Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging.

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Inorganic Chemistry of the Tripodal Picolinate Ligand Tpaa with Gallium(III) and Radiolabeling with Gallium-68.

We report here the improved synthesis of the tripodal picolinate chelator Tpaa, with an overall yield of 41% over five steps, in comparison to the previously reported 6% yield. Tpaa was investigated for its coordination chemistry with Ga(III) and radiolabeling properties with gallium-68 (68Ga). The obtained crystal structure for [Ga(Tpaa)] shows that the three picolinate arms coordinate to the Ga(III) ion, fully occupying the octahedral coordination geometry. This is supported by 1H NMR which shows that the three arms are symmetrical when coordinated to Ga(III). Assessment of the thermodynamic stability through potentiometry gives log KGa-Tpaa = 21.32, with a single species being produced across the range of pH 3.5-7.5. Tpaa achieved >99% radiochemical conversion with 68Ga under mild conditions ([Tpaa] = 6.6 µM, pH 7.4, 37 °C) with a molar activity of 3.1 GBq µmol-1. The resulting complex, [68Ga][Ga(Tpaa)], showed improved stability over the previously reported [68Ga][Ga(Dpaa)(H2O)] in a serum challenge, with 32% of [68Ga][Ga(Tpaa)] remaining intact after 30 min of incubation with fetal bovine serum.

Transition metal complexes of cyclam with two 2,2,2-trifluoroethylphosphinate pendant arms as probes for 19F magnetic resonance imaging.

A new cyclam-based ligand bearing two methylene(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent transition metal ions [Co(II), Ni(II), Cu(II), Zn(II)] was studied. The ligand was found to be very selective for the Cu(II) ion according to the common Williams-Irving trend. Complexes with all the studied metal ions were structurally characterized. The Cu(II) ion forms two isomeric complexes; the pentacoordinated isomer pc-[Cu(L)] is the kinetic product and the octahedral trans-O,O'-[Cu(L)] isomer is the final (thermodynamic) product of the complexation reaction. Other studied metal ions form octahedral cis-O,O'-[M(L)] complexes. The complexes with paramagnetic metal ions showed a significant shortening of 19F NMR longitudinal relaxation times (T1) to the millisecond range [Ni(II) and Cu(II) complexes] or tens of milliseconds [Co(II) complex] at the temperature and magnetic field relevant for 19F magnetic resonance imaging (MRI). Such a short T1 results from a short distance between the paramagnetic metal ion and the fluorine atoms (∼6.1-6.4 Å). The complexes show high kinetic inertness towards acid-assisted dissociation; in particular, the trans-O,O'-[Cu(L)] complex was found to be extremely inert with a dissociation half-time of 2.8 h in 1 M HCl at 90 °C. Together with the short relaxation time, it potentially enables in vitro/in vivo utilization of the complexes as efficient contrast agents for 19F MRI.

Bn2DT3A, a Chelator for 68Ga Positron Emission Tomography: Hydroxide Coordination Increases Biological Stability of [68Ga][Ga(Bn2DT3A)(OH)].

The chelator Bn2DT3A was used to produce a novel 68Ga complex for positron emission tomography (PET). Unusually, this system is stabilized by a coordinated hydroxide in aqueous solutions above pH 5, which confers sufficient stability for it to be used for PET. Bn2DT3A complexes Ga3+ in a hexadentate manner, forming a mer-mer complex with log K([Ga(Bn2DT3A)]) = 18.25. Above pH 5, the hydroxide ion coordinates the Ga3+ ion following dissociation of a coordinated amine. Bn2DT3A radiolabeling displayed a pH-dependent speciation, with [68Ga][Ga(Bn2DT3A)(OH)]- being formed above pH 5 and efficiently radiolabeled at pH 7.4. Surprisingly, [68Ga][Ga(Bn2DT3A)(OH)]- was found to show an increased stability in vitro (for over 2 h in fetal bovine serum) compared to [68Ga][Ga(Bn2DT3A)]. The biodistribution of [68Ga][Ga(Bn2DT3A)(OH)]- in healthy rats showed rapid clearance and excretion via the kidneys, with no uptake seen in the lungs or bones.

Cyclam with a phosphinate-bis(phosphonate) pendant arm is a bone-targeting carrier of copper radionuclides.

Ligands combining a bis(phosphonate) group with a macrocycle function as metal isotope carriers for radionuclide-based imaging and for treating bone metastases associated with several cancers. However, bis(phosphonate) pendant arms often slow down complex formation and decrease radiochemical yields. Nevertheless, their negative effect on complexation rates may be mitigated by using a suitable spacer between bis(phosphonate) and the macrocycle. To demonstrate the potential of bis(phosphonate) bearing macrocyclic ligands as a copper radioisotope carrier, we report the synthesis of a new cyclam derivative bearing a phosphinate-bis(phosphonate) pendant (H5te1PBP). The ligand showed a high selectivity to CuII over ZnII and NiII ions, and the bis(phosphonate) group was not coordinated in the CuII complex, strongly interacting with other metal ions in solution. The CuII complex formed quickly, in 1 s, at pH 5 and at a millimolar scale. The complexation rates significantly differed under a ligand or metal ion excess due to the formation of reaction intermediates differing in their metal-to-ligand ratio and protonation state, respectively. The CuII-te1PBP complex also showed a high resistance to acid-assisted hydrolysis (t1/2 2.7 h; 1 M HClO4, 25 °C) and was effectively adsorbed on the hydroxyapatite surface. H5te1PBP radiolabeling with [64Cu]CuCl2 was fast and efficient, with specific activities of approximately 30 GBq 64Cu per 1 µmol of ligand (pH 5.5, room temperature, 30 min). In a pilot experiment, we further demonstrated the excellent suitability of [64Cu]CuII-te1PBP for imaging active bone compartments by dedicated small animal PET/CT in healthy mice and subsequently in a rat femoral defect model, in direct comparison with [18F]fluoride. Moreover, [64Cu]CuII-te1PBP showed a higher uptake in critical bone defect regions. Therefore, our study highlights the potential of [64Cu]CuII-te1PBP as a PET radiotracer for evaluating bone healing in preclinical and clinical settings with a diagnostic value similar to that of [18F]fluoride, albeit with a longer half-life (12.7 h) than 18F (1.8 h), thereby enabling extended observation times.

Al(III)-NTA-fluoride: a simple model system for Al-F binding with interesting thermodynamics.

Al(iii) complexes are extensively studied as [18F]fluoride carriers in positron emission tomography. However, our limited knowledge on their thermodynamic and kinetic properties has hindered efforts to easily prepare radiochemically pure compounds while simultaneously reducing the overall labeling time. Thus, to improve our understanding of fluoride binding to coordinatively unsaturated Al(iii) complexes, we investigated the ternary system Al(iii)-H3NTA-F- (H3NTA = nitrilo-triacetic acid) by NMR, potentiometry and X-ray diffraction. Our results show that the [Al(NTA)] complex binds two water molecules, which are replaced by fluorides. Individual species and isomers show separate 19F NMR signals and different stability constants. The two available positions on the [Al(NTA)] complex feature significantly different properties in terms of basicity of the coordinated water molecules and preferential binding of fluoride anions. Fluorides are effectively bound in weakly acidic or neutral solutions, whereas hydroxido species are preferentially formed in alkaline solutions. Our experimental observations were rationalized by theoretical calculations: predictions of the energy ordering of complexes and isomers, interpretation of 19F NMR chemical shifts, and natural bonding orbital analysis. Radiolabeling of [Al(NTA)] with [18F]fluoride gave low yields that confirmed a high affinity of the complex for hydroxide anions.

Cross-Bridged Cyclam with Phosphonate and Phosphinate Pendant Arms: Chelators for Copper Radioisotopes with Fast Complexation.

Cross-bridged cyclam derivatives bearing two phosphonate (H4L1), bis(phosphinate) (H4L2), or phosphinate (H2L3) pendant arms were synthesized and studied with respect to their application as copper radioisotope carriers in nuclear medicine. The ligands show high macrocycle basicity (pK1 > 14) and high Cu(II) complex stability (log K = 20-24). The complexation and dissociation kinetics of the Cu(II) complexes were studied by ultraviolet-visible spectroscopy. Phosphonate Cu(II)-H4L1 and bis(phosphinate) Cu(II)-H4L2 complexes form very quickly, reaching quantitative formation within 1 s at pH ∼6 and millimolar concentrations. Conversely, the formation of the phosphinate complex Cu(II)-H2L3 is much slower (9 min at pH ∼6) due to the low stability of the out-of-cage reaction intermediate. All studied complexes are highly kinetically inert, showing half-lives of 120, 11, and 111 h for Cu(II)-H4L1, Cu(II)-H4L2, and Cu(II)-H2L3 complexes, respectively, in 1 M HClO4 at 90 °C. The high thermodynamic stability, fast formation, and extreme kinetic inertness of Cu(II) complexes indicate that phosphonate and bis(phosphinate) derivatives are promising ligands for nuclear medicine.

Improved Conjugation, 64-Cu Radiolabeling, in Vivo Stability, and Imaging Using Nonprotected Bifunctional Macrocyclic Ligands: Bis(Phosphinate) Cyclam (BPC) Chelators.

Bifunctional derivatives of bis(phosphinate)-bearing cyclam (BPC) chelators bearing a carboxylate, amine, isothiocyanate, azide, or cyclooctyne in the BP side chain were synthesized. Conjugations required no protection of phosphinate or ring secondary amine groups. The ring amines were not reactive (proton protected) at pH < ∼8. For isothiocyanate coupling, oligopeptide N-terminal α-amines were more suitable than alkyl amines, e.g., Lys ω-amine (p Ka ∼7.5-8.5 and ∼10-11, respectively) due to lower basicity. The Cu-64 labeling was efficient at room temperature (specific activity ∼100 GBq/µmol; 25 °C, pH 6.2, ∼100 ligand equiv, 10 min). A representative Cu-64-BPC was tested in vivo showing fast clearance and no nonspecific radioactivity deposition. The monoclonal anti-PSCA antibody 7F5 conjugates with thiocyanate BPC derivative or NODAGA were radiolabeled and studied in PC3-PSCA tumor bearing mice by PET. The radiolabeled BPC conjugate was accumulated in the prostate tumor with a low off-target uptake, unlike Cu-64-labeled NODAGA-antibody conjugate. The BPC chelators have a great potential for theranostic applications of the Cu-64/Cu-67 matched pair.

Lanthanide(iii) complexes of monophosphinate/monophosphonate DOTA-analogues: effects of the substituents on the formation rate and radiolabelling yield.

H4dota and its analogues are routinely used for complexation of lanthanide radioisotopes in nuclear medicine. Many of the radioisotopes have short half-lives and, thus, the complexation rate plays an important role. Notwithstanding that, the relationship between ligand structures and complexation rates is not well understood. Here we report a complexation study of H4dota and its analogues bearing one phosphonate or phosphinate pendant arm. The substituents on the phosphinate group were non-coordinating (-H) or contained another coordinating group (-CH2N(CH2COOH)2, -CH2PO2H2 or -CH2NH2). The basicity of ligands, stability of reaction intermediates, formation rates of CeIII complexes, and 177LuIII radiolabelling were studied. The complexation rates and labelling yields do not show any correlation with ligand basicity. In contrast, the additional chelating group attached to the pendant arm plays an important role. A decreased complexation rate and lower labelling yield were found for compounds bearing an additional amino group, whereas improved properties were found for the compound bearing a geminal bis(phosphinate) pendant arm. It indicates that the introduction of chelating pendant arms with acidic coordinating groups might be a promising strategy to improve radiolabelling of macrocyclic carriers with metal radioisotopes.

NOTA Complexes with Copper(II) and Divalent Metal Ions: Kinetic and Thermodynamic Studies.

H3nota derivatives are among the most studied macrocyclic ligands and are widely used for metal ion binding in biology and medicine. Despite more than 40 years of chemical research on H3nota, the comprehensive study of its solution chemistry has been overlooked. Thus, the coordination behavior of H3nota with several divalent metal ions was studied in detail with respect to its application as a chelator for copper radioisotopes in medical imaging and therapy. In the solid-state structure of the free ligand in zwitterionic form, one proton is bound in the macrocyclic cavity through a strong intramolecular hydrogen-bond system supporting the high basicity of the ring amine groups (log Ka = 13.17). The high stability of the [Cu(nota)]- complex (log KML = 23.33) results in quantitative complex formation, even at pH <1.5. The ligand is moderately selective for Cu(II) over other metal ions (e.g., log KML(Zn) = 22.32 and log KML(Ni) = 19.24). This ligand forms a more stable complex with Mg(II) than with Ca(II) and forms surprisingly stable complexes with alkali-metal ions (stability order Li(I) > Na(I) > K(I)). Thus, H3nota shows high selectivity for small metal ions. The [Cu(nota)]- complex is hexacoordinated at neutral pH, and the equatorial N2O2 interaction is strengthened by complex protonation. Detailed kinetic studies showed that the Cu(II) complex is formed quickly (millisecond time scale at cCu ≈ 0.1 mM) through an out-of-cage intermediate. Conversely, conductivity measurements revealed that the Zn(II) complex is formed much more slowly than the Cu(II) complex. The Cu(II) complex has medium kinetic inertness (τ1/2 46 s; pH 0, 25 °C) and is less resistant to acid-assisted decomplexation than Cu(II) complexes with H4dota and H4teta. Surprisingly, [Cu(nota)]- decomplexation is decelerated in the presence of Zn(II) ions due to the formation of a stable dinuclear complex. In conclusion, H3nota is a good carrier of copper radionuclides because the [Cu(nota)]- complex is predominantly formed over complexes with common impurities in radiochemical formulations, Zn(II) and Ni(II), for thermodynamic and, primarily, for kinetic reasons. Furthermore, the in vivo stability of the [Cu(nota)]- complex may be increased due to the formation of dinuclear complexes when it interacts with biometals.

Amino acid based gallium-68 chelators capable of radiolabeling at neutral pH.

Gallium-68 (68Ga) has been the subject of increasing interest for its potential in the production of radiotracers for diagnosis of diseases. In this work we report the complexation of 68Ga by the amino acid based tripodal chelate H3Dpaa, and two bifunctional derivatives, H3Dpaa.dab and H4Dpaa.ga, under a range of conditions with particular emphasis on the rapid complexation of 68Ga at pH 7.4. 100 µM H3Dpaa achieved a radiochemical yield of 95% at pH 7.4 in 5 minutes at 37 °C. The bifunctional derivatives H4Dpaa.ga and H3Dpaa.dab achieved 94% and 84% radiochemical yields, respectively, under the same conditions. The resulting Ga(iii) complexes show thermodynamic stabilities of log KGaDpaa = 18.53, log KGaDpaa.dab = 22.08, log KGaDpaa.ga = 18.36. Unfortunately, the resulting radiolabelled species do not present sufficient serum stability for in vivo application. Herein we show a flexible synthesis for bifunctional chelators based on amino acids that rapidly complex 68Ga under physiological conditions.

DOTA analogues with a phosphinate-iminodiacetate pendant arm: modification of the complex formation rate with a strongly chelating pendant.

The new ligand H6do3aPida combines the macrocyclic DOTA-like cavity and the open-chain iminodiacetate group connected through a coordinating phosphinate spacer. Its acid-base and coordination properties in solution were studied by potentiometry. Thermodynamic coordination characteristics of both chelating units are similar to those reported for H4dota and iminodiacetic acid themselves, respectively, so, macrocyclic and iminodiacetate units behave independently. The formation kinetics of the Ce(iii)-H6do3aPida complex was studied by UV-Vis spectrophotometry. Various out-of-cage intermediates were identified with 1 : 1, 1 : 2 and 2 : 1 ligand-to-metal ratios. The presence of the strongly coordinating iminodiacetate group significantly slows down the metal ion transfer into the macrocyclic cavity and, so, the formation of the in-cage complex is two orders of magnitude slower than that reported for the Ce(iii)-H4dota system. The kinetic inertness of the [Ce(do3aPida)]3- complex towards acid-assisted dissociation is comparable to that of the [Ce(dota)]- complex. The coordination modes of the ligand are demonstrated in the solid-state structure of [Cu4(do3aPida)(OH)(H2O)4]Cl·7.5H2O.

A DOTA based bisphosphonate with an albumin binding moiety for delayed body clearance for bone targeting.

Radiolabeled bisphosphonates are commonly used in the diagnosis and therapy of bone metastases. Blood clearance of bisphosphonates is usually fast and only 30%-50% of the injected activity is retained in the skeleton, while most of the activity is excreted by the urinary tract. A longer blood circulation may enhance accumulation of bisphosphonate compounds in bone metastases. Therefore, a chemically modified macrocyclic bisphosphonate derivative with an additional human albumin binding entity was synthesized and pharmacokinetics of its complex was evaluated. The DOTA-bisphosphonate conjugate BPAMD was compared against the novel DOTAGA-derived albumin-binding bisphosphonate DOTAGA(428-d-Lys)MBP (L1). The ligands were labeled with 68Ga(III) and were evaluated in in vitro binding studies to hydroxyapatite (HA) as well as to human serum albumin. The compounds were finally compared in in vivo PET and ex vivo organ distribution studies in small animals over 6h. Binding studies revealed a consistent affinity of both bisphosphonate tracers to HA. Small animal PET and ex vivo organ distribution studies showed longer blood retention of [68Ga]L1. [68Ga]BPAMD is initially more efficiently bound to the bone but skeletal accumulation of the modified compound and [68Ga]BPAMD equalized at 6h p.i. Ratios of femur epiphyseal plate to ordinary bone showed to be more favorable for [68Ga]L1 than for [68Ga]BPAMD due to the longer circulation time of the new tracer. Thus, the chemical modification of BPAMD toward an albumin-binding bisphosphonate, L1, resulted in a novel PET tracer which conserves advantages of both functional groups within one and the same molecule. The properties of this new diagnostic tracer are expected to be preserved in 177Lu therapeutic agent with the same ligand (a theranostic pair).

(177)Lu-labelled macrocyclic bisphosphonates for targeting bone metastasis in cancer treatment.

BACKGROUND: Metastatic bone lesion is a common syndrome of many cancer diseases in an advanced state. The major symptom is severe pain, spinal cord compression, and pathological fracture, associated with an obvious morbidity. Common treatments including systemic application of bisphosphonate drugs aim on pain reduction and on improving the quality of life of the patient. Particularly, patients with multiple metastatic lesions benefit from bone-targeting therapeutic radiopharmaceuticals. Agents utilizing beta-emitting radionuclides in routine clinical praxis are, for example, [(89)Sr]SrCl2 and [(153)Sm]Sm-EDTMP. No-carrier-added (n.c.a.) (177)Lu is remarkably suitable for an application in this scope. METHODS: Five 1,4,7,10-tetraazacyclododecane N,N',N'',N''-tetra-acetic acid (DOTA)- and DO2A-based bisphosphonates, including monomeric and dimeric structures and one 1,4,7-triazacyclononane-1,4-diacetic acid (NO2A) derivative, were synthesized and labelled with n.c.a. (177)Lu. Radio-TLC and high-performance liquid chromatography (HPLC) methods were successfully established for determining radiochemical yields and for quality control. Their binding to hydroxyapatite was measured in vitro. Ex vivo biodistribution experiments and dynamic in vivo single photon computed tomography (SPECT)/CT measurements were performed in healthy rats for 5 min and 1 h periods. Data on %ID/g or standard uptake value (SUV) for femur, blood, and soft-tissue organs were analyzed and compared with [(177)Lu]citrate. RESULTS: Radiolabelling yields for [(177)Lu]Lu-DOTA and [(177)Lu]Lu-NO2A monomeric bisphosphonate complexes were >98 % within 15 min. The dimeric macrocyclic bisphosphonates showed a decelerated labelling kinetics, reaching a plateau after 30 min of 60 to 90 % radiolabelling yields. All (177)Lu-bisphosphonate complexes showed exclusive accumulation in the skeleton. Blood clearance and renal elimination were fast. SUV data (all for 1 h p.i.) in the femur ranged from 3.34 to 5.67. The bone/blood ratios were between 3.6 and 135.6, correspondingly. (177)Lu-bisphosphonate dimers showed a slightly higher bone accumulation (SUVfemur = 4.48 ± 0.38 for [(177)Lu]Lu-DO2A(P(BP))2; SUVfemur = 5.41 ± 0.46 for [(177)Lu]Lu-DOTA(M(BP))2) but a slower blood clearance (SUVblood = 1.25 ± 0.09 for [(177)Lu]Lu-DO2A(P(BP))2; SUVblood = 1.43 ± 0.32 for [(177)Lu]Lu-DOTA(M(BP))2). CONCLUSIONS: Lu-complexes of macrocyclic bisphosphonates might become options for the therapy of skeletal metastases in the near future, since they show high uptake in bone together with a very low soft-tissue accumulation.

Cyclam Derivatives with a Bis(phosphinate) or a Phosphinato-Phosphonate Pendant Arm: Ligands for Fast and Efficient Copper(II) Complexation for Nuclear Medical Applications.

Cyclam derivatives bearing one geminal bis(phosphinic acid), -CH2PO2HCH2PO2H2 (H2L(1)), or phosphinic-phosphonic acid, -CH2PO2HCH2PO3H2 (H3L(2)), pendant arm were synthesized and studied as potential copper(II) chelators for nuclear medical applications. The ligands showed good selectivity for copper(II) over zinc(II) and nickel(II) ions (log KCuL = 25.8 and 27.7 for H2L(1) and H3L(2), respectively). Kinetic study revealed an unusual three-step complex formation mechanism. The initial equilibrium step leads to out-of-cage complexes with Cu(2+) bound by the phosphorus-containing pendant arm. These species quickly rearrange to an in-cage complex with cyclam conformation II, which isomerizes to another in-cage complex with cyclam conformation I. The first in-cage complex is quantitatively formed in seconds (pH ≈5, 25 °C, Cu:L = 1:1, cM ≈ 1 mM). At pH >12, I isomers undergo nitrogen atom inversion, leading to III isomers; the structure of the III-[Cu(HL(2))] complex in the solid state was confirmed by X-ray diffraction analysis. In an alkaline solution, interconversion of the I and III isomers is mutual, leading to the same equilibrium isomeric mixture; such behavior has been observed here for the first time for copper(II) complexes of cyclam derivatives. Quantum-chemical calculations showed small energetic differences between the isomeric complexes of H3L(2) compared with analogous data for isomeric complexes of cyclam derivatives with one or two methylphosphonic acid pendant arm(s). Acid-assisted dissociation proved the kinetic inertness of the complexes. Preliminary radiolabeling of H2L(1) and H3L(2) with (64)Cu was fast and efficient, even at room temperature, giving specific activities of around 70 GBq of (64)Cu per 1 µmol of the ligand (pH 6.2, 10 min, ca. 90 equiv of the ligand). These specific activities were much higher than those of H3nota and H4dota complexes prepared under identical conditions. The rare combination of simple ligand synthesis, very fast copper(II) complex formation, high thermodynamic stability, kinetic inertness, efficient radiolabeling, and expected low bone tissue affinity makes such ligands suitably predisposed to serve as chelators of copper radioisotopes in nuclear medicine.

Gallium(III) complexes of NOTA-bis (phosphonate) conjugates as PET radiotracers for bone imaging.

Ligands with geminal bis(phosphonic acid) appended to 1,4,7-triazacyclonone-1,4-diacetic acid fragment through acetamide (NOTAM(BP) ) or methylenephosphinate (NO2AP(BP) ) spacers designed for (68) Ga were prepared. Ga(III) complexation is much faster for ligand with methylenephosphinate spacer than that with acetamide one, in both chemical (high reactant concentrations) and radiolabeling studies with no-carrier-added (68) Ga. For both ligands, formation of Ga(III) complex was slower than that with NOTA owing to the strong out-of-cage binding of bis(phosphonate) group. Radiolabeling was efficient and fast only above 60 °C and in a narrow acidity region (pH ~3). At higher temperature, hydrolysis of amide bond of the carboxamide-bis(phosphonate) conjugate was observed during complexation reaction leading to Ga-NOTA complex. In vitro sorption studies confirmed effective binding of the (68) Ga complexes to hydroxyapatite being comparable with that found for common bis(phosphonate) drugs such as pamindronate. Selective bone uptake was confirmed in healthy rats by biodistribution studies ex vivo and by positron emission tomography imaging in vivo. Bone uptake was very high, with SUV (standardized uptake value) of 6.19 ± 1.27 for [(68) Ga]NO2AP(BP) ) at 60 min p.i., which is superior to uptake of (68) Ga-DOTA-based bis(phosphonates) and [(18) F]NaF reported earlier (SUV of 4.63 ± 0.38 and SUV of 4.87 ± 0.32 for [(68) Ga]DO3AP(BP) and [(18) F]NaF, respectively, at 60 min p.i.). Coincidently, accumulation in soft tissue is generally low (e.g. for kidneys SUV of 0.26 ± 0.09 for [(68) Ga]NO2AP(BP) at 60 min p.i.), revealing the new (68) Ga complexes as ideal tracers for noninvasive, fast and quantitative imaging of calcified tissue and for metastatic lesions using PET or PET/CT.

Thermodynamic and kinetic study of scandium(III) complexes of DTPA and DOTA: a step toward scandium radiopharmaceuticals.

Diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) scandium(III) complexes were investigated in the solution and solid state. Three (45)Sc NMR spectroscopic references suitable for aqueous solutions were suggested: 0.1 M Sc(ClO4)3 in 1 M aq. HClO4 (δSc =0.0 ppm), 0.1 M ScCl3 in 1 M aq. HCl (δSc =1.75 ppm) and 0.01 M [Sc(ox)4](5-) (ox(2-) = oxalato) in 1 M aq. K2C2O4 (δSc =8.31 ppm). In solution, [Sc(dtpa)](2-) complex (δSc = 83 ppm, Δν = 770 Hz) has a rather symmetric ligand field unlike highly unsymmetrical donor atom arrangement in [Sc(dota)](-) anion (δSc = 100 ppm, Δν = 4300 Hz). The solid-state structure of K8[Sc2(ox)7]⋅13 H2O contains two [Sc(ox)3](3-) units bridged by twice "side-on" coordinated oxalate anion with Sc(3+) ion in a dodecahedral O8 arrangement. Structures of [Sc(dtpa)](2-) and [Sc(dota)](-) in [(Hguanidine)]2[Sc(dtpa)]⋅3 H2O and K[Sc(dota)][H6 dota]Cl2⋅4 H2O, respectively, are analogous to those of trivalent lanthanide complexes with the same ligands. The [Sc(dota)](-) unit exhibits twisted square-antiprismatic arrangement without an axial ligand (TSA' isomer) and [Sc(dota)](-) and (H6 dota)(2+) units are bridged by a K(+) cation. A surprisingly high value of the last DOTA dissociation constant (pKa =12.9) was determined by potentiometry and confirmed by using NMR spectroscopy. Stability constants of scandium(III) complexes (log KScL 27.43 and 30.79 for DTPA and DOTA, respectively) were determined from potentiometric and (45)Sc NMR spectroscopic data. Both complexes are fully formed even below pH 2. Complexation of DOTA with the Sc(3+) ion is much faster than with trivalent lanthanides. Proton-assisted decomplexation of the [Sc(dota)](-) complex (τ1/2 =45 h; 1 M aq. HCl, 25 °C) is much slower than that for [Ln(dota)](-) complexes. Therefore, DOTA and its derivatives seem to be very suitable ligands for scandium radioisotopes.

Diffusion properties of conventional and calcium-sensitive MRI contrast agents in the rat cerebral cortex.

Calcium-sensitive MRI contrast agents can only yield quantitative results if the agent concentration in the tissue is known. The agent concentration could be determined by diffusion modeling, if relevant parameters were available. We have established an MRI-based method capable of determining diffusion properties of conventional and calcium-sensitive agents. Simulations and experiments demonstrate that the method is applicable both for conventional contrast agents with a fixed relaxivity value and for calcium-sensitive contrast agents. The full pharmacokinetic time-course of gadolinium concentration estimates was observed by MRI before, during and after intracerebral administration of the agent, and the effective diffusion coefficient D* was determined by voxel-wise fitting of the solution to the diffusion equation. The method yielded whole brain coverage with a high spatial and temporal sampling. The use of two types of MRI sequences for sampling of the diffusion time courses was investigated: Look-Locker-based quantitative T(1) mapping, and T(1) -weighted MRI. The observation times of the proposed MRI method is long (up to 20 h) and consequently the diffusion distances covered are also long (2-4 mm). Despite this difference, the D* values in vivo were in agreement with previous findings using optical measurement techniques, based on observation times of a few minutes. The effective diffusion coefficient determined for the calcium-sensitive contrast agents may be used to determine local tissue concentrations and to design infusion protocols that maintain the agent concentration at a steady state, thereby enabling quantitative sensing of the local calcium concentration.

PiB-Conjugated, Metal-Based Imaging Probes: Multimodal Approaches for the Visualization of ß-Amyloid Plaques.

In an effort toward the visualization of ß-amyloid plaques by in vivo imaging techniques, we have conjugated an optimized derivative of the Pittsburgh compound B (PiB), a well-established marker of Aß plaques, to DO3A-monoamide that is capable of forming stable, noncharged complexes with different trivalent metal ions including Gd(3+) for MRI and (111)In(3+) for SPECT applications. Proton relaxivity measurements evidenced binding of Gd(DO3A-PiB) to the amyloid peptide Aß1-40 and to human serum albumin, resulting in a two- and four-fold relaxivity increase, respectively. Ex vivo immunohistochemical studies showed that the DO3A-PiB complexes selectively target Aß plaques on Alzheimer's disease human brain tissue. Ex vivo biodistribution data obtained for the (111)In-analogue pointed to a moderate blood-brain barrier (BBB) penetration in adult male Swiss mice (without amyloid deposits) with 0.36% ID/g in the cortex at 2 min postinjection.

Methylene-bis[(aminomethyl)phosphinic acids]: synthesis, acid-base and coordination properties.

Three symmetrical methylene-bis[(aminomethyl)phosphinic acids] bearing different substituents on the central carbon atom, (NH(2)CH(2))PO(2)H-C(R(1))(R(2))-PO(2)H(CH(2)NH(2)) where R(1) = OH, R(2) = Me (H(2)L(1)), R(1) = OH, R(2) = Ph (H(2)L(2)) and R(1),R(2) = H (H(2)L(3)), were synthesized. Acid-base and complexing properties of the ligands were studied in solution as well as in the solid state. The ligands show unusually high basicity of the nitrogen atoms (log K(1) = 9.5-10, log K(2) = 8.5-9) if compared with simple (aminomethyl)phosphinic acids and, consequently, high stability constants of the complexes with studied divalent metal ions. The study showed the important role of the hydroxo group attached to the central carbon atom of the geminal bis(phosphinate) moiety. Deprotonation of the hydroxo group yields the alcoholate anion which tends to play the role of a bridging ligand and induces formation of polynuclear complexes. Solid-state structures of complexes [H(2)N=C(NH(2))(2)][Cu(2)(H(-1)L(2))(2)]CO(3)·10H(2)O and Li(2)[Co(4)(H(-1)L(1))(3)(OH)]·17.5H(2)O were determined by X-ray diffraction. The complexes show unexpected geometries forming dinuclear and cubane-like structures, respectively. The dinuclear copper(II) complex contains a bridging µ(2)-alcoholate group with the (-)O-P(=O)-CH(2)-NH(2) fragments of each ligand molecule chelated to the different central ion. In the cubane cobalt(II) complex, one µ(3)-hydroxide and three µ(3)-alcoholate anions are located in the cube vertices and both phosphinate groups of one ligand molecule are chelating the same cobalt(II) ion while each of its amino groups are bound to different neighbouring metal ions. All such three metal ions are bridged by the alcoholate group of a given ligand.