Pharmacokinetics of a High-Generation Dendrimer–Gd-DOTA 1 L. Henry Bryant, Jr, PhD, E. Kay Jordan, DVM, Jeff W. M. Bulte, PhD, 2 Vı ´t Herynek, PhD, Joseph A. Frank, MD RATIONALE AND OBJECTIVES Dendrimers are three-dimensional branching polymers that have received much attention as transfection agents (1), drug delivery agents (2), and magnetic resonance (MR) contrast agents (3–5). To be used as MR contrast agents, dendrimers have been conjugated with paramag- netic metal ion complexes. The large number of paramag- netic metal ion complexes that can be covalently attached per dendrimer molecule, which depends on the dendrimer generation, has resulted in a new class of macromolecular MR contrast agents. Unlike other macromolecular-based MR contrast agents such as albumin, dextran, and poly-L- lysine, dendrimers are discrete reagents with well-defined sizes and shapes. The paramagnetic metal ion complexes have included gadolinium chelated to the derivatized acy- clic ligand of diethylenetriaminepentaacetic acid (DTPA) and the derivatized macrocylic ligands of 1,4,7,10-tetra- azacyclododecane-1,4,7-triacetic acid (DO3A) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). In addition to these complexes, which primarily affect T1 relaxation, the dysprosium chelate of the deri- vatized macrocyclic ligand DOTA has been studied to develop a T2 selective contrast agent (6). These macro- molecular MR contrast agents are expected to have high relaxivities compared to the monomeric chelates through longer rotational correlation times (7,8). The dendrimer-based paramagnetic metal ion com- plexes investigated previously as MR contrast agents have involved the generation six or lower dendrimers. We have recently synthesized a series of high-generation (genera- tion [G] = 5, 7, 9, and 10) dendrimer-based gadolinium chelates of the derivatized macrocycle p-SCN-Bz-DOTA (5). The molar relaxivities were found to “saturate” for the G9- and G10-dendrimer–Gd-DOTA. As the next step toward the development of these high-generation den- drimer-based gadolinium chelates as potential in vivo MR contrast agents, we report the preliminary results of the blood clearance, biodistribution, and MR angiographic properties of the G9-dendrimer–Gd-DOTA in rats. These properties are also important in the design of targeted high-generation dendrimer-based MR contrast agents. MATERIALS AND METHODS Synthesis Ethylenediamine-core dendrimers were obtained from a commercial supplier (Starburst dendrimers; Dendritech, Midland, Mich). The G9-dendrimer–Gd-DOTA was pre- pared as described previously (5). Complete encapsulation of the gadolinium ion by the macrocycle as opposed to pre-encapsulation by the pendant donor groups was checked by two methods. The first method was based on the propensity of fluo- ride ions to precipitate gadolinium ions in solution (9). A 0.5 M NaF solution (0.5 mL; Aldrich, Milwaukee, Wis) was added to 0.5 mL of a 1 mM solution of either den- drimer–Gd-DOTA or Gd(CH 3 C0 2 ) 3 (Aldrich). An immedi- ate precipitate was observed after the addition of NaF to the Gd acetate solution. No precipitation was observed for the dendrimer–Gd-DOTA solution. The dendrimer–Gd- DOTA solution was incubated for 72 hours at room tem- perature and filtered. The relaxation rate before and after the addition of NaF was measured at 42 MHz and 23°C. Acad Radiol 2002; 9(suppl 1):S29–S33 1 From the Laboratory of Diagnostic Radiology Research (Warren Grant Magnuson Clinical Center) (L.H.B., E.K.J, J.W.M.B., J.A.F.) and the Neuro- imaging Branch (National Institute of Neurological Disorders and Stroke) (V.H.), National Institutes of Health, Bldg 10, Rm 1C660, Bethesda, MD 20892-1074. Address correspondence to L.H.B. 2 Current address: Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Md. © AUR, 2002 S29