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 x 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.