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Purpose: We have shown previously that carbogen (95% O2, 5% CO2) breathing by rodents can increase uptake of anticancer drugs into tumours. The aim of this study was to extend these observations to other rodent models using the anticancer drug 5-fluorouracil (5FU). 5FU pharmacokinetics in tumour and plasma and physiological effects on the tumour by carbogen were investigated to determine the locus of carbogen action on augmenting tumour uptake of 5FU. Methods: Two different tumour models were used, rat GH3 prolactinomas xenografted s.c. into nude mice and rat H9618a hepatomas grown s.c. in syngeneic Buffalo rats. Uptake and metabolism of 5FU in both tumour models with or without host carbogen breathing was studied non-invasively using fluorine-19 magnetic resonance spectroscopy (19F-MRS), while plasma samples from Buffalo rats were used to construct a NONMEM pharmacokinetic model. Physiological effects of carbogen on tumours were studied using 31P-MRS for energy status (NTP/Pi) and pH, and gradient-recalled echo magnetic resonance imaging (GRE-MRI) for blood flow and oxygenation. Results: In both tumour models, carbogan-induced GRE-MRI signal intensity increases of ∼60% consistent with an increase in tumour blood oxygenation and/or flow. In GH3 xenografts, 19F-MRS showed that carbogen had no significant effect on 5FU uptake and metabolism by the tumours, and 31P-MRS showed there was no change in the NTP/Pi ratio. In H9618a hepatomas, 19F-MRS showed that carbogen had no effect on tumour 5FU uptake but significantly p=0.0003) increased 5FU elimination from the tumour i.e. decreased the t½) and significantly (p=0.029) increased (53%) the rate of metabolism to cytotoxic fluoronucleotides (FNuct). The pharmacokinetic analysis showed that carbogen increased the rate of tumour uptake of 5FU from the plasma but also increased the rate of removal. 31P-MRS showed there were significant p ≥ 0.02) increases in the hepatoma NTP/Pi ratio of 49% and transmembrane pH gradient of 0.11 units. Conclusions: We suggest that carbogen can transiently increase tumour blood flow, but this effect alone may not increase uptake of anticancer drugs without a secondary mechanism operating. In the case of the hepatoma, the increase in tumour energy status and pH gradient may be sufficient to augment 5FU metabolism to cytotoxic FNuct, while in the GH3 xenografts this was not the case. Thus carbogen breathing does not universally lead to increased uptake of anticancer drugs. [ABSTRACT FROM AUTHOR]