Analysis of gene mutations and clastogenicity following short-term treatment with azathioprine in MutaMouse.

The mutagenicity and clastogenicity of the immunosuppressive drug azathioprine (AZA), a multitissue rodent carcinogen and IARC-classified human carcinogen, was investigated using transgenic lacZ mice (MutaMouse). Male animals (n = 5 per group) were dosed with AZA (10, 50, 100 mg/kg p.o. daily for 5 days), vehicle (n = 10), or the positive control, chlorambucil (15 mg/kg i.p., n = 3), and killed 24 hr or 25 days after the last treatment. Micronucleus assays were performed with bone marrow (24-hr samples) or peripheral blood (24-hr and 25-day samples) and DNA was extracted from bone marrow and liver for gene mutation analysis at the transgenic lacZ locus. AZA induced 5.3-111.3-fold increases in %MNPCE (P < 0.01) in bone marrow compared with 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 %MNRET and there was evidence of significant stem cell toxicity in both positive control and AZA treatment groups. By day 25, however, there was evidence of 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 control. Analysis of lacZ MF showed 1.4-1.6-fold increases in AZA 24-hr bone marrow samples, increasing to approximately 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, chlorambucil, 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 the clastogenicity of AZA in the mouse, and show that this compound induces gene mutations in bone marrow and liver, in vivo, at the highest dose and supports the view that AZA is a genotoxic carcinogen.

Comparative in-vivo genotoxicity of antiviral nucleoside analogues; penciclovir, acyclovir, ganciclovir and the xanthine analogue, caffeine, in the mouse bone marrow micronucleus assay.

Three purine nucleoside analogues, penciclovir (PCV), acyclovir (ACV) and ganciclovir (GCV), were assessed for in-vivo genotoxicity in the mouse bone marrow micronucleus assay, together with the xanthine (purine) analogue, caffeine (CAF). All these compounds exhibit anti-viral properties and the first three are marketed anti-viral drugs. All have been shown to be genotoxic in separate in-vitro and in-vivo studies. Because of their widespread use, we considered it important to directly compare their relative in-vivo genotoxic potencies as an aid to assessing their relative genotoxic risk to humans. Accordingly, two-dose (0 and 24 h)/single sample mouse micronucleus assays were performed on all four compounds. PCV and ACV appeared to give essentially arithmetic increases in induction of micronucleated polychromatic erythrocytes (MNPCE) with arithmetic increases in dose with apparent thresholds at approx. 1078 mumols/kg per day and 316 mumols/kg per day, respectively. The dose-response curve for GCV appeared more exponential, without a threshold, but with a no-effect dose of around 150 mumols/kg per day. With CAF, systemic toxicity allowed the assessment of only very weak effects, such that our estimate of a no-effect dose of 388 mumols/kg per day is subject to large errors. Taking into account magnitude of response, slope of dose-response curve and no-effect doses, the order of potency was GCV > ACV > (CAF?) > PCV. The relevance of these findings in terms of risk is uncertain.

Is polyploidy an important genotoxic lesion?

The importance of polyploidy as a genotoxic lesion is uncertain and there have been few publications and no reviews which have included data on spontaneous or induced polyploidy in routine genotoxicity screening. We have attempted to clarify some of the issues by reviewing the published literature and by reference to our historical data base for metaphase analysis of cultured human lymphocytes. In our studies on pharmaceutical compounds polyploidy was the lesion most often found, being induced by approximately 40% of the compounds tested. The mean spontaneous frequency was between 0.1 and 0.3%, and values for polyploidy induction were 5-fold to > 100-fold the spontaneous value. Spontaneous polyploids tended to be near-exact multiples of the haploid chromosome number whereas induced 'polyploids' were, in fact, very heteroploid with a wide range of chromosome numbers. Polyploidy induction often occurred at non-toxic concentrations, usually there were well defined no-effect (threshold) levels and it was unrelated to other genetic effects. Such observations would be expected for inducers of polyploidy because the target molecules are not DNA and for these non-DNA targets there is usually a degree of redundancy. Therefore, inducers of polyploidy are only likely to be a hazard for humans if they are positive at or below therapeutic concentrations. We conclude that polyploidy/near-polyploidy (shown as 'polyploidy' throughout) should be scored as accessory data which becomes important only when induction occurs at therapeutic levels.

Disappearance of immunoreactive alpha-melanotrophin from rat plasma following occlusion of different regions of the vascular bed.

The half-life for the disappearance of immunoreactive alpha-melanotrophin in plasma following intravenous injection of synthetic hormone was measured before and after occlusion of the blood supply to certain organs in the anaesthetized rat. Occlusion of the blood supply to the liver, gut, spleen and pancreas, and of the renal circulation caused non-significant increases in half-life of 2.8 and 10.7% respectively. The importance of peripheral tissues in the clearance of alpha-melanotrophin was demonstrated by the significant 63.4% increase in half-life caused by occlusion of the blood supply to sections of skeletal muscle, fat and skin.