In its evaluation, IARC considers experimental evidence of carcinogenicity but gives priority to human epidemiologic evidence.

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However, a team of FDA toxicology experts reviewed the lorcaserin NDA and interpreted the data related to the lorcaserin carcinogenicity studies.

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And there is no animal evidence either that it is a carcinogen, he added, saying that "phenylbutazone is not classifiable as to its carcinogenicity" according to the International Agency for Research on Cancer.

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Arena is going to need to agree with the FDA on an independent pathologist to figure out a path forward on the rat carcinogenicity studies it has already done and possibly do more animal experiments.

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He said there was "no convincing evidence" of phenylbutazone's carcinogenicity in humans "because in the individuals studied many other drugs had also been taken and any one of these might have caused the cancers seen".

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Moreover, the lorcaserin carcinogenicity studies were also thoroughly reviewed and discussed by the Center for Drug Evaluation and Research (CDER) Executive Carcinogenicity Assessment Committee, and the results of this scientific assessment were included in the background documents provided to the committee and discussed in Agency presentations at the Advisory Committee meeting.

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