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Complexes of Cu(ii) with studied ligands show very short NMR longitudinal relaxation times T1 of 19F nuclei and good T2*/T1 ratio. Thus, Cu(ii) can be considered as a suitable metal ion for the design of efficient 19F MRI/NMR relaxation agents.

A series of Cu(II) complexes with cyclam-based ligands containing two

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...

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.

A series of lanthanide(III) complexes of a monophosphinate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methyl[(N,N-dibenzylamino)methyl]phosphinic acid (H4do3apDBAm = H4L1), were prepared and their solid-state structures were studied using single-crystal X-ray diffraction. In all structures, the ligand anion was octadentately coordinated to the Ln(III) or Sc(III) ions similarly to other DOTA-like ligands, i.e. forming parallel N4- and O4-planes. The lighter lanthanide(III) complexes (till dysprosium) were nonacoordinated in the twisted square-antiprismatic (TSA) configuration with the apical coordination of water molecules or oxygen atoms from the neighbouring complex unit. The heavier lanthanide(III) complexes (from terbium) were found as the “anhydrous” octacoordinated twisted square-antiprismatic (TSA′) isomer. For the terbium(III) ion, both forms were structurally characterized. The structural data of the Ln(III)–H4L1 complexes and complexes of several related DOTA-like ligands were analysed. It clearly showed that the structural parameters for the square-antiprismatic (SA) isomers were clustered in a small range while those for the TSA/TSA′ isomers were significantly more spread. The analysis also gave useful information about the influence of various pendant arms on the structure of the complexes of the DOTA-like ligands. The twist angle (torsion) of the chelate ring containing a larger phosphorus atom was similar to those of the remaining three acetate pendants. It led to a larger separation of the N4⋯O4 planes and to smaller trans-O–Ln–O angles than the parameters found in the complexes of H4dota and its tetraamide derivatives dotam(R). It resulted in a relatively long bond between the metal ion and the coordinated water molecule. It led, together with the negative charge of the oxygen atoms forming the O4-plane, to an extremely fast water exchange rate reported for the Gd(III)–H4L1 complex and, generally, to a fast water exchange of Gd(III) complexes with the monophosphorus acid analogues of H4dota, H5do3ap/H4do3apR.

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.

In order to develop novel, more efficient, and/or selective contrast agents for magnetic resonance imaging (MRI), different

Aminoalkyl

International audience; In order to develop novel, more efficient and/or selective contrast agents for magnetic resonance imaging (MRI), different modi operandi are explored as alternatives to water-relaxation enhancement. In this work, cobalt(II/III) complexes of bis(N-trifluoroethy)cyclam derivatives with two acetate or two phosphonate pendant arms, H2te2f2a and H4te2f2p, were prepared and investigated. X-ray diffraction structures confirmed octahedral coordination with a very stable trans-III cyclam conformation and with fluorine atoms located about 5.3 Å from the metal center. The Co(II) complexes are kinetically inert, decomposing slowly even in 1 M aqueous HCl at 80 °C. The Co(II) complexes exhibited well-resolved paramagnetically-shifted NMR spectra. 2 These were interpreted with the help of quantum chemistry calculations. The 13 C NMR shifts of the trans-[Co II (te2f2p)] complex were successfully assigned based on spin density delocalization within the ligand molecule. The obtained spin density also helps to describe d-metal-induced NMR relaxation properties of 19 F nuclei, including the contribution of a Fermi contact relaxation mechanism. The paramagnetic complexes show convenient relaxation properties to be used as 19 F MRI contrast agents.

19F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable 19F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested. Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. 19F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo 19F MRI was performed at 4.7 T. 19F relaxation times strongly depended on the chelated lanthanide(III) ion. T1 ranged from 6.5 to 287 ms, T2 from 3.9 to 124.4 ms, and T2* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; 19F MRI in vivo confirmed the in vivo applicability of the probe. DOTP-based 19F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd3+ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

1H spin–lattice relaxation studies of water solutions of Bismuth-ethylenediamine-tetraacetic acid (Bi- EDTA), Bismuth-ethylenediamine-tetrakis(methylenephosphonic) acid (Bi-EDTP), Bismuth-1,4,7,10- tetraazacyclododecane-1,4,7,10-tetraacetic acid (Bi-DOTA), Bismuth-1,4,7,10-tetraazacyclodo decane-1,4,7,10-tetrakis(methylenephosphonic acid) (Bi-DOTP) and Bismuth-1,4,7,10-tetraazacyclo dodecane-1,4,7-triacetic acid (Bi-DO3A) have been performed in order to compare Quadrupole Relaxation Enhancement (QRE) effects with Paramagnetic Relaxation Enhancement (PRE) from the perspective of exploiting the first one as a novel contrast mechanism for Magnetic Resonance Imag- ing (MRI). The selected compounds can be considered as 209Bi counterparts of Gd3+ complexes. The relaxation experiments havebeenperformed in a broad frequency range of5 kHz–30 MHz.Therelax- ation contribution associated with QRE has been extracted from the data and compared with PRE. Similarities and differences between the two effects have been discussed.

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 (pKa ∼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, unlik...

H(3)nota 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 H(3)nota, the comprehensive study of its solution chemistry has been overlooked. Thus, the coordination behavior of H(3)nota 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 K-a = 13.17). The high stability of the [Cu(nota)](-) complex (log K-ML = 23.33) results in quantitative complex formation, even at pH Na(I) > K(I)). Thus, H(3)nota 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 c(Cu) approximate to 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 (tau(1/2) 46 s; pH 0, 25 degrees C) and is less resistant to acid-assisted decomplexation than Cu(II) complexes with H(4)dota and H(4)teta. Surprisingly, [Cu(nota)](-) decomplexation is decelerated in the presence of Zn(II) ions due to the formation of a stable dinuclear complex. In conclusion, H(3)nota 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.

Four dipicolylamine (DPA) derivatives bearing methylphosphonic or methylphosphinic acid (P–R; R = H, Me, CH2PO2H2) groups were synthesized. Their acid–base and coordination properties were studied by potentiometry, UV-Vis and NMR measurements. The phosphonate derivative shows increased basicity (log K1 = 8.39), whereas the phosphinate derivatives show decreased basicity (log K1 = 6–7) compared to the parent dipicolylamine. Consequently, the stability constants of the phosphonate complexes are 3 to 4 orders of magnitude higher than those of the phosphinate complexes. All ligands show excellent selectivity for Zn(II) over Ca(II) and Mg(II) ions. The structures of several Cu(II) and Ni(II) complexes in the solid state were determined by X-ray diffraction analysis. The complexes mostly show dimeric or polymeric structures and the two metal ions induce a different coordination geometry of the DPA group. The coordination geometry is always (pseudo)octahedral. The DPA fragment is bound in mer geometry in the Cu(II) complexes, whereas the Ni(II) complexes have fac geometry. In conclusion, the phosphonate and phosphinate derivatives of DPA are efficient complexing agents for divalent transition metal ions and the DPA-phosphinate grouping is a suitable fragment for sensing Zn(II) ions.

Selectivity and rate of complex formation with metal radionuclides are crucial parameters for the utilization of ligating systems in nuclear medicine. One of the very suitable metals used in these applications is copper, which has a number of radioisotopes with useful properties. The thermodynamic and kinetic properties of Cu(II) complexes with 10 macrocyclic 1,4,8-trimethylcyclam-based ligands having one coordinating acid pendant arm (Me3cyclam–R) were investigated in solution, allowing a direct comparison of the influence of the particular pendant arm on the properties of the complexes. They include the derivative with R = CH2CO2H (HL1) and a family containing various phosphorus acids R = CH2P(O)(OH)–X, where X = OEt (HL2); X = OH (H2L3); X = H (HL4); X = CH2CH2CO2H (H2L5); X = CH2P(O)(H)OH (H2L6); X = CH2P(O)(OH)2 (H3L7); X = CH2N(CH2C6H5)2 (HL8); X = CH2NH2 (HL9) and X = CH2N(CH2CO2H)2 (H3L10). For comparison, 1,4,8,11-tetramethylcyclam (TMC: R = Me) was used. The formation kinetics showed that ligands endowed with a coordinating pendant arm bind Cu(II) ions much faster when compared to TMC. At pH < 4, the fastest complexation was observed for acetate derivative HL1. At higher pH and, especially, at pH relevant for living and biocompatible systems (pH ≈ 6–7), the ligands with methylene(phosphonatomethyl)phosphinate and methylenephosphonate pendant arms (H3L7 and H2L3) showed the fastest complexation. Acid-assisted dissociation of Cu(II) complexes with the ligands endowed with a coordinating pendant arm is similar for all studied systems (τ1/2 = 7–35 min, 1 M HClO4, 25 °C). In contrast, the inertness of the Cu(II)–TMC complex is much lower (τ1/2 = 22 s under the same conditions). Potentiometric study of the selected ligands has confirmed a high thermodynamic selectivity of the studied ligands for Cu(II) binding over complexation of Ni(II) and Zn(II) (the differences between the stability constants reach 6–7 orders of magnitude). Fast complexation of Cu(II) at radio-level concentrations was observed, showing that the best ligands for potential in vivo use are those containing phosphonate or mixed geminal phosphinate–phosphonate pendant groups. One of the ligands (H2L3) and three Cu(II) complexes (of H2L5, H3L7 and H3L10) were structurally characterized by means of X-ray diffraction study. The predicted conformation I of the macrocycle was confirmed in all three complexes.

1,8-Bis(2,2,2-trifluoroethyl)cyclam (te2f) derivatives with two coordinating pendant arms involving methylenecarboxylic acid (H2te2f2a), methylenephosphonic acid (H4te2f2p), (2-pyridyl)methyl (te2f2py), and 2-aminoethyl arms (te2f2ae) in 4,11-positions were prepared, and their nickel(II) complexes were investigated as possible 19F MR tracers. The solid-state structures of several synthetic intermediates, ligands, and all complexes were confirmed by X-ray diffraction analysis. The average Ni···F distances were determined to be about 5.2 A. All complexes exhibit a trans-III cyclam conformation with pendant arms bound in the apical positions. Kinetic inertness of the complexes is increased in the ligand order te2f2ae ≪ te2f < te2f2py ≈ H4te2f2p ≪ H2te2f2a. The [Ni(te2f2a)] complex is the most kinetically inert Ni(II) complex reported so far. Paramagnetic divalent nickel caused a shortening of 19F NMR relaxation time down to the millisecond range. Solubility, stability, and cell toxicity were only satisfactor...

A new DOTA-like ligand H5do3aNP with a 2-[amino(methylphosphonic acid)]ethyl-coordinating pendant arm was prepared, and its coordinating properties were studied by NMR spectroscopy and potentiometry. The study revealed a rare slow exchange (on the 1H and 31P NMR time scale) between protonated and unprotonated complex species with a corresponding acidity constant pKA ∼ 8.0. This unusually slow time scale associated with protonation is caused by a significant geometric change from square-antiprismatic (SA) arrangement observed for protonated complex SA-[Eu(Hdo3aNP)]− to twisted-square-antiprismatic (TSA) arrangement found for deprotonated complex TSA-[Eu(do3aNP)]2–. This behavior results in simultaneous occurrence of the signals of both species in the 31P NMR spectra at approximately −118 and +70 ppm, respectively. Such an unprecedented difference in the chemical shifts between species differing by a proton is caused by a significant movement of the principal magnetic axis and by a change of phosphorus atom...

[Abstract] The solution dynamics of the Eu(III) complexes of H4dota (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracarboxylic acid) and H5do3ap (1,4,7,10-tetraazacyclododecane-4,7,10-tris(carboxymethyl)-1-methylphosphonic acid, bound in both monoprotonated and fully deprotonated forms) were investigated by using a combination of NMR measurements and DFT calculations. In solution, an equilibrium between the square antiprismatic (SAP) and twisted-square antiprismatic isomers (TSAP) of these complexes is present. These two isomers interconvert by rotation of the pendant arms or inversion of the cyclen chelate rings. 1D EXSY NMR spectra were used to determine these exchange rates with unprecedented accuracy. It was found that the two processes occur at different rates. Additional variable-temperature measurements allowed determination of the corresponding activation parameters for the two processes. DFT calculations were then used to obtain mechanistic information at the molecular level. The results show that the cyclen inversion pathway involves stepwise inversion of the four chelate rings formed upon metal ion coordination. However, the arm rotation process may operate through a synchronous rotation of the pendant arms or a stepwise mechanism depending on the system. A mixed cluster-continuum approach was required to improve the agreement between experimental and calculated activation parameters for the arm rotation process. The obtained results will aid the design of MRI contrast agents. Furthermore, the methodology developed in this work can be further applied for the investigation of other dynamic paramagnetic systems, e.g. peptides with Ln(III) probes or natively paramagnetic metalloproteins. Czech Science Foundation; 16-03156S Charles University; 1076016 Czech Republic. Ministry of Education; LTC 170607 Ministerio de Economía y Competitividad; CTQ2013-43243-P

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.

Protonation of a distant, noncoordinated group of metal-based magnetic resonance imaging contrast agents potentially changes their relaxivity. The effect of a positive charge of the drug on the human serum albumin (HSA)-drug interaction remains poorly understood as well. Accordingly, a (dibenzylamino)methylphosphinate derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was efficiently synthesized using pyridine as the solvent for a Mannich-type reaction of tBu3DO3A, formaldehyde, and Bn2NCH2PO2H2 ethyl ester. The ligand protonation and metal ion (Gd3+, Cu2+, and Zn2+) stability constants were similar to those of the parent DOTA, whereas the basicity of the side-chain amino group of the complexes (log KA = 5.8) was 1 order of magnitude lower than that of the free ligand (log KA = 6.8). The presence of one bound water molecule in both deprotonated and protonated forms of the gadolinium(III) complex was deduced from the solid-state X-ray diffraction data [gadolinium(III) and dysprosium(III)], from the square antiprism/twisted square antiprism (SA/TSA) isomer ratio along the lanthanide series, from the fluorescence data of the europium(III) complex, and from the 17O NMR measurements of the dysprosium(III) and gadolinium(III) complexes. In the gadolinium(III) complex with the deprotonated amino group, water exchange is extremely fast (τM = 6 ns at 25 °C), most likely thanks to the high abundance of the TSA isomer and to the presence of a proximate protonable group, which assists the water-exchange process. The interaction between lanthanide(III) complexes and HSA is pH-dependent, and the deprotonated form is bound much more efficaciously (∼13% and ∼70% bound complex at pH = 4 and 7, respectively). The relaxivities of the complex and its HSA adduct are also pH-dependent, and the latter is approximately 2-3 times increased at pH = 4-7. The relaxivity for the supramolecular HSA-complex adduct ( r1b) is as high as 52 mM-1 s-1 at neutral pH (at 20 MHz and 25 °C). The findings of this study stand as a proof-of-concept, showing the ability to manipulate an albumin-drug interaction, and thus the blood pool residence time of the drug, by introducing a positive charge in a side-chain amino group.

A macrobicyclic cyclen-based ligand containing a geminal bis(phosphinate) N,N′-bridge was synthesized via a Mannich-type reaction. The other two macrocycle amine groups were modified with two acetate pendants, yielding ligand H4L. The cyclen-bis(phosphinate) intermediate, the ligand salt Na0.5(H5L)(H3.5L)Cl·20H2O and CuII complex Li2[Cu(L)]·4H2O were characterized by single-crystal X-ray diffraction. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) and lanthanide(III) ions were determined by potentiometry. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) ions are similar to those of H4dota. Conversely, the stabilities of its Ln(III) complexes are ~ 7–10 orders of magnitude lower than those of H4dota complexes due to steric restrictions of the methylene-bis(phosphinate) side bridge. Therefore, these findings indicate that the ligand H4L is suitable for the complexation of transition metal ions.

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).

Complexation of Eu(III) ions with H 4 dota and its bis(phosphonate)-bearing analogs, H 8 do3aP BP with phosphinate spacer and H 7 dotam BP with carboxamide spacer, was studied by UV–Vis spectrophotometry and luminescence life-time measurements. The results show formation of two types of out-of-cage intermediates. The [EuL] oc intermediate is formed under metal ion excess, whereas [EuL 2 ] oc intermediate is formed under ligand excess. At room temperature, their conversion to in-cage complex is comparable but the [EuL 2 ] oc transformation into the in-cage complex is significantly faster at elevated temperatures. Both bis(phosphonate)-bearing ligands show slower complexation rate than H 4 dota. Comparison of reactivity of H 8 do3aP BP and H 7 dotam BP show important role of the spacer connecting the bis(phosphonate) group with the macrocycle. Presence of phosphinate in H 8 do3aP BP leads to significantly faster (3–4 orders of magnitude) complexation than in case of the amide spacer in H 7 dotam BP . Both bis(phosphonate)-bearing ligands show nonlinear dependence of the complexation rate on concentration of hydroxide anions due to changes in protonation state of the bis(phosphonate) moiety.

A novel macrocyclic DO3A derivative containing a linear diamine pendant arm, H3do3aNN, was prepared and its protonation and complexation properties were studied by means of potentiometry. It determined ligand consecutive protonation constants log K(An) = 12.62, 10.28, 9.67, 8.30, 3.30 and 1.58 and stability constants of selected lanthanide (Eu(iii), Yb(iii)) complexes log K(EuL) = 23.16 and log KYbL = 22.76. The complexes could be protonated on the pendant amino group(s) with log K(HLM) ≈ 5.6 and log K(H2LM) ≈ 4.8. Solution structures of both complexes were studied by NMR spectroscopy. The study revealed that the complex species exist exclusively in the form of twisted-square-antiprismatic (TSA) isomers. The complexes show significant pH dependence of the Chemical Exchange Saturation Transfer (CEST) between their amino groups and the bulk water molecules in the pH range of 5-8. Thus, the pH dependence of the magnetization transfer ratio of CEST signals can be used for pH determination using magnetic resonance imaging techniques in a pH range relevant for in vivo conditions.

The kinetic properties of Cu(ii) complexes of H4dota and its analogues with one (H5do3ap), two in the 1,7-position (trans-H6do2a2p), three (H7doa3p) and four (H8dotp) phosphonic acid pendant arms were investigated. The formation of a Cu(ii) complex with H4dota, trans-H6do2a2p and H8dotp at a slightly acidic pH is faster for the phosphonic acid derivatives than for H4dota, but with no simple dependence on the number of -CH2PO3H2 substituents (trans-H6do2a2p > H8dotp > H4dota; pH 4-6). Relative differences in the reactivity among the differently protonated species (HnL(x-)) of the same ligand are successively decreased with the more phosphonic acid groups in the ligand. The faster complexation is probably caused by the higher ability of phosphonates to bind the metal ion and/or to assist in the transfer of protons from the ring amine groups to the bulk water. The acid-assisted decomplexation kinetics of the complexes was followed in highly acidic solutions ([H(+)] = 0.01-5 M) and at different temperatures (15-70 °C) to determine the activation parameters of the reaction. The kinetic inertness of the Cu(ii) complexes follows the order: H4dota > H5do3ap > trans-H6do2a2p > H7doa3p > H8dotp. To obtain information on the influence of additional pendant arms, analogous data were obtained for trans-H2do2a. The ligand is less reactive than H4dota, but the kinetic inertness of its Cu(ii) complex is similar to that of the H4dota complex. As it was considered that the published thermodynamics data on the Cu(ii)-H8dotp system are probably incorrect, the system was re-investigated. It showed a very high stability for the [Cu(dotp)](6-) species and the easy formation of several Cu2L species in the presence of an excess of the metal ion. Also, the structure of the (H6doa3p)(-) anion in the solid state was determined. These experimental data demonstrate that the substitution of acetic acid pendant arms by methylphosphonic acid ones in H4dota-like ligands increases the rate of complexation but significantly decreases the kinetic inertness of the Cu(ii) complexes.

Objective 19F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable 19F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested. Methods Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. 19F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo 19F MRI was performed at 4.7 T. Results 19F relaxation times strongly depended on the chelated lanthanide(III) ion. T1 ranged from 6.5 to 287 ms, T2 from 3.9 to 124.4 ms, and T2* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; 19F MRI in vivo confirmed the in vivo applicability of the probe. Conclusion DOTP-based 19F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd3+ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

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.

In order to compare the coordination properties of 1,4,7-triazacyclononane (tacn) derivatives bearing varying numbers of phosphinic/carboxylic acid pendant groups towards 68Ga, 1,4,7-triazacyclononane-7-acetic-1,4-bis(methylenephosphinic) acid (NOPA) and 1,4,7- triazacyclononane-4,7-diacetic-1-[methylene(2-carboxyethyl)phosphinic] acid (NO2AP) were synthesized using Mannich reactions with trivalent or pentavalent forms of H-phosphinic acids as phosphorus components. Stepwise protonation constants logK1–3 12.06, 3.90 and 1.95, and stability constants with GaIII and CuII, logKGaL 24.01 and logKCuL 16.66, were potentiometrically determined for NOPA. Both ligands were labelled with 68Ga and compared with NOTA (tacn-N,N′,N″-triacetic acid) and NOPO, a TRAP-type [tacn-N,N′,N″- tris(methylenephosphinic acid)] chelator. At pH 3, NOPO and NOPA showed higher labelling efficiency (binding with lower ligand excess) at both room temperature and 95 °C, compared to NO2AP and NOTA. Labelling efficiency at pH = 0–3 correlated with a number of phosphinic acid pendants: NOPO &gt, &gt, NOPA &gt, NO2AP &gt, NOTA, however, it was more apparent at 95 °C than at room temperature. By contrast, NOTA was found to be labelled more efficiently at pH &gt, 4 compared to the ligands with phosphinic acids. Overall, replacement of a single phosphinate donor with a carboxylate does not challenge 68Ga labelling of TRAP-type chelators. However, the presence of carboxylates facilitates labelling at neutral or weakly acidic pH.

Der erhohte Metabolismus der Knochenmatrix kann zum einen mit der SPECT- und zum anderen mit der PET-Technik visualisiert werden. Als SPECT-Nuklid wird uberwiegend 99 mTc in Form von [99mTc]Bisphosphonat-Komplexen verwendet. Die PET bietet im Gegensatz zur SPECT eine hohere lokale und temporale Auflosung. Mit dem 68Ge/68Ga-Generatorsystem existiert ein mittlerweile bewahrtes PET-Pendant zum etablierten 99mTc-Generator. Neuartige Bisphosphonate (BP) in Kombination mit makrozyklischen Chelatoren sind derzeit weltweit Gegenstand der Forschung. Zur Komplexierung von Ga(III) werden haufig DOTA und NOTA sowie deren Derivate verwendet. Speziell im Fall von DOTA-Derivaten bieten DOTA-BP die Moglichkeit, auch 177Lu stabil zu koordinieren. Dies eroffnet vielversprechende Moglichkeiten fur Theranostika: [68Ga]NOTA-BP fur die PET/CT und – im Falle hoher SUV-Werte in den Knochenmetastasen – eine patientenindividuelle Therapie mit 177Lu-Analoga. Die vorliegende Ubersicht beschreibt den aktuellen Stand zur Diagnostik von Knochenmetastasen mit [68Ga]NO2APBP und [68Ga]BPAMD und die erfolgreiche Anwendung von [177Lu]BPAMD zur Therapie. Die neuen PET-Tracer werden mit [99mTc]MDP und [18F]Fluorid verglichen. Im Zusammenhang mit dem Potenzial von [68Ga]HBED-PSMA zur Visualisierung von Knochenmetastasen werden erstmals Ergebnisse im direkten Vergleich mit [68Ga]NO2APBP vorgestellt. Im Hinblick auf die Therapie werden die vorliegenden ex-vivo-Daten mit 223RA diskutiert. Abschliesend wird ein Ausblick gegeben auf die laufenden Weiterentwicklungen hin zu neuen 68Ga- bzw. 177Lu-markierten Bisphosphonaten mit weiter verbesserten pharmakologischen Eigenschaften.

1,8-Bis(2,2,2-trifluoroethyl)cyclam (te2f) derivatives with two coordinating pendant arms involving methylenecarboxylic acid (H

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.

Four geminal bis(phosphinic acids), namely, aminomethylbis(H-phosphinic acid) (H2L1) and 4-aminobutyl-1-hydroxy-1,1-bis(R-phosphinic acid) with R = H (H2L2), Me (H2L3) and CH2CH2COOH (H4L4), were studied. Their acid–base properties and coordination ability towards Cu2+, Ni2+ and Zn2+ ions were studied by potentiometry, UV/Vis spectroscopy and NMR spectroscopy. The amine group in H2L1 has a lower protonation constant (log Ka = 6.79) than those found for other studied bisphosphinates (log Ka = 10.75–11.05) with distant amine groups. The structure of [Ca(H2L2-O,O′)(HL2-O,O′)]Cl revealed an octahedral arrangement of the metal coordination sphere and a linear polymeric structure, which forms through eight-membered Ca(–O–P–O–)2Ca rings. The structure of [Cu(HL3-O,O′)2(H2O)]·5H2O shows two chelating bisphosphinate groups in an equatorial O4 environment. The structure of [Cu(H0.5L3-O,O′)(NO3)0.5]·2.25H2O shows two different coordination environments, one is an elongated tetragonal pyramid, and the other is a trigonal bipyramid with a bidentate nitrate ion.

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.

177 Lu radiolabeling of the first (G1-) or fourth (G4-) generation polyaminoamide (PAMAM) dendrimer conjugates with DOTA-like bifunctional chelator with one methylenepyridine-N-oxide pendant arm (DO3A-py NO-C ) stability of the radiolabeled species and their pharmacokinetic characteristics were evaluated in preclinical experiments. The results showed that the G1- and G4-dendrimer conjugates, modified in average with 7.5 or 57 DO3A-py NO-C chelating units, respectively, can also be labeled with 177 Lu with a high specific activity and radiochemical purity even at 37 °C. The radiolabeled species were stable for at least 24 h. Distribution profile of G1-dendrimer conjugate in organs and tissues of rats was more favorable than that of G4 one. On the other hand, the later dendrimer conjugate bears a substantially higher number of metal chelators per molecule enabling binding of a considerably larger number of radiometals. Our results indicate that an employment of dendrimer-chelate conjugates with bound radiometals might represent a prospective way for radiolabeling of biologically active target-specific macromolecules to obtain markedly high specific activity.

Thermodynamic and kinetic properties of Cu(II) complexes with macrocyclic cyclen-based ligands having four phosphonic acid monoester (H4dotpOEt = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylphosphonic acid monoethylester)) or four phosphinic acid (H4dotpH = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylphosphinic acid and H4dotpPh = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis[methyl(phenyl)phosphinic acid]) pendant arms were investigated. The formation kinetics carried out in pH range 1–5.5 (t = 25 °C, I = 0.1 M KCl) shows that reactive species are (H2L)2− and (HL)3− anions and their reactivity differs by 4–5 orders of magnitude. Among studied ligands, H4dotpH is the most reactive one. Dissociation of these copper(II) complexes was investigated in the presence of perchloric acid (I = 5 M (Na,H)ClO4, proton concentration range 0.1–5.0 M) and in temperature range 10–35 °C. All studied complexes are less kinetically inert than the [Cu(dota)]2− complex and the Cu(II) complex of H4dotpH is the least kinetically inert among the studied complexes, probably due to the lowest thermodynamic stability. These results show that the ligands probably cannot be employed for copper radioisotopes complexation but the data will be utilized in design of other macrocyclic ligands intended to be used in medicinal chemistry.

The bifunctional chelator NOPO (1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid]) shows remarkably high Ga(III) complexation efficiency and comprises one carboxylic acid moiety which is not involved into metal ion coordination. An improved synthetic protocol affords NOPO with 45% overall yield. Stepwise protonation constants (log Ka), determined by potentiometry, are 11.96, 5.22, 3.77, and 1.54; the stability constant of the Ga(III) complex is log KGaL = 25.0. Within 5 min, (68)Ga(III) incorporation by NOPO is virtually quantitative at room temperature between pH 3 and 4, and at 95 °C at pH ranging from 0.5 to 7, at NOPO concentrations of 30 μM and 10 μM, respectively. During amide bond formation at the distant carboxylate using the HATU coupling reagent, an intramolecular phosphinic acid ester (phosphilactone) is formed, which is cleaved during (68)Ga complexation or in acidic media, such as trifluoroacetic acid (TFA). Phosphilactone formation can also be suppressed by complexation of Zn(2+) prior to conjugation, the resulting zinc-containing conjugates nevertheless being suitable for direct (68)Ga-labeling. In AR42J (rat pancreatic carcinoma) xenografted CD-1 nude mice, (68)Ga-labeled NOPO-NaI(3)-octreotide conjugate ((68)Ga-NOPO-NOC) showed high and fully blockable tumor uptake (13.9 ± 5% ID/g, 120 min p.i., compared to 0.9 ± 0.4% ID/g with 5 mg/kg of nonlabeled peptide). Uptake in other tissues was generally below 3% ID/g, except appearance of excretion-related activity accumulation in kidneys. NOPO-functionalized compounds tend to be more hydrophilic than the corresponding DOTA- and NODAGA-conjugates, thus promoting fast and extensive renal excretion of (68)Ga-NOPO-radiopharmaceuticals. NOPO-functionalized peptides provide suitable pharmacokinetics in vivo and meet all requirements for efficient (68)Ga-labeling even at room temperature in a kit-like manner.

A killer application? Recently, fully integrated full-body positron-emission tomography (PET) and magnetic-resonance imaging (MRI) scanners were brought to market, allowing simultaneous recording of complementary 3D data sets. By using bimodal PET/MRI probes (see figure), in vivo 3D mapping of various parameters with medical relevance could become feasible.

Preparation of two benzylic bis(phosphonic acids) modified with primary amine or carboxylic acid groups on the benzene ring is described. These compounds were prepared and characterized in the form of both bis(phosphonate) tetraesters and corresponding free acids. The phosphonic acid esters are suitable for further derivation, mainly for conjugation through the amide bond. Mild conversion of the bis(phosphonate) esters to free acids using trimethylsilylbromide allowed to work with functional groups sensitive to conditions of acidic and/or alkaline hydrolysis. Three bis(phosphonate)-containing fluorescent probes were prepared from the building blocks, utilizing amide and sulfamide bonds as spacers. Dyes containing the dansyl group, rhodamine B, and fluorescein were chosen due to their common availability and low cost. The prepared bis(phosphonate)-building blocks and modified fluorescent probes were used for adsorption studies with hydroxyapatite, the commonly used model of bone tissue. Sorption ability of the prepared bis(phosphonate) compounds was similar to that of pamidronate.

International audience; The complexation ability of DOTA analogs bearing one methylenephosphonic (DO3AP) or methylene-phosphinic (DO3AP PrA and DO3AP ABn) acid pendant arm toward scandium was evaluated. Stability constants of their scandium(III) complexes were determined by potentiometry combined with 45 Sc NMR spectroscopy. The stability constants of the monophosphinate analogues are somewhat lower than that of the Sc–DOTA complex. The phosphorus acid moiety interacts with trivalent scandium even in very acidic solutions forming out-of-cage complexes; the strong affinity of the phosphonate group to Sc(III) precludes stability constant determination of the Sc–DO3AP complex. These results were compared with those obtained by the free-ion selective radiotracer extraction (FISRE) method which is suitable for trace concentrations. FISRE underestimated the stability constants but their relative order was preserved. Nonetheless , as this method is experimentally simple, it is suitable for a quick relative comparison of stability constant values under trace concentrations. Radiolabelling of the ligands with 44 Sc was performed using the radioisotope from two sources, a 44 Ti/ 44 Sc generator and 44m Sc/ 44 Sc from a cyclotron. The best radio-labelling conditions for the ligands were pH = 4, 70 °C and 20 min which were, however, not superior to those of the parent DOTA. Nonetheless, in vitro behaviour of the Sc(III) complexes in the presence of hydroxyapatite and rat serum showed sufficient stability of 44 Sc complexes of these ligands for in vivo applications. PET images and ex vivo biodistribution of the 44 Sc–DO3AP complex performed on healthy Wistar male rats showed no specific bone uptake and rapid clearance through urine.

The mono(pyridine-N-oxide) analog of the H4dota macrocylic ligand, H3do3a-pyNO, is capable of forming thermodynamically stable and kinetically inert Ln(III) complexes. Its Eu(III) and Tb(III) complexes display a strong long-lived fluorescence as a result of the antenna effect of the pyridine- N-oxide fluorophore in the reagent. It is shown that H3do3a-pyNO can be used as a fluorogenic reagent for the determination of Eu(III) and Tb(III) at pH 6.5 and cL 1 mM. At an excitation wavelength of 286 nm, the emission maxima are 615 nm (Eu(III)-complex), and 547 nm (Tb(III)complex). Detection limits are at concentrations around 1.0 mM and linearity of the method spans over 2 orders of magnitude. The method was applied to artificial and real samples (spiked mineral waters, extracts from cathode ray tube luminophore dust) and gave satisfactory results. The method is simple, rapid, and hardly interfered by other metal ions.