In vivo enhancement of 5-fluorouracil cytotoxicity to AKR leukemia cells by thymidine in mice.

The spleen colony assay was used to examine the effect of thymidine (dThd) on 5-fluorouracil (FUra) cytotoxicity in two transplantable leukemias, AKR (in AKR mice) and L1210 [in (BALB/c x DBA/2)F1 mice], in vivo. A large dose of dThd (10 mg/mouse) could not rescue these cell lines from FUra toxicity. Instead, when dThd was given within 1 hour before FUra, it enhanced FUra cytotoxicity by a factor between 100 and 1,000 in AKR leukemia. That dThd increased the cytotoxicity of FUra only by a factor of 3 in L1210 leukemia suggested a different mechanism of interaction of the two drugs in the two cell lines. Examination in hybrid mice capable of supporting the growth of both leukemias showed the enhancement to be tumor related rather than host related. We also demonstrated a dose-dependent effect of dThd injection 15 minutes before FUra in AKR leukemia. Concerning the kinetics of killing of AKR leukemia colony-forming units (LCFU) following the administration of dThd 15 minutes before FUra, LCFU survival continued to decrease for 24--36 hours following drug administration.