Amide conjugates of the DO3A-N-(alpha-amino)propionate ligand: leads for stable, high relaxivity contrast agents for MRI?

A novel synthetic methodology for preparing amide conjugates of the DO3A-N-(alfa-amino)propionate chelator is described, using the synthesis of the DO3A-N-(alfa-benzoylamido)propionate chelator as an illustrative example. The model Gd(DO3A-N-(alfa-benzoylamido)propionate) chelate displays accelerated water exchange, stability in a wide pH range and inertness towards transmetallation by Zn2+. The Gd(DO3A-N-(alafa-benzoylamido)propionate) complex is mainly excreted via the kidneys, producing a significant increase of the kidney medulla/cortex enhancement ratio in MR images of Wistar rats, reflecting probably its increased hydrophobicity compared to Gd(DTPA). The results presented suggest that Gd(DO3A-N-(alfa-amido)propionate) chelates can be valuable leads for preparing potentially safe high relaxivity MRI contrast agents.This work was financially supported by Fundação para a Ciência e a Tecnologia, Portugal: project PTDC/QUI/70063/2006, including a grant to C.I.O.M., grant SFRH/BD/63994/2009 to M.F.F. and grant SFRH/BD/46370/2008 to A.F.M. and Rede Nacional de RMN (REDE/1517/RMN/2005) for the acquisition of the Varian VNMRS 600 NMR spectrometer in Coimbra. T.B.R. was supported by a Marie Curie Fellowship (FP/-PEOPLE-2009-IEF 254380) and an EMBO Fellowship (ALTF 1145-2009). Financial support from La Ligue Contre le Cancer, France (E. T.), and from Ministerio de Ciencia e Innovación, Spain: projects SAF2011-23622 (S.C.) and CTQ2010-20960-C02-02 (P.L.-L.), and Comunidad de Madrid, Spain, project S2010/BMD-2349 (S.C. and P.L.-L). This work was carried out in the frame of the COST D38 Action “Metal Based Systems for Molecular Imaging” and COST TD1004

Novel Stealthy Gd(III)-DOTA/polymer Conjugates for Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is a routine diagnostic tool in modern clinical medicine. MRI has many advantages as a diagnostic imaging modality. It is noninvasive, delivers no radiation, and has excellent (submillimeter) spatial resolution. Some Gadolinium(III) complexes are commonly used to enhance the contrast between adjacent tissues when the resolution/sensitivity of MRI is too low. Because free Gd3+ is very toxic in doses required for MRI, Gd(III) is chelated by poly(amino-carboxylate) such as diethylenetriamine pentaacetic acid (DTPA) or 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Although DTPA/Gd3+ and DOTA/Gd3+ are water soluble, they have a very short circulation lifetime in blood, a low molecular weight and a short rotational time that make the contrast poor. To enhance the contrast, the Gd3+/complex doses have to be increased. In order to increase the sensitivity of the technique, while not increasing the concentration of the contrast agent, we were investigating different strategies to improve (i) the circulation lifetime in blood, (ii) the relaxation rate of Gd(III) (and consequently, the contrasting efficiency) and (iii) the targeting of the contrast agent. This presentation aims at reporting how a multifunctional (co)polymer can be designed and exploited for improving the contrasting ability and bioavailability of gadolinium-based complexes

Isotopic radiolabelling of the MR-contrast agent Gd-DOTA with Gd-147/149

Objectives Due to the new multimodal capabilities of in vivo imaging techniques, the supply of 147/149Gd appears interesting for SPECT or autoradiographic imaging with the aim to evaluate (new) MRI contrast agents. Although the production of 147/149Gd by irradiation of Sm and Eu has been published[1,2], an in vivo application of radiogadolinium has hitherto not been reported. Also the isolation of 147,149Gd from the more effective target material Eu was not described. Therefore, a bulk separation from the Eu target material was developed, followed by the authentic labeling of Gd-DOTA and a stability assessment of the complex in human blood serum (HBS). Methods A described separation of Sm2O3 and Gd2O3 with a Na-Hg amalgam[2] was systematically adapted for the new target material Eu2O3. The influence of extraction time, europium mass, amalgam mass and inert atmosphere was assessed. The resulting Eu/Gd mixture was absorbed on a DGA normal resin for the removal of excess sodium with 1 M HNO3. The lanthanides were eluted quantitatively with 0.1 M HCl, the resulting solution reduced to dryness and taken up in 0.5 ml pure water. Carrier-added and n.c.a.147/149Gd-DOTA was synthesized within 20 min at 90°C in a 0.1 M NaOAc buffer at pH 4-6 and the reaction monitored by radio-TLC. The stability of the complex in HBS was investigated at 37°C over a time period of 6 days. Results A 99.92% reduction of the Eu contamination down to 0.4 mg was achieved with the optimized Na-Hg amalgam extraction without loss of 147/149Gd. This was sufficient for first in vivo proof of principle tests with 147/149Gd-DOTA due to the strong chemical analogy of the neighboring lanthanides Gd and Eu. Removal of excess sodium was accomplished by a DGA resin enabling the application in an animal model. Carrier-added and n.c.a. 147,149Gd-DOTA were prepared with yields >99%. Additionally, over a time period of 6 days no decomposition or ion leaching was observed in HBS. Conclusions Bulk Eu was removed by amalgam extraction from c.a. as well as n.c.a 147/149Gd which was successfully used to label DOTA. This shows the possibility of an authentic labeling of (new) MRI contrast agents, allowing their in vitro or in vivo evaluation with autoradiography or SPECT. Acknowledgements The authors gratefully acknowledge the IKP-4 of FZJ for beam time and all cyclotron operators. References [1] Denzler F.-O. et al. (1997), Appl. Radiat. Isot., 48, 319. [2] Buchholz M. et al. (2014), Appl. Radiat. Isot., 91,

Gadolinium DOTA Chelates Featuring Alkyne Groups Directly Grafted on the Tetraaza Macrocyclic Ring: Synthesis, Relaxation Properties, "Click" Reaction, and High-Relaxivity Micelles

This paper reports on the synthesis and relaxivity properties of tetraacetic DOTA-type chelating agents featuring one or two alkyne groups directly grafted on the tetraaza macrocyclic ring and available for "click" reactions with azide-bearing substrates. The racemic DOTAma ligand bearing one alkyne group was obtained by a bisaminal template route. The same approach was used to prep. ligand DOTAda substituted by two alkyne groups located on two adjacent carbon atoms. The S,S enantiomer of DOTAda was also prepd. by a "crab-like" condensation. This ligand is the first example of a DOTA deriv. featuring two reactive functions adjacent to each other on the macrocyclic ring. A triacetic monoalkyne ligand (DO3ma) was also synthesized for comparison purposes. NMR studies indicate that the Yb(III) chelates of DOTAma and DOTAda adopt two conformations in solns. in which the tetraaza ring is rigidified. The hydration state of the Eu(III) chelates was detd. by luminescence spectroscopy, and the water exchange time of the Gd(III) complexes was measured by 17O NMR. Ring substitution accelerates the water exchange. These data were used to interpret nuclear magnetic relaxation dispersion curves of the Gd(III) chelates. Two long aliph. chains have been added to DOTAda by a "click" procedure to form the (C18)2DOTAda ligand. The corresponding Gd(III) complex forms micelles of unusually high relaxivity presumably because of the close proximity of the aliph. chains on the macrocyclic ring that ensures a rigid double anchoring into the micelles

Radiolabelling with isotopic mixtures of 52g/55^{52g/55}Mn(II) as a straight route to stable manganese complexes for bimodal PET/MR imaging

Radiolabelling using isotopic mixtures of 52g/55Mn(ii) offers fast and easy access to new small molecule PET/MR tracers, composed of chemically identical reporting units. trans-1,2-Diaminocyclohexane-N,N,N[prime or minute],N[prime or minute]-tetraacetic acid (CDTA) was radiolabelled with carrier-added 52gMn(ii) in >99% radiochemical yield, producing the first manganese-based bimodal PET/MR probe. The Mn-CDTA chelate was shown to be very stable to air oxidation and sufficiently inert to decomplexation in blood serum. These data sparked our interest in functionalized CDTA ligands for the design of optimized PET/MR tracers