The Role of Inorganic Chemistry in the Development of Radiometal Agents for Cancer Therapy

Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR3)2(Schiff base)]+ complexes

Complexes of the type trans-[Re(PR₃)₂(Schiff base)]⁺ (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu₄N)[ReOCl₄] with H₂sal₂en or H₂sal₂ibn followed by addition of a tertiary phosphine. The trans-[Re(PR₃)₂(sal₂en)]⁺ complexes 2-4 were stable in solution, whereas the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR₃)(sal₂ibn)]⁺ products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes is likely driven by steric interactions between the bulky backbone gem-dimethyl groups of the sal₂ibn ligand and the phosphine ligands. These complexes were isolated and characterized by ¹H and ¹³C NMR, FT-IR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. The results reported herein provide insight into the factors that drive trans-[Re(PR₃)₂(Schiff base)]⁺ complex formation. This will aid in the development of novel ^186/188Re therapeutic agents and the design of novel bifunctional N₂O₂ Schiff base ligands.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential ^99mTc diagnostic radiopharmaceuticals. The rhenium analogues to ^99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M^III(PR₃)₂(N₂O₂-Schiff base)]⁺ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf₂enH₂) with the M(V) starting materials ( nBu₄N)[TcOCl₄] and ( nBu₄N)[ReOCl₄] gave the monomeric products trans-[TcOCl(tmf₂en)] and trans-[ReOCl(tmf₂en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf₂en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re^III(PR₃)₂(tmf₂en)]⁺. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re^III/Re^IV and Re^III/Re^II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf₂en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt₃)₂(tmf₂en)]PF₆. A reversible Tc^III/Tc^II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc^III/Tc^IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.

Development of Biocompatible and Functional Polymeric Nanoparticles for Site-Specific Delivery of Radionuclides

Introduction

Encapsulation of biologically active molecules into nanoparticles (NPs), for site-specific delivery, is a fast growing area. These NPs must be biocompatible, non-toxic, and able to release their load in a controlled way. We have developed a series of NPs based on (bio)degradable and biocompatible poly(malic acid) derivatives, poly(benzyl malate) (PMLABe), with its PEG-grafted stealth analog and target-specific biotin-PEG-b-PMLABe one. A lipophilic radiotracer has then been encapsulated into these NPs.

Methods

Monomers were synthesized from dl-aspartic acid. PEG₄₂-b-PMLABe₇₃ and Biot-PEG₆₆-b-PMLABe₇₃ block copolymers were obtained by anionic ring-opening polymerization of benzyl malolactonate in presence of α-methoxy-ω-carboxy-PEG₄₂ and α-biotin-ω-carboxy-PEG₆₆ as initiators. NPs were prepared by nanoprecipitation. Size, polydispersity, and zeta potential were measured by dynamic light scattering (DLS) and zetametry. ^99mTc-SSS was prepared as previously described. Encapsulation efficacy was assessed by varying different parameters, such as encapsulation with preformed NPs or during their formation, influence of the solvent, and of the method to prepare the NPs. After decay, ^99mTc-loaded NPs were also analyzed by DLS and zetametry. NPs’ morphology was assessed by transmission electron microscopy.

Results

^99mTc-SSS was added during nanoprecipitation, using two different methods, to ensure good encapsulation. Radiolabeled NPs present increased diameters, with identical low polydispersity indexes and negative zeta potentials in comparison to non-radiolabeled NPs.

Conclusion

A radiotracer was successfully encapsulated, but some further optimization is still needed. The next step will be to modify these radiolabeled NPs with a hepatotrope peptide, and to replace ^99mTc with ¹⁸⁸Re for therapy. Our team is also working on drugs’ encapsulation and grafting of a fluorescent probe. Combining these modalities is of interest for combined chemo-/radiotherapy, bimodal imaging, and/or theranostic approach.

Peptide receptor radionuclide therapy: an overview

Peptide receptor radionuclide therapy (PRRT) is a site-directed targeted therapeutic strategy that specifically uses radiolabeled peptides as biological targeting vectors designed to deliver cytotoxic levels of radiation dose to cancer cells, which overexpress specific receptors. Interest in PRRT has steadily grown because of the advantages of targeting cellular receptors in vivo with high sensitivity as well as specificity and treatment at the molecular level. Recent advances in molecular biology have not only stimulated advances in PRRT in a sustainable manner but have also pushed the field significantly forward to several unexplored possibilities. Recent decades have witnessed unprecedented endeavors for developing radiolabeled receptor-binding somatostatin analogs for the treatment of neuroendocrine tumors, which have played an important role in the evolution of PRRT and paved the way for the development of other receptor-targeting peptides. Several peptides targeting a variety of receptors have been identified, demonstrating their potential to catalyze breakthroughs in PRRT. In this review, the authors discuss several of these peptides and their analogs with regard to their applications and potential in radionuclide therapy. The advancement in the availability of combinatorial peptide libraries for peptide designing and screening provides the capability of regulating immunogenicity and chemical manipulability. Moreover, the availability of a wide range of bifunctional chelating agents opens up the scope of convenient radiolabeling. For these reasons, it would be possible to envision a future where the scope of PRRT can be tailored for patient-specific application. While PRRT lies at the interface between many disciplines, this technology is inextricably linked to the availability of the therapeutic radionuclides of required quality and activity levels and hence their production is also reviewed.

Novel rhenium(III, IV, and V) tetradentate N2O2 Schiff base mononuclear and dinuclear complexes

Reaction of (Bu4N)[ReOCl4] with the tetradentate Schiff base ligand α,α'-[(1,1-dimethylethylene)dinitrilo]di-o-cresol (sal2ibnH2) yields cis-[Re(V)OCl(sal2ibn)], which quickly forms trans-[μ-O(Re(V)O(sal2ibn))2] in solution. The dinuclear complex can also be isolated by the addition of base (Et3N) to the reaction mixture. Conversely, the mononuclear complex can be trapped as cis-[Re(V)O(NCS)(sal2ibn)] by addition of (Bu4N)SCN to the reaction mixture. Reduction of cis-[Re(V)O(NCS)sal2ibn] with triphenylphosphine gives the rare trans-[Re(III)(NCS)(PPh3)(sal2ibn)] and unique μ-oxo Re(IV) dimer trans-[μ-O(Re(IV)(NCS)(sal2ibn))2]. All of the complexes were characterized by (1)H and (13)C NMR, FT-IR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), cyclic voltammetry and single crystal X-ray diffraction.

Synthesis and SAR of novel Re/99mTc-labeled benzenesulfonamide carbonic anhydrase IX inhibitors for molecular imaging of tumor hypoxia

Carbonic anhydrase IX (CA-IX) is upregulated in cancer in response to the hypoxic tumor microenvironment, making it an attractive molecular target for the detection of hypoxic solid tumors. A series of small molecule benzenesulfonamide based CA-IX inhibitors containing novel tridentate chelates complexed with the M(CO)(3) core (M = Re or (99m)Tc) were designed and synthesized. The in vitro binding affinity of the benzenesulfonamide rhenium complexes yielded IC(50) values ranging from 3 to 116 nM in hypoxic CA-IX expressing HeLa cells. One of the most potent compounds, 3d (IC(50) = 9 nM), was radiolabeled with technetium tricarbonyl ({(99m)Tc(CO)(3)}(+)) to afford the {(99m)Tc(CO)(3)}(+) complex in excellent yield and high purity. (99m)Tc(CO)(3)-3d bound specifically to CA-IX expressing hypoxic HeLa cells. This effort led to the identification of a diverse series of promising high affinity {(99m)Tc(CO)(3)}(+) radiolabeled CA-IX inhibitors with the potential to significantly impact diagnosis, staging, and treatment selection of hypoxic solid tumors.

Structural and Spectral Analyses of 2-[(2-Benzothiazolylmethyl)thio]-benzenamine and 2-[(2-Benzothiazolylmethyl)thio]-benzenamine hydrobromide

2-[2-benzothiazoylmethyl)thio]-benzenamine, which was first reported in 1898, was isolated from the reaction of bromoacetyl bromide and 2-aminothiophenol [1]. The product crystallized from an aqueous methanol solution of the reaction mixture to which nickel(II) acetate had been added. 2-[(2-benzothiazolylmethyl)thio]-benzenamine crystallized in the monoclinic system, in space group C2/c, with cell dimensions of a = 27.392 (19) Å, b = 4.730 (3) Å, and c = 23.686 (16) Å, β = 122.465 (6)°, V = 2589(3) Å(3), Z = 8 and refined to R = 0.0343 and R(w) = 0.0844. Crystallization from methanol yielded the product as the hydrobromide salt in the monoclinic space group Cc, with cell dimensions of a = 10.488 (3) Å, b = 33.404 (9) Å, c = 5.2578 (14) Å, β = 116.769(2)°, V = 1644.7(8) Å(3), Z = 4 and refined to R = 0.0296 and R(w) = 0.0600. Mass spectral and NMR analyses confirmed that the bulk and crystalline compound were all 2-[(2-benzothiazolylmethyl)thio]-benzenamine.

Design and synthesis of a bombesin peptide-conjugated tripodal phosphino dithioether ligand topology for the stabilization of the fac-[M(CO)3]+ core (M=(99 m)Tc or Re)

A new tumor-seeking tridentate topology consisting of a phosphino dithioether ((HOCH(2))(2)PCH(2)CH(2)S(CH(2))(n)CH(2)SR; PS(2)) ligand framework for the production of kinetically inert and in vivo stable facial [(99m)Tc(CO)(3)(PS(2))](+) or [Re(CO)(3)(PS(2))](+) is described. The X-ray crystal structure of fac-Re(CO)(3)(PS(2))PF(6) is reported. The bioconjugation strategies for incorporating bombesin (BBN) peptides on to the PS(2) tripodal framework and, thereby, de novo designing of GRP receptor-seeking Tc(PS(2)-BBN)(CO)(3) are developed.

Emergence and present status of Lu-177 in targeted radiotherapy: the Indian scenario

177Lu is presently considered to be a potential radionuclide for the development of agents for radionuclide therapy owing to its favorable nuclear decay characteristics [T 1/2 = 6.65 d, E β(max) = 0.497 MeV, E γ = 113 KeV (6.4%) and 208 KeV (11%)]. While the long half-life of this promising radioisotope offers distinct logistic advantage, particularly, in countries having limited reactor facilities, the feasibility of its large-scale production with adequate specific activity and excellent radionuclidic purity in medium flux research reactors constitute yet another desirable feature. Extensive studies have been carried out to optimize the production of this isotope, with high specific activity and radionuclidic purity by the (n,γ) route using the highest available flux and the optimum irradiation time. The gradual evolution of clin ical grade 177LuCl3 as a new radiochemical, ready for commercial deployment by Radiopharmaceuticals Division, Bhabha Atomic Research Centre, to nuclear medicine centers all over India was accomplished in 2010 in a stepwise manner with the commencement of the production of high specific activity 177Lu from enriched target in 2001. Research on 177Lu has demonstrated its immense potential in radiotherapeutic applications, a direct outcome of which has resulted in indigenous development of two agents viz. 177Lu-EDTMP and 177Lu-DOTA-TATE presently being evaluated in human patients for palliative care of bone pain due to skeletal metastases and treatment of malignancies of neuroendocrine origin, respectively. Using locally produced 177Lu, the radiolabeling of a plethora of other molecules with potential applicability in radiation synovectomy and targeted therapy of malignant tumors have been successfully demonstrated. A few of these agent such as a novel 177Lu-labeled porphyrin has shown considerable promise in initial studies and is presently evaluated. In the present article, our research efforts toward standardization of production methodology of 177Lu in high specific activity and its utilization in the devel opment of agents for targeted radiotherapy are being reported.

Oligopyrrole macrocycles: receptors and chemosensors for potentially hazardous materials

Oligopyrroles represent a diverse class of molecular receptors that have been utilized in a growing number of applications. Recently, these systems have attracted interest as receptors and chemosensors for hazardous materials, including harmful anionic species, high-valent actinide cations, and nitroaromatic explosives. These versatile molecular receptors have been used to develop rudimentary colorimetric and fluorimetric assays for hazardous materials.

Re(V) and Re(III) complexes with sal2phen and triphenylphosphine: rearrangement, oxidation and reduction

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'-o-phenylenebis(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 synthesis and toxicity of tripodal tricarbonyl rhenium complexes as radiopharmaceutical models

We report the synthesis and toxicity of a series of rhenium(I) tricarbonyl complexes incorporating the trisaminomethylethane (TAME) ligand. Compounds with the (TAME)Re(CO)(3)(+) cation were synthesized via several routes, including by use of Re(CO)(5)X precursors as well as the aqueous cation Re(CO)(3)(H(2)O)(3)(+). Salts of the formula [(TAME)Re(CO)(3)]X where X=Br(-), Cl(-), NO(3)(-), PF(6)(-) and ClO(4)(-) were evaluated using two cell lines: the monoclonal S3 HeLa line and a vascular smooth muscle cell line harvested from mice. All compounds have isostructural cations and differ only in the identity of the non-coordinating anion. None of the complexes exhibited any appreciable toxicity in the HeLa line up to the solubility limit. In the vascular smooth muscle cell line, the bromide salt exhibited some cytotoxicity, but this observation most likely results from the presence of bromide anion, which has been shown to have limited toxicity.

Labeling internalizing anti-epidermal growth factor receptor variant III monoclonal antibody with (177)Lu: in vitro comparison of acyclic and macrocyclic ligands

INTRODUCTION

The monoclonal antibody (mAb) L8A4, reactive with the epidermal growth factor receptor variant III (EGFRvIII), internalizes rapidly in glioma cells after receptor binding. Combining this tumor-specific mAb with the low-energy beta-emitter (177)Lu would be an attractive approach for brain tumor radioimmunotherapy, provided that trapping of the radionuclide in tumor cells after mAb intracellular processing could be maximized.

MATERIALS AND METHODS

L8A4 mAb was labeled with (177)Lu using the acyclic ligands [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A''-DTPA), 2-(4-isothiocyanatobenzyl)-diethylenetriaminepenta-acetic acid (pSCN-Bz-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylenetriaminepentaacetic acid (1B4M-DTPA), and the macrocyclic ligands S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and alpha-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA). Paired-label internalization and cellular processing assays were performed on EGFRvIII-expressing U87.DeltaEGFR glioma cells over 24 h to directly compare (177)Lu-labeled L8A4 to L8A4 labeled with (125)I using either iodogen or N-succinimidyl 4-guanidinomethyl-3-[(125)I]iodobenzoate ([(125)I]SGMIB). In order to facilitate comparison of labeling methods, the primary parameter evaluated was the ratio of (177)Lu to (125)I activity retained in U87.DeltaEGFR cells.

RESULTS

All chelates demonstrated higher retention of internalized activity compared with mAb labeled using iodogen, with (177)Lu/(125)I ratios of >20 observed for the three DTPA chelates at 24 h. When compared to L8A4 labeled using SGMIB, except for MeO-DOTA, internalized activity for (125)I was higher than (177)Lu from 1-8 h with the opposite behavior observed thereafter. At 24 h, (177)Lu/(125)I ratios were between 1.5 and 3, with higher values observed for the three DTPA chelates.

CONCLUSIONS

The nature of the chelate used to label this internalizing mAb with (177)Lu influenced intracellular retention in vitro, although at early time points, only MeO-DOTA provided more favorable results than radioiodination of the mAb via SGMIB.

Synthesis and characterization of the copper(II) complexes of new N2S2-donor macrocyclic ligands: synthesis and in vivo evaluation of the (64)Cu complexes

The aim of this work was to prepare a novel class of (64)Cu(II) labeled complexes with the new macrocyclic ligands 1,10-dithia-4,7-diazacyclododecane-3,8-dicarboxylic acid (NEC-SE, 1), 1,10-dithia-4,7-diazacyclotridecane-3,8-dicarboxylic acid (NEC-SP, 2) and 1,10-dithia-4,7-diazacyclotetradecane-3,8-dicarboxylic acid, (NEC-SB, 3 ) to evaluate the usefulness of these macrocycles for potential utility as (64)Cu(II) chelators. The corresponding non-radioactive complexes [Cu(NEC-SE)] x 3H(2)O (4), [Cu(NEC-SP)] x 3H(2)O (5) and [Cu(NEC-SB)] (6) were prepared and their (64)Cu-analogs, [(64)Cu(NEC-SE)] (7) and [(64)Cu(NEC-SP)] (8) and [(64)Cu(NEC-SB)] (9) were produced in >98% radiochemical purity. Rats were injected with complex 7, 8 or 9 and were euthanized at 1, 4 and 24 h. All three complexes are cleared from the blood over the first hour following injection but there is poor clearance of this activity over 24 h. A similar pattern of retention was noted in the liver where the levels of activity in this tissue at 1 h are not statistically different from those at 24 h. Molecular mechanics and DFT studies were performed on the complexes in order to gain insight into the lower stability.

Self-assembly in Schiff base lanthanide complexes — From supramolecular dimers to coordination polymers

A review is given of the latest contribution of the author’s research group to the coordination chemistry of Schiff base polyaza macrocycles and acyclic salicylaldimines. Focus is placed on the effectiveness of lanthanides in supramolecular self-assembly of the components leading to the formation of compounds with unusual properties and structures.Key words: Schiff base macrocycles, salicylaldimines, lanthanides, self-assembly, template synthesis.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

Metal complexes of cyclen and cyclam derivatives useful for medical applications: a discussion based on thermodynamic stability constants and structural data

A series of the most common chelators used in magnetic resonance imaging (MRI) and in radiopharmaceuticals for medical diagnosis and tumour therapy, H(4)dota, H(4)teta, H(8)dotp and H(8)tetp, is examined from a chemical point of view. Differences between 12- and 14-membered tetraazamacrocyclic derivatives with methylcarboxylate and methylphosphonate pendant arms and their chelates with divalent first-series transition metal and trivalent lanthanide ions are discussed on the basis of their thermodynamic stability constants, X-ray structures and theoretical studies.

Contribution of electrospray mass spectrometry for the characterization, design, and development of nitrido technetium and rhenium heterocomplexes as potential radiopharmaceuticals

Diagnostic nuclear medicine (NM) is among the imaging procedures (together with X-ray, computerized tomography, magnetic resonance, and echography) the clinicians can routinely adopt to image organs or tissues and related disorders. (99m)Tc-based agents are the radiopharmaceuticals of election in diagnostic NM because of the ideal physical properties of the 99mTc nuclide (t1/2 6.01 hr; Egamma 142 keV), low cost, and easy availability through the commercial 99Mo/99mTc generator, and chemical versatility of the element. In the last two decades the synergistic work of clinics, pharmacologists, and coordination chemists has had a tremendous impact in the development of new 99mTc-based radiopharmaceuticals through the recognition of the structure at the molecular level of the agent utilized. This has been achieved by studying the physico-chemical properties of the long-lived 99gTc (t1/2 2.11 x 10(5) year; Ebeta 292 keV) and third-row congener Re isostructural compounds. Electrospray ionization mass spectrometry (ESI-MS) and collision experiments (MS/MS) represent valuable analytical techniques suitable for the characterization of both technetium and rhenium complexes relevant to NM. Unequivocal structural identification of these bioinorganic compounds, either simple coordination complexes ("essential radiopharmaceuticals") or more sophisticated structures carrying bioactive fragments ("receptor-specific" radiopharmaceuticals), can be realized in combination with multinuclear NMR spectroscopy. MS/MS experiments provide useful information on the different metal-ligand bond strength, and comparison of the fragmentation profiles of isostructural technetium and rhenium compounds give additional details on the role played by the metal in determining preferred decomposition channels. The analysis of these data contribute to design novel synthetic strategies for the obtainment of technetium and rhenium compounds relevant to NM. The chemistry underlying the production of a new class of potential radiopharmaceuticals including a terminal nitrogen bond and a mixed coordination sphere comprising heterodiphosphines and/or dithiocarbamates (DTC) is presented in detail together with the ESI-MS and MS/MS investigations.

Synthesis, Characterization, and Structures of Indium In(DTPA-BA2) and Yttrium Y(DTPA-BA2)(CH3OH) Complexes (BA = Benzylamine): Models for111In- and90Y-Labeled DTPA-Biomolecule Conjugates

To explore structural differences in In3+, Y3+, and Lu3+ chelates, we prepared M(DTPA-BA2) complexes (M = In, Y, and Lu; DTPA-BA2 = N,N' '-bis(benzylcarbamoylmethyl)diethylenetriamine-N,N',N' '-triacetic acid) by reacting the trisodium salt of DTPA-BA2 with 1 equiv of metal chloride or nitrate. All three complexes have been characterized by elemental analysis, HPLC, IR, ES-MS, and NMR (1H and 13C) methods. ES-MS spectral and elemental analysis data are consistent with the proposed formula for M(DTPA-BA2) (M = In, Y, and Lu) and have been confirmed by the X-ray crystal structures of both In(DTPA-BA2) x 2H2O and Y(DTPA-BA2)(CH3OH) complexes. By a reversed-phase HPLC method, it was found that In(DTPA-BA2) is more hydrophilic than M(DTPA-BA2) (M = Y and Lu), most likely due to the dissociation of the two carbonyl oxygen donors in solution. The X-ray crystal structure of In(DTPA-BA2) revealed a rare example of an eight-coordinated In3+ complex with DTPA-BA2 bonding to the In3+ in a distorted square antiprism coordination geometry. Both benzylamine groups are in the trans position relative to the acetate-chelating arm that is attached to the central N atom. The Y3+ in Y(DTPA-BA2)(CH3OH) is nine-coordinated with an octadentate DTPA-BA2 and a methanol oxygen. The coordination geometry is best described as a tricapped trigonal prism. One benzylamine group is trans and the other cis to the acetate-chelating arm that is attached to the central N atom. All three M(DTPA-BA2) complexes (M = In, Y, and Lu) exist as at least three isomers in solution (approximately 10 mM), as shown by the presence of 6-8 overlapped 1H NMR signals from the methylene hydrogens of the benzylamine groups. The coordinated DTPA-BA2 remains rigid even at temperatures > 85 degrees C. The exchange rate between different isomers in M(DTPA-BA2) (M = In, Y, and Lu) is relatively slow at high concentrations (> 1.0 mM), but it is fast due to the partial dissociation and rapid interconversion of different isomers at lower concentrations ( approximately 10 mircroM). It is not surprising that M(DTPA-BA2) complexes (M = In, Y, and Lu) appear as a single peak in their respective HPLC chromatogram.

Synthesis, characterization, and X-ray crystal structure of In(DOTA-AA) (AA = p-aminoanilide): a model for 111In-labeled DOTA-biomolecule conjugates

This report describes the synthesis and structural characterization of the indium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-aminoanilide) (DOTA-AA), a model compound for (111)In-labeled DOTA-biomolecule conjugates. In(DOTA-AA) was prepared by reacting DOTA-AA with 1 equiv of InCl(3) in 0.5 M ammonium acetate buffer (pH approximately 6). It was characterized by spectroscopic methods (IR, ES-MS, and (1)H NMR), elemental analysis, and X-ray crystallography. For comparison purposes, we also prepared the complex Y(DOTA-AA). ES-MS and (1)H NMR data are consistent with the proposed structure. HPLC analysis using a reversed phase method shows that the retention time of In(DOTA-AA) is approximately 2.0 min shorter than that of Y(DOTA-AA), demonstrating that In(DOTA-monoamide) is more hydrophilic than Y(DOTA-monoamide). In the solid state, In(DOTA-AA) has a twisted square antiprismatic coordination geometry with all eight donor atoms (N(4)O(4)) bonded to the In center. The average In-N and In-O distances are almost identical to those of Y-N and Y-O bonds found in Y(DOTA-d-Phe-NH(2)) even though the ionic radius of Y(3+) is much longer than that of In(3+). It seems that In(3+) does not fit the coordination cavity of DOTA-AA perfectly. The (1)H NMR data clearly demonstrated that In(DOTA-AA) becomes fluxional at room temperature, most likely due to dissociation of the acetamide-oxygen, rotation of acetate chelating arms, and inversion of ethylenic groups of the macrocyclic ring. Results from this study and our previous studies (Liu, S.; Pietryka, J.; Ellars C. E.; Edwards D. S. Bioconjugate Chem. 2002, 13, 902-913) suggest that the In(3+) complex of DOTA-monoamide in the solid state might be different from that in solution due to dissociation of the carbonyl-oxygen donor. Although Y(3+) and In(3+) complexes of DOTA-monoamide are both eight-coordinate in the solid state, the difference in their solution structures is most likely responsible for their difference in lipophilicity.

Formation of two diverse classes of poly(amino-alkoxide) chelates and their mononuclear and polynuclear lanthanide(III) complexes

Factors that influence aggregation of lanthanide(III) (Ln(III)) ions to form polynuclear complexes were studied utilizing 1-aziridineethanol as a versatile source of macrocyclic and acyclic chelates. The facile ring-opening cyclo-oligomerization of 1-aziridineethanol leads to the formation of a series of polyaza cyclic oligomers (series A). In the presence of ethylenediamine, a competing N-alkylation reaction occurs to produce a new class of acyclic ligands (series B). The cyclo-oligomerization of four 1-aziridineethanol units is the most favorable process, leading to the formation of the 12-membered cyclen-type macrocycle, H(4)L(1) (1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraaza-cyclododecane). Ring-opening cyclo-oligomerization of 1-aziridineethanol in the presence of Ln(III) ions produces self-assembled mononuclear, tetranuclear, and pentanuclear compounds of H(4)L(1). In the presence of ethylenediamine, oligomerization of 1-aziridineethanol results in a dinuclear complex of an acyclic poly(amino-alkoxide) H(2)L(2). The coordinative unsaturation of (i) the alkoxy sites of [H(x)L(1)](x)(-)(4) (where x < 4) and (ii) Ln(III) ions in coordination numbers less than nine are critical factors in the formation of the polynuclear Ln(III) complexes. The identities of mononuclear, dinuclear, tetranuclear, and pentanuclear complexes herein discussed were established by X-ray crystallography.

Radiochemical investigations of 177Lu-DOTA-8-Aoc-BBN[7-14]NH2: an in vitro/in vivo assessment of the targeting ability of this new radiopharmaceutical for PC-3 human prostate cancer cells

Bombesin (BBN), a 14 amino acid peptide, is an analogue of human gastrin releasing peptide (GRP) that binds to GRP receptors (GRPr) with high affinity and specificity. The GRPr is over expressed on a variety of human cancer cells including prostate, breast, lung, and pancreatic cancers. The specific aim of this study was to identify a BBN analogue that can be radiolabeled with (177)Lu and maintains high specificity for GRPr positive prostate cancer tumors in vivo. A preselected synthetic sequence via solid phase peptide synthesis (SPPS) was designed to produce a DOTA-BBN (DOTA = 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid) conjugate with the following general structure: DOTA-X-Q-W-A-V-G-H-L-M-(NH(2)), where the spacer group, X = omega-NH(2)(CH(2))(7)COOH (8-Aoc). The BBN-construct was purified by reversed phase-HPLC (RP-HPLC). Electrospray Mass Spectrometry (ES-MS) was used to characterize both metallated and non-metallated BBN-conjugates. The new DOTA-conjugate was metallated with (177)Lu(III)Cl(3) or non-radioactive Lu(III)Cl(3). The (177)Lu(III)- and non-radiolabeled Lu(III)-conjugates exhibit the same retention times under identical RP-HPLC conditions. The (177)Lu-DOTA-8-Aoc-BBN[7-14]NH(2) conjugate was found to exhibit optimal pharmacokinetic properties in CF-1 normal mice. In vitro and in vivo models demonstrated the ability of the (177)Lu-DOTA-8-Aoc-BBN[7-14]NH(2) conjugate to specifically target GRP receptors expressed on PC-3 human prostate cancer cells.