Azathioprine-induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus

Management of squamous cell carcinoma in organ transplant recipients

The number of skin cancers continues to rise with well over one million cases of skin cancer expected in the United States this year. Optimal management depends on early detection and treatment. The consequences of skin cancer may be particularly devastating in organ transplant recipients. In this article, management of squamous cell carcinoma in the organ transplant recipient is discussed.

Azathioprine, a genotoxic agent to be considered non-genotoxic in man

Azathioprine, an immunosuppressive drug, has been used for 25 years. Azathioprine is rapidly converted into a number of metabolites after absorption. Maximum blood levels in experimental animals (mice) were 11.3 micrograms/ml after a dosage of 33.3 mg/kg. Generally, levels of less than 1 microgram/ml are found. As azathioprine is ineffective in hypoxanthine guanine phosphoribosyltransferase (HPRT)-deficient patients, it will be clear that for immunosuppressive activity azathioprine must be metabolised. Regarding mutagenic activity, its mutagenicity for bacteria seems irrelevant for man because the nitroimidazole moiety can be reduced by bacteria but not or hardly at all by mammalian tissues. So 6-mercaptopurine (a metabolite of azathioprine) and its metabolites should be regarded as the active compounds. In vitro azathioprine can induce chromosome aberrations and other cytogenetic events at high, non-physiological doses. However, in view of the low blood levels it is unlikely that azathioprine can induce chromosome aberrations in kidney transplant patients. It is more probable that azathioprine inhibits the elimination of such aberrant cells through its immunosuppressive activity. It should be pointed out that in microbial mutagenicity systems also, azathioprine concentrations that are not reached in patients are needed to obtain an increased mutation rate.

Induction of uterine hemangioendothelioma and lymphoma in (C57BL/6N x C3H/2N)F1 mice by oral administration of azathioprine

The tumorigenicity of 6-(1-methyl-4-nitro-5-imidazolyl)mercaptopurine (azathioprine), an immunosuppressant, was examined in (C57BL/6N X C3H/2N)F1 mice. Animals were divided into 6 groups with 50 mice in each group, and they were given powdered diet mixed with 0, 5 or 20 ppm azathioprine starting at 6 weeks and ending at 100 weeks of age. Female mice, given 20 ppm azathioprine, developed lymphomas and uterine hemangioendotheliomas at incidences of 12.5 and 14.6%, respectively, which were significantly higher than the incidences in control mice (P less than 0.05). Lymphoma and uterine hemangioendothelioma developed independently in different mice. On the other hand, the male mice given 20 ppm azathioprine had a significantly (P less than 0.05) lower incidence of hepatic tumor (0.5%) compared to the control mice (16%).

Absence of ecotropic or recombinant murine leukaemia virus in preleukaemic and leukaemic X-irradiated NZB mice

NZB mice X-irradiated with a single dose of 630 R when they were 1-month old developed a high incidence of histologically defined lymphocytic leukaemia 8--25 weeks later. We have screened for murine leukaemia viruses (MuLV) in the lymphoid tissues of 8 of these leukaemic mice, and in 8 "preleukaemic", apparently healthy NZBs killed 1 month post irradiation. Xenotropic, but not ecotropic or recombinant MuLV, was detected by in vitro co-cultivation of bone marrow, spleen and thymus with selectively permissive cell lines, followed by the immunofluorescence test for MuLV gs antigen, and the XC test. Our results are not consistent, therefore, with the concept that the factor causing the leukaemias was an oncogenic virus activated by X-irradiation.