Synthesis, characterization and in vitro cytotoxicity of gallium(III)-dithiocarbamate complexes.

A library of homoleptic mononuclear Ga(III) complexes of the general formula [Ga(DTC)3], where DTC is an alicyclic or a linear dithiocarbamate chelator, is reported. The complexes were prepared in high yields starting from Ga(NO3)3·6H2O and fully characterized by elemental analysis and IR and NMR spectroscopy. Crystals of five of these complexes were obtained. The antitumor activity of the newly synthesized compounds against a panel of human cancer cell lines was evaluated. The chemical nature of the DTC does not have a marked impact on the structural features of the final compound. X-ray crystal structure analyses revealed that all these complexes have a trigonal prismatic geometry with three identical chelating DTCs coordinating the Ga(III) ion. It is noteworthy that in complex 22, [Ga(NHEt)3] (NHEt = N-ethyldithiocarbamate), the asymmetric unit is formed by two independent and structurally different molecules. Cellular studies showed that all the synthesized Ga-DTC complexes exhibit marked cytotoxic activity, even against human colon cancer cells that are less sensitive to cisplatin. Among the tested compounds, 6 ([Ga(CEPipDTC)3], CEPipDTC = (ethoxycarbonyl)-piperidinedithiocarbamate) and 21 ([Ga(Pr-13)3], PR13 = 4 and N-(2-ethoxy-2-oxoethyl)-N-methyldithiocarbamate) are very promising derivatives, but they have no selectivity towards cancer cells. Nevertheless, the obtained data provide a foundation for developing gallium-dithiocarbamate complexes as anticancer agents.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer.

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., 68Ga, 64Cu, 177Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, 99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on 99mTc outscores that of PET agents and 99mTc-PSMA SPECT/CT may be a cost-effective alternative for 68Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [99mTc]Tc-tagged PSMA agents with particular attention to [99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [68Ga]Ga-PSMA11.

Impact of Different [Tc(N)PNP]-Scaffolds on the Biological Properties of the Small cRGDfK Peptide: Synthesis, In Vitro and In Vivo Evaluations.

Background: The [99mTc][Tc(N)(PNP)] system, where PNP is a bisphosphinoamine, is an interesting platform for the development of tumor 'receptor-specific' agents. Here, we compared the reactivity and impact of three [Tc(N)(PNP)] frameworks on the stability, receptor targeting properties, biodistribution, and metabolism of the corresponding [99mTc][Tc(N)(PNP)]-tagged cRGDfK peptide to determine the best performing agent and to select the framework useful for the preparation of [99mTc][Tc(N)(PNP)]-housing molecular targeting agents. Methods: cRGDfK pentapeptide was conjugated to Cys and labeled with each [Tc(N)(PNP)] framework. Radioconjugates were assessed for their lipophilicity, stability, in vitro and in vivo targeting properties, and performance. Results: All compounds were equally synthetically accessible and easy to purify (RCY ≥ 95%). The main influences of the synthon on the targeting peptide were observed in in vitro cell binding and in vivo. Conclusions: The variation in the substituents on the phosphorus atoms of the PNP enables a fine tuning of the biological features of the radioconjugates. ws[99mTc][Tc(N)(PNP3OH)]- and [99mTc][Tc(N)(PNP3)]- are better performing synthons in terms of labeling efficiency and in vivo performance than the [99mTc][Tc(N)(PNP43)] framework and are therefore more suitable for further radiopharmaceutical purposes. Furthermore, the good labeling properties of the ws[99mTc][Tc(N)(PNP3OH)]- framework can be exploited to extend this technology to the labeling of temperature-sensitive biomolecules suitable for SPECT imaging.

Water-Soluble [Tc(N)(PNP)] Moiety for Room-Temperature 99mTc Labeling of Sensitive Target Vectors.

The incorporation of bioactive molecules into a water-soluble [99mTc][Tc(N)(PNP)]-based mixed compound is described. The method, which exploits the chemical properties of the new [99mTc][Tc(N)(PNP3OH)]2+ synthon [PNP3OH = N,N-bis(di-hydroxymethylenphosphinoethyl)methoxyethylamine], was successfully applied to the labeling of small, medium (cysteine-functionalized biotin and c-RGDfK pentapeptide), and large molecules. Apomyoglobin was chosen as a model protein and derivatized via site-specific enzymatic reaction catalyzed by transglutaminase (TGase) with the H-Cys-Gly-Lys-Gly-OH tetrapeptide for the insertion in the protein sequence of a reactive N-terminal Cys for 99mTc chelation. Radiosyntheses were performed under physiological conditions at room temperature within 30 min. They were reproducible, highly specific, and quantitative. Heteroleptic complexes are hydrophilic and stable. Biodistributions of the selected compounds show favorable pharmacokinetics within 60 min post-injection and predominant elimination through the renal-urinary pathway. In a wider perspective, these data suggest a role of the [99mTc][Tc(N)(PNP)] technology in the labeling of temperature-sensitive biomolecules, especially targeting proteins for SPECT imaging.

Development of 177Lu-scFvD2B as a Potential Immunotheranostic Agent for Tumors Overexpressing the Prostate Specific Membrane Antigen.

The clinical translation of theranostic 177Lu-radiopharmaceuticals based on inhibitors of the prostate-specific 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 177Lu-labelled agent with a better pharmacokinetic profile than those previously reported. The labelled conjugated 177Lu-scFvD2B was obtained in high yield and stability. In vitro, 177Lu-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 177Lu-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 177Lu-scFvD2B has great potential in delivering ablative radiation doses to PSMA-expressing tumors, and warrants further studies to evaluate its preclinical therapeutic efficacy.

Improved synthesis on solid phase of dithiocarbamic cRGD-derivative and 99m Tc-radiolabelling.

In the field of angiogenesis, small cyclic pentapeptides containing the RGD motif are playing a relevant role for their high affinity and specificity for integrin receptors and for the possibility to act at both therapeutic and diagnostic level by inhibiting pathological angiogenesis and by serving as shuttles to deliver imaging-probe including SPECT/PET radionuclides to specific tissues. In the last decade, several new protocols were reported in literature for the direct synthesis of cyclic RDG either in solution or by SPPS. Here, we have elaborated and tested some alternative approaches using different resins and different protective groups. The introduction of the dithiocarbamate function, useful to complex radio-metals suitable for nuclear medicine applications, has also been considered and achieved.

[99mTc][Tc(N)(DASD)(PNP n)]+ (DASD = 1,4-Dioxa-8-azaspiro[4,5]decandithiocarbamate, PNP n = Bisphosphinoamine) for Myocardial Imaging: Synthesis, Pharmacological and Pharmacokinetic Studies.

[99mTc][TcN-DBODC(5) is the lead candidate of a class of cationic complexes proposed as myocardial imaging agents (MPIAs). Phase I clinical studies showed that its clinical properties were comparable to those of the commercially available agents. Thus, modification of [99mTc]TcN-DBODC(5), directed to obtain an ideal myocardial imaging without interference from the adjacent organ activities, is desirable. This work describes the pharmacological and pharmacokinetic development of four new complexes of general formula [99mTc][Tc(N)(DASD)(PNP n)]+, [99mTc]TcN-DASD( n) (DASD = 1,4-dioxa-8-azaspiro[4,5]decandithiocarbamate; PNP n = bisphosphinoamine), proposed as improved MPIAs. Among the tested compounds, [99mTc]TcN-DASD(5) and [99mTc]TcN-DASD(7) showed enhanced heart uptake compared with the gold standards, with a rapid liver washout and superior heart-to-liver ratio. These features might shorten the duration of imaging procedures below 30 min, consenting the early acquisition of high-quality images. In addition, mechanistic studies were performed in cellulo by using human drug-sensitive and drug-resistant cancer cell lines, obtaining results which might be conveniently applied to tumor imaging.

[99mTc][Tc(N)PNP43]-Labeled RGD Peptides As New Probes for a Selective Detection of αvß3 Integrin: Synthesis, Structure-Activity and Pharmacokinetic Studies.

New integrin-selective molecules suitable for therapeutic or imaging purposes are currently of interest in development of effective personalized medical platforms. RGDechi is a bifunctional peptide selective for integrin αvß3. Herein, RGDechi and three truncated derivatives functionalized with a cysteine (1-4) were synthesized and labeled with the [99mTc][Tc(N)PNP43]-synthon ([PNP43 = (CH3)2P(CH2)2N(C2H4OCH3)(CH2)2P(CH3)2]) (99mTc1-4) as a basis for selective integrin recognition. The pharmacological parameters of all radiolabeled peptides were assessed along with the pharmacokinetic profiles of the most promising 99mTc1 and 99mTc2 compounds both on healthy and melanoma-bearing mice. Their metabolism and metabolite identification are also reported. 99mTc1-2 are able to discriminate between endogenously expressed integrins αvß3 and αvß5 and possess favorable pharmacokinetics characterized by low liver uptake and rapid elimination from nontarget tissues resulting in positive target-to-nontarget ratios. Results are encouraging; the presented construct can be considered the starting point for the development of agents for the selective detection of αvß3 expression by SPECT.

Synthesis, Characterization, and Initial Biological Evaluation of [99m Tc]Tc-Tricarbonyl-labeled DPA-α-MSH Peptide Derivatives for Potential Melanoma Imaging.

α-Melanocyte stimulating hormone (α-MSH) derivatives target the melanocortin-1 receptor (MC1R) specifically and selectively. In this study, the α-MSH-derived peptide NAP-NS1 (Nle-Asp-His-d-Phe-Arg-Trp-Gly-NH2 ) with and without linkers was conjugated with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (DPA-COOH) and labeled with [99m Tc]Tc-tricarbonyl by two methods. With the one-pot method the labeling was faster than with the two-pot method, while obtaining similarly high yields. Negligible trans-chelation and high stability in physiological solutions was determined for the [99m Tc]Tc-tricarbonyl-peptide conjugates. Coupling an ethylene glycol (EG)-based linker increased the hydrophilicity. The peptide derivatives displayed high binding affinity in murine B16F10 melanoma cells as well as in human MeWo and TXM13 melanoma cell homogenates. Preliminary in vivo studies with one of the [99m Tc]Tc-tricarbonyl-peptide conjugates showed good stability in blood and both renal and hepatobiliary excretion. Biodistribution was performed on healthy rats to gain initial insight into the potential relevance of the 99m Tc-labeled peptides for in vivo imaging.

Transglutaminase-mediated conjugation and nitride-technetium-99m labelling of a bis(thiosemicarbazone) bifunctional chelator.

An assessment study involving the use of the transglutaminase (TGase) conjugation method and the nitride-technetium-99m labelling on a bis(thiosemicarbazone) (BTS) bifunctional chelating agent is presented. The previously described chelator diacetyl-2-(N4-methyl-3-thiosemicarbazone)-3-(N4-amino-3-thiosemicarbazone), H2ATSM/A, has been functionalized with 6-aminohexanoic acid (ε-Ahx) to generate the bifunctional chelating agent diacetyl-2-(N4-methyl-3-thiosemicarbazone)-3-[N4-(amino)-(6-aminohexanoic acid)-3-thiosemicarbazone], H2ATSM/A-ε-Ahx (1), suitable for conjugation to glutamine (Gln) residues of bioactive molecules via TGase. The feasibility of the TGase reaction in the synthesis of a bioconjugate derivative was investigated using Substance P (SP) as model peptide. Compounds 1 and H2ATSM/A-ε-Ahx-SP (2) were labelled with nitride-technetium-99m, obtaining the complexes [99mTc][Tc(N)(ATSM/A-ε-Ahx)] (99mTc1) and [99mTc][Tc(N)(ATSM/A-ε-Ahx-SP)] (99mTc2). The chemical identity of 99mTc1 and 99mTc2 was confirmed by radio/UV-RP-HPLC combined with ESI-MS analysis on the respective carrier-added products 99g/99mTc1 and 99g/99mTc2. The stability of the radiolabelled complexes after incubation in various environments was investigated. All the results were compared with those obtained for the corresponding 64Cu-analogues, 64Cu1 and 64Cu2. The TGase reaction allows the conjugation of 1 with the peptide, but it is not highly efficient due to instability of the chelator in the required conditions. The SP-conjugated complexes are unstable in mouse and human sera. However, indeed the BTS system can be exploited as nitride-technetium-99m chelator for highly efficient technetium labelling, thus making compound 1 worthy of further investigations for new targeted technetium and copper radiopharmaceuticals encompassing Single Photon Emission Computed Tomography and Positron Emission Tomography imaging.

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands.

The synthesis and characterization of a new series of neutral, six-coordinated compounds [ReIIIX3(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph2PCH2CH2)2NR (R = H, CH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3 and CH2CH2OCH3) are reported. Stable [ReIIIX3(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl3(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr3(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Melanoma targeting with [99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analogs: Effects of cyclization on the radiopharmaceutical properties.

The purpose of this study was to evaluate the effect of cyclization on the biological profile of a [99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analog. A lactam bridge-cyclized H-Cys-Ahx-ßAla3-c[Lys4-Glu-His-D-Phe-Arg-Trp-Glu10]-Arg11-Pro-Val-NH2 (NAP-NS2) and the corresponding linear H-Cys-Ahx-ßAla-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) peptide were synthetized, characterized by ESI-MS spectroscopy and their melanocortin-1 receptor (MC1R) binding affinity was determined in B16/F10 melanoma cells. The consistent [99mTc(N)(PNP3)]-labeled compounds were readily obtained in high specific activity and their stability and biological properties were assessed. As an example, the chemical identity of [99mTc(N)(NAP-NS1)(PNP3)]+ was confirmed by carrier added experiments supported by radio/UV HPLC analysis combined with ESI(+)-MS. Compared with the linear peptide, cyclization negatively affected the biological properties of NAP-NS2 peptide by reducing its binding affinity for MC1R and by decreasing the overall excretion rate of the corresponding [99mTc(N)(PNP3)]-labeled peptide from the body as well as its in vivo stability. [99mTc(N)(NAP-NS1)(PNP3)]+ was evaluated for its potential as melanoma imaging probe in murine melanoma model. Data from in vitro and in vivo studies on B16/F10 melanoma model of [99mTc(N)(NAP-NS1)(PNP3)]+ clearly evidenced that the radiolabeled linear peptide keeps its biological properties up on the conjugation to the [99mTc(N)(PNP3)]-building block. The progressive increase of the tumor-to-nontarget ratios over the time indicates a quite stable interaction between the radio-complex and the MC1R.

Folic acid-conjugated europium complexes as luminescent probes for selective targeting of cancer cells.

We report the synthesis of three optical probes (Eu(3+)⊂1, Eu(3+)⊂2, and Eu(3+)⊂3) having a luminescent Eu complex (signaling unit) bonded in different positions to folic acid (FA), the folate receptor (FR) targeting unit. The structures of the two regioisomers Eu(3+)⊂1 and Eu(3+)⊂2 were assigned by mass spectrometric experiments. The optical properties and stability of these probes were assessed in phosphate-buffered saline, cell culture medium, rat serum, and cellular lysate, and results indicated that they are chemically and photophysically stable. Cytotoxicity was studied with ovarian cancer cells having high (SKOV-3), intermediate (OVCAR-3), low (IGROV-1), or null (A2780) expression of FRs. The internalized probe, evaluated in SKOV-3, IGROV-1, and A2780 cells, was in the order Eu(3+)⊂2 > Eu(3+)⊂1 > Eu(3+)⊂3. No internalization was observed for A2780 cells. Such results, together with those obtained in competition experiments of FA versus Eu(3+)⊂2 and FA or Eu(3+)⊂2 versus (3)H-FA, indicate that internalization is receptor-mediated and that Eu(3+)⊂2 shows high selectivity and specificity for FR.

Reactivity of the [M(PS)2](+) building block (M = Re(III) and (99m)Tc(III); PS = phosphinothiolate) toward isopropylxanthate and pyridine-2-thiolate.

The coordination properties of isopropylxanthate (i-Pr-Tiox) and pyridine-2-thiolate (PyS) toward the [M(PS)2](+) moiety (M = Re and (99m)Tc; PS = phosphinothiolate) were investigated. Synthesis and full characterization of [Re(PS2)2(i-Pr-Tiox)] (Re1), [Re(PSiso)2(i-Pr-Tiox)] (Re2), [Re(PS2)2(PyS)] (Re3), and [Re(PSiso)2(PyS)] (Re4), where PS2 = 2-(diphenylphosphino)ethanethiolate and PSiso = 2-(diisopropylphosphino)ethanethiolate, and the structural X-ray analysis of complex Re3 were carried out. (99m)Tc analogues of complexes Re2 ((99m)Tc2) and Re4 ((99m)Tc4) were obtained in high radiochemical yield following a simple one-pot procedure. The chemical identity of the radiolabeled compounds was confirmed by chromatographic comparison with the corresponding rhenium complexes and by dual radio/UV HPLC analysis combined with ESI(+)-MS of (99g/99m)Tc complexes prepared in carrier-added conditions. The two radiolabeled complexes were stable with regard to trans chelation with cysteine, glutathione, and ethylenediaminotetraacetic acid and in rat and human sera. This study highlights the substitution-inert metal-fragment behavior of the [M(PS)2](+) framework, which reacts with suitable bidentate coligands to form stable hexacoordinated asymmetrical complexes. This feature makes it a promising platform on which to develop a new class of Re/Tc complexes that are potentially useful in radiopharmaceutical applications.

Novel [99mTcIII(PS)2(Ln)] mixed-ligand compounds (PS = phosphino-thiolate; L = dithiocarbamate) useful in design and development of TcIII-based agents: synthesis, in vitro, and ex vivo biodistribution studies.

A general procedure for the preparation of a new class of neutral six-coordinated mixed ligand [(99m)Tc(III)(PS)2(Ln)] compounds (PS = trisalkyl-phosphino-thiolate; Ln = dithiocarbamate) is reported as well as their in vitro stability and the ex vivo tissue distribution studies. [(99m)Tc(PS)2(Ln)] complexes were prepared in high yield in nearly physiologic conditions following a one-pot procedure. For instance, the chemical identity of [(99m)Tc(PSiso)2(L1)] (PSiso = 2-(diisopropylphosphino)ethanethiol; L1 = pyrrolidine dithiocarbamate) was determined by HPLC comparison with the corresponding (99g)Tc-complex. All complexes comprise the stable [(99m)Tc(III)(PS)2](+) moiety, where the remaining two coordination positions are saturated by a dithiocarbamate chelate, also carrying bioactive molecules (e.g., 2-methoxyphenylpiperazine). [(99m)Tc(PS)2(Ln)] complexes were inert toward ligand exchange reactions. No significant in vitro and in vivo biotransformation were observed, underlining their remarkable thermodynamic stability and kinetic inertness. These results could be conveniently utilized to devise a novel class of (99m)Tc(III)-based compounds useful in radiopharmaceutical applications.

Assessment of the best N(3-) donors in preparation of [M(N)(PNP)]-based (M=(99m)Tc-; (188)Re) target-specific radiopharmaceuticals: Comparison among succinic dihydrazide (SDH), N-methyl-S-methyl dithiocarbazate (HDTCZ) and PEGylated N-methyl-S-methyl dithiocarbazate (HO2C-PEG600-DTCZ).

Succinic dihydrazide (SDH), N-methyl-S-methyl dithiocarbazate (HDTCZ) and PEGylated N-methyl-S-methyl dithiocarbazate (HO2C-PEG600-DTCZ) are nitrido nitrogen atom donors employed for the preparation of nitride [M(N)]-complexes (M=(99m)Tc and (188)Re). This study aims to compare the capability and the efficiency of these three N(3-) group donors, in the preparation of [M(N)PNP]-based target-specific compounds (M=(99m)Tc, (188)Re; PNP=aminodiphosphine). For this purpose, three different kit formulations (SDH kit; HO2C-PEG600-DTCZ kit; HDTCZ kit) were assembled and used in the preparation of [M(N)(cys~)(PNP3)](0/+) complexes (cys~=cysteine derivate ligands). For each formulation, the radiochemical yield (RCY) of the [M(N)(~cys)(PNP3)] compounds, was determined by HPLC. The deviation of the percentage of RCY, due to changes in concentration of the N(3-) donors and of the exchanging ligand, was determined. For (99m)Tc, data clearly show that HDTCZ is the most efficient donor of N(3-); however, SDH is the most suitable nitrido nitrogen atom donor for the preparation of [(99m)Tc(N)(PNP)]-based target-specific agents with high specific activity. When HO2C-PEG600-DTCZ or HDTCZ are used in N(3-) donation, high amounts of the exchanging ligand (10(-4)M) were required for the formation of the final complex in acceptable yield. The possibility to use microgram amounts of HDTCZ also in [(188)Re(N)] preparation (0.050mg) reduces its ability to compete in ligand exchange reactions, minimizing the quantity of chelators required to obtain the final complex in high yield. This finding can be exploit for increasing the radiolabeling efficiency in [(188)Re(N)]-radiopharmaceutical preparations compared to the previously reported HDTCZ-based procedure, notwithstanding a purification process could be necessary to improve the specific activity of the complexes.

(99m)Tc(N)-DBODC(5), a potential radiolabeled probe for SPECT of multidrug resistance: in vitro study.

[(99m)Tc(N)(DBODC)(PNP5)](+) [DBODC is bis(N-ethoxyethyl)dithiocarbamato; PNP5 is bis(dimethoxypropylphosphinoethyl)ethoxyethylamine], abbreviated as (99m)Tc(N)-DBODC(5), is a lipophilic cationic mixed compound investigated as a myocardial imaging agent. The findings that this tracer accumulates in mitochondrial structures through a mechanism mediated by the negative mitochondrial membrane potential and that the rapid efflux of (99m)Tc(N)-DBODC(5) from nontarget tissues seems to be associated with the multidrug resistance (MDR) P-glycoprotein (P-gp) transport function open up the possibility to extend its clinical applications to tumor imaging and noninvasive MDR studies. The rate of uptake at 4 and 37 °C of (99m)Tc(N)-DBODC(5) was evaluated in vitro in selected human cancer cell lines and in the corresponding sublines before and after P-gp and/or MDR-associated protein (MRP) modulator/inhibitor treatment using (99m)Tc-sestamibi as a reference. The results indicated that (1) the uptake of both (99m)Tc(N)-DBODC(5) and (99m)Tc-sestamibi is correlated to metabolic activity of the cells and (2) the cellular accumulation is connected to the level of P-gp/MRP expression; in fact, an enhancement of uptake in resistant cells was observed after treatment with opportune MDR inhibitor/modulator, indicating that the selective blockade of P-gp/MRP prevented efflux of the tracers. This study provides a preliminary indication of the applicability of (99m)Tc(N)-DBODC(5) in tumor imaging and in detecting P-gp/MRP-mediated drug resistance in human cancer. In addition, the possibility to control the hydrophobicity and pharmacological activity of this heterocomplex through the variation of the substituents on the ligands backbone without affecting the P2S2 coordinating sphere makes (99m)Tc(N)-DBODC(5) a suitable scaffold for the preparation of a molecular probe for single photon emission computed tomography of MDR.

Chelating systems for (99m)Tc/(188)Re in the development of radiolabeled peptide pharmaceuticals.

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.

Synthesis, characterization and biological evaluation of [188Re(N)(cys ∼)(PNP)]+/0 mixed-ligand complexes as prototypes for the development of 188Re(N)-based target-specific radiopharmaceuticals.

We report on an efficient procedure for the preparation of [(188)Re(N)(PNP)]-based complexes (where PNP is diphosphinoamine) useful in the development of target-specific radiopharmaceuticals. The radiochemical yield of the compounds was optimized considering such reaction parameters as nature of the nitrido nitrogen donor, reaction times and pH level. The chemical identity of the (188)Re agents was determined by high-performance liquid chromatography comparison with the corresponding well-characterized cold Re compounds. (188)Re(N) mixed compounds have been evaluated with regard to stability toward transchelation with GSH and degradation by serum enzymes. The clearance of selected radiocompounds from normal tissues and their in vivo stability were evaluated in rats by biodistribution and imaging studies. [(188)Re(N)(cys ∼)(PNP)](+/0) mixed-ligand compounds were efficiently prepared in aqueous solution from perrhenate using a multistep procedure based on the preliminary formation of the labile (188)Re(III)-EDTA species, which easily undergo oxidation/ligand exchange reaction to afford the [(188)Re(V) ≡ N](2+) core in the presence of dithiocarbazate. The final mixed-ligand compounds were obtained, at 100 °C, by adding the two bidentate ligands to the buffered [(188)Re(V) ≡ N](2+) solution (pH 3.2-3.6). However, a relatively high amount of cys ∼ ligand was required to obtain a quantitative radiochemical yield. The complexes were stable toward reoxidation to perrhenate and ligand exchange reactions. In vivo studies showed rapid distribution and elimination of the complexes from the body. No specific uptakes in sensitive tissues/organs were detected. A positive correlation of the distribution of the complexes estimated with biodistribution studies (%ID) and with micro-SPECT semiquantification imaging analysis (standard uptake values) was observed. These results support the possibility of applying [(188)Re(N)(PNP)] technology to the preparation of target-specific agents.

Synthesis and biological evaluation of new [Tc(N)(PS)]-based mixed-ligand compounds useful in the design of target-specific radiopharmaceuticals: the 2-methoxyphenylpiperazine dithiocarbamate derivatives as an example.

This study presents the first application of a general procedure based on the use of the [Tc(N)Cl(PS)(PPh(3))] species (PS is an alkyl phosphinothiolate ligand) for the preparation of Tc(N) target-specific compounds. [Tc(N)Cl(PS)(PPh(3))] selectively reacts with an appropriate dithiocarbamate ligand (S(∧)Y) to give [Tc(N)(PS)(S(∧)Y)] compounds. 1-(2-Methoxyphenyl)piperazine, which displays a potent and specific affinity for 5HT(1A) receptors, was selected as a functional group and conjugated to the dithiocarbamate unit through different spacers (L( n )). [(99m)Tc(N)(PS)(L( n ))] complexes were prepared in high yield (more than 90%). The chemical identity of (99m)Tc complexes was determined by high performance liquid chromatography comparison with the corresponding (99g)Tc complexes. All complexes were found to be inert toward transchelation with an excess of glutathione and cysteine. No notable biotransformation of the native compound into different species by the in vitro action of the serum and liver enzymes was shown. Nanomolar affinity for the 5HT(1A) receptor was obtained for [(99m)Tc(N)(PSiso)L(3)] (IC(50) = 1.5 nM); a reduction of the affinity was observed for the other complexes as a function of the shortening of the alkyl chain interposed between the dithiocarbamate and the pharmacophore. Negligible brain uptake was found from in vivo distribution data of [(99m)Tc(N)(PSiso)L(3)]. The key finding of this study is that the complexes maintained good affinity and selectivity for 5HT(1A) receptors, and the IC(50) value for [(99g)Tc(N)(PSiso)L(3)] being comparable to the IC(50) value found for WAY 100635. This result confirmed the possibility of preparing [(99m)Tc(N)(PS)]-based target-specific compounds without affecting the affinity and selectivity of the bioactive molecules for the corresponding receptors.