Decay of tungsten-189.

We studied the decay of 189W as produced via the 192Os[n,alpha]189W reaction on a 99.9% isotopically enriched 192Os target. The irradiations were performed at the intense neutron beam facility at Cyclotron Research Center, Université Catholique Louvain-la-Neuve (CRC-UCL) (Belgium), where fast neutrons [En approximately 20 MeV, phi(n) = (4.8 +/- 0.3)10(11) ns(-1) cm(-2)] were generated by stopping 50 MeV deuterons on a thick Be target. The half-life of 189W was determined to be 9.3 +/- 0.3min, compared to 10.8 +/- 0.2min obtained from the 188W[n,gamma]189W reaction. The energies of the two predominant gamma-rays of 189W, 260.4 +/- 1.3 and 421.7 +/- 1.4 keV were in good agreement with that from the [n,gamma] reaction, however, the relative intensities of the two gamma-rays were not consistent. From the [n,gamma] reaction, the relative intensities of the 260 and 421 keV gamma-rays were 97 to 100, whereas from the [n,alpha] reaction the relative intensities were 100 to 77, respectively. Assuming a cross-section ratio of 20 +/- 5 for [n,p3n] to [n,alpha] reactions, a cross-section of 0.5 +/- 0.2 mb was suggested for the 192Os[n,alpha]189W reaction, and the absolute intensity of the 260 keV gamma-ray was estimated to be approximately 50%.

Development of a titanium tungstate-based 188W/188Re gel generator using tungsten of natural isotopic abundance.

The feasibility of developing titanium tungstate-based 188W/188Re gel generator using tungsten of natural isotopic abundance irradiated in a moderate flux reactor has been investigated. Influence of temperature, pH and eluent concentration on generator performance was studied. It was found that "post-formed" approach allows to construct gel generators with elution performance and 188Re elution yields very close to those of conventional alumina 188W/188Re generator. Curie-level 185W radionuclidic impurity presents a challenge during the processing of target material and subsequent elution of the generator. In the future use of semi-enriched with 186W target material (50-60% enrichment) would be beneficial in the development of titanium tungstate-based 188W/188Re gel generators.

Radionuclide production for therapeutic radiopharmaceuticals.

A fundamental task within the framework of a project searching for new radiopharmaceuticals for systemic therapy was the evaluation of the capabilities of the Portuguese Research Reactor (RPI) for the production of several important radionuclides. The feasibility of producing 64Cu, 77As, 153Sm, 165Dy, 166Ho, 170Tm, 177Lu, 186Re, 199Au and 111Ag in useful quantities was evaluated for the present RPI operation schedule (12 h cycles) and for continuous operation. The main evaluation criteria are expressed in terms of specific activity for continuous irradiation and/or 12 h cycle and the use of natural or enriched targets if necessary. Selected samples were irradiated and a comparison between measured activities and values calculated according to the irradiation schedule and using the same software was performed.

An efficient batch preparation of high specific activity.

The increasing demand for radiolabeled metaiodobenzylguanidine (mIBG) prompted the need to obtain the radiopharmaceutical by a reliable, routine and simple synthetic method for batch production. The production of mIBG labeled with either 123I or 124I has been optimized by modifying literature methods that involve solid-state exchange reaction on "cold" mIBG facilitated by ammonium sulfate. The radiochemical yield and purity of radioiodinated mIBG generally exceeded 80 and 98%, respectively, with specific activity of > 50 mCi/mg.

A prototype transduction tag system (delta LNGFR/NGF) for noninvasive clinical gene therapy monitoring.

The dramatic expansion of clinical gene therapy trials requires the development of noninvasive clinical monitoring procedures, which provide information about expression levels, expression kinetics, and spatial distribution of transduced therapeutic genes. With the development of such procedures, invasive sampling of tissue probes from patients potentially could be reduced significantly. In this study, an experimental platform for the rational design and in vitro testing of suitable receptor-ligand couples as components of future transduction tag systems for noninvasive gene therapy monitoring applications was developed. Initially, the feasibility of the delta LNGFR/nerve growth factor (NGF) transduction tag system was investigated; this system employs a mutated version of the low-affinity nerve growth factor receptor (p75mut or delta LNGFR) lacking the entire cytoplasmic domain. Specific binding of 125I-radiolabeled NGF was demonstrated for two stable delta LNGFR-transduced cell lines, but not for delta LNGFR-negative parental control cell lines. An additional binding analysis performed in a MicroImager directly confirmed binding of radiolabeled ligands (125I-NGF, 125I-anti-p75 monoclonal antibody) to the p75mut expressed on intact target cells, but not on control cells. Subsequent binding studies employing NGF radiolabeled with the positron-emitting isotope 124I demonstrated a specific binding for LNGFR+ PC12 cells. Consequently, the first in vitro proof of a transduction tag approach based on the specificity of the 124I-NGF/LNGFR interaction was provided, which opens up the possibility for future noninvasive positron emission tomography monitoring in clinical gene therapy trials.

Quantitative imaging of iodine-124 with PET.

UNLABELLED: PET is potentially very useful for the accurate in vivo quantitation of time-varying biological distributions of radiolabeled antibodies over several days. The short half-lives of most commonly used positron-emitting nuclides make them unsuitable for this purpose. Iodine-124 is a positron emitter with a half-life of 4.2 days and appropriate chemical properties. It has not been widely used because of a complex decay scheme including several high energy gamma rays. However, measurements made under realistic conditions on several different PET scanners have shown that satisfactory imaging and quantitation can be achieved. METHODS: Whole-body and head-optimized scanners with different detectors (discrete BGO, block BGO and BaF2 time-of-flight), different septa and different correction schemes were used. Measurements of resolution, quantitative linearity and the ability to quantitatively image spheres of different sizes and activities in different background activities were made using phantoms. RESULTS: Compared with conventional PET nuclides, resolution and quantitation were only slightly degraded. Sphere detectability was also only slightly worse if imaging time was increased to compensate for the lower positron abundance. CONCLUSION: Quantitative imaging with 124I appears to be possible under realistic conditions with various PET scanners.

Pharmacological characterization and positron emission tomography evaluation of 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide for investigations of central muscarinic cholinergic receptors.

4-[76Br]bromodexetimide and its inactive enantiomer 4-[76Br]bromolevetimide were prepared via electrophilic bromodesilylation using chloramine-T and no-carrier-added (NCA) [76Br]NH4. In vitro, Bmax measured on rat cortex membranes were 3.7 +/- 0.2 and < 0.07 pmol/mg protein for 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide, respectively. The kD of 4-[76Br]bromodexetimide was 1.9 +/- 0.3 nM. In vivo studies in rats showed specific uptake of 4-[76Br]bromodexetimide in cortex, striatum, thalamus and hippocampus. No specific uptake was observed with 4-[76Br]bromolevetimide. With [76Br]bromodexetimide, positron emission tomography (PET) studies in primates demonstrated a preferential accumulation of the radioactivity in the cortex and striatum which was displaced to the level of cerebellum by dexetimide. With 4-[76Br]bromolevetimide, the radioactivity concentrations in the cortex and striatum were similar to that of cerebellum.

Detection of malignant tumors using Tc-99m labeled Fab' fragments from a monoclonal antibody with specificity for D-dimer of cross-linked fibrin.

The authors present the case studies of two patients whose malignant tumors were detected with a Tc-99m labeled antifibrin monoclonal antibody (DD-3B6/22), which is specific for cross-linked fibrin. The first case was a malignant fibrous histiocytoma involving the proximal aspect of the left thigh, whereas in the second case, the patient was receiving treatment for a squamous cell carcinoma of the right mainstem bronchus. The results highlight the potential of this anti-D-dimer radiopharmaceutical for noninvasive detection of malignant tumors.

Radiosynthesis of [123I]N-methyl-4-iododexetimide and [123I]N-methyl-4-iodolevetimide: in vitro and in vivo characterisation of binding to muscarinic receptors in the rat heart.

[123I]N-methyl-4-iododexetimide, [123I]MIDEX, and its pharmacologically inactive enantiomer [123I]N-methyl-4-iodolevetimide, [123I]MILEV were prepared via electrophilic iododesilylation using Chloramine-T as oxidising agent followed by N-methylation using methyl iodide. The radiotracers were purified with semi-preparative HPLC with radiochemical yields of 80 +/- 11% (n = 6). The average time of synthesis was 100 min with specific activity > 2000 Ci/mmol. In vitro, the binding of [123I]MIDEX, after addition of carrier, measured on homogenates of rat atrium was Bmax = 4.5 +/- 0.4 pmol/mg protein, Kd = 3.3 +/- 0.2 nM while in the ventricle Bmax = 2.3 +/- 0.2 pmol/mg protein, Kd = 4.0 +/- 1.4 nM. In vitro, the binding of [123I]MILEV was non-specific. The in vivo biodistribution of [123I]MIDEX showed high uptake in the atrium (3.2% ID/g) and left and right ventricles (2.2, 2.5% ID/g respectively) at 5 min followed by clearance. High heart-to-lung and moderate liver-to-lung ratios were obtained during 60 min. Radioactivity in the atrium and ventricles was reduced by pre-administration of the m-AChR antagonist MQNB (1 mg/kg). Pretreatment of rats with other m-AChR ligands, pirenzapine (M1), methoctramine (M2) and 4-DAMP (M3) also resulted in reduction of [123I]MIDEX uptake with methoctramine being the most potent. [123I]MIDEX distribution in the rat heart was not significantly inhibited by pre-administration of selective adrenergic drugs. The uptake was highly stereoselective since the inactive enantiomer, [123I]MILEV, demonstrated very low myocardial retention. The stability of [123I]MIDEX was evaluated by performing a metabolite study on atrium samples which revealed unchanged radiotracer 60 min postinjection. These results suggest that [123I]MIDEX may be a useful single photon agent for in vivo imaging of myocardial m-AChR in humans with [123I]MILEV offering the potential of assessing non-specific binding of the active tracer.

Rapid characterization of chemically-modified proteins by electrospray mass spectrometry.

Electrospray mass spectrometry (ESI-MS) has been used to examine monoclonal antibodies (MAbs), antibody fragments (Fab and Fc), modified fragments, and a range of other chemically-modified proteins as part of a study aimed at establishing ESI-MS as a method for the characterization of radioimmunoconjugates. This has been approached from two angles. Firstly, ESI-MS of complexes formed between chelators and other small molecules conjugated to hen egg white lysozyme (HEL) (14 kDa) demonstrate the considerable advantages of this powerful new technique compared with existing methods for the characterization of chemically-conjugated proteins. Molecular weights can be determined rapidly to within 0.01-0.05% and with good sensitivity (10-50 pmol total), thus providing specific structural information and opening the way for ESI-MS to be applied widely for the structural characterization of radioimmunoconjugates. Secondly, the conditions for ESI-MS of intact antibodies and antibody fragments have been examined in detail, and we have shown that the addition of up to 10 biotin molecules to the 50 kDa Fab fragment can be easily detected in ESI mass spectra, thus demonstrating the potential for the characterization of modified MAb fragments and metabolites. Finally, the strengths and limitations of ESI-MS of intact antibodies are discussed, and these results indicate that it may only be possible to detect average shifts in the mass of intact antibodies following modification.

[166Dy]dysprosium/[166Ho]holmium in vivo generator.

A novel approach for the delivery of 166Ho (t1/2 = 26.6 h) to tissue is via the in vivo decay of its 81.5 h parent, 166Dy-an in vivo generator system. A critical question for the in vivo 166Dy/166Ho generator system is whether translocation of the daughter nucleus occurs. The in vitro and in vivo integrity of the [166Dy]Dy/166Ho-DTPA complex was investigated and results indicated that no translocation of the daughter nucleus occurs subsequent to beta- decay of 166Dy. Biodistribution studies of [166Dy]Dy-DTPA showed that the ratio of 166Dy/166Ho in bone remains constant (+/- 7%) over a 20 h period, indicating no significant in vivo loss of 166Ho from the complex. Increasing the in vivo residence time of [166Dy]Dy-DTPA complex attached to HSA gave similar results.

Human dosimetry and biodistribution of iodine-123-iododexetimide: a SPECT imaging agent for cholinergic muscarinic neuroreceptors.

UNLABELLED: Iodine-123-iododexetimide (IDEX) has recently been used for SPECT imaging of muscarinic cholinergic neuroreceptors (mAChR) in humans. We report the human radiation dosimetry, whole-body and normal cerebral distribution of IDEX. METHODS: Serial whole-body planar and brain SPECT scans were performed over 24 hr in four normal subjects. Organ activity was calculated from attenuation-corrected geometric mean counts from ROIs drawn over visible organs. Thigh activity was used for background subtraction. Organ absorbed doses and effective dose were calculated using the MIRD schema. Brain SPECT was performed 6 hr postinjection in ten normal subjects. ROIs placed over cortical and subcortical structures were used to determine brain distribution. RESULTS: The effective dose was 24.7 microSv/MBq. An average of 54% of IDEX remained in the body background. Decay-corrected brain uptake was 6.9% of injected dose at 1 hr, 8.6% at 6 hr and 8.1% at 24 hr. Regional brain distribution showed high uptake in striatum and cortex with low activity in thalamus and cerebellum. At 6 hr, activity relative to striatum was 70% for frontal and parietal cortex, 102% for occipital cortex, 54% for thalamus and 11% for cerebellum. CONCLUSION: Iodine-123-IDEX produced high quality SPECT images with activity at 6 hr reflecting the known distribution of mAChR receptors. The favorable dosimetry of IDEX and high synthetic yield (50%-70%) suggest it to be a suitable agent for clinical studies.

Is planar thallium-201/fluorine-18 fluorodeoxyglucose imaging a reasonable clinical alternative to positron emission tomographic myocardial viability scanning?

This comparative study was performed to determine whether a conventional planar gamma camera optimised for 511-keV imaging can reliably assess myocardial viability using the fluorine-18 fluorodeoxyglucose (FDG) metabolic tracer previously developed for positron emission tomography (PET). Twenty-seven patients with severe ischaemic cardiomyopathy (mean left ventricular ejection fraction: 20% +/- 9%) having clinically indicated nitrogen-13 ammonia/FDG PET myocardial viability studies consented to resting, four-view, planar myocardial thallium-201 perfusion and FDG metabolism imaging. The resultant PET and planar perfusion/metabolism images (PPI) were independently assessed for FDG defect size and perfusion/metabolism mismatch, using a four-point scale, in each of four vascular regions: apex, circumflex, left anterior and posterior descending coronary artery territories. Of 108 regions, 106 were evaluable (two not assessed by PET). There was complete agreement in 70% of coronary vascular territories, giving an unweighted kappa score of 0.56. Moreover, in 94% of segments agreement was within one grade. Interestingly, six of the seven differences of more than one grade occurred in the circumflex coronary territory, which was also the only region for which planar positron imaging underestimated FDG defect size. Three of four moderate areas of perfusion/metabolism mismatch seen with PET were also seen on PPI. PPI showed three small regions of mismatch not seen on PET, whilst the reverse occurred with one other small region of mismatch. Thus, for this PET protocol, PPI provides very similar information on the extent of regional FDG uptake and occurrence of mismatch.(ABSTRACT TRUNCATED AT 250 WORDS)

Iodine-123 N-methyl-4-iododexetimide: a new radioligand for single-photon emission tomographic imaging of myocardial muscarinic receptors.

Cardiac muscarinic receptor ligands suitable for positron emission tomography have previously been characterised. Attempts to develop radioligands of these receptors suitable for single-photon emission tomographic (SPET) imaging have not been successful due to high lung retention and high non-specific binding of previously investigated potential tracers. The purpose of this study was to evaluate the biodistribution and in vivo imaging characteristics of a new radiopharmaceutical, [123I]N-methyl-4-iododexetimide. Biodistribution studies performed in rats showed high cardiac uptake (2.4% ID/g) 10 min after injection with a heart to lung activity ratio of 5:1. Specificity and stereoselectivity of cardiac binding were demonstrated using blocking experiments in rats. Dynamic imaging studies in anaesthetised greyhounds demonstrated rapid and high myocardial uptake and low lung binding with stable heart to lung activity ratios of > 2.5:1 between 10 and 30 min, making SPECT imaging feasible. Administration of an excess of an unlabelled muscarinic antagonist, methyl-quinuclidinyl benzylate rapidly displaced myocardial activity to background levels and the pharmacologically inactive enantiomer, [123I]N-methyl-4-iodolevetimide, had no detectable cardiac uptake, indicating specific and stereoselective muscarinic receptor binding. SPET revealed higher activity in the inferior than in the anterior wall, this being consistent with previously described regional variation of cardiac parasympathetic innervation. [123I]N-methyl-4-iododexetimide shows promise as an imaging agent for muscarinic receptor distribution in the heart and may be helpful in evaluating diverse cardiac diseases associated with altered muscarinic receptor function, including heart failure and diabetic heart disease.

[11C]A-69024: a potent and selective non-benzazepine radiotracer for in vivo studies of dopamine D1 receptors.

[11C]A-69024, (+/-)-1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-[11C]methyl- 1,2,3,4-tetrahydroisoquinoline, is a specific and selective dopamine D1 radiotracer. The in vivo biodistribution of this novel radioligand in mice showed a high uptake in the striatum (6.7% ID/g) at 5 min, followed by clearance with a half-life of 16.1 min. As a measure of specificity, the striatal/cerebellar ratio reached a maximum of 7.4 at 30 min post-injection. Radioactivity in the striatum was reduced to the level of the cerebellum by pre-administration of the D1 antagonist SCH 23390 (1 mg/kg). Pretreatment of mice with spiperone (D2), 7-hydroxydipropylaminotetralin (7-OH-DPAT) (D3), clozapine (D4), ketanserin (5-HT2/5-HT2C), mazindol (monoamine reuptake), prazosin (alpha 1), and haloperidol (D2/sigma) had no inhibitory effect on [11C]A-69024 uptake in the striatum. The dextrorotatory enantiomer of the dopamine antagonist butaclamol inhibited striatal uptake, while the less active isomer (-)-butaclamol did not. [11C]A-69024 binding was inhibited by unlabeled A-69024 in a dose dependent manner (ED50 = 0.3 mg/kg) in the striatum while no change occurred in the cerebellum. [11C]A-69024 warrants further investigation as a PET ligand for examination of central dopamine D1 receptors in humans.

Planar cardiac F-18 fluorodeoxyglucose imaging with a conventional gamma camera.

OBJECTIVE: To examine the potential of an adapted gamma camera to image cardiac uptake of the positron emitting glucose analogue fluorine-18 fluorodeoxyglucose (FDG). DESIGN: Postprandial studies were performed in 19 patients (mean age, 56 +/- 9 years) with coronary disease and resting cardiac dysfunction who had undergone a routine clinical 7 min/view planar thallium-201 (Tl-201) stress reinjection or rest redistribution study. A glucose/insulin protocol was used and, an hour after FDG injection, 15-minute static planar myocardial images were acquired in the four views used for Tl-201 scanning. RESULTS: The diagnostic quality of FDG images was at least as good as that of their Tl-201 counterparts, with less liver background in all but one FDG study. In the left anterior oblique 45 degrees view uncorrected global myocardial FDG and stress Tl-201 counts were similar, but the FDG study had significantly higher peak myocardial to background ratios. CONCLUSION: Assessing regional cardiac FDG uptake and myocardial perfusion seems feasible with conventional gamma camera technology, providing a widely available and cost effective means of detecting hibernating myocardium. Similar equipment may appreciably reduce the need for positron emission tomography in a range of clinical conditions.

166Ho-microsphere liver radiotherapy: a preclinical SPECT dosimetry study in the pig.

Liver metastases cause the majority of deaths from colorectal cancer and response to chemotherapy is poor. Intrahepatic arterial 90Y-microspheres may induce tumour regression but the beta-radiation dose is variable and cannot be determined in patients. The 81 keV gamma emission of holmium-166 (166Ho) was used to determine, by single photon emission computed tomographic (SPECT) imaging, the beta-radiation absorbed dose to normal liver in pigs following intrahepatic arterial administration of 166Ho-microspheres. The SPECT system was calibrated with anthropomorphic liver phantoms containing known activity concentrations of 166Ho-chloride. The relationship of SPECT counts to phantom activity concentration was linear with a correlation coefficient of r = 0.996. The SPECT pattern of liver distribution following successive administrations of tracer activities of 166Ho-microspheres was similar. The ratio of initial to total SPECT estimates of mean activity concentration in regions of interest, from which anatomically matched biopsy samples were later obtained and counted in an ionization chamber, showed good correlation (r = 0.924). Prospective SPECT dosimetry performed on a tracer activity of 166Ho-microspheres predicted the total administered activity required to deliver a prescribed radiation absorbed dose of 25 Gy to the liver within an error of +/- 8%. This study demonstrates the feasibility of prospective control of the absorbed radiation dose to the critical normal organ by SPECT dosimetry on a tracer dose of 166Ho-microspheres prior to administration of a therapy dose.

Technetium-99 m labelling of DD-3B6/22 antifibrin monoclonal antibody fragment Fab' for thrombus imaging.

The antifibrin DD-3B6/22 monoclonal antibody Fab' fragment, a murine immunoglobulin, IgG3, has been labelled with technetium-99 m (99mTc) via a transchelation reaction, to specific activity in excess of 30 mCi/mg protein. The radiolabelling of Fab' was dependent on time, temperature, pH, antibody concentrations and nature of intermediary transchelation complex used. The resultant radioconjugate was stable in vitro and in vivo. Blood clearance of 99mTc-Fab' in rat followed two compartment kinetics with the half time of the fast phase being 0.5 h. The main route of excretion was via the kidneys with little uptake indicated by other tissues. The results suggest that the inherent specificity of the antibody, small molecular size, rapid plasma clearance, high specific radioactivity, together with the physical properties of the 99mTc label, combine to make this labelled monoclonal antibody (MoAb), potentially suitable as a radiopharmaceutical for the scintigraphic detection of thrombi in humans.

Induction of micronuclei in peripheral blood lymphocytes of patients treated for rheumatoid or osteo-arthritis of the knee with dysprosium-165 hydroxide macroaggregates or yttrium-90 silicate.

The advantages of radiation synovectomy, using preparations containing dysprosium-165 (Dy-165), over conventional materials based on yttrium-90 (Y-90) include a more ideal spectrum of decay energies, and a much shorter half-life permitting quicker and more efficient treatment. A new therapeutic agent, Dy-165 hydroxide macroaggregates, has been developed for the treatment of arthritis making use of these advantages. As part of a clinical trial of this material 42 patients were examined for micronucleus frequency in their peripheral blood lymphocytes prior to, and 2 weeks after, radiation synovectomy using Dy-165 hydroxide macroaggregates or Y-90 silicate. In the majority of patients from each treatment group no significant increase in micronucleus frequency was observed. This indicates that in these cases leakage of material from the site of treatment was not resulting in detectable irradiation of circulating cells irrespective of the choice of radiopharmaceutical. The maximum increase in micronucleus frequency observed corresponded to a radiation dose to circulating cells of approximately 0.3 Gray.

High-resolution positron emission tomography of human ovarian cancer in nude rats using 124I-labeled monoclonal antibodies.

PET has inherently high resolution and excellent contrast imaging and accurately measures radioactivity concentrations in vivo. When combined with specific immunological targeting it might provide a highly specific and sensitive radioimmunoscintigraphic tool. To investigate this we injected 124I-labeled MAb MX35 or MAb MH99 monoclonal antibodies (doses 200-400 mu Ci) intravenously into nude rats bearing subcutaneous human ovarian cancer xenografts (SK-OV-7 and SK-OV-3 cell lines). A melanoma cell line (SK-MEL-30) was used as a control tumor. These murine monoclonal antibodies react with cell-surface antigens expressed by most ovarian cancer cells, including the ovarian cell line used. Imaging was performed at 1-6 days using a high-resolution positron emission tomograph (PCR-I) with a spatial resolution of 4.5 mm. The slice thicknesses were 0.5 and 1.0 cm. Forty to seventy thousand coincident pulses were obtained per frame. The PET results were compared with those of autopsy and histology. Samples of blood, tumor, and normal tissues were obtained at various time points. PET calculation of isotope uptake ratios demonstrated specific localization of the antibodies in tumor, with ratios of tumor to normal tissue uptake as high as 6:1. Subcutaneous ovarian cancer nodules as small as 7 mm were identified with PET imaging. The results corresponded well with tissue sampling. Our findings suggest that PET imaging of tumors with 124I-labeled monoclonal antibodies may be useful in human diagnostic and therapeutic applications in ovarian cancer as well as other diseases.