Radioiodinated Capsids Facilitate In Vivo Non-Invasive Tracking of Adeno-Associated Gene Transfer Vectors.

Viral vector mediated gene therapy has become commonplace in clinical trials for a wide range of inherited disorders. Successful gene transfer depends on a number of factors, of which tissue tropism is among the most important. To date, definitive mapping of the spatial and temporal distribution of viral vectors in vivo has generally required postmortem examination of tissue. Here we present two methods for radiolabeling adeno-associated virus (AAV), one of the most commonly used viral vectors for gene therapy trials, and demonstrate their potential usefulness in the development of surrogate markers for vector delivery during the first week after administration. Specifically, we labeled adeno-associated virus serotype 10 expressing the coding sequences for the CLN2 gene implicated in late infantile neuronal ceroid lipofuscinosis with iodine-124. Using direct (Iodogen) and indirect (modified Bolton-Hunter) methods, we observed the vector in the murine brain for up to one week using positron emission tomography. Capsid radioiodination of viral vectors enables non-invasive, whole body, in vivo evaluation of spatial and temporal vector distribution that should inform methods for efficacious gene therapy over a broad range of applications.

Measurements of blood-brain barrier permeability in patients undergoing radiotherapy and chemotherapy for primary cerebral lymphoma.

Positron emission tomography (PET) has been used to measure changes in regional blood-brain barrier (BBB) permeability in patients with primary cerebral lymphoma undergoing radiotherapy and chemotherapy. The method employed is to measure the rate of wash-out of a radioactive tracer (68Ga-EDTA) from blood into brain tissue using time-sequence PET imaging. Preliminary studies carried out on patients with more common primary cerebral tumours show that time-activity data are reproducible to approximately 10%. Measurements made in 2 patients with primary cerebral lymphoma treated with initial chemotherapy showed significant changes in permeability in the region of the tumour. Within 5 weeks of the start of treatment, permeability values reached the levels of normal brain. No changes in BBB permeability in normal brain were seen immediately after radiotherapy.

A ticket to ride: peptide radiopharmaceuticals.

Over the past three decades, biospecific imaging agents have evolved from large proteins (i.e., antibodies) to antibody fragments (i.e., F(ab')2 and Fab fragments) to smaller "molecular recognition units" such as Fv fragments, antigen binding domain fragments and small biologically active peptides. The smaller size of these molecules confers desirable pharmacokinetic properties, such as higher target-to-background ratios and faster blood clearance, that are favorable for imaging. Molecular engineering techniques now permit the peptide to carry the radionuclide-binding group in its structure while maintaining high-affinity binding to the receptor site. An important component to this system is the ability to radiolabel these agents with high specific activity using short-lived radionuclides, particularly 99mTc. Recently, the application of small radiolabeled biologically active peptides for external imaging of a variety of biological processes has received considerable interest. These applications have ranged from the current widespread use of somatostatin analogs for imaging numerous types of tumors to the development of radiolabeled chemotactic peptides for infection imaging. In this review, we will describe many of the parameters for the rational development of peptide-based imaging agents, including: classes of peptides for imaging, methods for radiolabeling peptides, current biologically active peptide-based radiopharmaceuticals and future prospects for this new technology.

Quantitative in vivo measurements of tumor perfusion using rubidium-81 and positron emission tomography.

Rubidium-81 (t1/2 = 4.58 hr) was investigated as a tumor perfusion tracer in the VX2 carcinoma implanted into rabbit thigh muscle using a large-area, multiwire proportional chamber positron emission tomography (PET) system. Perfusion was determined using the arterial reference sample method, and the results from PET imaging were compared with postmortem tissue sampling. Absolute quantitation of tumor perfusion was achieved using external probes to estimate local extraction fraction. Redistribution of rubidium-81 (81Rb) was investigated using a dual-tracer technique. Average perfusion was found to be 13.5 and 3.7 ml/min/100 g in tumor and normal muscle, respectively. The extraction fraction as estimated from a two-compartment model ranged from 0.94 to 1.00. No significant redistribution of 81Rb was observed in these tissues. Nine patients with malignancies were studied using 81Rb and PET. Tumor perfusion in four patients with carcinoma of the breast was elevated by a factor of 1.8 (range 1.2-2.3) compared to contralateral normal breast.

Quantitative study of radioiodinated metaiodobenzylguanidine uptake in children with neuroblastoma: correlation with tumor histopathology.

Six children with neuroblastoma and one with ganglioneuroma received [125I] metaiodobenzylguanidine (MIBG) before major surgery. Uptake of [125I]MIBG in the excised tissues was measured by scintillation counting, and the material was submitted for histopathology. The ranges of uptake of [125I]MIBG, expressed as percent of the injected dose per gram of tissue, were as follows: for neuroblastoma 0.0013-0.071, for ganglioneuroma 0.0017-0.0028, and for non-neoplastic control tissues 0.0002-0.011. The quantitative uptake of [125I]MIBG by neuroblastoma varied between different patients and between different parts of individual tumors. The more undifferentiated tumors took up more [125I]MIBG and may be more likely to respond to targeted radiotherapy with MIBG.

Localization of radiolabeled chemotactic peptide at focal sites of Escherichia coli infection in rabbits: evidence for a receptor-specific mechanism.

UNLABELLED: The infection imaging properties of a high-affinity 99mTc-labeled chemotactic peptide receptor agonist (N-formyl-methionyl-leucyl-phenylalanine-lysine; N-For-MLFK) were compared with a low-affinity agonist (N-Acetyl-MLFK; N-Ac-MLFK), a moderate-affinity antagonist (N-isobutyloxycarbonyl-MLFK; N-IBoc-MLFK) and non-specific inflammation imaging agents. METHODS: All peptides were prepared by solid-phase methods and purified by high-performance liquid chromatography. The products were assayed in vitro for N-formyl-methionyl-leucyl-phenylalanine receptor binding and superoxide production. Three types of studies were performed in rabbits with Escherichia coli infection: (Study A) Four groups of six animals were coinjected with 99mTc-N-For-MLFK-hydrazinonicotinamide (N-For-MLFK-HYNIC) plus 111In-immunoglobulin G, 111In-red blood cells or 111In-diethylene triamine pentaacetic acid. (Study B) Three groups of six rabbits were coinjected with 111In-leukocytes plus 99mTc-N-For-MLFK-HYNIC, 99mTc-N-Ac-MLFK-HYNIC or 99mTc-N-IBoc-MLFK-HYNIC. (Study C) Two groups of six rabbits were injected with 99mTc-N-For-MLFK-HYNIC and 111In-leukocytes with and without an excess of antagonist. In all three studies, the radiopharmaceuticals were injected 24 hr after infection and dual photon (99mTc and 111In) gamma camera images were acquired at 2-3 and 16-18 hr later. Target-to-background (T/B) ratios were calculated for regions of interest drawn over the infected and contralateral normal tissue. RESULTS: N-For-MLFK, N-Ac-MLFK and N-IBoc-MLFK had EC50s for receptor binding of 2.0, 830 and 150 nM, respectively. The corresponding EC50s for superoxide production were 20.0, approximately 10(3) and > 10(4). Study A demonstrated that the T/B for 99mTc-N-For-MLFK-HYNIC was higher than for any of the nonspecific imaging agents (p < 0.001), and 111In-immunoglobulin G had a higher T/B ratio than 111In-diethylenetriamine pentaacetic acid (p < 0.01) or 111In-red blood cells (p = NS). Study B showed that 99mTc-N-For-MLFK-HYNIC had a higher T/B ratio than the other peptides (p < 0.001). 111In-leukocytes and 99mTc-N-IBoc-MLFK-HYNIC had comparable T/B ratios, which were higher than for 99mTc-N-Ac-MLFK-HYNIC (p < 0.05). Study C demonstrated that coinjection with an antagonist resulted in a significant reduction in the T/B ratio for 99mTc-N-For-MLFK-HYNIC (p < 0.001), but did not affect the T/B ratio for 111In-leukocytes. CONCLUSION: Nonspecific mechanisms contribute minimally to the localization of 99mTc-chemotactic peptide analogs at sites of infection and the majority of the accumulation appears to be receptor mediated. Also, chemotactic peptide receptor antagonists can be used for infection imaging. These results provide important new insights for future radiopharmaceutical development.

An improved tungsten-178/tantalum-178 generator system for high volume clinical applications.

Clinical utilization of the multiwire gamma camera (MWGC) requires low-energy radionuclides. The short-lived (9.3 min) tantalum-178 (178Ta) is ideally suited for the MWGC and can be produced from long-lived (21.7 day) tungsten-178 (178W) by a previously reported 178W/178Ta generator. This generator, however, is limited by sharp increase in breakthrough after elution of only 30-60 column-volumes. To optimize the 178W/178Ta generator for clinical use, varying eluant acid concentrations were evaluated. A reduced (from 0.1 to 0.03N) HCI concentration in the eluant, coupled with low operating temperatures (3 to 5 degrees C) allowed high (40 to 60%) 178Ta yield. Minimal 178W breakthrough (less than .01%) resulted, even after elution of more than 200 column-volumes. Each of six tested generators provided sterile, high activity (up to 100 mCi) 178Ta elutions for more than 30 days. Radiation dosimetry was estimated utilizing both human and animal biodistribution data. The whole body (critical organ) dose in adults and neonates were 1/20 (1/21) and 1/19 (1/50) respectively relative to that of technetium-99m (99mTc) as sodium pertechnetate. The optimized 178W/178Ta generator provides a commercially practical, safe source of low-energy radioisotope for the MWGC with substantial dosimetry advantages over 99mTc.

Metaiodobenzylguanidine: evaluation of its potential as a tracer for monitoring doxorubicin cardiomyopathy.

We evaluated alterations in cardiac adrenergic neuron activity and progression of left ventricular dysfunction in comparison with the severity of structural changes using a rat model of adriamycin cardiomyopathy. Rats were treated with adriamycin (2 mg/kg s.c. once a week) for 6, 7, 8 and 9 wk. Accumulation of 125I-metaiodobenzylguanidine (MIBG) 4 hr after intravenous administration was determined and left ventricular ejection fraction (LVEF) was calculated from gated blood-pool images. H & E and Masson-Trichrome stained specimens of the myocardium were examined by light microscopy. Histopathologic examination demonstrated dose-dependent myocyte damage, although there were no differences between the 8-wk and 9-wk groups. LVEF did not differ between controls and the 6-wk group (81.3% +/- 5.5% versus 82.1% +/- 4.8%, p = ns). LVEF began to decrease slightly in the 7-wk group (75.0% +/- 5.7%, p < 0.05) and showed a remarkable decrease in the 8-wk group (53.7% +/- 2.6%, p < 0.001). In the 9-wk group, LVEF diminished to 47.9% +/- 3.1% (p < 0.001), accompanied by massive pleural effusions and ascites. MIBG accumulation in the heart (%ID/heart) significantly and progressively diminished; 1.42% +/- 0.15% in the 6-wk group, 1.06% +/- 0.16% in the 7-wk group, 0.77% +/- 0.13% in the 8-wk group and 0.34% +/- 0.11% in the 9-wk group, respectively p < 0.001, compared to controls (1.99% +/- 0.30%). These results demonstrate that MIBG accumulation in the heart showed a greater and more linear dose-dependent decrease than LVEF. Furthermore, MIBG uptake was significantly reduced in the 6-wk group where only mild myocyte damage (isolated vacuolation or myofibrillar loss) was observed. Thus, MIBG may be a sensitive biochemical marker of adriamycin cardiomyopathy.

Myocardial substrate utilization and left ventricular function in adriamycin cardiomyopathy.

We evaluated alterations of substrate utilization in a rat model of adriamycin cardiomyopathy with deteriorating left ventricular function. Rats were treated with adriamycin (2 mg/kg), once a week for 6, 8, 9 and 10 wk. Fluorine-18-F-deoxyglucose (18F-FDG) and 125I-beta-methyl-branched fatty acid (125I-BMIPP) were used as tracers of glucose and fatty acid metabolism and 99mTc-hexakis (2-methoxyisobutyl-isonitrile) (99mTc-MIBI) was used as a myocardial blood flow tracer. Left ventricular ejection fraction (LVEF) calculated from gated blood pool images was used as an indicator of cardiac function. LVEF was normal in the 6-wk group (78.0% +/- 4.8%), abruptly decreased in the 8-wk group (43.1% +/- 10.1%) and further deteriorated in the 9-wk group (27.6% +/- 13.4%). Accumulation of 18F-FDG (%kgID/g) in the hearts of adriamycin treated animals progressively decreased compared to controls (2.19% +/- 0.38%); 1.47% +/- 0.42% (p < 0.01) at 6 wk, 1.22% +/- 0.27% (p < 0.001) at 8 wk, 0.69% +/- 0.56% (p < 0.001) at 9 wk and 0.50% +/- 0.08% (p < 0.001) at 10 wk. This decrease occurred earlier than the deterioration in LVEF. Myocardial accumulation of 125I-BMIPP decreased in the advanced stages of adriamycin cardiomyopathy and was well correlated with the decrease in 18F-FDG accumulation. However, the decrease was less profound than for 18F-FDG; 53.7% +/- 9.8% versus 31.6% +/- 25.4% of control at 9 wk (p = NS), 49.5% +/- 15.3% versus 22.6% +/- 3.5% of control at 10 wk (p < 0.05). Accumulation of 99mTc-MIBI did not differ between controls and the adriamycin treated groups. There were no differences in blood glucose levels between controls and adriamycin treatment groups. Both glucose and fatty acid utilization are decreased in adriamycin-induced cardiomyopathy and these critical impairments in energy metabolism are associated with heart failure. Impaired myocardial glucose utilization measured with 18F-FDG may be a particularly sensitive marker of adriamycin cardiomyopathy.

In vivo bioactivity and biodistribution of chemotactic peptide analogs in nonhuman primates.

The dose dependence of the effect of chemotactic peptide on peripheral leukocyte levels was measured in normal Rhesus monkeys. A 99mTc-labeled hydrazino nicotinamide (HYNIC) derivatized chemotactic peptide analog was used to study biodistribution and inflammation imaging in Rhesus monkeys. In normal animals the studies demonstrated that chemotactic peptide induced a clear dose-dependent reduction in peripheral leukocyte levels. The decrease in leukocyte number occurred almost immediately after injection and rapidly returned to baseline. Significant effects on differential WBC count, blood pressure, pulse rate or respiration rate were not detected. The lowest dose of peptide tested (10 ng/kg) had minimal effect on leukocyte level. The HYNIC derivatized peptide was prepared in excellent yield and purity, had biological activity similar to the native peptide and was readily labeled at specific activity of > 20,000 mCi/mumole. When approximately 0.5 mCi (< 2.0 ng/kg) of radiolabeled peptide was injected in monkeys with focal sites of mild sterile inflammation, a pattern of biodistribution similar to radiolabeled WBCs was observed and reductions in leukocyte levels were not detected. At 3 hr after injection, the site of inflammation was readily apparent with a target-to-background ratio of approximately 3:1. These studies demonstrate that radiolabeled chemotactic peptide analogs are effective agents for imaging sites of inflammation in monkeys. By radiolabeling at high specific activity, the effect of these reagents on peripheral leukocyte levels can be avoided.

Technetium-99m-labeled chemotactic peptides: comparison with indium-111-labeled white blood cells for localizing acute bacterial infection in the rabbit.

The biodistribution and infection imaging properties of 99mTc-labeled formyl-methionyl-leucyl-phenylalanyl-lysyl-hydrazinonicotinamide (99mTc-HP) were compared with 111In-labeled leukocytes (111In-WBCs) in rabbits with E. coli infections. Groups of six animals were co-injected with 1 mCi of 99mTc-HP plus 0.05 mCi of 111In-WBCs and serial scintigrams were acquired from 3 to 6 hr and 18 hr postinjection. After acquiring the final images, the animals were killed and biodistribution was determined. At all imaging times, the distributions of 99mTc-HP and 111In-WBCs were similar and the sites of infection were well visualized with both radiopharmaceuticals. The target (infected muscle) to background (contralateral normal muscle) ratios (T/B) were: 3.38 +/- 0.46, 3.80 +/- 0.37 and 10.87 +/- 1.44 for 99mTc-HP and 1.71 +/- 0.04, 1.81 +/- 0.26 and 3.79 +/- 0.83, for 111In-WBCs at 3, 6 and 18 hr postinjection, respectively. The average ratio of T/B ratios (99mTc-HP-to-111In-WBCs) was 2.99 +/- 1.88, with no value less than unity. T/B ratios calculated from direct tissue sampling were significantly higher for 99mTc-HP than for 111In-WBCs (33.6:1 versus 8.1:1, p < 0.01). These differences were primarily due to increased absolute accumulation of 99mTc-HP (0.102%ID/g versus 0.024%ID/g, p < 0.01) in infected muscle rather than a difference in accumulation in normal skeletal muscle. These results indicate that 99mTc-HP yields target-to-background ratios greater than or equal to those achievable with 111In-WBCs probably as a result of an increase in absolute accumulation at the site of infection.

Technetium-99m-labeled hydrazino nicotinamide derivatized chemotactic peptide analogs for imaging focal sites of bacterial infection.

We synthesized and evaluated four hydrazino nicotinamide (HYNIC) derivatized chemotactic peptide analogs: For-NleLFK-HYNIC (HP1), For-MLFK-HYNIC (HP2), For-MLFNH(CH2)6NH-HYNIC (HP3), and For-MLF-(D)-K-HYNIC (HP4), for in vitro bioactivity and receptor binding. The peptides were radiolabeled with 99mTc via a glucoheptonate co-ligand and their biodistribution determined in rats (n = 6/time point) at 5, 30, 60 and 120 min after injection. Localization of the peptides at sites of deep thigh Escherichia coli infection was determined by radioactivity measurements on excised tissues in rats (n = 6/time point) and rabbits as well as scintillation camera imaging in rabbits (n = 6). All peptides maintained biological activity (EC50s for O2 production by human PMNs: 12-500 nM) and the ability to bind to the oligopeptide chemoattractant receptor on human PMNs (EC50s for binding: 0.12-40 nM). After incubation with 99mTc-glucoheptonate, radiolabeled peptides were isolated by HPLC at specific activities of > 10,000 mCi/microM. Technetium-99m-labeled peptides retained receptor binding with EC50s < 10 nM. Blood clearance of all four peptides was rapid. Biodistributions of the individual peptides were similar, with low levels of accumulation in most normal tissues. In rats, all of the peptides concentrated at the infection sites (T/B ratio: 2.5-3:1) within 1 hr of injection. In rabbits, outstanding images of the infection sites were obtained, with T/B ratios of > 20:1 at 15 hr after injection. This study demonstrates that 99mTc-labeled chemotactic peptide analogs are effective agents for the external imaging of focal sites of infection.

Localization of indium-111-immunoglobulin G, technetium-99m-immunoglobulin G and indium-111-labeled white blood cells at sites of acute bacterial infection in rabbits.

Biodistribution and infection imaging properties of 111In-DTPA-IgG, 99mTc-hydrazino nicotinamide-IgG and 111In-WBC were compared in rabbits with E. coli infection. Groups of six rabbits were injected with 10 mCi of 99mTc-IgG plus 0.5 mCi of 111In-IgG or 1 mCi of 99mTc-IgG plus 0.05 mCi of 111In-WBC. At 4-5 and 18-20 hr, dual photon scintigrams were acquired. At both times, the distributions of 99mTc and 111In-IgG were nearly identical. The sites of infection were well visualized with all three radiopharmaceuticals. In the early images, the target-to-background ratios (T/B) for 111In and 99mTc-IgG determined by ROI analysis were 1.95 +/- 0.26 and 2.57 +/- 0.38 (p = NS). In the delayed images, the T/B ratios increased (p < 0.01) to 3.56 +/- 0.49 and 4.90 +/- 0.98. At both times, the T/B ratios for 111In-WBC were higher (p < 0.01); 4.17 +/- 0.78 at 4-5 hr and 8.52 +/- 1.52 at 18-20 hr. These results indicate that all three agents yield excellent images of infection sites. Although 111In-WBC had higher T/B ratios, the ease of preparation of the radiolabeled proteins makes them attractive alternatives for infection imaging.

The dose uptake ratio as an index of glucose metabolism: useful parameter or oversimplification?

UNLABELLED: The dose uptake ratio (DUR) has been used as a quantitative index of glucose metabolism for tumor classification and monitoring response to treatment. In order to provide consistent results, DUR measurements should be made when the concentration of tracer has reached a plateau. The time of this plateau cannot be identified from a single static acquisition. METHODS: In this study, we investigated the changes in DUR as a function of time in eight patients with stage III lung cancer. All patients underwent a quantitative dynamic 18F-FDG PET study before and after treatment and the data were analyzed with a three-compartment model. Using the fitted model parameters, the DUR was predicted at the plateau and intermediate times. RESULTS: Tumor concentrations of 18F-FDG did not reach a plateau within the 90 min of imaging in any of the pre-treatment studies and only in one case post-treatment. The average time to reach 95% of the plateau value pre-treatment was 298 +/- 42 min (range: 130-500 min); in post-treatment, it was 154 +/- 31 min (range: 65-240 min). The difference between the plateau DUR and the 60-min value was 46% +/- 6% pre-treatment and 17% +/- 5% post-treatment. CONCLUSIONS: These data indicate that DUR can vary widely with the time of measurement and that DUR should be interpreted with caution in any individual patient.

Synthesis of fluorine-18-labeled biotin derivatives: biodistribution and infection localization.

UNLABELLED: Recently there has been much interest in the exploitation of the high binding affinity of avidin/biotin as a means of targeting drugs and radionuclides for in vivo applications. We are interested in broadening the application of the avidin/biotin complex to PET. To this end we set out to prepare 18F-labeled biotin analogs. METHODS: Two 18F biotin derivatives, [3aS-(3a alpha,4 beta,6a alpha)]-hexahydro-2-oxo-1H-thieno[3,4- d]imidazole-4-(N-3-(1-[18F]fluoropropyl))pentanamide (1) and [3aS-(3a alpha,4 beta,6a alpha)]-tetrahydro-4-(5-(1-[18F]fluoropentyl)- 1H-thieno[3,4-d]imidazol-2(3H)-(2) were prepared with high specific activity (NCA) and evaluated for their potential in infection localization. RESULTS: Compound 1 binds to avidin and the biodistribution of these derivatives were studied in Escherichia coli infected rats. Half of the infected rats were treated with avidin 24 hr prior to intravenous injection of the 18F-labeled biotin analogs. Biotin 1, without avidin pretreatment, showed a selectivity of 6.08 +/- 1.12 for infection compared to normal muscle. With avidin pretreatment, selectivity increased slightly, giving an infection to normal muscle ratio of 6.39 +/- 0.96. In contrast, the biodistribution of biotin 2 indicated more binding to normal muscle with an infection to normal muscle ratio of 0.58 +/- 0.07. This lack of selectivity illustrates the importance of the side-chain amide group in infection localization. There was some defluorination of 1 and 2, as evidenced by increased 18F bone uptake after 60 min: 2.94 +/- 0.37 and 1.17 +/- 0.21 %IG/g +/- s.d., respectively. CONCLUSIONS: Biotin derivatives could be radiofluorinated with high specific activity. Biotin 1, is a potential positron tomography tracer for infection imaging.

SPECT imaging of dopamine transporter sites in normal and MPTP-Treated rhesus monkeys.

UNLABELLED: Parkinson's disease is characterized by degeneration of dopamine (DA) neurons and their terminals. Since these neurons contain dopamine transporters (DAT), radioligands that bind to these sites are promising radiopharmaceuticals for diagnosis and therapeutic monitoring of disease progression. We evaluated [123I]-2 beta-carbomethoxy- 3 beta-(4-fluorophenyl)-N-(1-iodoprop-1-en-3-yl)nortropane ([123I]IACFT) for SPECT imaging in an MPTP model of parkinsonism. METHODS: Three rhesus monkeys were imaged before and at 1 and 2 mo after treatment with MPTP. The SPECT results were correlated with motor behavior and PET imaging with [11C]-2 beta-carbomethoxy-3 beta-aryltropane ([11C]-CFT). Also, biodistribution was measured by planar imaging. RESULTS: In normal animals, striatal accumulation of radioactivity was rapid and peaked within 30 min. Striatal accumulation of [123I]IACFT was nearly completely displaceable with unlabeled CFT (1 mg/kg) but was not affected by a similar dose of the serotonin (5-HT) transport inhibitor, citalopram. The striatal to cerebellar ratio measured at 30 min, after injection of [123I]IACFT was significantly higher (p < 0.01) than with [11C]CFT; approximately 6; 1 versus approximately 2.5; 1. After MPTP treatment this ratio decreased to 1.02:1 with IACFT and 1.23:1 with [11C]CFT. Blood clearance of [123I]IACFT was rapid with a terminal t1/2 of approximately 30 min. HPLC of plasma samples demonstrated that the concentration of intact ligand decreases rapidly, approaching zero by 60 min. Low levels of accumulation were measured in extracranial tissues. CONCLUSION: These results demonstrate that [123I]IACFT is an excellent SPECT ligand for dopamine transporter sites that combines the critical characteristics of: (a) high striatal to cerebellar ratios, (b) high selectivity for dopamine versus 5-HT transporter sites, (c) convenient preparation at high-specific activity and radiochemical purity and (d) a striatal localization rate that is well matched to the physical t1/2 of 123I.

c-erbB2 protein overexpression in breast cancer as a target for PET using iodine-124-labeled monoclonal antibodies.

ICR 12, one of a panel of rat monoclonal antibodies recognizing the external domain of the human c-erb B2 proto-oncogene product, (Styles, 1990) was chosen as a candidate for radiolabeling with 124I for positron emission tomography of selected patients with breast cancer. By using N-bromosuccinimide (NBS), optimal labeling conditions were established using 125I. The labeling efficiency was determined using instant thin-layer chromatography (ITLC) and gel filtration (HPLC). The antibody was then labeled with the positron emitter 124I, and a labeling efficiency of 96% and immunoreactivity of 80%-90% was obtained. The product was stable, with less than 5% of the radiolabel being eluted after six days storage in plasma at 37 degrees C. Immunolocalization studies were performed in athymic mice bearing human breast carcinoma xenografts overexpressing the c-erb B2 gene product using as controls 125I labeled isotype-matched rat antibody, and antigen-negative tumors. Good uptake of 124I-labeled ICR12 was obtained in c-erb B2 expressing tumors (up to 12% injected dose per gram at intervals up to 120 hr), with localization indices of 3.4-6.2. Tumor xenografts of 6 mm diameter were successfully imaged with high resolution at 24, 48 and 120 hr using the RMH/ICR MUP-PET camera. We suggest that 124I-labeled ICR12 is a suitable agent to image and quantify immunolocalization in patients whose tumors overexpress the c-erb B2 proto-oncogene product.

A gamma camera for medical applications, using a multiwire proportional counter.

A multiwire proportional counter gamma camera, specifically designed for nuclear medicine applications, is portable and weighs less than 50 lb including shielding and collimator. The basic operating characteristics have been investigated with various radioactive sealed sources. The camera demonstrates a peak count rate of 850,000 cps, an intrinsic spatial resolution of 2.5 mm, and excellent image uniformity when used with x-ray sources in the range of 22-81 keV. Tests of the device with Ta-178--a very promising, short half-life (9.3 min), low-energy radionuclide--using 20 mCi injections provided images of quality comparable to those obtained from 15 mCi Tc-99m studies with conventional imaging devices. The camera used with Ta-178 offers particular promise in first-pass nuclear cardiology studies. Considerably improved study quality will likely result in this area because of the increased injectable dose levels offered by Ta-178 combined with the high-count rate capability and improved resolution.

Infection imaging with technetium-99m-labeled chemotactic peptide analogs.

The localization of occult sites of infection is frequently essential to the therapeutic management of critically ill patients. Current imaging procedures, including computed tomography, ultrasound, magnetic resonance imaging, and conventional radiography, rely primarily on focal changes in tissue density or composition to define the lesion. Typically, these are relatively late changes in the inflammatory process. Radionuclide procedures which have been used to localize early inflammatory processes, require a minimum of 12 hours, and usually 24 to 48 hours, from the time of injection to imaging. Clearly, a method of rapidly localizing sites of acute inflammation would be helpful for patient management. Recently, we developed methods for preparing analogs of the leukocyte chemoattractant peptide, N-formyl-methionyl-leucyl-phenylalanine (ForMLF), that can be radiolabeled for external imaging. In vitro, these compounds have bioactivity and neutrophil ForMLF receptor binding comparable with that of the native peptide. Studies in animals demonstrate that these agents bind to leucocytes in vivo, clear from the circulation rapidly, and localize at sites of Escherichia coli infection to an extent sufficient to yield external images early after injection. However, even at relatively low doses, the peptides elicit a profound but transient reduction in peripheral leukocyte levels. Despite this, if the peptides could be radiolabeled at very high specific activity, imaging might be possible with doses of peptide that do not reduce peripheral leukocyte levels. Recently, hydrazino nicotinamide--derivatized peptides were prepared and radiolabeled with technetium-99m at extremely high specific activity (> 20,000 mCi/mumole). In a rabbit model of E coli infection, these peptides yielded excellent images of sites of infection at injected doses that are approximately 1,000-fold below the level that produces a significant reduction in peripheral leukocytes. In addition, studies of thermally injured infected animals indicate that peptide localization may be infection selective, with very low levels of accumulation at sites of sterile inflammation.

Thermal injury in rats alters glucose utilization by skin, wound, and small intestine, but not by skeletal muscle.

The effects of thermal injury in rats on glucose utilization (Rg) by skin, wound, small intestine, and muscle in vivo has been determined using 2-[18F]-fluoro-2-deoxy-D-glucose (18FDG) 6 hours, 24 hours, and 3 weeks after injury. These results were compared with serum glucose and insulin levels at the same time points and with hexokinase activity in the tissues. Thermal injury had no significant effects on serum glucose levels; however, serum insulin levels were lower than sham values 6 hours after injury, the same as sham values 24 hours after injury, and significantly higher than sham values 3 weeks after injury. Rg of unburned skin was not changed at 6 or 24 hours after injury, but was increased at 3 weeks after injury. The Rg of the wound was almost zero at 6 and 24 hours after injury; however, at 3 weeks after injury, the wound area had a Rg two to three times higher than that of the normal skin of sham animals. Small intestine Rg was decreased 6 and 24 hours after injury, but was essentially normal by 3 weeks after injury. The Rg of muscle was unchanged at all of the time points tested. Total Rg was significantly higher in the animals 3 weeks after injury, with the increase primarily due to the increases in skin and wound. The increases in Rg were associated with changes in tissue hexokinase activity. The present data suggest that thermal injury to rats results in dramatic alterations in glucose utilization by the skin and wound, changes that may contribute to the overall alterations in carbohydrate metabolism during burn trauma.