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EPA’s Response to Selected Major Interagency Comments on the Interagency Science 

Consultation Draft IRIS Toxicological Review of Biphenyl

September 30, 2011

Purpose: 

The Integrated Risk Information System (IRIS) assessment development process of May 2009 

includes two steps (Step 3 and 6) where White House offices and other federal agencies can 

comment on draft assessments. The following are EPA’s responses to selected major 

interagency review comments received during the Interagency Science Consultation step (Step 3) 

for the draft IRIS Toxicological Review of Biphenyl (dated July 2011). All interagency 

comments provided were taken into consideration in revising the draft assessment prior to 

posting for public comment and external peer review. The complete set of all interagency 

comments is attached as an appendix to this document. 

For a complete description of the IRIS process, including Interagency Science Consultation, visit 

the IRIS website at www.epa.gov/iris.

July 2011 Interagency Science Consultation Draft IRIS Assessment—Selected Major 

Comments and Responses:

Topic #1: Cancer Weight of Evidence for Biphenyl - ATSDR commented that the peer 

reviewed literature provides very limited evidence of biphenyl carcinogenicity, as indicated by 

lack of genotoxicity, bladder tumors in male rats only following high doses and calculi formation, 

liver tumors in high dose female mice only (a species known for liver cancer susceptibility), and 

lack of promoting activity. 

EPA Response: As per EPA’s Guidelines for Carcinogen Risk Assessment (“Cancer 

Guidelines”) (U.S. EPA, 2005a), the cancer descriptor of “suggestive evidence of 

carcinogenic potential,” is appropriate when “a concern for potential carcinogenic effects 

in humans is raised, but the data are judged not sufficient for a stronger conclusion. This 

descriptor covers a spectrum of evidence associated with varying levels of concern for 

carcinogenicity, ranging from a positive cancer result in the only study on an agent to a 

single positive cancer result in an extensive database that includes negative studies in 

other species.” 
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The carcinogenic database for biphenyl includes evidence from chronic 2-year cancer 

bioassays showing an increase in the incidence of liver tumors (hepatocellular adenomas 

and carcinomas) in female BDF1 mice (Umeda et al., 2005) and urinary bladder tumors 

(transitional cell papillomas and carcinomas) in male F344 rats (Umeda et al., 2002). 

The induction of urinary bladder tumors in F344 male rats by dietary biphenyl exposure 

is a high-dose phenomenon closely related to the formation of urinary bladder calculi. 

Therefore, EPA concluded that at environmentally relevant exposure levels the database 

provides “suggestive evidence of carcinogenic potential” based on the findings of liver 

tumors. The limitations identified by ATSDR are addressed below. 

• Genotoxicity is a consideration in evaluating the mode of carcinogenic action for 

a chemical, but not in the determination of carcinogenic potential. As discussed 

in Section 4.5.2 (Genotoxicity) of the draft Toxicological Review of Biphenyl, the 

overall weight of evidence for biphenyl genotoxicity from short-term tests is 

negative or equivocal (also see Table B-2 in the Toxicological Review). Biphenyl 

did not induce mutations in a variety of bacterial test systems (in the absence or 

presence of exogenous metabolic activation), but in vitro assays of genotoxicity in 

mammalian test systems yielded a mix of negative and positive results, with 

positive results mostly in the presence of metabolic activation.

• With regard to the observation of bladder tumors in male mice at high doses, EPA 

agrees that induction of bladder tumors in male rats was related to calculus 

formation, and that this is a high-dose phenomenon. No risk of bladder tumors is 

expected at environmental exposures. This was taken into consideration when 

assigning the cancer descriptor of “suggestive.”

• The finding of liver tumors in one sex of mice (females) was also taken into 

consideration in assigning the cancer descriptor of “suggestive.” It is not 

uncommon to find an increased incidence of tumors in only one sex in a study; 

the absence of an increased incidence of liver tumors in male mice in the Umeda 

et al. (2005) bioassay does not negate the positive finding in female mice. EPA 

also recognized that some strains of mice have a high spontaneous incidence of 

liver tumors that can complicate interpretation of treatment-related tumors. In the 

case of BDF1 mice used in the Umeda et al. (2005) bioassay, the spontaneous 

incidence of liver tumors in concurrent control mice was about 6%. Compared to 

other strains of mouse, this background liver tumor incidence is not considered 

particularly high. EPA also obtained historical control data from the study 
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investigator (Dr. Umeda). The incidence of liver tumors (combined incidence of 

hepatocellular adenoma and/or carcinoma) in historical control female mice was 

7.1% (similar to the incidence in the concurrent control), with a range of 2-14%. 

The liver tumor incidences in all three groups of female mice treated with 

biphenyl exceeded both the concurrent control and the historical control range. 

Therefore, susceptibility to liver tumors in female mice did not appear to be an 

issue in the biphenyl bioassay in the mouse. 

• In response to ATSDR’s comment that biphenyl did not promote kidney cancer in 

rats that received an initiator, it is important to note that not all carcinogens are 

cancer promoters.

Issue #2: Selection of Tumor Incidence as the Point of Departure (POD) - ATSDR 

commented that the selection of an extra risk of 10% tumor incidence as a point of departure 

(POD) may be high for cancer effects. ATSDR suggested that an extra risk of 5% cancer 

incidence may be more acceptable. 

EPA Response: The tumor incidence at the point of departure (POD) does not reflect a 

level of acceptable risk; it is just a starting point for extrapolating to risks at lower 

exposures, through the use of a slope factor calculated from the POD on the doseresponse curve to the origin. In identifying the POD, EPA followed guidance provided in 

EPA’s Draft Benchmark Dose Technical Guidance Document (U.S. EPA, 2000) and 

EPA’s Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005). As noted in the 

Benchmark Dose Technical Guidance, a 10% response is at or near the limit of sensitivity 

in most cancer bioassays and in some noncancer bioassays as well. This was the case for 

the biphenyl cancer bioassay in the BDF1 mouse (Umeda et al., 2005) used to derive the 

cancer slope factor. An excess risk of 10% has been the BMR used most often for tumor 

data. The Toxicological Review was revised to clarify the use of a BMR of 10% extra 

risk to derive the biphenyl cancer slope factor.
Appendix

Comments on the Interagency Science Consultation Draft

IRIS Toxicological Review of Biphenyl
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Agency for Toxic Substances and Disease Registry (ATSDR) Comments on the Interagency 

Science Consultation Draft IRIS Toxicological Review of Biphenyl (dated July 2011)

Date: August 3, 2011

From: Agency for Toxic Substances and Disease Registry

Subject: Comments on EPA’s Toxicological Review of Biphenyl

To: Environmental Protection Agency

We appreciate the opportunity to review EPA’s Toxicological Review of Biphenyl. Overall, we 

found the draft IRIS Toxicological Review and fact sheet well-written and comprehensive in 

outlining the key studies. 

General Comments:

1. Page 8: section 3.2 Distribution

The EPA document indicates that only 0.1% of the dose was detected in animal genital tract and 

less than 1% of the administered dose remained in tissues after 96 hrs of exposure. Since EPA’s 

RfD was derived from the development study, can the author(s) of the documents add a 

discussion of the maternal blood (cord) levels and internal dose that cause fetal skeletal 

anomalies?

2. Page 10: section 3.3.1.2 Results from in vitro studies

The Benford et al. (1981) study revealed that levels of 2-hydroxylase metabolite in rats were 35 

times higher than in humans. Large amounts of 2-hydroxbiphenyl was associated with urinary 

bladder tumor formation in rats. Subsequently, the differences in metabolites concentrations 

suggest that humans may be less likely to develop bladder cancer than the male rats. This should 

be added to the discussion in the cancer section.

3. Page 16: Possible Relationships between Metabolites and Toxic Effects.
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The last sentence discusses gender differences in urine potassium concentrations, pH and calculi 

formation. In human urine, a 24 hr collection range is between 25-125 mEq/d depending on diet, 

while human ideal pH range is between 6-7 and can vary significantly. The question is how 

much resemblance is there between human and rat’s urine by gender? Please clarify.

4. Carcinogenicity of Biphenyl 

The reviewed literature provides very limited evidence of biphenyl carcinogenicity because:

a. Biphenyl is not genotoxic.

b. Renal tumor formation was found only in male rats and not in any other species. The 

tumor formation was observed in very high doses and followed calculi formation. 

Urinary bladder calculi induce continuous irritation and regeneration of urinary 

epithelium which may lead to cancer formation.

c. Even though male mice received a higher average dose than female mice, no cancer 

was detected in male mice.

d. Some mice strains, particularly females, such as C57BR/cdJ, are known for extreme 

liver cancer susceptibility.

e. Liver tumors occurred in very high doses.

f. Biphenyl did not promote kidney cancer in rats which received N-ethyl-Nhydroxyethylnitrosamine as initiator.

Due to the above, the weight of evidence for carcinogenicity determination is limited.

Page 45: Table 4-13 Summary of reproductive data in albino rats exposed to dietary 

biphenyl. 

Please add to the table footnotes that 0.1% = 105 mg/kg and 0.5%= 525 mg/kg/day.

5. Page 88: section 4.7.3.2.2

“Evidence of peroxisome proliferation was restricted to the 16,000 ppm group of female

mice ...” One should take into account that female mice have some independent increased risk 

of developing liver cancer when attributing peroxisome proliferation to the chemical in question 

or to dose response.

6. Page 94: table 5-1. Please define the X-axis as dose (mg/kg/day).

7. Page 112: 
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Selection of an extra risk of 10% tumor incidence as a point of departure may be high for cancer 

effects. An extra risk of 5% cancer incidence may be more acceptable.