B25716/1: a novel albumin-binding Gd-AAZTA MRI contrast agent with improved properties in tumor imaging

Abstract

The aim of this study is to describe the synthesis of, relaxometric characterization of, pharmacokinetic properties of, and animal imaging experiments with a new, low molecular weight gadolinium complex with high binding affinity toward serum albumin. The gadolinium(III) chelate (B25716/1) is based on the structure of the heptadentate ligand 1,4-bis(hydroxycarbonylmethyl)-6-[bis(hydroxycarbonylmethyl)]amino-6 methylperhydro-1,4-diazepine (AAZTA) covalently conjugated to an analogue of deoxycholic acid. The study was conducted as a comparison with that of an analogous complex based on the octadentate diethylenetriaminepentaacetic acid ligand B22956/1 (whose albumin binding properties were previously assessed). The structural modification with respect to B22956/1 leads to a system that can host two coordinated water molecules in fast exchange with bulk water with potential higher efficiency as an MRI contrast agent. On interaction with human serum albumin the expected-field-independent-relaxation enhancement is not observed, possibly as a consequence of the displacement of one of the two inner-sphere water molecules of the gadolinium complex. At clinically relevant magnetic fields, however, the plasma relaxivity of B25716/1 is markedly higher than that shown by B22956/1, owing to concomitant synergistic contributions from the electronic correlation time and water molecules in the second coordination sphere. The capability of B25716/1 to enhance tumor regions in magnetic resonance images was assessed in vivo at 3 T on a xenograft tumor mouse model prepared with PC-3 cells. B25716/1 displays signal enhancements approximately double those observed for B22956/1, in agreement with the findings of the in vitro relaxivity investigations.

Synthesis, Characterization, and Biodistribution of a Dinuclear Gadolinium Complex with Improved Properties as a Blood Pool MRI Agent

“…The change of slope observed in the M‐titration (Figure , top) at a (GdDTPA) 2 ‐Chol/HSA ratio of ∼3 may indicate the presence of three equivalent (i.e., with a similar K A value) binding sites, while best‐fitting parameters obtained by fitting E‐titration data (Figure , bottom) led to an affinity constant K A of (8.0±1.3)×10 3 m −1 (i.e., nK A =2.4×10 4 m −1 ) and a relaxivity of the bound fraction r 1b of 22.9±0.3 m m −1 s −1 . Because nK A , rather than K A alone, better expresses the affinity, the obtained value is similar to other products reported in the literature: B22956/1 has a K A of 4.5×10 4 m −1 , B25716/1 of 2×10 4 m −1 , MS‐325 of 1.1×10 4 m −1 , and the dimer described by Parac‐Vogt et al . of 1×10 4 m −1 .…”

“…In particular, the appearance of the relaxivity peak centered around 30–40 MHz is a clear indication of the binding of the paramagnetic complex to serum protein. The relaxivity was increased by about threefold, approaching typical values reported for albumin binding Gd complexes ,…”

“…Some parameters of the relaxation model were kept fixed during the fitting procedure, such as: the hydration number (set at 1), the distance between protons of the coordinated water and Gd ion (0.31 nm), the distance of closest approach of the outer‐sphere water protons (0.36 nm), the water diffusion constant in saline (2.2×10 −5 cm 2 s −1 ), saline with 35 g L −1 HSA (2.9×10 −5 cm 2 s −1 ), and in human plasma (2.28×10 −5 cm 2 s −1 , as estimated according to the Einstein–Smoluchowski law under the assumption of a spherical particle in a medium of viscosity η equal to 1.2 mPa s). All other parameters obtained from the data fitting were in the normal range observed for similar gadolinium chelates . A marked increase in the global rotational time (approximately two orders of magnitude) was observed in the presence of plasma proteins, thus fully supporting the occurrence of protein binding.…”

“…This latter experiment allowed the estimation of the exchange lifetime ( τ M ) of the water molecule coordinated to the paramagnetic ion, resulting to be 68±3 ns. This value is just shorter than for B22956/1, which displays a τ M value of 122 ns and similar to that of MS‐325 ( τ M 310 K =83 ns) . A τ M on the order of 100 ns is in the optimal range for attaining high relaxivity in the presence of long τ R , as occurs in the presence of binding to serum protein.…”

Synthesis of phosphonic analogues of AAZTA†AAZTA=6-Amino-6-methylperhydro-1,4-diazepine-N,N′,N″,N″-tetraacetic acid.† and relaxometric evaluation of the corresponding Gd(III) complexes as potential MRI contrast agents

“…For example, lipophilic AAZTA chelates were used to form lipid-based aggregates reaching high relaxivity values, 13 while bifunctional derivatives were designed to embody remote free functional groups devoted to conjugation purposes. 14 Other examples are reported in which the AAZTA complexes are functionalized with suitable hydrophobic substrates in order to prompt their interaction with serum albumin 15 …”