Gadolinium(III) DO3A macrocycles and polyethylene glycol coupled to dendrimers - Effect of molecular weight on physical and biological properties of macromolecular magnetic resonance imaging contrast agents

The macrocycle 1-(4-isothiocyanatobenzyl)amido-4,7,10-triacetic acid-tetraazacyclododecane (DO3A-bz-NCS) was synthesized and coupled to the terminal amine sites of a series of different generations (G(n)) of polyamidoamine or starburst dendrimers (n-SBDs) creating macromolecular polychelates. Gadolinium ion was added to the dendrimer polychelates for evaluating the parameters needed to create magnetic resonance imaging (MRI) contrast agents that have long blood circulation times. The resulting water soluble n-SBD-GdDO3As were mono-disperse and ranged from 11 Gd3+ ions per G(3) dendrimer (MW 18.4 kDalton (kDa)) to 57 Gd3+ ions per G(3) dendrimer (MW 61.8 kDa). Nuclear Magnetic Resonance Dispersion (NMRD) profiles revealed peak relaxivities up to 18.8 mM(-1) s(-1) at 25 MHz, with the magnitude increasing linearly as a function of molecular weight. Blood elimination half-life in rats increased with molecular weight ranging from 11(+/-5) min for 3-SBD-(GdDO3A)(24) (22 kDa) to 115(+/-8) min for the 5-SBD-(GdDO3A)(57) (61.8 kDa). Seven-day liver retention increased from 1 to over 40% over the same molecular weight range. The effects of grafting polyethylene glycol (PEG) onto n-SBD-GdDO3A polychelates were also studied. Relaxivities ranged from 11 to 14.9 mM(-1) s(-1), blood elimination half-lives increased significantly (range 33-1219 min) and the seven-day liver uptake dropped to 1-8% of the injected dose. However, no correlations between these measurements and molecular weight were found over the range studied (20.5-69.3 kDa). These results suggest that both the molecular weight and type of terminal group on the n-SBD-GdDO3A polychelates control the pharmacokinetics and biodistribution of the macromolecular contrast agent. The addition of covalently bound PEG to the n-SBD-GdDO3A surface significantly improved the biological performance of the contrast agents.