Synergistic Bone Marrow Toxicity of Pyrimethamine and Zidovudine in Murine in Vivo and in Vitro Models: Mechanism of Toxicity

Pyrimethamine (Pyr) is commonly used for treatment of toxoplasmic encephalitis in AIDS patients; however, in two clinical studies, an increased number of deaths were observed when Pyr was coadministered with zidovudine (ZDV). The BALB/c mouse was chosen as a model to study the mechanism underlying the unexpected toxicity from coadministration of these drugs. Daily administration by oral gavage of 60 mg/kg Pyr and 240 mg/kg ZDV resulted in 100% lethality after 30 days. These dose levels produced no effect when the drugs were given individually for the same period. Administration of combinations of Pyr and ZDV resulted in macrocytic anemia and leukopenia with synergistic decreases in lymphocyte and neutrophil numbers. To examine the mechanism of this hematotoxicity at the cellular level, mouse bone marrow colony-forming unit (mCFU) assays were employed. A combination of ZDV with various concentrations of Pyr resulted in synergistic decreases in numbers of erythroid and granulocyte-macrophage precursors (mCFU–E and mCFU–GM). mCFU–GM precursors appeared more sensitive than erythroid precursors to combinations of Pyr and ZDV. Incorporation of 14C-ZDV into cellular DNA was increased in a dose-dependent manner in the presence of increasing concentrations of Pyr in the mCFU–GM assay. This suggested that inhibition of dihydrofolate reductase by Pyr and accompanying inhibition of dTTP synthesis allows preferential incorporation of ZDV into DNA, with resulting strand breakage and cell death. 14C-ZDV incorporation was also observed when human GM cultures were analyzed, however, incorporation was less and required higher concentrations of Pyr.