Enhanced 5-fluorouracil cytotoxicity and elevated 5-fluoronucleotides in the rat Walker carcinosarcoma following methotrexate pre-treatment: a 19F-MRS study in vivo.

5-fluorouracil (5FU) is activated intracellularly to cytotoxic 5-fluoronucleotides (FNuct). These were detected non-invasively in rats bearing the Walker carcinosarcoma by 19F-magnetic resonance spectroscopy (MRS) following an i.v. bolus dose of 5FU (50 mg kg-1). Pre-treatment of the rats (3 to 24 h earlier) by methotrexate (MTX) (20 or 50 mg kg-1) did not affect the rate of 5FU disappearance but did significantly increase the rate of FNuct formation (P less than 0.002) and the final amount formed (P less than 0.02) as assessed by MRS in vivo. MTX (20 mg kg-1) caused substantially the same effects on FNuct formation (P less than 0.002 for rate and P less than 0.05 for the amount) when 5FU was administered i.p. although higher doses of 5FU (120 mg kg-1) were necessary to observe the 19F-signals. Quantitative analysis by MRS in vitro of extracts from the freeze-clamped tumours treated by 5FU i.v. confirmed that MTX pre-treatment increased FNuct formation 3-fold (P less than 0.05). Hplc quantitative analysis demonstrated that 50% of the FNuct was the cytotoxic nucleotide FUTP which was also increased 3-fold in MTX treated animals (P less than 0.05). Since the Walker tumour is probably sensitive to 5FU action via FUTP incorporation into RNA, these results suggested that drug regimes in which MTX preceded 5FU (MTX-5FU schedule) would be more cytotoxic that 5FU alone. At an MTX dose of 20 mg kg-1 24 h prior to 5FU there was significant inhibition of growth (P less than 0.05) compared to no treatment, MTX alone or the reverse schedule of 5FU-MTX. These results suggest MRS may be of clinical value in optimising chemotherapy using schedules where MTX precedes 5FU.