Rodent carcinogenicity tests need be no longer than 18 months: An analysisbased on 210 chemicals in the IARC monographs

The IARC Monographs (Vols 1-70) were studied to determine the time of onset of treatment-related tumorigenicity in long-term rodent studies for chemic als classified by IARC as having sufficient evidence of carcinogenicity in animals. The analysis excluded studies on metals and their salts, studies o n particulates, studies by parenteral routes of administration that resulte d in tumours only at the site of exposure, and studies that did not approxi mate to the current standard long-term rodent carcinogenicity bioassay, for instance transplacental or multigeneration studies, initiator-promoter stu dies, lung tumour essays in Strain A mice and studies in newborn animals. D ata from a total of 210 chemicals revealed that, overall, evidence of treat ment-related tumorigenicity was first apparent within 12 months for 66% of the chemicals and for only 7% were studies of longer than 18 months necessa ry. All IARC Group 1 chemicals were detected in animals within 18 months, a nd most within 12 months. Most of the tumour types that required more than 113 months for detection were of dubious relevance to human risk assessment . Termination of rodent carcinogenicity studies at 18 months or earlier wou ld greatly reduce the complications that arise in interpreting the findings in aged animals which often have defective hepatic or renal function and w ould also markedly reduce the time required for histopathological examinati on of dozens of tissues taken from the approximately 500 animals routinely employed in these studies, (C) 2000 Elsevier Science Ltd. Ail rights reserv ed.