Metabolism and actions of 2-deoxy-2-fluoro-D-galactose in vivo

Unlocking the Potential of HK2 in Cancer Metabolism and Therapeutics

Glycolysis is a tightly regulated process in which several enzymes, such as Hexokinases (HKs), play crucial roles. Cancer cells are characterized by specific expression levels of several isoenzymes in different metabolic pathways and these features offer possibilities for therapeutic interventions. Overexpression of HKs (mostly of the HK2 isoform) have been consistently reported in numerous types of cancer. Moreover, deletion of HK2 has been shown to decrease cancer cell proliferation without explicit side effects in animal models, which suggests that targeting HK2 is a viable strategy for cancer therapy. HK2 inhibition causes a substantial decrease of glycolysis that affects multiple pathways of central metabolism and also destabilizes the mitochondrial outer membrane, ultimately enhancing cell death. Although glycolysis inhibition has met limited success, partly due to low selectivity for specific isoforms and excessive side effects of the reported HK inhibitors, there is ample ground for progress. The current review is focused on HK2 inhibition, envisaging the development of potent and selective anticancer agents. The information on function, expression, and activity of HKs is presented, along with their structures, known inhibitors, and reported effects of HK2 ablation/inhibition. The structural features of the different isozymes are discussed, aiming to stimulate a more rational approach to the design of selective HK2 inhibitors with appropriate drug-like properties. Particular attention is dedicated to a structural and sequence comparison of the structurally similar HK1 and HK2 isoforms, aiming to unveil differences that could be explored therapeutically. Finally, several additional catalytic- and non-catalytic roles on different pathways and diseases, recently attributed to HK2, are reviewed and their implications briefly discussed.

Bringing physiology into PET of the liver

Several physiologic features make interpretation of PET studies of liver physiology an exciting challenge. As with other organs, hepatic tracer kinetics using PET is quantified by dynamic recording of the liver after the administration of a radioactive tracer, with measurements of time-activity curves in the blood supply. However, the liver receives blood from both the portal vein and the hepatic artery, with the peak of the portal vein time-activity curve being delayed and dispersed compared with that of the hepatic artery. The use of a flow-weighted dual-input time-activity curve is of importance for the estimation of hepatic blood perfusion through initial dynamic PET recording. The portal vein is inaccessible in humans, and methods of estimating the dual-input time-activity curve without portal vein measurements are being developed. Such methods are used to estimate regional hepatic blood perfusion, for example, by means of the initial part of a dynamic (18)F-FDG PET/CT recording. Later, steady-state hepatic metabolism can be assessed using only the arterial input, provided that neither the tracer nor its metabolites are irreversibly trapped in the prehepatic splanchnic area within the acquisition period. This is used in studies of regulation of hepatic metabolism of, for example, (18)F-FDG and (11)C-palmitate.

Nucleophilic radiosynthesis of 2-[18F]fluoro-2-deoxy-D-galactose from Talose triflate and biodistribution in a porcine model

INTRODUCTION

The galactose analogue 2-[(18)F]fluoro-2-deoxy-D-galactose (FDGal) is a promising positron emission tomography (PET) tracer for studies of regional differences in liver metabolic function and for clinical evaluation of patients with liver cirrhosis and patients undergoing treatment of liver diseases. However, there is an unmet need for routine production of FDGal from readily available starting material. In this study, we present the preparation of FDGal with high radiochemical purity and in amounts sufficient for clinical investigations from commercially available Talose triflate (1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulfonyl-β-D-talopyranose). In addition, the biodistribution of FDGal in the pig is presented.

METHODS

FDGal was prepared by nucleophilic fluorination of Talose triflate followed by basic hydrolysis. The entire synthesis was performed using the GE TRACERlab MX 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG) synthesizer and existing methods for quality control of FDG were applied. Biodistribution of FDGal was studied by successive whole-body PET recordings of two anaesthetized 37-kg pigs.

RESULTS

Up to 3.7 GBq sterile, pyrogen-free and no-carrier-added FDGal was produced with a radiochemical yield of 3.8±1.2% and a radiochemical purity of 98±1% (42 productions; yield is decay corrected). The adopted quality control methods for FDG were directly applicable for FDGal. Biodistribution studies in the pig revealed the liver and the urinary bladder as critical organs in terms of radiation dose.

CONCLUSION

Commercially available Talose triflate is a suitable starting material for routine productions of FDGal. The presented radiosynthesis and quality control methods allow for the production of pure, no-carrier-added FDGal in sufficient amounts for clinical PET-investigations of the liver.

Hepatic uptake and metabolism of galactose can be quantified in vivo by 2-[18F]fluoro-2-deoxygalactose positron emission tomography

Metabolism of galactose is a specialized liver function. The purpose of this PET study was to use the galactose analog 2-[(18)F]fluoro-2-deoxygalactose (FDGal) to investigate hepatic uptake and metabolism of galactose in vivo. FDGal kinetics was studied in 10 anesthetized pigs at blood concentrations of nonradioactive galactose yielding approximately first-order kinetics (tracer only; n = 4), intermediate kinetics (0.5-0.6 mmol galactose/l blood; n = 2), and near-saturation kinetics (>3 mmol galactose/l blood; n = 4). All animals underwent liver C15O PET (blood volume) and FDGal PET (galactose kinetics) with arterial and portal venous blood sampling. Flow rates in the hepatic artery and the portal vein were measured by ultrasound transit-time flowmeters. The hepatic uptake and net metabolic clearance of FDGal were quantified by nonlinear and linear regression analyses. The initial extraction fraction of FDGal from blood-to-hepatocyte was unity in all pigs. Hepatic net metabolic clearance of FDGal, K(FDGal), was 332-481 ml blood.min(-1).l(-1) tissue in experiments with approximately first-order kinetics and 15.2-21.8 ml blood.min(-1).l(-1) tissue in experiments with near-saturation kinetics. Maximal hepatic removal rates of galactose were on average 600 micromol.min(-1).l(-1) tissue (range 412-702), which was in agreement with other studies. There was no significant difference between K(FDGal) calculated with use of the dual tracer input (Kdual(FDGal)) or the single arterial input (Karterial(FDGal)). In conclusion, hepatic galactose kinetics can be quantified with the galactose analog FDGal. At near-saturated kinetics, the maximal hepatic removal rate of galactose can be calculated from the net metabolic clearance of FDGal and the blood concentration of galactose.

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.

Preparation of fluorine-18 labelled sugars and derivatives and their application as tracer for positron-emission-tomography

The usefulness of 18F-labelled carbohydrates, especially 2-deoxy-2-[18F]fluoro-D-glucose, to study pathophysiological processes in man non-invasively using positron-emission-tomography (PET) led to a widespread investigation of different 18F-labelled sugars and sugar derivatives. In consideration of the short half-life of fluorine-18 (T(1/2) = 110 min) synthetic strategies concerning precursor design, labelling conditions and deprotection of the intermediate compounds were developed to guarantee an efficient high radiochemical yield synthesis for diagnostic purposes. Besides some aspects of medical application of 2-deoxy-2-[18F]fluoro-D-glucose, a few synthetic strategies are described reflecting development work on promising 18F-labelled sugars for diagnostic purposes during the last two decades.

A chemoenzymatic synthesis of UDP-(2-deoxy-2-fluoro)-galactose and evaluation of its interaction with galactosyltransferase

Uridine 5'-diphospho-(2-deoxy-2-fluoro)galactose (UDP-2FGal), prepared and characterized for the first time by a chemoenzymatic method, was found to be a competitive inhibitor of beta-1,4-galactosyltransferase with a Ki value of 149 microM. This study supports that the glycosyltransferase reaction mechanism proceeds through a glycosidic cleavage transition state with sp2 character developed at the anomeric center.

Cytosine deaminase gene as a potential tool for the genetic therapy of colorectal cancer

The bacterial enzyme cytosine deaminase (CD) catalyzes the conversion of 5-fluorocytosine (5-FC) to the lethal 5-fluorouracil (5-FU) and so provides a useful system for selective killing of gene-modified mammalian tumor cells. Cloning of the CD gene from Escherichia coli and expression in human tumor cell lines enabled these cells to convert 3H-labeled 5-FC into 3H-5-FU. Two CD-expressing human tumor cell lines (adenocarcinoma cell line KM12 and glioblastoma cell line T1115) became 200-fold more sensitive to 5-FC than the nonexpressing parental cell lines. At least 90% of the cells are killed within 7 days. CD-expressing cells are able to kill nonexpressing cells when grown in the same culture flask (bystander effect). The CD gene may be used as a suicide system for in situ chemotherapy or as a safety mechanism abrogating the expression of other genes.

Metabolic pathway of 2-deoxy-2-[18F]fluoro-D-talose in mice: trapping in tissue after phosphorylation by galactokinase

To make clear the metabolic fate of 2-deoxy-2-[18F]fluoro-D-talose ([18F]FDT) in animals, the in vivo and in vitro metabolism of non-radioactive 2-deoxy-2-fluoro-D-talose (FDT) was investigated by 19F-NMR spectroscopy. Based on the 19F-NMR spectral analyses, 2-deoxy-2-fluoro-alpha-D-talose-1-phosphate (FDT-1-P) was identified as a single metabolite in the organs of tumor-bearing mice after FDT administration (60 mg/kg). In the liver, almost all FDT was converted to FDT-1-P within 10 min after FDT injection and the phosphate form remained unchanged for at least 3 h. FDT was well converted to FDT-1-P by galactokinase in vitro. The FDT-1-P formed, however, failed to convert to a uridylate derivative by treatment with galactose-1-phosphate uridyltransferase. The observed low affinity of galactose-1-phosphate uridyltransferase for the FDT-1-P could account for the accumulation mechanism of FDT-1-P in vivo. Similar metabolic studies of [18F]FDT with radio-TLC demonstrated the [18F]FDT-1-P as a single metabolite of [18F]FDT in the mouse liver. These results indicate that [18F]FDT enters a D-galactose metabolic pathway and undergoes a metabolic trapping in the [18F]FDT-1-P form by galactokinase in the tissues such as liver and tumor. Consequently, [18F]FDT is expected to be a new radiopharmaceutical for the measurement of galactokinase activity by positron emission tomography.

In vivo 19F nuclear magnetic resonance of a monofluorinated neuroleptic in the rat

In vivo 19F NMR measurements of the fluorinated neuroleptic melperone [4'-fluoro-4-(4-methylpiperidino)-butyrophenone hydrochloride] in the rat brain were performed using a geometrically optimized surface coil at 4.7 T. It was possible for the first time to detect a signal of a monofluorinated neuroleptic drug with a time resolution of 30 min after i.p. application. The kinetic time course of the investigated neuroleptic melperone was recorded over 6 h and showed that the half-life in the rat brain is 4.3 h. The total amount of drug and its metabolites in the brain was estimated to be 50 microM. the chemical shift of the 19F NMR signal shows the same upfield shift relative to that in aqueous solution as has been reported for trifluorinated neuroleptic drugs.

Inhibition of protein N-glycosylation by 2-deoxy-2-fluoro-D-galactose

The effects of 2-deoxy-2-fluoro-D-galactose (dGalF) on N- and O-glycosylation of proteins was studied in rat hepatocyte primary cultures and in human monocytes. In hepatocytes, dGalF at concentrations of 1 mM or higher completely inhibited N-glycosylation of alpha 1-antitrypsin and alpha 1-acid glycoprotein, whereas 4 mM-2-deoxy-D-galactose (dGal) only slightly impaired N-glycosylation. In monocytes, 1 mM- or 4 mM-dGalF blocked N-glycosylation of alpha 1-antitrypsin and of interleukin-6, while O-glycosylation of interleukin-6 remained unaffected. In monocytes, dGal had no effect on protein N-glycosylation. Addition of uridine effectively prevented the UTP deficiency induced by dGalF, but had no effect on the inhibition of protein N-glycosylation by dGalF. Using 19F-n.m.r. spectroscopy, 2-deoxy-2-fluoro-D-galactose 1-phosphate (dGalF-1-P), UDP-dGalF and UDP-dGlcF could be identified as the major metabolites of dGalF in hepatocytes as well as in monocytes. In conclusion, compared with dGal, dGalF is a more efficient inhibitor of protein N-glycosylation. The effect is not caused by the depletion of UTP induced by dGalF, but rather by metabolites of dGalF. dGalF is metabolized not only in hepatocytes but also in peripheral blood monocytes, which can be used for ex vivo studies of disturbances in D-galactose metabolism.

Synthesis and biodistribution of a fluorine-18 labeled analogue of D-talose: 2-deoxy-2-[18F]fluoro-D-talose

A fluorine-18 labeled analogue of D-talose, 2-deoxy-2-[18F]fluoro-D-talose ([18F]FDT), was synthesized via nucleophilic fluorination with [18F]fluoride ion and its biodistributions in animals were examined. Radiofluorination of benzyl 3,5,6-tri-O-benzyl-2-O-(trifluoromethanesulfonyl)-alpha-D-galac tof uranoside (5) with aminopolyether supported potassium [18F]fluoride (K18F/Kry222) in acetonitrile followed by deprotection of the [18F]fluorinated intermediate (6) with boron tribromide in CH2Cl2 gave [18F]FDT in an average radiochemical yield of 29% with a radiochemical purity greater than 98%. Biodistribution studies of [18F]FDT in mice bearing fibrosarcoma showed the highest uptake of radioactivity in the liver (34.9% dose/g), followed by the kidney (15.9%dose/g), the small intestine (12.9%dose/g) and fibrosarcoma (5.7%dose/g), at 30 min after i.v. administration. Although the radioactivity in the kidney and small intestine decreased with time, the uptake in the liver and the tumor slightly increased until 120 min. The high liver uptake of [18F]FDT was also observed in normal rats and this uptake was strongly inhibited by co-administration of D-galactose. These preliminary results suggest that [18F]FDT might be metabolized through the galactose metabolic pathway as analogously observed with 2-deoxy-2-[18F]fluoro-D-galactose which is an isomer with respect to carbon-2 of [18F]FDT, and that it may be another candidate for studying liver function by positron emission tomography.

Inhibition of glycolysis by 2-deoxygalactose in Saccharomyces cerevisiae

The enzymatic steps involved in the inhibition of glycolysis by 2-deoxygalactose in Saccharomyces cerevisiae have been investigated. Yeast, incubated with 2-deoxygalactose, accumulates up to 8 mM-2-deoxygalactose, 30 mM-2-deoxygalactose-1-phosphate and 0.25 mM-UDP-2-deoxygalactose and UDP-2-deoxyglucose. An inverse correlation between 2-deoxygalactose-1-phosphate content and rate of glycolysis has been observed. The intracellular concentration of glycolytic intermediates and related metabolites point to the hexokinase and phosphofructokinase steps as the targets for the inhibition of glycolysis by 2-deoxygalactose and rule out all other mechanisms that have been proposed to explain this inhibition.

2-Deoxy-2-fluoro-D-galactose protein N-glycosylation

2-Deoxy-2-fluoro-D-galactose (dGalF), added to the medium of primary cultured rat hepatocytes, inhibited N-glycosylation of membrane (gp 120) and secretory glycoproteins (alpha 1-macroglobulin) in a concentration-dependent manner. Complete inhibition of N-glycosylation was achieved at concentrations of 1 mM and above. At identical concentrations, 2-deoxy-2-fluoro-D-glucose (dGlcF) caused only incomplete inhibition of N-glycosylation. dGalF reduced incorporation of D-[2,6-3H]mannose into lipid-linked oligosaccharides indicating interference with their assembly in the dolichol cycle.

Locoregional administration of 5-fluoro-2'-deoxyuridine (FdUrd) in Novikoff hepatoma in the rat: effects of dose and infusion time on tumor growth and on FdUrd metabolite levels in tumor tissue as determined by 19F-NMR spectroscopy

The influence of infusion time and dose on the anticancer efficacy of 5-fluoro-2'-deoxyuridine (FdUrd) was investigated using a locoregional therapy model: Novikoff hepatoma transplanted i.m. into the thigh of Wistar rats and FdUrd infusion via a catheter implanted in the femoral artery. In experiment A the FdUrd dose (five daily doses of 12, 19 and 30 mg/kg) and the duration of administration (bolus, 1 h, 5 h, and 24 h) were varied. The change in tumor volume following treatment and the number of rats showing regression vs progression served as indicators of therapy response. The results showed a clear dose dependence, and for each infusion time the 30 mg/kg dose was the most effective, without any signs of general toxicity. At this dose the longest infusion time (24 h) was less effective (regression in three of six rats) compared with 1-h or 5-h treatments (four of five in regression). In experiment B either one or five daily FdUrd doses (15, 30, 60 mg/kg) were administered i.a. for the same infusion times used in experiment A. After treatment, tumors were explanted ex vivo and approximately 1-g tissues samples were immediately frozen in liquid nitrogen for storage. 19F-NMR spectroscopy at 11.7 T was used to quantify FdUrd metabolites [5-fluorouracil (FUra), alpha-fluoro-beta-alanine (F beta Ala), 5-fluorouracil nucleosides and nucleotides (F-Nuc)] in the solid tumor tissue samples (maintained at 4 degrees C) with a detection threshold of about 5 nmol/g. The metabolite signal pattern indicated that FdUrd is first converted to FUra, followed by anabolism primarily to nucleotides in the oxy form (e.g. FUTP). The total amount of fluorine detected in tumor tissue increased with dose and decreased with infusion time. For all treatments FNuc could be detected, even after 24 h infusion, and their levels showed a good linear correlation with the total F. The major catabolite F beta Ala was present in tumor at low levels that correlated poorly with total F, indicating recirculation from other organs (e.g. liver) as the main source. Thus, the NMR method can provide detailed information regarding the efficiency of locoregional treatment (catheter function, drug uptake and metabolism). Initial results of non-invasive in vivo NMR experiments are also presented.