Validation of brain tumour imaging with p-[123I]iodo-l-phenylalanine and SPECT

Targeted alpha therapy using astatine (211At)-labeled phenylalanine: A preclinical study in glioma bearing mice

Phenylalanine derivatives, which target tumors especially through L-type amino acid transporter-1 (LAT1), have elicited considerable attention. In this study, we evaluated the treatment effect of phenylalanine labeled with the alpha emitter astatine (²¹¹At-PA) in tumor bearing mice. The C6 glioma, U-87MG, and GL261 cell lines were subjected to a cellular ²¹¹At-PA uptake analysis that included an evaluation of the uptake inhibition by the system L amino acid transporter inhibitor 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH). BCH significantly inhibited para-²¹¹At-PA uptake in C6 glioma (12.2 ± 0.8%), U-87MG (27.6 ± 1.1%), and GL261 (12.6 ± 2.0%) cells compared to baseline, suggesting an uptake contribution by system L amino acid transporters. Subsequently, xenograft and allograft models were prepared by subcutaneously injecting C6 glioma (n = 12) or GL-261 cells (n = 12), respectively. C6 glioma mice received three ²¹¹At-PA doses (0.1, 0.5, or 1 MBq, n = 3/dose), while GL261 mice received one high dose (1 MBq, n = 7). ²¹¹At-PA exhibited a tumor growth suppression effect in C6 glioma models in a dose-dependent manner as well as in GL-261 models. This phenylalanine derivative labeled with astatine may be applicable as an alpha therapy that specifically targets system L amino acid transporters.

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.

Progress on the diagnosis and evaluation of brain tumors

Brain tumors are one of the most challenging disorders encountered, and early and accurate diagnosis is essential for the management and treatment of these tumors. In this article, diagnostic modalities including single-photon emission computed tomography, positron emission tomography, magnetic resonance imaging, and optical imaging are reviewed. We mainly focus on the newly emerging, specific imaging probes, and their potential use in animal models and clinical settings.

Systemic Endoradiotherapy with Carrier-Added 4-[(131)I]Iodo-L-Phenylalanine: Clinical Proof-of-Principle in Refractory Glioma

PURPOSE

To explore feasibility, tolerability, dosimetry and probable efficacy of intravenous endoradiotherapy with carrier-added 4-[(131)I]iodo-L-phenylalanine (c.a. (131)I-IPA) in refractory high-grade glioma.

METHODS

Two male patients (45 and 50 years), with long-standing, extensively pre-treated gliomas and evidence of progression underwent single intravenous injections of 2 and 4 GBq of c.a. (131)I-IPA, respectively. Tumour targeting was verified by (131)I-IPA single-photon emission computed tomography (SPECT). Metabolic and morphological changes indicative of tumour response were assessed by sequential [(18)F]fluoroethyltyrosine ((18)F-FET) positron emission tomography (PET) and contrast-enhanced magnetic resonance imaging (MRI) following therapy. Further monitoring included clinical state, safety laboratory, quality of life and dosimetry. Absorbed mean organ and whole-body doses were determined according to the Medical Internal Radiation Dose (MIRD) scheme using OLINDAEXM based on serial planar scintigraphy.

RESULTS

Both patients tolerated the treatment well. No evidence of acute or delayed organ toxicity was observed. (131)I-IPA accumulated in the tumour recurrences identified by MRI/(18)F-FET. In patient 1, PET showed progressively decreasing maximum standardised uptake values (SUVmax) over 10 months, indicating metabolic response, paralleled by reduced contrast enhancement and tumour volume on MRI. Progression occurred 18 months after therapy. Treatment was repeated using 6.6 GBq of (131)I-IPA, to which no response was observed. Patient 2, followed-up for 3 months after therapy, showed stable disease on MRI and PET. Mean absorbed whole body doses ranged from 0.13 to 0.17 mSv/MBq, with the highest absorbed organ doses to kidneys, bladder and heart (0.86-1.23; 0.49-0.6 and 0.45-0.56 mSv/MBq).

CONCLUSION

Systemic endoradiotherapy using up to 6.6 GBq of c.a.(131)I-IPA is not associated with clinically detectable toxicity. Measurable anti-tumour effects in gliomas were observed. (131)I-IPA warrants further evaluation as glioma therapy.

Synthesis, uptake mechanism characterization and biological evaluation of (18)F labeled fluoroalkyl phenylalanine analogs as potential PET imaging agents

INTRODUCTION

Amino acids based tracers represent a promising class of tumor metabolic imaging agents with successful clinical applications. Two new phenylalanine derivatives, p-(2-[(18)F]fluoroethyl)-L-phenylalanine (FEP, [(18)F]2) and p-(3-[(18)F]fluoropropyl)-L-phenylalanine (FPP, [(18)F]3) were synthesized and evaluated in comparison to clinically utilized O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET, [(18)F]1).

METHODS

FEP ([(18)F]2) and FPP ([(18)F]3) were successfully synthesized by a rapid and efficient two-step nucleophilic fluorination of tosylate precursors and deprotection reaction. In vitro cell uptake studies were carried out in 9L glioma cells. In vivo studies, 9L tumor xenografts were implanted in Fisher 344 rats.

RESULTS

FEP ([(18)F]2) and FPP ([(18)F]3) could be efficiently labeled within 90 min with good enantiomeric purity (>95%), good yield (11-37%) and high specific activity (21-69 GBq/μmol). Cell uptake studies showed FEP had higher uptake than FPP as well as reference ligand FET ([(18)F]1). Uptake mechanism studies suggested that FEP is a selective substrate for system L and prefers its subtype LAT1. In vivo biodistribution studies demonstrated FEP had specific accumulation in tumor cells and tumor to background ratio reached 1.45 at 60 min. Small animal positron emission tomography (PET) imaging studies showed FEP was comparable to FET for imaging rats bearing 9L tumor model. FEP had high uptake in 9L tumor compared to surrounding tissue and was quickly excreted through urinary tract.

CONCLUSION

Biological evaluations indicate that FEP ([(18)F]2) is a potential useful tracer for tumor imaging with PET.

Prospective study of p-[123I]iodo-L-phenylalanine and SPECT for the evaluation of newly diagnosed cerebral lesions: specific confirmation of glioma

PURPOSE

The differentiation between gliomas, metastases and gliotic or inflammatory lesions by imaging techniques remains a challenge. Gliomas frequently exhibit increased uptake of radiolabelled amino acids and are thus amenable to PET or SPECT imaging. Recently, p-[123I]iodo-L-phenylalanine (IPA) was validated for the visualization of glioma by SPECT and received orphan drug status. Here we investigated its diagnostic performance for differentiating indeterminate brain lesions.

METHODS

This prospective open study included 67 patients with newly diagnosed brain lesions suspicious for glioma (34 without and 33 with contrast enhancement in the MRI scan). Patients received 250 MBq IPA intravenously after overnight fasting. SPECT images at 30 min and 3 h post-injection were iteratively reconstructed and visually interpreted after image fusion with an MRI brain scan (fluid-attenuated inversion recovery sequence or T1-weighted contrast-enhanced image). Findings were correlated with results of stereotactic or open biopsies or serial imaging.

RESULTS

Twenty-seven low-grade (2 WHO I, 25 WHO II) and 24 high-grade gliomas (1 WHO III, 23 WHO IV), 3 metastases originating from lung cancer as well as 13 non-neoplastic lesions were proven. All non-neoplastic lesions and all metastases were negative with IPA SPECT. Forty gliomas were true-positive (TP) and 11 false-negative (FN) findings (8 WHO II, 1 WHO III, 2 WHO IV) occurred. There were no false-positive (FP) findings. For the differentiation of primary brain tumours and non-neoplastic lesions, sensitivity and specificity were 78 and 100%. In 34 lesions without contrast enhancement in MRI, IPA SPECT resulted in 17 TP, 8 true-negative, 9 FN and no FP findings (sensitivity 65%, specificity 100%).

CONCLUSION

In patients with suspected glioma, IPA SPECT shows a high specificity, but especially in low-grade gliomas FN findings may occur. Due to the high positive predictive value a positive finding allows a suspected glioma to be confirmed.

Synthesis and analysis of 2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine and their uptake in human glioma cell cultures in-vitro

2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine were prepared from the corresponding iodo and bromo derivatives using the Cu(+)-assisted nucleophilic exchange. 4-[211At]-L-phenylalanine was additionally prepared by destannylation of the BOC-derivatized 4-tributylstannyl-L-phenylalanine. Radiochemical yields of 2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine by nucleophilic exchange were 52-74% and 65-85%. Radiochemical yield of 4-[211At]-L-phenylalanine by electrophilic destannylation was 35-50%. HPLC sequence analysis showed that 2-[211At]-L-phenylalanine followed the halogen sequence (F<Cl<Br<I<At) whereas 4-[211At]-L-phenylalanine eluted between 4-Br-L-phenylalanine and 4-I-L-phenylalanine (F<Cl<Br<At<I), independent on the production pathway. Uptake of 4-[211At]-L-phenylalanine and 4-[131I]-L-phenylalanine in DBTRG-05MG glioma cells was inhibited by l-phenylalanine 7-fold and 6-fold, respectively.

Improved synthesis of no-carrier-added p-[124I]iodo-L-phenylalanine and p-[131I]iodo-L-phenylalanine for nuclear medicine applications in malignant gliomas

This work describes the synthesis and the tumor affinity testing of no-carrier-added (n.c.a.) p-[(124)I]iodo-L-phenyalanine ([(124)I]IPA) and n.c.a. p-[(131)I]iodo-l-phenyalanine ([(131)I]IPA) as radiopharmaceuticals for imaging brain tumors with PET and for radionuclid-based therapy, respectively. Parameters for labeling were optimized with regard to the amount of precursor, temperature and time. Thereafter, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were investigated in rat F98 glioma and in primary human A1207 and HOM-T3868 glioblastoma cells in vitro, followed by an in vivo evaluation in CD1 nu/nu mice engrafted with human glioblastoma. No-carrier-added [(124)I]IPA and n.c.a. [(131)I]IPA were obtained in 90+/-6% radiochemical yield and >99% radiochemical purity by iododestannylation of N-Boc-4-(tri-n-butylstannyl)-L-phenylalanine methylester in the presence of chloramine-T, followed by hydrolysis of the protecting groups. The total synthesis time, including the HPLC separation and pharmacological formulation, was less than 60 min and compatible with a clinical routine production. Both amino acid tracers accumulated intensively in rat and in human glioma cells. The radioactivity incorporation in tumor cells following a 15-min incubation at 37 degrees C/pH 7.4 varied from 25% to 42% of the total loaded activity per 10(6) tumor cells (296-540 cpm/1000 cells). Inhibition experiments confirmed that n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were taken up into tumor by the sodium-independent L- and ASC-type transporters. Biodistribution and whole-body imaging by a gamma-camera and a PET scanner demonstrated a high targeting level and a prolonged retention of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA within the xenotransplanted human glioblastoma and a primarily renal excretion. However, an accurate delineation of the tumors in mice was not possible by our imaging systems. Radioactivity accumulation in the thyroid and in the stomach as a secondary indication of deiodination was less than 1% of the injected dose at 24h p.i., confirming the high in vivo stability of the radiopharmaceuticals. In conclusion, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA are new promising radiopharmaceuticals, which can now be prepared in high radiochemical yields and high purity for widespread clinical applications. The specific and high-level targeting of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA to glioma cells in vitro and to glioblastoma engrafts in vivo encourages further in vivo validations to ascertain their clinical potential as agent for imaging and quantitation of gliomas with PET, and for radionuclid-based therapy, respectively.

Use of [123I]-2-iodo-L-phenylalanine as a tumor imaging agent in two dogs with synovial cell sarcoma

[123I]-iodo-L-phenylalanine was successfully evaluated for gamma camera imaging in vivo in tumor-bearing athymic mice and in humans with brain tumors. Here, we report the use of this tracer in two dogs with synovial cell sarcoma of the tarsus. [123I]-iodo-L-phenylalanine was quantitatively prepared as a kit formulation using the Cu(1+) +-assisted nucleophilic exchange. Rapid [123I]-2-iodo-L-phenylalanine tumor accumulation was observed with good tumor to background contrast and rapid clearance in these two dogs. This radiopharmaceutical is a promising alternative tumor tracer to overcome the known limitations of 18F-fluorodeoxyglucose and, when labelled with radioiodine-131, has the potential to be used for therapeutic purposes.

Intra-individual comparison of p-[123I]-iodo-L-phenylalanine and L-3-[123I]-iodo-α-methyl-tyrosine for SPECT imaging of gliomas

OBJECTIVES

Radioactive amino-acids accumulate in gliomas even with an intact blood-brain-barrier. L-3-[(123)I]-iodo-alpha-methyl-tyrosine (IMT) is well established for SPECT imaging of gliomas. Recently, we introduced p-[(123)I]-iodo-L-phenylalanine (IPA) for the characterisation of brain lesions. This study compares both tracers in glioma patients.

METHODS

Eleven patients with gliomas (1 WHO grade 1, 5 grade 2, 1 grade 3, 2 grade 4 gliomas, 1 unconfirmed upgrading and 1 post-therapeutic non-neoplastic lesion) underwent SPECT imaging with IPA (early and delayed acquisitions at 30 min and 3 h) and IMT (early only). Maximum tumour-to-brain ratios (TBR) were calculated using region-of-interest analysis to assess uptake of IMT and IPA. Imaging results were compared to histopathological findings.

RESULTS

Early TBRs of IMT and IPA were strongly correlated (r = 0.828, p = 0.002). TBRs were higher for IMT than IPA (1.95+/-0.50 versus 1.79+/-0.42; p < 0.05), but independent from tumour cell density (p > 0.1). Visual interpretation by different observers was more concordant for IMT-SPECT than IPA-SPECT (kappa 1.0 versus 0.774). No differences in early TBRs were observed between low-grade and high-grade gliomas for IMT (1.97+/-0.53 versus 2.21+/-0.44, p > 0.5) or IPA (1.70+/-0.23 versus 2.21+/-0.56, p = 0.167) with a trend to higher TBRs in low-grade tumours for IMT (p = 0.093). In contrast to the known wash-out of IMT, we observed persistent accumulation of IPA in gliomas.

CONCLUSIONS

IPA shows lower TBRs than IMT, especially in low-grade tumours, so IMT should be preferred for the delineation of low-grade gliomas by SPECT imaging. Due to its prolonged retention, however, IPA remains promising for therapeutic use in gliomas after labelling with I-131.