Multi-nuclear MRS and 19F MRI of 19F-labelled and 10B-enriched p-boronophenylalanine-fructose complex to optimize boron neutron capture therapy: phantom studies at high magnetic fields

Fluorinated borono-phenylalanine for optimizing BNCT: Enhancing boron absorption against hydrogen scattering for thermal neutrons

BACKGROUND

Boron-containing compounds, such as 4-borono-phenylalanine (BPA) are used as drugs for cancer treatment in the framework of Boron Neutron Capture Therapy (BNCT). Neutron irradiation of boron-rich compounds delivered to cancer cells triggers nuclear reactions that destroy cancer cells.

PURPOSE

We provide a modeling of the thermal neutron cross section of BPA, a drug used in Boron Neutron Capture Therapy (BNCT), to quantify the competing contributions of boron absorption against hydrogen scattering, for optimizing BNCT by minimizing the latter.

METHODS

We perform the experimental determination of the total neutron scattering cross section of BPA at thermal and epithermal neutron energies using neutron transmission measurements. We isolate the contribution related to the incoherent scattering by hydrogen atoms as a function of the neutron energy by means of the Average Functional Group Approximation, and we calculate the probability for a neutron of being absorbed as a function of the neutron energy both for BPA and for its variants where either one or all four aromatic hydrogen atoms are substituted by 19 F, and both for the samples with natural occurrence or enriched concentration of 10 B.

RESULTS

While referring to the already available literature for in vivo use of fluorinated BPA, we show that fluorine-rich variants of BPA increase the probability of neutrons being captured by the molecule. As the higher absorption efficiency of fluorinated BPA does not depend on whether the molecule is used in vivo or not, our results are promising for the higher efficiency of the boron neutron capture treatment.

CONCLUSIONS

Our results suggest a new advantage using fluorinated compounds for BNCT, in their optimized interaction with neutrons, in addition to their already known capability to be used for monitoring and pharmacokinetics studies using 19 F-Nuclear Magnetic Resonance or in 18 F-Positron Emission Tomography.

The Development of Boron Analysis and Imaging in Boron Neutron Capture Therapy (BNCT)

Boron neutron capture therapy (BNCT) is a selective biological targeted nuclide technique for cancer therapy. It has the following attractive features: good targeting, high effectiveness, and causes slight damage to surrounding healthy tissue compared with other traditional methods. It has been considered as one of the promising methods for the treatment of various cancers. Measuring ¹⁰B concentrations is vital for BNCT. However, the existing technology and equipment cannot satisfy the real-time and accurate measurement requirements, and more efficient methods are in demand. The development of methods and imaging applied in BNCT to help measure boron concentration is described in this review.

Multimodal evaluation of 19F-BPA internalization in pancreatic cancer cells for boron capture and proton therapy potential applications

PURPOSE

One of the obstacles to the application of Boron Neutron Capture Therapy (BNCT) and Proton Boron Fusion Therapy (PBFT) concerns the measurement of borated carriers' biodistribution. The objective of the present study was to evaluate the in vitro internalization of the ¹⁹F-labelled p-boronophenylalanine (¹⁹F-BPA) in the human cancer pancreatic cell line (PANC-1) for the potential application of BNCT and PBFT in pancreatic cancer. The ¹⁹F-BPA carrier has the advantage that its bio-distribution may be monitored in vivo using ¹⁹F-Nuclear Magnetic Resonance (¹⁹F NMR).

MATERIALS AND METHODS

The ¹⁹F-BPA internalization in PANC-1 cells was evaluated using three independent techniques on cellular samples left in contact with growing medium enriched with 13.6 mM ¹⁹F-BPA corresponding to a ¹¹B concentration of 120 ppm: neutron autoradiography, which quantifies boron; liquid chromatography hyphenated to tandem mass spectrometry and UV-Diode Array Detection (UV-DAD), which quantifies ¹⁹F-BPA molecule; and ¹⁹F NMR spectroscopy, which detects fluorine nuclei.

RESULTS

Our studies suggested that ¹⁹F-BPA is internalized by PANC-1 cells. The three methods provided consistent results of about 50% internalization fraction at 120 ppm of ¹¹B. Small variations (less than 15%) in internalization fraction are mainly dependent on the proliferation state of the cells.

CONCLUSIONS

The ability of ¹⁹F NMR spectroscopy to study ¹⁹F-BPA internalization was validated by well-established independent techniques. The multimodal approach we used suggests ¹⁹F-BPA as a promising BNCT/PBFT carrier for the treatment of pancreatic cancer. Since the quantification is performed at doses useful for BNCT/PBFT, ¹⁹F NMR can be envisaged to monitor ¹⁹F-BPA bio-distribution during the therapy.

A Chiron approach towards the stereoselective synthesis of polyfluorinated carbohydrates

The replacement of hydroxyl groups by fluorine atoms on hexopyranose scaffolds may allow access to the discovery of new chemical entities possessing unique physical, chemical and ultimately even biological properties. The prospect of significant effects generated by such multiple and controlled substitutions encouraged us to develop diverse synthetic routes towards the stereoselective synthesis of polyfluorinated hexopyranoses, six of which are unprecedented. Hence, we report the synthesis of heavily fluorinated galactose, glucose, mannose, talose, allose, fucose, and galacturonic acid methyl ester using a Chiron approach from inexpensive levoglucosan. Structural analysis of single-crystal X-ray diffractions and NMR studies confirm the conservation of favored ⁴C₁ conformation for fluorinated carbohydrate analogs, while a slightly distorted conformation due to repulsive 1,3-diaxial F···F interaction is observed for the trifluorinated talose derivative. Finally, the relative stereochemistry of multi-vicinal fluorine atoms has a strong effect on the lipophilicities (logP).

Polyfluorinated hexopyranoses display unique physical, chemical and biological properties, however their stereoselective synthesis is highly challenging. Here, the authors report a synthetic approach based on the chemical manipulation of inexpensive levoglucosan to obtain heavily fluorinated monosaccharides stereoselectively.

Electrophilic fluorination of stereodefined disubstituted silyl ketene hemiaminals en route to tertiary α-fluorinated carbonyl derivatives

A highly diastereoselective synthesis of tertiary α-fluoro carbonyl compounds is reported in only two chemical steps from a simple alkyne through the reaction of stereodefined fully substituted silyl ketene hemiaminal derivatives with Selectfluor.

Influence of fluorine substituents on the properties of phenylboronic compounds

Rapid development of research on the chemistry of boronic acids is connected with their applications in organic synthesis, analytical chemistry, materials’ chemistry, biology and medicine. In many applications Lewis acidity of boron atoms plays an important role. Special group of arylboronic acids are fluoro-substituted compounds, in which the electron withdrawing character of fluorine atoms influences their properties. The present paper deals with fluoro-substituted boronic acids and their derivatives: esters, benzoxaboroles and boroxines. Properties of these compounds, i.e. acidity, hydrolytic stability, structures in crystals and in solution as well as spectroscopic properties are discussed. In the next part examples of important applications are given.

NMR Techniques in Metabolomic Studies: A Quick Overview on Examples of Utilization

Metabolomics is a rapidly developing branch of science that concentrates on identifying biologically active molecules with potential biomarker properties. To define the best biomarkers for diseases, metabolomics uses both models (in vitro, animals) and human, as well as, various techniques such as mass spectroscopy, gas chromatography, liquid chromatography, infrared and UV-VIS spectroscopy and nuclear magnetic resonance. The last one takes advantage of the magnetic properties of certain nuclei, such as ¹H, ¹³C, ³¹P, ¹⁹F, especially their ability to absorb and emit energy, what is crucial for analyzing samples. Among many spectroscopic NMR techniques not only one-dimensional (1D) techniques are known, but for many years two-dimensional (2D, for example, COSY, DOSY, JRES, HETCORE, HMQS), three-dimensional (3D, DART-MS, HRMAS, HSQC, HMBC) and solid-state NMR have been used. In this paper, authors taking apart fundamental division of nuclear magnetic resonance techniques intend to shown their wide application in metabolomic studies, especially in identifying biomarkers.

Boron neutron capture therapy and 18F-labelled borophenylalanine positron emission tomography: a critical and clinical overview of the literature

Positron emission tomography (PET) is considered one of the most useful tool for molecular imaging both in clinical and preclinical research for in vivo assessing of biochemical and pharmacological processes. Boron neutron capture therapy (BNCT) is a biologically-targeted radiotherapy that can selectively hit the tumour cells, saving the surrounding normal tissue. Boron 10 ((10)B) is the isotope widely used for this purpose, and acts as killer for tumor cells, releasing highly reactive α and (7)Li-particles when it absorbs a thermal neutron. The basic requirements for a successful BNCT treatment are firstly that the boron-containing compound/material has to be delivered to the neoplastic tissue, and secondly the amount of boron atoms concentrated inside/around the cancer cells must be sufficient for an optimal therapeutic response. The irradiation of tissue or organ with therapeutic doses of thermal neutrons can lead to a selective, complete ablation of the malignant lesion. Specific carriers have been developed for BNCT: para-borophenylalanine (BPA), represents one of them and the most employed in clinical trials to preferentially deliver boron to the malignancy. For the in vivo examination of pharmacokinetic, accumulation and metabolism characteristics of L-B-BPA, a positron-labeled boronophenylalanine analogue, L-(18)F-(10)BPA was proposed and its pharmaco-properties were non-invasively evaluated by PET imaging. Herein, we summarize BNCT principles and applications, boron carrier and boron imaging with PET, PET-guided BNCT and other studied and employed tracers for PET in order to optimizeBNCT.

Quantitative magnetic resonance fluorine imaging: today and tomorrow

Fluorine (19F) is a promising moiety for quantitative magnetic resonance imaging (MRI). It possesses comparable magnetic resonance (MR) sensitivity to proton (1H) but exhibits no tissue background signal, allowing specific and selective assessment of the administrated 19F-containing compounds in vivo. Additionally, the MR spectra of 19F-containing compounds exhibited a wide range of chemical shifts (>200 ppm). Therefore, both MR parameters (e.g., spin-lattice relaxation rate R1) and the absolute quantity of molecule can be determined with 19F MRI for unbiased assessment of tissue physiology and pathology. This article reviews quantitative 19F MRI applications for mapping tumor oxygenation, assessing molecular expression in vascular diseases, and tracking labeled stem cells.

In vivo 19F MR imaging and spectroscopy for the BNCT optimization

The aim of this study was to evaluate in vivo the boron biodistribution and pharmacokinetics of 4-borono-2-fluorophenylalanine ((19)F-BPA) using (19)F MR Imaging ((19)F MRI) and Spectroscopy ((19)F MRS). The correlation between the results obtained by both techniques, (19)F MRI on rat brain and (19)F MRS on blood samples, showed the maximum (19)F-BPA uptake in C6 glioma model at 2.5h after infusion determining the optimal irradiation time. Moreover, the effect of L-DOPA as potential enhancer of (19)F-BPA tumour intake was assessed using (19)F MRI.

In vivo (19)F MRI and (19)F MRS of (19)F-labelled boronophenylalanine-fructose complex on a C6 rat glioma model to optimize boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is a promising binary modality used to treat malignant brain gliomas. To optimize BNCT effectiveness a non-invasive method is needed to monitor the spatial distribution of BNCT carriers in order to estimate the optimal timing for neutron irradiation. In this study, in vivo spatial distribution mapping and pharmacokinetics evaluation of the (19)F-labelled boronophenylalanine (BPA) were performed using (19)F magnetic resonance imaging ((19)F MRI) and (19)F magnetic resonance spectroscopy ((19)F MRS). Characteristic uptake of (19)F-BPA in C6 glioma showed a maximum at 2.5 h after compound infusion as confirmed by both (19)F images and (19)F spectra acquired on blood samples collected at different times after infusion. This study shows the ability of (19)F MRI to selectively map the bio-distribution of (19)F-BPA in a C6 rat glioma model, as well as providing a useful method to perform pharmacokinetics of BNCT carriers.