Synthesis, Radiolabeling, and Biological Evaluation of (R)- and (S)-2-Amino-5-[(18)F]fluoro-2-methylpentanoic Acid ((R)-, (S)-[(18)F]FAMPe) as Potential Positron Emission Tomography Tracers for Brain Tumors

Synthetic 18F labeled biomolecules that are selective and promising for PET imaging: major advances and applications

The concept of positron emission tomography (PET) based imaging was developed more than 40 years ago. It has been a widely adopted technique for detecting and staging numerous diseases in clinical settings, particularly cancer, neuro- and cardio-diseases. Here, we reviewed the evolution of PET and its advantages over other imaging modalities in clinical settings. Primarily, this review discusses recent advances in the synthesis of 18F radiolabeled biomolecules in light of the widely accepted performance for effective PET. The discussion particularly emphasizes the 18F-labeling chemistry of carbohydrates, lipids, amino acids, oligonucleotides, peptides, and protein molecules, which have shown promise for PET imaging in recent decades. In addition, we have deliberated on how 18F-labeled biomolecules enable the detection of metabolic changes at the cellular level and the selective imaging of gross anatomical localization via PET imaging. In the end, the review discusses the future perspective of PET imaging to control disease in clinical settings. We firmly believe that collaborative multidisciplinary research will further widen the comprehensive applications of PET approaches in the clinical management of cancer and other pathological outcomes.

Design, Synthesis, and Biological Evaluation of a Novel [18F]-Labeled Arginine Derivative for Tumor Imaging

To better diagnose and treat tumors related to arginine metabolism, (2S,4S)-2-amino-4-(4-(2-(fluoro-18F)ethoxy)benzyl)-5-guanidinopentanoic acid ([18F]7) was designed and prepared by introducing [18F]fluoroethoxy benzyl on carbon-4 of arginine. [18F]7 and 7 were successfully prepared using synthesis methods similar to those used for (2S,4S)-4-[18F]FEBGln and (2S,4S)-4-FEBGln, respectively. In vitro experiments on cell transport mechanisms showed that [18F]7 was similar to (2S,4S)4-[18F]FPArg and was transported into tumor cells by cationic amino acid transporters. However, [18F]7 can also enter MCF-7 cells via ASC and ASC2 amino acid transporters. Further microPET-CT imaging showed that the initial uptake and retention properties of [18F]7 in MCF-7 subcutaneous tumors were good (2.29 ± 0.09%ID/g at 2.5 min and 1.71 ± 0.09%ID/g at 60 min after administration), without significant defluorination in vivo. However, compared to (2S,4S)4-[18F]FPArg (3.06 ± 0.59%ID/g at 60 min after administration), [18F]7 exhibited lower tumor uptake and higher nonspecific uptake. When further applied to U87MG imaging, [18F]7 can quickly visualize brain gliomas (tumor-to-brain, 1.85 at 60 min after administration). Therefore, based on the above results, [18F]7 will likely be applied for the diagnosis of arginine nutrition-deficient tumors and efficacy evaluations.

Tinker, Tailor, Soldier, Spy: The Diverse Roles That Fluorine Can Play within Amino Acid Side Chains

Side chain-fluorinated amino acids are useful tools in medicinal chemistry and protein science. In this review, we outline some general strategies for incorporating fluorine atom(s) into amino acid side chains and for elaborating such building blocks into more complex fluorinated peptides and proteins. We then describe the diverse benefits that fluorine can offer when located within amino acid side chains, including enabling 19F NMR and 18F PET imaging applications, enhancing pharmacokinetic properties, controlling molecular conformation, and optimizing target-binding.

Characteristic Evaluation of a 11C-Labeled Leucine Analog, l-α-[5-11C]methylleucine, as a Tracer for Brain Tumor Imaging by Positron Emission Tomography

Amino acid transporters are upregulated in many cancer cells, and system L amino acid transporters (LAT1-4), in particular, LAT1, which preferentially transports large, neutral, and branched side-chain amino acids, are considered a primary target for cancer positron emission tomography (PET) tracer development. Recently, we developed a ¹¹C-labeled leucine analog, l-α-[5-¹¹C]methylleucine ([5-¹¹C]MeLeu), via a continuous two-step reaction of Pd⁰-mediated ¹¹C-methylation and microfluidic hydrogenation. In this study, we evaluated the characteristics of [5-¹¹C]MeLeu and also compared the sensitivity to brain tumors and inflammation with l-[¹¹C]methionine ([¹¹C]Met) to determine its potential for brain tumor imaging. Competitive inhibition experiments, protein incorporation, and cytotoxicity experiments of [5-¹¹C]MeLeu were performed in vitro. Further, metabolic analyses of [5-¹¹C]MeLeu were performed using a thin-layer chromatogram. The accumulation of [5-¹¹C]MeLeu in tumor and inflamed regions of the brain was compared with [¹¹C]Met and ¹¹C-labeled (S)-ketoprofen methyl ester by PET imaging, respectively. Transporter assay with various inhibitors revealed that [5-¹¹C]MeLeu is mainly transported via system L amino acid transporters, especially LAT1, into A431 cells. The protein incorporation assay and metabolic assay in vivo demonstrated that [5-¹¹C]MeLeu was neither used for protein synthesis nor metabolized. These results indicate that MeLeu is very stable in vivo. Furthermore, the treatment of A431 cells with various concentrations of MeLeu did not change their viability, even at high concentrations (∼10 mM). In brain tumors, the tumor-to-normal ratio of [5-¹¹C]MeLeu was more elevated than that of [¹¹C]Met. However, the accumulation levels of [5-¹¹C]MeLeu were lower than those of [¹¹C]Met (the standardized uptake value (SUV) of [5-¹¹C]MeLeu and [¹¹C]Met was 0.48 ± 0.08 and 0.63 ± 0.06, respectively). In brain inflammation, no significant accumulation of [5-¹¹C]MeLeu was observed at the inflamed brain area. These data suggested that [5-¹¹C]MeLeu was identified as a stable and safe agent for PET tracers and could help detect brain tumors, which overexpress the LAT1 transporter.

Radiolabeling and Biological Evaluation of Novel 99mTc-Nitrido and 99mTc-Oxo Complexes with 4-Methoxy-L-Phenylalanine Dithiocarbamate for Tumor Imaging

To develop novel radiolabeled amino acid tumor imaging agents, 4-methoxy-L-phenylalanine dithiocarbamate (MOPADTC) was synthesized successfully, and two kinds of 99mTc-labeled complexes ([99mTc]TcN-MOPADTC and [99mTc]TcO-MOPADTC) with high radiochemical purities (RCP > 95%) were obtained. The in vitro stability and partition coefficient were determined, and the results show that both of these complexes have good in vitro stability; [99mTc]TcO-MOPADTC is hydrophilic, while [99mTc]TcN-MOPADTC is slightly lipophilic. The biodistribution of [99mTc]TcN-MOPADTC and [99mTc]TcO-MOPADTC in mice bearing S180 tumors shows that the tumor uptake and tumor/muscle ratio of [99mTc]TcO-MOPADTC were higher than the tumor uptake and tumor/muscle ratio of [99mTc]TcN-MOPADTC. In addition, the tumor retention of [99mTc]TcO-MOPADTC is better than the tumor retention of [99mTc]TcN-MOPADTC. A competitive inhibition assay was performed, and the results indicate that [99mTc]TcO-MOPADTC may enter cells primarily via the L-alanine/L-serine/L-cysteine (ASC) system. Single-photon emission computed tomography (SPECT) imaging of [99mTc]TcO-MOPADTC shows obvious accumulation in tumor sites, suggesting that [99mTc]TcO-MOPADTC is a novel potential tumor-imaging agent.

The Utility of Conventional Amino Acid PET Radiotracers in the Evaluation of Glioma Recurrence also in Comparison with MRI

AIM

In this comprehensive review we present an update on the most relevant studies evaluating the utility of amino acid PET radiotracers for the evaluation of glioma recurrence as compared to magnetic resonance imaging (MRI).

METHODS

A literature search extended until June 2020 on the PubMed/MEDLINE literature database was conducted using the terms "high-grade glioma", "glioblastoma", "brain tumors", "positron emission tomography", "PET", "amino acid PET", "[¹¹C]methyl-l-methionine", "[¹⁸F]fluoroethyl-tyrosine", "[¹⁸F]fluoro-l-dihydroxy-phenylalanine", "MET", "FET", "DOPA", "magnetic resonance imaging", "MRI", "advanced MRI", "magnetic resonance spectroscopy", "perfusion-weighted imaging", "diffusion-weighted imaging", "MRS", "PWI", "DWI", "hybrid PET/MR", "glioma recurrence", "pseudoprogression", "PSP", "treatment-related change", and "radiation necrosis" alone and in combination. Only original articles edited in English and about humans with at least 10 patients were included.

RESULTS

Forty-four articles were finally selected. Conventional amino acid PET tracers were demonstrated to be reliable diagnostic techniques in differentiating tumor recurrence thanks to their high uptake from tumor tissue and low background in normal grey matter, giving additional and early information to standard modalities. Among them, MET-PET seems to present the highest diagnostic value but its use is limited to on-site cyclotron facilities. [¹⁸F]labelled amino acids, such as FDOPA and FET, were developed to provide a more suitable PET tracer for routine clinical applications, and demonstrated similar diagnostic performance. When compared to the gold standard MRI, amino acid PET provides complementary and comparable information to standard modalities and seems to represent an essential tool in the differentiation between tumor recurrence and other entities such as pseudoprogression, radiation necrosis, and pseudoresponse.

CONCLUSIONS

Despite the introduction of new advanced imaging techniques, the diagnosis of glioma recurrence remains challenging. In this scenario, the growing knowledge about imaging techniques and analysis, such as the combined PET/MRI and the application of artificial intelligence (AI) and machine learning (ML), could represent promising tools to face this difficult and debated clinical issue.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

Synthesis and preliminary evaluation of a novel glutamine derivative: (2S,4S)4-[18F]FEBGln

We report the preparation of a novel glutamine derivative, (2S,4S)-2,5-diamino-4-(4-(2-fluoroethoxy)benzyl)-5-oxopentanoic acid, (2S, 4S)4-[¹⁸F]FEBGln ([¹⁸F]4), through efficient organic and radiosyntheses. In vitro assays of [¹⁸F]4 using MCF-7 cells showed that it entered cells via multiple amino acid transporter systems including system L and ASC2 transporters but not through the system A transporter. [¹⁸F]4 showed promising properties for tumor imaging and may serve as a lead compound for further optimizing and targeting the system L transporter associated with enhanced glutamine metabolism in cancer cells.

18F-Branched-Chain Amino Acids: Structure-Activity Relationships and PET Imaging Potential

The large, neutral L-type amino acid transporters (LAT1-LAT4) are sodium-independent transporters that are widely distributed throughout the body. LAT expression levels are increased in many types of cancer, and their expression increases as cancers progress, leading to high expression levels in high-grade tumors and metastases. Because of the key role and overexpression of LAT in many types of cancer, radiolabeled LAT substrates are promising candidates for nuclear imaging of malignancies that are not well revealed by conventional radiotracers. The goal of this study was to examine the structure-activity relationships of a series of ¹⁸F-labeled amino acids that were predicted to be substrates of the LAT transport system. Methods: Using a photocatalytic radical fluorination, we prepared a series of 11 fluorinated branched-chain amino acids and evaluated them and their nonfluorinated parents in a cell-based LAT affinity assay. We radiofluorinated selected branched-chain amino acids via the same radical fluorination reaction and evaluated tumor uptake in U-87 glioma xenograft-bearing mice. Results: Structure-activity relationship trends observed in a LAT affinity assay were maintained in further in vitro studies, as well as in vivo using a U-87 xenograft model. LAT1 uptake was tolerant of fluorinated amino acid stereochemistry and chain length. PET imaging and biodistribution studies showed that the tracer (S)-5-¹⁸F-fluorohomoleucine had rapid tumor uptake, favorable in vivo kinetics, and good stability. Conclusion: By using an in vitro affinity assay, we could predict LAT-mediated cancer cell uptake in a panel of fluorinated amino acids. These predictions were consistent when applied to different cell lines and murine tumor models, and several new tracers may be suitable for further development as oncologic PET imaging agents.

Effect of α-Methyl versus α-Hydrogen Substitution on Brain Availability and Tumor Imaging Properties of Heptanoic [F-18]Fluoroalkyl Amino Acids for Positron Emission Tomography (PET)

Two [(18)F]fluoroalkyl substituted amino acids differing only by the presence or absence of a methyl group on the α-carbon, (S)-2-amino-7-[(18)F]fluoro-2-methylheptanoic acid ((S)-[(18)F]FAMHep, (S)-[(18)F]14) and (S)-2-amino-7-[(18)F]fluoroheptanoic acid ((S)-[(18)F]FAHep, (S)-[(18)F]15), were developed for brain tumor imaging and compared to the well-established system L amino acid tracer, O-(2-[(18)F]fluoroethyl)-l-tyrosine ([(18)F]FET), in the delayed brain tumor (DBT) mouse model of high-grade glioma. Cell uptake, biodistribution, and PET/CT imaging studies showed differences in amino acid transport of these tracer by DBT cells. Recognition of (S)-[(18)F]15 but not (S)-[(18)F]14 by system L amino acid transporters led to approximately 8-10-fold higher uptake of the α-hydrogen substituted analogue (S)-[(18)F]15 in normal brain. (S)-[(18)F]15 had imaging properties similar to those of (S)-[(18)F]FET in the DBT tumor model while (S)-[(18)F]14 afforded higher tumor to brain ratios due to much lower uptake by normal brain. These results have important implications for the future development of α-alkyl and α,α-dialkyl substituted amino acids for brain tumor imaging.