Elevation of plasma levels of fluorinated pyrimidines by guanosine 5'-monophosphate.

The plasma concentration of 5-fluorouracil (FUra) following the i.v. administration of FUra and guanosine 5'-monophosphate (GMP) or guanosine 5'-triphosphate (GTP) was markedly elevated. These values were more than 5-fold higher than those obtained with FUra alone over 60 min after administration. The elevation of plasma levels corresponded to the dose of GMP. Higher levels of FUra were maintained in the plasma after injection of inosine or inosine 5'-monophosphate in combination with FUra than after FUra alone, but they were lower than those induced by GMP or GTP. Moreover, plasma levels of two other fluorinated pyrimidines, 5'-deoxy-5-fluorouridine (DFUR) and 5-fluoro-2'-deoxycytidine (FdCyd), were also elevated by GMP. The combination of DFUR and GMP resulted in higher plasma levels of DFUR itself and FUra (12- and 10-fold, respectively, 30 min after treatment). After administration of FdCyd plus GMP, the plasma levels of FdCyd, 5-fluoro-2'-deoxyuridine, which is converted from FdCyd by cytidine deaminase, and FUra were 2-, 6-, and 7-fold higher, respectively, than those after FdCyd alone 30 min after treatment. Thus, GMP is the most effective compound for the maintenance of high plasma levels of fluorinated pyrimidines.