title: The interaction of MS-325 with human serum albumin and its effect on proton relaxation rates.
abstract: MS-325 is a novel blood pool contrast agent for magnetic resonance imaging currently undergoing clinical trials to assess blockage in arteries. MS-325 functions by binding to human serum albumin (HSA) in plasma. Binding to HSA serves to prolong plasma half-life, retain the agent in the blood pool, and increase the relaxation rate of water protons in plasma. Ultrafiltration studies with a 5 kDa molecular weight cutoff filter show that MS-325 binds to HSA with stepwise stoichiometric affinity constants (mM(-1)) of K(a1) = 11.0 +/- 2.7, K(a2) = 0.84 +/- 0.16, K(a3) = 0.26 +/- 0.14, and K(a4) = 0.43 +/- 0.24. Under the conditions 0.1 mM MS-325, 4.5% HSA, pH 7.4 (phosphate-buffered saline), and 37 degrees C, 88 +/- 2% of MS-325 is bound to albumin. Fluorescent probe displacement studies show that MS-325 can displace dansyl sarcosine and dansyl-L-asparagine from HSA with inhibition constants (K(i)) of 85 +/- 3 microM and 1500 +/- 850 microM, respectively; however, MS-325 is unable to displace warfarin. These results suggest that MS-325 binds primarily to site II on HSA. The relaxivity of MS-325 when bound to HSA is shown to be site dependent. The Eu(III) analogue of MS-325 is shown to contain one inner-sphere water molecule in the presence and in the absence of HSA. The synthesis of an MS-325 analogue, 5, containing no inner-sphere water molecules is described. Compound 5 is used to estimate the contribution to relaxivity from the outer-sphere water molecules surrounding MS-325. The high relaxivity of MS-325 bound to HSA is primarily because of a 60-100-fold increase in the rotational correlation time of the molecule upon binding (tau(R) = 10.1 +/- 2.6 ns bound vs 115 ps free). Analysis of the nuclear magnetic relaxation dispersion (T(1) and T(2)) profiles also suggests a decrease in the electronic relaxation rate (1/T(1e) at 20 MHz = 2.0 x 10(8) s(-1) bound vs 1.1 x 10(9) s(-1) free) and an increase in the inner-sphere water residency time (tau(m) = 170 +/- 40 ns bound vs 69 +/- 20 ns free).
authors.affiliation: EPIX Medical, Inc., 71 Rogers Street, Cambridge, MA 02142-1118, USA. pcaravan@epixmed.com
authors.forename: Peter
authors.initials: P
authors.lastname: Caravan
authors.forename: Normand J
authors.initials: NJ
authors.lastname: Cloutier
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Greenfield
authors.forename: Sarah A
authors.initials: SA
authors.lastname: McDermid
authors.forename: Stephen U
authors.initials: SU
authors.lastname: Dunham
authors.forename: Jeff W M
authors.initials: JW
authors.lastname: Bulte
authors.forename: John C
authors.initials: JC
authors.lastname: Amedio
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Looby
authors.forename: Ronald M
authors.initials: RM
authors.lastname: Supkowski
authors.forename: William DeW
authors.initials: WD
authors.lastname: Horrocks
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: McMurry
authors.forename: Randall B
authors.initials: RB
authors.lastname: Lauffer
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D003287:Contrast Media / Q000737:chemistry / Q000378:metabolism; D005682:Gadolinium; D006801:Humans; D007700:Kinetics; D008279:Magnetic Resonance Imaging; D009942:Organometallic Compounds / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011522:Protons; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism; D014462:Ultrafiltration
chemical_list: D003287:Contrast Media; D009942:Organometallic Compounds; D011522:Protons; D012709:Serum Albumin; D005682:Gadolinium; C109932:gadofosveset trisodium