Introduction In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated. Methods p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [18F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(−) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent. Results The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [18F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(−) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h. Conclusions We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [18F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa. Advances in Knowledge The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [18F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

Reactions of Re V , tetradentate Schiff base complexes with tertiary phosphines have previously yielded both rearranged Re V and reduced Re III complexes. To further understand this chemistry, the rigid diiminediphenol (N 2 O 2 ) Schiff base ligand sal 2 phen (N,N'-ophenylenebis(salicylaldimine)) was reacted with (n-Bu 4 N) [ReOCl 4 ] to yield trans-[ReOCl(sal 2 phen)] (1). On reaction with triphenylphosphine (PPh 3 ), a rearranged Re V product cis-[ReO(PPh 3 )(sal 2 phen*)]PF 6 (2), in which one of the imines was reduced to an amine during the reaction, and the reduced Re III products trans-[ReCl(PPh 3 )(sal 2 phen)] (4) and trans-[Re(PPh 3 ) 2 (sal 2 phen)] + (5) were isolated. Reaction of sal 2 phen with [ReCl 3 (PPh 3 ) 2 (CH 3 CN)] resulted in the isolation of [ReCl 2 (PPh 3 ) 2 (salphen)] (3). The compounds were characterized using standard spectroscopic methods, elemental analyses and single crystal X-ray crystallography.

The authors estimated human-equivalent internal radiation dose of 124I-MIBG using preclinical imaging data. As a reference, the effective dose estimation showed that 124I-MIBG would deliver less radiation dose than 124I-NaI, a radiotracer already being used in patients with thyroid cancer.

Introduction Development of radiolabeled octreotide analogues is of interest for targeting somatostatin receptor-positive tumors for diagnostic and therapeutic purposes. We are investigating a direct labeling approach for incorporation of a Re ion into octreotide analogues, where the peptide sequences are cyclized via coordination to Re rather than through a disulfide bridge. Methods Various octreotide analogue sequences and coordination systems (e.g., S2N2 and S3N) were synthesized and cyclized with non-radioactive Re. In vitro competitive binding assays with 111In-DOTA-Tyr3-octreotide in AR42J rat pancreatic tumor cells yielded IC50 values as a measure of somatostatin receptor affinity of the Re-cyclized analogues. Three-dimensional structures of Re-cyclized Tyr3-octreotate and its disulfide-bridged analogue were calculated from two-dimensional NMR experiments to visualize the effect of metal cyclization on the analogue’s pharmacophore. Results Only two of the eleven Re-cyclized analogues investigated showed moderate in vitro binding affinity toward somatostatin subtype 2 receptors. Three-dimensional molecular structures of Re- and disulfide-cyclized Tyr3-octreotate were calculated, and both of their pharmacophore turns appear to be very similar with minor differences due to metal coordination to the amide nitrogen of one of the pharmacophore amino acids. Conclusions Various Re-cyclized analogues were developed and analogue 4 had moderate affinity toward somatostatin subtype 2 receptors. In vitro stable studies that are in progress showed stable radiometal-cyclization of octreotide analogues via NS3 and N2S2 coordination forming 5- and 6- membered chelate rings. In vivo biodistribution studies are underway of 99m Tc- cyclized analogue 4.

Purpose [124I]m-iodobenzylguanidine (124I-mIBG) provides a quantitative tool for pretherapy tumor imaging and dosimetry when performed before [131I]m-iodobenzylguanidine (131I-mIBG) targeted radionuclide therapy of neuroblastoma. 124I (T1/2=4.2d) has a comparable half-life to that of 131I (T1/2=8.02d), and can be imaged by PET for accurate quantification of the radiotracer distribution. We estimated expected radiation dose in tumors from 131I-mIBG therapy using 124I-mIBG microPET/CT imaging data in a murine xenograft model of neuroblastoma transduced to express high levels of the human norepinephrine transporter (hNET). Procedures In order to enhance mIBG uptake for in vivo imaging and therapy, NB 1691-luciferase (NB1691) human neuroblastoma cells were engineered to express high levels of hNET protein by lentiviral transduction (NB1691-hNET). Both NB1691 and NB1691-hNET cells were implanted subcutaneously and into renal capsules in athymic mice. 124I-mIBG (4.2–6.5 MBq) was administered intravenously for microPET/CT imaging at 5 time points over 95 hours (0.5, 3–5, 24, 48, and 93–95 h median time points). In vivo biodistribution data in normal organs, tumors, and whole-body were collected from reconstructed PET images corrected for photon attenuation using the CT-based attenuation map. Organ and tumor dosimetry were determined for 124I-mIBG. Dose estimates for 131I-mIBG were made, assuming the same in vivo biodistribution as 124I-mIBG. Results All NB1691-hNET tumors had significant uptake and retention of 124I-mIBG, whereas unmodified NB1691 tumors did not demonstrate quantifiable mIBG uptake in vivo, despite in vitro uptake. 124I-mIBG with microPET/CT provided an accurate 3-dimensional tool for estimating the radiation dose that would be delivered with 131I-mIBG therapy. For example, in our model system, we estimated that the administration of 131I-mIBG in the range of 52.8 – 206 MBq would deliver 20 Gy to tumors. Conclusion The overexpression of hNET was found to be critical for 124I-mIBG uptake and retention in vivo. The quantitative 124I-mIBG PET/CT is a promising new tool to predict tumor radiation doses with 131I-mIBG therapy of neuroblastoma. This methodology may be applied to tumor dosimetry of 131I-mIBG therapy in human subjects using 124I-mIBG pretherapy PET/CT data.

A series of phosphoramidate-based prostate specific membrane antigen (PSMA) inhibitors of increasing lipophilicity were synthesized (4, 5, and 6), and their fluorine-18 analogs were evaluated for use as positron emission tomography (PET) imaging agents for prostate cancer. To gain insight into their modes of binding, they were also cocrystallized with the extracellular domain of PSMA. All analogs exhibited irreversible binding to PSMA with IC50 values ranging from 0.4 to 1.3 nM. In vitro assays showed binding and rapid internalization (80–95%, 2 h) of the radiolabeled ligands in PSMA(+) cells. In vivo distribution demonstrated significant uptake in CWR22Rv1 (PSMA(+)) tumor, with tumor to blood ratios of 25.6:1, 63.6:1, and 69.6:1 for [18F]4, [18F] 5, and [18F]6, respectively, at 2 h postinjection. Installation of aminohexanoic acid (AH) linkers in the phosphoramidate scaffold improved their PSMA binding and inhibition and was critical for achieving suitable in vivo imaging properties, positioning [18F]5 and [18F]6 as favorable candidates for future prostate cancer imaging clinical trials.

Introduction-To probe the interplay between radiotracer stability and somatostatin receptor affinity, Tyr 3 -octreotate and six variations of its peptide sequence, for which the Re-cyclized products were previously reported, were radiolabeled with 99m Tc and investigated for their in vitro stability.Methods-Radiolabeling of the peptides was effected by ligand exchange from 99m Tcglucoheptonate, and the desired products were purified by radio-RP-HPLC. The in vitro stability in phosphate-buffered saline, mouse serum, and cysteine solutions at physiological temperature and pH for all seven 99m Tc-cyclized peptides was determined by radio-RP-HPLC and radio-TLC. Normal CF-1 mouse biodistribution studies were performed for three of the 99m Tc-cyclized peptides.Results-Based on the fully characterized Re-cyclized peptide analogues, four 99m Tccoordination motifs were proposed for the 99m Tc-cyclized peptides. Technetium-99m-cyclized Tyr 3 -octreotate derivatives with N 2 S 2 metal coordination modes and large metal ring sizes were susceptible to oxidation and loss of 99m Tc in the form of 99m TcO 4 − , as evidenced by their instability in the various solutions under physiological conditions (15-58% intact at 24 h). As anticipated, the addition of a third cysteine to the sequence stabilized the 99m Tc metal coordination, and peptides with NS 3 coordination modes remained >85% intact out to 24 h. No significant differences were observed in the biodistribution studies performed with three peptides of varying stabilities.Conclusions-Improvements in stability were not sufficient to outweigh the low somatostatin receptor affinity for the peptides in this study. Further improvements in the peptide sequence and/ or metal coordination are needed to result in a radiodiagnostic/radiotherapeutic pair for targeting the somatostatin receptor.

Objective: The goal of the study was to investigate whether or not gram-negative organisms that secrete antichemotactic factors cause the nonaccumulation pattern of ¹¹¹In-oxine-labeled white blood cell (¹¹¹In-WBC) scans. Materials and Methods:Staphylococcus aureus (gram-positive) (group 1) was injected into 25 rabbits and Escherichia coli (gram-negative) (group 2) into another 25 to induce infection in the lumbar vertebrae or left thigh bone (femur). Sixteen successfully infected and surviving rabbits from each group were used for imaging and analysis. Of the 16 rabbits, each group included 8 with vertebral infection and 8 with femur infection. For imaging, each rabbit was injected intravenously with 11.1 MBq (300 μCi) ¹¹¹In-WBC, and images were acquired 24 h later. Microscopic histopathology was performed after decalcification to confirm osteomyelitis. Results: The ¹¹¹In-WBC accumulation was observed in 7 (87.5%) of the 8 rabbits infected with S. aureus in the vertebrae and thigh bone. Of the rabbits infected with the gram-negative vertebrae, 1 (12.5%) showed little accumulation of ¹¹¹In-WBC. Of the 8 rabbits with gram-negative-infected femurs, 1 had high accumulation and another had low accumulation of ¹¹¹In-WBC, while the rest did not show any uptake. Osteomyelitis was confirmed by histopathology in all the successfully infected rabbits used for imaging. Conclusion: In the majority of the gram-positive-infected rabbit vertebrae there was high accumulation of ¹¹¹In-WBC. However, no accumulation of ¹¹¹In-WBC was observed in most of the vertebrae infected with gram-negative organisms, which release antichemotactic factors that prevent adequate accumulation of WBC at the infected area.