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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
3'-Azido-3'-deoxythymidine, also known as Zidovudine, is one of a class of drugs called nucleoside reverse transcriptase inhibitors, which are among the most widely used therapeutic agents for the treatment of acquired immune deficiency syndrome and human immunodeficiency virus. Because HIV can infect immune cells such as CD4+ T cells, it is important to establish if drugs designed to treat the infection can impact the various components of the immune system. AZT exposure for 7 to 14 days in female mice has been demonstrated to affect the differentiation of thymus cells at daily doses greater than or equal to 500 mg/kg (McKallip et al., 1995). In that same study, the authors reported that AZT exposure produced no effects on the cellularity or various phenotypes of the spleen. At the present time, the effects of HIV/AIDS drugs on the developing immune system have not been established.
The National Toxicology Program requested that a study be conducted to evaluate effects of perinatal AZT exposure on the immune system. These developmental immunotoxicology studies were conducted in female B6C3F1 mice. C57BL/6 dams were exposed to AZT twice daily by oral gavage (134, 266, or 400 mg/kg/day), beginning one day after the conclusion of a 5-day mating period with C3H males. The first day of AZT treatment in the dams was designated as gestational day 6. Dams continued to receive AZT treatment until the B6C3F1 offspring were weaned on post-natal day 21. The B6C3F1 offspring were exposed via lactation and treated with a single dose of AZT by oral gavage from PND4 to PND20. After weaning on PND21 until the day of study termination (between PND49 and PND52), the B6C3F1 offspring were treated twice daily with AZT by oral gavage. AZT was prepared weekly as a solution in the vehicle 0.2% methylcellulose with 0.1% Tween 80. No usable sheep red blood cell high-salt release membrane antigen was available to evaluate serum immunoglobulin M anti-sRBC antibody levels. No other significant protocol or standard operating procedure deviations occurred during the study that affected the quality of the data and the ability to interpret the data with respect to the developmental immunotoxicity of AZT.
Exposure to AZT in utero and/or through lactation resulted in no significant effects on body weights at PND4. Significant decreases in both body weight and body weight gain were observed at PND9 in pups exposed to AZT at 400 mg/kg/day. This decrease was not significant at PND16. Terminal body weights were decreased at the 400 mg/kg/day dose in all studies, although the decrease in one study did not reach the level of statistical significance. Combined terminal body weights from the three studies were decreased 10% at the 400 mg/kg dose. Overall, the weights of the liver, spleen, thymus, lungs, and kidneys with adrenals were unaffected. Erythrocyte numbers were significantly decreased and mean corpuscular volume was significantly increased at all doses ≥ 134 mg/kg/day. In addition, mean corpuscular hemoglobin and blood platelet concentrations were significantly increased at the 134 and 266 mg/kg/day dose levels, but not at the 400 mg/kg/day dose. Reticulocytes were significantly increased, while blood leukocyte numbers and absolute numbers of lymphocytes in the peripheral blood were decreased at the 266 mg/kg/day dose only.
Spleen cell numbers and phenotypes were generally unaffected, with the exception of dose-related decreases in both the absolute number and the percentage of B cells. The humoral immune response, as evaluated using the antibody-forming cell assay was significantly decreased at the 400 mg/kg/day dose level when evaluated as Total Spleen Activity but not when evaluated as Specific Activity. No effects were observed on the mixed leukocyte response or anti-CD3-mediated spleen cell proliferation (measures of cell-mediated immunity) or on the cytolytic activity of natural killer cells, which is a measure of innate immunity. Bone marrow cell numbers were unaffected, while DNA synthesis of bone marrow cells was significantly decreased at all AZT doses ≥ 134 mg/kg/day. The numbers of the various bone marrow colony-forming units, i.e., CFU-GM, CFU-M, and CFU-E, were unaffected.
In conclusion, developmental exposure to AZT resulted in minimal effects on the immune system at doses up to 400 mg/kg/day in female B6C3F1 mice.
McKallip R.J., Nagarkatti M., & Nagarkatti P.S. (1995). Immunotoxicity of AZT: inhibitory effect on thymocyte differentiation and peripheral T cell responsiveness to gp120 of human immunodeficiency virus. Toxicol Appl Pharmacol, 131:53-62.