The basis for the antitumor activities of the exocyclic amino
nucleosides 4-amino-(ARPP) and 4-methoxy-8-(D-ribofuranosylamino)pyrimido[5,4-d]
pyrimidine (
MRPP) was investigated. The primary target of these
nucleosides appeared to be 5-phospho-alpha-D-ribofuranose-1-pyrophosphate (
PRPP) synthetase. MRPP-5'-monophosphate was a competitive inhibitor (Ki = 40 microM) of the activation of this
enzyme by the cofactor
inorganic phosphate (K alpha = 2.2 mM). Consequently, ARPP and
MRPP treatment of WI-L2 cultures rapidly inhibited both de novo
pyrimidine and
purine synthesis as well as the
nucleotide salvage reactions dependent on PRPP, ARPP or
MRPP treatment completely prevented [14C]
bicarbonate incorporation into
acid-soluble
pyrimidine and
purine nucleotides. The rate of salvage of [8-14C]
hypoxanthine to form
IMP was decreased by 85%. Treatment of cells with these agents caused a 50% reduction in the steady-state level of PRPP. When the capacity of the treated cells for sustained synthesis of PRPP was examined by
adenine incorporation, the rate of
adenine uptake was inhibited by greater than 50%. In vivo treatment of BDF1 mice with a single dose of ARPP (173 mg/kg) or
MRPP (62 mg/kg) extended the mean life span of the mice, which had been inoculated intraperitoneally 1 day earlier with 1 x 10(6) L1210 murine
leukemia cells, by 62 and 82% respectively. These studies indicate that
MRPP and ARPP inhibit
PRPP synthetase, and that
PRPP synthetase may be a viable target in the development of certain
antitumor agents.