Rotational Dynamics Account for pH-Dependent Relaxivities of PAMAM Dendrimeric, Gd-Based Potential MRI Contrast Agents

The EPTPA5- chelate, which ensures fast water exchange in GdIII complexes, has been coupled to three different generations (5, 7, and 9) of polyamidoamine (PAMAM) dendrimers through benzylthiourea linkages (H5EPTPA=ethylenepropylenetriamine-N,N,N,N,N-pentaacetic acid). The proton relaxivities measured at pH 7.4 for the dendrimer complexes G5-(GdEPTPA)111, G7-(GdEPTPA)253 and G9-(GdEPTPA)1157 decrease with increasing temperature, indicating that, for the first time for dendrimers, slow water exchange does not limit relaxivity. At a given field and temperature, the relaxivity increases from G5 to G7, and then slightly decreases for G9 (r1=20.5, 28.3 and 27.9 mM-1 s-1, respectively, at 37 °C, 30 MHz). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes. This originates from the pH-dependent rotational dynamics of the dendrimer skeleton, which was evidenced by a combined variable-temperature and multiple-field 17O NMR and 1H relaxivity study performed at pH 6.0 and 9.9 on G5-(GdEPTPA)111. The longitudinal 17O and 1H relaxation rates of the dendrimeric complex are strongly pH-dependent, whereas they are not for the [Gd(EPTPA)(H2O)]2- monomer chelate. The longitudinal 17O and 1H relaxation rates have been analysed by the Lipari-Szabo spectral density functions and correlation times have been calculated for the global motion of the entire macromolecule (tau_gO) and the local motion of the GdIII chelates on the surface (tau_lO), correlated by means of an order parameter S2. The dendrimer complex G5-(GdEPTPA)111 has a considerably higher tau_gO under acidic than under basic conditions (tau_gO298=4040 ps and 2950 ps, respectively), while local motions are less influenced by pH (tau_lO298=150 and 125 ps). The order parameter, characterizing the rigidity of the macromolecule, is also higher at pH 6.0 than at pH 9.9 (S2=0.43 vs 0.36, respectively). The pH dependence of the global correlation time can be related to the protonation of the tertiary amine groups in the PAMAM skeleton, which leads to an expanded and more rigid dendrimeric structure at lower pH. The increase of gO with decreasing pH is responsible for the pH dependent proton relaxivities. The water exchange rate on G5-(GdEPTPA)111 (kex298=150×106 s-1) shows no significant pH dependency and is similar to the one measured for the monomer [Gd(EPTPA)(H2O)]2-. The proton relaxivity of G5-(GdEPTPA)111 is mainly limited by the important flexibility of the dendrimer structure, and to a small extent, by a faster than optimal water exchange rate.