On-bead combinatorial synthesis and imaging of europium(III)-based paraCEST agents aids in identification of chemical features that enhance CEST sensitivity

Bio-instructive materials on-demand - combinatorial chemistry of peptoids, foldamers, and beyond

Combinatorial chemistry allows for the rapid synthesis of large compound libraries for high throughput screenings in biology, medicinal chemistry, or materials science. Especially compounds from a highly modular design are interesting for the proper investigation of structure-to-activity relationships. Permutations of building blocks result in many similar but unique compounds. The influence of certain structural features on the entire structure can then be monitored and serve as a starting point for the rational design of potent molecules for various applications. Peptoids, a highly diverse class of bioinspired oligomers, suit perfectly for combinatorial chemistry. Their straightforward synthesis on a solid support using repetitive reaction steps ensures easy handling and high throughput. Applying this modular approach, peptoids are readily accessible, and their interchangeable side-chains allow for various structures. Thus, peptoids can easily be tuned in their solubility, their spatial structure, and, consequently, their applicability in various fields of research. Since their discovery, peptoids have been applied as antimicrobial agents, artificial membranes, molecular transporters, and much more. Studying their three-dimensional structure, various foldamers with fascinating, unique properties were discovered. This non-comprehensive review will state the most interesting discoveries made over the past years and arouse curiosity about what may come.

Solid-phase synthesis and evaluation of tumour-targeting phenylboronate-based MRI contrast agents

Paramagnetic macrocycles functionalized with phenylboronic moieties have proven to be interesting for MRI applications based on their ability to recognize cancer cells and generate local contrast. However, full use of the potential of this class of compounds is hampered by laborious and inefficient synthetic and, especially, purification procedures. The amphiphilic character of water-soluble phenylboronates renders them difficult compounds to be prepared through conventional solution synthesis due to the tendency to aggregate and form adducts with other nucleophiles. The new strategy described herein exploits the advantage of solid-phase synthesis with the application of DEAM-PS resin for anchorage and the subsequent simplified derivatization of boronates. GdDOTA-EN-PBA and its fluorinated analogue GdDOTA-EN-F2PBA were synthesized in a much easier, faster and economically convenient way to achieve good yields and purity. Furthermore, the effect of electron-withdrawing fluorine atoms on the aromatic ring of the latter compound was investigated by comparing the physico-chemical properties of both compounds as well as their binding affinity towards melanoma cancer cells.

Solid phase synthesis in the development of magnetic resonance imaging probes

We review the use of the solid phase synthesis methodology for the preparation of diverse and potent MRI probes.

Identification of side arm-modified DOTA scaffolds as multi-site binding ligands for cancer cells over normal cells

The metal-chelated 1,4,7,10-tetraazacyclododecan-1,4,7,10-tetraacetic acid-tetraamide (DOTA) scaffold has been widely used as a contrast agent for diagnostic purposes in positron emission tomography (PET) and magnetic resonance imaging (MRI), but not as a biomarker targetable ligand. While the oxygen atoms at the stem of the four arms of the DOTA scaffold are needed for metal chelation, we previously introduced various physiochemical properties to extend these arms in a chemical library fashion to enhance the imaging contrast mechanism. We developed two such on-bead libraries, with 80 and 76 DOTA derivatives, where one arm was used to attach the DOTA scaffold onto resin beads and the other three arms were chemically modified. We now hypothesized that the chemical moieties used to modify these three arms can also recognize biomarkers on a cell surface. Therefore in this current study, we used such 76 derivatives of DOTA library to screen against HeLa cervical cancer cells. We found that two of the four 'hits' identified displayed higher binding towards HeLa cells than the unmodified parent DOTA. Furthermore, one of those 'hits' displayed better binding towards cervical and prostate cancer cells than lung and breast cancer cells and normal HBEC-3KT and RWPE1 cells. This indicates that this derivative can recognize a biomarker specific for certain types of cancer cells. If the compound has intrinsic activity, this can be used as a theranostic agent for real time therapy monitoring applications in the future. We believe that our DOTA derivative-based library approach can be applied to other types of cell and protein screens on various disease types in the future.

Solid-Phase-Supported Approach for the Preparation of Bioresponsive and Multifunctional MRI Probes

The development of bifunctional imaging probes can often be challenging with difficult and time-consuming solution phase chemistry protocols and purification techniques. A solid phase synthetic protocol was therefore utilized to produce a functionalized derivative of a potent bismacrocyclic calcium-responsive contrast agent for magnetic resonance imaging. Through a convenient building block approach, the applicability of this methodology in the preparation and simple future development of multifunctional imaging probes was demonstrated.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

A facile on-bead method for fully symmetric tetra-substituted DOTA derivatizations using peptoid moieties

The metal-chelated 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) has made a significant impact on the field of diagnostic imaging. This imaging mechanism is largely dependent on the four side arm functionalities around the DOTA scaffold. We previously demonstrated the effect of peptoid residue modification on these DOTA side arms, thereby conferring diverse physiochemical properties to the imaging mechanism. We generated two on-bead Eu(III)-DOTA libraries with three side arm modifications, where the remaining arm was used to attach DOTA onto the resin. However, having an on-bead fully symmetric tetra-substituted DOTA synthesis route can greatly improve the fields of diagnostic, therapeutic, and theragnostic agent development. Here, we report an efficient method for the synthesis of symmetric tetra-substituted DOTA derivatives by modification with peptoid moieties on all four arms using a conceptually unique solid-phase synthesis approach. Resins with different loading capacities were examined for synthesis feasibility and high loading resins were most effective. The reaction yields were also studied by varying the number of peptoid residues and incorporating different linkers. We have tested the binding ability of the tetra-substituted derivative with its previously tested tri-substituted analogs as model applications. Our protocol provides an efficient and facile on-bead synthesis route for fully symmetric tetra-substituted DOTA derivatizations.