Evaluation of single-dose RBC Pig-a and PIGRET assays in detecting the mutagenicity of thiotepa in rats

•We evaluated the PIGRET and RBC Pig-a assay in detecting thiotepa mutagenicity.•Rats were treated with thiotepa by single oral administration.•Same blood samples from the animals were used for both assay methods.•Increases in Pig-a mutant frequencies were detected only by the PIGRET assay.

The Pig-a assay, which uses reticulocytes (PIGRET assay) as target cells, is anticipated to detect mutagenicity at earlier time points than the RBC Pig-a assay, which uses all red blood cells as target cells. As part of a collaborative study conducted by the Mammalian Mutagenicity Study (MMS) Group, we evaluated the PIGRET and RBC Pig-a assays to detect Pig-a gene mutations induced by the carcinogen thiotepa.A single dose of thiotepa at 7.5, 15, and 30 mg/kg was administered to 8-week-old male Sprague-Dawley rats by oral gavage. PIGRET and RBC Pig-a assays were performed using peripheral blood collected from rats 7, 14, and 28 days after thiotepa administration (Day 0 as the day of administration), and the resulting Pig-a mutant frequencies (MFs) were compared. Increased Pig-a MF was observed from Day 7 onwards using the PIGRET assay. Pig-a MF remained fairly constant thereafter until Day 28 in the 30 mg/kg group, whereas it peaked on Day 14 in the 7.5 and 15 mg/kg groups. Using the RBC Pig-a assay, on the other hand, no significant increase in MF was observed at any of the dosages on Days 7, 14, or 28.These findings show that Pig-a gene mutations following a single dose of thiotepa were detected using the PIGRET assay but not the RBC Pig-a assay, which suggests that PIGRET assay is more suitable than RBC Pig-a assay for evaluating the in vivo mutagenicity by a single dose.