Synthesis of syn- and anti-1-amino-3-[18F]fluoromethyl-cyclobutane-1-carboxylic acid (FMACBC), potential PET ligands for tumor detection

The role of brown adipose tissue in branched-chain amino acid clearance in people

Brown adipose tissue (BAT) in rodents appears to be an important tissue for the clearance of plasma branched-chain amino acids (BCAAs) contributing to improved metabolic health. However, the role of human BAT in plasma BCAA clearance is poorly understood. Here, we evaluate patients with prostate cancer who underwent positron emission tomography-computed tomography imaging after an injection of 18F-fluciclovine (L-leucine analog). Supraclavicular adipose tissue (AT; primary location of human BAT) has a higher net uptake rate for 18F-fluciclovine compared to subcutaneous abdominal and upper chest AT. Supraclavicular AT 18F-fluciclovine net uptake rate is lower in patients with obesity and type 2 diabetes. Finally, the expression of genes involved in BCAA catabolism is higher in the supraclavicular AT of healthy people with high BAT volume compared to those with low BAT volume. These findings support the notion that BAT can potentially function as a metabolic sink for plasma BCAA clearance in people.

Fluorine-Effect-Enabled Photocatalytic 4-Exo-Trig Cyclization Cascade to Access Fluoroalkylated Cyclobutanes

Cyclobutanes are popular structural units in bioactive compounds and versatile intermediates in synthetic chemistry, but their synthesis is challenging owing to high ring strain. In this study, a novel method for highly regio- and diastereoselective synthesis of fluoroalkylcyclobutanes bearing vicinal quaternary and tertiary stereocenters is realized by a photocatalytic 4-exo-trig cyclization cascade of thioalkynes or trifluoromethylalkenes. Density functional theory calculations reveal that a unique fluorine effect, arising from hyperconjugative π→σ*C-F interactions, accounts for the regio-reversed radical addition at the sterically hindered alkene carbon, which facilitates an unprecedented 4-exo-trig ring closure. This chemistry enables the direct and controllable construction of medicinally valuable quaternary-carbon-containing cyclobutanes from readily available raw materials, nicely complementing the existing methods.

Potential Pitfall of Fluciclovine PET/CT in Castrate-Resistant Prostate Cancer With Adrenal Metastasis

ABSTRACT

An 83-year-old man with castrate-resistant prostate cancer underwent an 18 F-fluciclovine PET/CT scan, which was negative for local disease recurrence or locoregional lymphadenopathy, but there were multiple fluciclovine-avid bone metastases. In addition, mildly avid bilateral adrenal nodules were thought to be benign. However, on follow-up PET/CT 10 months later, while on additional therapy with enzalutamide, the bilateral nodules became mass lesions with interval decreased fluciclovine avidity. Adrenal metastases were suspected given their rapid growth, with subsequent CT-guided biopsy revealing metastatic prostate cancer without tumor necrosis. This false-negative case highlights the diagnostic challenge of fluciclovine PET in characterizing adrenal lesions.

18F-FDG and 18F-Fluciclovine Uptake in Poorly Differentiated Lung Adenocarcinoma in the Setting of Biochemically Recurrent Prostatic Adenocarcinoma

ABSTRACT

A 64-year-old man with a history of Gleason 7 (3 + 4) pT2cN0 prostatic adenocarcinoma status post prostatectomy underwent a fluciclovine PET/CT that showed a tracer-avid right upper lobe spiculated solitary pulmonary nodule. Follow-up FDG PET/CT showed a hypermetabolic right upper lobe spiculated solitary pulmonary nodule. Fine-needle aspiration was consistent with primary lung adenocarcinoma. Subsequently, right upper lobectomy was performed, and poorly differentiated lung adenocarcinoma was confirmed.

Fluorinated Rings: Conformation and Application

The introduction of fluorine atoms into molecules and materials across many fields of academic and industrial research is now commonplace, owing to their unique properties. A particularly interesting feature is the impact of fluorine substitution on the relative orientation of a C-F bond when incorporated into organic molecules. In this Review, we will be discussing the conformational behavior of fluorinated aliphatic carbo- and heterocyclic systems. The conformational preference of each system is associated with various interactions introduced by fluorine substitution such as charge-dipole, dipole-dipole, and hyperconjugative interactions. The contribution of each interaction on the stabilization of the fluorinated alicyclic system, which manifests itself in low conformations, will be discussed in detail. The novelty of this feature will be demonstrated by presenting the most recent applications.

Radiolabeling and Preclinical Evaluation of a New S-Alkylated Cysteine Derivative Conjugated to C-Substituted Macrocycle for Positron Emission Tomography

A new S-alkylated cysteine-derivatized tumor targeting agent, 2,2'-(12-(2-((2-acetamido-2-carboxyethyl)thio)acetamido)-11,13-dioxo-1,4,7,10-tetraazacyclotridecane-4,7-diyl)diacetic acid was developed for positron emission tomography (PET) imaging. N-Acetyl cysteine (NAC) was conjugated to ATRIDAT as a specific targeting agent toward L-type and ASC amino acid transporter systems in the oncogenic cells. NAC was attached via S-alkylation to prevent its incorporation at undesired recognition sites affecting the signal-to-noise ratio. NAC-ATRIDAT was subjected to gallium-68 complexation with >75% radiolabeling yield. The radiocomplex was purified through the tc18 cartridge to obtain 99.89% radiochemical yield. IC-50 of the NAC-ATRIDAT conjugate was 0.8 mM in A549 cells as evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazonium bromide assay. Binding affinity experiments on A549 cells showed noteworthy binding with K_D in the nanomolar range. A time course study showed a K_m value of 0.19 μM and V_max value of 0.49 pmol/μg protein/min showing reasonable tumor kinetics. Efflux studies showed that the synthesized radioligand is transported majorly by LAT followed by the ASC system. Clearance was found to be renal with 7.67 ± 1.48% ID/g uptake at 30 min which substantially declined to 0.52 ± 0.% ID/g at 4 h. A significant uptake of 10.06 ± 1.056% ID/g was observed at the tumor site in mice at 1 h. μPET images revealed a high contrast with a tumor-to-kidney ratio of 4.8 and a tumor-to-liver ratio of 35.85 at 1 h after injection. These preclinical in vitro and in vivo evaluation supports its potential on the way of becoming a successful ⁶⁸Ga-radiolabeled amino acid-based PET imaging agent.

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.

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.

Imaging of Prostate Cancer Using Fluciclovine

Prostate cancer is the most common cancer and the second leading cause of cancer death in men in the United States. Despite high disease prevalence, diagnosis and surveillance of the disease with conventional imaging are limited typically because of indolent biology. Functional imaging with advanced molecular techniques improves the ability to detect disease. Amino acids are building blocks of proteins, and intracellular transport of amino acids is upregulated in prostate cancer. This review provides a detailed overview of the use of F-18 fluciclovine PET in prostate cancer imaging.

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.

Simple preparation of new [(18) F]F-labeled synthetic amino acid derivatives with two click reactions in one-pot and SPE purification

New [(18) F]fluorinated 1,2,3-triazolyl amino acid derivatives were efficiently prepared from Huisgen 1,3-dipolar cycloaddition reactions, well known as click reaction. We developed two simultaneous click reactions in one-pot with a simple solid-phase extraction (SPE) purification method. [(18) F]fluoro-1-propyne was obtained at a 45% non-decay corrected radiochemical yield based on the [(18) F]fluoride ion. The one-pot and simultaneous two click reactions were performed with unprotected azido-alkyl amino acid, [(18) F]fluoro-1-propyne, and lipophilic additive alkyne to produce three synthetic amino acid derivatives, AMC-101 ([(18) F]-6a), AMC-102 ([(18) F]-6b), and AMC-103 ([(18) F]-6c) with 29%, 28%, and 24% of non-decay corrected radiochemical yields, respectively. All radiotracers indicated that radiochemical purities were >95% without any residual organic solvent. Our new method involving two click reactions in one-pot showed high radiochemical and chemical purity by easy removal of the residual precursor from the simultaneous two click reactions.

Synthesis and biological evaluation of (18)F-labeled Fluoroethoxy tryptophan analogues as potential PET tumor imaging agents

As a continuation of our research efforts toward the development of tryptophan-based radiotracers for tumor imaging with positron emission tomography (PET), three new fluoroethoxy tryptophan analogues were synthesized and evaluated in vivo. These new tracers (namely, 4-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]4-FEHTP), 6-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]6-FEHTP), and 7-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]7-FEHTP) carry the fluoroethoxy side chain either at positions 4-, 6-, or 7- of the indole core. Reference compounds and precursors were synthesized by multistep approaches. Radiosynthesis was accomplished by no-carrier-added nucleophilic (18)F-fluorination following either an indirect approach (O-alkylation of the corresponding hydroxytryptophan with [(18)F]fluoroethyltosylate) or a direct approach (nucleophilic [(18)F] fluorination using a protected mesyl precursor). Radiochemical yields (decay corrected) for both methods were in the range of 10-18%. Small animal PET imaging with xenograft-bearing mice revealed the highest tumor/background ratio for [(18)F]6-FEHTP which, in a direct comparison, outperformed the other two tryptophan tracers and also the well-established tyrosine analogue O-(2-[(18)F]fluoroethyl)-l-tyrosine ([(18)F]l-FET). Investigation of the transport mechanism of [(18)F]6-FEHTP in small cell lung cancer cells (NCI-H69) revealed that it is most probably taken up exclusively via the large neutral amino acid transporter(s) (LAT).

Synthesis and evaluation of ¹⁸F labeled FET prodrugs for tumor imaging

INTRODUCTION

O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET, [(18)F]1) is a useful amino-acid-based imaging agent for brain tumors. This paper reports the synthesis and evaluation of three FET prodrugs, O-(2-[(18)F]fluoroethyl)-L-tyrosyl-L-glycine (FET-Gly, [(18)F]2), O-(2-[(18)F]fluoroethyl)-L-tyrosyl-L-alanine (FET-Ala, [(18)F]3) and N-acetyl O-(2-[(18)F]fluoroethyl)-L-tyrosine (AcFET, [(18)F]4), which could be readily hydrolyzed to FET in vivo for tumor imaging. We investigated their metabolism in the blood and imaging properties in comparison to FET ([(18)F]1).

METHODS

Three new [(18)F]FET derivatives, 2-4, were prepared from their corresponding tosylate-precursors through nucleophilic fluorination and subsequent deprotection reactions. In vitro uptake studies were carried out in 9L glioma cancer cell lines. In vitro and in vivo hydrolysis studies were conducted to evaluate the hydrolysis of FET prodrugs in blood and in Fisher 344 rats. Biodistribution and PET imaging studies were then performed in rats bearing 9L tumors.

RESULTS

New FET prodrugs were prepared with 3-28% decay corrected radiochemical yields, good enantiomeric purity (>95%) and high radiochemical purity (>95%). FET-Gly ([(18)F]2), FET-Ala ([(18)F]3), and AcFET ([(18)F]4) exhibited negligible uptake in comparison to the high uptake of FET ([(18)F]1) in 9L cells. Metabolism studies of FET-Gly ([(18)F]2), FET-Ala ([(18)F]3), and AcFET ([(18)F]4) in rat and human blood showed that FET-Ala ([(18)F]3) was hydrolyzed to FET ([(18)F]1) faster than FET-Gly ([(18)F]2) or AcFET ([(18)F]4). Most of the FET-Ala (79%) was converted to FET ([(18)F]1) within 5min in blood in vivo. Biodistribution studies demonstrated that FET-Ala ([(18)F]3) displayed the highest tumor uptake. The tumor-to-background ratios of FET-Ala ([(18)F]3) and FET ([(18)F]1) were comparable and appeared to be better than those of FET-Gly ([(18)F]2) and AcFET ([(18)F]4). PET imaging studies showed that both FET ([(18)F]1) and FET-Ala ([(18)F]3) could visualize tumors effectively, and that they share similar imaging characteristics.

CONCLUSIONS

FET-Ala ([(18)F]3) demonstrated promising properties as a prodrug of FET ([(18)F]1), which could be used in PET imaging of tumor amino acid metabolism.

Synthesis and evaluation of 18F labeled alanine derivatives as potential tumor imaging agents

INTRODUCTION

This paper reports the synthesis and labeling of (18)F alanine derivatives. We also investigate their biological characteristics as potential tumor imaging agents mediated by alanine-serine-cysteine preferring (ASC) transporter system.

METHODS

Three new (18)F alanine derivatives were prepared from corresponding tosylate-precursors through a two-step labeling reaction. In vitro uptake studies to evaluate and to compare these three analogs were carried out in 9L glioma and PC-3 prostate cancer cell lines. Potential transport mechanisms, protein incorporation and stability of 3-(1-[(18)F]fluoromethyl)-L-alanine (L-[(18)F]FMA) were investigated in 9L glioma cells. Its biodistribution was determined in a rat-bearing 9L tumor model. PET imaging studies were performed on rat bearing 9L glioma tumors and transgenic mouse carrying spontaneous generated M/tomND tumor (mammary gland adenocarcinoma).

RESULTS

New (18)F alanine derivatives were prepared with 7%-34% uncorrected radiochemical yields, excellent enantiomeric purity (>99%) and good radiochemical purity (>99%). In vitro uptake of the L-[(18)F]FMA in 9L glioma and PC-3 prostate cancer cells was higher than that observed for the other two alanine derivatives and [(18)F]FDG in the first 1h. Inhibition of cell uptake studies suggested that L-[(18)F]FMA uptake in 9L glioma was predominantly via transport system ASC. After entering into cells, L-[(18)F]FMA remained stable and was not incorporated into protein within 2h. In vivo biodistribution studies demonstrated that L-[(18)F]FMA had relatively high uptake in liver and kidney. Tumor uptake was fast, reaching a maximum within 30 min. The tumor-to-muscle, tumor-to-blood and tumor-to-brain ratios at 60 min post injection were 2.2, 1.9 and 3.0, respectively. In PET imaging studies, tumors were visualized with L-[(18)F]FMA in both 9L rat and transgenic mouse.

CONCLUSION

L-[(18)F]FMA showed promising properties as a PET imaging agent for up-regulated ASC transporter associated with tumor proliferation.

PET imaging of glutaminolysis in tumors by 18F-(2S,4R)4-fluoroglutamine

Changes in gene expression, metabolism, and energy requirements are hallmarks of cancer growth and self-sufficiency. Upregulation of the PI3K/Akt/mTor pathway in tumor cells has been shown to stimulate aerobic glycolysis, which has enabled (18)F-FDG PET tumor imaging. However, of the millions of (18)F-FDG PET scans conducted per year, a significant number of malignant tumors are (18)F-FDG PET-negative. Recent studies suggest that several tumors may use glutamine as the key nutrient for survival. As an alternative metabolic tracer for tumors, (18)F-(2S,4R)4-fluoroglutamine was developed as a PET tracer for mapping glutaminolytic tumors.

METHODS

A series of in vitro cell uptake and in vivo animal studies were performed to demonstrate tumor cell addiction to glutamine. Cell uptake studies of this tracer were performed in SF188 and 9L glioblastoma tumor cells. Dynamic small-animal PET studies of (18)F-(2S,4R)4-fluoroglutamine were conducted in 2 animal models: xenografts produced in F344 rats by subcutaneous injection of 9L tumor cells and transgenic mice with M/tomND spontaneous mammary gland tumors.

RESULTS

In vitro studies showed that both transformed 9L and SF188 tumor cells displayed a high rate of glutamine uptake (maximum uptake, ≈ 16% dose/100 μg of protein). The cell uptake of (18)F-(2S,4R)4-fluoroglutamine by SF188 cells is comparable to that of (3)H-L-glutamine but higher than that of (18)F-FDG. The tumor cell uptake can be selectively blocked. Biodistribution and PET studies showed that (18)F-(2S,4R)4-fluoroglutamine localized in tumors with a higher uptake than in surrounding muscle and liver tissues. Data suggest that certain tumor cells may use glutamine for energy production.

CONCLUSION

The results support that (18)F-(2S,4R)4-fluoroglutamine is selectively taken up and trapped by tumor cells. It may be useful as a novel metabolic tracer for tumor imaging.

Detection of recurrent prostate carcinoma with anti-1-amino-3-18F-fluorocyclobutane-1-carboxylic acid PET/CT and 111In-capromab pendetide SPECT/CT

PURPOSE

To compare the diagnostic performance of the synthetic amino acid analog radiotracer anti-1-amino-3-fluorine 18-fluorocyclobutane-1-carboxylic acid (anti-3-(18)F-FACBC) with that of indium 111 ((111)In)-capromab pendetide in the detection of recurrent prostate carcinoma.

MATERIALS AND METHODS

This prospective study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained. Fifty patients (mean age, 68.3 years ± 8.1 [standard deviation]; age range, 50-90 years) were included in the study on the basis of the following criteria: (a) Recurrence of prostate carcinoma was suspected after definitive therapy for localized disease, (b) bone scans were negative, and (c) anti-3-(18)F-FACBC positron emission tomography (PET)/computed tomography (CT) and (111)In-capromab pendetide single photon emission computed tomography (SPECT)/CT were performed within 6 weeks of each other. Studies were evaluated by two experienced interpreters for abnormal uptake suspicious for recurrent disease in the prostate bed and extraprostatic locations. The reference standard was a combination of tissue correlation, imaging, laboratory, and clinical data. Diagnostic performance measures were calculated and tests of the statistical significance of differences determined by using the McNemar χ(2) test as well as approximate tests based on the difference between two proportions.

RESULTS

For disease detection in the prostate bed, anti-3-(18)F-FACBC had a sensitivity of 89% (32 of 36 patients; 95% confidence interval [CI]: 74%, 97%), specificity of 67% (eight of 12 patients; 95% CI: 35%, 90%), and accuracy of 83% (40 of 48 patients; 95% CI: 70%, 93%). (111)In-capromab pendetide had a sensitivity of 69% (25 of 36 patients; 95% CI: 52%, 84%), specificity of 58% (seven of 12 patients; 95% CI: 28%, 85%), and accuracy of 67% (32 of 48 patients; 95% CI: 52%, 80%). In the detection of extraprostatic recurrence, anti-3-(18)F-FACBC had a sensitivity of 100% (10 of 10 patients; 95% CI: 69%, 100%), specificity of 100% (seven of seven patients; 95% CI: 59%, 100%), and accuracy of 100% (17 of 17 patients; 95% CI: 80%, 100%). (111)In-capromab pendetide had a sensitivity of 10% (one of 10 patients; 95% CI: 0%, 45%), specificity of 100% (seven of seven patients; 95% CI: 59%, 100%), and accuracy of 47% (eight of 17 patients; 95% CI: 23%, 72%).

CONCLUSION

anti-3-(18)F-FACBC PET/CT was more sensitive than (111)In-capromab pendetide SPECT/CT in the detection of recurrent prostate carcinoma and is highly accurate in the differentiation of prostatic from extraprostatic disease.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.11102023/-/DC1.

Potential Biomarker of L-type Amino Acid Transporter 1 in Breast Cancer Progression

PURPOSE

L-type amino acid transporter 1 (LAT1) is essential for the transport of large neutral amino acids. However, its role in breast cancer growth remains largely unknown. The purpose of the study is to investigate whether LAT1 is a potential biomarker for the diagnosis and treatment of breast cancer.

METHODS

LAT1 mRNA and protein levels in breast cancer cell lines and tissues were analyzed. In addition, the effects of targeting LAT1 for the inhibition of breast cancer cell tumorigenesis were assessed with soft agar assay. The imaging of xenograft with anti-1-amino-3-[(18)F]fluorocyclobutane-1-carboxylic acid (anti-[(18)F]FACBC) PET was assessed for its diagnostic biomarker potential.

RESULTS

Normal breast tissue or low malignant cell lines expressed low levels of LAT1 mRNA and protein, while highly malignant cancer cell lines and high-grade breast cancer tissue expressed high levels of LAT1. In addition, higher expression levels of LAT1 in breast cancer tissues were consistent with advanced-stage breast cancer. Furthermore, the blockade of LAT1 with its inhibitor, 2-amino-bicyclo[2.2.1]heptane-2-carboxylic acid (BCH), or the knockdown of LAT1 with siRNA, inhibited proliferation and tumorigenesis of breast cancer cells. A leucine analog, anti-[(18)F]FACBC, has been demonstrated to be an excellent PET tracer for the non-invasive imaging of malignant breast cancer using an orthotopic animal model.

CONCLUSIONS

The overexpression of LAT1 is required for the progression of breast cancer. LAT1 represents a potential biomarker for therapy and diagnosis of breast cancer. Anti-[(18)F]FACBC that correlates with LAT1 function is a potential PET tracer for malignant breast tumor imaging.

(R,S)-anti-1-amino-2-[18F]fluorocyclopentyl-1-carboxylic acid: synthesis from racemic 2-benzyloxycyclopentanone and biological evaluation for brain tumor imaging with positron emission tomography

(R,S)-anti-1-amino-2-fluorocyclopentyl-1-carboxylic acid (2-FACPC, 4b) was radiolabeled in 39% yield starting from cyclic sulfamidate 12. The 9L gliosarcoma cells assays showed that 4b is mainly a substrate for the L-type amino acid transport with some affinity to the A-type. In rats bearing 9L gliosarcoma tumors, 4b displayed high tumor to brain ratio (10:1) at 120 min after injection. FACPC is an attractive candidate for imaging brain tumors with PET, and its isolated enantiomers are under investigation.

Synthesis, radiolabeling, and biological evaluation of (R)- and (S)-2-amino-3-[(18)F]fluoro-2-methylpropanoic acid (FAMP) and (R)- and (S)-3-[(18)F]fluoro-2-methyl-2-N-(methylamino)propanoic acid (NMeFAMP) as potential PET radioligands for imaging brain tumors

The non-natural amino acids (R)- and (S)-2-amino-3-fluoro-2-methylpropanoic acid 5 and (R)- and (S)-3-fluoro-2-methyl-2-N-(methylamino)propanoic acid 8 were synthesized in shorter reaction sequences than in the original report starting from enantiomerically pure (S)- and (R)-alpha-methyl-serine, respectively. The reaction sequence provided the cyclic sulfamidate precursors for radiosynthesis of (R)- and (S)-[(18)F]5 and (R)- and (S)-[(18)F]8 in fewer steps than in the original report. (R)- and (S)-[(18)F]5 and(R)- and (S)-[(18)F]8 were synthesized by no-carrier-added nucleophilic [(18)F]fluorination in 52-66% decay-corrected yields with radiochemical purity over 99%. The cell assays showed that all four compounds were substrates for amino acid transport and enter 9L rat gliosarcoma cells in vitro at least in part by system A amino acid transport. The biodistribution studies demonstrated that in vivo tumor to normal brain ratios for all compounds were high with ratios of 20:1 to115:1 in rats with intracranial 9L tumors. The (R)-enantiomers of [(18)F]5 and [(18)F]8 demonstrated higher tumor uptake in vivo compared to the (S)-enantiomers.

Stereoselective synthesis and biological evaluation of syn-1-amino-3-[18F]fluorocyclobutyl-1-carboxylic acid as a potential positron emission tomography brain tumor imaging agent

Amino acid syn-1-amino-3-fluoro-cyclobutyl-1-carboxylic acid (syn-FACBC) 12, the isomer of anti-FACBC, has been selectively synthesized and [(18)F] radiofluorinated in 52% decay-corrected yield using no-carrier-added [(18)F]fluoride. The key step in the synthesis of the desired isomer involved stereoselective reduction using lithium alkylborohydride/zinc chloride, which improved the ratio of anti-alcohol to syn-alcohol from 17:83 to 97:3. syn-FACBC 12 entered rat 9L gliosarcoma cells primarily via L-type amino acid transport in vitro with high uptake of 16% injected dose per 5 x 10(5) cells. Biodistribution studies in rats with 9L gliosarcoma brain tumors demonstrated high tumor to brain ratio of 12:1 at 30 min post injection. In this model, amino acid syn-[(18)F]FACBC 12 is a promising metabolically based radiotracer for positron emission tomography brain tumor imaging.

Non-natural amino acids for tumor imaging using positron emission tomography and single photon emission computed tomography

Amino acids are required nutrients for proliferating tumor cells, and amino acid transport is upregulated in many tumor types. Studies of radiolabeled amino acids in animals and humans demonstrate that amino acid based tracers have advantageous characteristics relative to 2-[(18)F]fluoro-2-deoxyglucose in certain tumors, particularly brain gliomas. Non-natural amino acids for tumor imaging generally have greater metabolic stability and can be labeled with longer-lived radionuclides for positron emission tomography and single photon emission computed tomography such as fluorine-18 and iodine-123. Amino acids enter cells via amino acid transport with varying selectivity based on their chemical structure. This review focuses on the rationale, biological basis, current status and future prospects of radiolabeled non-natural amino acids for tumor imaging and discusses various classes of these compounds including aromatic, alicyclic and alpha,alpha-dialkyl amino acids.

Synthesis and evaluation of [123I] labeled iodovinyl amino acids syn-, anti-1-amino-3-[2-iodoethenyl]-cyclobutane-1-carboxylic acid, and 1-amino-3-iodomethylene-cyclobutane-1-carboxylic acid as potential SPECT brain tumor imaging agents

syn- and anti-1-amino-3-[2-iodoethenyl]-cyclobutane-1-carboxylic acid (syn-, anti-IVACBC 16, 17) and their analogue 1-amino-3-iodomethylene-cyclobutane-1-carboxylic acid (gem-IVACBC 18) were synthesized and radioiodoinated with [(123)I] in 34-43% delay-corrected yield. All these amino acids entered 9L gliosarcoma cells primarily via L-type transport in vitro with high uptake of 8-10% ID/1 x 10(6) cells. Biodistribution studies of [(123)I]16, 17 and 18 in rats with 9L gliosarcoma brain tumors demonstrated high tumor to brain ratios (4.7-7.3:1 at 60 min post-injection). In this model, syn-, anti-, and gem-[(123)I]IVACBC are promising radiotracers for SPECT brain tumor imaging.

Syntheses of F-18 labeled fluoroalkyltyrosine derivatives and their biological evaluation in rat bearing 9L tumor

We hereby report the synthesis of four fluorine-18 labeled tyrosine derivatives, 3-(2-[(18)F]fluoroethyl)tyrosine ([(18)F]1, [(18)F]ortho-FET), 3-(3-[(18)F]fluoropropyl)tyrosine ([(18)F]2, [(18)F]ortho-FPT) O-methyl-[3-(2-[(18)F]fluoroethyl)]tyrosine ([(18)F]3, [(18)F]MFET), and O-methyl-[3-(3-[(18)F]fluoropropyl)]tyrosine ([(18)F]4, [(18)F]MFPT). The fluorine-18 labeled tyrosine derivatives were prepared by the displacement reaction of the ethyl and propyl tosylates with K[(18)F]/K2.2.2 in acetonitrile under no-carrier-added (NCA) conditions, followed by hydrolysis with 4N HCl. The biological properties of labeled compounds were evaluated in rats bearing 9L tumor after an intravenous injection and PET image was obtained. The tumor/blood and tumor/brain ratios were 2.06, 2.92 for [(18)F]1, 2.25, 4.05 for [(18)F]2, 2.88, 1.90 for [(18)F]3, and 2.00, 2.60 for [(18)F]4 at 60 min post injection, respectively. The PET image showed localized accumulation of PET tracers in 9L glioma of the rat.