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

Cis-4-[18F]fluoro-L-proline PET/CT molecular imaging quantifying liver collagenogenesis: No existing fibrotic deposition in experimental advanced-stage alcoholic liver fibrosis

Background and purpose

Heavy alcohol drinking-induced alcoholic fatty liver, steatohepatitis, and early-stage alcoholic liver fibrosis may progress to advanced-stage alcoholic liver fibrosis (AALF)/cirrhosis. The lack of non-invasive imaging techniques for the diagnosising collagenogenesis in activated hepatic stellate cells (HSCs) can lead to incurable liver fibrosis at the early reversible stage. Proline has been known as the most abundant amino acid of collagen type 1 synthesized by activated HSC with the transportation of proline transporter. cis-4-[18F]fluoro-L-proline ([18F]proline) was reported as a useful tool to quantify collagenogenesis in experimental alcoholic steatohepatitis. This study aims to use [18F]proline micro PET as non-invasive imaging to quantify liver collagenogenesis in HSC of experimental AALF.

Methods

AALF model was set up by a modified Lieber-DeCarli liquid ethanol diet for 12 weeks along with intraperitoneal injection (IP) of CCl 4 (0.5 ml/kg) between the 5th and 12th weeks. Controls were fed an isocaloric liquid diet and IP. PBS. In vitro [3H]proline uptake by HSCs isolated from livers was quantified using a liquid scintillation counter. Collagen type 1 production in HSCs culture medium was assayed by ELISA. Ex vivo liver collagen type 1 and proline transporter protein were compared between AALF rats (n = 8) and mice (n = 8). [3H]Proline uptake specificity in ex vivo liver tissues was tested using unlabeled proline and transporter inhibitor benztropine at different doses. Liver H&E, trichrome stain, and blood biochemistry were tested in rats and mice. In vivo, at varying times after instillation, dynamic and static [18F]proline micro PET/CT were done to quantify tracer uptake in AALF mice (n = 3). Correlation among liver collagen, liver SUVmax, normalized liver-to-brain ratio, normalized liver-to-thigh ratio, and fluoro-proline-induced collagen levels in ex vivo liver tissues were analyzed.

Results

In vitro HSCs study showed significant higher [3H]proline uptake (23007.9 ± 5089.2 vs. 1075.4 ± 119.3 CPM/mg, p < 0.001) in HSCs isolated from AALF rats than controls and so was collagen type 1 production (24.3 ± 5.8 vs. 3.0 ± 0.62 mg/ml, p < 0.001) in HSCs culture medium. Highly positive correlation between [3H]proline uptake and collagen type 1 by HSCs of AALF rats was found (r value = 0.92, p < 0.01). Ex vivo liver tissue study showed no significant difference in collagen type 1 levels between AALF rats (14.83 ± 5.35 mg/g) and AALF mice (12.91 ± 3.62 mg/g, p > 0.05), so was proline transporter expression between AALF rats (7.76 ± 1.92-fold) and AALF mice (6.80 ± 0.97-fold). Unlabeled fluoro-proline induced generation of liver tissue collagen type 1 and [3H]proline uptake were specifically blocked by transporter inhibitor. In vivo [18F]proline micro PET/CT imaging showed higher SUVmax in liver (4.90 ± 0.91 vs. 1.63 ± 0.38, p < 0.01), higher normalized liver/brain ratio (12.54 ± 0.72 vs. 2.33 ± 0.41, p < 0.01), and higher normalized liver/thigh ratio (6.03 ± 0.78 vs. 1.09 ± 0.09, p < 0.01) in AALF mice than controls, which are all positively correlated with fluoro-proline-induced levels of collagen in liver tissue (r value ≥ 0.93, p < 0.01) in AALF mice, but not correlated with existing liver collagen. Liver histology showed increased collagen in the liver of AALF mice. Blood serum ALT and AST levels were remarkably higher in AALF mice than in controls, but there is no significant difference in blood fibrotic parameters HA, A2M, TGFβ1, and MMP1.

Conclusions

[18F]proline micro PET/CT might be useful to visualize collagenogenesis in activated HSC of experimental AALF but fails to quantify existing liver collagen in AALF mice. [18F]proline has the potential sensitivity to assess the activity and severity of liver fibrosis.

Overview of PET Tracers for Brain Tumor Imaging

This article provides an overview of the key considerations for the development and application of molecular imaging agents for brain tumors and the major classes of PET tracers that have been used for imaging brain tumors in humans. The mechanisms of uptake, biological implications, primary applications, and limitations of PET tracers in neuro-oncology are reviewed. The available data indicate that several of these classes of tracers, including radiolabeled amino acids, have imaging properties superior to those of (18)F-fluorodeoxyglucose, and can complement contrast-enhanced magnetic resonance imaging in the evaluation of brain tumors.

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.

Comparison of F-18 FET-PET with F-18 FDG-PET for biopsy planning of non-contrast-enhancing gliomas

OBJECTIVE

The management of non-contrast-enhancing brain tumours largely depends on biopsy, which allows a differentiation of low-grade gliomas (LGG) from high-grade gliomas (HGG). The aim of this study was to compare positron emission tomography using 2-[(18)F]-fluoro-2-deoxy-D: -glucose (FDG-PET) and O-(2-[(18)F]-fluoroethyl)-L: -tyrosine (FET-PET) in terms of providing target regions for biopsies.

MATERIALS AND METHODS

Fifteen consecutive patients with newly diagnosed brain tumours (n = 11) or suspected recurrence of a known LGG (n = 4), in whom MRI demonstrated no contrast enhancement, were studied by both FET-PET and FDG-PET. FET-PET, FDG-PET and MRI data were fused, and then transferred to the neurosurgical navigation system, prior to neurosurgical interventions.

RESULTS

Histology showed HGG (WHO grade III) in 6/15 and LGG (WHO grade II) in 9/15 patients. FET-PET revealed an increased intratumoural tracer uptake in 8/9 LGG and in 5/6 HGG. FDG-PET depicted hypermetabolic spots in 2/9 LGG and in 4/6 HGG. In 6 patients we observed an increased intratumoural uptake of both tracers. In 4 of them, the area of highest FET accumulation in the tumour corresponded to the focus of increased FDG uptake.

CONCLUSIONS

FET-PET appears to be superior to FDG-PET for biopsy planning in non-contrast-enhancing brain tumours. FDG-PET does not provide any additional information in this issue.

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.

Imaging of fibrogenesis in patients with idiopathic pulmonary fibrosis with cis-4-[(18)F]-Fluoro-L: -proline PET

PURPOSE

Idiopathic pulmonary fibrosis (IPF) is a lethal lung disease for which no single diagnostic modality is able to evaluate the activity of the disease process. Cis-4-(18)F-fluoro-L: -proline ((18)F-proline) was shown in animal studies to be a reliable marker for fibrosis formation. We tested this candidate radioligand for imaging of fibrogenesis in patients with IPF.

METHODS

Five patients with IPF proven by lung biopsy and computed tomography were included. Furthermore, we also included one patient with non-specific interstitial pneumonia (NSIP) and scleroderma and one with NSIP and organising pneumonia. Positron emission tomography (PET) acquisition was performed 1, 2 and 3h after injection of 400MBq (18)F-proline. We scored (18)F-proline activity visually and quantitatively by calculating the activity in the regions of interest over lung, liver and mediastinum.

RESULTS

We found low uptake of (18)F-proline in the lungs of all patients with IPF. The highest uptake was seen at 2h post-injection, with a decline at 3h past injection. The differences in lung uptake between patients were small, except for one patient with NSIP and organising pneumonia who had a slightly higher (18)F-proline uptake. No significant correlations between (18)F-proline uptake and clinical parameters were found.

CONCLUSIONS

Due to the low pulmonary uptake of (18)F-proline in patients with IPF, (18)F-proline does not seem to be a suitable radioligand to evaluate the activity of fibrosis formation in patients with IPF. The low uptake in the lungs of patients with interstitial fibrosis may be explained by the slow nature of fibrogenesis or to the relatively low dose of proline that can be used.

Differentiation of tumour and inflammation: characterisation of [methyl-3H]methionine (MET) and O-(2-[18F]fluoroethyl)-L-tyrosine (FET) uptake in human tumour and inflammatory cells

PURPOSE

Previous studies suggest that radiolabelled amino acids could be superior to FDG in differentiating tumour and inflammation. Therefore the aim of this study was to investigate the uptake of FET and MET in human tumour and inflammatory cells and to investigate their uptake kinetics.

METHODS

For uptake studies, cells were incubated with 370 kBq FET or 3.7 kBq MET for 15 min. Kinetic studies were performed at variable concentrations of FET and MET. Competitive inhibition studies were done with BCH, MeAIB and L: -serine.

RESULTS

All inflammatory cells incorporated more MET than the tumour cells. The uptake of FET, in contrast, was significantly lower in all inflammatory cells than in the tumour cells. In tumour cells the uptake of MET was about five times the uptake of FET. The competitive inhibitors reduced uptake of both tracers to 20-40% in tumour cells and to 70% in inflammatory cells. Kinetic studies showed that MET and FET transport was saturable in all cells except macrophages and followed a Michaelis-Menten kinetic. Highest capacity (V (max)) and affinity (K (m)) for the uptake of MET was observed in granulocytes. Capacity and affinity for FET uptake were highest in the DHL-4 cells.

CONCLUSION

In contrast to MET, FET accumulated to a significantly greater extent in tumour cells than in inflammatory cells. The marked differences between tumour and inflammatory cells concerning FET and MET uptake suggest that FET and MET are substrates of different subtypes of the L system.

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.

Preclinical and clinical evaluation of O-[11C]methyl-l-tyrosine for tumor imaging by positron emission tomography

We performed preclinical and clinical studies of O-[11C]methyl-L-tyrosine, a potential tracer for imaging amino acid transport of tumors by positron emission tomography (PET). Examinations of the radiation-absorbed dose by O-[11C]methyl-L-tyrosine and the acute toxicity and mutagenicity of O-methyl-L-tyrosine showed suitability of the tracer for clinical use. The whole-body imaging of monkeys and healthy humans by PET showed low uptake of O-[11C]methyl-L-tyrosine in all normal organs except for the urinary track and bladder, suggesting that the O-[11C]methyl-L-tyrosine PET has the potential for tumor imaging in the whole-body. Finally, the brain tumor imaging was preliminarily demonstrated.