[PET-CT and PET-MRI of the prostate : From 18F-FDG to 68Ga-PSMA]. / PET-CT/-MRT der Prostata : Von 18F-FDG zu 68Ga-PSMA.

CLINICAL/METHODICAL ISSUE: In the last few years nuclear medical diagnostics have experienced a unprecedented renaissance in the diagnostics of prostate cancer, due to the availability of hybrid imaging with positron emission tomography computed tomography (PET/CT), PET magnetic resonance imaging (PET/MRI) and single photon emission computed tomography (SPECT) CT as well as the development of prostate-specific radiopharmaceuticals. METHODICAL INNOVATIONS: The use of fluorodeoxyglucose (FDG), which has been successfully implemented for many years in PET diagnostics, is only helpful in dedifferentiated tumors due to the biological characteristics of prostate cancer. New specific radiopharmaceuticals, such as choline-derivatives, which are incorporated into the prostate cancer cell and built into the cell membrane as well as the recently developed highly specific ligands for prostate-specific membrane antigen (PSMA) are revolutionizing prostate cancer imaging and (re-) staging. PRACTICAL RECOMMENDATIONS: The 68 Ga-labeled PSMA ligands for PET-CT and PET-MRI are highly specific tracers for primary diagnostics and detection of metastases of prostate carcinoma. In risk patients, which includes patients with intermediate and high-risk tumors, they have largely replaced choline-based PET-CT, especially in the case of very low PSA values <0.5 ng/ml in the diagnostics of recurrence. The use in the primary diagnostics as PET-MRI, also in combination with multiparametric MRI (mpMRI), is promising with respect to early diagnostics and image fusion-assisted biopsy as well as surgery and irradiation planning.

Radiolabelled RGD peptides for imaging and therapy.

Imaging of angiogenesis has become increasingly important with the rising use of targeted antiangiogenic therapies like bevacizumab (Avastin). Non-invasive assessment of angiogenic activity is in this respect interesting, e.g. for response assessment of such targeted antiangiogenic therapies. One promising approach of angiogenesis imaging is imaging of specific molecular markers of the angiogenic cascade like the integrin α(v)ß(3). For molecular imaging of integrin expression, the use of radiolabelled peptides is still the only approach that has been successfully translated into the clinic. In this review we will summarize the current data on imaging of α(v)ß(3) expression using radiolabelled RGD peptides with a focus on tracers already in clinical use. A perspective will be presented on the future clinical use of radiolabelled RGD peptides including an outlook on potential applications for radionuclide therapy.

Comparison of 3'-deoxy-3'-[¹8F]fluorothymidine positron emission tomography (FLT PET) and FDG PET/CT for the detection and characterization of pancreatic tumours.

PURPOSE: Despite recent advances in clinical imaging modalities, differentiation of pancreatic masses remains difficult. Here, we tested the diagnostic accuracy of molecular-based imaging including 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) positron emission tomography (PET) and [(18)F]fluorodeoxyglucose (FDG) PET/CT in patients with suspected pancreatic masses scheduled to undergo surgery. METHODS: A total of 46 patients with pancreatic tumours suspicious for malignancy and scheduled for resective surgery were recruited prospectively. In 41 patients, FLT PET and FDG PET/CT scans were performed. A diagnostic CT performed on a routine basis was available in 31 patients. FLT PET and FDG PET/CT emission images were acquired according to standard protocols. Tracer uptake in the tumour [FDG and FLT standardized uptake value (SUV)] was quantified by the region of interest (ROI) technique. For FDG PET/CT analysis, correct ROI placement was ensured via side-by-side reading of corresponding CT images. RESULTS: Of 41 patients, 33 had malignancy, whereas 8 patients had benign disease. Visual analysis of FDG and FLT PET resulted in sensitivity values of 91% (30/33) and 70% (23/33), respectively. Corresponding specificities were 50% (4/8) for FDG PET and 75% (6/8) for FLT PET. In the subgroup of patients with contrast-enhanced CT (n = 31), sensitivities were 96% (PET/CT), 88% (CT alone), 92% (FDG PET) and 72% (FLT PET), respectively. Mean FLT uptake in all malignant tumours was 3.0 (range SUV(max) 1.1-6.5; mean FDG SUV(max) 7.9, range 3.3-17.8; p < 0.001). CONCLUSION: For differentiation of pancreatic tumours, FDG PET and FDG PET/CT showed a higher sensitivity but lower specificity than FLT PET. Interestingly, visual analysis of FLT PET led to two false-positive findings by misinterpreting physiological bowel uptake as pathological FLT uptake in the pancreas. Due to the limited number of patients, the clinical value of adding FLT PET to the diagnostic workup of pancreatic tumours remains to be determined.

Evaluation of the novel myocardial perfusion positron-emission tomography tracer 18F-BMS-747158-02: comparison to 13N-ammonia and validation with microspheres in a pig model.

BACKGROUND: Positron-emission tomography (PET) tracers for myocardial perfusion are commonly labeled with short-lived isotopes that limit their widespread clinical use. 18F-BMS-747158-02 (18F-BMS) is a novel pyridaben derivative that was evaluated for assessment of myocardial perfusion by comparison with 13N-ammonia (13NH3) and with radioactive microspheres in a pig model. METHODS AND RESULTS: Fourteen pigs injected with 500 MBq of 13NH3 or 100 to 200 MBq of 18F-BMS underwent dynamic PET at rest and during pharmacological stress. In 8 of these pigs, 18F-BMS was injected during stress combined with transient, 2.5-minute constriction of the left anterior descending coronary artery. Radioactive microspheres were coinjected with 18F-BMS. Ratios of myocardial tracer uptake to surrounding tissues were determined, and myocardial blood flow was quantified by compartmental modeling. Both tracers showed high and homogeneous myocardial uptake. Compared with 13NH3, 18F-BMS showed higher activity ratios between myocardium and blood (rest 2.5 versus 4.1; stress 2.1 versus 5.8), liver (rest 1.2 versus 1.8; stress 0.7 versus 2.0), and lungs (rest 2.5 versus 4.2; stress 2.9 versus 6.4). Regional myocardial blood flow assessed with 18F-BMS PET showed good correlation (r=0.88, slope=0.84) and agreement (mean difference -0.10 [25th percentile -0.3, 75th percentile 0.1 mL x min(-1) x g(-1)]) with that measured with radioactive microspheres over a flow range from 0.1 to 3.0 mL x min(-1) x g(-1). The extent of defects induced by left anterior descending coronary artery constriction measured by 18F-BMS and microspheres also correlated closely (r=0.63, slope=1.1). CONCLUSIONS: 18F-BMS-747158-02 is a very attractive new PET perfusion tracer that allows quantitative assessment of regional myocardial perfusion over a wide flow range. The long half-life of 18F renders this tracer useful for clinical PET/CT applications in the workup of patients with suspected or proven coronary artery disease.

Synthesis, biological evaluation and radiolabelling by 18F-fluoroarylation of a dopamine D3-selective ligand as prospective imaging probe for PET.

Radical (18)F-fluoroarylation with fluorine-18-labelled arenediazonium chlorides has been successfully applied to the radiochemical synthesis of the dopamine D(3)-selective ligand SH 317 ([(18)F]8). SH 317 has been evaluated as a new PET ligand candidate by in vivo experiments.

[99cmTc]Tc-PSMA-I&S-SPECT/CT: experience in prostate cancer imaging in an outpatient center.

BACKGROUND: Prostate-specific membrane antigen (PSMA) SPECT imaging in prostate cancer (PCa) could be a valuable alternative in regions where access to PSMA-PET imaging is restricted. [99mTc]Tc-PSMA-I&S is a new 99mTc-labeled PSMA-targeting SPECT agent, initially developed for radio-guided surgery. We report on the diagnostic use of [99mTc]Tc-PSMA-I&S-SPECT/CT in PCa. RESULTS: [99mTc]Tc-PSMA-I&S-SPECT/CT was performed and evaluated in 210 outpatients with PCa at a single center. Patients were imaged for biochemical recurrence (BCR, n = 152, mean PSA 8.7 ng/ml), for primary staging of high-risk PCa (n = 12, mean PSA 393 ng/ml), and restaging in advanced recurrent PCa (n = 46, mean PSA 101.3 ng/ml). Number and location of positive lesions were determined for the different subgroups. For BCR, detection rates were calculated, defined as the proportion of scans with at least one PSMA-positive lesion. PSMA positive lesions were detected in 65.2% of all 210 patients. Tumor tissue was mainly detected in lymph nodes (59%), in the bone (42%), and in the prostate (fossa) (28%). In the subgroup of patients referred for detection of BCR the detection rate increased from 20% at a PSA level < 1 ng/ml to 82.9% and 100% at PSA levels > 4 ng/ml and > 10 ng/ml, respectively. In the subgroup of high-risk patients referred for primary staging, 42% demonstrated metastatic disease. Restaging of advanced recurrent PCa revealed detectability of PSMA positive tumor lesions in 85% of the scans. CONCLUSIONS: [99mTc]Tc-PSMA-I&S-SPECT/CT was useful in PSMA-targeted imaging of PCa at various clinical stages. At low PSA levels (< 4 ng/ml), detection rates of [99mTc]Tc-PSMA-I&S-SPECT/CT in BCR are clearly inferior to data reported for PET-imaging and should thus only be considered for lesion detection if imaging with PET is unavailable. However, at higher PSA levels (> 4 ng/ml) [99mTc]Tc-PSMA-I&S-SPECT/CT provides high detection rates in BCR. [99mTc]Tc-PSMA-I&S-SPECT/CT can also be used for primary staging and for restaging of advanced recurrent PCa. However, further studies are needed to assess the clinical value in these indications.

Introduction of functional groups into peptides via N-alkylation.

An optimized protocol for the mild and selective Fukuyama-Mitsunobu reaction was used for mono- and di- N-alkylation on solid support. Thereby, nonfunctionalized aliphatic and aromatic residues are quickly introduced into transiently protected, primary amines of a linear peptide. N-Alkylation can also be used to implement alkyl chains carrying (protected) functionalities suited for subsequent modification. Applicability of this method is demonstrated by various N-alkylated analogues of a cyclic CXCR4 receptor antagonist originally developed by Fujii et. al.

Peptides, multimers and polymers.

Due to their favorable properties and pharmacokinetics, peptides are often regarded as "agents of choice" for imaging and radiotherapy. Chemical strategies have been developed that allow their site specific labeling with various radionuclides for PET and SPECT, without compromising their biological integrity. Together with the overexpression of a wide range of peptide receptors and binding sites on tumor cells or matrix components, this class of compounds offers multiple imaging applications. Furthermore, radiolabeled peptides have great potential as carrier molecules for site-specific delivery of other signalling units, such as fluorescent moieties, cyctotoxic compounds or metals for magnetic resonance imaging. In addition, great efforts have been made to exploit the favorable characteristics of peptides for the development of larger constructs, such as multimeric ligands, polymer-peptide conjugates and "peptide-coated" liposomes and nanoparticles. Some peptides have already entered clinical routine application; some are currently being evaluated in clinical studies. However, a variety of peptides is still "waiting" to enter the imaging arena. This chapter presents a brief overview of the highly active field of peptide radiopharmaceuticals and the future potential of multimeric and polymeric peptide constructs.

From interventionist imaging to intraoperative guidance: New perspectives by combining advanced tools and navigation with radio-guided surgery. / De la imagen intervencionista a la guía intraoperatoria: nuevas perspectivas combinando herramientas avanzadas y navegación con la cirugía radioguiada.

The integration of medical imaging technologies into diagnostic and therapeutic approaches can provide a preoperative insight into both anatomical (e.g. using computed tomography, magnetic resonance imaging, or ultrasound), as well as functional aspects (e.g. using single photon emission computed tomography, positron emission tomography, lymphoscintigraphy, or optical imaging). Moreover, some imaging modalities are also used in an interventional setting (e.g. computed tomography, ultrasound, gamma or optical imaging) where they provide the surgeon with real-time information during the procedure. Various tools and approaches for image-guided navigation in cancer surgery are becoming feasible today. With the development of new tracers and portable imaging devices, these advances will reinforce the role of interventional molecular imaging.

Fluorine-18 labeling of peptides and proteins.

The pool of promising peptides worthy of investigation and evaluation for clinical use is continuously filled from different sources. Driven by the promising results obtained with peptides addressing somatostatin-2 receptor positive (sst2+) neuroendocrine tumours, other peptides targeting further receptor systems are being studied and evaluated. Progress in profiling the density and incidence of peptide hormone receptors in human cancer has initiated and will further promote research on the corresponding peptidic binders. In addition, industrial pharmaceutical research will be another significant source of peptides in the future. A recent prognosis revealed that about 50% of the drugs entering clinical trials in the next years will be peptides. The extensive research activities in genomics and proteomics will point out and quantify new and already known target structures upregulated in specific diseases. Based on the knowledge of their endogenous ligands or via selection of suitable candidates by phage display, suitable peptide ligands for e.g. membrane associated receptors can be identified and thus allow targeting of such binding sites. Thus, bioactive peptides specifically addressing relevant molecular targets are expected to become an important class of tracers, also due to the possibility of bridging imaging with therapeutic approaches. In this brief overview a summary of methods and strategies for the 18F-labeling of peptides and proteins is given.

Gender dependent rate of metabolism of the opioid receptor-PET ligand [18F]fluoroethyldiprenorphine.

AIM: The morphinane-derivate 6-O-(2-[(18)F]fluoroethyl)-6-O-desmethyldiprenorphine ([(18)F]FDPN) is a nonselective opioid receptor ligand currently used in positron emission tomography (PET). Correction for plasma metabolites of the arterial input function is necessary for quantitative measurements of [(18)F]FDPN binding. A study was undertaken to investigate if there are gender dependent differences in the rate of metabolism of [(18)F]FDPN. METHODS: The rate of metabolism of [(18)F]FDPN was mathematically quantified by fitting a bi-exponential function to each individual's dynamic metabolite data. RESULTS: No statistically significant gender differences were found for age, weight, body mass index or dose. However, significant differences (p < 0.01) in two of the four kinetic parameters describing the rate of metabolism were found between the two groups, with women metabolizing [(18)F]FDPN faster than men. These differences were found in the contribution of the fast and slow kinetic components of the model describing the distribution of radioactive species in plasma, indicating a higher rate of enzyme-dependent degradation of [(18)F]FDPN in women than in men. CONCLUSION: The findings reinforce the need for individualized metabolite correction during [(18)F]FDPN-PET scans and also indicate that in certain cases, grouping according to gender could be performed in order to minimize methodological errors of the input function prior to kinetic analyses.

Noninvasive imaging of alpha(v)beta3 integrin expression using 18F-labeled RGD-containing glycopeptide and positron emission tomography.

The alpha(v)beta3 integrin is an important cell adhesion receptor involved in tumor-induced angiogenesis and tumor metastasis. Here we describe the 18F-labeling of the RGD-containing glycopeptide cyclo(-Arg-Gly-Asp-D-Phe-Lys(sugar amino acid)-) with 4-nitrophenyl 2-[18F]fluoropropionate and the evaluation of this compound in vitro and in tumor mouse models. Binding assays with isolated immobilized alpha(v)beta3, alpha(v)beta5, and alpha(IIb)beta3 as well as in vivo studies using alpha(v)beta3-positive and -negative murine and xenotransplanted human tumors demonstrated receptor-specific binding of the radiolabeled glycopeptide yielding high tumor:background ratios (e.g., 120 min postinjection: tumor:blood, 27.5; tumor:muscle, 10.2). First imaging results using a small animal positron emission tomograph suggest that this compound is suitable for noninvasive determination of the alpha(v)beta3 integrin status and therapy monitoring.

Investigation of transport mechanism and uptake kinetics of O-(2-[18F]fluoroethyl)-L-tyrosine in vitro and in vivo.

UNLABELLED: The aim of the study was to investigate the transport mechanism and uptake kinetics of the new 18F-labeled amino acid O-(2-[18F]fluoroethyl)-L-tyrosine (L-[18F]FET) and D-[18F]FET in human SW 707 colon carcinoma cells and the in vivo biodistribution of this tracer in SW 707 tumor-bearing mice. METHODS: SW 707 cells were incubated with L- and D-[18F]FET under physiologic amino acid concentrations with and without the competitive transport inhibitors 2-amino-2 norbornane-carboxylic acid and a-(methylamino)isobutyric acid plus serine. For the investigation of the transport capacity, unlabeled L-FET was added to the samples. In addition, xenotransplanted mice were injected intravenously with L-[18F]FET; killed 10, 30, 60 and 120 min after injection; and the radioactivity concentration in different organs was measured in a gamma counter. RESULTS: The in vitro kinetic experiments showed a fast initial uptake of L-[18F]FET into the cells up to 6 min, followed by a nearly constant tracer concentration. The accumulation factor, calculated as the ratio between intracellular and extracellular tracer concentration, ranged from 3.0 to 5.0. In comparison, D-[18F]FET did not accumulate in the cells. Washing the cells in medium at 37 degrees C, after a 30-min incubation with L-[F-18]FET, led to a rapid decrease of radioactivity, which demonstrates the bidirectional transport. In addition, experiments with increasing concentrations of unlabeled L-FET indicated a linear correlation between L-FET uptake rate and the extracellular concentration. Results of transport inhibition experiments with the specific competitive inhibitors demonstrated that the uptake of L-FET into SW 707 cells was caused mainly (>80%) by the transport system L. In the in vivo studies, the half-life (t1/2 beta) of L-[18F]FET in the plasma was determined to be 94 min and the uptake into the brain increased to 120 min with a brain-to-blood ratio of 0.86. The xenotransplanted tumor showed higher uptake of L-[18F]FET (>6 %ID/g) at 30 and 60 min than all other organs, except the pancreas. The tumor-to-blood ratio reached about 2 between 30 and 120 min. CONCLUSION: L-[18F]FET, which is transported by the specific amino acid transport system L, seems to be a potential amino acid tracer for tumor imaging and therapy monitoring with PET.

Synthesis and radiopharmacology of O-(2-[18F]fluoroethyl)-L-tyrosine for tumor imaging.

UNLABELLED: The aim of the study was to develop a simple 18F-labeled amino acid as a PET tracer for cerebral and peripheral tumors. O-(2-[18F]fluoroethyl)-L-tyrosine (L-[18F]FET) was synthesized and biologically evaluated. Results of the first human PET study are reported. METHODS: No carrier added (n.c.a.) and D-[18F]FET were prepared by 18F-fluoroethylation of L- and D-tyrosine in a two-step procedure. Biodistribution studies were performed in mice. The metabolic fate of L-[18F]FET was investigated in plasma, brain, tumor and pancreatic tissue samples using chromatographic procedures. Tumor uptake studies were performed in mammary carcinoma-bearing mice and in mice with the colon carcinoma SW 707. In a human PET study, a 59-y-old man with a recurrent astrocytoma was imaged using n.c.a. L-[18F]FET. RESULTS: Synthesis of [18F]FET was accomplished in about 50 min with an overall radiochemical yield of 40%. The uptake of L-[18F]FET in the brain of mice reached a level >2% ID/g between 30 and 60 min postinjection. The brain uptake of the D-isomer was negligible, indicating blood-brain barrier penetration by a specific amino acid transport system. L-[18F]FET is not incorporated into proteins. High-performance liquid chromatography (HPLC) analysis of brain, pancreas and tumor homogenates as well as plasma samples of mice at 10, 40 or 60 min postinjection showed only unchanged L-[18F]FET. Activity uptake in the bone did not exceed 2% ID/g at 40 min postinjection. The brain uptake of L-[18F]FET in mice bearing mammary carcinomas and colon carcinomas reached 7.1%+/-1.2% ID/g and 6.4%+/-1.7% ID/g 1h postinjection, respectively. In the first human study, L-[18F]FET-PET allowed a clear delineation of a recurrent astrocytoma. Thirty-five minutes postinjection, the tumor-to-cortex ratio was >2.7. A tumor-to-blood ratio >1.5 was reached at 30 min postinjection and continued to increase. No significant activity accumulation was observed in peripheral organs after approximately 40 min postinjection. CONCLUSION: The high in vivo stability of L-[18F]FET, its fast brain and tumor uptake kinetics, its low accumulation in nontumor tissue and its ease of synthesis strongly support further evaluation of L-[18F]FET as an amino acid tracer for cerebral and peripheral tumors.

6-O-(2-[18F]fluoroethyl)-6-O-desmethyldiprenorphine ([18F]DPN): synthesis, biologic evaluation, and comparison with [11C]DPN in humans.

UNLABELLED: 6-O(2-[18F]fluoroethyl)-6- -desmethyldiprenorphine ([18F]DPN) was developed and biologically evaluated. Results of animal experiments, binding studies in vivo, and a human PET study are reported and compared with those of [11C]DPN. METHODS: [18F]DPN was obtained by 18F-fluoroethylation of 3-O-trityl-6-O-desmethyldiprenorphine and subsequent deprotection in good radiochemical yields (23% +/- 7%; 100 min; 37 TBq/mmol). Binding of [18F]DPN to mu, kappa, and delta opioid receptors was shown by autoradiography studies on rat brain slices. Quantification of cerebral opioid receptor binding in men was performed by spectral analysis of a dynamic PET scan (25 frames, 90 min) after intravenous application of 63 MBq [18F]DPN (36 GBq/micromol) and correction for metabolites. RESULTS: [18F]DPN shows high affinity to opioid receptors. Parametric images (impulse response function at 60 min) of this human study showed a binding pattern of [18F]DPN equal to that of a control group (n = 9 healthy volunteers) after administration of [11C]DPN. CONCLUSION: The advantage of the longer half-life of 18F will allow extended scanning periods, more flexible interventions (e.g., displacement studies), and DPN to be available to PET centers without an on-site cyclotron.

Glycosylated RGD-containing peptides: tracer for tumor targeting and angiogenesis imaging with improved biokinetics.

UNLABELLED: The alpha(v)beta3 integrin plays an important role in metastasis and tumor-induced angiogenesis. Targeting with radiolabeled ligands of the alpha(v)beta3 integrin may provide information about the receptor status and enable specific therapeutic planning. Previous studies from our group resulted in tracers that showed alpha(v)beta3-selective tumor uptake. However, these first-generation compounds predominantly revealed hepatobiliary excretion with high radioactivity found in the liver. In this report, the synthesis and biological evaluation of the first glycosylated RGD-containing peptide (RGD-peptide) for the noninvasive imaging of alpha(v)beta3 expression are described. METHODS: Peptides were assembled on a solid support using fluorenylmethoxycarbonyl-coupling protocols. The precursor cyclo(-Arg-Gly-Asp-D-Tyr-Lys(SAA)-) GP1 was synthesized by coupling 3-acetamido-2,6-anhydro-4,5,7-tri-O-benzyl-3-deoxy-beta-D-glycero-D-gulo-heptonic acid (SAA(Bn3)) with cyclo(-Arg(Mtr)-Gly-Asp(OtBu)-D-Tyr(tBu)-Lys-) and subsequent removal of the protection groups. Iodine labeling was performed by the Iodo-Gen method (radiochemical yield > 50%). The in vitro binding assays were performed using purified immobilized alpha(IIb)beta3, alpha(v)beta5, and alpha(v)beta3 integrins. For in vivo experiments, nude mice bearing xenotransplanted melanomas and mice with osteosarcomas were used. RESULTS: The glycosylated peptide 3-iodo-Tyr4-cyclo(-Arg-Gly-Asp-D-Tyr-Lys(SAA)-) GP2 showed high affinity and selectivity for alpha(v)beta3 in vitro (50% inhibitory concentration = 40 nmol/L). Pretreatment studies indicate specific binding of [125I]GP2 on alpha(v)beta3-expressing tumors in vivo. Comparison of the pharmacokinetics of [125I]GP2 and [125I]-3-iodo-Tyr4-cyclo(-Arg-Gly-Asp-D-Tyr-Val-) [125I]P2 revealed for [125I]GP2 an increased activity concentration in the blood (e.g., 3.59 +/- 0.35 percentage injected dose [%ID]/g vs. 1.72 +/- 0.44 %ID/g at 10 min postinjection) and a significantly reduced uptake in the liver (e.g., 2.59 +/- 0.24 %ID/g vs. 21.96 +/- 2.78 %ID/g at 10 min postinjection). Furthermore, a clearly increased activity accumulation in the tumor was found (e.g., 3.05 +/- 0.31 %ID/g vs. 0.92 +/- 0.16 %ID/g at 240 min postinjection), which remained almost constant between 60 and 240 min postinjection. This resulted in good tumor-to-organ ratios for the glycosylated tracer (e.g., 240-min postinjection osteosarcoma model: tumor-to-blood = 16; tumor-to-muscle = 7; tumor-to-liver = 2.5), which were confirmed by the first gamma-camera images of osteosarcoma-bearing mice at 240 min postinjection. CONCLUSION: This study demonstrates that the introduction of a sugar moiety improves the pharmakokinetic behavior of a hydrophobic peptide-based tracer. Additionally, this alpha(v)beta3-selective glycosylated radioiodinated second-generation tracer GP2 shows high tumor uptake and good tumor-to-organ ratios that allow noninvasive visualization of alpha(v)beta3-expressing tumors and monitoring therapy with alpha(v)beta3 antagonists. Finally, the favorable biokinetics make the glycosylated RGD-peptide a promising lead structure for tracers to quantify the alpha(v)beta3 expression using PET.

Radiolabeled alpha(v)beta3 integrin antagonists: a new class of tracers for tumor targeting.

UNLABELLED: The alpha(v)beta3 integrins play an important role during tumor metastasis and tumor-induced angiogenesis. Targeting of this receptor may provide information about the receptor status of the tumor and enable specific therapeutic planning. Cyclo(-Arg-Gly-Asp-D-Phe-Val-) has been shown to be a selective alpha(v)beta3 integrin antagonist with high affinity. In this study we describe the synthesis and biological evaluation of [125I]-3-iodo-D-Tyr4-cyclo(-Arg-Gly-Asp-D-Tyr-Val-) ([125I]P2), [125I]-3-iodo-Tyr5-cyclo(-Arg-Gly-Asp-D-Phe-Tyr-) ([125I]P4) and the negative control peptide [1251]-3-iodo-D-Tyr4-cyclo(-Arg-D-Ala-Asp-Tyr-Val-) ([125I]P6). METHODS: Peptides were assembled on a solid support using fluorenylmethoxycarbonyl amino acid coupling protocols. Radioiodination was performed using the iodogen method. The in vitro binding assays were performed using isolated, immobilized alphaIIbeta3 and alpha(v)beta3 integrins. Expression of the alphaVbeta3 receptor on the different tumors was validated by immunohistochemical methods using alpha(v) and alpha(v)beta3 specific antibodies. For biodistribution studies, nude mice with melanoma M21 or mammary carcinoma MaCaF and BALB/c mice with osteosarcoma were used. RESULTS: The in vitro binding assays demonstrate that the introduction of tyrosine and subsequent iodination have no influence on the high affinity and selectivity for alpha(v)beta3. Immunohistochemical staining clearly indicates the presence of the alpha(v)beta3 integrins on the tumor tissue of the melanoma and the osteosarcoma. Pretreatment and displacement studies show specific binding of [125I]P2 on melanoma M21-bearing nude mice and osteosarcoma-bearing BALB/c mice but less specific binding on mammary carcinomas. [125I]P2 exhibits fast elimination kinetics. The accumulation in the tumor 10 min postinjection is 2.07 +/- 0.32 %ID/g for the melanoma M21 and 3.50 +/- 0.49 %ID/g for the osteosarcoma and decreases to 1.30 +/- 0.13 %ID/g and 2.03 +/- 0.49 %ID/g 60 min postinjection, respectively. [125I]P4 shows even faster elimination kinetics, resulting in a tumor accumulation of 0.40 +/- 0.10 %ID/g 60 min postinjection for the osteosarcoma-bearing BALB/c mice. Both peptides reveal predominately hepatobiliary excretion. For [1251]P2, this also is confirmed by autoradiography. The negative control peptide [125I]P6 shows no specific activity accumulation. CONCLUSION: [125I]P2 exhibits high affinity and selectivity for the alpha(v)beta3 integrin in vitro and in vivo and, thus, represents the first radiolabeled alpha(v)beta3 antagonist for the investigation of angiogenesis and metastasis in vivo.

Opioid receptors in the human cerebellum: evidence from [11C]diprenorphine PET, mRNA expression and autoradiography.

Little is known regarding opioid receptors in the human cerebellum. The present [11C]diprenorphine PET study investigated opioid receptor binding in the human cerebellum in vivo, and showed a differential binding level in cerebellar cortex, vermis and dentate nuclei. The additional study in vitro of opioid receptors in human cerebellar cortex and rat brain corroborated the presence of opioidergic mechanisms in the human cerebellum in contrast to the rat. A differential cellular distribution pattern was detected for the three major opioid receptors investigated. For the mu-receptor, and at a lower level for the kappa-receptor, mRNA expression was mainly observed over granule cells. Binding sites were most prominent in the molecular layer. For the delta-receptor no signal was detected. The consideration of cerebellar opioidergic mechanisms and the distribution patterns of the various opioid receptors may promote the understanding of cerebellar function and of opioidergic pharmacology in the human.

Central pain after pontine infarction is associated with changes in opioid receptor binding: a PET study with 11C-diprenorphine.

Using 18F-fluorodeoxyglucose and 11C-diprenorphine positron emission tomography (PET), we investigated alterations in glucose metabolism and opioid receptor binding in a patient with central poststroke pain, which developed after a small pontine hemorrhagic infarction. In comparison with normal databases, reduced 11C-diprenorphine binding was more accentuated than the hypometabolism on the lateral cortical surface contralateral to the symptoms, and a differential abnormal distribution between the tracers was seen in pain-related central structures. These results show that 11C-diprenorphine PET provides unique information for the understanding of central poststroke pain.