System a amino acid transport-targeted brain and systemic tumor PET imaging agents 2-amino-3-[(18)F]fluoro-2-methylpropanoic acid and 3-[(18)F]fluoro-2-methyl-2-(methylamino)propanoic acid

The Role of Amino Acids in the Diagnosis, Risk Assessment, and Treatment of Breast Cancer: A Review

This review summarizes the role of amino acids in the diagnosis, risk assessment, imaging, and treatment of breast cancer. It was shown that the content of individual amino acids changes in breast cancer by an average of 10-15% compared with healthy controls. For some amino acids (Thr, Arg, Met, and Ser), an increase in concentration is more often observed in breast cancer, and for others, a decrease is observed (Asp, Pro, Trp, and His). The accuracy of diagnostics using individual amino acids is low and increases when a number of amino acids are combined with each other or with other metabolites. Gln/Glu, Asp, Arg, Leu/Ile, Lys, and Orn have the greatest significance in assessing the risk of breast cancer. The variability in the amino acid composition of biological fluids was shown to depend on the breast cancer phenotype, as well as the age, race, and menopausal status of patients. In general, the analysis of changes in the amino acid metabolism in breast cancer is a promising strategy not only for diagnosis, but also for developing new therapeutic agents, monitoring the treatment process, correcting complications after treatment, and evaluating survival rates.

Preclinical and Clinical Use of Indigenously Developed 99mTc-Diethylenetriaminepentaacetic Acid-Bis-Methionine: l-Type Amino Acid Transporter 1-Targeted Single Photon Emission Computed Tomography Radiotracer for Glioma Management

A new era in tumor classification, diagnosis, and prognostic evaluation has begun as a consequence of recent developments in the molecular and genetic characterization of central nervous system tumors. In this newly emerging era, molecular imaging modalities are essential for preoperative diagnosis, surgical planning, targeted treatment, and post-therapy evaluation of gliomas. The radiotracers are able to identify brain tumors, distinguish between low- and high-grade lesions, confirm a patient's eligibility for theranostics, and assess post-radiation alterations. We previously synthesized and reported the novel l-type amino acid transporter 1 (LAT-1)-targeted amino acid derivative in light of the use of amino acid derivatives in imaging technologies. Further, we have developed a single vial ready to label Tc-lyophilized kit preparations of diethylenetriaminepentaacetic acid-bis-methionine [DTPA-bis(Met)], also referred to as methionine-diethylenetriaminepentaacetic acid-methionine (MDM) and evaluated its imaging potential in numerous clinical studies. This review summarizes our previous publications on 99mTc-DTPA-bis(Met) in different clinical studies such as detection of breast cancer, as a prognostic marker, in detection of recurrent/residual gliomas, for differentiation of recurrent/residual gliomas from radiation necrosis, and for comparison of 99mTc-DTPA-bis(Met) with 11C-L-methionine (11C-MET), with relevant literature on imaging modalities in glioma management.

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.

Synthesis, Radiolabeling, and Biological Evaluation of the trans-Stereoisomers of 1-Amino-3-(fluoro-18F)-4-fluorocyclopentane-1-carboxylic Acid as PET Imaging Agents

The enantiomeric non-natural cyclic amino acids (3R,4R)-1-amino-3-fluoro-4-(fluoro-¹⁸F)cyclopentane-1-carboxylic acid and (3S,4S)-1-amino-3-fluoro-4-(fluoro-¹⁸F)cyclopentane-1-carboxylic acid ([ ¹⁸ F]5) have been prepared as a racemic mixture in 1.3% decay corrected radiochemical yield and in greater than 99% radiochemical purity. [ ¹⁸ F]5 is transported primarily via system L with some transport occurring via system ASC, as assessed in rat 9L gliosarcoma, human U87 ΔEGFR glioblastoma, and human DU145 androgen-independent prostate carcinoma tumor cells. In rats bearing intracranial 9L gliosarcoma, [ ¹⁸ F]5 gave tumor to contralateral brain tissue ratios of up to 2.8. Biodistribution studies in healthy rats demonstrated that bladder accumulation is delayed until 10 min postinjection.

Synthesis, Radiolabeling, and Biological Evaluation of the cis Stereoisomers of 1-Amino-3-Fluoro-4-(fluoro-18F)Cyclopentane-1-Carboxylic Acid as PET Imaging Agents

The non-natural cyclic amino acids (1S,3R,4S)-1-amino-3-fluoro-4-(fluoro-¹⁸F)cyclopentane-1-carboxylic acid ([¹⁸F]9) and (1S,3S,4R)-1-amino-3-fluoro-4-(fluoro-¹⁸F)cyclopentane-1-carboxylic acid ([¹⁸F]28) have been prepared in 10 and 1.7% decay corrected radiochemical yield, respectively, and in greater than 99% radiochemical purity. Cell assays in rat 9L gliosarcoma, human U87 ΔEGFR glioblastoma, and human DU145 androgen-independent prostate carcinoma tumor cells indicated that both compounds are substrates for amino acid transport primarily by system L, with some transport taking place via system ASC. In rats with 9L gliosarcoma, [¹⁸F]9 and [¹⁸F]28 provided high tumor to normal brain tissue ratios, with maximal ratios of 3.5 and 4.1, respectively. Biodistribution studies in healthy rats confirmed that both compounds are BBB permeable and that bladder accumulation is low until at least 5 min post injection.

Amino Acid Metabolism as a Target for Breast Cancer Imaging

Amino acids are an alternate energy source to glucose, and amino acid metabolism is up-regulated in multiple malignancies, including breast cancers. Multiple amino acid radiotracers have been used to image breast cancer with unique strengths and weaknesses. ¹¹C-methionine uptake correlates with S-phase fraction in breast cancer and may be useful for evaluation of treatment response. Invasive lobular breast cancers may demonstrate greater ¹⁸F-fluciclovine avidity than ¹⁸F-fluorodeoxyglucose. Thus, different histologic subtypes of breast cancer may use diverse metabolic pathways and may be better imaged by different tracers.

Synthesis and biological properties of radiohalogenated α,α-disubstituted amino acids for PET and SPECT imaging of amino acid transporters (AATs)

Fluorine-18 and iodine-123 labeled nonnatural alicyclic and methyl branched disubstituted α,α-amino acids are a diverse and useful class of tumor imaging agents suitable for positron emission tomography and single photon emission computed tomography. These tracers target the increased expression of the cell membrane amino acid transporter systems L, ASC, and A exhibited by many human tumor cells. The most established clinical use for these radiolabeled amino acids is imaging primary and recurrent gliomas and primary, recurrent, and metastatic prostate cancer. This review focuses on the synthesis, radiolabeling, and amino acid transport mechanism of a series of nonnatural fluorine-18 and iodine-123 labeled analogs of 1-aminocyclobutane-1-carboxylic acid, 1-aminocyclopentane-1-carboxylic acid, α-aminoisobutyric acid, and α-methylaminoisobutyric acid.

Small Molecule PET Tracers for Transporter Imaging

As the field of PET has expanded and an ever-increasing number and variety of compounds have been radiolabeled as potential in vivo tracers of biochemistry, transporters have become important primary targets or facilitators of radiotracer uptake and distribution. A transporter can be the primary target through the development of a specific high-affinity radioligand: examples are the multiple high-affinity radioligands for the neuronal membrane neurotransmitter or vesicular transporters, used to image nerve terminals in the brain. The goal of a radiotracer might be to study the function of a transporter through the use of a radiolabeled substrate, such as the application of 3-O-[¹¹C]methyl]glucose to measure rates of glucose transport through the blood-brain barrier. In many cases, transporters are required for radiotracer distributions, but the targeted biochemistries might be unrelated: an example is the use of 2-deoxy-2-[¹⁸F]FDG for imaging glucose metabolism, where initial passage of the radiotracer through cell membranes requires the action of specific glucose transporters. Finally, there are transporters such as p-glycoprotein that function to extrude small molecules from tissues, and can effectively work against successful uptake of radiotracers. The diversity of structures and functions of transporters, their importance in human health and disease, and their role in therapeutic drug disposition suggest that in vivo imaging of transporter location and function will continue to be a point of emphasis in PET radiopharmaceutical development. In this review, the variety of transporters and their importance for in vivo PET radiotracer development and application are discussed. Transporters have thus joined the other major protein targets such as G-protein coupled receptors, ligand-gated ion channels, enzymes, and aggregated proteins as of high interest for understanding human health and disease.

Evaluation of Prostate Cancer with Radiolabeled Amino Acid Analogs

Conventional imaging of prostate cancer has limitations related to the frequently indolent biology of the disease. PET is a functional imaging method that can exploit various aspects of tumor biology to enable greater detection of prostate cancer than can be provided by morphologic imaging alone. Radiotracers that are in use or under investigation for targeting salient features of prostate cancer include those directed to glucose, choline, acetate, prostate-specific membrane antigen, bombesin, and amino acids. The tumor imaging features of this last class of radiotracers mirror the upregulation of transmembrane amino acid transport that is necessary in carcinomas because of increased amino acid use for energy requirements and protein synthesis. Natural and synthetic amino acids radiolabeled for PET imaging have been investigated in prostate cancer patients. Early work with naturally occurring amino acid-derived radiotracers, such as l-¹¹C-methionine and l-1-¹¹C-5-hydroxytryptophan, demonstrated promising results, including greater sensitivity than ¹⁸F-FDG for intraprostatic and extraprostatic cancer detection. However, limitations with naturally occurring amino acid-derived compounds, including metabolism of the radiotracer itself, led to the development of synthetic amino acid radiotracers, which are not metabolized and therefore more accurately reflect transmembrane amino acid transport. Of the synthetic amino acid-derived PET radiotracers, anti-1-amino-3-¹⁸F-fluorocyclobutane-1-carboxylic acid (¹⁸F-FACBC or ¹⁸F-fluciclovine) has undergone the most promising translation to human use, including the availability of simplified radiosynthesis. Several studies have indicated advantageous biodistribution in the abdomen and pelvis with little renal excretion and bladder activity-characteristics beneficial for prostate cancer imaging. Studies have demonstrated improved lesion detection and diagnostic performance of ¹⁸F-fluciclovine in comparison with conventional imaging, especially for recurrent prostate cancer, although issues with nonspecific uptake limit the potential role of ¹⁸F-fluciclovine in the diagnosis of primary prostate cancer. Although work is ongoing, recently published intrapatient comparisons of ¹⁸F-fluciclovine with ¹¹C-choline reported higher overall diagnostic performance of the former, especially for the detection of disease relapse. This review is aimed at providing a detailed overview of amino acid-derived PET compounds that have been studied for use in prostate cancer imaging.

Amino acid transport system - A substrate predicts the therapeutic effects of particle radiotherapy

L-[methyl-¹¹C]Methionine (¹¹C-Met) is useful for estimating the therapeutic efficacy of particle radiotherapy at early stages of the treatment. Given the short half-life of ¹¹C, the development of longer-lived ¹⁸F- and ¹²³I-labeled probes that afford diagnostic information similar to ¹¹C-Met, are being sought. Tumor uptake of ¹¹C-Met is involved in many cellular functions such as amino acid transport System-L, protein synthesis, and transmethylation. Among these processes, since the energy-dependent intracellular functions involved with ¹¹C-Met are more reflective of the radiotherapeutic effects, we evaluated the activity of the amino acid transport System-A as an another energy-dependent cellular function in order to estimate radiotherapeutic effects. In this study, using a carbon-ion beam as the radiation source, the activity of System-A was evaluated by a specific System-A substrate, alpha-[1-¹⁴C]-methyl-aminoisobutyric acid (¹⁴C-MeAIB). Cellular growth and the accumulation of ¹⁴C-MeAIB or ¹⁴C-Met were evaluated over time in vitro in cultured human salivary gland (HSG) tumor cells (3-Gy) or in vivo in murine xenografts of HSG tumors (6- or 25-Gy) before and after irradiation with the carbon-ion beam. Post 3-Gy irradiation, in vitro accumulation of ¹⁴C-Met and ¹⁴C-MeAIB decreased over a 5-day period. In xenografts of HSG tumors in mice, tumor re-growth was observed in vivo on day-10 after a 6-Gy irradiation dose, but no re-growth was detected after the 25-Gy irradiation dose. Consistent with the growth results, the in vivo tumor accumulation of ¹⁴C-MeAIB did not decrease after the 6-Gy irradiation dose, whereas a significant decrease was observed after the 25-Gy irradiation dose. These results indicate that the activity of energy dependent System-A transporter may reflect the therapeutic efficacy of carbon-ion radiotherapy and suggests that longer half-life radionuclide-labeled probes for System-A may also provide widely available probes to evaluate the effects of particle radiotherapy on tumors at early stage of the treatment.

PET Tracers Beyond FDG in Prostate Cancer

Conventional anatomical imaging with CT and MRI has limitations in the evaluation of prostate cancer. PET is a powerful imaging technique, which can be directed toward molecular targets as diverse as glucose metabolism, density of prostate-specific membrane antigen receptors, and skeletal osteoblastic activity. Although 2-deoxy-2-18F-FDG-PET is the mainstay of molecular imaging, FDG has limitations in typically indolent prostate cancer. Yet, there are many useful and emerging PET tracers beyond FDG, which provide added value. These include radiotracers interrogating prostate cancer via molecular mechanisms related to the biology of choline, acetate, amino acids, bombesin, and dihydrotestosterone, among others. Choline is used for cell membrane synthesis and its metabolism is upregulated in prostate cancer. 11C-choline and 18F-choline are in wide clinical use outside the United States, and they have proven most beneficial for detection of recurrent prostate cancer. 11C-acetate is an indirect biomarker of fatty acid synthesis, which is also upregulated in prostate cancer. Imaging of prostate cancer with 11C-acetate is overall similar to the choline radiotracers yet is not as widely used. Upregulation of amino acid transport in prostate cancer provides the biologic basis for amino acid-based radiotracers. Most recent progress has been made with the nonnatural alicyclic amino acid analogue radiotracer anti-1-amino-3-18F-fluorocyclobutane-1-carboxylic acid (FACBC or fluciclovine) also proven most useful for the detection of recurrent prostate cancer. Other emerging PET radiotracers for prostate cancer include the bombesin group directed to the gastrin-releasing peptide receptor, 16β-18F-fluoro-5α-dihydrotestosterone (FDHT) that binds to the androgen receptor, and those targeting the vasoactive intestinal polypeptide receptor 1 (VPAC-1) and urokinase plasminogen activator receptor (uPAR), which are also overexpressed in prostate cancer.

Transport of haloacids across biological membranes

Haloacids are considered to be environmental pollutants, but some of them have also been tested in clinical research. The way that haloacids are transported across biological membranes is important for both biodegradation and drug delivery purposes. In this review, we will first summarize putative haloacids transporters and the information about haloacids transport when studying carboxylates transporters. We will then introduce MCT1 and SLC5A8, which are respective transporter for antitumor agent 3-bromopyruvic acid and dichloroacetic acid, and monochloroacetic acid transporters Deh4p and Dehp2 from a haloacids-degrading bacterium. Phylogenetic analysis of these haloacids transporters and other monocarboxylate transporters reveals their evolutionary relationships. Haloacids transporters are not studied to the extent that they deserve compared with their great application potentials, thus future inter-discipline research are desired to better characterize their transport mechanisms for potential applications in both environmental and clinical fields.

Study of LAT1 Expression in Brain Metastases: Towards a Better Understanding of the Results of Positron Emission Tomography Using Amino Acid Tracers

Positron emission tomography using radiolabeled amino acid (PET-AA) appears to be promising in distinguishing between recurrent tumour and radionecrosis in the follow-up of brain metastasis (BM). The amino acid transporter LAT1 and its cofactor CD98, which are involved in AA uptake, have never been investigated in BM. The aim of our study was to determine and compare the expression of LAT1 and CD98 in BM and in non-tumoral brain tissue (NT). The expression of LAT1 and CD98 were studied by immunohistochemistry in 67 BM, including 18 BM recurrences after radiotherapy, in 53 NT, and in 13 cases of patients with previously irradiated brain tumor and investigated by [18F] FDOPA-PET. LAT1 and CD98 expression were detected in 98.5% and 59.7% of BM respectively and were significantly associated with BM tissue as compared to NT (p<0.001). LAT1 expression in recurrent BM was significantly increased as compared to newly occurring BM. Ten cases investigated by [18F] FDOPA-PET corresponding to recurrent BM displayed significant [18F] FDOPA uptake and LAT1 overexpression whereas three cases corresponding to radionecrosis showed no or low uptake and LAT1 expression. LAT1 expression level and [18F] FDOPA uptake were significantly correlated. In conclusion, we hypothesized that BM may overexpress the AA transporter LAT1. We have shown that LAT1 overexpression was common in BM and was specific for BM as compared to healthy brain. These results could explain the specific BM uptake on PET-AA.

Synthesis and Biological Evaluation of (S)-Amino-2-methyl-4-[(76)Br]bromo-3-(E)-butenoic Acid (BrVAIB) for Brain Tumor Imaging

The novel compound, (S)-amino-2-methyl-4-[(76)Br]bromo-3-(E)-butenoic acid (BrVAIB, [(76)Br]5), was characterized against the known system A tracer, IVAIB ([(123)I]8). [(76)Br]5 was prepared in a 51% ± 19% radiochemical yield with high radiochemical purity (≥98%). The biological properties of [(76)Br]5 were compared with those of [(123)I]8. Results showed that [(76)Br]5 undergoes mixed amino acid transport by system A and system L transport, while [(123)I]8 had less uptake by system L. [(76)Br]5 demonstrated higher uptake than [(123)I]8 in DBT tumors 1 h after injection (3.7 ± 0.4% ID/g vs 1.5 ± 0.3% ID/g) and also showed higher uptake vs [(123)I]8 in normal brain. Small animal PET studies with [(76)Br]5 demonstrated good tumor visualization of intracranial DBTs up to 24 h with clearance from normal tissues. These results indicate that [(76)Br]5 is a promising PET tracer for brain tumor imaging and lead compound for a mixed system A and system L transport substrate.

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

A novel (18)F-labeled α,α-disubstituted amino acid-based tracer, 2-amino-5-[(18)F]fluoro-2-methylpentanoic acid ([(18)F]FAMPe), has been developed for brain tumor imaging with a longer alkyl side chain than previously reported compounds to increase brain availability via system L amino acid transport. Both enantiomers of [(18)F]FAMPe were obtained in good radiochemical yield (24-52% n = 8) and high radiochemical purity (>99%). In vitro uptake assays in mouse DBT gliomas cells revealed that (S)-[(18)F]FAMPe enters cells partly via sodium-independent system L transporters and also via other nonsystem A transport systems including transporters that recognize glutamine. Biodistribution and small animal PET/CT studies in the mouse DBT model of glioblastoma showed that both (R)- and (S)-[(18)F]FAMPe have good tumor imaging properties with the (S)-enantiomer providing higher tumor uptake and tumor to brain ratios. Comparison of the SUVs showed that (S)-[(18)F]FAMPe had higher tumor to brain ratios compared to (S)-[(18)F]FET, a well-established system L substrate.

Imaging tumor metabolism using positron emission tomography

Positron emission tomography (PET) is an extraordinarily sensitive clinical imaging modality for interrogating tumor metabolism. Radiolabeled PET substrates can be traced at subphysiological concentrations, allowing noninvasive imaging of metabolism and intratumoral heterogeneity in systems ranging from advanced cancer models to patients in the clinic. There are a wide range of novel and more established PET radiotracers, which can be used to investigate various aspects of the tumor, including carbohydrate, amino acid, and fatty acid metabolism. In this review, we briefly discuss the more established metabolic tracers and describe recent work on the development of new tracers. Some of the unanswered questions in tumor metabolism are considered alongside new technical developments, such as combined PET/magnetic resonance imaging scanners, which could provide new imaging solutions to some of the outstanding diagnostic challenges facing modern cancer medicine.