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IRIS

IRIS Bimonthly Public Meeting (Oct 2014)

EPA hosted a Bimonthly IRIS public meeting to provide an opportunity for the public to give input and participate in an open discussion regarding preliminary materials that were prepared for IRIS chemicals prior to the development of the draft assessment. This included the following chemicals:

  • Diisononyl Phthalate (DINP)
  • Hexavalent Chromium (Cr(VI))

Meeting Objective:

The objective of this public meeting was to obtain input from stakeholders and the public on the studies and data that will be used in the assessments that are under development. Specifically, EPA was seeking input on preliminary materials including draft literature searches and associated search and screening strategies, the approach for selecting primary studies to be included in the evidence tables, the approach for evaluating methodological features of studies, evidence tables, and exposure-response arrays prior to the development of the IRIS assessments for diisononyl phthalate and hexavalent chromium.

Dates:

The meeting was held on October 29-30 , 2014 from 9:00am - 5:00 pm Eastern Time.

Location:

The meeting was held at the EPA Conference Center at 2777 South Crystal Drive, Arlington, Virginia 22202. The meeting was also streamed by webinar/teleconference for remote participants.

Meeting Agenda:

On October 29-30, 2014, EPA hosted a public meeting/webinar in Arlington, VA, to provide an opportunity for the public to give input and participate in an open discussion regarding several IRIS chemical assessments that are under development. See the final agenda:

Meeting Materials:

The chemicals and associated discussion materials include:

Diisononyl Phthalate (DINP): Teneille Walker, Assessment Manager

Key Science Questions:

Science Question 1: Liver effects, including spongiosis hepatis. Liver effects including changes in weight and serum markers of liver functions and histopathological alterations have been observed following exposure to diisononyl phthalate (DINP). In rats, a dose-dependent increase in the incidence of spongiosis hepatis has been reported following chronic exposures (see Section 3.3.1 of the Preliminary Materials). Spongiosis hepatis/cystic degeneration is observed in aging rats, particularly males, less commonly in aging mice, and to a greater extent following exposure to some liver carcinogens (Thoolen et al., 2010; Karbe and Kerlin, 2002). Whether spongiosis hepatis/cystic degeneration represents a preneoplastic change or a non-neoplastic change has been the subject of scientific debate (Bannasch, 2003; Karbe and Kerlin, 2002). EPA is seeking public discussion of DINP-induced liver effects, including spongiosis hepatis.

Science Question 2: Male reproductive effects. Various animal studies report evidence for DINP-induced reproductive effects that are considered indicative of the phthalate syndrome in rats, including decreased testicular testosterone production, sperm motility, anogenital distance, male reproductive organ weight, and increased incidence of multinucleated gonocytes, nipple retention and Leydig cell aggregates (see Section 3.3.3 of the Preliminary Materials). However, malformations associated with phthalate syndrome effects (e.g., hypospadias and cryptorchidism) have not been observed. EPA is seeking public comment on the evidence for DINP-induced male reproductive toxicity.

Science Question 3: Human relevance of testicular xenograft studies. Studies using human testicular tissue xenografts and ex-vivo tissue culture preparations have raised questions about the human relevance of some of the testis-specific endpoints measured in experimental rodents exposed to phthalates. Recent reviews have suggested limitations in these studies, including variability in the human population, small sample size, and gestational age of the human tissues (Albert and Jégou, 2014; CPSC, 2014). EPA is seeking public discussion of the relevance of the xenograft and ex-vivo tissue studies.

Science Question 4: Human relevance of mononuclear cell leukemia. Animal studies have reported that DINP exposure is associated with increased incidence of mononuclear cell leukemia (MNCL) in rats and mice (see Section 3.3.6 of the Preliminary Materials). There are no human epidemiological studies on DINP exposure and cancer. The human relevance of MNCLs observed in animals has been a topic of scientific debate. EPA is seeking public discussion of the human relevance of MNCL in rats and mice following exposure to DINP.

Science Question 5: Transparency and utility of mechanistic data. The preliminary materials include an inventory of mechanistic data intended to facilitate subsequent identification of potential mode(s) of action and development of adverse outcome pathways for the health effects of DINP (EPA is presenting this material to stimulate public discussion and has not yet conducted mode-of-action or adverse-outcome-pathway analyses). EPA is seeking public discussion on (1) the transparency and utility of the presentation of mechanistic data, and (2) how mechanistic data can inform the hazard identification and dose-response analysis for each hazard.

References

  • Bannasch, P. (2003) Comments on R. Karbe and R. L. Kerlin (2002). Cystic degeneration/spongiosis hepatis (Toxicol Pathol 30(2):216–227). Toxicol Pathol 31:566–570.
  • CPSC (Consumer Product Safety Commission) (2014). Chronic Hazard Advisory Panel on Phthalates and Phthalate Alternatives. Bethesda, MD: U.S. Consumer Product Safety Commission. Available online at Chronic Hazard Advisory Panel (CHAP) on Phthalates (accessed July, 2014).
  • Karbe, E; Kerlin, RL. (2002) Cystic degeneration/spongiosis hepatis in rats. Toxicol Pathol 30(2):216-227.
  • Thoolen, B; Maronpot, RR; Harada, T; et al. (2010) Proliferative and nonproliferative lesions of the rat and mouse hepatobiliary system. Toxicol Pathol 38:5S–81S.

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Hexavalent Chromium (Cr(VI)): Catherine Gibbons & Alan Sasso, Assessment Managers

Key Science Questions:

Science Question 1: Methodological considerations for evaluating epidemiology studies. Section 1.2.4 of the Preliminary Materials describes methodological characteristics that will be considered in EPA's evaluation of hexavalent chromium epidemiology studies, including aspects of the study design affecting the internal or external validity of the results (e.g., population characteristics and representativeness, exposure and outcome measures, confounding, data analysis) that could influence or limit interpretation of the human data. EPA is seeking public discussion of whether there is additional literature that has a bearing on methodological considerations specific to hexavalent chromium, and in particular literature relevant to evaluation of exposure measures for populations occupationally-exposed to hexavalent chromium.

Science Question 2: Inhalation cancer dose-response modeling. At the June 2014 IRIS Bimonthly Public Science meeting, we discussed adopting the cancer classification "known human carcinogen by inhalation" from previous assessments by national and international health agencies and focusing the review of the human lung cancer evidence on those studies that might improve the quantitative dose-response analysis of the inhalation cancer data. Inclusion criteria for selecting inhalation lung cancer studies are presented in Table 1-3 of the Preliminary Materials, and the inhalation lung cancer studies that will be the focus of this health assessment are summarized in Table 2-8. EPA is seeking public discussion of the selection of human studies in the lung cancer evidence table, and whether there are additional studies with data useful for quantitative dose-response modeling of cancer inhalation risk that should be included.

In addition, EPA intends to evaluate whether data sets from multiple human studies can be combined in conducting quantitative dose-response modeling of lung cancer data (e.g., using approaches for meta analysis). In particular, EPA is interested in public discussion of issues with combining data sets that use different exposure metrics, exposure levels, and populations.

Science Question 3: Toxicokinetic considerations for dose-response. In September 2013, EPA convened a public workshop webinar on the topic of inter- and intraspecies differences in gastrointestinal toxicokinetics of ingested hexavalent chromium. Structural and functional differences between the gastrointestinal tracts of rodents and humans were discussed at this workshop. See hexavalent chromium September 2013 workshop website.

Interspecies parameters relevant to hexavalent chromium-induced toxicity are well defined for the stomach. In particular, stomach pH is lower in humans, and low pH is known to increase the rate at which hexavalent chromium is reduced to trivalent chromium (a detoxification pathway). However, toxicologically-relevant parameters and interspecies differences are not as well defined for other tissues in the digestive system. For tissues downstream of the stomach (small and large intestine), humans exhibit higher pH than rodents. Furthermore, significant differences exist in the proportional sizes and surface areas of the small intestine between species, and the distribution of relevant uptake transporters is unknown. It is known that tissues upstream of the stomach (oral cavity, esophagus) exhibit fast emptying times, meaning that hexavalent chromium exposure to these tissues occurs for a shorter duration than other digestive tract tissues. However, these upstream tissues contain fewer components capable of extracellularly reducing hexavalent chromium. EPA is seeking discussion of physiological or histological aspects of the small intestine and oral cavity of mice, rats, and humans that may be significant factors in interpreting toxicity data for those tissues.

Section 3 of the Preliminary Materials identified physiologically-based pharmacokinetic (PBPK) models available for hexavalent chromium. EPA is evaluating the use of these models for dose-response assessment, and seeks scientific discussion regarding the strengths and uncertainties in model assumptions and application. For example:

  • Hexavalent chromium uptake (normalized by volume of small intestine) estimated by PBPK modeling has previously been applied as an internal dose-metric. EPA seeks discussion of the appropriateness of other additional internal dose metrics that may be used for dose-response modeling of effects in the small intestine (for example, hexavalent chromium concentrations in the gastrointestinal tract or hexavalent chromium uptake normalized by a toxicologically-relevant surface area). It has been suggested that mice are susceptible to hexavalent chromium-induced gastrointestinal tract toxicity due to depletion of stomach reducing agents (Thompson et al., 2013). However, ongoing work by EPA scientists (Schlosser and Sasso, In Press) indicates that the reducing agent might not be depleted at high hexavalent chromium concentrations, and that no well-defined toxicokinetic threshold exist. As a result, species differences and interindividual variation in gastric reduction of hexavalent chromium in vivo might depend more on stomach physiology and pH than on total gastric reduction capacity. EPA seeks input on data regarding the variability of gastric parameters in both rodent and human populations. Such parameters for the stomach may include pH and emptying time, size, acid production, and microbiota.

[Related comments submitted by Sean Hays (Summit Toxicology)]:

  • The internal dose measure used to support the risk assessment needs to consider its relation to the mode of action (MOA), as well as the confidence in the PBPK model's ability to predict it.
     
  • It has been suggested that the depletion of gastric reducing agents in high-dose mice under the conditions of the NTP bioassay may contribute to a nonlinear tumor dose-response (Proctor et al., 2012).
     
  • Key assumptions for the gastric reduction model for hexavalent chromium include: (1) the number of reducing agent pools included in the model; and (2) the form of the pH-dependency for the reduction rate constant(s).

Science Question 4: Mechanistic studies database. The most recent literature search in February 2014 identified 806 mechanistic studies relevant to hexavalent chromium. In order to respond to the 2011 recommendations of the National Research Council for a consistent, transparent, and systematic approach for the identification, evaluation, and integration of data for assessing hazards to human health (NRC, 2011), information from mechanistic studies of hexavalent chromium was extracted into an Excel spreadsheet (see Section 4 of the Preliminary Materials) as a preliminary step in the process of database organization and analysis. Study information on dose, route, duration, or findings have not been entered into the spreadsheet. EPA considers this approach appropriate at this early stage in assessment development, i.e., before human health hazards have been identified and specific mode of action hypotheses constructed and evaluated. At this early stage, we did not adopt the approach taken by Kushman et al. (2013), which describes a targeted search of a curated database after a specific hazard had been identified. Are there aspects of the database, including the preliminary designation of mechanistic categories, that could be improved? Are there any studies missing from this database?

[Comment related to mechanistic information suggested by Mark Harris (ToxStrategies, Inc.) on behalf of the American Chemistry Council, and Ted Simon (Ted Simon LLC)]: Many scientists and regulators believe that mode of action should be considered very early in the evaluation process and throughout the analysis to enable effective use of data in human health risk assessment (Meek et al., 2014, Simon et al., 2014). Such consideration can focus efforts and resources on key endpoints and relevant data. For example, are the available data more consistent with a direct mutagenic or indirect nonmutagenic mode of action in the etiology of hexavalent chromium-induced tumors in the small intestine of mice? Two hypotheses exist in the scientific literature to explain the observed dose-dependent increase in small intestinal tumors in mice exposed to hexavalent chromium in drinking water (McCarroll et al., 2010; Thompson et al., 2011). Such tumors were not observed in similarly exposed rats. Discussion is sought on the modes of action proposed in these papers and on the utility of considering them at different stages in the assessment process.

Science Question 5: Chromium-DNA adducts. The ability of chromium (in the trivalent oxidation state, intracellularly) to covalently bind DNA molecules has been shown by several investigators experimentally in vitro (reviewed in Zhitkovich, 2005); however, these adducts have not been observed in vivo. EPA is seeking public discussion on the feasibility of chromium-DNA adducts forming in vivo and how predictive these lesions are of mutagenic potential.

Science Question 6: In vitro mutagenicity/genotoxicity studies of hexavalent chromium. In 2010, EPA released an external review draft IRIS assessment for hexavalent chromium. In the final peer review report (2011), it was emphasized by several reviewers that for in vitro studies of mutagenicity and genotoxicity, positive results are only observed following very high concentration exposures that are toxic to the cells exposed. In vitro studies often rely on using high exposures to induce effects to allow experimental investigation of mechanistic events. EPA is seeking discussion of the utility of these studies to inform mechanistic evaluation for hexavalent chromium.

Science Question 7: [New issue suggested by Deborah Proctor (ToxStrategies, Inc.) on behalf of the Electric Power Research Institute (EPRI)]: In vivo mutagenicity/genotoxicity studies of hexavalent chromium. Several in vivo genotoxicity/mutagenicity assays have been conducted for hexavalent chromium, including in target tissues of carcinogenicity (e.g. intestine). Some of these in vivo studies have employed high, carcinogenic concentrations by relevant routes of exposure and have indicated negative results with respect to genotoxicity/mutagenicity. Discussion is sought of the utility of these studies to inform mechanistic evaluation for hexavalent chromium.

Science Question 8: Definitions of mutagenicity and genotoxicity. For this assessment, the IRIS Program is considering using the following definitions found in the EU Technical Guidance on Risk Assessment (1996):

"The chemical and structural complexity of the chromosomal DNA and associated proteins of mammalian cells, and the multiplicity of ways in which changes to the genetic material can be affected, make it difficult to give precise, discrete definitions.

Mutagenicity refers to the induction of permanent transmissible changes in the amount or structure of the genetic material of cells or organisms. These changes, 'mutations,' may involve a single gene or gene segment, a block of genes, or whole chromosomes. Effects on whole chromosomes may be structural and/or numerical.

Genotoxicity is a broader term and refers to potentially harmful effects on genetic material, which may be mediated directly or indirectly, and which are not necessarily associated with mutagenicity. Thus, tests for genotoxicity include tests which provide an indication of induced damage to DNA (but not direct evidence of mutation) via effects such as unscheduled DNA synthesis (UDS), sister chromatid exchange (SCE) or mitotic recombination, as well as tests for mutagenicity."

EPA is seeking public discussion of this proposal.

References

  • European Commission. (1996) Technical guidance document in support of commission directive 93/67/EEC on risk assessment for new notified substances and commission regulation (EC) No 1488/94 on risk assessment for existing substances, Part I.
  • Kushman, ME; et al. (2013) A systematic approach for identifying and presenting mechanistic evidence in human health assessments. Regul Toxicol Pharmacol 67: 266-277.
  • McCarroll, N; et al. (2010). An evaluation of the mode of action framework for mutagenic carcinogens case study II: chromium (VI). Environ Mol Mutagen 51: 89-111.
  • National Research Council (NRC). (2011) Review of the Environmental Protection Agency's Draft IRIS Assessment of Formaldehyde. Washington, DC: National Academies Press.
  • Proctor, DM; et al. (2012) Hexavalent chromium reduction kinetics in rodent stomach contents. Chemosphere 89: 487-93.
  • Schlosser, PM; Sasso, AF. (2014) A revised model of ex-vivo reduction of hexavalent chromium in human and rodent gastric juices. Toxicol Appl Pharmacol 280: 352-361.
  • Thompson et al. (2011) Investigation of the mode of action underlying the tumorigenic response induced in B6C3F1 mice exposed orally to hexavalent chromium. Toxicol Sci 123: 58-70.
  • Thompson, CM; et al. (2013). Assessment of the mode of action underlying development of rodent small intestinal tumors following oral exposure to hexavalent chromium and relevance to humans [Review]. Crit Rev Toxicol 43: 244-274.
  • Zhitkovich, A. (2005) Importance of chromium-DNA adducts in mutagenicity and toxicity of chromium (VI) [Review]. Chem Res Toxicol 18: 3-11.

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General Comments

The public is encouraged to identify additional science issues that they would like to join us in discussing.

Note: The Cr(VI) materials released for this meeting are the second set of preliminary materials released on this chemical. The materials include updated information on the literature search and screening strategy, approaches for the selection of human studies of Cr(VI) for hazard identification, presentation of critical human studies in evidence tables, and a preliminary summary of toxicokinetic and mechanistic studies pertinent to the assessment of Cr(VI). The first set of preliminary materials released in April 2014, Preliminary Materials for the Integrated Risk Information System (IRIS) Toxicological Review of Hexavalent Chromium Part 1: Experimental Animal Studies, presented the planning and scoping summary, problem formulation information, and evidence tables for experimental animal data for the health effects of Cr(VI).