The amphiphilic gadolinium complex MS-325 ((trisodium-{(2-(
R)-[(4,4-diphenylcyclohexyl) phosphonooxymethyl]diethylenetriaminepentaacetato) (aquo)gadolinium(III)}) is a contrast agent for magnetic resonance angiography(MRA). MS-325 consists of two slowly interconverting diastereoisomers,
A and
B (65:35 ratio), which can beisolated at pH > 8.5 (Tyekl
r, Z.; Dunham, S. U.; Midelfort, K.; Scott, D. M.; Sajiki, H.; Ong, K.; Lauffer, R. B.;Caravan, P.; McMurry, T. J.
Inorg. Chem. 2007,
46, 6621-6631). MS-325 binds to human serum albumin (HSA)in plasma resulting in an extended plasma half-life, retention of the agent within the blood compartment, and anincreased relaxation rate of water protons in plasma. Under physiological conditions (37
C, pH 7.4, phosphatebuffered saline (PBS), 4.5% HSA, 0.05 mM complex), there is no statistical difference in HSA affinity or relaxivitybetween the two isomers (
A 88.6 ± 0.6% bound,
r1 = 42.0 ± 1.0 mM
-1 s
-1 at 20 MHz;
B 90.2 ± 0.6% bound,
r1 = 38.3 ± 1.0 mM
-1 s
-1 at 20 MHz; errors represent 1 standard deviation). At lower temperatures, isomer
A hasa higher relaxivity than isomer
B. The water exchange rates in the absence of HSA at 298 K,
kA298 = 5.9 ± 2.8× 10
6 s
-1,
kB298 = 3.2 ± 1.8 × 10
6 s
-1, and heats of activation,
HA = 56 ± 8 kJ/mol,
HB = 59 ± 11 kJ/mol,were determined by variable-temperature
17O NMR at 7.05 T. Proton nuclear magnetic relaxation dispersion (NMRD)profiles were recorded over the frequency range of 0.01-50 MHz at 5, 15, 25, and 35
C in a 4.5% HSA in PBSsolution for each isomer (0.1 mM). Differences in the relaxivity in HSA between the two isomers could be attributedto the differing water exchange rates.