Influence of pH on the uptake of 5-fluorouracil into isolated tumour cells

Abstract

Summary To investigate the possible dependence of 5-fluorouracil (5FU) uptake in tumours on the intra-(pH) and extracellular (pH.) pH, a pH gradient (ApH) was imposed across the plasma membrane of ascites tumour cells in vitro, similar to that known to occur in some solid tumours in vivo, by incubation in media of PHe 5-8. A 2:1 (intracellular/extracellular) accumulation of radiolabelled 5FU occurred after 5 min incubation of the cells with 0.5 mm 5FU at pHe of 5.0, 5.5 or 6.0. 5FU metabolism is slow under these conditions, and 5FU uptake was not affected by longer incubations up to 20 min, nor by the absence of a sodium gradient. pHi was estimated from the distribution of the weak acid, (Naguib et al, 1985). 5FU metabolism has been extensively studied by 19F magnetic resonance spectroscopy (MRS) in cultured cells, and in vivo in tumour-bearing animals and patients (for review see Findlay and Leach, 1994). 5FU cytotoxicity is caused by irreversible inhibition of thymidylate synthase (TS) via the generation of the anabolite 5-fluoro-2-deoxyuridine monophosphate (FdUMP) leading to an inhibition of DNA synthesis, and by incorporation of 5-fluorouridine-5-triphosphate (FUTP) into RNA (FU-RNA). The degree of sensitivity to either, or both, of these cytotoxic nucleotides is tissue dependent (Heidelberger et al, 1983). TS inhibition tends to be favoured by prolonged exposure of low 5FU concentrations (approximately 15 giM) for several days, whereas FU-RNA formation tends to be favoured by brief exposures to high concentrations (1 mM) for a few hours (see Aschele et al, 1992). The intracellular concentrations of 5FU and its metabolites are also dependent on the transport and uptake of 5FU into the target cells (Heidelberger et al, 1983).'9F-MRS studies have shown that 5FU appears to be retained longer in tumours than in normal tissues Received 10 March 1997 Revised 11 August 1997 Accepted 20 August 1997 Correspondence to: M Stubbs Guerquin-Kem et al, 1991;Findlay et al, 1993). Enhanced retention has also been shown to be significantly associated with response Findlay et al, 1993), perhaps because higher concentrations of 5FU will sustain its anti-tumour effects by favouring the lasting presence of toxic metabolites at target tissue sites (Peters et al, 1993). The elimination half-lives (t112) of 5FU in the VX2 tumour of the rabbit , and the Walker 256 adenocarcinoma of the rat (El-Tahtawy and Wolf, 1991), were found to be about 1 h (63-73.2 and 42.2-59.4 min respectively), and greatly exceed the t 12 of 5FU in rat plasma (Au et al, 1983), which is similar to that reported in humans (5-15 min) (Cohen et al, 1982). Presant et al (1994) found that the response of patients to 5FU could be predicted from the rate of loss of tumour 5FU signal, measured by '9F-MRS. These observations suggest that elucidation of the mechanisms by which 5FU accumulates in tumours could be of clinical significance as they may aid the development of more rational combination chemotherapy.The pH of tumours measured non-invasively with 31...

Serial In vivo Spectroscopic Nuclear Magnetic Resonance Imaging of Lactate and Extracellular pH in Rat Gliomas Shows Redistribution of Protons Away from Sites of Glycolysis

“…These differences seem to favor tumor growth and metastasis (2)(3)(4)(5)(6) and can also be exploited to target therapies (7)(8)(9)(10)(11). Inhibition of pH-regulating mechanisms in tumor cells can reduce proliferation, invasiveness, and metastasis (2,(12)(13)(14).…”

Cytosine deaminase and thymidine kinase gene therapy in a Dunning rat prostate tumour model: absence of bystander effects and characterisation of 5-fluorocytosine metabolism with 19F-NMR spectroscopy

“…[48][49][50] Since the solubility of 5-FU increases with increasing pH (pK a = approximately 7.7 for the equilibrium between neutral and negatively charged species 39 ), a higher pH within tumour cells compared with the interstitium can result in intracellular trapping. 51 Preliminary 19 F-NMR measurements with the pH indicator 6-fluoropyrodoxal (unpublished) for our AT-1 tumour model indicate that ⌬pH = approximately -0.4 in vivo.…”

VatuximabTM: Optimizing Therapeutic Strategies for Prostate Cancer Based on Dynamic MR Tumor Oximetry

“…However, to date the method has not been translated to the routine clinical setting, since most clinical MRI scanners lack a 19 F capability. PFCs have been observed in patients following administration as adjuvant to radiotherapy and as residues in the eye, where they are used as tamponades during retinal surgery (127,137,138 (140)(141)(142)(143). Even then, R 1 sensitivity to oxygen is low (B'= 0.0002 s -1 /torr) and to temperature is high.…”

“…In tumors with lower pH, the retention of 5FU is considerably enhanced [139][140][141]. This has prompted 216 J.-X.…”