Proton-decoupled 19F spectroscopy of 5-FU catabolites in human liver

Non-Invasive Analysis of Human Liver Metabolism by Magnetic Resonance Spectroscopy

The liver is a key node of whole-body nutrient and fuel metabolism and is also the principal site for detoxification of xenobiotic compounds. As such, hepatic metabolite concentrations and/or turnover rates inform on the status of both hepatic and systemic metabolic diseases as well as the disposition of medications. As a tool to better understand liver metabolism in these settings, in vivo magnetic resonance spectroscopy (MRS) offers a non-invasive means of monitoring hepatic metabolic activity in real time both by direct observation of concentrations and dynamics of specific metabolites as well as by observation of their enrichment by stable isotope tracers. This review summarizes the applications and advances in human liver metabolic studies by in vivo MRS over the past 35 years and discusses future directions and opportunities that will be opened by the development of ultra-high field MR systems and by hyperpolarized stable isotope tracers.

Magnetic resonance spectroscopy of cancer metabolism and response to therapy

Magnetic resonance spectroscopy allows noninvasive in vivo measurements of biochemical information from living systems, ranging from cultured cells through experimental animals to humans. Studies of biopsies or extracts offer deeper insights by detecting more metabolites and resolving metabolites that cannot be distinguished in vivo. The pharmacokinetics of certain drugs, especially fluorinated drugs, can be directly measured in vivo. This review briefly describes these methods and their applications to cancer metabolism, including glycolysis, hypoxia, bioenergetics, tumor pH, and tumor responses to radiotherapy and chemotherapy.

An 19F Magnetic Resonance–Based In Vivo Assay of Solid Tumor Methotrexate Resistance: Proof of Principle

PURPOSE

Studies in oncology have implicated multiple molecular mechanisms as contributors to intrinsic and acquired tumor resistance to antifolate therapy. Here we show the utility of an (19)F-labeled methotrexate (FMTX) with (19)F magnetic resonance to differentiate between sensitive and resistant tumors in vivo and thus predict therapeutic response.

EXPERIMENTAL DESIGN

Human sarcoma xenografts in nude mice were used in this study. The sarcoma cell lines chosen for this study (HT-1080, HS-16, and M-805) are well characterized in terms of their methotrexate sensitivity and molecular mechanisms of resistance. The pharmacokinetics of tumor uptake/washout of FMTX were monitored via in vivo (19)F magnetic resonance spectroscopy (pulse/acquire with surface coil localization) following an i.v. bolus injection. Response post-therapy, following leucovorin rescue, was monitored via tumor growth.

RESULTS

The three tumor models show differences in both the peak concentrations of tumor FMTX and the dynamics of uptake/retention. These differences are most pronounced for time points late in the magnetic resonance observation period (225-279 minutes post-injection). A statistically significant linear correlation between tumor tissue concentrations of FMTX at these late time points and therapeutic response in the days/weeks post-treatment is shown (R = 0.81, F = 9.27, P < 0.001). Interestingly, a 400 mg/kg i.v. bolus injection of FMTX is a more potent cytotoxic agent in vivo against methotrexate-sensitive tumors than is the parent compound (P = 0.011).

CONCLUSIONS

In principle, the assay method described herein could be implemented in the clinic as a diagnostic tool to make decisions regarding therapeutic protocol for the treatment of osteosarcoma on a case-by-case basis.

Gallbladder localization of (19)F MRS catabolite signals in patients receiving bolus and protracted venous infusional 5-fluorouracil

The hepatobiliary distribution of 5-fluorouracil (5FU) catabolites was investigated in nine patients. Using fluorine 3D-chemical shift imaging, four patients receiving protracted venous infusion of 5FU demonstrated catabolite localized to the gallbladder. No hepatobiliary fluorine signals were detected in three patients whose gallbladders were absent or abnormal. Signals from the gallbladder showed a 2.2-2.4 ppm high-frequency shift from alpha-fluoro-beta-alanine, suggesting the presence of alpha-fluoro-beta-alanine-bile-acid conjugates. 3D-chemical shift imaging of two patients receiving bolus 5FU revealed alpha-fluoro-beta-alanine to be localized to the liver within 1 hr of administration. In one patient examined 4 hr after bolus administration, catabolite signal was detected only in the gallbladder.

1H decoupling for in vivo (19)F MRS studies using the time-share modulation method on a clinical 1.5 T NMR system

Time-share modulation has been implemented successfully on a clinical 1.5 T nuclear magnetic resonance (NMR) system to perform in vivo (19)F[-(1)H] decoupling. It uses commercially available radiofrequency (RF) filters, a transistor-transistor logic (TTL)-controlled attenuator, and a double-resonant RF surface coil suitable for (19)F magnetic resonance spectroscopy (MRS) studies in vivo. This decoupling scheme gives a threefold improvement in signal-to-noise ratio performance compared with the conventional WALTZ-4 decoupling scheme, when significant interaction exists between the decoupler and the receiver. It can also eliminate receiver saturation (i.e., saturation due to coupling of the strong proton decoupling pulse to the (19)F coil) without the need for a high-isolation coil system, thus allowing flexibility in coil geometry. This method has been demonstrated in vivo in patients receiving 5-fluorouracil chemotherapy. Magn Reson Med 44:5-9, 2000.

19F-MRS studies of fluorinated drugs in humans

The use of 19F-NMR as a noninvasive probe to measure directly the pharmacokinetics of drugs at their target (effector) site(s) is illustrated in this article by human studies with 5-fluorouracil (5-FU). This drug, and several of its metabolites, have been measured in vivo in animals and in patients using standard clinical MRI systems. Using a pharmacokinetic imaging approach the parameter that can be measured most readily is the tumoral t(1/2) of 5-FU. Patients whose tumoral t(1/2) of 5-FU is equal to/greater than 20 min are designated as "trappers", and those whose tumoral t(1/2) of 5-FU is less are nontrappers. Trapping of 5-FU in tumors is a necessary, albeit not a sufficient condition, for response. Problems associated with the technical aspects of these measurements have been discussed, as well as how modulators and other agents will affect the tumoral t(1/2) of 5-FU. The rationale for the biological processes underlying the fate of 5-FU in humans has been illustrated with the use of a 12 compartment model, where several of the steps have been discussed and the consequences of their inhibition/stimulation related to the noninvasive studies that can be performed with modulators of the action of 5-FU. These 19F-NMR studies have now been extended to other fluoropyrimidines, some of which are prodrugs of 5-FU, and others where the fluorine atoms are on the ribose ring. These studies also reveal information that has both scientific and clinical significance. The studies presented here illustrate some of the potential and some of the usefulness of 19F-MRS in patient management and in drug development. It is a technique that has proven itself.

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

Fluoropyrimidine chemotherapy relies on the intracellular anabolic conversion of 5-fluorouracil and the corresponding nucleosides to cytotoxic fluorinated nucleotides (F-Nuctd), such as 5-fluorouridine-5'-triphosphate (FUTP) or 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP), which can be detected by 19F-NMR spectroscopy. We have made 19F-NMR signal assignments at 11.7 T and 4 degrees C for model solutions containing 5-fluorouracil (FUra), 5-fluorouridine (FUrd), 5-fluoro-2'-deoxyuridine (FdUrd), 5-fluorouridine-5'-monophosphate (FUMP), FdUMP, 5-fluorouridine-5'-diphosphate (FUDP), FUTP and 5-fluorouridine-5'-diphospho(1)-alpha-D-glucose (FUDPG), and we have studied the effects of pH over the range 4.5-7.8, of Mg2+ concentration and addition of EDTA. This information provides a basis for the analysis of 19F-NMR spectra obtained from cells, tissues or extracts following fluoropyrimidine treatment.