A Risk Assessment Perspective: Application of Mode of Action and Human Relevance Frameworks to the Analysis of Rodent Tumor Data

When the United States Environmental Protection Agency (USEPA) published guidelines for carcinogen risk assessment in 1986, it provided essentially no guidance on how to consider mechanistic information. The guidelines simply pointed out that ‘‘The Agency will review each assessment as to the evidence on carcinogenesis mechanisms and other biological or statistical evidence that indicates the suitability of a particular extrapolation model.’’ (USEPA, 1986). At that time, mechanisms of carcinogenesis were largely unknown and empirically difficult to approach. Nonetheless, as mechanistic data became available, the Agency considered it in formulating science policies, e.g., alpha2u-globulin–induced rat renal tumors (USEPA, 1991) and thyroid follicular cell rodent tumors and disruption of thyroid homeostasis (USEPA, 1998) or in chemical-specific assessments, e.g., bladder tumors induced by melamine through formation of urinary calculi (USEPA, 1988). In 1996, the Agency formally proposed to make information on the mode of carcinogenic action a pivotal point of the cancer risk assessment process (USEPA, 1996). Because complete knowledge of how an agent causes cancer is unlikely to exist (certainly for the near term), the 1996 guidelines put forth the notion of understanding mode of action versus mechanism of action. The former being a less detailed biochemical description of events than is meant by mechanism of action. The mode of action is sufficient evidence to draw a reasonable working conclusion concerning the agent’s influence on key processes. The mode of action concept permits information on precursor events to be evaluated and incorporated into the risk assessment process in a realistic way. The most frequent comment about the Agency’s 1996 proposed revision of its cancer risk assessment guidelines was that more guidance was needed on how to evaluate an agent’s mode of action. A few comments raised concern that the new USEPA guidelines would not be public health protective and opposed using mode of action data. In response, in 1999, EPA incorporated a mode of action framework analysis (USEPA, 1999) in conjunction with work by the International Programme for Chemical Safety (IPCS) (SonichMullin et al., 2001). This framework provided assurance that a rigorous and transparent approach would be taken to using mode of action data in the Agency’s cancer risk assessments, and it was well received by the scientific community. The mode of action framework is derived from considerations for causality used in epidemiologic investigations, which were originally articulated by Hill (1965). The Hill criteria were later modified by the U.S. Department of Health and Human Services (USHHS, 1982) and by Faustman et al. (1997) to extend to experimental animal studies. The publication of the USEPA 1999 framework provided an important tool with which the Agency can promote and formalize the use of mode of action data in cancer assessment. Mode of action data have come into play in several ways in EPA risk assessments. It has been critical in informing the dose–response relationship below the experimental observable range of tumors, and thus it is useful in establishing more appropriate guidance levels for environmental contaminants. The Agency’s traditional linear default extrapolation procedure for modeling tumor incidence has been a longstanding and often controversial issue. The 1986 USEPA guidelines provided only one dose–response default, namely the linear extrapolation procedure. The 1999 (and 1996) guidelines advanced this issue by providing two default extrapolation approaches—one linear and and one nonlinear. Application of the nonlinear default extrapolation procedure (typically a margin of exposure analysis or determination of a reference dose/ concentration) is based on the mode of action understanding about the agent. It was acknowledged in the 1999 (1996) guidelines that ‘‘true thresholds’’ are experimentally difficult to This paper does not necessarily reflect the views and policies of the U.S. Environmental Protection Agency. The opinions expressed within this paper reflect the views of the authors. 1 To whom correspondence should be addressed. Fax: þ1-703-5147; E-mail: dellarco.vicki@epamail.epa.gov.