Pyrazofurin (PYF), a C-riboside, inhibited the replication of cultured Novikoff rat
hepatoma cells, HeLa cells, and mouse L-cells at concentrations as low as 0.1 to 10 muM, but Novikoff cells were more sensitive than the cells of the other two cell lines. Inhibition of cell replication was completely prevented by the presence of 0.1 to 1 mM
uridine in the medium, and partly by the presence of other
pyrimidine, but not
purine nucleosides. A 2- to 4-hr treatment of the cells with 10 muM PYF resulted in a 2-fold increase in the rate of incorporation of
uridine into the
acid-soluble pool and
nucleic acids, while the rate of incorporation of
adenosine into
RNA was reduced about 85%. The incorporation of
adenosine and
deoxyuridine into
DNA were reduced about 85 and 50%, respectively. The results are consistent with the view that PYF inhibits the de novo synthesis of
pyrimidine nucleosides. The inhibition of cell replication seems to be due mainly to an inhibition of
DNA rather than
RNA synthesis, resulting from a rapid depletion of the
pyrimidine deoxynucleotide pool, since addition of
thymidine and
deoxycytidine reversed the inhibition of
DNA synthesis and cell replication by PYF. PYF must enter the cells to exert its toxicity since the toxicity of PYF was reduced 70 to 80% by the presence of 8 muM
Persantin, a potent inhibitor of the facilitated and simple diffusion of various substrates, in the medium. If PYF is incorporated via normal
nucleoside salvage pathways, its affinity for the
nucleoside transport system(s) and
kinases, must be low since, even at a concentration of 1 mM, it had only a slight effect on the initial rates of incorporation of various
nucleosides into the
nucleotide pool.