Currently, receptor based radiopharmaceuticals have received great attention in molecular imaging and radiotherapy of cancer, and provide a unique tool for target-specific delivery of radionuclides to pathological tissues. In this context, receptor binding peptides represent an attractive class of target vectors for Nuclear Medicine purposes. The rich chemistry of the group 7 elements elaborated in past years, has allowed the development of different procedures for the preparation of radiolabeled peptides in high yield. This, joint to the use of solid-phase peptide synthesis, has opened the possibility to explore new strategies for approaching the design of new class of radiolabeled receptor-targeted peptides, and to create new versatilities in targeting vehicle design e.g. in synthesis of metal-cyclized peptides or of multivalent targeting agents. This review provides an overview on several aspects of the development of new (99m)Tc/(188)Re-peptide based target specific radiopharmaceuticals, in particular on the synthetic strategies employed for modifying molecular vectors, and the application of the different metal-cores and/or building block for preparing high specific activity agents.

(99m)Tc(N)-DBODC5 is a cationic mixed compound under clinical investigation as potential myocardial imaging agent. In spite of this, analogously to the other cationic (99m)Tc-agents, presents a relatively low first-pass extraction. Thus, modification of (99m)Tc(N)-DBODC(5) direct to increase its first-pass extraction keeping unaltered the favorable imaging properties would be desirable. This work describes the synthesis and biological evaluation of a series of novel cationic (99m)Tc-nitrido complexes, of general formula [(99m)TcN(DTC-Ln)(PNP)](+) (DTC-Ln= alicyclic dithiocarbamates; PNP = diphosphinoamine), as potential radiotracers for myocardial perfusion imaging. The synthesis of cationic (99m)Tc-(N)-complexes were accomplished in two steps. Biodistribution studies were performed in rats and compared with the distribution profiles of (99m)Tc(N)-DBODC5 and (99m)Tc-Sestamibi. The metabolisms of the most promising compounds were evaluated by HPLC methods. Biological studies revealed that most of the complexes have a high initial and persistent heart uptake with rapid clearance from nontarget tissues. Among tested compounds, 2 and 12 showed improved heart uptake with respect to the gold standard (99m)Tc-complexes with favorable heart-to-liver and slightly lower heart-to-lung ratios. Chromatographic profiles of (99m)Tc(N)-radioactivity extracted from tissues and fluids were coincident with the native compound evidencing remarkable in vivo stability of these agents. This study shows that the incorporation of alicyclic dithiocarbamate in the [(99m)Tc(N)(PNP)](+) building block yields to a significant increase of the heart uptake at early injection point suggesting that the first-pass extraction fraction of these novel complexes may be increased with respect to the other cationic (99m)Tc-agents keeping almost unaltered the favorable target/nontarget ratios.

The synthesis and characterization of a new series of neutral, six-coordinated mixed-ligand compounds [M(III)(PS)2(L)] (M = Re; (99)Tc), where PS is bis(arylalkyl)- or trialkylphosphinothiolate and L is dithiocarbamate, are reported. Stable [M(III)(PS)2(L)] complexes were easily synthesized, in good yield, starting from precursors where the metal was in different oxidation states (III, V, and VII), involving ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, positive-ion electrospray ionization mass spectrometry, multinuclear NMR spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. All complexes are constituted by the presence of the [M(III)(PS)2](+) moiety, where two phosphinothiolate ligands are tightly bound to the metal and the remaining two positions are saturated by a dithiocarbamate chelate, also carrying bulky bioactive molecules [e.g., (2-methoxyphenyl)piperazine]. X-ray analyses were performed on crystalline specimens of four different Re/(99)Tc compounds sharing a distorted trigonal-prismatic geometry, with a P2S4 coordination donor set. The possibility of easily preparing these [M(III)(PS)2(L)] complexes, starting from the corresponding permetalate anions, in mild reaction conditions and in high yield, lays the first stone to the preparation of a new series of M(III)-based (M = (99m)Tc/(188)Re) compounds potentially useful in theragnostic applications.

The clinical translation of theranostic 177 Lu-radiopharmaceuticals based on inhibitors of the prostatespecific membrane antigen (PSMA) has demonstrated positive clinical responses in patients with advanced prostate cancer (PCa). However, challenges still remain, particularly regarding their pharmacokinetic and dosimetric properties. We developed a potential PSMA-immunotheranostic agent by conjugation of a single-chain variable fragment of the IgGD2B antibody (scFvD2B) to DOTA, to obtain a 177 Lu-labelled agent with a better pharmacokinetic profile than those previously reported. The labelled conjugated 177 Lu-scFvD2B was obtained in high yield and stability. In vitro, 177 Lu-scFvD2B disclosed a higher binding and internalization in LNCaP (PSMA-positive) compared to PC3 (negative control) human PCa cells. In vivo studies in healthy nude mice revealed that 177 Lu-scFvD2B present a favorable biokinetic profile, characterized by a rapid clearance from non-target tissues and minimal liver accumulation, but a slow wash-out from kidneys. Micro-SPECT/CT imaging of mice bearing pulmonary microtumors evidenced a slow uptake by LNCaP tumors, which steadily rose up to a maximum value of 3.6 SUV at 192 h. This high and prolonged tumor uptake suggests that 177 Lu-scFvD2B has great potential in delivering ablative radiation doses to PSMA-expressing tumors, and warrants further studies to evaluate its preclinical therapeutic efficacy. Background. Prostate cancer (PCa) is the second leading cause of cancer deaths for adult men in the Western world. Although radical prostatectomy and local radiotherapy are largely successful for patients with localized cancer, available treatments for metastatic PCa have demonstrated weak curative efficacy 1. Consequently, new tools to improve the detection of recurrent PCa, and particularly to identify and treat distant metastases, are imperatively needed. The prostate-specific membrane antigen (PSMA) is one of the most promising targets for the development of PCa theranostic agents. PSMA is overexpressed in 95% of prostate cancers and its expression levels progressively increase in high-grade tumors and in metastatic disease, up to 1,000 times more than in normal cells 2. Among the several PSMA-targeting molecules that have been developed, the radiolabeled Glu-ureido-based PSMA inhibitors are gaining much interest due to their high uptake by PSMA-positive cancer cells, and low background and excellent contrast in cancer imaging, even in small metastases 3,4. Theranostic agents such as 177 Lu-PSMA-617,