Albumin binding, relaxivity, and water exchange kinetics of the diastereoisomers of MS-325, a gadolinium(III)-based magnetic resonance angiography contrast agent.

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 be isolated at pH > 8.5 (TyeklAr, 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 an increased relaxation rate of water protons in plasma. Under physiological conditions (37 degrees C, pH 7.4, phosphate buffered saline (PBS), 4.5% HSA, 0.05 mM complex), there is no statistical difference in HSA affinity or relaxivity between 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 has a higher relaxivity than isomer B. The water exchange rates in the absence of HSA at 298 K, kA298 = 5.9 +/- 2.8 x 10(6) s(-1), kB298 = 3.2 +/- 1.8 x 10(6) s(-1), and heats of activation, DeltaHA = 56 +/- 8 kJ/mol, DeltaHB = 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 degrees C in a 4.5% HSA in PBS solution for each isomer (0.1 mM). Differences in the relaxivity in HSA between the two isomers could be attributed to the differing water exchange rates.

Enhancing reactivity via structural distortion.

To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))]PF(6) (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))-S(pers)]PF(6) (2) using PEt(3). (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))]PF(6) (1), a nitrile hydratase model compound previously reported by our group, except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry, opens a S-Fe-N angle by approximately 10 degrees, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S = 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K(eq) describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N(3) or 3-N(3) in MeOH is approximately 10 times larger for the (Et,Pr) system. In CH(2)Cl(2), azide binds approximately 3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N(3) relative to 1-N(3). The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N(3) and 1-N(3) are much faster than one would expect for low-spin Fe(III). Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.

Imide Transfer Properties and Reactions of the Magnesium Imide ((THF)MgNPh)(6): A Versatile Synthetic Reagent.

Imide transfer properties of ((THF)MgNPh)(6) (1) and the synthesis of the related species {(THF)MgN(1-naphthyl)}(6).2.25THF (2), via the reaction of dibutylmagnesium with H(2)N(1-naphthyl), in a THF/heptane mixture are described. Treatment of 1 with Ph(2)CO, 4-Me(2)NC(6)H(4)NO, t-BuNBr(2) (3), PCl(3), or MesPCl(2) (Mes = 2,4,6-Me(3)C(6)H(2)-) leads to the isolation of Ph(2)CNPh (4), 4-Me(2)NC(6)H(4)NNPh (5), t-BuNNPh (6), (PhNPCl)(2) (7), or (MesPNPh)(2) (8) in moderate yield. Reaction between 1 and GeCl(2).dioxane, SnCl(2), or PbCl(2) affords the M(4)N(4) (M = Ge, Sn, Pb) cubane imide derivative (GeNPh)(4) (9), [(SnNPh)(4).{MgCl(2)(THF)(4)}](infinity) (10), (SnNPh)(4).0.5PhMe (11), or (PbNPh)(4).0.5PhMe (12). Interaction of 1 with Ph(3)PO, (Me(2)N)(3)PO, or Ph(2)SO furnishes the complex (Ph(3)POMgNPh)(6) (13), {(Me(2)N)(3)POMgNPh}(6).2PhMe (14), or (Ph(2)SOMgNPh)(6) (15). The addition of 3 equiv of MgBr(2) to 1 gives 1.5 equiv of ((THF)Mg)(6)(NPh)(4)Br(4) (16) in quantitative yield, whereas treatment of 16 with 4 equiv of 1,4-dioxane is an alternative synthetic route to 1. Compounds 2, 3, 9, 10, and 14 were characterized by X-ray crystallography. The reactions demonstrate that 1 is a versatile and useful reagent for the synthesis of a variety of main group imides. Crystal data at 130 K with Mo Kalpha (lambda = 0.710 73 Å) radiation for 3 or Cu Kalpha (lambda = 1.541 78 Å) radiation for 2, 9, 10, and 14: 2, C(93)H(108)Mg(6)N(6)O(7.25), a = 28.101(7) Å, b = 35.851(7) Å, c = 36.816(7) Å, Z = 2, space group Fddd, R = 0.068 for 3500 (I > 2sigma(I)) data; 3, C(4)H(9)Br(2)N, a = 6.682(2) Å, b = 10.834(3) Å, c = 11.080(3) Å, alpha = 66.25(2) degrees, beta = 89.88(2) degrees, gamma = 82.53(2) degrees, Z = 4, space group P&onemacr;, R = 0.038 for 2043 (I > 2sigma(I)) data; 9, C(24)H(20)Ge(4)N(4), a = 10.749(2) Å, b = 12.358(3) Å, c = 35.818(7) Å, Z = 8, space group Pbca, R = 0.040 for 2981 (I > 2sigma(I)) data; 10, C(40)H(52)Cl(2)MgN(4)O(4)Sn(4), a = 12.770(3) Å, b = 13.554(3) Å, c = 25.839(5) Å, Z = 4, space group P2(1)2(1)2(1), R = 0.040 for (I > 2sigma(I)) data; 14, C(86)H(154)Mg(6)N(4)O(6)P(6), a = 22.478(4) Å, b = 16.339(3) Å, c = 29.387(6) Å, Z = 4, space group Pbcn, R = 0.081 for 4696 (I >2sigma(I)) data.