L-[3-¹⁸F]-α-methyltyrosine uptake by lymph node metastasis is a predictor of complete response to CRT in esophageal cancer

BACKGROUND/AIM

The amino acid positron emission tomography (PET) tracer [(18)F]-3-fluoro-alpha-methyltyrosine ((18)F-FAMT) is known to be highly specific for malignancies. We evaluated the accumulation of (18)F-FDG or (18)F-FAMT in lymph nodes (LN) prior to definitive chemoradiotherapy (CRT) for esophageal cancer.

PATIENTS AND METHODS

We retrospectively reviewed 30 patients with esophageal squamous cell carcinoma. All patients received definitive CRT. The relationship between the accumulation of (18)F-FDG PET or (18)F-FAMT PET in LNs prior to CRT and clinical outcomes was assessed.

RESULTS

A correlation was observed between LNs in which most of (18)F-FAMT was accumulated and complete response (CR) rate, but was not for (18)F-FDG. Additionally, for (18)F-FAMT, the CR rate was significantly higher in the LN accumulated lesion ≤ 1 group than in the LN accumulated lesion >2 group.

DISCUSSION

To predict the outcome of definitive CRT in patients with esophageal cancer, it is important to evaluate the LN status.

Use of pre-treatment 18F-FAMT PET to predict patient survival in squamous cell carcinoma of the esophagus treated by curative surgery

BACKGROUND

[18F]-3-fluoro-alpha-methyl tyrosine (18F-FAMT) as an amino acid tracer in positron emission tomography (PET) has been widely investigated in several tumor types. Herein we investigated the clinical significance of 18F-FAMT PET uptake as a prognostic marker together in our updated data of patients with esophageal cancer.

PATIENTS AND METHODS

We retrospectively assessed the treatment outcomes of 42 patients with histologically-confirmed esophageal cancer. The survival rate was analyzed using the median peak standardized uptake value (SUV) with 2.2 as the cut-off value.

RESULTS

FAMT uptakes were significantly correlated with factors reflecting tumor progression. Moreover, a significant correlation was observed between FAMT uptake and disease-free survival (p=0.023). Moreover, on evaluation of individual lymph node groups, the specificity and positive predictive value were significantly higher for 18F-FAMT-PET than for 18F-FDG-PET and computed tomography (CT).

CONCLUSION

18F-FAMT is an important pre-treatment diagnostic modality and its accumulation is a good predictor of disease-free survival (DFS) in patients with operable esophageal cancer.

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.

123I-IMT SPECT for evaluation of the response to radiation therapy in high grade gliomas: a feasibility study

Assessing response to radiation therapy in patients with high grade gliomas is needed upon making decisions toward further therapy strategies. Currently used standard imaging tools such as CT and MRI are not sensitive enough to detect early therapy effects. We prospectively investigated if single photon emission computed tomography (SPECT) using radiolabelled amino acid derivate (123)I-methyltyrosine (IMT) would be useful for this aim. 10 patients with histologically proven high grade gliomas, who were candidates for radiation therapy, were enrolled in this investigation. All patients were examined by IMT SPECT before radiation therapy and 4 weeks after the initiation of the hypofractionated application of 40 Gy. Patients were followed up for 39 months; the tumours to background ratios (T/B) for IMT under/before radiation therapy were correlated to survival times. Initially, SPECT depicted an abnormal intratumoural IMT uptake in all patients (mean T/B ratios 1.37-1.87). In four out of 10 patients, the mean T/B ratios decreased by more than 10% under radiation therapy. In six other patients, the BQ decreased by less than 10% or increased. There were no significant correlations between the degree of changes in T/B and survival (r = -0.1, p = 0.973). Serial IMT SPECT measurements allow detection of changes in amino acid accumulation in high-grade gliomas under radiation therapy. However, these changes seem to possess no prognostic value in respect to survival prediction.

Detection of maleate-induced Fanconi syndrome by decreasing accumulation of 125I-3-iodo-alpha-methyl-L-tyrosine in the proximal tubule segment-1 region of renal cortex in mice: a trial of separate evaluation of reabsorption

OBJECTIVE

Fanconi syndrome is a renal dysfunction characterized by various combinations of renal tubular transport dysfunction involving amino acids, glucose, protein and other substances. Most reabsorption of amino acids occurs in proximal renal tubule segment 1 (S1). The present study evaluated the possibility of early detection of drug-induced Fanconi syndrome, based on decreased renal accumulation of 125I-3-iodo-alpha-methyl-L-tyrosine (125I-IMT), an amino acid transport marker, in the S1 region of renal cortex. The present experimental model used maleate (MAL)-induced Fanconi syndrome in mice. Results were compared between 125I-IMT and 3 other clinical renal radiopharmaceuticals: 99mTc-2,3-dimercaptosuccinic acid (99mTc-DMSA); 99mTc-mercaptoacetylglycylglycylglycine (99mTc-MAG3); and 99mTc-diethylenetriaminepentaacetic acid (99mTc-DTPA).

METHODS

Male ddY mice (age, 6 weeks; body weight, 25 g) were used to create a Fanconi model of renal dysfunction. A single dose of maleate disodium salt was administered by intraperitoneal injection (6 mmol/kg). Hematoxylin and eosin (HE) staining of the renal cortex, renal autoradiography and measurement of renal radioactivity of labeled compounds were performed at 30, 60, 90 and 120 min after MAL injection. At 5 min after injection of labeled compounds (18.5 kBq for accumulation experiment, 670 kBq for autoradiography), animals were sacrificed by ether overdose and kidneys were removed. For the accumulation experiment, radioactivity was measured using a well-type scintillation counter. For autoradiography, 20-microm sections of frozen kidney were used with Bio-Imaging Analyzer.

RESULTS

At 30 min after MAL injection, HE staining showed pyknosis in some proximal tubule cells. At that time, accumulations of 125I-IMT and 99mTc-DMSA in the S1 region were approximately 67% and 55% of control levels (p < 0.005). MAL increased accumulation of 99mTc-DTPA in the S1 region, but had no effect on accumulation of 99mTc-MAG3 in the S 1 region.

CONCLUSIONS

Decreased accumulation of 123I-IMT in the S1 region appears to represent a useful marker for detection of MAL-induced Fanconi syndrome. In future, we plan to assess the efficacy of using 125I-IMT to monitor renal dysfunction induced by nephrotoxic clinical drugs.

Evaluation of D-isomers of O-11C-methyl tyrosine and O-18F-fluoromethyl tyrosine as tumor-imaging agents in tumor-bearing mice: comparison with L- and D-11C-methionine

The aim of this study was to investigate whether D-amino acid isomers of O-(11)C-methyl tyrosine ((11)C-CMT) and O-(18)F-fluoromethyl tyrosine ((18)F-FMT) were better than the corresponding L-isomers as tumor- detecting agents with PET in comparison with the difference between L- and D-methyl-(11)C-methionine ((11)C-MET).

METHODS

L- and D-(11)C-MET, (11)C-CMT, and (18)F-FMT were injected intravenously into BALB/cA Jcl-nu mice bearing HeLa tumor cells. At 5, 15, 30, and 60 min after injection, normal abdominal organs and xenotransplanted HeLa cells were sampled, and the uptake of each ligand was determined. Metabolic analyses of these compounds in the plasma were also performed. Accumulation of the d-isomers of (11)C-MET, (11)C-CMT, and (18)F-FMT in turpentine-induced inflammatory tissue was assayed in comparison with (18)F-FDG. The whole-body distribution of each tracer was imaged with a planar positron imaging system (PPIS).

RESULTS

Although the tumor uptake (standardized uptake value [SUV]) levels of the D-isomers of (11)C-MET, (11)C-CMT, and (18)F-FMT were 261%, 72%, and 95% of each L-isomer 60 min after administration, the tumor-to-blood ratios of these D-isomers were 130%, 140%, and 182% of the corresponding L-isomers. In the blood, the D-isomers of these labeled compounds revealed a relatively faster elimination rate compared with their L-isomers, with a high peak uptake in the blood and kidney 5 min after administration. Compared with the natural amino acid ligand l-(11)C-MET, the uptake of L-isomers of (11)C-CMT and (18)F-FMT was relatively low and stable in the abdominal organs, whereas D-isomers revealed much lower levels and faster clearance rates compared with corresponding L-isomers. Among the abdominal organs, the pancreas showed a relatively high uptake of (11)C-CMT and (18)F-FMT; the uptake of these D-isomers was much lower than that of L-isomers. Pretreatment with cycloheximide, a protein synthesis inhibitor, resulted in a marked reduction of L-(11)C-MET uptake and a slight reduction of D-(11)C-MET uptake into protein fractions, whereas no significant changes were detected with L- and D-(11)C-CMT and (18)F-FMT. D-Isomers of (11)C-MET, (11)C-CMT, and (18)F-FMT did not accumulate in turpentine-induced inflammatory tissue, where (18)F-FDG revealed a high uptake. Whole-body imaging with a PPIS provided consistent distribution data obtained from the tissue dissection analysis.

CONCLUSION

These results suggest that D-isomers of (11)C-CMT and (18)F-FMT could be potentially better tracers than L- and D-(11)C-MET for tumor imaging with PET.

Early effects of irradiation on [(123)I]-IMT and [(18)F]-FDG uptake in rat C6 glioma cells

BACKGROUND

Single-photon emission computed tomography (SPECT) using 3-[(123)I]-iodo-L-alpha-methyltyrosine ([(123)I]-IMT) and positron emission tomography (PET) using 2-[(18)F]-fluoro-2-deoxy-D-glucose ([(18)F]-FDG) are valuable tools for the distinction between viable tumor and radionecrosis in patients receiving radiotherapy for high-grade gliomas. However, to date, little is known about the early effects of radiation on [(123)I]-IMT and [(18)F]-FDG uptake in gliomas.

MATERIAL AND METHODS

To determine the early effects of irradiation on [(123)I]-IMT and [(18)F]-FDG uptake in gliomas, in vitro studies were performed using rat C6 glioma cells. The glioma cells were irradiated with 20 Gy which is a common dose applied to patients receiving intraoperative radiotherapy. Subsequently, the early kinetics of [(123)I]-IMT and [(18)F]-FDG uptake in glioma cells were monitored for 3 days.

RESULTS

Micromorphometric examinations of the irradiated glioma cells revealed that about 25% of the viable cells transformed into giant cells. [(123)I]-IMT uptake per 10(5) viable glioma cells was unchanged on the 1st day post irradiation, but showed a significant increase on the 2nd and 3rd day following radiotherapy (p < 0.01). In addition, there was a moderate increase in [(18)F]- FDG accumulation per 10(5) viable glioma cells during the first 3 days after irradiation (p < 0.05). The maximum increase in early [(123)I]-IMT uptake 1 h after application surpassed that of [(18)F]-FDG (p < 0.01).

CONCLUSION

Rat C6 glioma cells show an early increase in [(123)I]-IMT and [(18)F]-FDG uptake following irradiation which may be partly due to giant cell formation. These data suggest that [(123)I]-IMT SPECT and [(18)F]-FDG PET may be promising procedures for the early prediction of the therapeutic response of gliomas to radiotherapy.

The partial D2-like dopamine receptor agonist terguride acts as a functional antagonist in states of high and low dopaminergic tone: evidence from preweanling rats

RATIONALE

In adult rats, the partial D(2)-like agonist terguride acts as an antagonist at normosensitive D(2)-like post-synaptic receptors, while it acts as an agonist at the same receptors during states of low dopaminergic tone.

OBJECTIVE

The purpose of the present study was to determine whether partial D(2)-like agonists exhibit both antagonistic and agonistic actions during the preweanling period.

METHODS

In experiments 1 and 2 (examining the agonistic actions of terguride), preweanling rats were either given an escalating regimen of amphetamine to induce a state of amphetamine withdrawal or pretreated with the tyrosine hydroxylase inhibitor AMPT. Distance traveled was measured after rats were injected with saline, terguride (0.4-1.6 mg/kg), or the full D(2)-like receptor agonist NPA (0.01 mg/kg). In experiment 3 (examining the antagonistic actions of terguride), preweanling rats were pretreated with terguride 30 min before they were tested with saline, NPA (0.05 mg/kg), or amphetamine (1.5 mg/kg).

RESULTS

NPA had an exaggerated locomotor activating effect when tested under conditions of amphetamine withdrawal, while the partial D(2)-like agonist did not enhance distance traveled under any circumstance. Similarly, NPA increased and terguride did not affect the distance-traveled scores of AMPT-pretreated rats. In experiment 3, terguride pretreatment significantly reduced the distance traveled of amphetamine-treated and NPA-treated rats.

CONCLUSIONS

The behavioral evidence indicates that, during the preweanling period, terguride antagonizes D(2)-like post-synaptic receptors in a state of high dopaminergic tone; however, there is no evidence that terguride is capable of stimulating D(2)-like post-synaptic receptors during states of low dopaminergic tone.

Characterization of 123I-iodo-α-methyltyrosine transport in rat lymphoma cells

(123)I-Iodo-alpha-methyltyrosine (IMT) transport into lymphomas has not been fully characterized. In rat Nb2-11C and Nb2-Sp lymphoma cell lines, linear uptake of (123)I-IMT occurred rapidly within 5-10 min. Eadie-Hoftee plots of (123)I-IMT uptake gave apparent Km's of 8.34+/-1.17 and 9.64+/-1.05 microM for Nb2-11C and Nb2-Sp cells, respectively, and involved the L and B(0,+) systems. In lymphoma-bearing rats, injected (123)I-IMT accumulated rapidly in the primary tumors but gave a low tumor-to-background ratio of 2:1. (123)I-IMT was transported rapidly into lymphoma cells both in vitro and in vivo, but low target-to-nontarget ratio may not make (123)I-IMT practical for scanning in vivo.

Renal accumulation and excretion of radioiodinated 3-iodo-alpha-methyl-L-tyrosine

OBJECTIVE

We investigated mechanisms of renal accumulation of radioiodinated 3-iodo-alpha-methyl-L-tyrosine (IMT), which has been used clinically for tumor imaging and as an amino acid transport marker in studies of brain and pancreas function.

METHODS

In this study, we used 125I- or 123I-labeled IMT ([125I]IMT or [123I]IMT) as the transport marker. Partition coefficients of [125I]IMT were determined for hypothetic urine at pH ranging from 5 to 8. The examination of uptake and inhibition of [125I]IMT was performed using normal human renal proximal tubule epithelial cells (RPTEC), which are characteristic of the proximal convoluted tubule. The plasma protein binding ratio of [125I]IMT was determined using rats. In the in vivo experiments using mice, we examined biodistribution and excretion inhibition, and performed whole body autoradiography. Also, renal SPECT using [123I]IMT was performed using a normal canine.

RESULTS

Very low lipophilicity of [125I]IMT in hypothetic urine suggests that a carrier-mediated pathway contributes to its marked kidney accumulation. [125I]IMT uptake into RPTEC was significantly inhibited by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) in a sodium-dependent manner, suggesting reabsorption mainly via system B0 in apical membrane of proximal tubule. Plasma protein binding ratio of IMT was 45.4 +/- 5.6%. At 6 hr after administration of IMT to mice, excretion via urinary tract was 77.51% of injected dose, and excretion into feces was 0.25%. Furosemide, ethacrynic acid and probenecid inhibited tubular secretion of [125I]IMT in mice. We obtained very clear autoradiographs of mouse renal cortex and a canine renal SPECT image (S2-like region).

CONCLUSIONS

We believe that [123I]IM-T is useful for kidney imaging. In future studies, we plan to examine the use of [123I]IMT in diagnosis of disease.

Tracers to monitor the response to chemotherapy: in vitro screening of four radiopharmaceuticals

OBJECTIVES

It has been postulated that radiopharmaceuticals can be used to predict the therapeutic response to (chemo)therapy, which could lead to individualized treatment regimens. In this study, 18F-deoxyglucose, 99mTc-tetrofosmin, 125I-deoxyuridineribose, and 125I-methyltyrosine were tested for this purpose.

METHODS

The uterine sarcoma cell line MES-SA (MDR-) and its multidrug resistant variant, MES-SA/Dx5 (MDR+), were used. The MDR+ cells express high levels of P-glycoprotein, which makes them relatively resistant to various chemotherapeutic agents. Cells were cultured in the presence of escalating concentrations of doxorubicin, and the cellular uptake of the radiopharmaceuticals was determined.

RESULTS

Decreasing 18F-deoxyglucose uptake at escalating doxorubicin concentrations reflected the chemosensitivity of the cells: 18F-deoxyglucose uptake in the MDR- cells was reduced to 40% of the baseline level in the presence of 1 microM of doxorubicin, compared to 74% in the MDR+ cells. The 125I-deoxyuridineribose uptake in MDR- cells was reduced to 2% of the baseline level when cultured at a concentration of 1 microM of doxorubicin, while this was 79% in the MDR+ cells. The same trend was observed with 125I-methyltyrosine. The enhanced doxorubicin chemosensitivity of MDR+ cells in the presence of verapamil, a modulator of P-glycoprotein, was reflected by the reduced uptake of 18F-deoxyglucose, 125I-deoxyuridineribose, and 125I-methyltyrosine. Furthermore, baseline 99mTc-tetrofosmin uptake in MDR+ cells was more than six-fold lower than in MDR- cells.

CONCLUSION

In the presence of doxorubicin, the uptake of 18F-deoxyglucose, 125I-deoxyuridineribose and, to a lesser extent, 125I-methyltyrosine is more pronouncedly reduced in MDR- cells than in MDR+ cells. The reversal of doxorubicin-resistance of MDR+ cells by verapamil was also reflected by the uptake of 18F-deoxyglucose, 125I-deoxyuridineribose, and 125I-methyltyrosine. 99mTc-tetrofosmin uptake reflected P-glycoprotein expression without exposure to doxorubicin.

Transcellular transport of radioiodinated 3-iodo-α-methyl-L-tyrosine across monolayers of kidney epithelial cell line LLC-PK1

OBJECTIVE

3-[123I]iodo-alpha-methyl-L-tyrosine ([123I]IMT) is an imaging agent for amino acid transport. In order to obtain fundamental data related to tumor imaging with [123I]IMT and renal physiological accumulation of [123I]IMT, we investigated the transport characteristics of [125I]IMT in porcine kidney epithelial cell line LLC-PK1 using cell monolayers grown on microporous membrane filters.

METHODS

LLC-PK1 monolayers were created on a collagen-coated microporous (3 microm) membrane (4.7 cm2). To examine transcellular transport (secretion and reabsorption) and accumulation, the monolayers were incubated for up to 90 min at 37 degrees C with 18.5 kBq [125I]IMT in Dulbecco's phosphate-buffered saline (pH 7.4) as an uptake solution. After incubation, transcellular transport was assessed by quantifying the radioactivity of the solutions on each side of the monolayer. For the accumulation experiment, the cells were solubilized in NaOH solution, and the radioactivity was quantified. For the inhibition experiment, the inhibitor was added at a final concentration of 1 mM. For the pH dependence experiment, the pH of the apical-side uptake solution was varied from pH 5 to pH 8. Transport of [14C]Tyr was examined for comparison.

RESULTS

Bi-directional transcellular transport of [125I]IMT was observed, corresponding to secretion and reabsorption in proximal tubule. Accumulation of [125I]IMT from the basolateral side (1.62 +/- 0.15%) and the apical side (2.62 +/- 0.35%) was observed at 90 min. 2-Amino-bicyclo[2,2,1]heptane-2-carboxylic acid (a specific inhibitor of system L), L-Tyr (mother compound of [125I]IMT) and 2-aminoisobutyric acid (an inhibitor of system L and A) inhibited both directional transport (p < 0.01) and accumulation (p < 0.01). 2-(Methylamino)isobutyric acid (a specific inhibitor of system A) appeared to inhibit transport and accumulation, but the results were not significant. Decreasing apical pH significantly enhanced accumulation of [125I]IMT from both sides (p < 0.001), whereas accumulation of mother L-Tyr was significantly suppressed.

CONCLUSIONS

The inhibition experiments suggest that the main contributor to [125I]IMT transport is system L, rather than Na+ -dependent transport, in both apical and basolateral membrane. [125I]IMT was transported by the system that transported L-Tyr, but the observed pH dependence of transport suggests that different mechanisms are involved in accumulation of [125I]IMT and [14C]Tyr.

Characterization of uptake of 3-[131I]iodo-α-methyl-L-tyrosine in human monocyte-macrophages

The radiopharmaceutical 3-[(123)I]iodo-alpha-methyl-L-tyrosine ([(123)I]IMT) can be used to study amino acid transport by single-photon emission tomography (SPET). In order to evaluate the potential contribution of [(123)I]IMT accumulation in macrophages to overall uptake values measured in neoplastic lesions in vivo, we studied the mechanisms governing the uptake of this tracer by human monocyte-macrophages (HMMs). HMMs were isolated from healthy human donors by density gradient centrifugation using Ficoll methods. The human glioblastoma cell line U-138 MG (GLIOs) was obtained from American Type Culture Collection. Using multiwell dishes, cells were incubated in phosphate buffered saline or an equivalent sodium-free buffer with 50 kBq [(131)I]IMT per well. [(131)I]IMT uptake was quantified as % injected dose per mass of protein within each culture well. Several natural and artificial amino acids were used as potential transport inhibitors both in sodium-containing and sodium-free medium. [(131)I]IMT uptake was significantly lower in HMMs than in GLIOs (34 +/- 2 %/mg (40 min) vs. 507 +/- 50 %/mg at 30 minutes of incubation, respectively; p < 0.01). Endotoxin (LPS) significantly increased [(131)I]IMT uptake in HMMs by a factor of approximately 2. Transport into non-stimulated HMMs was exclusively sodium-independent and inhibitable by BCH, but not by MeAIB. Under LPS stimulation exclusively, there was in addition also a sodium-dependent inhibition of [(131)I]IMT uptake by L-arginine and MeAIB, albeit to a minor extent. [(131)I]IMT accumulation in HMMs is mainly mediated via an L-like amino acid transport system and increases on HMM activation by LPS. LPS may induce an additional Na(+)-dependent transport system in HMMs. The considerably lower [(131)I]IMT uptake in HMMs than in GLIOs suggests that overall uptake values of this tracer measured by SPET in tumors are not significantly affected by [(123)I]IMT accumulation in macrophages within the neoplastic lesion.

Comparison of O-(2-18F-fluoroethyl)-L-tyrosine PET and 3-123I-iodo-alpha-methyl-L-tyrosine SPECT in brain tumors

The aim of this study was to compare PET with O-(2-(18)F-fluoroethyl)-L-tyrosine ((18)F-FET) and SPECT with 3-(123)I-iodo-alpha-methyl- L-tyrosine ((123)I-IMT) in patients with brain tumors.

METHODS

Twenty patients with a suspected brain tumor were investigated by (18)F-FET PET, (123)I-IMT SPECT, and MRI within 3 wk. Region-of-interest analyses were performed on coregistered PET/SPECT/MRI images and the tumor-to-brain ratio (TBR), muscle-to-brain ratio (MBR), cerebellum-to-brain ratio (CerBR), and sinus-to-brain ratio (SBR) were calculated. In addition, the presence of tumor and the discrimination of anatomic structures on (18)F-FET PET and (123)I-IMT SPECT images were visually determined by 3 observers who were unaware of clinical data.

RESULTS

The TBR of (18)F-FET and (123)I-IMT uptake in cerebral tumors showed a highly significant correlation (r = 0.96; P < 0.001). In the visual analysis for the presence or absence of tumors, no differences for (123)I-IMT SPECT and (18)F-FET PET were found in 19 of 20 patients; in one patient a low-grade glioma was only identified on (18)F-FET PET images but not on (123)I-IMT SPECT images. The contrast between tumor and normal brain was significantly higher in (18)F-FET PET (TBR, 2.0 +/- 0.9) than in (123)I-IMT SPECT (TBR, 1.5 +/- 0.5). The discrimination of anatomic structures yielded a significantly better score on (18)F-FET PET images (rating score, 2.6 +/- 0.9) compared with (123)I-IMT SPECT images (rating score, 1.7 +/- 0.9). The uptake of (18)F-FET in the muscles was significantly higher compared with (123)I-IMT (MBR (18)F-FET, 1.4 +/- 0.3; MBR (123)I-IMT, 0.6 +/- 0.2; P < 0.001) and (18)F-FET demonstrated a significantly higher blood-pool radioactivity than (123)I-IMT (SBR (18)F-FET, 1.3 +/- 0.2; SBR (123)I-IMT, 0.8 +/- 0.2; P < 0.001).

CONCLUSION

The significant correlation of the TBRs of (18)F-FET and (123)I-IMT indicates that clinical experiences of brain tumor diagnostics with (123)I-IMT SPECT might be valid for (18)F-FET PET although substantial differences of the physiologic behavior were identified in extracerebral tissue. As (18)F-FET PET allows improved discrimination of anatomic structures and the tumor-to-brain contrast was significantly superior compared with (123)I-IMT SPECT scans, the results are encouraging for further evaluation of (18)F-FET for imaging brain tumors.

Evidence in locomotion test for the functional heterogeneity of ORL-1 receptors

1. The ORL1 agonists nociceptin and Ro 64-6198 were compared in their ability to modify spontaneous locomotor activity in male NMRI mice not habituated to the test environment. 2. Higher doses of nociceptin (>5 nmol i.c.v.) reduced whereas lower doses (<1 nmol i.c.v.) stimulated locomotor activity. Both effects were blocked by the putative ORL1 antagonists [NPhe1]nociceptin(1-13)NH2 (10 nmol i.c.v.) and UFP101 (10 nmol, i.c.v.). The effects were also blocked by naloxone benzoylhydrazone (1 mg x kg(-1) s.c.), but not by the nonselective opioid antagonist naloxone (1 mg x kg(-1) s.c.). 3 In contrast to nociceptin, the synthetic ORL1 agonist Ro 64-6198 (0.01-1.0 mg x kg(-1) i.p.) produced monophasic inhibition of locomotor activity, which was insensitive to the treatment with [NPhe1]nociceptin(1-13)NH2 or naloxone benzoylhydrazone. Treatment with UFP101 abolished the locomotor inhibition induced by Ro 64-6198 (1.0 mg x kg(-1)), whereas naloxone (1.0 mg x kg(-1), s.c.) further increased the locomotor-inhibitory effects. 4. Naloxone benzoylhydrazone (0.3; 1.0 and 3.0 mg x kg(-1) s.c.) increased locomotor activity, although the effect was statistically significant only with the highest dose used. 5. Pretreatment with the tyrosine hydroxylase inhibitor H44-68 totally eliminated the motor-stimulatory effects of low doses of nociceptin, probably via dopamine depletion. 6. The results suggest that nociceptin stimulates locomotor activity at low doses if dopamine activity is intact. High doses of nociceptin and all the tested doses of Ro 64-6198 seem to interact with a functionally different subset of ORL1 receptors. In addition, the effects of Ro 64-6198 are modulated by tonic opioid receptor activity.

Comparative biodistribution of iodinated amino acids in rats: selection of the optimal analog for oncologic imaging outside the brain

3-(123)I-Iodo-alpha-methyltyrosine ((123)I-3-IMT) is used for the detection of residual and recurrent brain tumors. The application of (123)I-3-IMT for the study of extracerebral malignancies is limited by its marked and rapid renal uptake. In this study, we compared the tumor uptake, biodistribution, and specificity of 5 structurally related iodinated amino acids with those of (123)I-3-IMT. The aim was to select the optimal analog for oncologic imaging outside the brain.

METHODS

We studied 3-(123)I-iodotyrosine ((123)I-3-IT), 2-(123)I-iodotyrosine ((123)I-2-IT), (123)I-iodo-azatyrosine ((123)I-IAzaT), 2-(123)I-iodophenylalanine ((123)I-2-IPhe), and 4-(123)I-iodophenylalanine ((123)I-4-IPhe). Tumor uptake and renal uptake in sarcoma-bearing rats were measured by use of in vivo dynamic imaging. The differential uptake ratio (average counts per pixel of the region of interest divided by the average counts per pixel inside the total body) and rates of tracer accumulation (K(1) values) were calculated. Results were compared with the values obtained for (123)I-3-IMT in the same rat. Tracers that demonstrated high tumor uptake were labeled with (125)I and coinjected with (18)F-FDG in rats with turpentine-induced acute inflammation. After 30 min, the rats were sacrificed and dissected. Amino acid tracer uptake in organs and tissues was measured, and the increase in uptake in the inflamed muscle was expressed relative to the increase in (18)F-FDG uptake.

RESULTS

Tumor uptake and K(1) values for (123)I-2-IT and (123)I-2-IPhe were comparable to those for (123)I-3-IMT. (123)I-4-IPhe showed high tumor uptake but a reduced K(1) value because of high blood-pool activity. (123)I-3-IT and (123)I-IAzaT did not accumulate markedly in tumor tissue. Renal accumulation of (123)I-2-IT, (123)I-2-IPhe, and (123)I-4-IPhe was at least 6 times lower than that of (123)I-3-IMT. (18)F-FDG uptake was markedly increased in areas of acute inflammation (215%). The increases for (125)I-3-IMT and (125)I-4-IPhe were 35.5% and 22.2%, respectively, of the increase for (18)F-FDG. Almost no increase was found for (125)I-2-IT (3.3%) and (125)I-2-IPhe (2.8%).

CONCLUSION

(123)I-2-IT and (123)I-2-IPhe are promising tracers for oncologic imaging outside the brain. (123)I-2-IT has the advantage of an established kit for radiosynthesis.

An artificial amino acid, 4-iodo-L-meta-tyrosine: biodistribution and excretion via kidney

We evaluated the use of radiolabeled 4-iodo-L-meta-tyrosine as an amino acid transport marker. The pharmacologic features of this compound, particularly the biodistribution and excretion, were examined by conducting in vivo and in vitro studies using 4-(125)I-iodo-L-meta-tyrosine (4-(125)I-mTyr). Results obtained for L-(14)C-Tyr and 3-(125)I-iodo-alpha-methyl-L-tyrosine ((125)I-IMT) were used for comparison.

METHODS

In vivo biodistribution studies of 4-(125)I-mTyr were performed in male ddY mice. Urinary excretion of 4-(125)I-mTyr and (125)I-IMT with administration of probenecid was studied. Local distribution of 4-(125)I-mTyr and (125)I-IMT in kidney was visualized by autoradiography. We performed metabolite analysis of 4-(125)I-mTyr in mice. For in vitro studies, reabsorption mechanisms of 4-(125)I-mTyr were compared with those of (125)I-IMT and the parent L-(14)C-Tyr using superconfluent monolayers of the porcine kidney epithelial cell line LLC-PK(1) in medium containing inhibitor (L-Tyr, D-Tyr, and 2,4-dinitrophenol), in Na(+)-free medium, and at 4 degrees C.

RESULTS

4-(125)I-mTyr demonstrated high accumulation in the pancreas and kidney and comparable brain uptake to that of (125)I-IMT. Blood clearance of 4-(125)I-mTyr was faster than that of (125)I-IMT. Three hours after administration, >70% of 4-(125)I-mTyr was excreted via the urine, whereas <5% was found in the feces. Renal autoradiography revealed moderate accumulation of 4-(125)I-mTyr and high accumulation of (125)I-IMT in the renal cortex. Probenecid further reduced accumulation of 4-(125)I-mTyr and (125)I-IMT in the kidney as well as urinary excretion. At 30 min after tracer injection, intact free 4-(125)I-mTyr accounted for >98.1% of the total present in kidney and >96.3% in urine. Protein incorporation was not observed. Uptake of 4-(125)I-mTyr into LLC-PK(1) cell monolayers was remarkably reduced by 5 mmol/L L-Tyr (4.6%) and incubation at 4 degrees C (15.6%) but was reduced by 5 mmol/L D-Tyr (50.0%). L-(14)C-Tyr and (125)I-IMT showed similar results; however, uptake of (125)I-IMT was enhanced by 0.1 mmol/L 2,4-dinitrophenol (165.1%), an inhibitor of generation of energy-rich phosphates.

CONCLUSION

The artificial amino acid 4-(125)I-mTyr demonstrated high metabolic stability, rapid blood clearance, rapid urinary excretion, and similar biodistribution to other radiolabeled L-Tyr analogs. 4-(125)I-mTyr can be a competitive substrate of L-Tyr reabsorption. However, 4-(125)I-mTyr demonstrates different pharmacologic features than those of (125)I-IMT, particularly in renal handling. 4-(125)I-mTyr may potentially be applied as a new amino acid transport marker.

Isoform selectivity of 3-125I-iodo-alpha-methyl-L-tyrosine membrane transport in human L-type amino acid transporters

3-(123)I-Iodo-alpha-methyl-L-tyrosine ((123)I-IMT) has been developed for SPECT of amino acid transport imaging. We examined the isoform selectivity of (125)I-IMT transport of the 2 human L-type amino acid transporters, hLAT1 and hLAT2, with human 4F2hc-coexpressed Xenopus laevis oocytes.

METHODS

An uptake study of (125)I-IMT was performed using transporter-expressed X. laevis oocytes. Oocytes were injected with 17.6 ng of hLAT1 or hLAT2 complementary RNA (cRNA) and 7.4 ng of h4F2hc cRNA in a molar ratio of 1:1. Two days after injection, the uptake of (125)I-IMT was measured in the Na(+)-free uptake solution containing 18.5 kBq of noncarrier-added (125)I-IMT. After incubation for 30 min at room temperature, radioactivity of the oocytes was determined.

RESULTS

Of the 2 hLAT isoforms and h4F2hc-coexpressed X. laevis oocytes, (125)I-IMT uptake via hLAT1 was 5.95-fold higher than that via hLAT2 (P < 0.005).

CONCLUSION

(125)I-IMT transport was hLAT1 selective. Investigations on the isoform selectivity of (125)I-IMT transport with other transporters are anticipated.

Characterization of 3-[125I]iodo-alpha-methyl-L-tyrosine transport via human L-type amino acid transporter 1

We examined transport of 3-[(125)I]iodo-alpha-methyl-L-tyrosine ([(125)I]IMT) in Xenopus laevis oocytes co-expressing human L-type amino acid transporter 1 (a component of system L) and human 4F2hc. Human LAT1 mediated transport of [(125)I]IMT. [(125)I]IMT uptake was decreased by the presence of L-isomers of Cys, Leu, Ileu, Phe, Met, Tyr, His, Trp and Val and D-isomers of Leu, Phe and Met. Human LAT1-mediated [(125)I]IMT uptake was highly stereoselective for the L-isomers of Tyr, His, Trp, Val and Ileu. To examine the effects of 3-iodination and alpha-methylation on IMT transport, kinetic parameters of IMT were compared with those of mother Tyr and 3-[(125)I]iodo-L-tyrosine (3-I-Tyr). Uptake of Tyr, 3-I-Tyr and [(125)I]IMT followed Michaelis-Menten kinetics, with K(m) values of 29.0 +/- 5.1, 12.6 +/- 6.1 and 22.6 +/- 4.1 microM, respectively. Neither the alpha-methyl group nor the size of the 3-iodinated Tyr residue was an obstacle to transport via hLAT1. Furthermore, affinity of IMT for hLAT1 is higher than that of the natural parent tyrosine. The level of efflux mediated by hLAT1 was highly stimulated by extracellularly applied L-Leu, suggesting exchange of [(125)I]IMT and L-Leu via hLAT1.

Increased tumor uptake of 3-(123)I-Iodo-L-alpha-methyltyrosine after preloading with amino acids: an in vivo animal imaging study

3-(123)I-Iodo-L-alpha-methyltyrosine (3-IMT) is an amino acid analog used for tumor imaging. Specific accumulation is mediated mainly by the system L amino acid transport system. System L activity is known to increase when cells are loaded with amino acids. The aim of our study was to measure the effects of amino acid preload on (123)I-3-IMT tumor uptake and image contrast in a rat tumor model using in vivo dynamic imaging.

METHODS

Rhabdomyosarcoma (R1M) tumor-bearing rats underwent 2 dynamic (123)I-3-IMT studies on separate days: 1 baseline study and 1 after intraperitoneal injection (0.25 mmol/kg) of a single amino acid (arginine, proline, glutamate, asparagine, tryptophan, or phenylalanine) administered 30 min before intravenous injection of 18.5 MBq (123)I-3-IMT. A (99m)Tc-labeled human serum albumin study was performed on each rat for the calculation of the blood-pool activity inside the tumor. Time-activity curves were generated for tumor, contralateral background region, kidney, heart, and total body. Tumor uptake was corrected for blood-pool and background activity. Image contrast was calculated as the ratio between tumor and background activity. The rate (K(1)) of tracer entering the tumor was obtained using Patlak analysis. A displacement study was performed on a separate group of rats, in which a high dose of phenylalanine was administered 40 min after (123)I-3-IMT injection.

RESULTS

(123)I-3-IMT accumulation in tumor reached a plateau 10 min after injection. Tumor uptake on the baseline scans correlated well with tumor size (r = 0.92). After preloading, tumor uptake and contrast increased in all conditions: arginine, +26% and +26%; proline, +15% and +13%; glutamate, +14% and +9%; asparagine, +19% and +15%; tryptophan, +36% and 11%; phenylalanine, +22% and + 13%. K(1) values also increased. Administration of an afterload with phenylalanine induced a significant displacement of (123)I-3-IMT tumor accumulation.

CONCLUSION

Prior amino acid administration increases (123)I-3-IMT tumor accumulation and image contrast. This effect can be explained by the increased antiporter activity of the amino acid transport system L in preloaded conditions. Our results indicate that the fasted state might not be the optimal metabolic condition to study tumor accumulation of L-transported tracers such as (123)I-3-IMT. Amino acid administration before (123)I-3-IMT injection could improve tumor uptake and image contrast.

3-[(123)I]Iodo-alpha-methyl-L-tyrosine: uptake mechanisms and clinical applications

3-[(123)I]Iodo-alpha-methyl-L-tyrosine (IMT) is an artificial amino acid which has gained considerable interest in Nuclear Medicine in the last two decades. Although the tracer is not incorporated into proteins it exhibits high uptake in brain tumors and appears to be a valuable tool especially for the diagnostic evaluation and therapy planning of patients with cerebral gliomas. In this paper the present knowledge of the uptake mechanisms and the clinical applications of IMT are reviewed and the clinical perspectives discussed.

Prognostic significance of amino acid transport imaging in patients with brain tumors

OBJECTIVE

To evaluate the prognostic significance of presence, intensity, and extent of amino acid uptake in patients with suspected primary or recurrent brain tumors.

METHODS

We retrospectively analyzed 181 consecutive studies of amino acid uptake using single-photon emission computed tomography and the amino acid l-[3-(123)I]iodo-alpha-methyltyrosine (IMT). In a blinded analysis, all studies were evaluated for presence, maximal uptake (IMT(max)), and extent (IMT(ext)) of focal tracer uptake.

RESULTS

The most frequent tumors were 53 astrocytomas (World Health Organization Grade I-III), 41 glioblastomas, 16 metastases, 13 oligodendrogliomas (Grade II-III), and 10 medulloblastomas. The other patients exhibited various parenchymal tumors or nonneoplastic lesions. IMT uptake was present in 69% of the patients with IMT(max) ranging from 1.4 to 6.2. IMT(max) and IMT(ext) were significant predictors of survival in the whole group. When the group was divided according to primary versus recurrent tumor, only the primary tumors achieved a high level of significance (P < 0.01). When patients without any IMT uptake were excluded from the analysis, statistical significance for both IMT(max) and IMT(ext) was lost. Multiple regression analysis, including IMT(max), IMT(ext), age, and tumor grade, revealed only extent of IMT uptake as an independent predictor of prognosis.

CONCLUSION

Absence of IMT uptake is a significant predictor of long-term survival in patients with suspected primary or recurrent brain tumors. Only the extent of a given lesion provided minor supplementary prognostic information as compared with histopathology and age. These findings suggest caution in relating high amino acid uptake values to poor prognosis, despite the capability of amino acid imaging to help determine the presence and extent of gliomas.

3-[123I]Iodo-L-alpha-methyl tyrosine transport into human fibroblasts and comparison with Ewing's sarcoma cells

The cellular transport systems and the transport kinetics of [123I]IMT uptake into non-malignant extracranial cells were characterized for the first time. Human fibroblasts were chosen as non-malignant extracranial cells as they are found ubiquitous in the body. [123I]IMT is exclusively transported into fibroblasts via the sodium independent system L. An apparent Michaelis constant K(m) = 116.2 +/- 18.9 microM and a maximum transport velocity V(max) = 191.6 +/- 13.9 pmol x (10(6) cells)(-1) x min(-1) were calculated for the sodium-independent transport. These results were compared with those determined in two malignantly transformed extracranial cell lines, the human Ewing's sarcoma cell lines VH-64 and CADO-ES-1.

Correlation between postoperative 3-[(123)I]iodo-L-alpha-methyltyrosine uptake and survival in patients with gliomas

The aim of this study was to evaluate the prognostic value of SPECT imaging using the amino acid analog 3-[(123)I]iodo-L-alpha-methyltyrosine (IMT) in patients with gliomas.

METHODS

One hundred fourteen consecutive patients with newly diagnosed gliomas were examined by IMT SPECT (low-grade glioma, n = 12; anaplastic astrocytoma or oligodendroglioma, n = 46; glioblastoma, n = 56). Seventy-one of these patients had undergone tumor resection 4-6 wk before SPECT imaging (group A). Forty-three patients with unresectable tumors were examined after stereotactic biopsy (group B). IMT uptake at the site of the tumor was assessed visually and quantified relative to a contralateral reference region (IMT uptake ratio). After IMT SPECT, all patients were treated with conformal radiotherapy. The median follow-up time was 27 mo.

RESULTS

In group A, focal IMT uptake at the resection site was visible in 52 of 71 patients (73%). Median survival was only 13 mo in these patients, whereas median survival was reached in patients without focal IMT uptake (P = 0.02). Furthermore, the intensity of IMT uptake significantly correlated with survival: patients with an IMT uptake ratio > 1.7 were at a 4.6 times higher risk of death than were patients with a lower IMT uptake (P < 0.001). The IMT uptake ratio remained a significant prognostic factor when age and grading were included in a multivariate model. In contrast, IMT uptake did not correlate with survival in group B (P = 0.95).

CONCLUSION

In patients with unresectable high-grade gliomas, IMT uptake appears not to correlate with the biologic aggressiveness of tumor cells. Nevertheless, the clear association between focal IMT uptake after tumor resection and poor survival suggests that IMT is a specific marker for residual tumor tissue. Therefore, IMT SPECT is expected to become a valuable tool for the planning and monitoring of local therapeutic modalities.

Kinetic parameters of 3-[(123)I]iodo-L-alpha-methyl tyrosine ([(123)I]IMT) transport in human GOS3 glioma cells

The radiolabelled amino acid 3-[(123)I]iodo-L-alpha-methyl tyrosine ([(123)I]IMT) is a promising tool for the diagnosis and monitoring of brain tumors using single-photon emission tomography (SPECT). However, little is known about the precise kinetics of [(123)I]IMT uptake in human glioma cells. The kinetic analysis of [(123)I]IMT transport in human GOS3 glioma cells yielded a high-affinity apparent Michaelis constant (K(m) = 20.1 +/- 1.5 microM). The maximum transport velocity (V(max)) amounted to 34.8 +/- 1.9 nmol/mg protein/10 min. Competitive inhibition experiments revealed that [(123)I]IMT transport is mediated principally by the sodium-independent system L.

High uptake of L-3-[123I]iodo-alpha-methyl tyrosine in pilocytic astrocytomas

Despite a favourable prognosis, pilocytic astrocytomas may exhibit signs of malignancy on various neuroimaging modalities. This retrospective analysis was conducted to determine whether scintigraphic features of malignancy are also found on single-photon emission tomography (SPET) using L-3-[123I]iodo-alpha-methyl tyrosine (IMT) as a tracer. Twenty patients with pilocytic astrocytomas were retrospectively selected from a large series of patients referred for the evaluation of primary or recurrent brain tumours. IMT SPET was performed in 16 patients, positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose (FDG) was available in 10 of the patients and SPET using technetium-99m tetrofosmin or thallium-201 had been performed in 11. Image analysis was performed using standard protocols to determine how many patients exceeded the respective thresholds of malignancy. Features of malignancy were found in 7/16 IMT SPET studies, in 7/10 FDG PET studies and in 7/11 of the residual SPET investigations. A significant correlation of tumour size and IMT uptake in primary pilocytic astrocytomas indicated partial volume effects to partly account for the differential uptake behaviour (n = 10, r = 0.87, P < 0.05). Differences in IMT uptake in primaries (1.7 +/- 0.6, n = 10) and in recurrent tumours (2.3 +/- 0.7, n = 6) did not attain statistical significance. IMT SPET results indicative of malignancy are regularly found in pilocytic astrocytomas, despite their good prognosis. No uptake may be detected in largely cystic or in small tumours.

In vitro characterization of the influx of 3-[125I]iodo-L-alpha-methyltyrosine and 2-[125I]iodo-L-tyrosine into U266 human myeloma cells: evidence for system T transport

The aim of this study was to investigate the cellular uptake mechanisms responsible for the accumulation of 3-[(125)I]iodo-L-alpha-methyltyrosine ((125)I-3-IMT) and 2-[(125)I]iodo-L-tyrosine ((125)I-2-IT), two radiotracers for metabolic tumor imaging, using single-photon emission tomography, into U266 human myeloma cancer cells. Time course and concentration dependency of (125)I-3-IMT uptake was assessed. Kinetic parameters were calculated using an Eadie Hofstee plot. A set of competitive inhibitors of the main amino acid transport systems was used for the discrimination of the transporters responsible for the uptake of (125)I-3-IMT and (125)I-2-IT. Protein incorporation of both tracers was determined using acid precipitation. The measured maximum velocity for (125)I-3-IMT transport was 4.199 nmol per mg protein 20 s(-1), and the Michaelis constant was 107.9 microM. Addition of 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH), a competitive inhibitor of System L, reduced the influx by 39.0+/-3.3% for (125)I-3-IMT and 66.3+/-0.9% for (125)I-2-IT. The BCH-insensitive influx was further reduced by Tryptophan (Trp) by 43.8+/-3.5% for (125)I-3-IMT and 15.3+/-1.3% for (125)I-2-IT. This suggests involvement of System T transport. We measured <2% of radioactivity in the acid precipitable fractions of both tracers with no increase in time. We conclude that the influx of (125)I-3-IMT and (125)I-2-IT into U266 human myeloma cells is mediated by both System L and System T amino acid transporters. The kinetic parameters suggest that elevated plasma levels of aromatic amino acids will reduce (123)I-3-IMT uptake in myeloma patients. Both tracers do not enter protein synthesis significantly.

Characterization of 3-[123I]iodo-L-alpha-methyl tyrosine ([123I]IMT) transport into human Ewing's sarcoma cells in vitro

3-[(123)I]Iodo-L-alpha-methyl tyrosine ([(123)I]IMT) scintigraphy of extracranial malignant tumors has been described, but little is known about the transport systems involved in [(123)I]IMT uptake into extracranial tumor cells. Here, the precise kinetics of [(123)I]IMT transport into human Ewing's sarcoma cells (VH-64) was determined. The apparent Michaelis constant was of high affinity value (K(m)=41.7+/-3.9 microM) and maximum transport velocitiy amounted to V(max)=20.7+/-0.6 nmol x mg protein(-1) x 10 min(-1). Inhibition experiments revealed the predominance of [(123)I]IMT uptake via sodium-independent system L.

Uptake mechanisms of L-3-[125I]iodo-alpha-methyl-tyrosine in a human small-cell lung cancer cell line: comparison with L-1

The radiolabelled amino acid analogue L-3-[125I]iodo-alpha-methyl-tyrosine (IMT) is under evaluation in brain tumours, where it reflects amino acid transport activity, but is also taken up in many other tumour types. This study investigated the uptake mechanism of IMT in tumour cells not derived from brain tumours, in comparison with the native amino acid 14C-tyrosine (Tyr) from which IMT is derived. Human GLC4 small-cell lung cancer cells in log-phase were incubated with IMT and Tyr. Tracer uptake was determined in various buffers, incubation periods, concentrations of specific amino acid transport blockers, pH and temperature. IMT uptake was very fast, reaching a plateau within 5 min, while Tyr kept on accumulating for > 60 min. Based on steady-state experiments, > 90% of IMT uptake could be attributed to amino acid transport activity. The L transport system was the most important, both for IMT and Tyr. IMT uptake into GLC4 tumour cells is almost completely the result of amino acid transport activity (especially the L system) and is very similar to Tyr uptake. Therefore, also outside the brain, IMT is a metabolic tracer that may reflect the increased amino acid transport that is characteristic for malignant tumours.

Characterization of 3-[(123)I]iodo-L-alpha-methyl tyrosine transport in astrocytes of neonatal rats

3-[(123)I]Iodo-L-alpha-methyl tyrosine ((123)I-IMT) is used for diagnosis and monitoring of brain tumours by means of single-photon emission tomography. As recently shown, (123)I-IMT is predominantly mediated into rat C6 glioma cells by sodium-independent system L for large neutral amino acids. Until now, (123)I-IMT transport in non-neoplastic glial cells has not been examined. Therefore, the aim of this study was to examine the cellular pathways and precise transport kinetics of (123)I-IMT uptake into astrocytes of neonatal rats. In particular sodium-independent (123)I-IMT transport into neonatal astrocytes was compared with sodium-independent (123)I-IMT uptake into neoplastic rat C6 glioma cells. Competitive inhibition experiments showed that (123)I-IMT is exclusively transported via sodium-independent system L into the neonatal astrocytes (92%). Kinetic analysis of sodium-independent (123)I-IMT uptake into neonatal astrocytes and into C6 glioma cells revealed apparent Michaelis constants K(M) = 13.9 +/- 0.5 microM and K(M) = 33.9 +/- 4.1 microM, respectively, which are in the same range of K(M) values as those recently determined for amino acid transport into neoplastic and non-neoplastic glial cells. Indeed, the K(M) values in the micromolar range correspond to the expression of the LAT-1 subunit of system L both in the neonatal astrocytes and in C6 glioma cells. However, sodium-independent maximum transport velocities (V(max)) differed significantly between neonatal astrocytes and C6 glioma cells (11.1 +/- 0.3 and 39.9 +/- 3.3 nmol/mg protein/10 min, respectively).

3-[123I]iodo-alpha-methyl-L-tyrosine transport and 4F2 antigen expression in human glioma cells

3[(123)I]iodo-alpha-methyl-L-tyrosine is a tracer of amino acid transport in brain tumors using single-photon emission-computed tomography and predominantly transported by amino acid transport system L. The 4F2 antigen has been identified to be linked to system L-like transport and is assumed to be a part of the transporter protein. We demonstrated that system L-mediated transport of IMT and 4F2 antigen expression are dependent on proliferation rate of human glioma cells and significantly correlated with each other.

Investigation of iodine-123-labelled amino acid derivatives for imaging cerebral gliomas: uptake in human glioma cells and evaluation in stereotactically implanted C6 glioma rats

In developing iodine-123-labelled amino acid derivatives for imaging cerebral gliomas by single-photon emission tomography (SPET), we compared p-[123I]iodo-L-phenylalanine (IPA), L-[123I]iodo-1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (ITIC) and L-3-[123I]iodo-alpha-methyltyrosine (IMT) with regard to their uptake in human glioblastoma T99 and T3868 cells, and thereafter studied the mechanisms promoting the cellular uptake. The potential of the 123I-iodinated agents for use as SPET radiopharmaceuticals was evaluated in healthy experimental rats as well as in rats with stereotactically implanted C6 gliomas. The radiopharmaceutical uptake into glioblastoma cells was rapid, temperature and pH dependent, and linear during the first 5 min. Equilibrium was reached after 15-20 min, except in the case of ITIC, the initial uptake of which gradually decreased from 15 min onwards. The radioactivity concentration in glioma cells following 30-min incubation at 37 degrees C (pH 7.4) varied from 11% to 35% of the total activity per million cells (ITIC < IMT < or = IPA). Competitive inhibition experiments using alpha-(methylamino)-isobutyric acid and 2-amino-2-norbornane-carboxylic acid, known as specific substrates for systems A and L, respectively, as well as representative amino acids preferentially transported by system ASC, indicated that IPA, like IMT, is predominantly mediated by the L and ASC transport systems, while no significant involvement of the A transport system could be demonstrated. By contrast, none of the three principal neutral amino acid transport systems (A, L and ASC) appear to be substantially involved in the uptake of ITIC into glioblastoma cells. Analysis of uptake under conditions that change the cell membrane potential, i.e. in high K+ medium, showed that the membrane potential plays an important role in ITIC uptake. Alteration of the mitochondrial activity by means of valinomycin or nigericin induces a slight increase or decrease in the radiopharmaceutical uptake, suggesting a minor contribution of the mitochondria in the uptake. IPA, IMT and ITIC passed the blood-brain barrier, and thereafter showed efflux from the brain. The radioactivity concentration in healthy rat brain 15 min following intravenous injection varied from 0.07% (ITIC) to 0.27% ID/g (IPA). In comparison, the brain uptake in the stereotactically implanted C6 glioma rats was substantially higher (up to 1.10% ID/g 15 min p.i.), with tumour-to-background ratios greater than 4. These data indicate that IPA and ITIC, like IMT, exhibit interesting biological characteristics which hold promise for in vivo brain tumour investigations by SPET.

Transport mechanisms of 3-[123I]iodo-alpha-methyl-L-tyrosine in a human glioma cell line: comparison with [3H]methyl]-L-methionine

The amino acid analog 3-[(123)I]iodo-alpha-methyl-L-tyrosine (IMT) is under clinical evaluation as a SPECT tracer of amino acid transport in brain tumors. This study investigated the carrier systems involved in IMT transport in human glioma cells in comparison with [3H-methyl]-L-methionine (3H-MET).

METHODS

Human glioma cells, type 86HG-39, were cultured and incubated for 1 min at 37 degrees C with IMT and 3H-MET in the lag phase (1.2 d after seeding), exponential growth phase (3 d after seeding), and plateau phase (8 d after seeding). Experiments were performed in the presence and absence of Na+, during inhibition of system L amino acid transport by 2-aminobicyclo[2.2.1 ]heptane-2-carboxylic acid (BCH), and during inhibition of system A amino acid transport by 2-(methylamino)-isobutyric acid (MeAIB).

RESULTS

IMT and 3H-MET uptake decreased by 55%-73% when the cells entered from the exponential growth phase into the plateau phase (P< 0.05; n = 3-11). Inhibition by BCH reduced uptake of IMT in the lag phase, exponential growth phase, and plateau phase by 90%-98% (P < 0.001; n = 3-6) and the uptake of 3H-MET by 73%-83% (P < 0.001; n = 3-11). In a Na+-free medium 3H-MET uptake was reduced by 23%-33% (P < 0.05; n = 3-11), whereas IMT uptake was not significantly different. MeAIB showed no significant effect on IMT or 3H-MET uptake in either phase.

CONCLUSION

Transport of both IMT and 3H-MET depends on the proliferation rate of human glioma cells in vitro and is dominated by BCH-sensitive transport. These data indicate that system L is induced in rapidly proliferating glioma cells and is the main contributor to the uptake of both tracers. 3H-MET transport showed a minor Na+ dependency that was not attributable to system A. The similarity of transport mechanisms of both tracers emphasizes the clinical equivalence of IMT SPECT and (11)C-MET PET for the diagnostic evaluation of gliomas.

Sequential scintigraphic strategy for the differentiation of brain tumours

Both thallium-201 and iodine-123 alpha-methyltyrosine (123I-IMT) have been shown to be useful in the diagnostic evaluation of brain tumours. The aim of this study was to investigate the respective contributions of 201Tl and 123I-IMT single-photon emission tomography (SPET) in the non-invasive evaluation of intracerebral tumours. We analysed 65 patients with the following brain tumours: 8 non-neoplastic lesions, 4 meningiomas, 12 low-grade gliomas, 28 high-grade gliomas, 11 metastases and 2 high-grade lymphomas. 201Tl SPET and 123I-IMT SPET were performed [start of 201Tl SPET: 15 min p.i. (early) and 180 min p.i. (delayed); start of 123I-IMT SPET: 15 min p.i.]. The intensity of uptake was quantified as the ratio between tracer accumulation in the tumour and in the contralateral hemisphere. None of the non-neoplastic lesions or low-grade gliomas expressed marked 201Tl uptake. All malignant tumours except one small metastasis and all meningiomas except one small, cystic and degenerated lesion showed significant 201Tl accumulation [Tl(15')>2.0]; 123I-IMT uptake was either absent or intermediate in non-malignant lesions except in two low-grade gliomas; the highest levels were observed in high-grade gliomas followed by metastases and lymphomas (mean IMT: 2.7 vs. 2.1 vs. 1.8), with metastases showing a high variability in 123I-IMT uptake (range: 0.8-3.6). Using 201Tl to distinguish non-neoplastic lesions from malignant tumours and meningiomas, 63 of 65 patients were characterised correctly. In the latter group, high-grade gliomas were correctly identified in 27 of 28 cases by their amino acid uptake. It is concluded that the combination of 201Tl and 123I-IMT surpasses the accuracy of each single test in the differentiation of space-occupying lesions of the brain. Based on these preliminary results, a sequential strategy is proposed involving an initial 201Tl SPET study and an additional 123I-IMT SPET study in the event of positive 201Tl uptake.

Kinetics of 3-[(123)I]iodo-l-alpha-methyltyrosine transport in rat C6 glioma cells

3-[(123)I]Iodo-l-alpha-methyltyrosine ((123)I-IMT) is used for the diagnosis and monitoring of brain tumours by means of single-photon emission tomography (SPET). To date, little has been known about the system for the transport of (123)I-IMT into brain tumour cells. It is assumed that (123)I-IMT is transported by a specific carrier for large, neutral amino acids (L-system). In this study, rat C6 glioma cells were used to characterize the uptake system of (123)I-IMT and to investigate its precise kinetics. The time course of (123)I-IMT uptake into the cells was examined for a range of 1-60 min. (123)I-IMT uptake rates with varying concentrations of (123)I-IMT (2. 5-50 microM) in the medium were quantified to assess the kinetic parameters of (123)I-IMT transport. Furthermore, competition of (123)I-IMT with other amino acids was investigated to identify the distinct transport systems involved in (123)I-IMT uptake. (123)I-IMT uptake into C6 glioma cells was linear for approximately 10 min and reached a steady-state level within 30 min. The analysis of the rate of uptake of (123)I-IMT at different concentrations was concordant with the predominance of a single uptake system. The apparent Michaelis constant (K(m)) of (123)I-IMT was 26.2+/-1.9 microM, and the maximum transport velocity (V(max)) was 35.4+/-1.7 nmol/mg protein per 10 min. 77%+/-10% of (123)I-IMT transport was sodium independent and 23%+/-3% was sodium dependent. Competitive inhibition of (123)I-IMT uptake by 2-aminobicyclo[2.2. 1]heptane-2-carboxylic acid, alpha-(methylamino)isobutyric acid and naturally occurring amino acids revealed a major (123)I-IMT transport via the sodium-independent system L (72%) and a minor uptake via the sodium-dependent system B(0,+) (17%). Our results show that (123)I-IMT transport into C6 glioma cells is principally mediated by the L-system and to a minor extent by the B(0,+)-system. The kinetic parameters of (123)I-IMT uptake are in the range of those of naturally occurring amino acids.

Feasibility of tumor imaging using L-3-[iodine-123]-iodo-alpha-methyl-tyrosine in extracranial tumors

L-3-[123I]-Iodo-alpha-methyl-tyrosine (IMT) is a modified amino acid. It is reported to be avidly taken up in brain tumors, reflecting amino acid transport and is suitable for SPECT.

METHODS

To determine whether tumors outside the brain can also accumulate this tracer, we injected 300-450 MBq IMT into 20 patients with different tumors [5 breast cancers, 4 lung tumors (1 benign), 2 carcinoid liver metastases, 4 soft-tissue tumors (1 benign), 3 malignant lymphomas and 2 primary brain tumors]. Tumor size ranged from 1-12 cm. Imaging was repeated after radiotherapy in two patients with breast cancer. Histology was available in all cases. Dynamic scans, whole-body imaging and SPECT were performed during the first hour and 3 hr after injection. Plasma samples were analyzed for IMT, free 1231 and other metabolites.

RESULTS

All primary tumors were visualized. Tumor-to-background ratios ranged from 1.1 to 3.8 on planar and from 1.3 to 6.2 on SPECT images. Tumor uptake peaked in the first hour. Two carcinoid lesions in the liver tumors exhibited no IMT uptake above liver background. Tumor-to-background ratios in a benign bone inflammatory process and a focal pulmonary vasculitis were less than 1.2 (planar) and 1.9 (SPECT) and could be differentiated from uptake in all malignant nonbrain tumors. IMT was rapidly cleared from the plasma [3.6% +/- 0.6% (mean +/- s.d.) injected dose/liter at 10 min postinjection]. Minor in vivo deiodination was present (<1% of injected dose 1 hr postinjection). No other metabolites were found. Normal distribution consists of some uptake in the brain, liver, spleen, muscles, pancreatic region and intestinal structures and massive uptake and excretion in the kidneys and bladder.

CONCLUSION

IMT has potential as a metabolic tracer in tumors outside the brain.

Comparison of iodotyrosines and methionine uptake in a rat glioma model

This study compares brain tumor imaging with 3-[123I]iodo-alpha-methyl-L-tyrosine (IMT) and 3-[123I/125I]iodo-O-methyl-alpha-methyl-L-tyrosine (OMIMT) to that with [methyl-3H]-L-methionine (Met) in a rat glioma model by double-tracer autoradiography.

METHODS

Cells of the glioma clone F-98 were implanted stereotactically into the right basal ganglia of 22 Fischer 344 rats. After 8 days of tumor growth, the animals simultaneously were injected with a mixture of either 123I-IMT and 3H-Met (n=5), 123I-OMIMT and 3H-Met (n=8) or 123I-IMT and 125I-OMIMT (n=9). The animals were killed 15 min after the tracer injection and cryosections of the tumor-bearing brain area were exposed to phosphor-imaging plates both immediately and after the decay of 123I. Tumor-to-brain ratios (T/B) and intratumoral distribution of the different tracers and of the cresyl violet staining of the tissue were compared.

RESULTS

There was a significant correlation of the T/B ratios between all tracers (IMT versus Met: r=0.97, n=5, p < 0.01; OMIMT versus Met: r=0.94, n=8, p < 0.001; OMIMT versus IMT: r=0.95, n=9, p < 0.001). Intratumoral tracer distribution was similar for all tracers and the extent of tumor labeling was identical to that of the histological tumor extent. Mean values of the T/B ratios, however, were lower for IMT (2.81+/-0.78, n=14, mean+/-s.d., p < 0.01 compared with Met) and for OMIMT (2.03+/-0.57, n=17, p < 0.01 compared with Met) than for Met (3.86+/-1.12, n=13).

CONCLUSION

This study confirms that tumor imaging with IMT is similar to that of Met but T/B ratios of IMT are lower. OMIMT intratumoral tracer distribution and tumor size are similar to Met and IMT, but the T/B contrast is rather low and makes this amino acid less suitable for clinical application.

Whole-body kinetics and dosimetry of L-3--123I-iodo-alpha-methyltyrosine

The synthetic amino acid L-3--123I-iodo-alpha-methyltyrosine (IMT) is currently under clinical evaluation as a single-photon emission tomography (SPET) tracer of amino acid uptake in brain tumours. So far, dosimetric data in respect of IMT are not available. Therefore we investigated the whole-body distribution of IMT in six patients with cerebral gliomas and the radiation doses were estimated. Whole-body scans were acquired at 1.5, 3 and 5 h after i.v. injection of 370-550 MBq IMT. The bladder was voided prior to each scan and the radioactivity excreted in the urine was measured. Based on the MIRD-11 method and the updated MIRDOSE3, the mean absorbed doses for various organs and the effective dose were calculated from geometric means of the anterior and posterior whole-body scans using seven source organs and the residence time. IMT was predominantly excreted by the kidneys (52.8%+/-11.5% at 1.5 h p.i., 63.0%+/-15.7% at 3 h p.i. and 74.6%+/-9.8% at 5 h p.i.). No organ system other than the urinary tract showed significant retention of the tracer. Early whole-body scans revealed slightly increased tracer uptake in the liver and in the bowel. Highest absorbed doses were found for the urinary bladder wall (0.047 mGy/MBq), the kidneys (0.010 mGy/MBq), the lower large intestinal wall (0.011 mGy/MBq) and the upper large intestinal wall (0.008 mGy/MBq). The effective dose according to ICRP 60 was estimated to be 0.0073 mSv/MBq for adults. This leads to an effective dose of 3.65 mSv in a typical brain SPET study using 500 MBq IMT. The MIRDOSE3 scheme yielded similar results. Thus, in spite of the relatively high tracer dose required for optimal brain scanning, radiation exposure in SPET studies with IMT is in the normal range of routine nuclear medicine investigations.

Radioiodine-labelled alpha-methyl-tyrosine in malignant melanoma: cell culture studies and results in patients

Tyrosine is a precursor of melanin synthesis and might thus present a valuable marker for melanoma. The aim of this study was to evaluate the uptake of alpha-methyl-tyrosine (AMT) in melanoma cell cultures and to assess its usefulness as a radiopharmaceutical for staging melanoma patients with whole-body scintigraphy. Melanoma (M19-cell lines) and fibroblast (negative control) cell cultures were incubated with 125I-AMT and the radioactive uptake in the cell lines was measured in a gamma-counter over 24 h. For in vivo studies, planar whole-body scintigraphy and single photon emission computed tomography (SPECT) of the tumour region was performed following injection of 250-350 MBq 123I-AMT in six patients with known melanoma metastases. Findings were compared with results of whole-body positron emission tomography using 18F-fluorodeoxyglucose (FDG-PET) as a standard of reference. Fibroblasts showed an unchanged uptake of (mean +/- SD) 0.56 +/- 0.09% 15 min and 0.066 +/- 0.09% 24 h, respectively, after incubation of 125I-AMT, whereas there was an increased uptake in melanoma cell cultures over time from 0.9 +/- 0.05% to 7.5 +/- 1.6%. In staging melanoma patients, the sensitivity of whole-body AMT-scintigraphy compared with FDG-PET was 37% (10 of 27 metastases). AMT is transported and metabolized to a high extent in melanoma cells and 123I-AMT is accumulated in melanoma metastases. Owing to its low sensitivity, however, the clinical use of whole-body AMT scintigraphy cannot be recommended.

Amino acid transport after transient global ischemia in rats: quantitative autoradiographic study using 3-[125I]iodo-alpha-methyl-L-tyrosine

We studied the influence of reperfusion on amino acid transport of the brain after transient global ischemia in rats. The animals were subjected to 30-min four-vessel occlusion according to the procedures developed by Pulsinelli prior to recirculation for 3, 6, 24, 48 and 72 h. We used 3-[125I]iodo-alpha-methyl-L-tyrosine as an autoradiographic tracer for selective cerebral amino acid transport maker. Following 30-min global ischemia, uptakes of 3-[125I]iodo-alpha-methyl-L-tyrosine were significantly (P < 0.05) lower in substantia nigra, striatum and ventral tegmental area (6, 24, 48 and 72 h post-reperfusion), but significantly (P < 0.05) higher in cortex and thalamus (3 and 6 h post-reperfusion). The influence of transient global ischemia on cerebral amino acid transport manifested region-specific three different patterns; namely, suppression, acceleration and no change in amino acid transport. The influence of transient ischemia on catecholamine-synthesizing brain sites is most remarkable.

[Uptake of DL-3-123I-iodo-alpha-methyltyrosine in recurrent brain tumors]

DL-3-123I-iodo-alpha-methyltyrosine (123I-IMT) is a radiopharmacon which concentrates in brain tumors and can be employed in SPECT. We performed 20 studies in 16 patients after neurosurgery for malignant brain tumors (localization of the primary tumor by CT/MRI). Tumor/non-tumor ratios (T/NT) were calculated in ROI-technique. In 17 cases there was a recurrence or tumor remnant. 14/17 were detectable by increased uptake (T/NT 1.43-2.25). The scans were correlated with CT/MRI studies and validated by biopsy (6/14) or follow-up. All 3 patients without recurrence (neuroradiological follow-up over 6-24 months) had a negative scan. 123I-IMT scintigraphy provides complementary information to CT and MRI. In equivocal neuroradiological or clinical cases it may be valuable in the detection of tumor recurrences and allows an earlier onset of therapy.

Canine SPECT studies for cerebral amino acid transport by means of 123I-3-iodo-alpha-methyl-L-tyrosine and preliminary kinetic analysis

We have already reported that 123I-3-iodo-alpha-methyl-L-tyrosine (123I-L-AMT) is superior as a single-photon emitter labeled radiopharmaceutical reflecting cerebral amino acid transport. In this study, we investigated the distribution of 123I-L-AMT in the canine head by means of SPECT and kinetically analyzed the data in the brain. As a result, clear SPECT images of the canine brain were obtained. Kinetic analysis with a 2-compartment model, including or expressing membrane transport of the amino acid, was performed with time-activity curves in the arterial blood and in the cerebral region. The results of the analysis coincided closely with the experimental data and the relevance of the model was strongly suggested. Therefore 123I-L-AMT is considered to be useful as a single photon radiopharmaceutical which enables us to measure the cerebral amino acid transport rate.

Pharmacodynamic and pharmacokinetic interactions between alpha-methylparatyrosine and phenobarbitone

The hypnosis and hypothermia induced by phenobarbitone (100 mg/kg i.p.) were greatly potentiated by combined treatment with alpha-methylparatyrosine (alpha-MPT, 250 mg/kg i.p.). alpha-MPT per se produced sedation and hypothermia. Measurement of blood and brain levels of phenobarbitone in rats treated with phenobarbitone alone or phenobarbitone plus alpha-MPT revealed that the latter delayed the disappearance rates of phenobarbitone from both brain and plasma. These results suggest an interaction at the site of distribution, metabolism and/or excretion of phenobarbitone. The possibility of a pharmacodynamic interaction involving neurotransmitters is also discussed.

Direct n.c.a. electrophilic radioiodination of tyrosine analogues; their in vivo stability and brain-uptake in mice

In order to improve tracers for amino acid transport studies with SPET we have radioiodinated methylated tyrosines and compared their brain uptake and in vivo deiodination in mice. O-methylation not only leads to a higher lipophilicity and hence significantly higher brain uptake with a maximum of 5% dose/g for 3-[123I]iodo-O-methyl-L-alpha-methyltyrosine (OMIMT) but also significantly prevents in vivo deiodination. High n.c.a. radioiodination yields (> or = 80%) are obtained for the activated aromatic compounds L-tyrosine and L-alpha-methyltyrosine using Iodo-gen iin a heterogeneous aqueous system. Direct n.c.a. radioiodination of the less-activated O-methyl analogues has been achieved in reasonable yields (60%) with Iodo-gen in homogeneous TFA solutions containing about 10% of water.

Brain uptake of alpha-[14C]methyl-para-tyrosine in the rat

The blood-brain permeabilities of L-[3H]tyrosine and the tyrosine hydroxylase (TH) inhibitor alpha-[14C]methyl-para-tyrosine ([14C]AMPT) were determined in rat striatum, a brain region rich in TH activity, and in other brain regions containing relatively little TH activity. In striatum, the unidirectional clearance rate (K1) for L-[3H]tyrosine (6.2 ml hg-1 min-1) was significantly greater than the rates for L-[14C]AMPT (2.8 ml hg-1 min-1) and D-[14C]AMPT (0.8 ml hg-1 min-1). The apparent volume of distribution (Vf) for L-[14C]AMPT in striatum (72.5 +/- 4.0 ml hg-1) did not differ from the Vf in other brain regions. The homogeneous distribution of L-[14C]AMPT in rat brain indicates that labeled AMPT is unsuitable for the study of TH in vivo by quantitative autoradiography.

Evaluation of 3-[18F]fluoro-alpha-fluoromethyl-p-tyrosine as a tracer for striatal tyrosine hydroxylase activity

3-[18F]Fluoro-alpha-fluoromethyl-p-tyrosine (3-F-FMPT) was evaluated as a tracer for CNS tyrosine hydroxylase (TH) activity in rodents and in a rhesus monkey. Results of in vitro experiments using rat striatal homogenates showed that the introduction of fluorine into the 3-phenyl position did not significantly alter the ability of FMPT to act as a TH-activated L-aromatic amino acid decarboxylase (L-AAAD) inhibitor. These studies further showed that 3-F-FMPT-induced L-AAAD inhibition was dose-dependent. Furthermore, striatal homogenates prepared from rats pretreated with the potent TH inhibitor alpha-methyl-p-tyrosine was found to have diminished 3-F-FMPT-induced L-AAAD inhibition. However, despite these promising in vitro results, the biodistribution of this compound in mice showed low brain uptake and fast clearance through the kidneys. A PET study using a Rhesus monkey injected with 3-[18F]F-FMPT confirmed the results obtained in mice, i.e. negligible brain uptake but high localization in the bladder. We conclude that 3-[18F]F-FMPT would not be useful as a tracer for cerebral TH activity.

A new approach to the integrity of dual blood-brain barrier functions of global ischemic rats. Barrier and carrier functions

BACKGROUND AND PURPOSE

We studied the influence of reperfusion on carrier and barrier functions of the blood-brain barrier after transient global ischemia in rats.

METHODS

We used iodine-125-labeled 3-iodo-alpha-methyl-L-tyrosine and carbon-14-labeled sucrose as tracers for studying carrier and barrier functions of the blood-brain barrier, respectively. Brain uptakes of these two tracers were measured in Wistar rats subjected to either 15- or 30-minute four-vessel occlusion developed by Pulsinelli and Brierly before recirculation for 3, 6, 24, 48, and 72 hours. Tracer (5 microCi) was injected intravenously in each rat 30 minutes before killing the animal.

RESULTS

Following 15- or 30-minute ischemia, [14C]sucrose uptakes were significantly higher at 3 and 6 hours of reperfusion before recovery to control values after reperfusing for 24 to 48 hours in almost all brain regions. However, a rebound in radioligand uptake was significantly manifested in some sites at 72 hours after reperfusion (p < 0.05 to p < 0.01). Uptakes of 125I-3-iodo-alpha-methyl-L-tyrosine were brain site-dependent: significantly (p < 0.05) higher in cortex (3 and 48 hours after reperfusion) and thalamus (3, 6, and 48 hours after reperfusion) but significantly (p < 0.05 to p < 0.01) lower in striatum, cortex (72 hours after reperfusion), and midbrain (6, 24, and 72 hours after reperfusion). Because the [14C]sucrose uptake in brain was 10% lower than that of 125I-3-iodo-alpha-methyl-L-tyrosine, the change in absolute transport of the latter tracer was approximated to its brain uptake.

CONCLUSIONS

The carrier and barrier functions of the blood-brain barrier should be evaluated separately. The radioligand 125I-3-iodo-alpha-methyl-L-tyrosine may serve as a useful tool to evaluate the carrier function of the blood-brain barrier after transient cerebral ischemia in rats.