CARCINOGENICITY THRESHOLD DETERMINATIONS ON A CASE-BY-CASE BASIS
CARCINOGENICITY THRESHOLD DETERMINATIONS ON A CASE-BY-CASE BASIS should be recognized as an appropriate approach to the assessment of chemical carcinogens, PMA maintained in recent comments on the White House Office of Science & Technology Policy (OSTP) draft document on carcinogenicity evaluation. PMA said that while OSTP does not make "any conclusive statements regarding the existence of or lack of existence of thresholds," it "fails to acknowledge that solid experimental evidence exists to demonstrate that a few chemicals do produce cancer in animals by mechanism involving a threshold." The assn. cited the food additive selenium as having a recognized threshold for carcinogenicity. FDA "concluded that selenium's capacity to induce liver damage at excessively high doses may be associated with a higher incidence of liver cancer but that such capacity does not warrant its classification as a carcinogen at nutritionally-required levels," PMA commented. Since at least one regulatory agency has acknowledged that "for at least some chemicals thresholds do exist," PMA said, it is "important to state this fact and to allow scientists the flexibility of case-by-case analysis as to whether the concept applies to a particular substance." OSTP published a draft document of its review of the science and associated principles of chemical carcinogens in the May 22 Federal Register. The purpose of the document is to "articulate a view of carcinogenesis that scientists generally hold in common today," and to derive a series of principles "that can be used to establish specific guidelines for assessing carcinogenic risk," OSTP explained. One of OSTP's principles states that "agents found carcinogenic in animal studies . . . are considered suspect human carcinogens." PMA stated that, as currently worded, "the principle could be misconstrued to imply that, once a chemical is associated with cancer in animals, it must forever be considered a suspect human carcinogen." The assn. recommended the principle be revised to state "agents found carcinogenic in animal studies . . . should be further evaluated for their potential carcinogenicity in humans, thus permitting a more definitive classification." With respect to high dosing in long-term animal studies, PMA said that while OSTP's document "contains strong inferences that excessive dosage can confuse interpretation, little attention is given to the fact that the animal model can be so disrupted by overdosage that the findings may have no application to the test animal itself in a normal physiological state and certainly none to man." The assn. recommended that test doses exceeding human exposure levels be considered appropriate "as long as the doses are not so excessive that the data are confounded."
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