19F-[1H] nuclear Overhauser effect and proton decoupling of 5-fluorouracil and alpha-fluoro-beta-alanine

Solution dynamics of 5-fluorouracil entrapped in poly lactic-co-glycolic acid (PLGA) microsphere-A study with 1D selective NMR methods

In this report, our main focus is to introduce a set of one-dimensional (1D) NMR methods based on chemical shift, relaxation, and magnetization transfer, namely, NOE and chemical exchange involving selective pulse excitation to study the solution dynamics of drug in free and encapsulated state within polymeric microsphere. In this regard 5-fluorouracil (5-FU) loaded poly lactic-co-glycolic acid (PLGA) microspheres are prepared as model system via standard water-in-oil-in-water emulsification method. One-dimensional 1 H and 19 F nuclear magnetic resonance (NMR) spectra of 5-FU in presence of PLGA microspheres presented a significant change in linewidth and relaxation rates compared with free 5-FU confirming encapsulation. Furthermore, loss of coupling pattern in 1 H and 19 F NMR of PLGA encapsulated 5-FU as compared with free 5-FU suggests an enhanced -NH and -H2 O protons exchange dynamics in the interior of the microsphere indicating hydrated microsphere cavity. Quantification of exchange dynamics in case of free and PLGA-encapsulated 5-FU was attempted employing 1D selective NOESY and 1D multiply selective inversion recovery experiments. Analysis of the exchange rates confirmed existence of more than one kind of water population within the cavity as mentioned in an earlier solid state NMR report.

Fluorine (19F) MRS and MRI in biomedicine

Shortly after the introduction of (1)H MRI, fluorinated molecules were tested as MR-detectable tracers or contrast agents. Many fluorinated compounds, which are nontoxic and chemically inert, are now being used in a broad range of biomedical applications, including anesthetics, chemotherapeutic agents, and molecules with high oxygen solubility for respiration and blood substitution. These compounds can be monitored by fluorine ((19)F) MRI and/or MRS, providing a noninvasive means to interrogate associated functions in biological systems. As a result of the lack of endogenous fluorine in living organisms, (19)F MRI of 'hotspots' of targeted fluorinated contrast agents has recently opened up new research avenues in molecular and cellular imaging. This includes the specific targeting and imaging of cellular surface epitopes, as well as MRI cell tracking of endogenous macrophages, injected immune cells and stem cell transplants.

Optimization of localized 19F magnetic resonance spectroscopy for the detection of fluorinated drugs in the human liver

Fluorine MR spectroscopy ((19)F MRS) is an indispensable tool for assessing the pharmacokinetics of fluorinated drugs. Since the metabolism of 5-fluorouracil (5FU), a frequently used cytotoxic drug, is expected to be different in normal liver and in tumor tissue, spatial localization is required for detection by MRS. In this study, three independent signal-to-noise ratio (SNR) optimizations were combined to enable chemical shift imaging (CSI) as a localization method in the detection of 5FU and its metabolites in tumor tissue. First, the hardware was optimized by using circularly polarized coils together with integrated preamplifiers. Second, the optimal pulse angle (Ernst angle) was determined on the basis of T(1) relaxation time measurements of 5FU. Finally, averaging of CSI phase-encoding steps was optimized by using the applied Hamming filter as a weighting function. The combination of these three methods enables the in vivo detection of 5FU and alpha-fluoro-beta-alanine (FBAL) by (19)F MRS, localized in three dimensions in tumor and liver tissue at a time resolution of 4 min at 1.5 Tesla.

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.

Decoupling: theory and practice. II. State of the art: in vivo applications of decoupling

Current methods for broadband heteronuclear decoupling are reviewed from a historical perspective. The principal concern is that decoupling should be effective over a wide range of chemical shifts without undue radiofrequency heating of the sample, particularly when human patients are involved. Continuous-wave methods are the least efficient in this respect, followed by noise decoupling. Composite pulse schemes offer a more effective use of radiofrequency power, while adiabatic passage methods are the most efficient of all. Bi-level decoupling employs a low level of radiofrequency irradiation during the relaxation delay to maintain the nuclear Overhauser effect, with a higher level during signal acquisition in order to decouple over a wide frequency band. All decoupling sequences introduce cycling sidebands into the observed spectrum, and schemes are described to minimize the intensity of these artifacts. In part II, practical applications of decoupling methods are examined in the context of in vivo spectroscopy, where the improvements in sensitivity and resolution through broadband decoupling can be critical for solving clinical problems. Attention is focused on the regulatory limits on power deposition in these experiments. A tabulation of the existing work on decoupling in biological tissue is presented, mainly involving 31P and 13C spectroscopy in vivo or in vitro.

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

An RF network and a dual-tuned surface coil are described for obtaining proton-decoupled, NOE enhanced 19F spectra from a whole body clinical imager operating at 1.5 Tesia. The network removes 19F frequency noise from the decoupler transmitter, and prevents preamplifier saturation from high-level decoupling signals. Proton decoupling of 19F spectra was optimized using a sample of urine containing 5-fluorouracil (5-FU) and its catabolite fluoro-beta-alanine (FBAL). Proton-decoupled 19F spectroscopy in vivo is demonstrated by obtaining both nonlocalized spectra and spectra localized with three-dimensional chemical shift imaging from the liver of patients undergoing 5-FU chemotherapy.