Assignment and pH dependence of the 19F-NMR resonances from the fluorouracil anabolites involved in fluoropyrimidine chemotherapy

Synthesis of 5-Fluorouracil Polymer Conjugate and 19F NMR Analysis of Drug Release for MRI Monitoring

To monitor the release of fluorinated drugs from polymeric carriers, a novel 19F MRI enzyme-responsive contrast agent was developed and tested. This contrast agent was prepared by conjugation of 5-fluorouracil (5-FU) to hyperbranched poly(N,N-dimethylacrylamide) (HB-PDMA) via an enzyme-degradable peptide linker. Due to the different molecular sizes, the release of 5-FU from the 5-FU polymer conjugate resulted in a sufficiently substantial difference in spin-spin T2 19F NMR/MRI relaxation time that enabled differentiating between attached and released drug states. The 5-FU polymer conjugate exhibited a broad signal and short T2 relaxation time under 19F NMR analysis. Incubation with the enzyme induced the release of 5-FU, accompanied by an extension of T2 relaxation times and an enhancement in the 19F MRI signal. This approach is promising for application in the convenient monitoring of 5-FU drug release and can be used to monitor the release of other fluorinated drugs.

Methodological Developments for Metabolic NMR Spectroscopy from Cultured Cells to Tissue Extracts: Achievements, Progress and Pitfalls

This is a broad overview and critical review of a particular group of closely related ex vivo and in vitro metabolic NMR spectroscopic methods. The scope of interest comprises studies of cultured cells and excised tissue, either intact or after physicochemical extraction of metabolites. Our detailed discussion includes pitfalls that have led to erroneous statements in the published literature, some of which may cause serious problems in metabolic and biological interpretation of results. To cover a wide range of work from relevant research areas, we consider not only the most recent achievements in the field, but also techniques that proved to be valid and successful in the past, although they may not have generated a very significant number of papers more recently. Thus, this comparative review also aims at providing background information useful for judiciously choosing between the metabolic ex vivo/in vitro NMR methods presented. Finally, the methods of interest are discussed in the context of, and in relation to, other metabolic analysis protocols such as HR-MAS and cell perfusion NMR, as well as the mass spectrometry approach.

(19)F MRSI of capecitabine in the liver at 7 T using broadband transmit-receive antennas and dual-band RF pulses

Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. (19)F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non-invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi-channel transmit-receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of (19)F detection protocols. The antennas were broadband optimized to facilitate both the (1)H (298 MHz) and (19)F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B1(+) simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B1(+) and B1(-) information provided at the (1)H frequency for the optimization of B1(+) and B1(-) at the (19)F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual-band RF pulse was designed and evaluated. Finally, (19)F MRS(I) measurements were performed to detect (19)F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, (19)F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set-up for in vivo detection of metabolic rates and drug distribution in the body.

Development of an LC-MS/MS assay for the quantitative determination of the intracellular 5-fluorouracil nucleotides responsible for the anticancer effect of 5-fluorouracil

5-Fluorouracil (5-FU) and its oral prodrug capecitabine are among the most widely used chemotherapeutics. For cytotoxic activity, 5-FU requires cellular uptake and intracellular metabolic activation. Three intracellular formed metabolites are responsible for the antineoplastic effect of 5-FU: 5-fluorouridine 5'-triphosphate (FUTP), 5-fluoro-2'-deoxyuridine 5'-triphosphate (FdUTP) and 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP). In this paper, we describe the development of an LC-MS/MS assay for quantification of these active 5-FU nucleotides in peripheral blood mononuclear cells (PBMCs). Because the intracellular 5-FU nucleotide concentrations were very low, maximization of the release from the cell matrix and minimization of interference were critical factors. Therefore, a series of experiments was performed to select the best method for cell lysis and nucleotide extraction. Chromatography was optimized to obtain separation from endogenous nucleotides, and the effect of different cell numbers was examined. The assay was validated for the following concentration ranges in PBMC lysate: 0.488-19.9 nM for FUTP, 1.66-67.7 nM for FdUTP and 0.748-30.7 nM for FdUMP. Accuracies were between -2.2 and 7.0% deviation for all analytes, and the coefficient of variation values were ≤ 4.9%. The assay was successfully applied to quantify 5-FU nucleotides in PBMC samples from patients treated with capecitabine and patients receiving 5-FU intravenously. FUTP amounts up to 3054 fmol/10(6) PBMCs and FdUMP levels up to 169 fmol/10(6) PBMCs were measured. The FdUTP concentrations were below the lower limit of quantification. To our knowledge, this is the first time that 5-FU nucleotides were quantified in cells from patients treated with 5-FU or capecitabine without using a radiolabel.

Quantitative (19)F imaging of nmol-level F-nucleotides/-sides from 5-FU with T(2) mapping in mice at 9.4T

A unique acquisition method is proposed for quantitative, high-sensitivity (19)F MR spectroscopic imaging for the study of drug distribution aiming at nmol-level metabolite information in mice. The use of fast spin echo (FSE) at 9.4T allowed us to obtain whole-body images with minimal effect of magnetic susceptibility and to acquire several metabolite signals simultaneously by the method of interleaved multifrequency selection. Modified 2-shot FSE was designed for simultaneous, high-sensitivity (19)F imaging and T(2) mapping. A time course study including all the main metabolites at 10-minute resolution was attained with an oral dose of 1-2 mmol 5-fluorouracil (5-FU) (130-260 mg)/kg in mice. With acquisition parameters optimized for in vivo T(2) of 40 ms, images of F-nucleotides/-sides, effective anabolites of the anticancer drug 5-FU, were obtained at the level of 200 nmol in the tumor for all the mice studied with a linear correlation (R = 0.96) between image intensity and the quantity determined in the excised tissue. The method exhibits potential capability of molecular imaging with a variety of (19)F-labeled compounds and drug evaluation.

Fluorine-19 or phosphorus-31 NMR spectroscopy: A suitable analytical technique for quantitative in vitro metabolic studies of fluorinated or phosphorylated drugs

Fluorine-19 or phosphorus-31 NMR (19F NMR or 31P NMR) spectroscopy provides a highly specific tool for identification of fluorine- or phosphorus-containing drugs and their metabolites in biological media as well as a suitable analytical technique for their absolute quantification. This article focuses on the application of in vitro 19F or 31P NMR to the quantitative metabolic studies of some fluoropyrimidine or oxazaphosphorine drugs in clinical use. The first part presents an overview of the advantages (non-destructive and non-selective direct quantitative study of the biological matrices) and limitations (expensive cost of the spectrometers, limited mass or concentration sensitivity) of NMR spectroscopy. The second part deals with the criteria to be considered for successful quantification by NMR (uniform excitation over the entire spectral width of the spectrum, resonance signals properly characterised by taking into account T1 values and avoiding NOE enhancements, optimisation of the data processing, choice of a suitable standard reference). The third and fourth parts report some examples of quantification of 5-fluorouracil, its prodrug capecitabine, 5-fluorocytosine and their metabolites in bulk solutions (biofluids, tissue extracts, perfusates and culture media) and heterogeneous media (excised tissues and packed intact cells) as well as cyclophosphamide and ifosfamide in biofluids. These two parts emphasise the high potential of in vitro 19F or 31P NMR for absolute quantification, in a single run, of all the fluorine- or phosphorus-containing species in the matrices analysed. The limit of quantification in bulk solutions is 1-3 microM for 19F NMR and approximately 10 microM for 31P NMR. In heterogeneous media analysed with 19F NMR, it is 2-5 nmol in excised tissues and cell pellets.

Monitoring fluoropyrimidine metabolism in solid tumors with in vivo (19)F magnetic resonance spectroscopy

(19)Fluorine magnetic resonance spectroscopy ((19)F MRS) offers unique possibilities for monitoring the pharmacokinetics of fluoropyrimidines in vivo in tumors and normal tissue in a non-invasive way, both in animals and in patients. This method may therefore be useful for predicting response to fluoropyrimidine-based therapy with or without the effects of modulating agents, and this may be of value for the individualization of anticancer therapy and the strategic development of new anticancer drugs. (19)F MRS has been very valuable in elucidating the basic aspects of fluoropyrimidine metabolism, especially in animal studies. Studies in humans have indicated its clinical potential, but widespread application has been hampered by the relatively low detection sensitivity of the method. The recent introduction of clinical MR scanners with magnetic fields above 1.5 T may stimulate increased clinical use of (19)F MRS.

Novel NMR approach to assessing gene transfection: 4-fluoro-2-nitrophenyl-beta-D-galactopyranoside as a prototype reporter molecule for beta-galactosidase

Gene therapy holds great promise for the treatment of diverse diseases. However, widespread implementation is hindered by difficulties in assessing the success of transfection in terms of spatial extent, gene expression, and longevity of expression. The development of noninvasive reporter techniques based on appropriate molecules and imaging modalities may help to assay gene expression. 4-Fluoro-2-nitrophenyl-beta-D-galactopyranoside (PFONPG) is a novel prototype NMR-sensitive molecule, which is highly responsive to the action of beta-galactosidase (beta-gal), the product of the lacZ gene. The molecule is stable in solution and with respect to wild-type cells, but the enzyme causes very rapid liberation of the aglycone, accompanied by color formation and a 19F NMR chemical shift of 5-10 ppm, depending on pH. Since the product is pH-sensitive, this opens the possibility for direct pH determinations at the site of enzyme activity. Molecular and 19F NMR characteristics of PFONPG in solution, blood, and prostate tumor cells are presented. This prototype molecule facilitates a novel approach for assaying gene activity in vivo.

Correlation between 5-fluorouracil metabolism and treatment response in two variants of C26 murine colon carcinoma

Following an i.p. dose of 150 mg x kg(-1) 5-fluorouracil (5-FU), drug uptake and metabolism over a 2-h period were studied by in vivo (19)F magnetic resonance spectroscopy (MRS) for the murine colon carcinoma lines C26-B (5-FU-insensitive; n=11) and C26-10 (5-FU-sensitive; n=15) implanted s.c. in Balb/C mice. Time courses for tumour growth, intracellular levels of FdUMP, thymidylate synthase (TS) activity, and 5-FU in RNA were also determined, and the effects of a 9.5-min period of carbogen breathing, starting 1 min before drug administration, on MRS-detected 5-FU metabolism and tumour growth curves were examined. Both tumour variants generated MRS-detectable 5-FU nucleotides and showed similar initial growth inhibition after treatment. However, the growth rate of C26-B tumours returned to normal, while the sensitive C26-10 tumours, which produced larger fluoronucleotide pools, still showed moderate growth inhibition. Carbogen breathing did not significantly influence 5-FU uptake or fluoronucleotide production but did significantly enhance growth inhibition in C26-10 tumours. While both tumour variants exhibited incorporation of 5-FU into RNA and inhibition of TS via FdUMP, clearance of 5-FU from RNA and recovery of TS activity were greater for the insensitive C26-B line, indicating that these processes, in addition to 5-FU uptake and metabolism, may be important determinants of drug sensitivity and treatment response.

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

The rat prostate tumour cell line R3327 AT-1 was transfected with a gene coding for a fusion protein comprised of cytosine deaminase (CD from E. coli) and thymidine kinase (TK from Herpes simplex virus, HSV-1). The resulting AT-1/CDglyTK cell line was sensitive to the prodrug 5-fluorocytosine (IC(50) = 78 microM, 96-h incubation) via CD and to ganciclovir (GCV, IC(50) = 1 microM, 96 h) via TK. Subcutaneous tumours generated from 100% CDglyTK(+) cells responded well to 5-FC therapy (500 mg/kg, i.p., 14 daily treatments, four out of seven animals in remission) and to GCV therapy (30 mg/kg, i.p., 14 daily treatments, five of six animals in remission). However, experiments with mixtures of CDglyTK(+) and CDglyTK(-) cells showed low levels of connexins (intercellular gap junctions) and no bystander effect for nontransfected cells using either 5-FC or GCV therapy. Furthermore, (19)F-NMR spectroscopy showed that incubation of cultured CDglyTK(+) cells with 774 microM 5-FC for 16 h resulted in the following intracellular concentrations: 5-FC = 314 microM, 5-FU = 52 microM, cytotoxic fluoronucleotides = 163 microM; extracellular 5-FU reached only 6.4 microM. Thus, in this model system intracellular trapping of 5-FU (slow export) contributes to the failure of the CD/5-FC bystander effect via an extracellular route.

Direct in vivo observation of 5-fluorouracil release from a prodrug in human tumors heterotransplanted in nude mice: a magnetic resonance study

A glucuro-conjugated carbamate derivative of 5-fluorouracil (5-FU), originally designed as a prodrug for antibody-directed enzyme prodrug therapy (ADEPT) application, has been used for direct in vivo observation of in situ 5-FU generation in two human colon tumors heterotransplanted in nude mice. Because of the very fast elimination of glucuro-conjugated drugs, this observation required intratumoral injection. These tumors, when becoming necrotic, are rich enough in beta-glucuronidase to allow (19)F magnetic resonance spectroscopy monitoring, at the tumor level, of both prodrug elimination and 5-FU liberation without preliminary treatment by a specifically targeted enzyme conjugate. Convenient tumors have been selected by magnetic resonance imaging (MRI) on the basis of a correlative study between MRI and conventional histology. This contribution is the first report evidencing such a direct intra-tumoral conversion of a glucuro-conjugated prodrug into the expected active drug. This method, which should allow overall estimation of the beta-glucuronidase content of tumors, might also be helpful for selecting tumors as specific targets for non-toxic glucuro-conjugated prodrugs without prior treatment with a fusion protein.