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High relaxivity macromolecular contrast agents based on the conjugation of gadolinium chelates to the interior and exterior surfaces of MS2 viral capsids are assessed. The proton nuclear magnetic relaxation dispersion (NMRD) profiles of the conjugates show up to a five-fold increase in relaxivity, leading to a peak relaxivity (per Gd 3+ ion) of 41.6 mM -1 s -1 at 30 MHz for the internally modified capsids. Modification of the exterior was achieved through conjugation to flexible lysines, while internal modification was accomplished by conjugation to relatively rigid tyrosines. Higher relaxivities were obtained for the internally modified capsids, showing that (i) there is facile diffusion of water to the interior of capsids and (ii) the rigidity of the linker attaching the complex to the macromolecule is important for obtaining high relaxivity enhancements. The viral capsid conjugated gadolinium hydroxypyridonate complexes appear to possess two inner-sphere water molecules (q = 2) and the NMRD fittings highlight the differences in the local motion for the internal (τ Rl = 440 ps) and external (τ Rl = 310 ps) conjugates. These results indicate that there are significant advantages of using the internal surface of the capsids for contrast agent attachment, leaving the exterior surface available for the installation of tissue targeting groups. MANUSCRIPT TEXTMagnetic resonance imaging (MRI) is a routinely used noninvasive diagnostic technique, providing detailed images without the use of ionizing radiation. Although the resolution of MRI is excellent, its dynamic range is relatively narrow because of the limited variation in relaxation rates exhibited by water protons in vivo. When these differences are insufficient to distinguish between adjacent tissues, contrast enhancement is often achieved through the administration of synthetic agents that increase the water proton relaxation rates in accessible locations. Gadolinium complexes are the most often used for this purpose, with more than 10 million MRI studies being performed through their use each year. [2][3][4] The current commercially available Gd(III)-based contrast agents use poly(aminocarboxylate) chelates, and typically gram quantities of Gd must be injected to reach concentrations sufficient for usable contrast enhancement. However, this strategy is more difficult to apply to the specific imaging of biomarkers present in low (μM -nM) concentrations. To distinguish these sites from the background signal, targetable contrast agents will undoubtedly require significantly improved contrast enhancement efficiencies. 3,5 2 MRI contrast agents are commonly evaluated on the basis of relaxivity (r 1p ), which describes their ability to increase the longitudinal relaxation rate of nearby water molecule protons per millimolar concentration of agent applied. 6 Strategies for enhancing the relaxivity of contrast agents include increasing the number of bound water molecules (q), optimizing the water-residence time (τ M ) and increasing the rotational correlati...

ConspectusMagnetic resonance imaging (MRI) is a particularly effective tool in medicine because of its high depth penetration (1 mm to 1 m) and ability to resolve different soft tissues. The MRI signal is generated by the relaxation of in vivo water molecule protons. MRI images can be improved by administering paramagnetic agents, which increase the relaxation rates of nearby water protons, thereby enhancing the MRI signal. The lanthanide cation Gd 3+ is generally used because of its favorable electronic properties; high toxicity, however, necessitates strongly coordinating ligands to keep Gd 3+ completely bound while in the patient. In this Account, we give a coordination chemistry overview of contrast agents (CAs) based on Gd-hydroxypyridinone (HOPO), which show improved MRI contrast and high thermodynamic stabilities.Tris-bidentate HOPO-based ligands developed in our laboratory were designed to complement the coordination preferences of Gd 3+ , especially its oxophilicity. The HOPO ligands provide a hexadentate coordination environment for Gd 3+ in which all the donor atoms are oxygen. Because Gd 3+ favors eight or nine coordination, this design provides two to three open sites for inner-sphere water molecules. These water molecules rapidly exchange with bulk solution, hence affecting the , following which he began his faculty appointment at UC Berkeley. His research interests encompass microbial iron transport, the development of actinide sequestration agents, lanthanide-based luminescent and MRI agents, and supra-molecular metal complex clusters for host-guest chemistry and catalysis. NIH Public Access Author ManuscriptAcc Chem Res. Author manuscript; available in PMC 2010 July 21. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript relaxation rates of bulk water molecules. The parameters affecting the efficiency of these contrast agents have been tuned to improve contrast while still maintaining a high thermodynamic stability for Gd 3+ binding. The Gd-HOPO-based contrast agents surpass current commercially available agents because of a higher number of inner-sphere water molecules, rapid exchange of inner-sphere water molecules via an associative mechanism, and a long electronic relaxation time. The contrast enhancement provided by these agents is at least twice that of commercial contrast agents, which are based on polyaminocarboxylate ligands.Advances in MRI technology have made significant contributions to the improvement of clinical diagnostics by allowing visualization of underlying pathology. However, understanding the mechanism of a disease at the molecular level requires improved imaging sensitivity. The ultimate goal is to visually distinguish between different disease targets or markers-such as enzymes, hormones, proteins, or small molecules-at biologically relevant concentrations (millimolar to nanomolar). Although MRI techniques can provide images of the organs and tissues in which these biomarkers are regulated, the high sensitivity required to visualize the biological t...

Two high-relaxivity nanoscale magnetic resonance contrast agents have been built using bacteriophage MS2 as a biomolecular scaffold. Protein capsid shells were functionalized on either the exterior or interior surface to display multiple copies of an aldehyde functional group. Subsequently, approximately 90 heteropodal bis(hydroxypyridonate)terephthalamide ligands were attached to these sites through oxime condensation reactions. Upon complexation with Gd3+, contrast agents with ionic relaxivities of up to 41.6 mM-1 s-1 (30 MHz, 25 degrees C) and total molecular relaxivities of up to 3900 mM-1 s-1 (30 MHz, 25 degrees C) were produced. Capsids sequestering the Gd-chelates on the interior surface (attached through tyrosine residues) not only provided higher relaxivities than their exterior functionalized counterparts (which relied on lysine modification) but also exhibited improved water solubility and capsid stability. The attachment functional cargo to the interior surface is envisioned to minimize its influences on biodistribution, yielding significant advantages for tissue targeting by additional groups attached to the capsid exterior.