EPA/690/R-23/005F | September 2023 | FINAL United States Environmental Protection Agency xvEPA Provisional Peer-Reviewed Toxicity Values for l-Phenyl-l-(2,4-dimethylphenyl)-ethane (PXE) (CASRN 6165-52-2) U.S. EPA Office of Research and Development Center for Public Health and Environmental Assessment ------- A United States Environmental Protection %#UI JTT,Agency EPA/690/R-23/005F September 2023 https://www.epa.gov/pprtv Provisional Peer-Reviewed Toxicity Values for 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane (PXE) (CASRN 6165-52-2) Center for Public Health and Environmental Assessment Office of Research and Development U.S. Environmental Protection Agency Cincinnati, OH 45268 ------- AUTHORS, CONTRIBUTORS, AND REVIEWERS CHEMICAL MANAGER Kyoungju Choi, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH CONTRIBUTORS Allison L. Phillips, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH Lucina E. Lizarraga, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH SCIENTIFIC TECHNICAL LEAD Lucina E. Lizarraga, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH DRAFT DOCUMENT PREPARED BY SRC, Inc. 7502 Round Pond Road North Syracuse, NY 13212 PRIMARY INTERNAL REVIEWERS Q. Jay Zhao, PhD, MPH, DABT Center for Public Health and Environmental Assessment, Cincinnati, OH Jeffry L. Dean II, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH PRIMARY EXTERNAL REVIEWERS Eastern Research Group, Inc. 110 Hartwell Avenue Lexington, MA 02421-3136 PPRTV PROGRAM MANAGEMENT Teresa L. Shannon Center for Public Health and Environmental Assessment, Cincinnati, OH Allison L. Phillips, PhD Center for Public Health and Environmental Assessment, Cincinnati, OH J. Phillip Kaiser, PhD, DABT Center for Public Health and Environmental Assessment, Cincinnati, OH Questions regarding the content of this PPRTV assessment should be directed to the U.S. EPA Office of Research and Development (ORD) Center for Public Health and Environmental Assessment (CPHEA) website at https://ecomments.epa.gov/pprtv. in 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- TABLE OF CONTENTS COMMONLY USED ABBREVIATIONS AND ACRONYMS v BACKGROUND 1 QUALITY ASSURANCE 1 DISCLAIMERS 2 QUESTIONS REGARDING PPRTVs 2 1. INTRODUCTION 3 2. REVIEW OF POTENTIALLY RELEVANT DATA (NONCANCER AND CANCER) 7 2.1. HUMAN STUDIES 10 2.2. ANIMAL STUDIES 10 2.3. OTHER DATA (SHORT-TERM TESTS, OTHER EXAMINATIONS) 10 3. DERIVATION 01 PROVISIONAL VALUES 15 3.1. DERIVATION OF ORAL REFERENCE DOSES 15 3.2. DERIVATION OF INHALATION REFERENCE CONCENTRATIONS 15 3.3. SUMMARY OF NONCANCER PROVISIONAL REFERENCE VALUES 15 3.4. CANCER WEIGHT-OF-EVIDENCE DESCRIPTOR 15 3.5. DERIVATION OF PROVISIONAL CANCER RISK ESTIMATES 16 APPENDIX A. SCREENING NONCANCER PROVISIONAL VALUES 17 APPENDIX B. REFERENCES 27 iv 1-Phenyl-1-(2,4-dimethylphenyl)-ethane ------- COMMONLY USED ABBREVIATIONS AND ACRONYMS a2u-g alpha 2u-globulin IVF in vitro fertilization ACGIH American Conference of Governmental LC50 median lethal concentration Industrial Hygienists LD50 median lethal dose AIC Akaike's information criterion LOAEL lowest-observed-adverse-effect level ALD approximate lethal dosage MN micronuclei ALT alanine aminotransferase MNPCE micronucleated polychromatic AR androgen receptor erythrocyte AST aspartate aminotransferase MOA mode of action atm atmosphere MTD maximum tolerated dose ATSDR Agency for Toxic Substances and NAG 7V-acetyl-P-D-glucosaminidase Disease Registry NCI National Cancer Institute BMC benchmark concentration NO A F.I. no-observed-adverse-effect level BMCL benchmark concentration lower NTP National Toxicology Program confidence limit NZW New Zealand White (rabbit breed) BMD benchmark dose OCT ornithine carbamoyl transferase BMDL benchmark dose lower confidence limit ORD Office of Research and Development BMDS Benchmark Dose Software PBPK physiologically based pharmacokinetic BMR benchmark response PCNA proliferating cell nuclear antigen BUN blood urea nitrogen PND postnatal day BW body weight POD point of departure CA chromosomal aberration PODadj duration-adjusted POD CAS Chemical Abstracts Service QSAR quantitative structure-activity CASRN Chemical Abstracts Service registry relationship number RBC red blood cell CBI covalent binding index RDS replicative DNA synthesis CHO Chinese hamster ovary (cell line cells) RfC inhalation reference concentration CL confidence limit RfD oral reference dose CNS central nervous system RGDR regional gas dose ratio CPHEA Center for Public Health and RNA ribonucleic acid Environmental Assessment SAR structure-activity relationship CPN chronic progressive nephropathy SCE sister chromatid exchange CYP450 cytochrome P450 SD standard deviation DAF dosimetric adjustment factor SDH sorbitol dehydrogenase DEN diethylnitrosamine SE standard error DMSO dimethylsulfoxide SGOT serum glutamic oxaloacetic DNA deoxyribonucleic acid transaminase, also known as AST EPA Environmental Protection Agency SGPT serum glutamic pyruvic transaminase, ER estrogen receptor also known as ALT FDA Food and Drug Administration SSD systemic scleroderma FEVi forced expiratory volume of 1 second TCA trichloroacetic acid GD gestation day TCE trichloroethylene GDH glutamate dehydrogenase TWA time-weighted average GGT y-glutamyl transferase UF uncertainty factor GSH glutathione UFa interspecies uncertainty factor GST glutathione-S'-transfcrase UFC composite uncertainty factor Hb/g-A animal blood-gas partition coefficient UFd database uncertainty factor Hb/g-H human blood-gas partition coefficient UFh intraspecies uncertainty factor HEC human equivalent concentration UFl LOAEL-to-NOAEL uncertainty factor HED human equivalent dose UFS subchronic-to-chronic uncertainty factor i.p. intraperitoneal U.S. United States of America IRIS Integrated Risk Information System WBC white blood cell Abbreviations and acronyms not listed on this page are defined upon first use in the PPRTV assessment. v 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F DRAFT PROVISIONAL PEER-REVIEWED TOXICITY VALUES FOR 1 -PHENYL-1 -(2,4-DIMETHYLPHENYL)-ETHANE (PXE; CASRN 6165-52-2) BACKGROUND A Provisional Peer-Reviewed Toxicity Value (PPRTV) is defined as a toxicity value derived for use in the Superfund program. PPRTVs are derived after a review of the relevant scientific literature using established U.S. Environmental Protection Agency (U.S. EPA) guidance on human health toxicity value derivations. The purpose of this document is to provide support for the hazard and dose-response assessment pertaining to chronic and subchronic exposures to substances of concern, to present the major conclusions reached in the hazard identification and derivation of the PPRTVs, and to characterize the overall confidence in these conclusions and toxicity values. It is not intended to be a comprehensive treatise on the chemical or toxicological nature of this substance. Currently available PPRTV assessments can be accessed on the U.S. EPA's PPRTV website at https://www.epa.gov/pprtv. PPRTV assessments are eligible to be updated on a 5-year cycle and revised as appropriate to incorporate new data or methodologies that might impact the toxicity values or affect the characterization of the chemical's potential for causing toxicologically relevant human-health effects. Questions regarding nomination of chemicals for update can be sent to the appropriate U.S. EPA eComments Chemical Safety website at https://ecomments.epa.gov/chemicalsafetv/. QUALITY ASSURANCE This work was conducted under the U.S. EPA Quality Assurance (QA) program to ensure data are of known and acceptable quality to support their intended use. Surveillance of the work by the assessment managers and programmatic scientific leads ensured adherence to QA processes and criteria, as well as quick and effective resolution of any problems. The QA manager, assessment managers, and programmatic scientific leads have determined under the QA program that this work meets all U.S. EPA quality requirements. This PPRTV assessment was written with guidance from the CPHEA Program Quality Assurance Project Plan (PQAPP), the QAPP titled Program Quality Assurance Project Plan (PQAPP) for the Provisional Peer- Reviewed Toxicity Values (PPRTVs) and Related Assessments/Documents (L-CPAD-0032718-QP), and the PPRTV assessment development contractor QAPP titled Quality Assurance Project Plan—Preparation of Provisional Toxicity Value (PTV) Documents (L-CPAD-0031971-QP). As part of the QA system, a quality product review is done prior to management clearance. A Technical Systems Audit may be performed at the discretion of the QA staff. All PPRTV assessments receive internal peer review by at least two CPHEA scientists and an independent external peer review by at least three scientific experts. The reviews focus on whether all studies have been correctly selected, interpreted, and adequately described for the purposes of deriving a provisional reference value. The reviews also cover quantitative and qualitative aspects of the provisional value development and address whether uncertainties associated with the assessment have been adequately characterized. 1 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F DISCLAIMERS The PPRTV document provides toxicity values and information about the toxicologically relevant effects of the chemical and the evidence on which the value is based, including the strengths and limitations of the data. All users are advised to review the information provided in this document to ensure that the PPRTV used is appropriate for the types of exposures and circumstances at the site in question and the risk management decision that would be supported by the risk assessment. Other U.S. EPA programs or external parties who may choose to use PPRTVs are advised that Superfund resources will not generally be used to respond to challenges, if any, of PPRTVs used in a context outside of the Superfund program. This document has been reviewed in accordance with U.S. EPA policy and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. QUESTIONS REGARDING PPRTVS Questions regarding the content of this PPRTV assessment should be directed to the U.S. EPA ORD CPHEA website at https://ecomments.epa.gov/pprtv. 2 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F 1. INTRODUCTION 1-Phenyl-1-(2,4-dimethylphenyl)-ethane (PXE), CASRN 6165-52-2, is a discrete organic chemical; it is a hydrocarbon containing both aromatic and aliphatic carbons (Figure 1). PXE is listed with the U.S. EPA Substance Registry Services and the Toxic Substances Control Act's public inventory (U.S. EPA. 2022c. d). It is listed on the European Chemicals (EC) inventory and is preregistered with Europe's Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) program (ECHA. 2022). There are no data available on the production of PXE in the United States or commercial uses reported for PXE (NLM. 2022b; U.S. EPA. 2022d). Synonyms of 1-phenyl-1-(2,4-dimethylphenyl)-ethane appearing in these databases and other sources include 2,4-dimethy-l-(l-phenylethyl)benzene, 1 -phenyl- 1-metaxylyl-ethane, 1-phenyl-1-(2,4-xylyl)ethane and 4-(l-phenylethyl)-w-xylene. The empirical formula for PXE is Ci6Hi8. The physicochemical properties for PXE are provided in Table 1. There are no experimental physicochemical property data available for PXE; therefore, all property data presented are estimates from the U.S. EPA CompTox Chemicals Dashboard version 2.2.1 and EPI Suite™. PXE is slightly soluble in water and has moderate vapor pressure (U.S. EPA. 2012). Its moderate vapor pressure indicates that it may volatilize from dry soil surfaces and will exist in the vapor phase in air. In the atmosphere, vapor-phase PXE has an estimated half-life of 0.5 days, based on its estimated rate of reaction with photochemically-produced hydroxyl radicals (U.S. EPA. 2012). At ambient temperatures, the potential for volatilization from water surfaces or moist soil surfaces is expected to be moderate, based on its estimated Henry's law constant. The estimated soil adsorption coefficient (K oc) values for PXE indicate the potential for sorption to soil is high. Based on its Log Koc value, PXE is classified to be slightly mobile in soils by the Food & Agriculture Organization of the United Nations (FAO) (U.S. EPA. 2012). Hydrolysis is not expected to be an important fate process due to the lack of hydrolysable functional groups in this chemical. Figure 1. l-Phenyl-l-(2,4-dimethylphenyl)-ethane (PXE) (CASRN 6165-52-2) Structure 3 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 1. Physicochemical Properties of PXE (CASRN 6165-52-2) Property (unit) Value3 Molecular formula Cl6Hl8 Molecular weight (g/mol) 210.32 Physical state NA Boiling point (°C) 301 Melting point (°C) 29°C Density (g/cm3 at 25°C) 0.962 Vapor pressure (mm Hg at 25°C) 2.7 x 10-3 Vapor density NA Water solubility (mol/L) 1.46 x 10-6 Log octanol-water partition coefficient (log Kow) 5.29 pKa (unitless) NA Henry's law constant (atm-m3/mol at 25°C) 5.25 x 10-4 Soil adsorption coefficient Koc (L/kg) 2.29 x 103 Atmospheric OH rate constant (cm3/molecule-sec at 25°C) 2.14 x 10-11 Atmospheric half-life (d) 0.5 (calculated using a 12-hday; 1.5 x 106OH/cm3)b Flash point (°C) 142 aUnless otherwise noted, data were extracted from the U.S. EPA CompTox Chemicals Dashboard (2,4-dimethyl- 1 -(1 -phehvlet hy 1 )ben/ene. CASRN 6165-52-2. https://comptox.epa.gov/dashboard/DTXSID50884231: accessed July 31, 2023). All values are predicted averages unless otherwise specified. bU.S. EPA (2012) (EPI Suite™ estimates using SMILES CC(C1 =CC=CC=C 1 )C 1 =CC=C(C)C=C 1C. EPI = Estimation Programs Interface; NA = not applicable; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane; SMILES = Simplified Molecular Input Line Entry System; U.S. EPA = U.S. Environmental Protection Agency. A summary of available toxicity values for PXE from the U.S. EPA and other agencies/organizations is provided in Table 2. 4 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 2. Summary of Available Toxicity Values for PXE (CASRN 6165-52-2) Source (parameter)3 Value (applicability) Notes Reference0 Noncancer IRIS NV NA U.S. EPA (2022b) HEAST NV NA U.S. EPA (2011b) DWSHA NV NA U.S. EPA (2018) ATSDR NV NA ATSDR (2022) WHO NV NA WHO (2022); IPCS (2021) CalFPA NV NA CalEPA (2022. 2020) OSHA NV NA OSHA (2020. 2017a. 2017b) NIOSH NV NA NIOSH (2018) ACGIH NV NA ACGIH (2022) DOE (PAC) NV NA DOE (2018) Cancer IRIS NV NA U.S. EPA (2022b) HEAST NV NA U.S. EPA (2011b) DWSHA NV NA U.S. EPA (2018) NTP NV NA NTP (2021) IARC NV NA IARC (2022) CalEPA NV NA CalEPA (2022. 2020) ACGIH NV NA ACGIH (2022) aSources: ACGIH = American Conference of Governmental Industrial Hygienists; ATSDR = Agency for Toxic Substances and Disease Registry; CalEPA = California Environmental Protection Agency; DOE = U.S. Department of Energy; DWSHA = Drinking Water Standards and Health Advisories; HEAST = Health Effects Assessment Summary Tables; IARC = International Agency for Research on Cancer; IRIS = Integrated Risk Information System; NIOSH = National Institute for Occupational Safety and Health; NTP = National Toxicology Program; OSHA = Occupational Safety and Health Administration; WHO = World Health Organization. Parameters: PAC = protective action criteria. °Reference date is the publication date for the database and not the date the source was accessed. NA = not applicable; NY = not available; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane. Literature searches were conducted in November 2018 and October 2022, and updated mostly recently in July 2023 for studies relevant to the derivation of provisional toxicity values for PXE. Searches were conducted using the U.S. EPA's Health and Environmental Research Online (HERO) database of scientific literature. HERO searches for the following databases: PubMed, TOXLINE1 (including TSCATS1), Scopus, and Web of Science. The National Technical Reports Library (NTRL) was searched for government reports from 2018 through 'Note that this version of TOXLINE is no longer updated (https://www.nlm.nib.gov/databases/download/toxlinesubset.html): therefore, it was not included in the literature search update from October 2022 or July 2023. 5 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F September 20202. The following resources were searched outside of HERO for health-related values: American Conference of Governmental Industrial Hygienists (ACGIH), U.S. Agency for Toxic Substances and Disease Registry (ATSDR), California Environmental Protection Agency (CalEPA), Defense Technical Information Center (DTIC), European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC), European Chemicals Agency (ECHA), the U.S. EPA Chemical Data Access Tool (CDAT), the U.S. EPA ChemView, the U.S. EPA Integrated Risk Information System (IRIS), the U.S. EPA Health Effects Assessment Summary Tables (HEAST), the U.S. EPA Office of Water (OW), International Agency for Research on Cancer (IARC), the U.S. EPA TSCATS2/TSCATS8e, the U.S. EPA High Production Volume (HPV) Chemicals via International Programme on Chemical Safety (IPCS) INCHEM, Japan Existing Chemical Data Base (JECDB), Organisation for Economic Cooperation and Development (OECD) Screening Information Data Sets (SIDS), OECD International Uniform Chemical Information Database (IUCLID), OECD HPV, U.S. National Institute for Occupational Safety and Health (NIOSH), U.S. National Toxicology Program (NTP), the U.S. EPA Office of Water (OW) Drinking Water Standards and Health Advisories, U.S. Occupational Safety and Health Administration (OSHA), and World Health Organization (WHO). 2NTRL was a subset of TOXLINE until December 2019 when TOXLINE was discontinued. Searches of NTRL were conducted starting in 2018 to ensure that references were not missed due to delays in importing items into the database. 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane 6 ------- EPA/690/R-23/005F 2. REVIEW OF POTENTIALLY RELEVANT DATA (NONCANCER AND CANCER) As summarized in Tables 3A and 3B, no short-term, subchronic, chronic, or reproductive/developmental toxicity studies of PXE in humans or animals exposed by oral or inhalation routes adequate for deriving provisional toxicity values were identified. The phrase "statistical significance" and term "significant," used throughout the document, indicate a p-value of < 0.05 unless otherwise specified. 7 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 3A. Summary of Potentially Relevant Noncancer Data for PXE (CASRN 6165-52-2) Category Number of Male/Female, Strain, Species, Study Type, Reported Doses, Study Duration Dosimetry Critical Effects NOAEL LOAEL Reference (comments) Notes Human 1. Oral (mg/kg-d) ND 2. Inhalation (mg/m3) ND Animal 1. Oral (mg/kg-d) ND 2. Inhalation (mg/m3) ND LOAEL = lowest-observed-adverse-effect level; ND = no data; NOAEL = no-observed-adverse-effect level; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane. 8 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 3B. Summary of Potentially Relevant Cancer Data for PXE (CASRN 6165-52-2) Category Number of Male/Female, Strain, Species, Study Type, Reported Doses, Duration Dosimetry Critical Effects Reference (comments) Notes Human 1. Oral (mg/kg-d) ND 2. Inhalation (mg/m3) ND Animal 1. Oral (mg/kg-d) ND 2. Inhalation (mg/m3) ND ND = no data; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane. 9 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F 2.1. HUMAN STUDIES No studies were located regarding the toxicity or carcinogenicity of PXE in humans after oral or inhalation exposure. 2.2. ANIMAL STUDIES No studies were located regarding the toxicity or carcinogenicity of PXE in animals after oral or inhalation exposure. 2.3. OTHER DATA (SHORT-TERM TESTS, OTHER EXAMINATIONS) No genotoxicity or other supporting studies were identified for PXE. However, some limited data were located for mixtures containing PXE (see Table 4). Male rats treated with 100 mg/kg-day of SAS-296, a commercial preparation containing 7% PXE and 93% 1-phenyl-1-orthoxylyl-ethane, daily for 1 month showed increases in relative liver weights. Statistically significant decreases in lipid profiles (free cholesterol, total cholesterol, and/or phospholipids) in serum and the liver were observed, as were other significant increases in liver enzyme (alkaline phosphatase), and free fatty acids in serum (Hasegawa et al.. 1982a). A companion toxicokinetics study performed in rats using a single oral dose showed that SAS-296 (a mixture of PXE and 1-phenyl-1-orthoxylyl-ethane) was absorbed through the gastrointestinal tract, rapidly cleared from the blood and widely distributed throughout the body, with large amounts initially found in the liver and slight accumulation in fat (Hasegawa et al.. 1982b). The percent dissipation of SAS-296, enzyme kinetics, and Km value determined by toxicokinetic experiments from the same group using rat liver microsomes supported the rapid disappearance and altered composition of SAS-296 observed in vivo. However, PXE dissipation rates were not accurately measured due to the lack of the saturation of substrate to enzymatic reaction (Hasegawa et al.. 1982b). In skin painting experiments, the test material, described only as a diaryl-alkane phenyl-xylyl-ethane synthetic fluid (and presumed to be a mixture of PXE, PXE isomers, and other structurally related compounds) was negative for tumorigenicity and carcinogenicity in two strains of SENCAR mice or hr/hr Oslo mice treated twice per week with 100% of diaryl-alkane phenyl-xylyl-ethane for 18 months (Iversen. 1990). Lower concentrations of diaryl-alkane phenyl-xylyl-ethane in acetone (20 or 40%) did not initiate or promote tumors in studies with dimethylbenz[a]anthracene or 12-O-tetradecanoylphorbol 13-acetate (Iversen. 1990). 10 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 4. Other Studies Test Materials and Methods Results Conclusions References Supporting studies in animals following oral exposure Short-term Mixture: SAS-296 (contains 7% PXE and 93% 1-phenyl- 1 -ortho xy ly 1-ethane) Male JCL:SD rats (5-6/group) were administered SAS-296, via gavage (no vehicle specified) at a dose of 100 mg/kg-day for 1 mo. Body-weight change during the experiment was recorded. Rats were sacrificed ~2 h after the final dose. Blood and liver were collected for limited serum and liver biochemistry measurements (unfasted). Select organs (liver, kidney, heart, spleen, and brain) were weighed. Gross and microscopic examinations were not performed. Reported changes included decreased body weight gain, increased relative liver weight, increased serum ALP, and lipid profile changes in serum (increased fatty acids; decreased cholesterol and phospholipids) and liver (increased phospholipids and pyruvate; decreased cholesterol, triglycerides, glycogen and glycolipids). Repeated exposure to SAS-296 produced effects on the liver in treated rats; the potential contribution of PXE to the observed toxicologically relevant effects is unclear. Haseeawa et al. (1982a) Supporting studies in animals following dermal exposure Carcinogenicity Mixture: diary 1-alkane phenyl-xylyl-ethane (presumed to be a mixture of PXE, PXE isomers, and other structurally related compounds; mixture composition uncharacterized in the study) In a skin painting study, hr/hr Oslo mice and SENCAR mice (16/sex/group; 24/sex for controls) were dermally exposed to 0% (vehicle), 20 or 40% (SENCAR mice), or 100% (Oslo mice) diary 1-alkane phenyl-xylyl-ethane in an acetone vehicle, 2 times/wk for 18 mo. A positive control group was administered 51.2 |ig or 25.6 |ig of DMBA. 100 |iL of the test solutions were applied to the backs of mice and the applications sites were left uncovered. Mice were observed to lick each other after painting and an "oil smell" was apparent for up to 4 h; therefore, oral and inhalation exposures were also likely. Body and organ weights were not recorded. Signs of skin and general toxicity were noted but not actively monitored. Animals were monitored 1 time/wk for skin tumors. Necropsies were performed "whenever possible." Tumors in other organs were examined histologically along with areas showing skin toxicity (eczematous changes, pigmentation, or ulcerations). No increases in incidences of skin tumors or differences in tumor rates or in overall tumor yields were observed in either hr/hr Oslo or SENCAR mice, compared to controls. Death rates were reported to increase in hr/hr mice, but data were not provided. Signs of toxicity (hyperplasia, ulcers, mast cell collections) were reported as "scant" (data were not provided). Diaryl-alkane phenyl-xylyl ethane was nontumorigenic in hr/hr Oslo or SENCAR mice; the potential contribution of PXE to the lack of observed effects is unclear. Iversen (1990) 11 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 4. Other Studies Test Materials and Methods Results Conclusions References Tumor initiation and promotion Mixture: diary 1-alkane phenyl-xylyl-ethane (presumed to be a mixture of PXE, PXE isomers, and structurally related compounds; mixture composition uncharacterized in the study) Tumor promotion tests were conducted in hr/hr Oslo and SENCAR mice (16/sex/strain/group; 24/sex for controls). hr/hr Oslo mice were dermally treated: once with 51.2 |ig DMBA and then with 40% diaryl-alkane phenyl- xylyl-ethane in acetone 2 times/wk for 18 mo; with 2.6 |ig DMBA 2 times/wk for 10 wk followed by applications of 40% diaryl-alkane phenyl-xylyl-ethane 2 times/wk for 18 mo; with 2.6 |ig DMBA alternating with 20% diaryl- alkane phenyl-xylyl-ethane in acetone 2 times/wk for 20 wk; or 100% PXE 2 times/wk for 18 mo. Negative control mice were administered acetone 2 times/wk for the duration of the experiment. Other groups of control mice received single applications of 51.2 or 25.6 |ig DMBA only, 2.6 |ig of DMBA 2 times/wk for 5 wk, or 2.6 |ig DMBA alternating with acetone 2 times/wk for 10 wk. SENCAR mice were dermally treated: once with 51.2 |ig DMBA and followed by 20% diaryl-alkane phenyl- xylyl-ethane in acetone 2 times/wk for 18 mo; once with 20% diaryl-alkane phenyl-xylyl-ethane in acetone and with 10 nmol TPA 2 times/wk for 18 mo; 2.6 ng DMBA alternating with 20% diaryl-alkane phenyl-xylyl-ethane in acetone 2 times/wk for lOwk; or 20 and 40% diaryl-alkane phenyl-xylyl-ethane in acetone 2 times/wk for 18 mo. Negative control mice were administered acetone 2 times/wk for the duration of the experiment. Other groups of control mice received single application of 51.2 |ig DMBA; 2.6 |ig of DMBA 2 times/wk for 10 wk; or 2.6 |ig of DMBA alternating with acetone 2 times/wk for 10 wk. Body and organ weights were not recorded. Signs of skin and general toxicity were noted but not actively monitored. Animals were monitored 1 time/wk for skin tumors. Necropsies were performed "whenever possible." Tumors in other organs were examined histologically along with areas showing skin toxicity (eczematous changes, pigmentation, or ulcerations). Treatment with diaryl-alkane phenyl-xylyl- ethane did not enhance or diminish DMBA-induced tumorigenesis in either mouse strain in any experiment. There were no significant tumorigenic responses after initiation with 20% diaryl-alkane phenyl- xylyl-ethane in acetone and promotion with TPA in SENCAR mice. Diaryl-alkane phenyl-xylyl ethane was not a tumor promoter or a tumor initiator; the potential contribution of PXE to the lack of observed effects is unclear. Iversen (1990) 12 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 4. Other Studies Test Materials and Methods Results Conclusions References Metabolism and Toxicokinetics In vivo Distribution Mixture: SAS-296 (contains PXE and 1 -phenyl-1 -orthoxy ly 1- ethane; administered mixture composition uncharacterized in the study) Groups of 8-wk-old male JCL-SD rats (number per group not specified) were administered SAS-296 orally either by a single dose of 100 mg/kg or daily doses of 100 mg/kg-day for 1 mo (performed simultaneously with the 1-mo toxicity study described above). Rats were sacrificed at 0, 2, 4, 24, and 48 h after the single dose, and at 2, 4, 24 h, and 7 d after the final dose in the repeated dose experiment. Multiple organs and tissues were excised at each sacrifice and analyzed by GC for SAS-296 components. Sinsle dose: SAS-296 was absorbed through the gastrointestinal tract and was rapidly cleared from the blood and widely distributed throughout the body. After a single dose of SAS-296, large amounts were initially found in the liver, followed by fat. After continuous administration, SAS-296 showed some potential for accumulation in fat. Haseeawa • At 2 h: SAS-296 peaked and was primarily observed in the liver followed by fat, with smaller amounts in other organs. • At 4 h: SAS-296 was nearly absent in the liver and continued to increase in total body fat and in subcutaneous fat for 24 h. • At 48 h: SAS-296 was only present in subcutaneous fat and body fat, although at lower levels than at 24 h. • Composition of each SAS-296 component in varying in organs changed over time in comparison to material administered, which the study authors attributed to different rates of metabolism for the two components (see in vitro study, below). Reoeat dose: • At 2 h: SAS-296 was primarily distributed to body fat, followed by subcutaneous fat, and to a lesser extent in the liver, heart, kidney, and brain. • SAS-296 continued to accumulate in body and subcutaneous fat and concentrations peaked at 24 h postdosing and then declined. SAS-296 was still present in fat after 7 d. Concentrations in all other organs and blood were near zero at 7 d. • SAS-296 concentrations in liver declined rapidly over time. et al. (1982b) 13 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 4. Other Studies Test Materials and Methods Results Conclusions References In vitro Rat liver microsomes Mixture: SAS-296 (contains PXE and 1 -phenyl-1 -ortho xylyl- ethane; mixture composition uncharacterized in the study) In a reaction mixture, 2.5 mL of freshly prepared rat liver homogenates were incubated with SAS-296 (final concentrations of PXE at 2.6, 3.6, or 4.6 ng/mL and 1-phenyl-l-orthoxylyl-ethane at 1.1, 1.6, or 2.0 |ig/mL) for 2 h at 37°C. Concentrations of PXE and 1-phenyl-1-orthoxylyl-ethane were analyzed at time zero and after 2 h and chemical dissipation rates, Michaelis constants (KM) and maximum reaction velocities (Vmax) were determined. • After 2 h, PXE concentrations were reduced by 23-32%, and concentrations of 1-phenyl-1-orthoxylyl-ethane were reduced by 58-60%. • Km and Vmax values were 11.2 ng/mL and 3.6 ng/mL/g/h for PXE and 16.2 ng/mL and 10 iig/mL/g/h for 1-phenyl-1-orthoxylyl- ethane, reflecting the higher enzyme affinity and lower maximal velocity of enzyme- catalyzed reaction for PXE. The percent dissipation of SAS-296 and KM and Vmax values are supportive of the rapid disappearance and altered isomer composition of SAS-296 in rats. However, PXE dissipation rates were not accurately measured due to the lack of the saturation of substrate to enzymatic reaction. Haseeawa et al. (1982b) ALP = alkaline phosphatase; DMBA = dimethylbenz[a]anthracene; GC = gas chromatography; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane; TPA = 12-O-tetradecanoylphorbol 13-acetate. 14 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F 3. DERIVATION OF PROVISIONAL VALUES 3.1. DERIVATION OF ORAL REFERENCE DOSES No studies were located regarding toxicity of PXE to humans or animals via oral exposure. Due to the lack of oral toxicity data for PXE, subchronic and chronic provisional reference doses (p-RfDs) could not be derived directly. Instead, the derivation of oral toxicity values was attempted using an alternative analogue approach, but a suitable analogue with available toxicity values was not identified (see Appendix A). 3.2. DERIVATION OF INHALATION REFERENCE CONCENTRATIONS No studies were located regarding toxicity of PXE to humans or animals via inhalation exposure. Due to the lack of inhalation toxicity data for PXE, subchronic and chronic provisional reference concentrations (p-RfCs) could not be derived directly. Instead, the derivation of inhalation toxicity values was attempted using an alternative analogue approach, but a suitable analogue with available toxicity values was not identified (see Appendix A). 3.3. SUMMARY OF NONCANCER PROVISIONAL REFERENCE VALUES Table 5 presents a summary of noncancer provisional reference values. Table 5. Summary of Noncancer Reference Values for PXE (CASRN 6165-52-2) Toxicity Type (units) Species/ Critical p-Reference POD POD Principal Sex Effect Value Method (HED/HEC) UFc Study Subchronic p-RfD (mg/kg-d) NDr Chronic p-RfD (mg/kg-d) NDr Subchronic p-RfC (mg/m3) NDr Chronic p-RfC (mg/m3) NDr HEC = human equivalent concentration; HED = human equivalent dose; NDr = not determined; POD = point of departure; p-RfC = provisional reference concentration; p-RfD = provisional reference dose; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane; UFC = composite uncertainty factor. 3.4. CANCER WEIGHT-OF-EVIDENCE DESCRIPTOR No oral or inhalation studies have been conducted to assess the carcinogenicity of PXE. Under the U.S. EPA Cancer Guidelines (U.S. EPA. 2005). there is "Inadequate Information to Assess the Carcinogenic Potential' of PXE by oral or inhalation exposure (see Table 6). 15 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table 6. Cancer WOE Descriptor for PXE (CASRN 6165-52-2) Possible WOE Descriptor Designation Route of Entry (oral, inhalation, or both) Comments "Carcinogenic to Humans" NS NA The available data do not support this descriptor. "Likely to be Carcinogenic to Humans " NS NA The available data do not support this descriptor. "Suggestive Evidence of Carcinogenic Potential" NS NA The available data do not support this descriptor. "Inadequate Information to Assess Carcinogenic Potential" Selected Both No adequate information is available to assess the carcinogenic potential of PXE by the inhalation or oral routes of exposure. "Not Likely to be Carcinogenic to Humans" NS NA The available data do not support this descriptor. NA = not applicable; NS = not selected; PXE = 1-phenyl-1-(2,4-dimethylphenyl)-ethane; WOE = weight of evidence. 3.5. DERIVATION OF PROVISIONAL CANCER RISK ESTIMATES Due to a lack of carcinogenicity data, derivation of cancer risk estimates is precluded (see Table 7). Table 7 Summary of Cancer Risk Estimates for PXE (CASRN 6165-52-2) Toxicity Type (units) Species/Sex Tumor Type Cancer Risk Estimate Principal Study p-OSF (mg/kg-d) 1 NDr p-IUR (lng/in3) 1 NDr NDr = not determined; p-IUR = provisional inhalation unit risk; p-OSF = provisional oral slope factor; PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane. 16 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F APPENDIX A. SCREENING NONCANCER PROVISIONAL VALUES Due to the lack of evidence described in the main Provisional Peer-Reviewed Toxicity Value (PPRTV) assessment, it is inappropriate to derive provisional toxicity values for PXE. However, some information is available for this chemical, which although insufficient to support derivation of a provisional toxicity value under current guidelines, may be of limited use to risk assessors. In such cases, the Center for Public Health and Environmental Assessment (CPHEA) summarizes available information in an appendix and develops a "screening value." Appendices receive the same level of internal and external scientific peer review as the provisional reference values to ensure their appropriateness within the limitations detailed in the document. Users of screening toxicity values in an appendix to a PPRTV assessment should understand that there could be more uncertainty associated with deriving an appendix screening toxicity value than for a value presented in the body of the assessment. Questions or concerns about the appropriate use of screening values should be directed to the CPHEA. APPLICATION OF AN ALTERNATIVE ANALOGUE APPROACH (METHODS) The analogue approach allows for the use of data from related compounds to calculate screening values when data for the compound of interest are limited or unavailable. Details regarding searches and methods for analogue analysis are presented in Wang et al. (2012) and Lizarraga et al. (2023). Three types of potential analogues (structural, metabolic, and toxicity- like) are identified to facilitate the final analogue chemical selection. The analogue approach may or may not be route specific or applicable to multiple routes of exposure. All information is considered together as part of the final weight-of-evidence (WOE) approach to select the most suitable analogue both toxicologically and chemically. An expanded analogue identification approach was developed to collect a comprehensive set of candidate analogues for the compounds undergoing U.S. Environmental Protection Agency (U.S. EPA) PPRTV screening-level assessment. As described below, this method includes application of a variety of tools and methods for identifying candidate analogues that are similar to the target chemical based on chemical structure and key features; metabolic relationships; or related toxic effects and mechanisms of action. To identify structurally-related compounds, an initial pool of analogues is identified using automated tools, including ChemlDplus3 (Nl.M. 2022a). the CompTox Chemicals Dashboard (U.S. EPA, 2022a). and the Organisation for Economic Co-operation and Development (OECD) Quantitative Structure-Activity Relationship (QSAR) Toolbox (OECD. 2021). to conduct structural similarity searches. Additional analogues identified as ChemlDplus-related substances, parent, salts, and mixtures, and CompTox-related substances are considered. CompTox Generalized Read-Across (GenRA) analogues are collected using the methods available on the publicly available GenRA version 3.2, which may include Morgan fingerprints, Torsion fingerprints, ToxPrints and ToxCast, Tox21, and ToxRef data. For compounds that have very few analogues identified by structure similarity using a similarity threshold of 0.8 or 80%, substructure searches in the QSAR Toolbox may be performed, or similarity searches may be rerun using a reduced similarity threshold (e.g., 70 or 60%). The compiled list of candidate analogues is batch run through the CompTox Chemicals Dashboard where QSAR-ready 3The National Library of Medicine (NLM) retired ChemlDplus in December 2022. 17 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F simplified molecular-input line-entry system (SMILES) notations are collected and toxicity data availability is determined (e.g., from the Agency for Toxic Substances and Disease Registry [ATSDR], Office of Environmental Health Hazard Assessment [OEHHA], California Environmental Protection Agency [CalEPA], the U.S. EPA Integrated Risk Information System [IRIS], PPRTV assessments). The batch output information is then uploaded into the Chemical Assessment Clustering Engine (C hem ACE) (U.S. HP A. 201 la), which clusters the chemicals based on chemical fragments and displays the toxicity data availability for each candidate. The ChemACE output is reviewed by an experienced chemist, who narrows the list of structural analogues based on known or expected structure-toxicity relationships, reactivity, and known or expected metabolic pathways. Toxicokinetic studies tagged as potentially relevant supplemental material during screening were used to identify metabolic analogues (metabolites and metabolic precursors). Metabolites were also identified from the two OECD QSAR Toolbox version 4.4 metabolism simulators (in vivo rat metabolism simulator and rat liver S9 metabolism simulator). Targeted PubMed searches were conducted to identify metabolic precursors and other compounds that share any of the observed or predicted metabolites identified for the target chemical. Metabolic analogues are then added to the pool of candidate analogues and toxicity data availability is determined (e.g., from ATSDR, OEHHA, CalEPA, U.S. EPA IRIS, PPRTV assessments). In vivo toxicity data for the target chemical (if available) are evaluated to determine whether characteristic toxicity associated with a particular mechanism of toxicity was observed (e.g., cholinesterase inhibition, inhibition of oxidative phosphorylation). In addition, in vitro mechanistic data tagged as potentially relevant supplemental material during screening or obtained from tools including GenRA version 3.2, ToxCast/Tox21, and Comparative Toxicogenomics Database (CTD) (CTD. 2022; Davis et al.. 2021) were evaluated for this purpose. Data from CompTox Chemicals Dashboard ToxCast/Tox 21 are collected to determine bioactivity of the target chemical in in vitro assays that may indicate potential mechanism(s) of action. The GenRA option within the Dashboard also offers an option to search for analogues based on similarities in activity in ToxCast/Tox21 in vitro assays. Using the ToxCast/Tox21 bioactivity data, nearest neighbors identified with similarity indices of >0.5 may be considered potential candidate analogues. The CTD (CTD. 2022; Davis et al.. 2021) is searched to identify compounds with gene interactions similar to interactions induced by the target chemical; compounds with gene interactions similar to the target chemical (with a similarity index >0.5) may be considered potential candidate analogues. These compounds are then added to the pool of candidate analogues, and toxicity data availability is determined (e.g., from ATSDR, OEHHA, CalEPA, the U.S. EPA IRIS, PPRTV assessments). The application of a variety of different tools and methods to identify candidate analogues serves to minimize the limitations of any individual tool with respect to the pool of chemicals included, chemical fragments considered, and methods for assessing similarity. Further, the inclusion of techniques to identify analogues based on metabolism and toxicity or bioactivity expands the pool of candidates beyond those based exclusively on structural similarity. The specific tools described above used for the expanded analogue approach searches were selected because they are publicly available, supported by U.S. and OECD agencies, updated regularly, and widely used. 18 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Analogue Search Results for PXE Candidate analogues for PXE were identified based on structural, metabolic, and toxicity/mechanisms/mode-of-action (MOA) relationships. For candidates identified through these approaches, the U.S. EPA (IRIS and PPRTV), ATSDR, and CalEPA sources were searched for subchronic, intermediate, and chronic oral and inhalation toxicity values. Details are provided below. Identification of Structural Analogues with Established Toxicity Values Table A-l summarizes the candidate structural analogues for PXE. PXE is not a member of an existing OECD or New Chemical category. Candidate structural analogues for PXE were identified using the U.S. EPA CompTox Chemistry Dashboard, OECD QSAR Toolbox, and ChemlDplus tools. Using similarity searches, 411unique structural analogues were identified in the Dashboard version 2.2.1, GenRA version 3.2, OECD QSAR Toolbox version 4.4, and ChemlDplus. Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 J v J HjC Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure Dashboard (Tanimoto) AND OECD QSAR Toolbox (Dice) AND ChemlDplus 1,4-dimethyl-2-( 1 -phenylethyl)benzene (6165-5 l-l)b Y ^CH3 Dashboard (Tanimoto) AND OECD QSAR Toolbox (Dice) l-methyl-2-(l-phenylethyl)benzene (40766-30-1) CH, Dashboard (Tanimoto) AND OECD QSAR Toolbox (Dice) l,2-Dimethyl-3-(l-phenylethyl)benzene (40766-3l-2)b ch3 H,C. III |j ch3 19 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure Dashboard (Tanimoto) AND OECD QSAR Toolbox (Dice) l,2-Dimethyl-4-(l-phenylethyl)benzene (6196-95-8) CHS ch3 Dashboard (Tanimoto) only l-Methyl-2-[l-(4-methylphenyl)ethyl]benzene (5080-10-4) Dashboard (Tanimoto) only 4-Benzyl-1,2-dimethylbenzene (13 540-56-2) ch3 Dashboard (Tanimoto) only 1 -Methyl-2-[(3-methylphenyl)methyl]benzene (21895-13-6) Dashboard (Tanimoto) only l-Benzyl-2,4-dimethylbenzene (28122-28-3) Dashboard (Tanimoto) only 2-Benzyl-l,3-dimethylbenzene (28122-29-4) H3C. CH3 fl ^ 20 1-Phenyl-1-(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure Dashboard (Tanimoto) only 1 -Methy 1-3 -(1 -phenylethyl)benzene (32341-91-6) Dashboard (Tanimoto) only 1 -Benzy 1-2,3-dimethylbenzene (32518-97-1) H,C Dashboard (Tanimoto) only 1,2-Dimethy 1-3 -[(2-methylphenyl)methyljbenzene (41888-01-1) H,c. A. CH, ^11 Dashboard (Tanimoto) only l-Methyl-4-[l-(4-methylphenyl)ethyl]benzene (530-45-0) CH3 Dashboard (Tanimoto) only l,4-Dimethyl-2-[(3-methylphenyl)methyl]benzene (61819-81-6) Dashboard (Tanimoto) only 1,3 -Dimethyl-2-[(3 -methylphenyl)methyljbenzene (721 -34-6) 21 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure Dashboard (Tanimoto) only l,4-Dimethyl-2-[(4-methylphenyl)methyl]benzene (721-45-9) Dashboard (Tanimoto) only 2,4-Dimethyl-1 -[(3 -methylphenyl)methyl]benzene (721 -54-0) [ ch3 H3C. ch3 Dashboard (Tanimoto) only l,2-Dimethyl-4-[(2-methylphenyl)methyl]benzene (721-80-2) CH3 CHj Dashboard (Tanimoto) only l,2-Dimethyl-4-[l-(3-methylphenyl)ethyl]benzene (874811-05-9) H3Cs> Jl ch3 ch3 Dashboard (Tanimoto) only l-Methyl-2-[(2-methylphenyl)methyl]benzene (1335-47-3) 22 1-Phenyl-1-(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure Dashboard (Tanimoto) only 2-Benzyl-1,4-dimethylbenzene (13 540-50-6) ^ H: Dashboard (Tanimoto) only l-Benzyl-2-methylbenzene (713-36-0) Dashboard (Tanimoto) only 1 -Ethyl-2-( 1 -phenylethyl)benzene (18908-70-8) ^-CH3 H3C Dashboard (Tanimoto) only 1 -Ethy 1-3-(1 -phenylethyl)benzene (18908-71-9) Dashboard (Tanimoto) only l-Benzyl-2-ethylbenzene (28122-25-0) H^C 23 1-Phenyl-1-(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-l. Candidate Structural Analogues Identified for PXE ch3 ch3 Tool (method)3 Analogue (CASRNs) Selected for Toxicity Value Searchesb Structure ChemlDplus0 only l,3-Dimethyl-2-(l-phenylethyl)benzene (81749-29-3) CH3 aAll software tools set to 80% similarity threshold for analogue identification. bOECD QSAR Toolbox reported that repeated dose toxicity data are available in the Japanese NITE database. °ChemIDplus structural similarity search algorithms embedded in the software. NITE = National Institute of Technology and Evaluation; OECD = Organisation for Economic Co-Operation and Development; PXE = 1-phenyl-1-(2,4-dimethylphenyl)-ethane; QSAR = quantitative structure-activity relationship. After eliminating analogues containing metals or deuterated compounds, the remaining list of analogues was reviewed by a chemist with expertise in read-across. The following criteria for determining PXE analogues were applied as part of the expert review: (1) the presence of a methylene or 1,1-ethylidene bridge connecting the two aromatic rings (compounds with other hydrocarbon moieties connecting the two aromatic rings were excluded because additional substitutions would impact the steric rotation of the molecule and would block the bridge atom from metabolism ([or reactivity in general]); (2) compounds with any other atom (such as oxygen or sulfur) at the bridge or substituted on the structure were excluded because this could change the activation/reactivity of the aromatic rings; (3) consistent with the structure of PXE, methyl groups and/or ethyl/ethynyl group on the rings were limited to no more than two per ring or one per ring, respectively, because more or larger substitutions would result in steric hinderance, decrease the solubility, and increase the log Kow of the compound; and (4) the presence of methyl or ethyl/ethynyl groups, as they are potential sites for metabolism. Of the 411 unique structural analogues identified by similarity searches, only 26 met the criteria above and were carried forward as candidate structural analogues (see Table A-l). No toxicity values were identified for any of the 26 candidate structural analogues. Identification of Toxicokinetic Precursors or Metabolites with Established Toxicity Values PubMed searches (searching "1-phenyl-1-(2,4-dimethylphenyl)-ethane" or "6165-52-2" and "metabolite") were conducted to identify metabolic precursors to PXE. No metabolic precursors were identified. No metabolites were identified for PXE in the scientific literature. Predicted metabolites were queried with the OECD QSAR Toolbox version 4.4 using the in vivo rat metabolism simulator and the rat liver S9 metabolism simulator. PubMed was also searched to identify other compounds that are metabolized to one of the 33 predicted metabolites of PXE (searching the metabolite name [none of the metabolites had CASRNs] and "metabolite"); no 24 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F compounds that share at least one metabolite with PXE were identified. Table A-2 summarizes the 33 candidate metabolic analogues for PXE identified by the OECD QSAR Toolbox. Searches for relevant toxicity values available from the U.S. EPA, ATSDR, or CalEPA for the candidate metabolic analogues of PXE did not identify toxicity values for any of the predicted metabolites. Table A-2. Candidate Metabolic Analogues of PXE Relationship to PXE Compound3 Metabolic precursor None identified [3 -methy l-4-( 1 -methyl-1 -pheny lethy l)pheny 1] methanol [5 -methy l-2-( 1 -pheny lethy l)pheny 1] methanol 2-(2,4-dimethyl-phenyl)-2-phenyl)ethanol 5-[ 1 -(4-hydroxyphenyl)ethyl] -2,4-dimethylphenol 3 -[ 1 -(2,4-dimethylphenyl)ethyl]phenol 2,4-dimethyl-5-(l-phenylethyl)phenol 2,6-dimethyl-3-(lphenylethyl)phenol 4-[ 1 -(2,4-dimethylphenyl)ethyl]phenol 4-[ 1 -(2,hydroxymethyl-4-methylphenyl)-ethyl] -phenol 4- [ 1 - [2-methy l-4-(hy droxy methy l)pheny 1] ethy lphenol 4-[l-(2,4-dimethylphenyl)-2-hydroxy]ethylphenol 3 -methyl-4-( 1 -phenylethyl)benzoic acid 3 -methy l-4-( 1 -pheny lethy l)benzaldehyde 5 -methyl-2-( 1 -phenylethyl)benzoic acid 5 -methy l-2-( 1 -pheny lethy l)benzaldehyde 2-(2,4-dimethylphenyl)-2-phenylacetic acid Predicted metabolite 2-(2,4-dimethyl-phenyl)-2-pheny lacetaldehyde 4-(2-hydroxy-1 -pheny lethy l)-3 -methy lbenzaldehyde 2-(4-hydroxymethyl-2-methy lphenyl)-2-pheny lacetaldehyde 2-(4-hydroxymethyl-2-methy lphenyl)-2-phenylethanol (2-hydroxymethyl-4-methylphenyl)-2-pheny lacetaldehyde 2-(2-hydroxy-l-phenylethyl)-5-methy lbenzaldehyde 2-(2-hydroxymethyl-4-methylphenyl)-2-phenylethanol 4-hydroxy-5-methyl-2-(l-phenylethyl)benzaldehyde 4-hydroxymethyl-2-methyl-5-( 1 -pheny lethy l)phenol 2-hydroxy-5-methyl-4-(l-phenylethyl)benzaldehyde 2-hydroxymethyl-4-methyl-5-( 1 -pheny l-ethyl)-phenol 5-(2-hydroxy-l-phenylethyl)-2,4-dimethy lphenol 2-hydroxymethyl-5-(2-hydroxy-l-phenyl-ethyl)-4-methyl-phenol 4-hydroxymethyl-5-(2-hydroxy-l-phenylethyl)-2-methylphenol 4-[ 1 -(2,4-dimethylphenyl)ethyl] -benzene-1,2-diol 2-( 1 -pheny lethy l)-3,5-dimethy lphenol 25 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F Table A-2. Candidate Metabolic Analogues of PXE Relationship to PXE Compound3 2-(5-hydroxy-2,4-dimethylphenyl)-2-phenylacetaldehyde 1 -(2,4-dimethylphenyl)-1 -phenylethane- l-hydroperoxideb 2-( 1 -phenylethyl)-2,5-dimethylphenol Shares common metabolite(s) None identified aNo CASRNs are available for these metabolites. bChemical structure is unstable or otherwise unsuitable for use as PXE analogue. PXE = l-phenyl-l-(2,4-dimethylphenyl)-ethane. Identification of Analogues on the Basis of Toxicity/Mechanistic/MOA) Information and Established Toxicity Values No toxicity or mechanistic/MOA data relevant for identifying candidate analogues for PXE were identified in the scientific literature. The GenRA option version 3.2 within the Dashboard version 2.2.1 offers the ability to search for analogues based on similarities in activity in ToxCast/Tox21 in vitro assays; however, there were no bioactivity data for PXE, so this was not further investigated. The CTD did not have an entry for PXE. Summary Searches for metabolic, structural, and toxicity/mechanistic analogues for PXE yielded a total of 59 unique candidate analogues: 33 metabolism-related analogues and 26 structural analogues. No candidate analogues were identified on the basis of having similar characteristic toxicity or mechanisms/MOAs. None of the candidate analogues have oral or inhalation toxicity values from the U.S. EPA, ATSDR, or CalEPA. Therefore, no suitable candidate analogues were identified to calculate screening oral or inhalation toxicity values. ORAL NONCANCER TOXICITY VALUES Derivation of Subchronic and Chronic Screening Provisional Reference Doses Subchronic and chronic provisional reference doses could not be derived due to the lack of an appropriate analogue having oral toxicity values. INHALATION NONCANCER TOXICITY VALUES Derivation of Subchronic and Chronic Screening Provisional Reference Concentrations Subchronic and chronic provisional reference concentrations could not be derived due to the lack of an appropriate analogue having inhalation toxicity values. 26 1 -Phenyl-1 -(2,4-dimethylphenyl)-ethane ------- EPA/690/R-23/005F APPENDIX B. REFERENCES ACGIH. (2022). 2022 TLVs and BEIs: Based on the documentation of the threshold limit values for chemical substances and physical agents & biological exposure indices. In 2022 TLVs and BEIs: Based on the documentation of the threshold limit values for chemical substances and physical agents & biological exposure indices. Cincinnati, OH. ATSDR. (2022). Toxic substances portal: Toxicological profiles [Database], Atlanta, GA. Retrieved from https://www.atsdr.cdc.gov/toxprofiledocs/index.html CalEPA. (2020). Consolidated table of OEHHA/CARB approved risk assessment health values. Sacramento, California, https://ww2.arb.ca.gov/resources/documents/consoiidated-tabie- oehha-carb-approved-risk-assessment-heal th-values CalEPA. (2022). OEHHA chemical database [Database], Sacramento, CA: Office of Environmental Health Hazard Assessment. Retrieved from https://oehha.ca.gov/chemicals ("I'D. (2022). 1-Phenyl- l-(2,4-dimethylphenyl)-ethane (PXE): Comparative toxicogenomics database [Database]: MDI Biological Laboratory. Retrieved from http://ctdbase.org/ Davis. AP; Grondin. CJ: Johnson. RJ; Sciakv. D; Wiegers. J; Wiegers. TC; Mattinglv. CJ. (2021). The comparative toxicogenomics database (CTD): update 2021 [Database], Raleigh, NC: MDI Biological Laboratory. North Carolina State University. Retrieved from http://ctdbase.org/ DOE. (2018). Table 2: Protective Action Criteria (PAC) Rev. 29a based on applicable 60-minute AEGLs, ERPGs, or TEELs. https://edms3.energy.gov/pac/docs/Revision 29A Tabte2.pdf ECHA. (2022). Registered substances: Helsinki, Finland. 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