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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
Pentamidine isethionate (PIT) is an aromatic diamidine which was synthesized originally for the therapy of trypanosomiasis. It has found increasing usefulness in the treatment of pneumocystosis. Pentamidine isethionate has been effective in treating AIDS patients for pneumocystis carinii pneumonia. It is for this reason that it was selected for immunotoxicity studies. The objective of this study was to determine if pentamidine isethionate, administered subcutaneously, altered the immune status. NTP Tier I & selected Tier II assays were performed. Female C57Bl/6 mice were treated with 10, 17 or 25 mg/kg pentamidine isethionate for 5 days a week for four weeks by the subcutaneous route. Since pentamidine isethionate accumulates in the tissues and appears in the urine for six to eight weeks after cessation of therapy, the lymphoid organs were most likely to be continuously exposed to the drug. Since the immune system is targeted by the HIV virus a! nd because antiviral therapy is being used after detection of antibody to HIV, detecting and understanding potential actions on the immune system is important.
The objective of this study was to determine the potential of pentamidine isethionate to compromise the immune system. Pentamidine isethionate was administered by the subcutaneous injection daily for 5 days per week for 4 weeks. The studies were divided into standard toxicology studies, which provided the base against which to compare the immunology studies, and the immunology studies, which provided data on specific cellular targets of the drug.
Executive Summary Table 1 (ES-1) shows a summary of the standard toxicology studies. Treatment with pentamidine isethionate in doses as high as 25 mg/kg/day for 22 days over a 30 day period were tolerated by adult female C57Bl/6 mice. There was less than 1% mortality from exposure and no overt signs of toxicity. There were no histopathologic findings in the lung, mesenteric lymph node, spleen, thymus, liver or kidney that could be attributed to pentamidine isethionate treatment. Body weight changes associated with pentamidine isethionate treatment were minimal. As expected, the comparative control, azathioprine, produced a significant weight loss from exposure to 10 mg/kg/day for 30 days. Two organs were affected by exposure to the high dose (25 mg/kg) of pentamidine isethionate. These included a 13% decrease in thymus weight and a 9% increase in kidney weight. Pentamidine isethionate treatment was associated with a decrease in the erythroid elements a! s seen in decreases in erythrocytes, hemoglobin, and hematocrit. The leukocyte elements were not significantly affected by pentamidine isethionate treatment. Bone marrow cellularity, DNA synthetic ability, and the number of CFU-M and CFU-GM stem cells were unaffected by treatment with pentamidine isethionate. Azathioprine produced the expected decrease in cellularity and number of stem cells with a concomitant compensatory increase in DNA synthesis. The no effect level for the standard toxicological parameters was not reached. Pentamidine isethionate affected the erythrocyte number at the lowest dose (10 mg/kg) tested.
Executive Summary Table 2 (ES-2) summarizes the immunology studies of mice treated with pentamidine isethionate. Treatment of mice with pentamidine isethionate produced no effects on indicators of humoral immunity, such as changes in B cell number, proliferative ability or ability to differentiate into antibody producing cells to a T-dependent antigen. T lymphocyte numbers were slightly decreased in pentamidine isethionate-treated mice but the decrease did not translate into functional changes. Spleen cell response to the T-dependent antigen was unaffected, indicating the regulatory T cells were similar to those from control mice. Proliferative capacity, as measured by response to T cell mitogens and to allogenic cells, was unaffected by pentamidine isethionate treatment. Differentiation and the killing mechanism of T cells were intact in mice treated with pentamidine isethionate as seen in a CTL response which did not differ from the control group. Ma! crophages, derived from the peritoneal cavity of mice treated with pentamidine isethionate, were more responsive to activation by gamma interferon than control mice which may be a response to the local granuloma produced at the injection site. Natural killer cell activity was unaffected by pentamidine isethionate treatment.
The no effect level for these studies was below 10 mg/kg and the erythroid elements were one of the most sensitive parameters. The immune system is not a target for toxicity by pentamidine isethionate.