Analysis of gene mutations and clastogenicity following short-term treatment with azathioprine in Muta (TM) Mouse

The mutagenicity and clastogenicity of the immunosuppressive drug azathiopr ine (AZA), a multitissue rodent carcinogen and IARC-classified human carcin ogen, was investigated using transgenic lacZ mice (Muta(TM)Mouse). Male ani mals (n = 5 per group) were dosed with AZA (10, 50, 100 mg/kg p.o. daily fo r 5 days), vehicle (n = 10), or the positive control, chlorambucil (15 mg/k g i.p., n = 3), and killed 24 hr or 25 days after the last treatment. Micro nucleus assays were performed with bone marrow (24-hr samples) or periphera l blood (24-hr and 25-day samples) and DNA was extracted from bone marrow a nd liver For gene mutation analysis at the transgenic lacZ locus. AZA induc ed 5.3-111.3-fold increases in %MNPCE (P < 0.01) in bone marrow compared wi th vehicle control, accompanied by 4.4-5.6-fold increases in %MNRETs (P < 0 .01) in peripheral blood. Chlorambucil caused a 14.5-fold increase in %MNRE T and there was evidence of significant stem cell toxicity in both positive control and AZA treatment groups. By day 25, however, there was evidence o f substantial recovery of the bone marrow as determined by the frequency of RET, and the %MNRET in all treatment groups was the same as the vehicle co ntrol. Analysis of lacZ MF showed 1.4-1.6-fold increases in AZA 24-hr bone marrow samples, increasing to similar to 2.0-fold above concurrent controls by day 25 (medium dose P < 0.05, high dose P < 0.01). For liver, there was a 2-fold increase in MF (P < 0.05) in the 24-hr sample at the highest dose only, and increases of 1.3-1.5-fold by day 25 in the medium (P < 0.05) and high (P = 0.055) dose groups, respectively. The positive control, chloramb ucil, induced 2-3-fold increases (P < 0.01) in mean MF in both bone marrow (25-day sample) and liver (24-hr and 25-day samples). These data confirm th e clastogenicity of AZA in the mouse, and show that this compound induces g ene mutations in bone marrow and liver, in vivo, at the highest dose and su pports the view that AZA is a genotoxic carcinogen. (C) 1999 Wiley-Liss, In c.