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Synthesis and evaluation of 2-amino-4-[18F]fluoro-2-methylbutanoic acid (FAMB): relationship of amino acid transport to tumor imaging properties of branched fluorinated amino acids

https://doi.org/10.1016/S0969-8051(03)00025-8Get rights and content

Abstract

Radiolabeled amino acids represent a promising class of tumor imaging agents, and the determination of the optimal characteristics of these tracers remains an area of active investigation. A new 18F-labeled branched amino acid, 2-amino-4-[18F]fluoro-2-methylbutanoic acid (FAMB), has been prepared in 36% decay-corrected yield using no-carrier-added [18F]fluoride. In vitro uptake assays with rat 9L gliosarcoma cells suggest that [18F]FAMB was transported primarily via the L type amino acid transport system. In vivo studies with [18F]FAMB demonstrated tumor to normal brain ratios of 14:1 in rats with intracranial 9L gliosarcoma tumors at 60 minutes after injection. Comparison of [18F]FAMB with structurally related 18F-labeled branched amino acids demonstrated that A type transport in vitro was positively correlated with the tumor to brain ratios observed in vivo.

Introduction

The development of amino acids labeled with 11C, 18F and 123I has produced a number of tumor imaging agents that have shown clinical utility in human cancer patients. Of these, L-[11C]methionine (MET), O-(2-[18F]fluoroethyl) tyrosine (FET) and 3-[123I]iodo-L-α-methyl tyrosine (IMT) have been the most extensively studied in humans [12], [16], [31], [32]. Radiolabeled amino acids appear to possess significant advantages over 2-[18F]fluorodeoxyglucose (FDG) for imaging brain tumors, primarily due to the lower uptake of amino acids in normal brain relative to [18F]FDG [5], [12], [27]. Normal cerebral cortex shows high physiological uptake of [18F]FDG which can reduce tumor to brain ratios and complicate interpretation of FDG-PET studies, and inflammatory tissue which may occur after treatment of brain tumors often has high uptake of [18F]FDG. Additionally, evidence suggests that amino acid tumor imaging agents may provide diagnostic information that complements studies performed with [18F]FDG in the evaluation of systemic tumors [12], [16].

Amino acids enter normal and neoplastic cells through carrier proteins in the cell membrane with varying substrate selectivities. In most mammalian cells, the most abundant amino acid transporters systems are system A, system L, system ASC and system Gly [16], [21], [23], [24]. All of these transport systems are sodium-dependent except system L. Currently, it appears that the rate of amino acid transport by tumor cells may be more important for amino acid accumulation than the rate of protein synthesis [12], [16]. Both A type and L type transport have been shown to be increased in tumor cells relative to normal tissue, and these transport systems have been the major focus of the development of amino acids for tumor imaging [1], [12], [20], [23], [30]. However, direct comparisons of radiolabeled A type and L type substrates for tumor imaging have been limited. The behavior of structurally distinct amino acids may be difficult to compare directly due to differences in physicochemical and biological properties not related to transport. Structurally related radiolabeled amino acids with varying selectivities for A type and L type transport would provide useful tools for studying the contribution of these transport systems to the tumor imaging properties of amino acids.

As part of a program to develop and characterize radiohalogenated amino acids for tumor imaging, we have prepared and evaluated a new 18F-labeled amino acid, 2-amino-4-[18F]fluoro-2-methylbutanoic acid (FAMB, compound 7). Our group has focused on radiolabeled non-natural amino acids because they may offer improved metabolic stability over radiolabeled natural amino acids and may allow relatively simple analysis of tracer kinetics. The impetus to evaluate this compound arose from the promising biologic behavior of the recently reported 18F-labeled analogues of α-aminoisobutyric acid (AIB), 2-amino-3-[18F]fluoro-2-methylpropanoic acid (FAMP) and 3-[18F]fluoro-2-methyl-2-(methylamino)propanoic (N-MeFAMP) [18]. Both [18F]FAMP and [18F]N-MeFAMP are substrates for the A type amino acid transporter and have demonstrated very high tumor to brain ratios in rats with intracranial 9L gliosarcoma tumors. We were particularly interested in the effects of extending the fluorinated side chain of FAMP on radiolabeling, biological transport and tumor uptake. In this report, we describe the radiosynthesis of [18F]FAMB and the evaluation of its tumor imaging properties through amino acid uptake assays and biodistribution studies in normal and tumor-bearing rats. Additionally, the results obtained with [18F]FAMB were compared with those previously reported for [18F]FAMP and [18F]N-MeFAMP to provide structure-activity data for this class of compounds (see Fig. 1).

Section snippets

General

All reagents used were obtained from commercially available sources. Solvents used in reactions were purchased from Aldrich Chemicals (Milwaukee, WI USA) while solvents for chromatography were obtained from VWR Scientific Products (West Chester, PA USA). Melting points are uncorrected and were determined in capillary tubes using a Mel-Temp II apparatus (Laboratory Devices, Inc., Holliston, MA USA). 1H NMR spectra were recorded on a Varian spectrometer at 400 MHz and referenced to the NMR

Chemistry

The synthetic strategy for obtaining the radiosynthetic precursor of [18F]FAMB focused on the preparation of the tosylate 5 via a hydantoin intermediate. As discussed subsequently in the Section 3.2, the use of a sulfonyl ester precursor to produce [18F]FAMB was expected to succeed where an analogous sulfonyl ester precursor had failed for the synthesis of [18F]FAMP. This reaction sequence is shown in Scheme 1 and is similar to the method used in the preparation of intermediates in the

Conclusions

The data presented here show that [18F]FAMB can be produced in good radiochemical yield (36% decay-corrected) and high radiochemical purity (>99%) from a tosylate precursor using no-carrier-added [18F]fluoride. In vitro amino acid uptake assays suggest that [18F]FAMB entered 9L gliosarcoma cells primarily via L type amino acid transport but not via A type transport. Administration of [18F]FAMB to rats with intracranial 9L gliosarcoma tumors led to tumor to brain ratios of 14:1 at 60 minutes

References (33)

  • P. Heiss et al.

    Investigation of transport mechanism and uptake kinetics of O-(2-[18F]fluoroethyl)-L-tyrosine in vitro and in vivo

    J. Nucl. Med.

    (1999)
  • T. Inoue et al.

    Biodistribution studies on L-3-[fluorine-18]fluoro-alpha-methyl tyrosinea potential tumor-detecting agent

    J. Nucl. Med.

    (1998)
  • T. Inoue et al.

    18F alpha-methyl tyrosine PET studies in patients with brain tumors

    J. Nucl. Med.

    (1999)
  • P.L. Jager, W. Vaalburg, J. Pruim, E.G. de Vries, K.J. Langen, D.A. Piers, Radiolabeled amino acids: basic aspects and...
  • K. Kubota et al.

    Tumor detection with carbon-11-labelled amino acids

    Eur J. Nucl. Med.

    (1984)
  • K.J. Langen et al.

    Transport mechanisms of 3-[123I]iodo-alpha-methyl-L-tyrosine in a human glioma cell linecomparison with [3H]methyl]-L-methionine

    J. Nucl. Med.

    (2000)
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