the formulated product. Thus, “technical grade” is synonymous with the formulated material. Little toxicity information is available regarding pure or analytical-grade metam-sodium. The following discussion of technical grade/formulated metam-sodium toxicity briefly covers a number of published studies and the results of toxicity studies submitted to governmental agencies in support of metam-sodium registration.

107.2  Acute toxicity Metam-sodium is slightly to moderately acutely toxic depending on the route of exposure. The following toxicity values pertain to technical grade metam-sodium (U.S. EPA, 1994a). All of the following values were obtained with standard acute toxicity studies designed to determine the dose or concentration that causes death to 50% of the test animals (LD50 or LC50): The acute LD50 for technical grade metam-sodium (43.7% a.i.) is reported as 870 mg/kg for male rats and 924 mg/kg for female rats. The combined (male and female) LD50 is 896 mg/kg (placing the compound into Toxicity Category III (U.S. EPA, 1994a) or similarly classified as slightly toxic (LD50  5–15 g/kg; Klaassen, 1986). l The acute dermal LD50 of technical grade metamsodium (43.7%) applied to male and female rabbits is 368 mg/kg (Toxicity Category III). l The acute inhalation LC50 of aerosolized technical grade metam-sodium (42%) in rats is 2.275 mg/l (Toxicity Category III). l Technical grade metam-sodium (42%) was found to be slightly irritating to the eyes of New Zealand White rabbits (Toxicity Category III). l

Technical grade metam-sodium (42%) is irritating to the shaved skin of male and female rabbits (Liggett and McRae, 1991) and is classified as a moderate to severe dermal irritant (Toxicity Category II). l Metam-sodium (42%) was also found to be a skin sensitizer to guinea pigs using the delayed contact hypersensitivity test (Parcell and Denton, 1991). l

Acute studies conducted with 32.7% metam-sodium showed similar but milder results than the above cited data for the 42% formulated compound. Jowa (1998) reported the following values for multiple studies conducted with metam-sodium: The acute oral LD50 for 32.7% metam-sodium varied from 1294 to 1415 mg/kg for male rats and 1350 to 1428 mg/kg for female rats. l The acute dermal LD50 for 32.7% metam-sodium varied from 1012 to 3500 mg/kg in rabbits. l The acute inhalation LC50 varied from 4.7 to 5.4 mg/1 for male rats exposed to 32.7% metamsodium for four hours. l In one eye irritation study with rabbits, 32.7% metamsodium was found to be a mild irritant, but in another study it was found to be nonirritating. l Dermal irritation studies with rabbits exposed to 32.7% metam-sodium showed that the compound was a severe irritant in one study and was corrosive in another study. l Testing guinea pigs with 32.5% metam-sodium in the Buehler test resulted in sensitization. l

Standardized acute toxicity studies provide limited information regarding subtle toxic effects and are not designed to establish a no observed effect level (NOEL). To further understand the sublethal effects of a compound, lower dose levels or concentrations are required.

107.3  Subchronic toxicity Effects of metam-sodium exposure over longer periods vary with the species tested and route of administration. A variety of toxicity studies have shown that there is a definite dose–response effect to metam-sodium (U.S. EPA, 1992, 1993). At very low doses levels there is no evidence of toxicity, but as the dose level increases the prevalence and severity of toxic effects increases. In a 90-day study (Whiles, 1991), male and female mice were administered metam-sodium in drinking water at dose levels of 0, 0.018, 0.088, 0.35, or 0.62 mg/ml (2.7, 11.7, 52.4, or 78.7 mg/kg/day for males; 3.6, 15.2, 55.4, or 83.8 mg/kg/ day for females). No treatment-related mortality, morbundity, or clinical signs of toxicity were observed during the 90-day study period. Treatment-related statistically significant decreases in mean body weight were observed in both males and females at dose levels of 0.35 and 0.62 mg/ml. Treatment-related changes in hematology parameters

Chapter | 107  Metam-Sodium

were noted at doses as low as 0.088 mg/ml for females and 0.62 mg/ml for males. The lowest effect level was determined to be 0.088 mg/ml (11.7 mg/kg/day for males, 15.2 mg/kg/day for females) based on urinary bladder lesions observed in both males and females and in statistically significant decreases in hemoglobin, red blood cell, and hematocrit in females. The NOEL for systemic toxicity was 0.018 mg/ml (2.7 and 3.6 mg/kg/day for males and females, respectively). In another 90-day metam-sodium study (Allen, 1991), male and female rats received metam-sodium in the drinking water at nominal dose levels of 0, 0.018, 0.089, and 0.443 mg/ml (1.7, 8.1 and 26.9 mg/kg/day for males; 2.5,9.3, and 30.6 mg/kg/day for females). Systemic toxicity was evident by significant decreases in food and water consumption, decreased body weight gain, and histological changes in the nasal cavity olfactory epithelium in both males and females receiving metam-sodium at 0.443 mg/ml. Renal tubular dilation and basophilia along with increases in blood and protein in the urine were also observed in 0.443 mg/ml rats. In both males and females receiving 0.089 mg/ml there were significant decreases in red blood cell count and hematocrit. Females at the 0.089 mg/ml dose level also had a significant decrease in group mean body weight and decreased body weight gain (11%) when compared to controls. Based on the results of this study the NOEL was 0.018 mg/ml (1.7 mg/kg/day for males; 2.5 mg/kg/day for females). In a subchronic dog study (Brammer, 1992), metamsodium (43.15% purity) was administered by gelatin capsule to male and female beagles at nominal dose levels of 0, 1, 5, or 10 mg/kg/day once daily for 13 weeks. Toxic effects were observed at all dose levels tested but were primarily evident at the 5 and 10 mg/kg/day dose levels. Decreased body weight and body weight gain were observed in males and females receiving metam-sodium at 10 mg/kg/day. There were no significant clinical effects at 1 or 5 mg/kg/day and no ophthalmoscopic abnormalities in any animals. Regurgitation within 30–60 minutes of dosing occurred throughout the study in the 10 mg/kg/ day group and on isolated occasions in the 5 mg/kg/day dogs. There was no regurgitation in the 1 mg/kg/day dosing group. In dogs receiving 5 and 10 mg/kg/day there were changes in hematologic parameters (increases in cell volume, cell hemoglobin, neutrophils, and monocytes; decreases in mean corpuscular hemoglobin concentration); significant increases in plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and gammaglutamyltransferase; increased blood, urobilinogen, bilirubin, and protein in the urine; and microscopic evidence of hepatitis). One female receiving 1 mg/kg/day showed increased plasma ALT. Biliary duct proliferation with inflammatory cell infiltration (less severe than hepatitis) was observed in one male and one female at the 5 mg/kg/day dose level and in one female at the 1 mg/kg/day dose level. No evidence of tumors were found in this study. Toxic effects appeared to be doseand time-related. For female dogs, no systemic NOEL was established (NOEL 1 mg/kg/day) due to increases in plasma

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ALT and biliary duct proliferation with inflammatory cell infiltration observed in a single female from the 1 mg/kg/day dose group. For male dogs, the systemic NOEL is 1 mg/kg/ day. The lowest observed effect level (LOEL) of 5 mg/kg/day is based on statistically significant increases in plasma ALT, AST, and alkaline phosphatase, and the increased incidence of hepatitis and bile duct proliferation. In order to further study the effects of metam-sodium on the liver of dogs, a study was conducted at the dose level that caused moderate to marked hepatitis in all dogs during the 90-day study described above (Brammer, 1993). One male and one female beagle dog received metam-sodium (43.14% purity) in a gelatin capsule daily at a dose level of 10 mg/kg. Dosing of each dog continued until there were elevations in plasma enzyme activities (or other clinical signs) indicative of liver toxicity. Following cessation of dosing, each dog was monitored until the enzyme activities returned to normal or prestudy levels. Dosing ceased after 12 weeks of dosing for the female and after 13 weeks for the male. Recovery was monitored for 8 weeks. After Week 6 the plasma ALT levels in the female began to increase and by Week 10 they were over 200 IU/L. In the male, elevated plasma ALT was noted at Week 9 and exceeded 200 UI/L by Week 11. In both dogs, plasma ALP levels gradually increased until dosing ceased. Following cessation of dosing, ALT levels increased during the first recovery week then gradually declined to normal levels after 8 weeks. Plasma ALP decreased in the female dog immediately after cessation of dosing and by Recovery Week 4 was less than prestudy values. In the male, ALP continued to rise during the first recovery week then gradually decreased so that by Recovery Week 5, ALP values were less than prestudy values. In both dogs, ALP levels continued to fall until study termination. At study termination, there were no macroscopic abnormalities in either dog and liver weights were normal. Microscopic evaluations revealed that there was a minimal or slight increase in the number of pigmented macropahges/Kupffer cells in the liver, but this is a common finding in beagle dogs of this strain (Alderley Park). The significant elevations in plasma ALT and ALP levels found in this study are consistent with the findings of the previous 90-day dog study at the same dose level (10 mg/kg/day) and are indicative of liver injury. However, after cessation of exposure, enzyme levels returned to normal, with full recovery eight weeks after the last exposure to metam-sodium. There was no evidence of liver injury at the end of the study. These findings confirm the reversible nature of induced liver effects from subchronic exposure to relatively high levels of metam-sodium.

107.4  Genetic toxicity Metam-sodium is not mutagenic but has been shown to be directly cytotoxic to bacteria, fungi, and mammalian cells. Metam-sodium has been tested and found to be negative in

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both in vitro and in vivo genetic toxicology assays covering a range of genetic toxicology endpoints including mutations, cytoge-netics, and DNA repair. There is evidence that at high enough dose levels, exposure to metam-sodium can be immunotoxic, with response evident in a dose-dependent manner. A review of metam-sodium genetic toxicity studies (Mackay, 1996) concluded that “[M]etam sodium shows no in vitro or in vivo genotoxic activity in a series of assays conducted up to concentrations/dose levels inducing significant toxicity in the target cells/animals.” In a bacterial gene mutation assay using Salmonella typhimurium strains TA92, TA98, TA100, TA1535, TA1537, and TA1538 in the presence and absence of metabolic activation (AROCHLOR 1254-induced rat liver S9 mix) there were no significant increases in the number of revertant colonies in any of the strains or S9 combinations tested. l In a Chinese hamster ovary mammalian cell gene mutation (HGRPT locus) assay metam-sodium was tested in the presence and absence of metabolic activation. There was no evidence of any reproducible dose-related effects of metam-sodium on mutation frequency or evidence of in vitro mutagenic activity. l In two in vitro cytogenetic assays using human lymphocytes there was no evidence of clastogenic activity from metam-sodium treatment when tested at concentrations up to those limited by toxicity and/or cytotoxic effects on chromosomal morphology. l The first study found an increase in aberrant cells at concentration levels that caused severe cytotoxicity (20 g/ml without S9 mix; 40 and 20 g/ml with S9 mix) and therefore were unsuitable to be included in the evaluation of clastogenic potential. At concentration levels of 1, 5, and 10 (g/ml with and without S9 mix, there were no increases in the percentage of aberrant cells. l In the second in vitro human lymphocyte clastogenic study, metam-sodium at concentrations of 2.5, 20, and 30 g/ml in the absence of S9 mix and 5, 20, and 40 (g/ml with S9 were tested. No statistically or biologically significant increases in the percentage of aberrant cells were observed at any of the metamsodium concentrations tested in the absence of the S 9 mix. There was a small statistical increase in the number of aberrations observed in the 40 g/ml test concentration with the S9 mix, but the values observed were well within the historical solvent control range and do not indicate clastogenic activity. l In an in vitro unscheduled DNA synthesis assay using primary rat hepatocytes treated with metam-sodium there was no evidence of induction of DNA repair, even in cultures treated with toxic concentrations of metam-sodium. l In an in vivo Chinese hamster bone marrow chromosomal aberration assay there was no evidence of any l

polyploidy inducing effect of metam-sodium nor was there evidence of any clastogenic activity. l When metam-sodium was administered to CD-1 mice in an in vivo mouse bone marrow micronucleus test, there was no evidence of clastogenic activity in the mouse bone marrow when tested up to the maximum tolerated dose level for both male and female mice. There were no statistically or biologically significant increases in the incidence of micronucleated polychromatic erythrocytes. U.S. EPA Tox Oneliners report on the results of genetic studies submitted to and reviewed by the U.S. EPA. Jowa (1998) reported on the same genetic studies submitted to and reviewed by the California Environmental Protection Agency (Cal EPA). In some cases, the results presented by Jowa did not agree with conclusions of the U.S. EPA. In two separate Ames studies using multiple strains of Salmonella typhimurium (TA 1535, 1537, 1538, 92, 98, and 100) up to 2500 g/plate with and without activation (S9 mix), metam-sodium did not induce mutations and the results were negative. l In a study with yeast (Sacchromyces cerevisiae strain D4) with and without S9 mix, metam-sodium did not induce mutations and the results were negative. l In an in vitro study with Bacillus subtilis, metamsodium did not cause DNA damage. l According to the Cal EPA review, equivocal results were obtained for a REC assay in Bacillus subtilis H17 and M45 (/ S9). l According to the U.S. EPA review, metam-sodium (42.2%) is not a recombinogenic agent (i.e., causes DNA damage) to Bacillus subtilis strains H17 and M45 at concentrations up to 150 l/well. l In an in vitro study using cultured lymphocytes procured from a single male human donor, there was evidence of possible aberrant chromosomes. However, according to the Metam-sodium Task Force, the scientific validity of this study is under question since the aberrant chromosomes were observed only at concentration levels that were clearly cytotoxic to the cells. When the cells from noncytotoxic concentration levels were evaluated, there was no indication of any clastogenic activity. l In a mammalian cytogenetic study with Chinese hamsters, metam-sodium did not induce cytogenic effects l According to the Cal EPA review, there was evidence of polyploidy in Chinese hamster ovary cells at dose levels of 150 and 300 mg/kg. l According to the US EPA review, metam-sodium (42.2%) had a negative response (no effect) in the Chinese hamster bone marrow cytogenetic assay at concentrations of 150, 300, and 600 mg/kg. l Metam-sodium was found to be negative in an unscheduled DNA synthesis study with primary rat hepatocyte culture. l

Chapter | 107  Metam-Sodium

A study was conducted to assess the immunotoxicological and selected general toxicological effects of metam-sodium (Pruett et al., 1992). Metam-sodium was administered to female B6C3F1 mice at 200 mg/kg/day for 3, 5, 10, or 14 days. Selected organ weights were measured, hematological and bone parameters were examined, changes in thymus and spleen lymphocyte subpopulations were evaluated, and production of antibody-forming cells in vitro was measured. Major effects of metam-sodium administration included decreased thymus weight at all time points; increased spleen weight and bone marrow cellularity after 10 or 14 days of exposure; significant decreases in mature lymphocytes in the thymus and spleen; decrease in thymocytes; and decreased body weight. According to Pruett and co-workers (1992), overall patterns of change indicate that metam-sodium rapidly depletes most CD4  CD8  thymocytes, more slowly depletes a smaller number of mature lymphocytes in the thymus and spleen, and induces compensatory and/or detoxication mechanisms after 10–14 days of exposure. Pruett and co-workers (1992) conducted subsequent experiments to assess selected immune function parameters after exposure to metam-sodium. Metam-sodium was administered for seven days (either orally or dermally) and immunological assays were conducted on Day 8. Mice receiving metam-sodium orally at dose levels of 50 to 300 mg/kg showed substantial, dose-dependent suppression of NK cell activity. Evaluation of humoral responses indicated that the cellular and molecular components required for humoral immune responses are not major targets for the acute effects of metam-sodium. There was no suppression of antibody production in vivo or splenocyte responses to mitogens or allogeneic lymphocytes in vitro, which indicates that the lymphocytes which survive metam-sodium exposure are still able to proliferate and differentiate and are not significantly impaired with regard to function. The authors also noted that the pattern of thymic subpopulation changes is consistent with direct or indirect induction of apoptosis. These studies showed that immunological parameters could be significantly suppressed in the absence of a significant decrease in body weight, suggesting that most of the effects of metam-sodium on the immune system are not secondary to generalized toxicity. In response to reports that metam-sodium is immunotoxic, a series of in vivo and in vitro studies was conducted with metam-sodium and other dithiocarbamates (Padgett et al., 1992). Metam-sodium in distilled water was administered orally via daily gavage to female mice for seven days at dose levels of 0, 150, 225, or 300 mg/kg. Body weight was not significantly decreased at any dose level, but thymus weight was significantly decreased in mice receiving metam-sodium at dose levels of 225 and 300 mg/ kg. In tests of splenic NK cell activity, metam-sodium at dose levels of 225 and 300 mg/kg was found to significantly inhibit NK activity. This study also demonstrated

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that metam-sodium was directly cytotoxic to lymphoid cells in vitro, but that cytotoxic potency in vitro does not correlate well with immunological changes in vivo.

107.5  Developmental and reproductive toxicity Developmental studies in two different species found evidence of increased fetal loss, increased skeletal variations, and developmental delays from oral administration of metam-sodium to pregnant animals at dose levels that also caused overt maternal toxicity. Visceral or skeletal abnormalities were not present at low dose levels but increased in incidence and severity with increasing dose (U.S. EPA, 1991). In a multigeneration reproductive study, metamsodium did not affect reproductive performance, even at toxic dose levels (U.S. EPA, 1994a). A developmental study with rats receiving metamsodium (Hellwig and Hildebrand, 1987) indicated that there were significant maternal and fetal effects at higher dose levels and that these effects were dose-related. An aqueous solution of metam-sodium (42.2%) was administered at 0, 10, 40, or 120 mg/kg by gavage to pregnant Wistar rats on Days 6–15 of gestation. Body weight gains were significantly decreased in dams receiving metam-sodium at dose levels of 40 and 120 mg/kg during the dosing period. Cesarean section observations revealed that there was a statistically significant increase in the percentage of postimplantation loss and a significant decrease in the percentage of live fetuses per dam at the 10 and 120 mg/kg dose levels, but not at the 40 mg/kg dose level. It is possible that the effects observed at the 10 mg/kg dose level were statistical anomalies, but this remains unconfirmed in the absence of a review of the individual data, which were not available. All other parameters in the 10 mg/kg group were comparable to controls, including the total number of live fetuses and live fetuses per dam. Since there were no statistically significant changes in Cesarean section observations in the 40 mg/kg group, it is likely that the statistically significant changes in percentage of live fetuses per dam and the percentage of postimplantation loss in the 10 mg/kg group are not treatment-related. The only abnormal finding observed during the macroscopic examination of the fetuses was meningocele (hernial protrusion of the meniges through a bony defect) in two fetuses from one litter in the 120 mg/kg dose group. Since this is a rare finding that was not present in historical controls, this anomaly was considered to be treatment-related. Skeletal evaluations of the fetuses revealed an increased incidence of variations and a delay in the development of fetuses in the 40 and 120 mg/kg dose groups. Fetal weights were significantly reduced in the 120 mg/kg group. The NOEL for fetal and maternal effects was 10 mg/kg. In another rat developmental toxicity study (Tinston, 1993) groups of pregnant rats were administered

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metam-sodium at dose levels of 0, 5, 20, or 60 mg/kg/day on Days 7–16 (inclusive) of gestation. Maternal toxicity evidenced by reduced body weight gain, reduced food consumption, and the presence of clinical signs (piloerection, salivation, and urinary incontinence) occurred at the 20 and 60 mg/kg/day dose levels. Body weight gain and food consumption were marginally reduced at the 5 mg/kg/day dose level but there were no treatment-related clinical signs. In both the 20 and 60 mg/kg/day dose groups there was an increase in fetal effects (reduced fetal weights, reduced ossification of manus and pes, and increased incidences of minor skeletal defects and/or variants). The no observed adverse effect level (NOAEL) for maternal toxicity or fetal effects in this study was 5 mg/kg/day. In a teratology/developmental study (Hellwig, 1987), pregnant Himalayan rabbits were administered a 42.2% aqueous solution of metam-sodium at dose levels of 0, 10, 30, or 100 mg/kg by gavage from gestation Days 6 through 18. Evaluation of body weight data revealed a treatment-related decrease in body weight gain in the 100 mg/kg dams. There were no statistically significant treatment-related effects noted in food consumption or food efficiency. Cesarean section observations revealed statistically significant decreases in the total number of live fetuses and statistically significant increases in total re-sorptions in the 30 and 100 mg/kg/day groups. Macroscopic fetal examinations revealed meningocele and spina bifida in one rabbit in one litter in the 100 mg/kg/day group (it is not clear from the data if the findings were in the same rabbit or in two separate rabbits). Due to the rarity of this event and that it was also present in the rat developmental study, this abnormality is considered to be treatment-related. There were no treatment-related effects noted from the visceral examinations. Skeletal examinations revealed no treatment-related effects. However, these examinations were done using acceptable European methods that have not been validated by EPA and are not considered to be comparable with U.S. EPA-accepted methods. In another rabbit developmental toxicity study (Hodge, 1993) groups of pregnant rabbits were administered metam-sodium at dose levels of 0, 5, 20, or 60 mg/kg/day on Days 8–20 (inclusive) of gestation. At the 60 mg/kg/day dose level, dams showed marked weight loss and reduced food consumption. At 20 mg/kg/day, body weight of dams was slightly reduced. There were no observable effects noted on dams at the 5 mg/kg/day dose level. Fetal examinations revealed a marked increase in embryonic lethality at the 60 mg/kg/day maternal dose level and changes in ossification pattern at the 20 and 60 mg/kg/day maternal dose levels. The NOAEL for maternal and developmental toxicity was 5 mg/kg/day. In a multigeneration reproduction study (Milburn, 1993), Alpk:ApfSD rats received metam-sodium in drinking water at the following concentrations: 0,0.01,0.03, or 0.1 mg/ml. These concentrations corresponded to dose levels of 0, 1.2, 3.2, or 11.5 mg/kg/day for males and 0, 1.8, 3.9,

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or 13.5 mg/kg/day for females. After the first 10 weeks of treatment, animals were mated on a one-to-one ratio. Males were then removed from their cages and females were allowed to give birth and raise pups. At 21 days of age, pups from the parental (F0) generation were selected as parents for the Fl generation. In parents, body weights were marginally reduced in rats receiving 0.10 mg/ml (the highest concentration tested) during the premating period and markedly reduced during pregnancy and lactation. Water consumption was reduced in the 0.10 mg/ml rats throughout the study and to a lesser extent in the 0.03 mg/ml group. In offspring, there was a marginal reduction in food consumption during the premating period in the F0 and Fl rats in the 0.10 mg/ml group, but there were no effects on food consumption in the 0.01 and 0.03 mg/ml treatment groups. Offspring body weights and total litter weights were reduced in the 0.10 mg/ml group in both generations. There were no effects on any of the reproductive parameters at any treatment level for parents or offspring. Histopathological evaluations indicated increased changes in the epithelium of the nasal passages of the F0 and Fl adult females in the 0.10 mg/ml groups. This effect was not observed in 0.10 mg/ml adult males or in male or female offspring of either generation. No treatment-related histopathological changes were observed in rats receiving metam-sodium at concentrations of 0.01 or 0.03 mg/ml. Metam-sodium did not affect reproductive performance at any dose level tested. Evidence of toxicity was observed only at the highest concentration level tested, i.e., 0.1 mg/ml. In adult female rats receiving metam-sodium at the 0.1 mg/ml concentration level (13.5 mg/kg/day), evidence of systemic toxicity consisted of (1) duct hypertrophy of Bowman’s gland with loss of alveolar cells, (2) degeneration, disorganization, and/or atrophy of the olfactory epithelium, and (3) dilation of the Bowman’s gland ducts. Changes in Bowman’s glands were accompanied in all affected animals by degeneration, disorganization, and/or atrophy of the olfactory epithelium. In pups in the 0.1 mg/ml group, evidence of toxicity consisted of a 14% decrease in mean pup weight on Day 22 for the F1 generation, a 16% decrease in mean body weight gain for F2 litters, and decreases of 8–9% in testes and epididymis weight in male pups in the F1a and F2a litters. The NOEL for systemic toxicity (adults and pups) was 0.03 mg/ml. The NOEL for reproductive effects was 0.1 mg/ml (11.5 mg/kg/day for males, 13.5 mg/kg/day for females).

107.6  Chronic/oncogenicity toxicity A two-year combined chronic toxicity/carcinogenicity study demonstrated that metam-sodium shows no carcinogenic potential in rats (Thomassen, 1998; U.S. EPA, 1994b). However, a two-year carcinogenicity study in mice

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Table 107.1  Summary of Metam-Sodium No Observed Effect Levels (NOELs) and Lowest Observed Effect Levels (LOELs) Study

Species

Dosing duration

Dose levels

NOEL

LOEL effects

90-day drinking water

Mouse

90 days

0, 0.018, 0.088, 0.35, and 0.62 mg/ml

0.018 mg/ml

0.088 mg/ml

0, 2.7, 11.7, 52.4, and 78.7 mg/kg/day ( ) 0, 3.6, 15.2, 55.4, and 83.8 mg/kg/day ( )

2.7 mg/kg/day

0,0.018, 0.089, and 0.443 mg/ml

0.018 mg/ml

  urinary bladder lesions  decreases in hemoglobin, RBC, and hematocrit

●

90-day drinking water

Rat

90 days

●

3.6 mg/kg/day 0.089 mg/ml  decreased body weight and body weight gain

●

90-day oral

Dog

90 days

0, 1.7, 8.1, and 26.9 mg/kg/day ( ) 0, 2.5, 9.3, and 30.6 mg/kg/day ( )

1.7 mg/kg/day

0, 1, 5, and 10 mg/kg/day

1 mg/kg/day

2.5 mg/kg/day

  decreases in RBC and hematocrit

●

5 mg/kg/day  increased plasma ALT, AST, and ALP ●  hepatitis and bile duct proliferation ●

1 mg/kg/ day

1 mg/kg/day   increased plasma ALT   bile duct proliferation

● ●

Developmental toxicity

Rat

10 days

0, 10, 40, and 120 mg/kg/day

10 mg/kg/day

40 mg/kg/day ●  decreased maternal weight gain ●  increased fetal skeletal variations ●  delay in development

Developmental toxicity

Rat

10 days

0, 5, 20, and 60 mg/kg/day

5 mg/kg/day

20 mg/kg/day ●  decreased maternal weight gain ●  reduced food consumption ●  maternal clinical signs ●  increased fetal skeletal variations ●  reduced ossification of manus and pes ●  reduced fetal weights

Developmental toxicity

Rabbit

13 days

0, 10, 30, and 100 mg/kg/day

10 mg/kg/ day—fetal

30 mg/kg/day—fetal ●  decreases in live fetuses ●  increased resorptions 100 mg/kg/day—maternal ●  decreased body weight gain

30 mg/kg/ day—maternal Developmental toxicity

Rabbit

13 days

0, 5, 20, and 60 mg/kg/day

5 mg/kg/day

20 mg/kg/day ●  reduced maternal body weights ●  change in fetal ossification pattern

Multigeneration

Rat

Chronic

0,0.01, 0.03, and 0.1 mg/ml

0.03 mg/ml— systemic

0.1 mg/ml—systemic toxicity

0, 1.2, 3.2, and 11.5 mg/ kg/day ( )

 wchanges in Bowman’s gland and olfactory epithelium (adults)

●

(Continued)

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Table 107.1  (Continued) Study

Species

Dosing duration

Dose levels

NOEL

0, 1.8, 3.9, and 13.5 mg/ kg/day ( )

LOEL effects  decreased mean pup weight   0.1 mg/ml ●

0.1 mg/ml— reproductive Carcinogenicity: twoyear drinking

Rat

Chronic

0, 0.019, 0.056, and 0.19 mg/ml 0, 1.3, 3.9, and 12.0 mg/ kg/day ( ) 0, 2.3, 6.2, and 16.2 mg/ kg/day ( )

0.056 mg/ml

0.19 mg/ml ●  decreased body weight gain ●  decreased food consumption, food efficiency and water consumption ●  changes in hematology and clinical chemistry ●  abnormalities in nasal cavity, voluntary muscle and sciatic nerve

Carcinogenicity: twoyear drinking

Mouse

Chronic

0, 0.019, 0.074, and 0.23 mg/ml 0, 1.6, 6.5, and 27.7 mg/ kg/day ( ) 0, 2.3, 8.7, and 29.9 mg/ kg/day ( )

0.019 mg/ml

0.074 mg/ml ●  increased liver weight ●  changes in kidney and epididymis weights

1-year oral

Dog

Chronic

0, 0.05, 0.1, and 1.0 mg/kg/day

0.1 mg/kg/day

1.0 mg/kg/day ●  increase in hepatocyte and liver macrophage/Kupffer cells ●  increased plasma ALT

Acute neurotoxicity

Rat

Single dose

0, 22, 324, and 647 mg/kg

22 mg/kg

22 mg/kg ●  reduced ambulatory and total motor activity

Subchronic neurotoxicty

Rat

13 weeks

0, 0.02, 0.06, and 0.2 mg/ml

0.06 mg/ml ( )

0, 1.4, 5.0, and 12.8 mg/ kg/day ( ) 0, 2.3, 7.0, and 15.5 mg/ kg/day ( )

0.02 mg/ml ( )

0.2 mg/ml ( ) 0.06 mg/ml ( ) ●  decreased body weight gain

revealed an increased incidence of angiosarcoma in mice at higher dose levels (U.S. EPA, 1994c). A one-year study with dogs showed no evidence of carcinogenicity but evidence of liver damage similar to but less severe than the effects (that were shown to be reversible) observed in previous subchronic metam-sodium dog studies. In a two-year combined chronic toxicity/carcinogenicity study with Wistar rats (Rattray, 1994), metamsodium (43.14% a.i.) was administered in drinking water at concentration levels of 0, 0.019, 0.056, or 0.19 mg/ml (achieved dosages of 0, 1.3, 3.9, or 12.0 mg/kg/day for males and 0, 2.3, 6.2, or 16.2 mg/kg/day for females). There was no evidence of an adverse effect of metamsodium on the survival or rats. There were no ophthalmological changes associated with metam-sodium treatment. Evidence of toxicity was present in both males and females at the highest concentration level tested, i.e.,

0.19 mg/ml. At 0.19 mg/ml, male and female rats had decreased mean body weight gain for Weeks 1–13 (12% for males, 16% for females) and for Weeks 1–105 (18% for males, 20% for females). Food consumption, food efficiency, and water consumption were significantly decreased for both males and females receiving 0.19 mg/ml metamsodium. Effects were also observed in 0.19 mg/ml male and female hematology (decreased red blood cells, hemoglobin, and hematocrit) and clinical chemistry (decreased cholesterol and triglycerides). Nasal passages were identified as the target organ. Microscopic abnormalities of the nasal cavity were mainly confined to 0.19 mg/ml animals. These changes included (1) an increased incidence of rhinitis, (2) hypertrophy of Bowman’s ducts/glands, (3) atrophy and adenitis of Steno’s gland, and (4) hyperplasia and degeneration of olfactory epithelium. The incidence of degenerative myopathy of voluntary muscle was similar in all

Chapter | 107  Metam-Sodium

groups, including controls. However, there was an increase in the severity of myopathy in animals in the 0.019 mg/ml group. There was no indication of an increased incidence of neoplasia or early onset of tumors from treatment with metam-sodium. Evaluation of the tumor incidence demonstrated that metam-sodium shows no carcinogenic potential in rats (Rattray, 1994; U.S. EPA, 1994b). The NOEL for both male and female Wistar rats was 0.056 mg/ml. The Cal EPA, Department of Pesticide Regulation evaluated the tumor data from the two-year metam-sodium drinking-water study in rats and concluded that there was a possible tumorigenic effect at the 0.056 mg/ml concentration level (U.S. EPA, 1995). According to the Cal EPA review, the incidence of hemangiosarcoma (8/64) was increased at this dose, in relation to the control incidence (0/64) and the high dose (0.19 mg/ml) incidence (3/64). The hypothesis that this could be a positive response was based on the positive findings in the two-year mouse study and that this increased incidence could be based on decreased body weight observed at the high dose in relation to other doses. When the U.S. EPA (1995) re-evaluated the tumor data for their Carcinogenicity Classification, they did not find the effect that the Cal EPA found (presumably because the Cal EPA analysis did not exclude animals that died before observation of the first tumor). However, there was a significant pairwise comparison in the incidence of hemangiosarcoma in male rats at the 0.019 and 0.056 mg/ml (1.3 and 3.9 mg/kg/day) levels when compared to controls. The U.S. EPA also considered debatable the hypothesis of increased incidence of hemangiosarcoma at the mid-dose level based on decreased body weight in male rats at the high dose level. Rats in this study were not fed a calorie-restricted diet, nor was their access to food controlled. In addition, the decreases in body weight gain were observed for both male and female rats, although the preponderance of hemangiomas/hemangiosarcomas was observed only in male rats. In addition, the time to tumor formation was observed at approximately the same time in all dose levels. In calorierestricted studies, the numbers of tumors are often reduced in conjunction with a delay in the time to tumor formation. In response to the position taken by Cal EPA, the two-year drinking water study with Wistar rats was reviewed and compared to an expanded historical control data base for Wistar rats that at the time of their original review was not available (Thomassen, 1998). Hemangiomatous tumors (heman-gioma and/or hemangiosarcoma) were observed only in rats sacrificed at termination of the study (Study Week 105) or in rats that were found dead or were euthanized due to their clinical condition (moribund or to prevent suffering). No hemangiomatous tumors were observed in rats euthanized during the interim sacrifice at Study Week 53. Therefore, tumor analysis (as was done by the U.S. EPA) should exclude animals that were sacrificed or died prior to the first observance of a heman-gioma or hemangiosarcoma. Statistically significant increased incidence of hemangiosarcomas occurred only in

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the 0.019 and 0.056 mg/ml males and not in the 0.19 mg/ml males, although the actual numbers were very similar (3/49 at 0.019 mg/ml and 3/51 at 0.19 mg/ml). Three possible explanations for reduced tumor incidence with an increase in treatment are: (1) the high dose of metam-sodium exceeded the maximum tolerated dose and had a negative impact on the tumor response in the high dose males; (2) reduced body weight associated with reduced tumor incidence accounted for the difference (as suggested by the Cal EPA reviewer); or (3) biological variability was responsible. Neither the U.S. EPA nor the Cal EPA thought the maximum tolerated dose had been exceeded. The U.S. EPA carcinogenicity peer review panel did not believe that reduced body weight accounted for the reduced tumor incidence (U.S. EPA, 1995). However, the possibility that biological variability could account for the effect was hampered by the lack of historical control data for Wistar rats. Hemangiomatous tumors (variously diagnosed as angiomas and angiosarcomas, hemangiomas and hemangiosarcomas, and lymphangiomas and lymphangiosarcomas) are common in some but not all strains of Wistar rats (Bomhard, 1992; Bomhard et al., 1986; Crain, 1958; Deerberg et al., 1980; Kroes et al., 1981; Rehm et al., 1984). The reported incidences of these tumors in Wistarderived rats used in European laboratories vary considerably, but there are reports of up to a 74% incidence for males and 44% for females (Rehm et al., 1984). These reports also indicate that there is a definite propensity for development of tumors in the lymph nodes, particularly the mesenteric lymph nodes of male Wistar rats. Although Zeneca Central Toxicology Laboratory did not have an historical control data base for Wistar rats used in this study, there was a large historical control tumor data base compiled by several European laboratories utilizing Wistarderived rats (49 studies ranging in duration from 24 to 31 months). This data base was published as the RITA Wistar Rat Control Tumor Data Base (Thomassen, 1998). Information presented in the RITA control data base is consistent with the types and numbers of tumors observed in the metam-sodium two-year rat study. Based on a thorough review of the original study and comparisons with the RITA control data base for Wistar rats, it was concluded that: Metam-sodium is not a carcinogen in the rat. The reduced number of hemangiosarcomas in the high dose male rats in the metam-sodium study is not due to reduced caloric intake. l The natural distribution and incidence of spontaneously occurring hemangiosarcomas in untreated male Wistar rats can account for the distribution and incidence of hemangiosarcomas observed in male rats treated with metam-sodium. l l

In a two-year carcinogenicity study in mice (Horner, 1994), metam-sodium (43.15%) was administered in the drinking water to C57BL/10JfCD-l/Alpk mice for 104 weeks at

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nominal concentration levels of 0, 0.019, 0.074, or 0.23 mg/ml (actual achieved doses of 0, 1.6, 6.5, or 27.7 mg/kg/day for males and 0, 2.3, 8.7, or 29.9 mg/kg/day for females). Metamsodium did not adversely affect survival of mice at any dose level. Clinical signs of toxicity were considered to be unremarkable. Male and female mice receiving metam-sodium at 0.074 and 0.23 mg/ml had dose-related and statistically significant increases in absolute liver weight when compared to controls (111% and 119% for 0.074 mg/ml males and females, respectively; 135% and 122% for 0.23 mg/ml mice). At 0.23 mg/ml, male mice had decreased body weight gain of 14% for Weeks 1–13 and 20% for Weeks 1–104. Food consumption was unaffected during the early part of the study, but during Weeks 24 to 52 there were statistically significant decreases in food consumption for the 0.074 and 0.23 mg/ml male mice. Decreases in food consumption were not observed for female mice. Water consumption was significantly decreased for both males and females in the 0.23 mg/ml group during the study’s first week, but by Week 9, males in the 0.23 mg/ml group had significantly increased water consumption. By Week 11, water consumption was significantly increased for both 0.074 and 0.23 mg/ml males. By Week 48, water consumption for all groups (male and female) were approximately equal to controls. Hematological investigations showed no significant treatment-related effects at any dose level. Macroscopic observations revealed several changes in 0.23 mg/ml mice including liver appearance (accentuated lobular pattern, pale), subcutaneous tissue masses, urinary bladder wall thickening, and reduced incidence of enlarged seminal vesicles. Several changes were noted in liver, kidney, and epididymis weights at 0.074 and 0.23 mg/ml treatment levels. Microscopic evaluations revealed several non-neoplastic effects in 0.23 mg/ml mice but also revealed evidence of neoplastic changes at this same dose level. There was evidence of dose-dependent metam-sodium induced carcinogenicity in mice. In both males and females at the 0.23 mg/ml treatment level, there was an increased incidence of hepatic adenoma and angiosarcoma, splenic angiosarcoma, subcutaneous tissue angiosarcoma, and a single incidence of a urinary bladder transitional cell papilloma in one high dose male and a single incidence of urinary bladder transitional cell carcinoma in one high dose female. The overall incidence of angiosarcoma, regardless of site, increased for both males and females in the 0.23 mg/ml treatment group when compared to concurrent as well as historical controls. The no observed effect level for neoplastic changes is 0.074 mg/ml. According to the U.S. EPA (1994c), there was equivocal evidence of a possible increase in splenic angiosarcoma at 0.074 mg/ml [something that the study author and registrants believe is related to the difficulty in determining the primary site(s) of angiosarcoma]. There was no evidence of increased tumors at the lowest dose level. In the U.S. EPA’s (1994c) evaluation of the two-year mouse study, the reviewers suggested that the dosing levels in this study

Hayes’ Handbook of Pesticide Toxicology

were adequate due to the degree of toxicity (increased liver weights, non-neoplastic changes in bladder, and tumors) observed in both males and females at the 0.23 mg/ml treatment level. According to the U.S. EPA (1994c) based on the significant increase observed in liver weight in male and female mice, the LOEL is considered to be 0.074 mg/ml, which is the NOAEL for neoplastic changes. In a one-year toxicity study (Brammer, 1994), metamsodium was administered orally to beagle dogs at dose levels of 0, 0.05, 0.1, or 1.0 mg/kg/day. Animals were observed daily for food consumption, evidence of gastro-intestinal upset, and changes in clinical condition. Animals also received detailed clinical evaluations weekly and complete veterinary examinations (including ophthalmoscopy) every three months. Blood chemistry, hematology, urine chemistry, and cytology evaluations were conducted at regular intervals throughout the study. At study termination, each animal received a full necropsy and histopathological evaluation of selected tissues. Throughout the study, there were no overt signs of toxicity at any dose level and all dogs remained in good health. There were no toxicologically significant effects on body weight, food consumption, clinical condition, or on the incidence of gastro-intestinal effects (i.e., vomiting, loose stools, etc.). There were no ophthal-moscopic abnormalities nor were there significant changes in hematology or urinalysis or in organ weights. There were no macroscopic findings that could be attributed to treatment with metamsodium. Microscopic evaluations revealed a slight increase in hepatocyte and macrophage/Kupffer cells in the liver of one female dog dosed at 1.0 mg/kg/day. This same female also had significant elevations in plasma alanine transaminase activity. These changes were similar to but less severe than those observed in previous subchronic dog studies with metam-sodium and are considered to be treatment-related. Therefore, the NOEL for this study was 0.1 mg/kg/day.

107.7  Nurotoxicity Metam-sodium is not neurotoxic based on evidence from neurotoxicity studies. In an acute neurotoxicity study (Lamb, 1993), male and female Sprague–Dawley Crl: CD®BR rats received metamsodium (43.15%) orally at doses of 0, 50, 750, or 1500 mg formulated metam-sodium/kg body weight or 0, 22, 324, or 647 mg a.i./kg. Mortality was observed at the l500 mg/kg dose level (males 31%, females 19%). Signs of systemic toxicity were observed at the 750 and 1500 mg/kg dose levels and included changes in posture, palpebral closure, respiratory rate, arousal, rearing activity, time to first step, olfactory and pupil responses, tail pinch response, hindlimb strength, body temperature, and body weight. Lacrimation and salivation were also noted among some animals at both the 750 and 1500 mg/kg dose levels. Reductions in ambulatory and motor activity were observed at the 50 mg/kg dose level and above

Chapter | 107  Metam-Sodium

on Day 0 (day of dosing) yet there were no treatment-related effects on the functional observational battery in the 50 mg/kg dose group. No signs indicative of neurotoxicity were observed at any dose level. There was no significant change in brain cholinesterase (ChE) activity at any dose level and there were no signs of cholinergic effects at any dose level. There were no treatment related differences in brain weight or dimensions in any treatment group. Histopathological evaluations of brain and nervous system tissues showed no evidence of neurotoxicity. According to the U.S. EPA (1994d), plasma and RBC ChE activity levels were reduced in 1500 mg/kg male and female rats 24 hours postdose (6% and 12% for male plasma and RBC ChE, respectively; 24% and 14% for females). [Although statistically significant, none of these decreases in ChE activity are considered to be biologically relevant as all decreases are well within the range of normal variation and are below the thresholds set by the World Health Organization (JMPR, 1995; WHO, 1990) and other regulatory agencies (Carlock et al., 1999).] Based on the results of this study, the 1500 mg/kg dose level was considered the NOAEL in males and females for acute neurotoxicity while the 50 mg/kg/day dose level was considered the LOEL for acute systemic toxicity (based on reduced motor activity). In a subchronic neurotoxicity study (Allen, 1991), male and female Sprague-Dawley rats were given metam-sodium (43.15%) in drinking water at concentration levels of 0, 0.02, 0.06, or 0.2 mg/ml for 13 weeks (achieved dosages of 0, 1.4, 5.0, or 12.8 mg/kg/day for males and 0, 2.3, 7.0, and 15.5 mg/ kg/day for females). Male and female rats administered 0.2 mg metam-sodium/ml drinking water showed reductions in body weight, food consumption, and water consumption. Similar effects were observed in females at the 0.06 mg/ml concentration level. Body weight gain was reduced 14% for the 0.2 mg/ ml males and the 0.06 mg/ml females, and 18–21% for the 0.2 mg/ml females. Food utilization was slightly reduced in males at the 0.2 mg/ml level. Reduced water consumption was also observed in males at the 0.06 mg/ml level and in females at the 0.02 mg/ml level. All of these effects were considered to be a consequence of poor potability of the drinking water rather than toxicity of metam-sodium. A functional observational battery and comprehensive neuropathological examination of the peripheral and central nervous systems revealed no evidence of any effects attributable to treatment with metam-sodium. Since there was no evidence of a neurotoxic effect from metamsodium, the NOAEL for neurotoxicity is 0.2 mg/ml.

107.8  Other studies (Mammalian) In vitro percutaneous absorption of metam-sodium through rat and human skin was evaluated (Clowes, 1993). Metamsodium was applied at dose levels of 940 and 94.0 g/cm2.

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Ten hours after dermal application, the skins were washed to determine how much of the dose could be removed from the skin surface, receptor fluid was analyzed, and the proportion of the dose remaining associated with the skin after washing and the amount absorbed were quantified. The absorption of metam-sodium was found to be dose and time dependent through both rat and human skin. The highest amount of metam-sodium absorbed was through rat skin from the 940 g/cm2 application (mean 200 g/cm2; 21.3% of the applied dose at 10 hours). A correspondingly smaller amount was absorbed from the 94.0 g/cm2 application through rat skin (mean 18.2 g/cm2; 19.4% at 10 hours). Absorption through cadaver human skin was 2.19% for the 940 g/cm2 dose (mean 20.6 g/cm2 ) and 12.2% (mean 11.5 g/cm2) of the applied dose. Absorption of metamsodium through both rat and human skin increased with time but at a decreasing rate over the 10 hour period. The percentage of the dose remaining in the skin increased with decreasing dose. There was less metam-sodium absorbed through human skin than rat skin, and at the highest dose level there was approximately a 10-fold decrease in absorption of metam-sodium by human skin when compared to rat skin. An in vivo percutaneous (dermal) absorption study in the rat (Stewart, 1992) showed that metam-sodium and/ or its radiolabeled degradation products are only poorly absorbed following a single dermal application to the rat. Radiolabeled 14C metam-sodium was applied to male rats in aqueous solution at nominal dose levels of 0.1, 1, and 10 mg/animals. A glass saddle containing an activated charcoal filter to adsorb any volatile radioactivity evaporating from the skin surface protected the application site. Four animals from each group were evaluated at 1, 2, 10, and 24 hours after treatment for radioactivity in the excrement, in and on the skin, and in the body. Another four animals per group had the treatment area washed 10 hours after administration. Radioactivity in the excrement was monitored over a total of 72 hours prior to evaluations of the skin and body. Overall mean recoveries of radioactivity were in the range of 83.5 to 95.7% of the applied dose. The extent of absorption was similar at each dose level with an overall mean of approximately 3%. In general, absorption increased with time. Levels of absorbed material 24 hours postapplication for the 0.1, 1, and 10 mg/animal dose levels were approximately 7.5, 50, and 231 g equivalents of 14C metam-sodium, respectively. Substantial quantities of the nonabsorbed dose were recovered from the charcoal, suggesting that metam-sodium or its degradation products are highly volatile. At the 0.1, 1, and 10 mg/animal dose levels, the amounts of metam-sodium absorbed over a 10 hour exposure period were 2.4, 3.7, and 1.5% of the applied dose, respectively. Absorbed radioactivity was either eliminated in urine or exhaled and subsequently trapped in expired air traps. Less than 0.7% of the applied dose was recovered in feces and the recovery of radioactivity from

Hayes’ Handbook of Pesticide Toxicology

2304

the carcass ranged from below the limit of detection to 1.2%. Following applications of metam-sodium at 1 and 10 mg/animal, concentrations of radioactivity in blood and plasma peaked at one hour postdose. Levels of radioactivity in blood and plasma in the 0.1 mg/animal group were below the limit of detection. This study showed that (1) metamsodium is poorly absorbed following a single dermal application; (2) absorbed radioactivity is rapidly excreted, primarily via the urine and expired air; and (3) washing the application site with soap and water effectively removes the majority of the applied dose. A further dermal absorption study with metam-sodium showed that absorption for rats was only 2.5% of the applied dose (U.S. EPA, 1994a). Radiolabeled 14C metam-sodium was applied to shaved dorso-lumbar skin sites of rats at concentrations of 8.6, 86.2, or 862 (g/cm2. Animals were exposed to metam-sodium for 1, 2, 10, 24, or 72 hours. At the end of the study, total dermal absorption after 72 hours was determined to be 2.5% of total applied dose. Since metam-sodium is poorly absorbed dermally, human skin surfaces are acidic, and sweat is approximately pH5, metam-sodium that may come in contact with skin is expected to rapidly degrade prior to absorption.

107.9  Metabolism After oral ingestion, metam-sodium is rapidly absorbed, metabolized, and excreted from the body. Exhalation and excretion in the urine are the major elimination pathways after oral exposure. Metam-sodium is poorly absorbed following dermal application but the metam-sodium that is absorbed is rapidly excreted, primarily through the urine and expired air. In study of biokinetics and metabolism, radiolabeled metam-sodium [14C] (purity  99%) was administered to Sprague–Dawley rats at dose levels of 10 or 100 mg/kg (Hawkins et al., 1987). Blood, urine, and feces were tested for radioactivity for up to seven days postdosing while expired air was collected up to 72 hours postdose. The results of this study showed that metam-sodium was rapidly and completely absorbed after oral ingestion. Radioactivity in plasma reached a maximum level in 1 hour and decreased to near background levels by 24 hours. Animals receiving [14C] metam-sodium at 10 mg/kg eliminated approximately 25% of the total radioactivity through the urine during the first 8 hours. By 168 hours, 55% of the total activity had been eliminated through the urine and 3–4% through feces. At the 100 mg/kg dose, 18% of the total activity had been eliminated in the urine by 8 hours and 40% by 168 hours. Within 24 hours, expired air from 10 mg/kg rats contained approximately 32% of the radioactivity with 1% MITC, 15% carbon disulfide (CS2)/ carbonyl sulfide (COS), and 17% carbon dioxide. At 24 hours for the 100 mg/kg dose level, expired air contained

approximately 48% of the total radioactivity with 24% MITC, 18% CS2/COS, and 6% C02. Negligible amounts of radiolabeled material were expired from 24 to 72 hours at either dose level. Approximately 98% of the radioactivity had been eliminated by the seventh day, with only 2% of the activity remaining in the tissues. The highest concentration of radioactivity was found in the thyroid, but significant concentrations were also found in the liver, kidneys, and lungs. Analysis of the urinary metabolites found that glutathione conjugation with MITC is the source of the major urinary metabolite, N-acetyl-S-(N-methylthiocarbamoyl)-lcysteine, which accounted for 21% of the excreted dose. No evidence for glucuronide or sulfate conjugates of the metabolites of metam-sodium was found. Based on the results of this study, it appears that metam-sodium degrades to either CS2 or MITC in the stomach (accelerated by the stomach pH). MITC is eliminated either through exhalation or in the urine after glutathione conjugation in the liver. CS2 is eliminated by exhalation or further metabolized in the liver to CO2 prior to elimination. Therefore, two different metabolic pathways, CS2 metabolism and MITC conjugation, are involved in urinary elimination. At higher dose levels, saturation of the metabolic processes results in greater exhalation of unmetabolized products. The in vivo dermal absorption study conducted by Stewart (1992), which was described previously, further demonstrated that (1) metam-sodium is poorly absorbed following a single dermal application; (2) absorbed radioactivity is rapidly excreted, primarily via the urine and expired air; and (3) washing the application site with soap and water effectively removes the majority of the applied dose. In both this study and the Hawkins et al. (1987) metabolism study, peak blood and plasma radioactivity levels occurred one hour after dosing.

References Allen (1991). Bomhard, E. (1992). Frequency of spontaneous tumors in Wistar rats in 3-month studies. Exp. Toxic. Pathol. 44, 381–392. Bomhard, E., Karbe, E., and Loesser, E. (1986). Spontaneous tumors of 2000 Wistar TNO/W.70 rats in two-year carcinogenicity studies. J. Environ. Path. Toxicol. Oncol. 7, 35–52. Brammer, A. (1992). “Metam-Sodium: 90-Day Oral Dosing Study in Dogs.” Unpublished study (Rep. CTL/P/3679) conducted by Zeneca Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. Submitted by Metam-sodium Task Force. Brammer, A. (1993). “Metam-Sodium: Assessment of Recovery in Dogs.” Unpublished study (Rep. CTL/L/5204) conducted by Zeneca Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. Submitted by Metam-sodium Task Force. Brammer, A. (1994). “Metam-Sodium: 1-Year Oral Toxicity Study in Dogs.” Unpublished study (Rep. CTL/P/4196) conducted by Zeneca Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. Submitted by Metam-sodium Task Force.

Chapter | 107  Metam-Sodium

Carlock, L. L., Chen, W. L., Gordon, E. B., Killeen, J. C., Manley, A., Meyer, L. S., Mullin, L. S., Pendino, K. J., Percy, A., Sargent, D. E., Seaman, L. R., Svanborg, N. K., Stanton, R. H., Tellone, C. I., and Van Goethem, D. L. (1999). Regulating and assessing risks of cholinesterase-inhibiting pesticides: Divergent approaches and interpretations. J. Toxicol. Environ. Health. B 2, 105–160. Clowes, H. M. (1993). “Metam Sodium: In Vitro Absorption through Rat and Human Skin.” Unpublished study (Rep. CTL/P/4118) conducted by Zeneca Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. Submitted by Metam-sodium Task Force. Crain, R. C. (1958). Spontaneous tumors in the Rochester strain of the Wistar rat. Amer. J. Pathol. 34, 311–335. Deerberg, F., Rapp, K., Rehm, S., and Pitterman, W. (1980). Genetic and environmental influences on lifespan and diseases in Han: Wistar rats. Mech. Ageing Devel. 14, 333–343. Hawkins, D. B., Elsom, L. F., and Girkin, G. (1987). “The Biokinetics and Metabolism of 14C-Metam in the Rat.” Unpublished study conducted by Huntingdon Research Centre, UK. Submitted by BASF Corporation, Research Triangle Park, NC. Hellwig, J. (1987). “Report on the Study of the Prenatal Toxicity of Metam-Sodium (Aqueous Solution) in Rabbits after Oral Administration (Gavage).” Unpublished study (Project 38R0232/8579) conducted by BASF Aktiengesellschaft, Federal Republic of Germany. Submitted by BASF Corporation Chemicals Division, Parsippany, NJ. Hellwig, J., and Hildebrand, B. (1987). “Report on the Study of the Prenatal Toxicity of Metam-Sodium in Rats after Oral Administration (Gavage).” Unpublished study (Rep. 87/0128) conducted by BASF Aktiengesellschaft, West Germany. Submitted by BASF Corporation, Research Triangle Park, NC. Hodge, M. C. E. (1993). “Metam Sodium: Developmental Toxicity Study in the Rabbit.” Unpublished study (Rep. CTL/P/4035) conducted by Zeneca Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. Submitted by Metam-sodium Task Force. Homer, S. A. (1994). “Metam-Sodium: Two Year Drinking Study in Mice.” Unpublished study (Rep. CTL/P/4095) conducted by Zeneca Central Toxicology Laboratory, Cheshire, UK. Submitted by Metamsodium Task Force. JMPR (Joint Meeting on Pesticide Registrations, World Health Organization) (1995). “Pesticide Residues in Food—1995.” FAO Plant Production and Protection Paper 133, p. 4. Jowa, L. (1998). Metam: Animal toxicology and human risk assessment. In “Toxicology and Risk Assessment: Principles, Methods, and Applications” (A. M. Fan and L. W. Chang, eds.), p. 619. Dekker, New York. Klaassen, C. D. (1986). Chapter 2: Principles of toxicology. In “Cassarett and Doull’s Toxicology: The Basic Science of Poisons” (C. D. Klaassen, M. O. Amdur, and J. Doull, eds.), pp. 11–32. McGraw– Hill, New York. Kroes, R., Garbis-Berkvens, J. M., de Vries, T., and van Nesselrooy, H. J. (1981). Histopathological profile of a Wistar rat stock including a survey of the literature. J. Gerontol. 36, 259–279. Lamb, I. C. (1993). “An Acute Neurotoxicity Study of Metam-Sodium in Rats (Definitive).” Unpublished study (Study WIL-188009) conducted by WIL Research Laboratories, Inc., Ashland, OH. Submitted by Metam-sodium Task Force, Los Angeles, CA. Liggett, M. P., and McRae, L. A. (1991). “Skin Irritation to Rabbits with Metam-Sodium.” Unpublished study (Study 90997D/UCB 368/SE) conducted by Huntingdon Research Centre, Ltd., UK. Submitted by UCB Chemicals Corporation, Norfolk, VA.

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Alternative to Methyl Bromide for Fruit and Vegetable Production.” US EPA document posted at http://earthl.epa.gov/ozone/mbr/ metams.htm. Whiles, A. J. (1991). “Metam-Sodium: 90-Day Drinking Water Study in Mice with a 28-Day Interim Kill.” Unpublished Report (Rep. CTL/ P/3185) conducted at ICI Central Toxicology Laboratory, Alderley Park, Macclesfield, Cheshire, UK. World Health Organization (WHO) (1990). “Environmental Health Criteria 104. Principles for the Toxicological Assessment of Pesticide Residues in Food.” International Program on Chemical Safety, World Health Organization, Geneva.