Transport of cis- and trans-4-[(18)F]fluoro-L-proline in F98 glioma cells

Application of chiral chromatography in radiopharmaceutical fields: A review

Chiral column chromatography (CCC) is a revolutionary analytical methodology for the enantioseparation of novel positron emission tomography (PET) tracers in the primary stages of drug development. Due to the different behaviors of tracer enantiomers (e.g. toxicity, metabolism and side effects) in administrated subjects, their separation and purification is a challenging endeavor. Over the last three decades, different commercial chiral columns have been applied for the enantioseparation of PET-radioligand (PET-RL) or radiotracers (PET-RT), using high-performance liquid chromatography (HPLC). The categorization and reviewing of them is a vital topic. This review presents a brief overview of advances, applications, and future prospectives of CCC in radiopharmaceutical approaches. In addition, the effective chromatographic parameters and degravitation trends to enhance enantioseparation resolution are addressed. Moreover, the application and potential of chiral super fluidical chromatography (CSFC) as an alternative for enantioseparation in the field of radiopharmaceutical is discussed. Finally, the crucial application challenges of CCC are explained and imminent tasks are suggested.

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.

Biological evaluation of new [(18) F]F-labeled synthetic amino acid derivatives as oncologic radiotracers

The present study evaluated the tumoral uptake of the novel synthetic amino acid positron emission tomography (PET) tracers (S)-2-amino-3-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)propanoic acid (AMC-101), (S)-2-amino-4-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)butanoic acid (AMC-102), and (S)-2-amino-5-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)pentanoic acid (AMC-103), all of which are (S)-2-amino-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)alkyl acids. In vitro cellular uptake was investigated using the rat glioma cell lines 9L and C6. In vitro competitive inhibition tests were performed to identify the involvement of specific amino acid transporters. In vivo dynamic PET images of 9L xenograft tumor-bearing model mice were acquired over 2 h after AMC administration. [(18) F]FDOPA PET studies were performed with and without S-carbidopa pretreatment for comparison. All three AMCs exhibited good in vitro cell uptake through the L and alanine-serine-cysteine transporters and enabled clear tumor visualization on PET, leaving the brain devoid of the tracer. Thirty minutes after injection, the mean tumor standardized uptake values were 1.59 ± 0.05, 1.89 ± 0.27, and 1.74 ± 0.13 for AMC-101, AMC-102, and AMC-103, respectively. Although the tumor uptake values of AMCs were lower than that of [(18) F]FDOPA with S-carbidopa pretreatment, AMCs enabled higher contrast images with lower background activity compared with [(18) F]FDOPA with S-carbidopa pretreatment. Our results indicate the potential uses of these new synthetic amino acids as oncologic radiotracers.

4-Fluoroprolines: Conformational Analysis and Effects on the Stability and Folding of Peptides and Proteins

Proline is unique among proteinogenic amino acids because a pyrrolidine ring links its amino group to its side chain. This heterocycle constrains the conformations of the main chain and thus templates particular secondary structures. Proline residues undergo post-translational modification at the 4-position to yield 4-hydroxyproline, which is especially prevalent in collagen. Interest in characterizing the effects of this modification led to the use of 4-fluoroprolines to enhance inductive properties relative to the hydroxyl group of 4-hydroxyproline and to eliminate contributions from hydrogen bonding. The strong inductive effect of the fluoro group has three main consequences: enforcing a particular pucker upon the pyrrolidine ring, biasing the conformation of the preceding peptide bond, and accelerating cis/trans prolyl peptide bond isomerization. These subtle, yet reliable modulations make 4-fluoroproline-incorporation a complement to traditional genetic approaches for exploring structure-function relationships in peptides and proteins, as well as for endowing peptides and proteins with conformational stability.

Synthesis and Characterization of ¹⁸F-Interleukin-8 Using a Cell-Free Translation System and 4-¹⁸F-Fluoro-L-Proline

Macromolecules such as proteins are attracting increasing interest for molecular imaging. We previously proposed a novel strategy for preparing macromolecules labeled with a PET radionuclide, (11)C, using a cell-free translation system with (11)C-methionine. However, macromolecules tend to exhibit slower kinetics, thus requiring a longer scanning time. Here, we expand our strategy using (18)F, which has a longer half-life, with the cell-free translation system with 4-(18)F-fluoro-L-proline ((18)F-FPro). We evaluated (18)F-interleukin-8 ((18)F-IL-8) produced by this method in vitro and in vivo to provide a proof of concept of our strategy.

METHODS

We tested some fluorinated amino acids to be incorporated into a protein. Trans-(18)F-FPro was radiolabeled from the corresponding precursor. (18)F-IL-8 was produced using the cell-free translation system with trans-(18)F-FPro instead of natural L-proline with incubation at 37°C for 120 min. An in vitro binding assay of (18)F-IL-8 was performed using IL-8 receptor-expressing cells. After intravenous administration of (18)F-IL-8, in vivo PET imaging of IL-8 receptor-expressing xenograft-bearing mice was performed using a small-animal PET system.

RESULTS

FPro was identified as an amino acid incorporated into the protein. (18)F-IL-8 was successfully prepared using the cell-free translation system and trans-(18)F-FPro with the radiochemical yield of 1.5% (decay-corrected) based on trans-(18)F-FPro. In vitro binding assays of (18)F-IL-8 demonstrated its binding to IL-8 receptor. In vivo PET imaging demonstrated that (18)F-IL-8 clearly accumulated in IL-8 receptor-expressing xenografts in mice, unlike trans-(18)F-FPro.

CONCLUSION

(18)F-IL-8 produced by this method binds to IL-8 receptors in vitro, and (18)F-IL-8 PET clearly visualizes its target receptor-expressing xenograft in vivo. Therefore, this technique might be useful for labeling macromolecules and performing preclinical evaluations of proteins of interest in vitro and in vivo.

Radiosynthesis and biological evaluation of N-[18F]labeled glutamic acid as a tumor metabolic imaging tracer

We have previously reported that N-(2-[18F]fluoropropionyl)-L-methionine ([18F]FPMET) selectively accumulates in tumors. However, due to the poor pharmacokinetics of [18F]FPMET in vivo, the potential clinical translation of this observation is hampered. In this study, we rationally designed and synthesized [18F] or [11C]labeled N-position L-glutamic acid analogues for tumor imaging. N-(2-[18F]fluoropropionyl)-L-glutamic acid ([18F]FPGLU) was synthesized with a 30±10% (n = 10, decay-corrected) overall radiochemical yield and a specific activity of 40±25 GBq/μmol (n = 10) after 130 min of radiosynthesis. In vitro cell experiments showed that [18F]FPGLU was primarily transported through the XAG(-) system and was not incorporated into protein. [18F]FPGLU was stable in urine, tumor tissues, and blood. We were able to use [18F]FPGLU in PET imaging and obtained high tumor to background ratios when visualizing tumors tissues in animal models.

Radiosynthesis and biological evaluation of 5-(3-[18F]fluoropropyloxy)-L-tryptophan for tumor PET imaging

INTRODUCTION

[(18)F]FDG PET has difficulty distinguishing tumor from inflammation in the clinic because of the same high uptake in nonmalignant and inflammatory tissue. In contrast, amino acid tracers do not accumulate in inflamed tissues and thus provide an excellent opportunity for their use in clinical cancer imaging. In this study, we developed a new amino acid tracer 5-(3-[(18)F]Fluoropropyloxy)-L-tryptophan ([(18)F]-L-FPTP) by two-step reactions and performed its biologic evaluation.

METHODS

[(18)F]-L-FPTP was prepared by [(18)F]fluoropropylation of 5-hydroxy-L-tryptophan disodium salt and purification on C18 cartridges. The biodistribution of [(18)F]-L-FPTP was determined in normal mice and the incorporation of [(18)F]-L-FPTP into tissue proteins was investigated. In vitro competitive inhibition experiments were performed with Hepa1-6 hepatoma cell lines. [(18)F]-L-FPTP PET imaging was performed on tumor-bearing and inflammation mice and compared with [(18)F]-L-FEHTP PET.

RESULTS

The overall uncorrected radiochemical yield of [(18)F]-L-FPTP was 21.1 ± 4.4% with a synthesis time of 60 min, the radiochemical purity was more than 99%. Biodistribution studies demonstrate high uptake of [(18)F]-L-FPTP in liver, kidney, pancreas, and blood at the early phase, and fast clearance in most tissues over the whole observed time. The uptake studies in Hepa1-6 cells suggest that [(18)F]-L-FPTP is transported by the amino acid transport system B(0,+), LAT2 and ASC. [(18)F]-L-FPTP displays good stability and is not incorporated into proteins in vitro. PET imaging shows that [(18)F]-L-FPTP can be a better potential PET tracer for differentiating tumor from inflammation than [(18)F]FDG and 5-(3-[(18)F]fluoroethyloxy)-L-tryptophan ([(18)F]-L-FEHTP), with high [(18)F]-L-FPTP uptake ratio (2.53) of tumor to inflammation at 60 min postinjection.

CONCLUSIONS

Using [(18)F]fluoropropyl derivatives as intermediates, the new tracer [(18)F]-L-FPTP was achieved with good yield and radiochemical purity, and the biological evaluation results of [(18)F]-L-FPTP showed that it was a hopeful tracer for PET 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.

cis-4-[(18)F]-Fluoro-l-proline fails to detect peripheral tumors in humans

System A amino acid transport is increased in transformed and malignant cells. The amino acid 4-cis[(18)F]fluoro-l-proline (cis-[(18)F]FPro) has been shown to be a substrate of the System A amino acid carrier. In this pilot study, we investigated the diagnostic potential of cis-[(18)F]FPro in patients with various tumors in comparison with [(18)F]fluorodeoxyglucose-positron emission tomography (FDG-PET).

METHODS

Eight patients (seven females, one male, age range 43-77 years) with large primary, recurrent or metastatic tumors of different histologies were included in this study. One patient had a recurrent non-Hodgkin lymphoma; two patients, metastatic colon or rectal cancer; one, a metastatic endometrial cancer; one, a multiple myeloma; one, an Ewing sarcoma; one, a metastatic breast cancer and one, a gastrointestinal stromal tumor. PET scans of the trunk were acquired at 1 h after intravenous injection of 400 MBq cis-[(18)F]FPro and compared to PET scans with [(18)F]FDG.

RESULTS

None of the tumors or metastatic lesions in this series of patients demonstrated relevant uptake of cis-[(18)F]FPro. In contrast, all tumors with exception of the multiple myeloma showed an intensive uptake of [(18)F]FDG. The mean standardized uptake value of cis-[(18)F]FPro in the tumor or metastases was significantly lower than that of [(18)F]FDG uptake (1.7+/-0.6 vs. 5.7+/-3.0; n=8; P<.01).

CONCLUSION

Although other System A-specific tracers have shown relevant tumor uptake, cis-[(18)F]FPro fails to detect most types of human tumors. Based on these results, we cannot recommend a further evaluation of this tracer as a tumor-seeking agent.

Preferred transport of O-(2-[18F]fluoroethyl)-d-tyrosine (d-FET) into the porcine brain

Amino acids are valuable tracers for brain tumor imaging with positron emission tomography (PET). In this study the transport of O-(2-[(18)F]fluoroethyl)-D-tyrosine (D-FET) across the blood-brain barrier (BBB) was studied with PET in anesthetized piglets and patients after subtotal resection of brain tumors and compared with O-(2-[(18)F]fluoroethyl)-L-tyrosine (L-FET) and 3-O-methyl-6-[(18)F]fluoro-L-DOPA (L-OMFD). In piglets, compartmental modeling of PET data was used to calculate the rate constants for the blood-brain (K(1)) and the brain-blood (k(2)) transfer of D-FET, L-FET and L-OMFD. In patients standardized uptake values (SUVs) were calculated in brain cortex and lesions. Additionally, affinity determinations on various amino acid transporters (LAT1, LAT2, PAT1, XPCT) were performed in vitro using unlabeled D-FET, L-FET and L-OMFD. The initial brain uptake of D-FET in piglets was more than two-fold higher than that of l-FET, whereas the initial brain uptake of D-FET in patients was similar to that of L-FET. Calculation of K(1) and k(2) from the brain uptake curves and the plasma input data in piglets revealed about 4- and 2-fold higher values for D-FET compared to L-FET and L-OMFD, respectively. The distribution volume of D-FET in the piglet brain was slightly higher than that of L-FET as it was also found for most other organs. In brain tumor patients, initial D-FET uptake in the brain was similar to that of L-FET but showed faster tracer washout. L-FET uptake remained rather constant and provided a better delineation of residual tumor than D-FET. In conclusion, our data indicate considerable differences of stereoselective amino acid transport at the BBB in different species. Therefore, the results from animal experiments concerning BBB amino acid transport may not be transferable to humans.

Transport of L-proline, L-proline-containing peptides and related drugs at mammalian epithelial cell membranes

Membrane transport of L-proline has received considerable attention in basic and pharmaceutical research recently. Of the most recently cloned members of the solute carrier family, two are "proline transporters". The amino acid transporter PAT1, expressed in intestine, kidney, brain and other organs, mediates the uptake of proline and derivatives in a pH gradient-dependent manner. The Na(+)-dependent proline transporter SIT1, cloned in 2005, exhibits the properties of the long-sought classical IMINO system. Proline-containing peptides are of interest for several reasons. Many biologically important peptide sequences contain highly conserved proline residues. Xaa-Pro peptides are very often resistant to enzymatic hydrolysis and display, in contrast to Pro-Xaa peptides, a high affinity to the H(+)/peptide cotransporter PEPT1 which is expressed in intestinal, renal, lung and biliary duct epithelial cells. Furthermore, several orally available drugs are recognized by PEPT1 as Xaa-Pro analogues due to their sterical resemblance to small peptides.

Preferred stereoselective transport of the D-isomer of cis-4-[18F]fluoro-proline at the blood-brain barrier

Generally, L-amino acids are preferably transported into mammalian cells compared with their D-isomers, and only L-amino acids are incorporated into proteins. Former studies, however, indicated that D-[H]proline is accumulated in the brain of mice after injection, while L-[3H]proline is not. We investigated the differential cerebral uptake of the D- and L-isomers of the PET tracer cis-4-[18F]fluoroproline (D-/L-cis-FPro) and of D-/L-[3H]proline (D-/L-Pro) in rats by dual tracer autoradiography and the uptake of D-cis-FPro in two human subjects by PET. The standardized uptake value (SUV) of D-cis-FPro in the cerebral cortex of rats 2 h p.i. was 3.05+/-1.18 (n=9) versus 0.06+/-0.01 (n=4) for L-cis-FPro (P<0.001) and 1.29+/-0.27 (n=4) for D-Pro versus 0.30+/-0.14 (n=9) for L-Pro (P<0.001). Analysis of the rat brain tissue after injection of D-cis-FPro (n=3) revealed no radioactivity in the proteins but a relevant part in the form of L-trans-FPro. The PET studies yielded a four- to five-fold higher SUV and influx rate constant in the human cortex for D-cis-FPro than for L-cis-FPro. We conclude that D-cis-FPro and D-Pro are preferably transported at the blood-brain barrier compared with their L-isomers and isomerized to the L-form within the brain. Thus, D-Pro in the plasma might be a source of intracerebral L-proline, which has been shown to act as a modulator of excitatory neurotransmission.

Imaging of gliomas with Cis-4-[18F]fluoro-L-proline

Tumor imaging with cis-4-[18F]fluoro-L-proline (cis-FPro) was compared to that of L-[3H]proline and L-[3H]methionine in F98 rat gliomas by dual-tracer autoradiography. All tracers exhibited high accumulation in the tumors but in the normal brain significant uptake was observed for L-[3H]methionine only. Tumor extent on autoradiograms with L-[3H]proline and L-[3H]methionine was identical to that of histological staining while autoradiograms of cis-FPro showed diffuse uptake in the penumbra of some tumors. First PET studies in 7 patients with cerebral gliomas demonstrated accumulation of cis-FPro in tumor areas with enhancement of Gd-DTPA on MR scans. Uptake of cis-FPro in normal brain tissue was negligible. In one patient with a glioblastoma accumulation of cis-FPro was also found in two brain areas without enhancement of Gd-DTPA on MR scans. Control of MRI suggested tumor growth in these areas at further follow up. Our results indicate that in most gliomas increased cis-FPro uptake is restricted to areas with disruption of the BBB which limits its clinical utility.

Transport of pharmacologically active proline derivatives by the human proton-coupled amino acid transporter hPAT1

Several proline derivatives such as L-azetidine-2-carboxylic acid, cis-4-hydroxy-L-proline, and 3,4-dehydro-DL-proline prevent procollagen from folding into a stable triple-helical conformation, thereby reducing excessive deposition of collagen in fibrotic processes and the growth of tumors. This study was performed to investigate whether the recently discovered human proton-coupled amino acid transporter 1 (hPAT1) is capable of transporting such pharmacologically relevant proline derivatives and also GABA analogs. Uptake of L-[3H]proline and [3H]glycine in Caco-2 cells was Na+-independent but strongly H+-dependent. The L-proline uptake was saturable and mediated by a single transport system (hPAT1) with an affinity constant of 2.0 +/- 0.2 mM. The uptake of L-[3H]proline was inhibited by D-proline, trans-4-hydroxy-L-proline, cis-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-DL-proline, L-azetidine-2-carboxylic acid, 3-amino-1-propanesulfonic acid, D- and L-pipecolic acid, l-thiaproline, and many others. Apical uptake and transepithelial flux of L-[3H]proline across Caco-2 cell monolayers were strongly inhibited by proline derivatives in proportions corresponding to their respective affinity constants at hPAT1. The basolateral to apical flux of L-[3H]proline was only 8% of that in the opposite direction. Apical uptake of unlabeled L-proline, cis-4-hydroxy-L-proline, and L-azetidine-2-carboxylic acid was stimulated by an inside directed H+ gradient 2- to 3-fold. Total apical to basolateral flux of proline derivatives was moderately correlated with their inhibitory potency for L-[3H]proline uptake and flux inhibition. We conclude that 1) the substrate specificity of hPAT1 is very much broader than so far reported and 2) the system accepts therapeutically relevant proline and GABA derivatives. hPAT1 is a promising candidate for new ways of oral drug delivery.

Comparison of fluorotyrosines and methionine uptake in F98 rat gliomas

The transport mechanisms of O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET) and 2-[(18)F]fluoro-L-tyrosine (FTyr) were compared to those of [(3)H]-Methyl-L-methionine (MET) in F98 rat glioma cells in vitro and by tumor imaging by ex vivo dual tracer autoradiography in F98 rat gliomas. Both, FET and FTyr exhibited similar transport characteristics in F98 glioma cells compared to MET, i.e. mainly a sodium dependent transport similar to system B(0,+) and sodium independent transport via system L. Radioactivity of FET in the acid precipitable fraction was <1% after 120 min incubation time while FTyr and MET exhibited a 15-18% incorporation into proteins. Comparison of FET and FTyr with MET uptake in F98 rat gliomas demonstrated a significant correlation of tumor to brain ratios and a similar intratumoral tracer distribution pattern.