Effect of l-Histidinol on the Metabolism of 5-Fluorouracil in the BALB/c × DBA/8 F1 Murine Tumor System

l-Histidinol, a structural analogue of histidine, which transiently inhibits proliferation, can protect cells from the toxic effects of proliferation-dependent chemotherapeutic agents such as 5-fluorouracil (FUra). In the BALB/c × DBA/8 F1 (hereafter called CD8F1) murine tumor system, l-histidinol protected mice from FUra-induced leukopenia, weight loss, and mortality; however, the therapeutic index was not improved since l-histidinol also protected the tumor against the toxic effects of FUra. In order to understand the mechanism of this protection, we examined the effects of l-histidinol on the metabolism of FUra. Results indicate that l-histidinol had no effect on the phosphoribosyl pyrophosphate levels in tumor, the activation of FUra to nucleotides or levels of free 5-fluorodeoxyuridine monophosphate in either tumor or bone marrow. l-Histidinol (7 mg/mouse, every 2 h for 5 doses) reduced RNA and DNA synthesis, as measured by 32P incorporation in vivo , by approximately one-half in tumor, and by 70% in bone marrow. This in turn resulted in reduced incorporation of FUra into RNA in both tumor and bone marrow. At 2 h, 4 h, and 24 h after FUra administration the level of FUra in RNA was 24–37% less in both tumor and bone marrow of mice that received l-histidinol with FUra. Using 32P as a monitor of overall RNA synthesis, the [3H]FUra/32P ratio remained unchanged, suggesting that the reduction of FUra incorporation into RNA was due to decreased RNA synthesis rather than a decrease in the number of FUra molecules per RNA chain. In contrast, l-histidinol had no effect on the in vivo inhibition of thymidylate synthetase by 5-fluorodeoxyuridine monophosphate as measured by the incorporation of [3H]-2′-deoxyuridine into DNA or on the percentages of thymidylate synthetase in the free versus 5-fluorodeoxyuridine monophosphate-bound state. We conclude that l-histidinol reduces FUra toxicity by reducing FUra incorporation into RNA. Since the major mechanism of action in the CD8F1 breast tumor system is the incorporation of FUra into RNA, the reduction in toxicity and antitumor activity observed when l-histidinol is combined with FUra is consistent with the observed reduction in tumor and bone marrow RNA containing incorporated FUra residues.