Pyrimethamine (5-[4-chlorophenyl]-6-ethyl-2,4-pyrimidinediamine,PYM) is a folic acid antagonist with antiparasitic activity commonlyused in combination with sulfadiazine to treat toxoplasmosis inAIDS patients also receiving 3'-azido-3'-deoxythymidine (AZT).At doses used in the treatment of toxoplasmosis, there may beleukopenia, thrombocytopenia, pancytopenia, and hypersensitivityreactions including erythema multiforme, skin eruptions, uticaria, anaphalactoid reactions, and several other allergic responses. In addition, previous studies by Luster et al.demonstrated that some of the anti-HIV drugs have adverse effectson the immune system. These adverse responses raise a concern that PYM and AZT in combination may have adverse effects on theimmune system.
3'-Azido-3'-deoxythymidine and pyrimethamine were nominated to the NTP for toxicological evaluation and were selected for immunotoxicity studies by the chemical manager. Thus, the purpose of these studies was to determine the potential effects of 3'-azido-3'-deoxythymidineand pyrimethamine on the immune system.
The study reported here is an initial range-finding study conductedin female B6C3F1 mice. The animals weretreated daily for 14 days by oral gavage alone or in combination. In-life phase of the study was conducted between 9 September 1994 and 23 September 1994. PYM and AZT were prepared as solutions in Maalox Therapeutic Concentrate (T.C.).
The baseline toxicology studies are summarized in Table ES-1. In this range-finding study of orally administered PYM at 2.5, 5 and 10 mg/kg/day along with AZT at 200 mg/kg/day, AZT at 200mg/kg/day alone, or PYM at 10 mg/kg/day for 14 days, there were no effects on body weight or rate of weight gain. PYM+AZT had no effect on organ weights. The erythrocyte count, hemoglobin, and hematocrit were decreased by PYM+AZT and by AZT alone while the MCV and MCH were slightly increased. Reticulocyte and leukocyte counts were unaffected. The number of bone marrow cells per femur was increased by PYM and AZT given alone, but in combination there was no effect. DNA synthesis was reduced by PYM but not by AZT,and the high dose of PYM+AZT stimulated DNA synthesis.
Table ES-2 summarizes the immunology studies. PYM alone did not significantly affect the AFC response. AZT at 200 mg/kg reduced the IgM AFC response by about one-third as did the samedose level of PYM+AZT. Neither PYM nor AZT altered the serum titerof anti-sRBC, but the high dose combination reduced the serumtiter by one-third. AZT increased the proliferative response ofresponder cells and responder+stimulator as did the high doseof PYM+AZT. Both PYM and AZT stimulated NK cell activity withPYM being 2-3 times as active as AZT at the doses used. PYM+AZTincreased NK cell activity 40-50%.
Overall, combined treatment with PYM and AZT in the dose rangeof about 10 mg/kg/day PYM with 200 mg/kg/day AZT for 14 days hadmoderate effects on the immune system without serious systemictoxicity. There was an impaired AFC response coupled with a one-third reduction in the anti-sRBC serum titer. NK cell activity per 107spleen cells was stimulated by combination treatment.