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.