Synthesis, Radiolabelling and In Vitro Characterization of the Gallium-68-, Yttrium-90- and Lutetium-177-Labelled PSMA Ligand, CHX-A''-DTPA-DUPA-Pep.

Since prostate-specific membrane antigen (PSMA) has been identified as a diagnostic target for prostate cancer, many urea-based small PSMA-targeting molecules were developed. First, the clinical application of these Ga-68 labelled compounds in positron emission tomography (PET) showed their diagnostic potential. Besides, the therapy of prostate cancer is a demanding field, and the use of radiometals with PSMA bearing ligands is a valid approach. In this work, we describe the synthesis of a new PSMA ligand, CHX-A''-DTPA-DUPA-Pep, the subsequent labelling with Ga-68, Lu-177 and Y-90 and the first in vitro characterization. In cell investigations with PSMA-positive LNCaP C4-2 cells, KD values of ≤14.67 ± 1.95 nM were determined, indicating high biological activities towards PSMA. Radiosyntheses with Ga-68, Lu-177 and Y-90 were developed under mild reaction conditions (room temperature, moderate pH of 5.5 and 7.4, respectively) and resulted in nearly quantitative radiochemical yields within 5 min.

PET and MRI reveal early evidence of neurodegeneration in spinocerebellar ataxia type 17.

UNLABELLED: Spinocerebellar ataxia type 17 (SCA17) is a rare autosomal dominantly inherited neurodegenerative disorder presenting with a variable phenotype including ataxia, dystonia, chorea, and parkinsonism, as well as cognitive impairment. We evaluated morphologic and functional imaging characteristics to elucidate evidence of neurodegeneration in SCA17, even in the presymptomatic stage of the disease. METHODS: Nine individuals of 3 large SCA17 pedigrees, including 4 presymptomatic mutation carriers, underwent cranial 3-dimensional MRI volumetry, as well as multitracer PET with (18)F-FDG, (11)C-d-threo-methylphenidate, and (11)C-raclopride. Healthy subjects showing no signs of a neurologic or psychiatric disease served as controls. RESULTS: MRI volumetry revealed atrophy of the cerebellum and caudate nucleus in manifesting patients (P = 0.04 and 0.05, respectively) and in presymptomatic mutation carriers (P = 0.04 and 0.01, respectively). PET demonstrated decreased glucose metabolism in the striatum, as well as in the cuneus, cingulum, and parietal lobe, in all SCA17 patients and presymptomatic mutation carriers. In addition, PET was closely correlated with motor performance as assessed by the Scale for the Assessment and Rating of Ataxia (P = 0.037) and Unified Parkinson Disease Rating Scale (P = 0.05) and with cognitive function as assessed by the Mini-Mental Status Examination (P = 0.037). Furthermore, (11)C-raclopride PET showed impairment of the postsynaptic dopaminergic compartment of the putamen and caudate nucleus not only in manifest SCA17 patients (P = 0.04 and 0.008, respectively) but also in yet-unaffected mutation carriers (P = 0.05 and 0.05, respectively). The degree of postsynaptic dopaminergic dysfunction was associated with impairment of motor performance. In contrast, significant presynaptic dopaminergic deficits assessed with (11)C-d-threo-methylphenidate PET were not detected. CONCLUSION: MRI volumetry, as well as (11)C-raclopride and (18)F-FDG PET, reveal neuronal dysfunction and neurodegeneration even in the presymptomatic stage and may serve as markers for disease activity in upcoming interventional trials on SCA17.

Significant impact of different oxygen breathing conditions on noninvasive in vivo tumor-hypoxia imaging using [¹8F]-fluoro-azomycinarabino-furanoside ([¹8F]FAZA).

BACKGROUND: [18F]FAZA is a PET biomarker with great potential for imaging tumor hypoxia. Aim of our study was to compare [18F]FAZA uptake in mice with subcutaneous exogenous CT26 colon carcinomas and endogenous polyoma middle-T (PyV-mT) mammary carcinomas and to analyze the influence of different breathing protocols in CT26 colon carcinomas as well as the reversibility or irreversibility of [18F]FAZA uptake. METHODS: We injected subcutaneous CT26 colon carcinoma or polyomavirus middle-T (PyV-mT) mammary carcinoma-bearing mice intravenously with18F-FAZA and performed PET scans 1-3 h post injection (p.i.). To analyze the impact of oxygen supply in CT26 carcinomas we used three different breathing protocols: (P0) air; (P1) 100% oxygen 1 h prior injection until 3 h p.i.; (P2) 100% oxygen breathing starting 2 min prior tracer injection until 1 h p.i. and during the PET scans; mice were breathing air between the 2 h and 3 h 10 min static scans. Normalized PET images were analyzed by using defined regions of interest. Finally, some mice were dissected for pimonidazole immunohistochemistry. RESULTS: There was no difference in18F-FAZA uptake 1-3 h p.i. between the two carcinoma types (CT26: 1.58 ± 0.45%ID/cc; PyV-mT: 1.47 ± 0.89%ID/cc, 1 h p.i., tumor size < 0.5 cm3). We measured a significant tracer clearance, which was more pronounced in muscle tissue (P0). The [18F]FAZA tumor-to-muscle-ratios in CT26 colon carcinoma-bearing mice 2 h and 3 h, but not 1 h p.i. were significantly higher when the mice breathed air (P0: 3.56 ± 0.55, 3 h) compared to the oxygen breathing protocols (P1: 2.45 ± 0.58; P2: 2.77 ± 0.42, 3 h). Surprisingly, the breathing protocols P1 and P2 showed no significant differences in T/M ratios, thus indicating that the crucial [18F]FAZA uptake phase is during the first hour after [18F]FAZA injection. Importantly, the muscle clearance was not affected by the different oxygen breathing conditions while the tumor clearance was lower when mice were breathing air. CONCLUSION: Exogenous CT26 colon carcinomas and endogenous polyoma middle-T (PyV-mT) mammary carcinomas showed no differences in [18F]FAZA uptake 1-3 h p.i. Our analysis using various breathing protocols with air (P0) and with pure oxygen (P1, P2) clearly indicate that [18F]FAZA is an appropriate PET biomarker for in vivo analysis of hypoxia revealing an enhanced tracer uptake in tumors with reduced oxygen supply. [18F]FAZA uptake was independent of tumor-type.

Assessment of PET tracer uptake in hormone-independent and hormone-dependent xenograft prostate cancer mouse models.

UNLABELLED: The pharmacokinetics of (18)F-fluorodeoxythymidine (FLT), (18)F-FDG, (11)C-choline, and (18)F-fluoroethylcholine (FEC) in 2 hormone-independent (PC-3, DU145) and 2 hormone-dependent (CWR22, PAC120) prostate cancer xenograft mouse models were evaluated by PET and compared by immunohistochemistry. Further investigation was performed to determine whether PET can detect early changes in tumor metabolism after androgen ablation therapy through surgical castration. METHODS: PET was performed on 4 consecutive days. In addition, the CWR22 and PAC120 tumor models were surgically castrated after the baseline measurement and imaged again after castration. The tracer uptake was analyzed using time-activity curves, percentage injected dose per volume (%ID/cm(3)), and tumor-to-muscle ratio (T/M). RESULTS: Regarding the hormone-independent prostate tumor models, (18)F-FLT showed the best T/M and highest %ID/cm(3) in PC-3 (2.97 ± 0.63 %ID/cm(3)) and DU145 (2.06 ± 0.75 %ID/cm(3)) tumors. (18)F-FDG seemed to be the tracer of choice for delineation of the PC-3 tumors but not for the DU145 tumors. Using (11)C-choline (PC-3: 1.33 ± 0.29 %ID/cm(3), DU145: 1.60 ± 0.27 %ID/cm(3)) and (18)F-FEC, we did not find any significant uptake in the tumors, compared with muscle tissue. Regarding the hormone-dependent prostate tumor models, the CWR22 model showed a highly significant (P < 0.01) decrease in tumor (18)F-FDG uptake from 4.11 ± 1.29 %ID/cm(3) to 2.19 ± 1.45 %ID/cm(3) after androgen ablation therapy. However, the (18)F-FLT, (11)C-choline, or (18)F-FEC tracers did not provide sufficient uptake or reliable information about therapy response in CWR22 tumors. The PAC120 model showed a significant increase in (18)F-FLT tumor uptake (P = 0.015) after androgen ablation therapy. The accumulation of (18)F-FEC (before: 2.32 ± 1.01 %ID/cm(3), after: 1.36 ± 0.39 %ID/cm(3)) was found to be the next highest after (18)F-FDG (before: 2.45 ± 0.93 %ID/cm(3), after: 2.18 ± 0.65 %ID/cm(3)) in PAC120 tumors before castration and is better suited for monitoring therapy response. CONCLUSION: This comprehensive study in 2 hormone-dependent and 2 hormone-independent prostate tumor mouse models shows that (18)F-FLT and (18)F-FDG are the most appropriate tracers for delineation of PC-3, DU145 (except (18)F-FDG), and CWR22 tumors, but not for PAC120 tumors. (18)F-FEC and (11)C-choline, in particular, revealed insufficient T/M ratio in the prostate tumor models. The results may indicate that radiolabeled choline and choline derivatives compete with a high concentration of the precursor dimethylaminoethanol, resulting in reduced uptake in small-rodent tumor models, a hypothesis that is currently under investigation in our laboratory.

Radiosynthesis of a new PSMA targeting ligand ([18F]FPy-DUPA-Pep).

Due to the specificity of expression of PSMA (prostate specific membrane antigen) particularly in prostate cancer cells (e.g. LNCaP), numerous PSMA ligands have been synthesized until now. In the current study, we synthesized DUPA-Pep having 2-[3-(1,3-dicarboxypropyl)ureido]pentanedioic acid (DUPA) linked via 8-aminooctanoic acid to two phenylalanine residues and chose 6-[(18)F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester [(18)F]FPy-TFP as a prosthetic group for coupling. [(18)F]FPy-DUPA-Pep was obtained in a radiochemical yield of 48±0.9% (decay uncorrected) within 50 min with a chemical purity of >98%.

Central serotonin transporter levels are associated with stress hormone response and anxiety.

RATIONALE: Negative mood states are characterized by both stress hormone dysregulation and serotonergic dysfunction, reflected by altered thalamic serotonin transporter (5-HTT) levels. However, so far, no study examined the individual association between cortisol response and cerebral in vivo 5-HTT levels in patients suffering from negative mood states. OBJECTIVE: The objective of this cross-sectional study was to assess the interrelation of cortisol response, thalamic 5-HTT levels, and anxiety in healthy subjects and two previously published samples of patients with unipolar major depression (UMD) and obsessive-compulsive disorder (OCD), controlling for age, gender, 5-HTT genotype, smoking, and seasonality. METHODS: Regional 5-HTT levels and cortisol response to dexamethasone-corticotropin (Dex-CRH) challenge were assessed in consecutive samples of medication-free patients suffering from UMD (N = 10) and OCD (N = 10), and 20 healthy volunteers. The intervention used was combined Dex-CRH test and [(11)C]DASB positron emission tomography. The main outcome measures were: 5-HTT binding potential (BP(ND)) in a predefined thalamic ROI, cortisol response defined as the maximum cortisol increase in the combined Dex-CRH-test, and state of anxiety from the state-trait-anxiety inventory. RESULTS: Reduced thalamic 5-HTT BP(ND) was associated with increased cortisol response (r = -0.35, p < 0.05; in patients: r = -0.53, p < 0.01) and with increased state anxiety (r = -0.46, p < 0.01), surviving correction for age, gender, 5-HTT genotype, smoking, and seasonality (p < 0.05). The 5-HTT genotype, on the contrary, was not significantly associated with cortisol response (p = 0.19) or negative mood (p = 0.23). CONCLUSION: The association between stress hormone response, thalamic 5-HTT levels, and anxiety in patients suffering from negative mood states suggests an interaction between two major mechanisms implicated in negative mood states in humans.

PET imaging of prostate cancer xenografts with a highly specific antibody against the prostate-specific membrane antigen.

UNLABELLED: Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, is highly expressed by virtually all prostate cancers and is currently the focus of several diagnostic and therapeutic strategies. We have previously reported on the generation of several monoclonal antibodies (mAb) and antibody fragments that recognize and bind with high affinity to the extracellular domain of cell-adherent PSMA. This article reports the in vivo behavior and tumor uptake of the radiolabeled anti-PSMA mAb 3/A12 and its potential as a tracer for PET. METHODS: The mAb 3/A12 was conjugated with the chelating agent 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) and radiolabeled with (64)Cu. Severe combined immunodeficient mice bearing PSMA-positive C4-2 prostate carcinoma xenografts were used for small-animal PET imaging. Mice with PSMA-negative DU 145 tumors served as controls. For PET studies, each animal received 20-30 microg of radiolabeled mAb corresponding to an activity of 7.6-11.5 MBq. Imaging was performed 3, 24, and 48 h after injection. After the last scan, the mice were sacrificed and tracer in vivo biodistribution was measured by gamma-counting. RESULTS: Binding of the mAb 3/A12 on PSMA-expressing C4-2 cells was only minimally influenced by DOTA conjugation. The labeling efficiency using (64)Cu and DOTA-3/A12 was 95.3% +/- 0.3%. The specific activity after (64)Cu labeling was between 327 and 567 MBq/mg. After tracer injection, static small-animal PET images of mice with PSMA-positive tumors revealed a tumor-to-background ratio of 3.3 +/- 1.3 at 3 h, 7.8 +/- 1.4 at 24 h, and 9.6 +/- 2.7 at 48 h. In contrast, no significant tracer uptake occurred in the PSMA-negative DU 145 tumors. These results were confirmed by direct counting of tissues after the final imaging. CONCLUSION: Because of the high and specific uptake of (64)Cu-labeled mAb 3/A12 in PSMA-positive tumors, this ligand represents an excellent candidate for prostate cancer imaging and potentially for radioimmunotherapy.

Biodistribution and imaging of 1-(2-deoxy-beta-d-ribofuranosyl)-2,4-difluoro-5-[123/125I]iodobenzene (dRF[(123/125)I]IB), a nonpolar thymidine-mimetic nucleoside, in rats and tumor-bearing mice.

1-(2-Deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (dRFIB) is a putative bioisostere of iododeoxyuridine (IUdR). The advantages of dRFIB over IUdR for in vivo studies include resistance to both phosphorolytic cleavage of the nucleoside bond and de-iodination. dRFIB was radioiodinated (dRF(123/125)IB) by copper-catalyzed exchange using commercial sodium [(123/125)I]iodide. The in vivo biodistribution of dRF[(125)I]IB in BALBc mice and imaging of dRF[(123)I]IB in Sprague-Dawley rats are reported. In vivo data for rats show rapid clearance of radioactivity from blood (>95%ID in 15 minutes), extensive excretion in urine (56%ID/24 hours), concentration in the hepatobiliary-small intestine system and very little fecal excretion (approximately 3%ID/24 hours). Pharmacokinetic data for dRF[(125)I]IB (i.v. 48.7 ug/kg) in rats (t(1/2)[h] = 0.51 +/- 0.14, AUC(inf)[microg.min/mL] = 3.7 +/- 0.4, Cl[L/kg/h] = 0.75 +/- 0.12, Vss[L/kg] = 0.96 +/- 0.18) confirm previously reported dose-dependent pharmacokinetics. Scintigraphic images of rats dosed with dRF[(123)I]I were compatible with rapid soft-tissue clearance and extensive accumulation of radioactivity in bladder/urine and liver/small intestine. In tumor-bearing mice, thyroid and stomach radioactivity was indicative of moderate deiodination. An unidentified polar radioactive metabolite was detected in serum.

Radioiodination of 1-(2-deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (dRFIB), a putative thymidine mimic nucleoside for cell proliferation studies.

Iodine-124 was produced via the (124)Te(p,n)(124)I reaction by 15 MeV proton irradiation of an in-house solid mass tellurium dioxide target, using the Tübingen PETtrace (General Electric Medical Systems) cyclotron. 1-(2-Deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (dRFIB), a stable, non-polar thymidine mimic nucleoside, was synthesized in 5 steps following a literature method, for radioiodination with [(124)I] iodide via isotope exchange in the presence of copper sulphate and ammonium sulphate in methanol-water. The radiolabelling procedure was optimized with respect to temperature, amount of dRFIB, amount of sodium hydroxide and reaction time, to produce radiochemical yields of up to 85% with a 1-h reaction at 140 degrees C. With routine I-124 production of 30 MBq/run, relatively high specific activities, approaching 100 MBq/mmol, can be expected. The activation energy for dRFIB radioiodination was calculated from temperature-time RCY data to be approximately 100 kJ/mol using no-carrier-added [(124)I]iodide.

Simultaneous PET-MRI: a new approach for functional and morphological imaging.

Noninvasive imaging at the molecular level is an emerging field in biomedical research. This paper introduces a new technology synergizing two leading imaging methodologies: positron emission tomography (PET) and magnetic resonance imaging (MRI). Although the value of PET lies in its high-sensitivity tracking of biomarkers in vivo, it lacks resolving morphology. MRI has lower sensitivity, but produces high soft-tissue contrast and provides spectroscopic information and functional MRI (fMRI). We have developed a three-dimensional animal PET scanner that is built into a 7-T MRI. Our evaluations show that both modalities preserve their functionality, even when operated isochronously. With this combined imaging system, we simultaneously acquired functional and morphological PET-MRI data from living mice. PET-MRI provides a powerful tool for studying biology and pathology in preclinical research and has great potential for clinical applications. Combining fMRI and spectroscopy with PET paves the way for a new perspective in molecular imaging.

The specificity and sensitivity of transcranial ultrasound in the differential diagnosis of Parkinson's disease: a prospective blinded study.

BACKGROUND: Increased echogenicity of the substantia nigra (SN), as determined by transcranial sonography (TCS), is characteristic of idiopathic Parkinson's disease (iPD). The results of initial retrospective studies indicate that this ultrasound sign is specific for iPD and can help to differentiate it from atypical parkinsonian syndromes (aPS); however, these early studies were done in patients with later disease stages and known clinical diagnosis. We aimed to determine the diagnostic value of TCS in the early stages of parkinsonian syndromes, when the clinical symptoms often do not enable a definite diagnosis to be made. METHODS: 60 patients who presented with the first, but still unclear, clinical symptoms of parkinsonism had TCS in this prospective blinded study. Investigators were blinded to the results of the clinical investigations, the ultrasound findings, and the diagnosis at time of investigation. The patients were followed-up every 3 months for 1 year to assess and re-evaluate the clinical symptoms. The patients in whom a clinical diagnosis could not be made with certainty were investigated with raclopride PET or dopamine transporter single-photon emission computed tomography (SPECT), or both. FINDINGS: A clinical diagnosis of parkinsonism could not be established at baseline in 38 patients. At 12 months, 39 patients were clinically categorised as having iPD. Compared with endpoint diagnosis, the sensitivity of TCS at baseline was 90%7% and the specificity was 82.4%; the positive predictive value of TCS for iPD was 92.9% and the classification accuracy was 88.3%. INTERPRETATION: TCS is an easy to implement, non-invasive, and inexpensive technique that could help in the early differential diagnosis of parkinsonian syndromes. The routine use of TCS in the clinic could enable disease-specific therapy to be started earlier. FUNDING: Michael J Fox Foundation for Parkinson's Research.

Intratumoral spatial distribution of hypoxia and angiogenesis assessed by 18F-FAZA and 125I-Gluco-RGD autoradiography.

UNLABELLED: The hypoxia-inducible factor-1 alpha (HIF-1 alpha) activates angiogenesis in response to cellular hypoxia, suggesting a spatial correlation between angiogenesis and tissue hypoxia. METHODS: Using digital autoradiography of coinjected 18F-labeled azomycin arabinoside (8F-FAZA) (assessing regional hypoxia) and a glycosylated RGD-containing peptide (125I-3-iodo-dTyr(4)-cyclo(-Arg-Gly-Asp-dTyr-Lys(SAA)-), or 125I-Gluco-RGD) (assessing angiogenesis via binding to alpha v beta 3 integrin receptors on endothelial cells) performed on 22 EMT6 tumor xenografts, we investigated the intratumoral spatial distribution of these tracers. We applied a Bayesian bivariate image analysis using the mean tumor-to-muscle ratio as a discriminator, resulting in 4 groups: FAZA high/RGD high (Q1), FAZA low/RGD high (Q2), FAZA low/RGD low (Q3), and FAZA high/RGD low (Q4). In an additional 18 xenografts, the immunohistochemically derived HIF-1 alpha protein distribution was compared with 18F-FAZA autoradiography. Animals were divided into groups breathing either room air or carbogen (95% oxygen, 5% CO2) for 4 h until sacrifice. RESULTS: Under room air conditions, roughly 60% of the tumor surface displayed a spatial coupling of 18F-FAZA and 125I-Gluco-RGD uptake: either high (Q1) or low (Q3) uptake for both tracers, with Q1 indicating spatial association of hypoxia and angiogenesis and Q3 indicating adequate oxygenation without active angiogenesis. However, the remaining approximately 40% of the tumor surface showed discordant 18F-FAZA and 125I-Gluco-RGD uptake, indicating that hypoxia and angiogenesis are not necessarily spatially linked to each other and highlighting substantial intratumoral heterogeneity of the 18F-FAZA and 125I-Gluco-RGD uptake. Although carbogen breathing conditions significantly decreased the mean 18F-FAZA tumor-to-muscle ratio, no significant changes were observed for 125I-Gluco-RGD, indicating that an acute increase in tumor oxygenation did not influence alpha v beta 3 integrin receptor expression. The HIF-1 alpha-positive (HIFpos) tumor cell fraction was not significantly influenced by breathing conditions and covered between 0% and 35% of the total tumor section surface. However, the HIFpos tumor section surface was much smaller than the tumor section surface of increased 18F-FAZA uptake, suggesting that both markers are identifying distinctly different biologic processes associated with hypoxia. CONCLUSION: The study revealed a substantial spatial discordance of the 18F-FAZA and 125I-Gluco-RGD tumor distribution suggesting that hypoxia and angiogenesis are not necessarily spatially linked in malignancies. These results may prove essential in developing advanced targeted systemic chemotherapeutic approaches (such as combinations of hypoxia-activated cytotoxins and antiangiogenic drugs) for hypoxic tumors.

[11C]PIB binding in Parkinson's disease dementia.

[(11)C]PIB ((11)C-6-OH benzothiazole) reflects the regional distribution of amyloid (beta-sheeted proteins) in patients with Alzheimer's disease (AD). Proteinaceous inclusions in Parkinson's disease with dementia (PDD), so-called Lewy bodies, also consist of fibrillar, misfolded proteins, chiefly alpha-synuclein. To test whether PDD subjects show specific amyloid binding in vivo and whether this could reflect fibrillar alpha-synuclein accumulation, we investigated 10 PDD subjects with [(11)C]PIB-PET. Radioligand binding was compared to that in 11 control and 6 AD subjects. Furthermore, postmortem sections of 4 patients with Parkinson's disease (PD), therefrom 2 with dementia (PDD), and of 6 controls were stained with PIB to evaluate the histological distribution of the fluorescent ligand in the brainstem. In PET, only 2 PDD patients displayed increased PIB binding to cortical amyloid comparable to AD patients. The other 8 patients showed control-like cortical findings but elevated PIB binding in the pons and mesencephalon. Fluorescence microscopy showed PIB binding to Lewy bodies and neuromelanin in the substantia nigra of PD and PDD brainstem sections, but not in controls. These data suggest that PIB-PET can be used to further differentiate PDD with respect to cortical amyloid. Furthermore, we provide evidence that--in addition to nonspecific binding--PIB uptake in the brainstem may also reflect PDD related amyloid.

Decarbonylation of multi-substituted [18F]benzaldehydes for modelling syntheses of 18F-labelled aromatic amino acids.

For 18F-labelling of aromatic amino acids such as tyrosine and DOPA simple and efficient procedures have been under development for quite a while. The direct introduction of 18F using [18F]fluoride can principally be realized by nucleophilic aromatic substitution (SNAr). However, this requires the presence of an appropriate leaving group and an auxiliary substituent, which is able to reduce the electron density of the benzene ring. Furthermore, this auxiliary substituent should be removable easily after the introduction of 18F. The electron-withdrawing formyl substituent meets both requirements. It facilitates nucleophilic attack in the ortho and/or para position and is easily removed in a decarbonylation reaction mediated by Wilkinson's catalyst (RhCl(PPh3)3). In order to evaluate the reaction conditions for a possible synthesis of 2-[18F]fluoro-5-hydroxyphenylalanine ([18F]-m-tyrosine), 2-[18F]fluoro-4-hydroxyphenylalanine ([18F]-p-tyrosine) or 2-[18F]fluoro-4,5-hydroxyphenylalanine ([18F]FDOPA), the dependence of the decarbonylation reaction on solvent, temperature, reaction time and catalyst concentration was studied using appropriate model compounds. Optimum yields of 81%, 89% and 88% could be achieved using benzonitrile as solvent and 2M equivalents of RhCl(PPh3)3 (based on labelling precursor) at 150 degrees C reaction temperature within 20 min reaction time for compounds modelling [18F]-m-tyrosine, [18F]-p-tyrosine and [18F]FDOPA, respectively.

Synthetic approaches and bio-distribution studies of [11C]methyl-phenidate.

PURPOSE: The purpose of this study was a) to present a facilitated method for the preparation and workup of [11C]d-threo-methylphenidate ([11C]d-threo-MP) (a ligand that was shown to bind selectively to the presynaptic dopaminergic transporters) from [11C]methyliodide ([11C]CH3I), b) to demonstrate that the ligand can as well be produced by an alternative labeling method employing [11C]diazomethane as the labeling agent and c) to present biodistribution data for this tracer obtained in rats. METHODS: 11C-labeling with [11C]CH3I was performed using either [d-threo-1-(2-nitrophenylsulfanyl)piperidin-2-yl]phenyl-acetic acid (d-threo-N-NPS-ritalinic acid) under addition of sodium hydroxide as base or the previously prepared sodium salt of d-threo-N-NPS-ritalinic acid. The two approaches were compared with regard to radiochemical yield and purification procedures needed in order to obtain a sufficiently pure tracer solution for human use. For the alternative reaction pathway using [11C]diazo-methane as the labeling agent the reaction was performed with d-threo-N-NPS-ritalinic acid. The biodistribution of [11C]d-threo-MP was determined in rats at 5, 10 and 30 min post injection of the tracer. RESULTS: The application of the sodium salt of d-threo-N-NPS-ritalinic acid as precursor resulted in higher radiochemical yields than the use of the free acid under basic conditions, the yields were 20 +/- 8% and 6 +/- 3%, respectively for the final isolated product (based on [11C]CH3I starting activity). The alternative labeling approach by means of [11C]diazomethane as the labeling agent was demonstrated to give radiochemical yields of 76 +/- 8% (based on [11C]diazomethane starting activity, determined by HPLC analysis of the crude reaction mixture before final work-up) within shorter process times. Based on [11C]methane starting activity both approaches result in similar yields (17% and 15%, respectively) Biodistribution studies in rats revealed a low blood activity (0.09% injected dose/g (% ID/g)) at 5 min post injection (p.i.), as well as a relatively high liver uptake (15.9% ID at 30 min) compared to a lower kidney uptake (3.2% ID at 30 min). Brain uptake was 0.9% ID/g already 5 and 10 min p.i.. CONCLUSIONS: The application of the sodium salt of d-threo-N-NPS-ritalinic acid as precursor for the radiosynthesis of [11C]d-threo-MP reduces the amount of [11C]methanol formed from the reaction of [11C]CH3I with sodium hydroxide, that is added to generate the carboxylic anion of d-threo-N-NPS-ritalinic acid needed for labeling with [11C]CH3I. The purification process could be simplified (omission of one solid phase extraction step), resulting in an easily automated process for the production of the tracer. The preparation of [11C]d-threo-MP by means of [11C]diazomethane as the labeling agent appears to be an interesting alternative to the [11C]CH3I methods because of shorter overall process times and high labeling yields. Biodistribution data show a rapid extraction of the tracer from the blood pool. Tracer excretion seems to take place predominantly via the hepatic pathway since liver uptake at 30 min was considerably higher than kidney uptake. [11C]d-threo-MP exhibits a rapid and sufficiently high brain uptake in rats.