Sequential and parallel dual labeling of nanoparticles using click chemistry

Carbonic Anhydrase IX Targeting Mn(II)-Based Magnetic Resonance Molecular Imaging Probe for Hypoxia Tumors

Physiological hypoxic conditions in the tumor microenvironment and consequential overexpression of carbonic anhydrase IX (CA IX) are two characteristics shared by numerous types of solid malignant tumors. Early detection with hypoxia assessment is crucial to improve the prognosis and therapy outcomes of hypoxia tumors. Herein, using acetazolamide (AZA) as a CA IX-targeting moiety, we design and synthesize an Mn(II)-based MR imaging probe (named AZA-TA-Mn) incorporating AZA and two Mn(II) chelates of Mn-TyEDTA on a rigid triazine (TA) scaffold. The per Mn relaxivity of AZA-TA-Mn is 2-fold higher than its monomeric Mn-TyEDTA, which allows it for low-dose imaging of hypoxic tumors. In a xenograft mice model of esophageal squamous cell carcinoma (ESCC), a low dosage of AZA-TA-Mn (0.05 mmol/kg) can selectively produce prolonged and stronger contrast enhancement in the tumor compared to the non-specific Gd-DTPA (0.1 mmol/kg). A competition study of co-injection of free AZA and Mn(II) probes confirms the in vivo tumor selectivity of AZA-TA-Mn, resulting in a more than 2.5-fold decreased tumor-to-muscle contrast-to-noise ratio (ΔCNR) at 60 min post-injection. MR imaging results were further supported by the quantitative analysis of Mn tissue levels, as the co-injection of free AZA resulted in significantly reduced Mn accumulation in tumor tissues. Finally, immunofluorescence staining of tissue sections confirms the positive correlation between the tumor accumulation of AZA-TA-Mn and CA IX overexpression. Hence, using CA IX as the hypoxia biomarker, our results illustrate a practical strategy for the development of novel imaging probes for hypoxic tumors.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Modular Assembly of Multimodal Imaging Agents through an Inverse Electron Demand Diels-Alder Reaction

The combination of two imaging probes on the same biomolecule gives access to targeted bimodal imaging agents that can provide more accurate diagnosis, complementary information, or that may be used in different applications, such as nuclear imaging and fluorescence guided surgery. In this study, we demonstrate that dichlorotetrazine, a small, commercially available compound, can be used as a modular platform to easily assemble various imaging probes. Doubly labeled tetrazines can then be conjugated to a protein through a biorthogonal IEDDA reaction. A series of difunctionalized tetrazine compounds containing various chelating agents and fluorescent dyes was synthesized. As a proof of concept, one of these bimodal probes was conjugated to trastuzumab, previously modified with a constrained alkyne group, and the resulting dual-labeled antibody was evaluated in a mouse model, bearing a HER2-positive tumor. A significant uptake into tumor tissues was observed in vivo, by both SPECT-CT and fluorescence imaging, and confirmed ex vivo in biodistribution studies.

A palette of background-free tame fluorescent probes for intracellular multi-color labelling in live cells

A palette of background-free tame fluorescent probes were designed and applied to intracellular multi-color labelling in live cells.

A multi-color labelling technique for visualizing multiple intracellular apparatuses in their native environment using small fluorescent probes remains challenging. This approach requires both orthogonal and biocompatible coupling reactions in heterogeneous biological systems with minimum fluorescence background noise. Here, we present a palette of BODIPY probes containing azide and cyclooctyne moieties for copper-free click chemistry in living cells. The probes, referred to as ‘tame probes’, are highly permeable and specific in nature, leaving no background noise in cells. Such probes, which are rationally designed through optimized lipophilicity, water solubility and charged van der Waals surface area, allow us to demonstrate rapid and efficient concurrent multi-labelling of intracellular target components. We show that these probes are capable of not only labelling organelles and engineered proteins, but also showing the intracellular glycoconjugates’ dynamics, through the use of metabolic oligosaccharide engineering technology in various cell types. The results demonstrated in this study thus provide flexibility for multi-spectral labelling strategies in native systems in a high spatiotemporal manner.

Facile Quenching and Spatial Patterning of Cylooctynes via Strain-Promoted Alkyne-Azide Cycloaddition of Inorganic Azides

Little is known about the reactivity of strain-promoted alkyne-azide cycloaddition (SPAAC) reagents with inorganic azides. We explore the reactions of a variety of popular SPAAC reagents with sodium azide and hydrozoic acid. We find that the reactions proceed in water at rates comparable to those with organic azides, yielding in all cases a triazole adduct. The azide ion's utility as a cyclooctyne quenching reagent is demonstrated by using it to spatially pattern uniformly doped hydrogels. The facile quenching of cyclooctynes demonstrated here should be useful in other bioorthogonal ligation techniques in which cyclooctynes are employed, including SPANC, Diels-Alder, and thiol-yne.

B, Wang SH. Dendrimer antibody conjugate to target and image HER-2 overexpressing cancer cells

Although many breast and lung cancers overexpress human epidermal growth factor receptor-2 (HER-2), no methods currently exist for effective and early detection of HER-2-positive cancers. To address this issue, we designed and synthesized dendrimer-based novel nano-imaging agents that contain gold nanoparticles (AuNPs) and gadolinium (Gd), conjugated with the humanized anti-HER-2 antibody (Herceptin). Generation 5 (G5) polyamidoamine (PAMAM) dendrimers were selected as the backbone for the nano-imaging agents due to their unique size, high ratio of surface functional groups and bio-functionality. We modified G5 PAMAM dendrimer surface with PEG and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelators to encapsulate AuNPs and complex Gd. These dendrimer entrapped AuNPs were further conjugated with Herceptin through copper-catalyzed azide- alkyne click reaction to construct the nano-imaging agent Au-G5-Gd-Herceptin. The targeted nano-imaging agent bound selectively to HER-2 overexpressing cell lines, with subsequent internalization into the cells. More importantly, non-targeted nano-imaging agent neither bound nor internalized into cells overexpressing HER-2. These results suggest that our approach could provide a platform to develop nano-diagnostic agents or nano-therapeutic agents for early detection and treatment of HER-2-positive cancers.

Cyclopropenone-caged Sondheimer diyne (dibenzo[a,e]cyclooctadiyne): a photoactivatable linchpin for efficient SPAAC crosslinking

The first fully conjugated bis-cyclopropenone (photo-DIBOD), a derivative of dibenzo[a,e][8]annulene, has been synthesized. 350-420 nm irradiation of this robust compound results in the efficient formation of dibenzo [a,e] cyclooctadiyne, an unstable, but useful SPAAC cross-linking reagent. Since photo-DIBO doesn't react with organic azides, this method allows for the spatiotemporal control of the ligation of two azide-tagged substrates.

Recent advances in click chemistry applied to dendrimer synthesis

Dendrimers are monodisperse polymers grown in a fractal manner from a central point. They are poised to become the cornerstone of nanoscale devices in several fields, ranging from biomedicine to light-harvesting. Technical difficulties in obtaining these molecules has slowed their transfer from academia to industry. In 2001, the arrival of the "click chemistry" concept gave the field a major boost. The flagship reaction, a modified Hüisgen cycloaddition, allowed researchers greater freedom in designing and building dendrimers. In the last five years, advances in click chemistry saw a wider use of other click reactions and a notable increase in the complexity of the reported structures. This review covers key developments in the click chemistry field applied to dendrimer synthesis from 2010 to 2015. Even though this is an expert review, basic notions and references have been included to help newcomers to the field.