Click Chemistry for Drug Delivery Nanosystems

Facile method to radiolabel glycol chitosan nanoparticles with (64)Cu via copper-free click chemistry for MicroPET imaging

An efficient and straightforward method for radiolabeling nanoparticles is urgently needed to understand the in vivo biodistribution of nanoparticles. Herein, we investigated a facile and highly efficient strategy to prepare radiolabeled glycol chitosan nanoparticles with (64)Cu via a strain-promoted azide-alkyne cycloaddition strategy, which is often referred to as click chemistry. First, the azide (N3) group, which allows for the preparation of radiolabeled nanoparticles by copper-free click chemistry, was incorporated to glycol chitosan nanoparticles (CNPs). Second, the strained cyclooctyne derivative, dibenzyl cyclooctyne (DBCO) conjugated with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator, was synthesized for preparing the preradiolabeled alkyne complex with (64)Cu radionuclide. Following incubation with the (64)Cu-radiolabeled DBCO complex (DBCO-PEG4-Lys-DOTA-(64)Cu with high specific activity, 18.5 GBq/μmol), the azide-functionalized CNPs were radiolabeled successfully with (64)Cu, with a high radiolabeling efficiency and a high radiolabeling yield (>98%). Importantly, the radiolabeling of CNPs by copper-free click chemistry was accomplished within 30 min, with great efficiency in aqueous conditions. In addition, we found that the (64)Cu-radiolabeled CNPs ((64)Cu-CNPs) did not show any significant effect on the physicochemical properties, such as size, zeta potential, or spherical morphology. After (64)Cu-CNPs were intravenously administered to tumor-bearing mice, the real-time, in vivo biodistribution and tumor-targeting ability of (64)Cu-CNPs were quantitatively evaluated by microPET images of tumor-bearing mice. These results demonstrate the benefit of copper-free click chemistry as a facile, preradiolabeling approach to conveniently radiolabel nanoparticles for evaluating the real-time in vivo biodistribution of nanoparticles.

Click to bind: metal sensors

The copper-catalyzed azide-alkyne "click" cycloaddition reaction is an efficient coupling reaction that results in the formation of a triazole ring. The wide range of applicable substrates for this reaction allows the construction of a variety of conjugated systems. The additional function of triazoles as metal-ion ligands has led to the click reaction being used for the construction of optical sensors for metal ions. The triazoles are integral binding elements, which are formed in an efficient modular synthesis. Herein, we review recent examples of triazoles as a metal-binding element in conjugated metal-ion sensors.

Efficient functionalization of oxide-free silicon(111) surfaces: thiol-yne versus thiol-ene click chemistry

Thiol-yne click (TYC) chemistry was utilized as a copper-free click reaction for the modification of alkyne-terminated monolayers on oxide-free Si(111) surfaces, and the results were compared with the analogous thiol-ene click (TEC) chemistry. A wide range of thiols such as 9-fluorenylmethoxy-carbonyl cysteine, thio-β-d-glucose tetraacetate, thioacetic acid, thioglycerol, thioglycolic acid, and 1H,1H,2H,2H-perfluorodecanethiol was immobilized using TYC under photochemical conditions, and all modified surfaces were characterized by static water contact angle measurements, X-ray photoelectron spectroscopy (including a simulation thereof by density functional calculations), and infrared absorption reflection spectroscopy. Surface-bound TYC proceeds with an efficiency of up to 1.5 thiols per alkyne group. This high surface coverage proceeds without oxidizing the Si surface. TYC yielded consistently higher surface coverages than TEC, due to double addition of thiols to alkyne-terminated monolayers. This also allows for the sequential and highly efficient attachment of two different thiols onto an alkyne-terminated monolayer.

Multifunctional nanoparticles: cost versus benefit of adding targeting and imaging capabilities

Nanoparticle-based drug delivery systems have been developed to improve the efficacy and reduce the systemic toxicity of a wide range of drugs. Although clinically approved nanoparticles have consistently shown value in reducing drug toxicity, their use has not always translated into improved clinical outcomes. This has led to the development of "multifunctional" nanoparticles, where additional capabilities like targeting and image contrast enhancement are added to the nanoparticles. However, additional functionality means additional synthetic steps and costs, more convoluted behavior and effects in vivo, and also greater regulatory hurdles. The trade-off between additional functionality and complexity is the subject of ongoing debate and the focus of this Review.

Synthesis of water-soluble camptothecin-polyoxetane conjugates via click chemistry

Water-soluble camptothecin (CPT)-polyoxetane conjugates were synthesized using a clickable polymeric platform P(EAMO) that was made by polymerization of acetylene-functionalized 3-ethyl-3-(hydroxymethyl)oxetane (i.e., EAMO). CPT was first modified with a linker 6-azidohexanoic acid via an ester linkage to yield CPT-azide. CPT-azide was then click coupled to P(EAMO) in dichloromethane using bromotris(triphenylphosphine)copper(I)/N,N-diisopropylethylamine. For water solubility and cytocompatibility improvement, methoxypolyethylene glycol azide (mPEG-azide) was synthesized from mPEG 750 g mol(-1) and click grafted using copper(II) sulfate and sodium ascorbate to P(EAMO)-g-CPT. (1)H NMR spectroscopy confirmed synthesis of all intermediates and the final product P(EAMO)-g-CPT/PEG. CPT was found to retain its therapeutically active lactone form. The resulting P(EAMO)-g-CPT/PEG conjugates were water-soluble and produced dose-dependent cytotoxicity to human glioma cells and increased γ-H2AX foci formation, indicating extensive cell cycle-dependent DNA damage. Altogether, we have synthesized CPT-polymer conjugates able to induce controlled toxicity to human cancer cells.

X-ROS signaling: rapid mechano-chemo transduction in heart

We report that in heart cells, physiologic stretch rapidly activates reduced-form nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) to produce reactive oxygen species (ROS) in a process dependent on microtubules (X-ROS signaling). ROS production occurs in the sarcolemmal and t-tubule membranes where NOX2 is located and sensitizes nearby ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). This triggers a burst of Ca(2+) sparks, the elementary Ca(2+) release events in heart. Although this stretch-dependent "tuning" of RyRs increases Ca(2+) signaling sensitivity in healthy cardiomyocytes, in disease it enables Ca(2+) sparks to trigger arrhythmogenic Ca(2+) waves. In the mouse model of Duchenne muscular dystrophy, hyperactive X-ROS signaling contributes to cardiomyopathy through aberrant Ca(2+) release from the SR. X-ROS signaling thus provides a mechanistic explanation for the mechanotransduction of Ca(2+) release in the heart and offers fresh therapeutic possibilities.