The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of the NTP or the U.S. Government.
2',3'-Dideoxyadenosine (ddA), one of several nucleoside analogs with in vitro anti-HIV activity, was considered as a potential candidate for the treatment of patients with Acquired Immunodeficiency Syndrome (AIDS). The compound is rapidly converted in vivo to dideoxyinosine and in phase I clinical trials has been shown to cause nephritis. Since AIDS patients already represent an immunologically compromised population, it is important to establish the immunotoxicity of any potential AIDS therapy. This study was performed to determine the potential effects of ddA on the immune system of mice.
ddA was provided by the National Cancer Institute (RK-04-243 and NSC-98700, Lot 231-B-2) and was greater than 99% pure as determined by HPLC. The drug was prepared in sodium acetate buffer, pH 7.0, to which was added 0.5% methylcellulose. Female C57BL/6 mice were dosed for 22 days over a 30 day period with either vehicle, 87.5, 175, or 350 mg/kg of ddA by gastric intubation in a volume of 0.1ml/10g body weight. A positive control group was employed and received 3 mg/kg of methotrexate (Sigma Chemical Corp.) administered intraperitoneally at the same time. For each of the immune parameters examined, 7-8 mice per treatment group were used unless indicated otherwise in the tables.
There were no treatment-related mortalities or histopathologic changes associated with ddA exposure. However, body and thymus weights in the high dose group were significantly decreased while kidney weights were increased (data not shown). The immune and bone marrow studies are summarized in Table 1. ddA produced a dose-dependent decrease in the IgM antibody PFC response to sheep erythrocytes. Antibody suppression occurred in the presence of decreased numbers of splenic B lymphocytes but in the absence of any effect on the proliferative responsiveness to mitogens (Con A, PHA, or LPS), allogeneic cells (MLR) or anti Ig/IL4. The cytotoxic T cell and natural killer cell responses were also unaffected by ddA treatment. Thus, the suppression of the IgM antibody response may be associated with a specific effect on B lymphocytes. Bone marrow parameters were unaffected by ddA except for increases in macrophage colony formation and bone marrow cellularity observed in the 350 mg/kg treatment group.
The host resistance data are presented in Table 2. Mice dosed with ddA showed an increased resistance to Listeria monocytogenes. This protection may be associated with an increase in macrophage activity since an increase in macrophage tumoricidal activity was observed in treated mice (data not shown). The decreased resistance to Streptococcus pneumoniae may be associated with the decrease in humoral immunity. There was no change in resistance to B16F10 melanoma in ddA treated mice.
Under these experimental conditions, dideoxyadenosine had a selective action on the immune system with specificity for humoral immunity.