Potential nanostructured contrast agents based on Gd(III) complexes with keplerate polyanions

Gd(III) molecular complexes are currently used in clinical magnetic resonance imaging to enhance contrast. In fact, these chelates reduce the relaxation times of water protons in the tissues where they are distributed allowing a better tissue characterization. However, relatively high local concentrations of the complex are required and different strategies aiming at signal amplification are being proposed. Gd(III) complexes packed into nanosized species constitute an alternative to molecular complexes, showing increased contrast due to slowed down rotational dynamics together with improved kinetic and thermodynamic stability. [1,2,3] In particular, nanosized polyoxomolybdates (keplerates) encapsulating Gd3+ ions were identified as a promising basis for the formulation of contrast agents. [4,5] In this work, nanosized hydrophilic colloids were prepared by self-assembly of keplerate-based polyanions, Gd3+ ions and PEO-PPO-PEO triblock copolymers in water (Figure 1). [4, 5] We focused on two keplerates carrying different building blocks, [{Mo6O21}12{Mo2O4(CH3COO)}30]42- and [{Mo6O21}12{Mo2O4(HPO4)}30]72-, with the aim of revealing the possible variation of the binding capacity of CH3COO- versus HPO42- towards Gd3+. 1H FFC NMR relaxometry was applied to determine r1 as a function of Larmor frequency in the 10 kHz to 40 MHz range. Relaxivity dispersions, showing a maximum at around 30-40 MHz (Figure 2), clearly indicated the incorporation of Gd(III) complexes in nanostructures. In order to get insight into the binding mode of Gd3+, the dispersions were acquired on samples characterized by different Gd3+: keplerate molar ratios. An analysis of the dispersions is in progress aimed at understanding the mechanism of contrast enhancement in these systems. Acknowledgments This work was partially supported by the CA15209 COST Action (EURELAX). Reference [1] J. Wahsner, E. M. Gale, A. Rodríguez- Rodríguez, P. Caravan, Chem. Rev., 2018, DOI: 10.1021/acs.chemrev.8b00363. [2] M. Botta, L. Tei, Eur. J. Inorg. Chem., 2012, 1945-1960. [3] P. Hermann, J. Kotek, V. Kubí?ek, I. Luke?, Dalton Trans., 2008, 3027-3047. [4] J. Elistratova, B. Akhmadeev, A. Gubaidullin, V. Korenev, M. Sokolov, I. Nizameev, A. Stepanov, I. Ismaev, M. Kadirov, A. Voloshina, A. Mustafina, New J. Chem., 2017, 41, 5271-5275. [5] J. Elistratova, B. Akhmadeev, V. Korenev, M. Sokolov, I. Nizameev, A. Gubaidullin, A. Voloshina, A. Mustafina, Soft Matter, 2018, 14, 7916-7925.