Title: A medium‐term rat liver bioassay for rapid <i>in vivo</i> detection of carcinogenic potential of chemicals
Abstract: A reliable medium‐term bioassay system for rapid detection of carcinogenic potential of chemicals in the human environment has been developed. The 8‐week‐protocol consists of 2 stages; male F344 rats are given a single intraperitoneal injection of diethylnitrosamine (200 mg/kg) for initiation of liver carcinogenesis, followed by a 6‐week test chemical treatment starting 2 weeks thereafter. Test chemicals are usually given in the diet or the drinking water and in the 2nd week of test chemical treatment, all rats are subjected to two‐thirds partial hepatectomy in order to induce regenerative cell replication. The end‐point marker is the glutathione S‐transferase placental form (GST‐P)‐positive hepatic focus, the numbers and sizes of which are analyzed using an image‐analyzer and expressed as values per unit liver section (1 cm 2 ). When the yield of GST‐P‐positive foci is significantly enhanced (P<0.05) over the control value, a chemical is judged to possess carcinogenic or promotion potential for the liver. Among 313 chemicals already tested in this system in our laboratory, 30/31 (97%) mutagenic hepatocarcinogens and 29/33 (88%) non‐mutagenic hepatocarcinogens gave positive results. Ten out of 43 (23%) agents known to be carcinogenic in organs other than the liver were also positive. It is particularly important that only one of 48 non‐carcinogens gave a very weak positive result, so that the system has a very low false‐positivity rate. It is now well documented that the assay system is highly effective for detecting hepatocarcinogens, bridging the gap between traditional long‐term carcinogenicity tests and short‐term screening assays. At the Fourth International Conference on Harmonization, our medium‐term liver bioassay based on an initiation and promotion protocol was recommended in the guidelines as an acceptable alternative to the long‐term rodent carcinogenicity test. (Cancer Sci 2003; 94: 3–8)