Synthesis and characterization of monophosphinic acid DOTA derivative: A smart tool with functionalities for multimodal imaging.

A new facile synthetic strategy was developed to prepare bifunctional monophosphinic acid Ln-DOTA derivatives, Gd-DO2AGAPNBn and Gd- DO2AGAPABn. The relaxivities of the Gd-complexes are enhanced compared to Gd-DOTA. Monophosphinic acid arm of these Gd-complexes affords enhancement of inner sphere water exchange rate due to its steric bulkiness. The different functionalities of DO2AGAPNBn were appended in trans positions and are designed to conjugate identical or different vectors according to the potential applications. The conjugation of Gd-DO2AGAPABn with E3 peptide known to target apoptosis was successfully performed and in vivo MRI allowed cell death detection in a mouse model.

Fluorescent magnetic nanoparticles for cell labeling: flux synthesis of manganite particles and novel functionalization of silica shell.

Novel synthetic approaches for the development of multimodal imaging agents with high chemical stability are demonstrated. The magnetic cores are based on La0.63Sr0.37MnO3 manganite prepared as individual grains using a flux method followed by additional thermal treatment in a protective silica shell allowing to enhance their magnetic properties. The cores are then isolated and covered de novo with a hybrid silica layer formed through the hydrolysis and polycondensation of tetraethoxysilane and a fluorescent silane synthesized from rhodamine, piperazine spacer, and 3-iodopropyltrimethoxysilane. The aminoalkyltrialkoxysilanes are strictly avoided and the resulting particles are hydrolytically stable and do not release dye. The high colloidal stability of the material and the long durability of the fluorescence are reinforced by an additional silica layer on the surface of the particles. Structural and magnetic studies of the products using XRD, TEM, and SQUID magnetometry confirm the importance of the thermal treatment and demonstrate that no mechanical treatment is required for the flux-synthesized manganite. Detailed cell viability tests show negligible or very low toxicity at concentrations at which excellent labeling is achieved. Predominant localization of nanoparticles in lysosomes is confirmed by immunofluorescence staining. Relaxometric and biological studies suggest that the functionalized nanoparticles are suitable for imaging applications.

Gadolinium- and manganite-based contrast agents with fluorescent probes for both magnetic resonance and fluorescence imaging of pancreatic islets: a comparative study.

Three magnetic resonance (MR)/fluorescence imaging probes were tested for visualization, cellular distribution, and survival of labeled pancreatic islets in vitro and following transplantation. As T(1) contrast agents (CAs), gadolinium(III) complexes linked to ß-cyclodextrin (Gd-F-ßCD) or bound to titanium dioxide (TiO2 @RhdGd) were tested. As a T(2) CA, perovskite manganite nanoparticles (LSMO@siF@si) were examined. Fluorescein or rhodamine was incorporated as a fluorescent marker in all probes. Islets labeled with gadolinium(III) CAs were visible as hyperintense spots on MR in vitro, but detection in vivo was inconclusive. Islets labeled with LSMO@siF@si CA were clearly visible as hypointense spots or areas on MR scans in vitro as well as in vivo. All CAs were detected inside the islet cells by fluorescence. Although the vitality and function of the labeled islets was not impaired by any of the tested CAs, results indicate that LSMO@siF@si CA is a superior marker for islet labeling, as it provides better contrast enhancement within a shorter scan time.

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.

Gadolinium complexes of monophosphinic acid DOTA derivatives conjugated to cyclodextrin scaffolds: efficient MRI contrast agents for higher magnetic fields.

Middle-molecular-weight MRI contrast agents based on conjugates of a phosphinic acid DOTA analogue, 1,4,7,10-tetraazacyclododecane-4,7,10-triacetic-1-{methyl[(4-aminophenyl)methyl]phosphinic acid} (DO3AP(ABn)), with amino-substituted cyclodextrins were prepared and studied by a variety of physico-chemical methods. The conjugates were formed by reaction of the corresponding isothiocyanate with per-6-amino-α/ß-cyclodextrin and were complexed with the Ln(III) ion to get the final complexes, (LnL)(6)-α-CD and (LnL)(7)-ß-CD. Solution structure of the complexes was estimated by investigation of the Eu(III) complexes. The Gd(III) conjugate complexes are endowed with a short water residence time (τ(M) ∼ 10-15 ns at 298 K) and a high abundance of the twisted-square antiprismatic diastereoisomer. They show a high (1)H relaxivity at high fields due to a convenient combination of the fast water exchange rate and the slow rate of the molecular tumbling given by their macromolecular nature. The (1)H relaxation enhancements per molecule of a contrast agent (CA) are very high reaching for a larger (GdL)(7)-ß-CD conjugate ∼140 s(-1) mM(-1) and ∼100 s(-1) mM(-1) at 25 °C and magnetic fields 1.5 T and 3 T, respectively, which is the highest reported longitudinal relaxivity for kinetically stable contrast agents of an intermediate molecular mass (<10 kDa) with one water molecule in the first coordination sphere.

Mn2+ complexes with 12-membered pyridine based macrocycles bearing carboxylate or phosphonate pendant arm: crystallographic, thermodynamic, kinetic, redox, and 1H/17O relaxation studies.

Mn(2+) complexes represent an alternative to Gd(3+) chelates which are widely used contrast agents in magnetic resonance imaging. In this perspective, we investigated the Mn(2+) complexes of two 12-membered, pyridine-containing macrocyclic ligands bearing one pendant arm with a carboxylic acid (HL(1), 6-carboxymethyl-3,6,9,15-tetraazabicyclo[9.3.1] pentadeca-1(15),11,13-triene) or a phosphonic acid function (H(2)L(2), 6-dihydroxyphosphorylmethyl-3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene). Both ligands were synthesized using nosyl or tosyl amino-protecting groups (starting from diethylenetriamine or tosylaziridine). The X-ray crystal structures confirmed a coordination number of 6 for Mn(2+) in their complexes. In aqueous solution, these pentadentate ligands allow one free coordination site for a water molecule. Potentiometric titration data indicated a higher basicity for H(2)L(2) than that for HL(1), related to the electron-donating effect of the negatively charged phosphonate group. According to the protonation sequence determined by (1)H and (31)P pH-NMR titrations, the first two protons are attached to macrocyclic amino groups whereas the subsequent protonation steps occur on the pendant arm. Both ligands form thermodynamically stable complexes with Mn(2+), with full complexation at physiological pH and 1:1 metal to ligand ratio. The kinetic inertness was studied via reaction with excess of Zn(2+) under various pHs. The dissociation of MnL(2) is instantaneous (at pH 6). For MnL(1), the dissociation is very fast (k(obs) = 1-12 × 10(3) s(-1)), much faster than that for MnDOTA, MnNOTA, or the Mn(2+) complex of the 15-membered analogue. It proceeds exclusively via the dissociation of the monoprotonated complex, without any influence of Zn(2+). In aqueous solution, both complexes are air-sensitive leading to Mn(3+) species, as evidenced by UV-vis and (1)H NMRD measurements and X-ray crystallography. Cyclic voltammetry gave low oxidation peak potentials (E(ox) = 0.73 V for MnL(1) and E(ox) = 0.68 V for MnL(2)), in accordance with air-oxidation. The parameters governing the relaxivity of the Mn(2+) complexes were determined from variable-temperature (17)O NMR and (1)H NMRD data. The water exchange is extremely fast, k(ex) = 3.03 and 1.77 × 10(9) s(-1) for MnL(1) and MnL(2), respectively. Variable-pressure (17)O NMR measurements have been performed to assess the water exchange mechanism on MnL(1) and MnL(2) as well as on other Mn(2+) complexes. The negative activation volumes for both MnL(1) and MnL(2) complexes confirmed an associative mechanism of the water exchange as expected for a hexacoordinated Mn(2+) ion. The hydration number of q = 1 was confirmed for both complexes by (17)O chemical shifts. A relaxometric titration with phosphate, carbonate or citrate excluded the replacement of the coordinated water molecule by these small endogenous anions.

Mn2+ complexes of 1-oxa-4,7-diazacyclononane based ligands with acetic, phosphonic and phosphinic acid pendant arms: stability and relaxation studies.

A new class of macrocyclic ligands based on 1-oxa-4,7-diazacyclononane was synthesized and their Mn(2+) complexes were investigated with respect to stability and relaxation properties. Each ligand has two pendant arms involving carboxylic (H(2)L(1)--1-oxa-4,7-diazacyclononane-4,7-diacetic acid), phosphonic (H(4)L(2)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphonic acid)), phosphinic (H(2)L(3)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphinic acid)) or phenylphosphinic (H(2)L(4)--1-oxa-4,7-diazacyclononane-4,7-bis[methylene(phenyl)phosphinic acid]) acid moieties. H(2)L(3) and H(2)L(4) were synthesized for the first time. The crystal structure of the Mn(2+) complex with H(2)L(4) confirmed a coordination number of 6 for Mn(2+). The protonation constants of all ligands and the stability constants of their complexes with Mn(2+) and some biologically or biomedically relevant metal ions were determined by potentiometry. The protonation sequence of H(2)L(3) was followed by (1)H and (31)P NMR titration and the second protonation step was attributed to the second macrocyclic nitrogen atom. The potentiometric data revealed a relatively low thermodynamic stability of the Mn(2+) complexes with all ligands investigated. For H(2)L(3) and H(2)L(4), full Mn(2+) complexation cannot be achieved even with 100% ligand excess. The transmetallation of MnL(1) and MnL(2) with Zn(2+) was too fast to be followed at pH 6. Variable temperature (1)H NMRD and (17)O NMR measurements have been performed on MnL(1) and MnL(2) to provide information on water exchange and rotational dynamics. The (17)O chemical shifts indicate hydration equilibrium between mono- and bishydrated species for MnL(1), while MnL(2) is monohydrated. The water exchange is considerably faster on MnL(1) (k(ex)(298) = 1.2 × 10(9) s(-1)) than on MnL(2) (k(ex)(298) = 1.2 × 10(7) s(-1)). Small endogenous anions (phosphate, carbonate, citrate) do not replace the coordinated water in either of the complexes, but they induce their slow decomposition. All Mn(2+) complexes are stable toward air-oxidation.

Phosphonate-titanium dioxide assemblies: platform for multimodal diagnostic-therapeutic nanoprobes.

Multimodal imaging-therapeutic nanoprobe TiO(2)@RhdGd was prepared and successfully used for in vitro and in vivo cell tracking as well as for killing of cancer cells in vitro. TiO(2) nanoparticles were used as a core for phosphonic acid modified functionalities, responsible for contrast in MRI and optical imaging. The probe shows high (1)H relaxivity and relaxivity density values. Presence of fluorescent dye allows for visualization by means of fluorescence microscopy. The applicability of the probe was studied, using mesenchymal stem cells, cancer HeLa cells, and T-lymphocytes. The probe did not exhibit toxicity in any of these systems. Labeled cells were successfully visualized in vitro by means of fluorescence microscopy and MRI. Furthermore, it was shown that the probe TiO(2)@RhdGd can be changed into a cancer cell killer upon UV light irradiation. The above stated results represent a valuable proof of a principle showing applicability of the probe design for diagnosis and therapy.

Dissociation kinetics of Mn2+ complexes of NOTA and DOTA.

The kinetics of transmetallation of [Mn(nota)](-) and [Mn(dota)](2-) was investigated in the presence of Zn(2+) (5-50-fold excess) at variable pH (3.5-5.6) by (1)H relaxometry. The dissociation is much faster for [Mn(nota)](-) than for [Mn(dota)](2-) under both experimental and physiologically relevant conditions (t(½) = 74 h and 1037 h for [Mn(nota)](-) and [Mn(dota)](2-), respectively, at pH 7.4, c(Zn(2+)) = 10(-5) M, 25 °C). The dissociation of the complexes proceeds mainly via spontaneous ([Mn(nota)](-)k(0) = (2.6 ± 0.5) × 10(-6) s(-1); [Mn(dota)](2-)k(0) = (1.8 ± 0.6) × 10(-7) s(-1)) and proton-assisted pathways ([Mn(nota)](-)k(1) = (7.8 ± 0.1) × 10(-1) M(-1) s(-1); [Mn(dota)](2-)k(1) = (4.0 ± 0.6) × 10(-2) M(-1) s(-1), k(2) = (1.6 ± 0.1) × 10(3) M(-2) s(-1)). The observed suppression of the reaction rates with increasing Zn(2+) concentration is explained by the formation of a dinuclear Mn(2+)-L-Zn(2+) complex which is about 20-times more stable for [Mn(dota)](2-) than for [Mn(nota)](-) (K(MnLZn) = 68 and 3.6, respectively), and which dissociates very slowly (k(3)∼10(-5) M(-1) s(-1)). These data provide the first experimental proof that not all Mn(2+) complexes are kinetically labile. The absence of coordinated water makes both [Mn(nota)](-) and [Mn(dota)](2-) complexes inefficient for MRI applications. Nevertheless, the higher kinetic inertness of [Mn(dota)](2-) indicates a promising direction in designing ligands for Mn(2+) complexation.

Gallium(III) complexes of DOTA and DOTA-monoamide: kinetic and thermodynamic studies.

Given the practical advantages of the (68)Ga isotope in positron emission tomography applications, gallium complexes are gaining increasing importance in biomedical imaging. However, the strong tendency of Ga(3+) to hydrolyze and the slow formation and very high stability of macrocyclic complexes altogether render Ga(3+) coordination chemistry difficult and explain why stability and kinetic data on Ga(3+) complexes are rather scarce. Here we report solution and solid-state studies of Ga(3+) complexes formed with the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, (DOTA)(4-), and its mono(n-butylamide) derivative, (DO3AM(Bu))(3-). Thermodynamic stability constants, log K(GaDOTA) = 26.05 and log K(GaDO3AM(Bu)) = 24.64, were determined by out-of-cell pH-potentiometric titrations. Due to the very slow formation and dissociation of the complexes, equilibration times of up to ∼4 weeks were necessary. The kinetics of complex dissociation were followed by (71)Ga NMR under both acidic and alkaline conditions. The GaDOTA complex is significantly more inert (τ(1/2) ∼12.2 d at pH = 0 and τ(1/2) ∼6.2 h at pH = 10) than the GaDO3AM(Bu) analogue (τ(1/2) ∼2.7 d at pH = 0 and τ(1/2) ∼0.7 h at pH = 10). Nevertheless, the kinetic inertness of both chelates is extremely high and approves the application of Ga(3+) complexes of such DOTA-like ligands in molecular imaging. The solid-state structure of the GaDOTA complex, crystallized from a strongly acidic solution (pH < 1), evidenced a diprotonated form with protons localized on the free carboxylate pendants.

Towards MRI contrast agents responsive to Ca(II) and Mg(II) ions: metal-induced oligomerization of dota-bisphosphonate conjugates.

In magnetic resonance imaging (MRI), paramagnetic complexes are utilized as contrast agents. Much attention has been paid to the development of new contrast agents responsive to pH, temperature or concentration of various components of body liquids. We report a new type of MRI probe sensing the concentrations of calcium and magnesium in biological media. The ligand do3ap(BP) combines a dota-like chelator with a bisphosphonate group. In the complex, the Gd(III) ion is entrapped in the macrocyclic cavity whereas the bisphosphonate group is not coordinated and therefore is available for coordination with endogenous metal ions. In the presence of metal ions, Gd-do3ap(BP) appears to show formation of coordination oligomers leading to an unprecedented increase in r(1) up to 200-500%. The extremely high relaxivity response makes this type of compound interesting for further studies as MRI ion-responsive probes for biomedical research.

Cyclodextrin-based bimodal fluorescence/MRI contrast agents: an efficient approach to cellular imaging.

A novel bimodal fluorescence/MRI probe based on a cyclodextrin scaffold has been synthesized and characterized. The final agent employs the fluorescein (F) functionality as a fluorescence marker and the Gd(III) complex of a macrocyclic DOTA-based ligand (GdL) having one aminobenzyl-phosphinic acid pendant arm as an MRI probe, and has a statistical composition of (GdL)(6.9)-F(0.1)-beta-CD. Slow rotational dynamics (governed by a very rigid cyclodextrin scaffold) combined with fast water exchange (ensured by the chosen macrocyclic ligand) resulted in a high relaxivity of approximately 22 s(-1) mM(-1) per Gd(III) or approximately 150 s(-1) mM(-1) per molecule of the final conjugate (20 MHz, 25 degrees C). In vitro labelling of pancreatic islets (PIs) and rat mesenchymal stem cells has been successfully performed. The agent is not cytotoxic and is easily internalized into cells. The labelled cells can be visualized by MRI, as proved by the detection of individual labelled PIs. A fluorescence study performed on mesenchymal stem cells showed that the agent stays in the intracellular space for a long time.

A triazacyclononane-based bifunctional phosphinate ligand for the preparation of multimeric 68Ga tracers for positron emission tomography.

For application in positron emission tomography (PET), PrP9, a N,N',N''-trisubstituted triazacyclononane with methyl(2-carboxyethyl)phosphinic acid pendant arms, was developed as (68)Ga(3+) complexing agent. The synthesis is short and inexpensive. Ga(III) and Fe(III) complexes of PrP9 were characterized by single-crystal X-ray diffraction. Stepwise protonation constants and thermodynamic stabilities of metal complexes were determined by potentiometry. The Ga(III) complex possesses a high thermodynamic stability (log K([GaL])=26.24) and a high degree of kinetic inertness. (68)Ga labeling of PrP9 is possible at ambient temperature and in a wide pH range, also at pH values as low as 1. This means that for the first time, the neat eluate of a TiO(2)-based (68)Ge/(68)Ga generator (typically consisting of 0.1 M HCl) can be directly used for labeling purposes. The rate of (68)Ga activity incorporation at pH 3.3 and 20 degrees C is higher than for the established chelators DOTA and NOTA. Tris-amides of PrP9 with amino acid esters were synthesized to act as models for multimeric peptide conjugates. These conjugates exhibit radiolabeling properties similar to those of unsubstituted PrP9.

Mn(2+) complexes with pyridine-containing 15-membered macrocycles: thermodynamic, kinetic, crystallographic, and (1)H/(17)O relaxation studies.

Given its five unpaired d-electrons, long electronic relaxation time, and fast water exchange, Mn(2+) is a potential candidate for contrast agent application in medical magnetic resonance imaging. Nevertheless, the design of chelators that ensure stable Mn(2+) complexation and optimal relaxation properties remains a coordination chemistry challenge. Here, we report the synthesis of two pyridine-containing ligands L1 and L2, with 15-membered triaza-dioxa-crown and pentaaza-crown ether macrocycles, respectively, and the characterization of their Mn(2+) complexes. Protonation constants of the ligands and stability constants of various metal complexes were determined by potentiometry. The presence of the pyridine in the macrocyclic ring induces rigidity of the complexes which results in a greater thermodynamic stability with respect to the nonpyridine analogues. Solid-state structures of MnL1 and MnL2 confirmed seven-coordination of Mn(2+) with Cl(-) and H(2)O in axial positions. The dissociation kinetics of MnL2 in the presence of Zn(2+) were followed by relaxometric measurements. They proved the prime importance of the proton-assisted dissociation while the zinc(II)-assisted pathway is not important at physiological pH. For MnL1, the dissociation was too fast to be studied by conventional relaxivity measurements under pH 6. A combined (17)O NMR and (1)H NMRD study on MnL1 and MnL2 yielded the parameters that govern the relaxivity of these complexes. The water exchange rate for MnL1, k(ex)(298) = 0.38 x 10(7) s(-1), is the lowest value ever reported for a Mn(2+) complex, while a considerably higher value was obtained for MnL2 (k(ex)(298) = 6.9 x 10(7) s(-1)). Anion binding was studied by relaxometric titrations. They revealed weak interactions between MnL2 and phosphate or citrate, leading to the formation of monohydrated species. Overall, the incorporation of a pyridine into a polyaza macrocycle scaffold has several beneficial effects on the Mn(2+) chelates with respect to potential MRI contrast agent applications: (i) The thermodynamic and the kinetic stability of the complexes is increased. (ii) The rigidified ligand backbone results in higher coordination numbers of the metal ion, allowing for two inner-sphere water molecules in aqueous solution.

Bone-seeking probes for optical and magnetic resonance imaging.

In clinical practice the imaging of bone tissue is based almost exclusively on x-ray or radiochemical methods. Alternative methods, such as MRI and optical imaging, can provide not only anatomical, but also physiological information, due to their ability to reflect the properties of body fluids (temperature, pH and concentration of ions). In this article we review bone targeting probes for MRI and fluorescence imaging. As bone targeting is mainly associated with phosphonate and bisphosphonate derivatives, we also focus on their sorption behavior. Also discussed in detail is the limitation of using bone-targeting probes for MRI and optical imaging mainly due to their long-time retention in bone tissue and the low permeability of tissues for light.

Densely packed Gd(III)-chelates with fast water exchange on a calix[4]arene scaffold: a potential MRI contrast agent.

A pyridine-N-oxide functionalized DOTA analogue has been conjugated to a calix[4]arene and the corresponding Gd-complex was characterized with respect to its suitability as MRI contrast agent. The compound forms spherical micelles in water with a cmc of 35 microM and a radius of 8.2 nm. The relaxivity of these aggregates is 31.2 s(-1) mM(-1) at 25 degrees C and 20 MHz, which corresponds to a molecular relaxivity of 125 s(-1) mM(-1). The high relaxivity mainly originates from the short tau(M) (72.7 ns) and the size of the micelles. The interaction with bovine serum albumin (BSA) was studied and an observed relaxivity of up to 40.8 s(-1) mM(-1) (163.2 s(-1) mM(-1) per binding place) at 20 MHz and 37 degrees C was found in the presence of 2.0 mM protein.

PAMAM dendrimers conjugated with an uncharged gadolinium(III) chelate with a fast water exchange: the influence of chelate charge on rotational dynamics.

A bifunctional ligand, DO3A-py(NO-C) (DO3A-py(NO-C) = 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), was attached to different generations (G0, G1, G2, and G4) of ethylenediamine-cored PAMAM dendrimers (PAMAM = polyamidoamine). The gadolinium(III) complex of this ligand possesses one molecule of water in its first coordination sphere and has a unique combination of a short water residence lifetime (tau(M) = 34 ns), a neutral overall charge, and a possibility for rigid attachment to molecules bearing primary amino groups. These favorable properties predestine the ligand for constructions of highly efficient nanosized contrast agents for magnetic resonance imaging (MRI). The coupling reaction between the carboxylic group on the pyridine-N-oxide moiety of the protected ligand molecule and primary amines in the dendrimers was achieved by an active ester method under nonaqueous conditions using the coupling agent TBTU. This reaction afforded conjugates with high loadings (80-100% of the theoretically available primary amines) and of high purity. The gadolinium(III) complexes of the conjugates were studied by variable-temperature 17O NMR and 1H NMRD measurements. The water residence lifetime (tau(M) = 55 ns) found in the largest conjugate G4-[Gd(H2O)(do3a-py(NO-C))]57, though somewhat longer compared to the "monomeric" complex, is still short enough not to limit the relaxivity. Surprisingly, compared with analogous conjugates with negatively charged chelates, the prepared uncharged compounds displayed much faster global rotational correlation times (tau(g)) and lower relaxivities. This phenomenon can be explained on the basis of Coulomb interactions. The motion of the charged chelates is restrained due to interactions with their counterions and the chelates themselves, while the uncharged chelates are not affected. Comparison of the PAMAM-based conjugates bearing uncharged and (1-)-charged chelates based on relaxometric data, 1H DOSY spectra, and SAXS measurements reveals that tau(g) reflects the rotational motion of large segments (dendrons) of the conjugates rather than that of the whole macromolecule.

Gd(iii) complex of a monophosphinate-bis(phosphonate) DOTA analogue with a high relaxivity; Lanthanide(iii) complexes for imaging and radiotherapy of calcified tissues.

A new phosphinic-acid DOTA-like ligand, DO3AP(BP), containing a geminal bis(phosphonic acid) moiety as a highly effective bone-seeking group, was synthesized in high yield. Its crystal structure was determined by X-ray analysis. Complexation with lanthanide(iii) ions occurs under mild conditions (pH = 8-9, 25 degrees C, 2-3 h). (1)H, (31)P, and (17)O NMR spectroscopy show that DO3AP(BP) forms nine-coordinated lanthanide(iii) complexes with one water molecule in the first coordination sphere except for Ln = Er-Lu, which have in addition a species without lanthanide(iii)-bound water. Selective formation of only two diastereomers (out of four possible) suggests that the coordinated phosphinate phosphorus atom occurs exclusively in one of the enantiomeric forms. The ratio of the twisted square antiprism (TSA) and square antiprism (SA) diastereomers changes along the lanthanide series; the gadolinium(iii) complex has about 35% of the TSA species. The bis(phosphonate) moiety remains free for anchoring to osseous tissue. The (1)H longitudinal relaxivity of the Gd-DO3AP(BP) complex (r(1) = 7.4 s(-1) mM(-1), 20 MHz, 25 degrees C, pH = 7.5) is unexpectedly high compared to that of other monohydrated chelates of similar size thanks to a significant contribution from the second hydration sphere. The water residence time tau(M)(298) is 198 ns. Further increase in the relaxivity was observed in the presence of Zn(ii), Mg(ii) or Ca(ii) ions, due to formation of coordination polymers. Slowing down of the tumbling rate of the Gd-DO3AP(BP) complex upon adsorption on hydroxyapatite also leads to an increase of the relaxivity (r(1) = 17 s(-1) mM(-1), 20 MHz, 25 degrees C, pH = 7.5).

Complexation and biodistribution study of 111In and 90Y complexes of bifunctional phosphinic acid analogs of H4dota.

In this study, the complexation rates of two new phosphinate H(4)dota (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) analogs, H(5)do3ap(PrA) and H(4)do3ap(ABn), and H(4)dota itself were compared under identical conditions (H(5)do3ap(PrA)=10-{[(2-carboxyethyl)hydroxyphosphoryl]methyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid; H(4)do3ap(ABn)=10-{[(4-aminophenyl)hydroxyphosphoryl]methyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). The biodistribution of their (111)In and (90)Y complexes in healthy rats was also studied. Unlike the observation obtained under "chemical" conditions where differences between the ligands were observed no such differences in complexation rates were found under radiochemical conditions. The ligands bind the radiometals similarly to H(4)dota. So, "chemical" formation kinetic data should be transferred into radiochemical conditions only with high care and "radiochemical" complexation experiments should be run as part of standard in vitro studies with new ligands considered as potential radiopharmaceuticals. Pharmacokinetic studies in rats showed similar distribution characteristics for both phosphinate H(4)dota analogs radiolabelled with (111)In and (90)Y when compared with that of the (111)In-H(4)dota complex. No specific uptake in any organ and tissue of rats was determined. The phosphinate complexes are not accumulated in calcified tissues.