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Proposed OPP Policy on Determining the Need for In- Utero/Perinatal Carcinogenicity Testing on a Pesticide

8 August 1997

Prepared by :
Karl Baetcke, HED, OPP
William Burnam, HED, OPP
Nancy McCarroll, HED, OPP
Richard Hill, OPPTS
Mike Ioannou, HED, OPP
Susan Makris, HED, OPP
Whang Phang, HED, OPP

A Description of a Proposed Process for Determining the Need for Perinatal Carcinogenicity Studies of Pesticides

BACKGROUND

The National Academy of Sciences in the 1993 report, Pesticides in the Diets of Infants and Children, recommended that EPA develop toxicity testing procedures that specifically evaluate the vulnerability of infants and children to pesticides exposure. In addition, the Food Quality Protection Act of 1996 (P.L. 104-170) states, "EPA shall assess the risk of the pesticide chemical residue based on available information concerning the special susceptibility of infants and children to the pesticide chemical residues, including ... effects of in utero exposure to pesticide chemicals" (SEC. 405)(VI)(C)(II). The Health Effects Division of the Office of Pesticide Programs (OPP) reviewed whether perinatal carcinogenesis bioassay would be of greater value in evaluating the carcinogenic potential of pesticides for infants and children than the standard carcinogenesis bioassay. In October 1996, the OPP presented its findings and conclusions (USEPA, 1996) to the Scientific Advisory Panel (SAP) for comment and recommendations.

OPP had observed in perinatal and standard carcinogenicity studies that: (a) qualitatively, the same tumors are generally produced and (b) quantitatively, perinatal carcinogenicity dosing may or may not result in a higher tumor incidence than standard dosing. When there is a greater response, it is difficult to determine if the effect is due to an enhanced sensitivity during development or to greater total dosing.

These observations led OPP to conclude that currently available data do not support routine application of perinatal carcinogenicity testing in lieu of the standard carcinogenesis bioassay. The available data are not very robust. Therefore, OPP will assess the need for a perinatal carcinogenicity study on a case-by-case basis.

The SAP agreed with the OPP's conclusion that there was not an adequate basis for the routine use of perinatal carcinogenesis tests. The SAP also stated that there is no convincing evidence indicating that perinatal testing would necessarily increase the sensitivity of the standard carcinogenesis bioassay.

OPP also requested the SAP to suggest factors that might be considered in determining the need for a perinatal carcinogenesis bioassay. To facilitate this evaluation, the SAP was provided with the criteria employed by the Center for Food Safety and Applied Nutrition, Food and Drug Administration (FDA). These criteria are:

  1. Compounds whose lowest "effect" level is less than 200-times the expected human exposure.
  2. Compounds which are used as non-nutritive additives and whose exposure exceeds 0.25 mg/kg/day.
  3. Compounds which are considered as nutritive additives.
  4. Compounds with reproductive toxicity or teratogenic activity.
  5. Any compound with data indicating differences in affected organs in utero studies which require further study.
  6. Compounds with other data (reproductive and developmental toxicity) indicating a need for in utero exposure.

After reviewing the FDA criteria for considering chemicals as candidates for perinatal carcinogenicity testing, the SAP suggested that items 5 and 6 were especially applicable to OPP; the other criteria were either too broad or not relevant.

PROPOSED SCIENCE POLICY

This section of the paper presents a proposed evaluation process that OPP will use in determining the need for a perinatal carcinogenicity test. It builds upon the criteria used by FDA and incorporates the thinking of scientists in the field. The first section is a generic list of observations on chemical carcinogens that serve as the basis for the development of the factors which OPP deems necessary to consider. The factors developed from these observations are presented in Section 2 with a discussion of the weight-of-the-evidence process which the OPP proposes to use in assessing the factors and determining the need for perinatal carcinogenicity testing.

1. Basis for the Policy

Several observations serve as a basis for the proposed science policy. They influence:

  1. whether developing organisms may manifest unique carcinogenic responses not seen in the exposed adults;
  2. whether developing organisms may be more sensitive than mature organisms to carcinogenic stimuli; and
  3. whether there may be a need for perinatal cancer testing.

a. Data on diethylstilbestrol (DES) indicate that the induction of developmental abnormalities during gestation may predispose the individual to carcinogenic effects later in life that are different from those produced in adults (IARC, 1979; Anderson et al., 1991).

b. Differences in the ability to metabolize chemicals between developing and adult organisms can alter the sensitivity to carcinogenic substances. For example, certain mutagenic carcinogens [e.g., diethylnitrosamine (DEN)] are metabolized poorly to mutagenic moieties by fetal tissue as compared to adults and are less effective transplacentally (Vesselinovitch et al., 1979). On the other hand, certain mutagenic polycyclic aromatic hydrocarbons, PAHs (e.g., 3-methylcholenthrene) are more potent carcinogens in utero(Tomatis, 1979). It has been speculated that increased fetal inducibility of the mixed function oxidase system by PAHs is associated with increased in utero tumorigenesis (Anderson et al., 1991).

c. No consistent picture has emerged concerning the use of genetic toxicology test results in predicting the need for perinatal versus standard cancer testing. The decidedly different outcomes reported for the two mutagenic carcinogens discussed in item b above illustrate this point.

d. Significant cell division in tissues and organs during development can result in the enhanced expression of mutations due to the reduced time available for repair of DNA lesions that are induced chemically or occur spontaneously. Additionally, certain embryonic cells (e.g., brain cells) lack key DNA repair enzymes.

e.Some carcinogens often act by inducing organ-specific cytotoxicity, increasing cell proliferation, altering terminal differentiation of cells and/or disrupting apoptosis.

f. Components of the immune system become fully functional at varying times during development (Roitt, Brostoff, and Male, 1993).

2.Factors to be Considered

As a matter of science policy, OPP has developed a set of proposed factors based on the above observations that should be considered in selecting pesticides as potential candidates for perinatal carcinogenicity testing.

a. The pesticide causes adverse effects in studies with dosing in utero or during early postnatal development that:

  1. are qualitatively different from those produced in adults (e.g., liver effects in the adults and uterine or testicular effects in the offspring).
  2. suggest potential carcinogenic effects (e.g., hyperplasia, dysplasia, inhibition of apoptosis, inhibition of terminal differentiation).
  3. are quantitatively different from those in adults (i.e., occurring at significantly lower exposures).

b. Anticipated pesticide exposures during in utero and/or postnatal developmental periods are generally high in comparison to those later in life.

c. The structure-activity-relationship analysis indicates an association with a chemical that has shown increased sensitivity in perinatal carcinogenicity tests.

d. Margins of exposures between doses producing adverse effects and anticipated exposure are smaller during development than during adulthood.

Additional factors that should be considered are based on the ability of OPP to obtain data from sources other than standard toxicology testing requirements. For example, information on the potential of a pesticide to be transferred to milk can be obtained from residue chemistry data. When concerns arise for potential transplacental effects, OPP has the authority under its testing requirements to request special studies. For some chemicals, particularly those that have long-standing registrations, a literature review may yield information pertinent to evaluating the need for special testing.

e. Pesticides that have been shown to form adducts with the DNA of fetal tissue.

f. The pesticide is transported from maternal circulation across the placenta to the developing fetus and is detected in breast milk.

g. The developing animal can readily metabolize the pesticide to the expected active carcinogenic moiety.

h. The pesticide has been found to cause biologically relevant effects due to a modification of the biological activity of estrogenic and/or androgenic receptor complexes or other hormonally related molecular targets that indicate a potential for endocrine disruption and increased sensitivity of the developing animal.

3. Weight-of-the-Evidence Approach

OPP is proposing to use the above factors (listed under item 2) in a-weight-of-the-evidence approach in deciding whether a perinatal carcinogenicity study is needed.

During the process of consideration, OPP will carefully evaluate the information available on these factors for each chemical and determine whether the combined information on all factors leads to a coherent conclusion about the chemical and the appropriate testing.

Pesticides that are positive for mutagenic effects in multiple test systems and which have been shown to be positive in standard cancer bioassays, are expected also to be positive in perinatal cancer tests. Risks from these pesticides would usually be evaluated by a linear low-dose extrapolation procedure. In the presence of good anticipated exposure estimates, calculated risks can be expected to be protective for various population subgroups, including infants and children. Since a comparison of the available data between standard and perinatal cancer bioassays have indicated only quantitatively minor differences, additional testing in the perinatal carcinogenesis bioassay probably would not significantly affect the overall risk assessment. Therefore, a perinatal carcinogenicity study may not be warranted for mutagenic carcinogens. However, in certain cases such as with ethylnitrosourea, an established direct-acting mutagenic carcinogen and potent transplacental carcinogen (Tomatis, 1979; Swenberg et al., 1972; Grossi-Paoletti et al., 1970) further investigation may be required. Therefore, the OPP will give careful consideration to the nature of the cancer response, the adequacy of the exposure assessment (i.e., potential infant exposure), potential dose-response-relationship, and possible risks, before reaching a final decision regarding the need for additional testing.

Should a perinatal carcinogenicity study be required, the study protocol will be designed on a chemical-by-chemical basis, in consultation with other agencies and with the registrant. It is recognized that no single study design would be suitable for all chemicals.

Finally, OPP emphasizes that the proposed factors (in item 2 above) will be evaluated from time to time for their adequacy. Appropriate modifications will be made as new information becomes available.

REFERENCES

Anderson, L.M., Jones, A.B., & Rice, J.M. (1991). Meeting Report: Perinatal Carcinogenesis: Current Directions. Br. J. Cancer 63:1025-1028.

Grossi-Paoletti, E., Paoletti, P.,& Schiffer, D., et al. (1970). Experimental brain tumors induced in rats by nitrosourea derivatives. Part 2. Morphological aspects of nitrosourea tumors obtained by transplacental induction. J. Neuro. Sci 11:573-581.

IARC (1979), IARC Monographs on Diethylstilbestrol Dipropionate. Lyon, 21: 173-231.

Roitt, I.M., Brostoff, J., & Male, D.K. (1993). Immunology (Third Edition). Mosby, London.

Swenberg, J.A., Koestner, A., Wechsler, W. et al. (1972). Quantitative aspects of transplacental tumor induction with ethylnitrosourea in rats. Cancer Res. 32:2656-2660.

Tomatis, L. (1979). Prenatal Exposure to Chemical Carcinogens and Its Effects on Subsequent Generations. Natl. Cancer Inst. Monogr. 51:159-184.

Vesselinovitch, S.D., Rao, K.V.N., & Mihailovich, N. (1979). Neoplastic Response of Mouse Tissues During Perinatal Age Periods and Its Significance in Chemical Carcinogenesis. Natl. Cancer Inst. Monogr. 51: 239-250.

US Environmental Protection Agency (1996) Comparison of effects of chemicals with combined perinatal and adult exposure vs. adult only exposure in carcinogenesis bioassay. (100 KB, PDF)

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