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The Interaction of MS-325 with Human Serum Albumin and Its Effect on Proton Relaxation Rates

Journal of the American Chemical Society2002Vol. 124(12), pp. 3152–3162

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Peter Caravan, Normand J. Cloutier, Matthew T. Greenfield, Sarah A. McDermid, Stephen U. Dunham, Jeff W. M. Bulte, John C. Amedio, Richard Looby, R.M. Supkowski, William DeW. Horrocks, Thomas J. McMurry, Randall B. Lauffer

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).

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Abstract

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