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

Abstract

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.