Efficient construction of therapeutics, bioconjugates, biomaterials and bioactive surfaces using azide-alkyne "click" chemistry

Biological evaluation of avidin-based tumor pretargeting with DOTA-Triazole-Biotin constructed via versatile Cu(I) catalyzed click chemistry

BACKGROUND

The biotin-avidin interaction remains a gold standard for the two-step pretargeting approach to image tumor sites. We aim to develop two-step pretargeting systems utilizing (99m)Tc labeled biotin functionalized macrocyclic chelating agents synthesized using the highly efficient Cu(I) catalyzed azide-alkyne cycloaddition for potential radioimaging applications.

METHODS

A facile synthesis of DOTA-Triazole-Biotin, radiocomplexation with (99m)Tc and the pretargeting protocol is described. The synthesis features Cu(I) catalyzed click conjugation between biotinylated azide and propynyl functionalized DO3A. (99m)Tc radiolabeling was performed to detect the accumulation of avidin as the pretargeting agent. Cytotoxicity was determined using the trypan blue exclusion assay, macrocolony, and MTT assay. Cell uptake studies were performed using radiolabeled DOTA-Triazole-Biotin and compared with avidin treated cells for 2 h. Tumor imaging was performed in U-87MG cell line implanted tumor bearing nude mice and uptake of the radiotracer was estimated.

RESULTS

All compounds have been successfully characterized by NMR and MS spectroscopy. More than 96% radiolabeling efficiency was obtained and the radioconjugate exhibited sufficient stability under physiological conditions.

CONCLUSION

To summarize, a new candidate for avidin based two-step pretargeting of tumors has been synthesized and evaluated for potential imaging and diagnostic applications. The chelate possesses high stability under physiological conditions, exhibits effective interaction with its avidin target, and low nonspecific retention in vivo.

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) and beyond: new reactivity of copper(I) acetylides

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a widely utilized, reliable, and straightforward way for making covalent connections between building blocks containing various functional groups. It has been used in organic synthesis, medicinal chemistry, surface and polymer chemistry, and bioconjugation applications. Despite the apparent simplicity of the reaction, its mechanism involves multiple reversible steps involving coordination complexes of copper(I) acetylides of varying nuclearity. Understanding and controlling these equilibria is of paramount importance for channeling the reaction into the productive catalytic cycle. This tutorial review examines the history of the development of the CuAAC reaction, its key mechanistic aspects, and highlights the features that make it useful to practitioners in different fields of chemical science.

Chemoselective ligation techniques: modern applications of time-honored chemistry

Chemoselective ligation techniques enable the selective modification of proteins and other biomolecules in dilute aqueous solution. Importantly, these reactions occur at or near physiological pH and are compatible with the complex array of functional groups commonly found in biological macromolecules including proteins, nucleotides, and carbohydrates, allowing conjugation reactions to be carried out on unprotected substrates. Recently, a growing number of reactions with established utility in synthetic organic chemistry have been shown to have surprising utility in the context of biological molecules in aqueous media. In this review we highlight several promising reactions that may have widespread applicability in the generation of new materials based on biological macromolecules.

Design and fabrication of magnetic nanoparticles for targeted drug delivery and imaging

Magnetic nanoparticles (MNPs) represent a class of non-invasive imaging agents that have been developed for magnetic resonance (MR) imaging. These MNPs have traditionally been used for disease imaging via passive targeting, but recent advances have opened the door to cellular-specific targeting, drug delivery, and multi-modal imaging by these nanoparticles. As more elaborate MNPs are envisioned, adherence to proper design criteria (e.g. size, coating, molecular functionalization) becomes even more essential. This review summarizes the design parameters that affect MNP performance in vivo, including the physicochemical properties and nanoparticle surface modifications, such as MNP coating and targeting ligand functionalizations that can enhance MNP management of biological barriers. A careful review of the chemistries used to modify the surfaces of MNPs is also given, with attention paid to optimizing the activity of bound ligands while maintaining favorable physicochemical properties.

Conjugation of ligands to the surface of preformed liposomes by click chemistry

Click chemistry represents a new bioconjugation strategy that can be used to conveniently attach various ligands to the surface of preformed liposomes. This efficient and chemoselective reaction involves a Cu(I)-catalyzed azide-alkyne cycloaddition, which can be performed under mild experimental conditions in aqueous media. Here, we describe the application of a model click reaction to the conjugation, in a single step of unprotected alpha-1-thiomannosyl ligands, functionalized with an azide group to liposomes containing a terminal alkyne-functionalized lipid anchor. Excellent coupling yields were obtained in the presence of bathophenanthrolinedisulphonate, a water soluble copper-ion chelator, acting as a catalyst. No vesicle leakage was triggered by this conjugation reaction and the coupled mannose ligands were exposed at the surface of the liposomes. The major limitation of Cu(I)-catalyzed click reactions is that this conjugation is restricted to liposomes made of saturated (phospho)lipids. Efficient copper-free azide-alkyne click reactions are, however, being developed, which should alleviate this constraint in the future.

Aza-dibenzocyclooctynes for fast and efficient enzyme PEGylation via copper-free (3+2) cycloaddition

A strained aza-dibenzocyclooctyne was prepared via a high-yielding synthetic route. Copper-free, strain-promoted click reaction with azides showed excellent kinetics, and a functionalised aza-cyclooctyne was applied in fast and efficient PEGylation of enzymes.

Fluorogenic click reaction

Fluorogenic Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reactions have emerged as a powerful tool for bioconjugation, materials science, organic synthesis and drug discovery. This review highlights the design of the recent development of fluorogenic CuAAC reactions as well as their applications.

Hydrogels in regenerative medicine

Hydrogels, due to their unique biocompatibility, flexible methods of synthesis, range of constituents, and desirable physical characteristics, have been the material of choice for many applications in regenerative medicine. They can serve as scaffolds that provide structural integrity to tissue constructs, control drug and protein delivery to tissues and cultures, and serve as adhesives or barriers between tissue and material surfaces. In this work, the properties of hydrogels that are important for tissue engineering applications and the inherent material design constraints and challenges are discussed. Recent research involving several different hydrogels polymerized from a variety of synthetic and natural monomers using typical and novel synthetic methods are highlighted. Finally, special attention is given to the microfabrication techniques that are currently resulting in important advances in the field.

Synthesis of linear polyether polyol derivatives as new materials for bioconjugation

Linear polyether polyol (PEP) consisting of glycidol as repeating units is a flexible hydrophilic aliphatic polymer. The polyether main chain is similar to the widely used, biocompatible polymer poly(ethylene glycol) (PEG). While linear PEG has one or two terminal hydroxyl group(s), linear PEP distinguishes itself by the large number of pendant hydroxyl groups along the polyether main chain. We propose that this property of PEP represents a major advantage over PEG, namely, by providing multiple anchorage points and increasing the possibility for introducing different functional groups. As a first step to establishing PEP as a bioconjugation material, we modified the pendant hydroxyl groups on PEP and prepared a series of mono- and heterobifunctional derivatives with the potential to join various drug entities and biomolecules. The synthesis methods and the results of characterization are reported here.