Binding constant determination of WIN 22169, a novel polymeric ligand.

WIN 22169 is a co-polymer containing approximately 11 repeating units of polyoxyethylene and diethylenetriamine pentaacetic acid (DTPA). WIN 66368, a magnetic resonance imaging (MRI) contrast agent, is the gadolinium III complex of WIN 22169. WIN 22169 has been characterized with respect to its equivalent weight, acidity constants and excess acid or base, as well as its metal ion binding constants. The logs of the equilibrium binding constants of the ligand to Gd3+, Ca2+, Zn2+ and Cu2+ were found to be 16.6, 7.47, 12.2 and 14.0. The Gd selectivity constant, a measure of the preferential binding of the ligand toward Gd3+ versus the three in vivo ions: Ca2+, Zn2+ and Cu2+, of WIN 66368 was calculated to be 7.9. This value compares favourably to that for Gd DTPA which has a Gd selectivity constant of 7.04.