# Sulfonylureas: Screening Analysis of Toxicological Profiles to Consider Whether a Candidate Common Mechanism Group Can Be Established.

Office of Pesticide Programs Office of Chemical Safety and Pollution Prevention U.S. Environmental Protection Agency Washington, DC

September 9, 2015Page 3 of 13 analysis.html). This document provides guidanc e on how to screen groups of pesticides for cumulative evaluation using a two-step approach beginning with the evaluation of available toxicological information and if necessary, follo wed by a risk-based screening approach. This framework supplements the existing guidance documents for establishing common mechanism groups (CMGs) 1 and conducting cumulative risk assessments (CRA) 2 . The Agency has utilized this framework for the sulfonylureas and determined that although the sulfonylureas share some chemical and toxicological characteristics, the to xicological database does not support a testable hypothesis for a common mechanism of action. No further mechanistic data are required and no further cumulative evaluation is necessary for the sulfonylureas.

Chemical PC Code CAS Number Bensulfuron-methyl 128820 83055-99-6 Chlorimuron-ethyl 128901 90982-32-4 Chlorsulfuron 118601 64902-72-3 Ethametsulfuron 129091 97780-06-8 Flazasulfuron 119011 104040-78-0 Foramsulfuron 122020 173159-57-4 Halosulfuron 128721 135397-30-7 Imazosulfuron 118602 122548-33-8Iodosulfuron-methylsodium 122021 144550-36-7

Mesosulfuron-methyl 122009 208465-21-8 Metsulfuron 122010 868680-84-6 Nicosulfuron 129008 111991-09-4 Orthosulfamuron 108209 213464-77-8 Primisulfuron-methyl 128973 86209-51-0 Prosulfuron 129031 94125-34-5 Rimsulfuron 129009 122931-48-0 Sulfometuron-methyl 122001 74222-97-2 Sulfosulfuron 085601 141776-32-1 Thifensulfuron-methyl 128845 79277-27-3 Triasulfuron 128969 82097-50-5 Tribenuron-methyl 128887 101200-48-0 Trifloxysulfuron-sodium 119009 199119-58-9 Triflusulfuron-methyl 129002 126535-15-7

1 Guidance For Identifying Pesticide Chemicals and Other S ubstances that have a Common Mechanism of Toxicity (USEPA, 1999) 2 Guidance on Cumulative Risk Assessment of Pesticide Chemicals That Have a Common Mechanism of Toxicity (USEPA, 2002)Page 4 of 13 Sulfonylureas: Screening Analysis of Toxicological Profiles to Consider Whether a Candidate Common Mechanism Group Can Be Established.

1.0 Introduction

Sulfonylureas (SU) are a group of herbicides that control weeds through inhibition of the enzyme acetolactate synthase (ALS), which catalyzes the fi rst step in the synthesis of branched-chain amino acids. They are used as pre- and post-em ergent herbicides on a wide variety of crops, pasture land, rangeland, industrial sites, and turf grass. Currentl y, the SUs are undergoing registration review.

EPA’s Office of Pesticide Programs has developed a draft guidance document entitled Pesticide Cumulative Risk Assessment: Fr amework for Screening Analysis (http://www.epa.gov/oppfead1/ cb/csb_page/updates/2015/framework-for-screeninganalysis.html). This document provides guidanc e on how to screen groups of pesticides for cumulative evaluation using a two-step approach beginning with the evaluation of available toxicological information and, if necessary, follo wed by a risk-based screening approach. This framework supplements the existing guidance documents for establishing common mechanism groups (CMGs) 3 and conducting cumulative risk assessments (CRA) 4 . Consistent with the framework, the toxicological knowledgebase for th e SUs was evaluated in terms of information on mode of action (MOA)/adverse outcome pathway (AOP), chemical structural similarity, and toxicological profile to determin e if the available data supports establishing a candidate CMG for this class of pesticides.

2.0 MOA/AOP

The herbicidal MOA for SUs is inhibition of ALS, an enzyme that catalyzes the biosynthesis of three branched-chain amino acids (valine, leucine, and isoleucine), all of which are essential for plant growth. In contrast, no mammalian MOA data was available for the SUs. While humans do have an ALS enzyme, its function is unknown (J outel et al., 1996), and therefore the human relevance of ALS inhibition is uncertain. Of note, valine, leucine, and isoleucine are essential amino acids, meaning they cannot be synthesized by mammals and must be ingested, indicating that the role of ALS in humans, if any, is likely different than in plants. Given that there is no mammalian MOA data for the SUs and that the function of ALS in humans is unknown, ALS does not provide a foundation for estab lishing a candidate common mechanism group

. 2.1 Chemical Structural Similarity

All of the registered SUs contai n a central sulfonylurea structure that terminates in different structures depending on the SU as depicted in Figure 2.1 and Appendix A. The SUs can be further subcategorized based on the substitu ent on the urea side of the molecule (R 2 ). For the registered SUs this is either a subs tituted pyrimidine or triazine ring.

3 Guidance For Identifying Pesticide Chemicals and Other S ubstances that have a Common Mechanism of Toxicity (USEPA, 1999) 4 Guidance on Cumulative Risk Assessment of Pesticide Chemicals That Have a Common Mechanism of Toxicity (USEPA, 2002)Page 5 of 13 In contrast to the relative similarity of th e substituents on the urea side, the substituents terminating the sulfonyl end of the molecule (R ′ ) show a higher degree of structural variability. Due to this variability, the registered SUs could not be meaningfully subcategorized based on the substituents on the sulfonyl end of the molecule.

In summary, although the degree of similarity va ries depending on the substituents on either end of the chemical, the SUs can be considered to have a shared core chemical structure.

Figure 2.1. Sulfonylurea chemical structure.

2.2 Toxicological Profile

The Pesticide Cumulative Risk Assessment: Framework for Screening Analysis document states that reviewers should conduct an in itial review of the experimental toxicology data from the data submitted for pesticide registration for a group of pesticides to evaluate the extent to which common patterns of effects are observed among the members of the potential class. The document also emphasizes that effects at lower do ses, particularly those used in deriving points of departure (PoDs) for single pesticide risk a ssessments, get more weight in the screening analysis. With these points in mind, the SUs were first screened to determine if there were common durations of exposure in which the eff ects observed for deriving the PoDs could be compared for common effects. Based on an initia l screen, it was determined that only a small number of the SUs had an endpoint chosen for acut e dietary risk assessment (8/23) and therefore, the studies chosen for acute di etary exposure would not be useful for comparing across the SUs as a class. In contrast, all of the SUs had ch ronic dietary endpoints, except for foramsulfuron where no toxicity was observed in any study and therefore only qualitativ e exposure assessments were performed. Since all of the quantitative assessments fo r the SUs had a chronic dietary endpoint and the chronic dietary endpoint would generally represen t the most sensitive effects for each SU, the effects used for deriving the chronic dietary endpoints were focused on to determine if there were common patterns of toxicological effects across the SUs.

For each SU, the chronic study used for the chroni c dietary endpoint was evaluated to determine the toxicological effects at the LOAEL and the target organs or general effects (decreased body weight/body weight gain) were noted. The full toxi city profile of each SU was also evaluated to make sure the target organs identified and effects observed were consistent with the effects seen throughout the database of that ch emical. Once the chronic endpoint for each SU was evaluated, the target organs were compared across the SUs as a class to see if th ere was a clear pattern indicating a common toxicological effect. Furthermore, since stru cturally the SUs could be subdivided based on the substituent on the urea portion of the molecule, as described in section 2.1, the target organs of each subclass were compared to see if there was a clear association between the identified target organs and the substituent on the urea side of the molecule.Page 6 of 13 The chronic dietary endpoints and the target organs identified ar e summarized in Appendix B. The target organs/effects obs erved were as follows:

 The hematopoietic system

o Effects on the hematopoietic system were observed in 8/23 SUs (5/14, pyrimidine; 3/9, triazinine).

o Effects were consistent across the SUs (s igns of anemia and microscopic changes in the spleen and bone marrow).

 The liver

o Liver effects were observed in 7/23 SU s (5/14, pyrimidine; 2/9, triazinine).

o The liver effects observed were non-specific (increased liver weights and elevated enzymes) and were not informative for dete rmining if they were the result of a common mechanism of toxicity.

 General body weight effects

o Effects on body weight were observed in 6/23 SUs (5/14, pyrimidine; 7/9, triazinine).

o Decreases in body weight ar e non-specific and not likely to be derived from a common toxicity pathway.

 Other target organs

o Kidney (3/23)

o Thyroid (2/23)

o Urinary tract (1/23)

o Testes (1/23)

o Thymus (1/23)

It should be pointed out that the numbers above do not indicate that, for example, only 7 SUs showed evidence of liver effects. More SUs th an outlined above had evidence of a particular effect; however, the effects were either seen inconsistently acro ss the toxicity profile for the individual SU or were seen at higher doses and not considered a major target organ for that SU.

Despite the fact that th e most sensitive effects following chro nic exposure were similar for some of the SUs, there was no consistent target orga n/effect across the class that would be supportive of a common apical outcome. Further subdividing the SUs into subclasses based on the urea substituent did not result in a clear association between a target organ a nd either subclass. Taken together, the toxicological profiles of the SU s did not support a candida te CMG determination.

3.0 Conclusions from CMG Screening Anal ysis and Options for Further CRA

The SUs share a core chemical structure with va rying degrees of structural similarity based on individual substituents on either side of the molecule. In addition, the SUs share a pesticidal MOA (inhibition of ALS), although the func tion of ALS in humans is unknown and the relevance of this MOA in humans in unclear. Based on toxicity studies , the SUs do not share a common toxicological profile; in stead the target organs vary among the class and are often unspecific, such as changes in body weight or ge neral effects on the liver. Further dividing the SUs into subclasses based on the urea substituent di d not result in a clear a ssociation of a target organ with any particular substructure.Page 7 of 13 Based on the weight of the evidence, considering the lack of common toxicological profile of the SUs, the uncertainty in the human relevance of ALS inhibition, and the lack of mammalian MOA data, a testable hypothesis for a common mechanism of action ca nnot be identified. Therefore, the Agency concludes that no common mechanism of toxicity exists among these pesticides and a CRA approach is not a ppropriate for this class of pesticides.

4.0 References

Joutel, A; Ducros, A; Alamowitch, S; et al. (1996 ). A human homolog of bacterial acetolactate synthase genes maps within the CADASIL critical region. Genomics 38(2):192-198.Page 8 of 13 Appendix A. Sulfonylurea chemical stru ctures and struct ural subcategory

Chemical Name Structure Subcategory based on urea substituent Bensulfuron-methyl pyrimidine

Chlorimuron-ethyl pyrimidine

Chlorsulfuron triazine

Ethametsulfuron triazine

Flazasulfuron pyrimidine

Foramsulfuron pyrimidine

Halosulfuron pyrimidine

Imazosulfuron pyrimidine

Iodosulfuron-methyl-sodium triazine

Mesosulfuron-methyl pyrimidine

Metsulfuron triazinePage 9 of 13 Chemical Name Structure Subcategory based on urea substituent Nicosulfuron pyrimidine

Orthosulfamuron pyrimidine

Primisulfuron-methyl pyrimidine

Prosulfuron triazine

Rimsulfuron pyrimidine

Sulfometuron-methyl pyrimidine

Sulfosulfuron pyrimidine

Thifensulfuron-methyl triazine

Triasulfuron triazinePage 10 of 13 Chemical Name Structure Subcategory based on urea substituent Tribenuron-methyl triazine

Trifloxysulfuron-sodium pyrimidine

Triflusulfuron-methyl triazinePage 11 of 13 Appendix B. Sulfonylurea Chronic Diet ary Endpoints and Target Organs

Chemical Name Chronic dietary endpoint 1 Target Organ Subcategory 2Bensulfuronmethyl Increased liver enzymes and liver weight (N= 21.4, L= 237.3) Liver pyrimidine

Chlorimuronethyl Hematological changes and atrophy of thymus (N=9, L= 42.7) Hematopoietic, thymus pyrimidine

Chlorsulfuron Decreased body weight (N=5, L=25) Body weight triazine

Ethametsulfuron Decreased body weight and body weight gain (N=86.9, L=483) Body weight triazine

Flazasulfuron Chronic nephropathy (N=1.3, L=13.3) Kidney, liver pyrimidine

Foramsulfuron No toxicity observed in any study No endpoint pyrimidine

Halosulfuron Decreased body weight gain, hematological changes, and clinical chemistry changes (N=10, L=40) Hematopoietic, body weight pyrimidine

Imazosulfuron Thyroid hypertrophy (N=75, L=150) thyroid pyrimidine

Iodosulfuronmethyl-sodium Hematopoietic histopathology (N=7.3, L=43.7) Hematopoietic triazine

Mesosulfuronmethyl Increased mucous secretion (N=155, L=574) Other pyrimidine

Metsulfuron Decreased body weight and body weight gain (N=25, L=250) Body weight triazine

Nicosulfuron Decreased body weight gain and increased kidney/liver weight (N= 125, L= 500) Body weight pyrimidine

Orthosulfamuron Decreas ed body weight gain and hepatotoxicity/nephrotoxicity (N=5, L=500) Kidney, liver, hematopoietic, body weight pyrimidine

Primisulfuronmethyl Effects on blood cells, increased liver weight with histopathology, and thyroid hyperplasia (N=25, L= 125) Hematopoietic, liver, thyroid pyrimidine

Prosulfuron Decreased body weight gain, hematological findings, and hepatotoxicity (N=5.3, L=54) Hematopoietic, liver, body weight triazine

Rimsulfuron Decreased body weight gain and liver effects (N=11.8, L=121) Body weight, liver pyrimidinePage 12 of 13 Chemical Name Chronic dietary endpoint 1 Target Organ Subcategory 2Sulfometuronmethyl Decreased body weight and hemolytic anemia (N=27.5. L=148.5) Hematopoietic, body weight pyrimidine

Sulfosulfuron Urinary tract pathology (N=24, L=244.2) Urinary tract pyrimidine

Thifensulfuronmethyl Decreased body weight and body weight gain (N=4.3, L=128) Body weight triazine

Triasulfuron Centrilobular hepatocytomegaly (N=1.2, L=129) Liver triazine

Tribenuronmethyl Decreased body weight gain, increased liver enzymes, and increased urinary volume (N=0.8, L=8.16) Body weight triazine

Trifloxysulfuronsodium Tubular atrophy in kidney (N=23.7, L= 99.3) Kidney pyrimidine

Triflusulfuronmethyl Decreased body weight/body weight gain, hematological changes, and interstitial cell hyperplasia (N=2.44, L=30.6) Body weight, hematopoietic, testes triazine

1 N= NOAEL, L=LOAEL. All values presented in mg/kg/day.

2 Subcatergory is based on the substitu ent on the urea side of the molecule.Page 13 of 13