Marc Jennewein

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine.

Purpose: We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats. Experimental Design: Bavituximab was labeled with 74As (β+, T1/2 17.8 days) or 77As (β−, T1/2 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats. Results: Clear images of the tumors were obtained using planar γ-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium. Conclusions: These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

A no-carrier-added 72Se/72As radionuclide generator based on distillation

Abstract Arsenic-72 is a positron emitting isotope with promising properties for syntheses of 72As-labelled radiopharmaceuticals for future application in positron emission tomography. This work describes the radiochemical separation of no-carrier-added 72Se from cyclotron irradiated germanium targets and the development of a 72Se/72As radionuclide generator, avoiding the addition of any selenium carrier. Using a vertical quartz tube device, no-carrier-added 72As is nearly quantitatively released from various chloride salt solutions containing 72Se within 10 min at a temperature of 100 °C in an HCl gas flow. The kinetics of the 72Se/72As isotope generator has been studied in relation to temperature, salt charge, and redox-stability. Under optimised conditions, 72Se remains almost quantitatively (>99.7%) in solution.

A no-carrier-added 72Se/72As radionuclide generator based on solid phase extraction

Summary 72As-labelled radiopharmaceuticals could be a valuable resource for Positron Emission Tomography (PET). In particular, the long half-life of 72As (T1/2 = 26 h) facilitates the observation of long-term physiological or metabolic processes, such as the enrichment and distribution of antibodies in tumor tissue. This work describes the primary radiochemical separation of no-carrier-added (nca) 72Se from cyclotron irradiated germanium targets and the development of a polystyrene type solid-phase extraction based 72Se/72As radionuclide generator, avoiding the addition of any selenium carrier. The irradiated germanium target is dissolved in HFconc and selenium is reduced with hydrazine dihydrochloride. The nca 72Se(0) is adsorbed on a solid-phase extraction cartridge, representing the generator column. The 72As is eluted using various aqueous solvents with 40–60% yield and < 0.1% 72Se content. To be able to study the radiopharmaceutical arsenic chemistry, subsequent chemical modification of the nca 72As eluates to nca [72As]AsI3 provides a versatile radioarsenic labelling synthon.

Separation and purification of no-carrier-added arsenic from bulk amounts of germanium for use in radiopharmaceutical labelling

Abstract Radioarsenic labelled radiopharmaceuticals could add special features to molecular imaging with positron emission tomography (PET). For example the long physical half-lives of 72As (T1/2=26 h) and 74As (T1/2=17.8 d) in conjunction with their high positron branching rates of 88% and 29%, respectively, allow the investigation of slow physiological or metabolical processes, like the enrichment and biodistribution of monoclonal antibodies in tumour tissue or the characterization of stem cell trafficking. A method for separation and purification of no-carrier-added (nca) arsenic from irradiated metallic germanium targets based on distillation and anion exchange is developed. It finally converts the arsenic into an *As(III) synthon in PBS buffer and pH 7 suitable for labelling of proteins via As-S bond formations. The method delivers radioarsenic in high purity with separation factors of 106 from germanium and an overall yield from target to labelling synthon of >40%. In a proof-of-principle experiment, the monoclonal antibody Bevacizumab, directed against the human VEGF receptor, was labelled with a radiochemical yield >90% within 1 h at room temperature with nca 72/74/77As.

UTSW Small Animal Positron Emission Imager

A Small Animal Imager (SAI) for PET has been designed, built, tested in phantoms, and applied to investigations in mice and rats. The device uses principles based on gamma-ray induced scintillation in crossed fiber optic detectors connected to Position Sensitive Photomultiplier Tubes (PSPMT). Each detector consists of an epoxied stack of 28 layers of 135 round 1 mm BCF-10 scintillating plastic fibers. The overlap region forms a 13.5times13.5times2.8 cm3 detector volume. Scintillating light from the fibers is detected by two (X and Y directions) Hamamatsu R-2486 PSPMTs with 16 anode wires in each of two orthogonal directions. A centroid-finding algorithm gives the position of a light cluster on the face (photocathode) of a PSPMT. The accuracy of the reconstruction of an interaction position is essentially independent of light cluster position. This translates to a nearly isotropic photon response for the entire detector. The system has been used to test several 3D image reconstruction algorithms, software modifications, and improvements. The sensitivity (~12.6 cps/kBq at 9 cm inner diameter) and sub-millimeter spatial resolution (better than 1 mm in phantoms) obtained with an iterative algorithm incorporating system modeling make the SAI a relatively inexpensive high performance animal imager. The SAI is currently being used for imaging experiments in mice and rats

Visualisation of a somatostatin receptor-expressing tumour with 67Ga-DOTATOC SPECT.

In comparison to In-DTPAOC (Octreoscan), galliumlabelled DOTATOC shows better binding affinity to human somatostatin receptor subtype 2 and improved pharmacology in vivo [1, 2]. Especially if Ga-DOTATOC and PET/ CT are applied, somatostatin receptor-expressing tumour tissue is excellently visualised. However, SPECT is still a more widely available imaging method. Here we present the first visualisation of a human somatostatin receptorexpressing tumour with Ga-DOTATOC SPECT/CT. A 65-year-old man with known mesenteric lymph node metastases of a surgically removed carcinoid in the small bowel received 180 MBq of In-DTPAOC and, 1 week later, 230 MBq of Ga-DOTATOC (50 MBq/μg specific activity). All metastases detected with In-DTPAOC (a) could be visualised with Ga-DOTATOC as well. Scans of Ga-DOTATOC (SPECT/CT) were performed less than 4 h post injection to generate excellent images of the mesenteric manifestations (b), with a higher tumour to background ratio compared to that on In-DTPAOC images. The presence of only faint renal Ga-DOTATOC uptake constitutes a further favourable characteristic of this radiolabelled peptide. a

Projection and Pinhole-Based Data Acquisition for Small-Animal SPECT Using Storage Phosphor Technology

Three-dimensional Single Photon Emission Computed Tomography (SPECT) can provide high-resolution insight into biomolecular distribution and pharmacokinetics. However, instrument availability and distribution is limited at present,and imaging times can be considerable. To evaluate the large array of novel agents which are becoming available,we find that storage phosphor-based in vivo imaging can provide an important,rapid-throughput transition from the traditional ex vivo sacrifice/gamma counting and autoradiography to full-time course SPECT.