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Cell-penetrating peptides (CPPs) have emerged as versatile tools to increase the intracellular accumulation of different kinds of cargoes. For an efficient cellular uptake and drug delivery, their organization into a distinct and stable secondary structure at the outer surface of the plasma membrane is a hallmark and supports optimal lipid-peptide interactions. Incorporation of hydrophobic moieties, such as carboranes (CBs), has the potential to increase the lipophilicity of peptides, and thus, to facilitate the formation of secondary structures. Herein, we present synthesis and biophysical as well as biological characterization of carborane-CPP conjugates having incorporated one or more CB clusters. Our results highlight the possibility to modulate the secondary structure of CPPs by the addition of CB's leading to constructs with altered membrane activity and promising use in terms of nucleic acid delivery.

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In the present article we describe the creation of a small carboranylcarboxamide compound library followed by a screening campaign at the soluble epoxide hydrolase (sEH). We identified meta-carboranyl alkylamides, -anilides, and -benzylamides as potent sEH inhibitors. Furthermore, we optimized the scaffolds and we derived structure-activity relationships. The most potent benzylamide 33 (MS1) was similar to a previously reported adamantane derivative and gave an IC50 value of 0.07 µM for meta- and 0.08 µM for para-carborane at isolated sEH. The ortho-derivative suffered deboronation. The results underline the potential of carboranes as non-natural 3-D pharmacophores to extend the chemical space in drug discovery.

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Substituted xanthine derivatives are important bioactive molecules. Herein we report on a new, practical synthesis of 6-amino-5-carboxamidouracils, the main building blocks for the preparation of 8-substituted xanthines, by condensation of 5,6-diaminouracil derivatives and various carboxylic acids using the recently developed non-hazardous coupling reagent COMU (1-[(1-(cyano-2-ethoxy-2-oxoethylideneaminooxy)dimethylaminomorpholinomethylene)]methanaminium hexafluorophosphate). Optimized reaction conditions led to the precipitation of pure products after only 5 to 10 min of reaction time. The method tolerates a variety of substituted 5,6-diaminouracil and carboxylic acid derivatives as starting compounds resulting in most cases in more than 80% isolated yield. Regioselectivity of the reaction yielding only the 5-carboxamido-, but not the 6-carboxamidouracil derivatives, was unambiguously confirmed by single X-ray crystallography and multidimensional NMR experiments. The described method represents a convenient, fast access to direct precursors of 8-substituted xanthines under mild conditions without the necessity of hazardous coupling or chlorinating reagents.

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An overexpression of the transmembrane ATP-binding cassette transporter G2 (ABCG2, BCRP) in cancer tissues is supposed to play a role in the multidrug resistance (MDR) of tumors resulting in an inefficient chemotherapy. Therefore, co-administration of selective and non-toxic ABCG2 inhibitors is a promising strategy for improving the efficacy of chemotherapy by blocking ABCG2-mediated export of the cytostatic drugs. In the present study, we designed a small library of 38 novel compounds containing a heteroaryl-phenyl scaffold possessing several (bioisosteric) moieties, and twelve new precursors. We investigated the library for ABCG2 inhibition, for the selectivity against MDR-involved efflux pump ABCB1 (P-gp) and for toxicity. Structure activity relationship (SAR) studies revealed that, at least a phenylheteroaryl-phenylamide scaffold is necessary for observing an ABCG2 inhibition. 4-Methoxy-N-(2-(2-(6-methoxypyridin-3-yl)-2H-tetrazol-5-yl)phenyl)benzamide (43) exhibited a high potency (IC50 = 61 nM)), selectivity, low intrinsic toxicity and reversed the ABCG2-mediated drug resistance in presence of only 0.1 µM.

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Lipids are defined as apolar molecules comprising as major classes fatty acids and fatty acid esters of normally natural origin. Non-natural components, such as dicarba-closo-dodecaboranes (in short carboranes) can also form acids and esters, which reveal lipid-like properties. Carboranes are synthetic boron clusters featuring ten BH and two CH vertices, organized in icosahedral shape. The highly hydrophobic clusters are organic-inorganic hybrid constructs and can be modified at both the cluster boron and the cluster carbon atoms via adjusted organic reactions. Here, we report the synthesis and characterization of lipid esters from meta-carborane-1-carboxylic acid using a new coupling reagent strategy. Carboranyl esters from long-chain alcohols revealed wax-like properties.

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Amide bond formation is one of the most important chemical reactions. In peptide and organic chemistry, the application of amide coupling reagents is a routine strategy, but surprisingly not in carborane chemistry. Thus, we now report a fast, safe, and robust protocol to couple amines to m- and p-dicarba-closo-dodecaborane-1-carboxylic acids. The procedure comprises the activation of carboxylic acid with the coupling reagent (1-cyano-2-ethoxy-2-oxoethylidenaminooxy)(dimethylamino)morpholinocarbenium hexafluorophosphate, extraction of the product using the hydrophobic nature of the cluster, and a straightforward chromatographic purification. The protocol allows access to a variety of carborane-organic hybrid molecules suitable for application in multiple areas.

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B-Cyanodicarba-closo-dodecaboranes are a poorly explored class of compounds due to their complex synthetic availability. Now, we report a fast, atom-efficient, and high-yielding synthesis. We obtained the cyano derivatives by reacting B-iododicarba-closo-dodecaboranes with copper(I) cyanide under both palladium catalysis and microwave irradiation. We successfully applied this method to 9-iodo-o-, 9-iodo-m-, and 2-iodo-p-dicarba-closo-dodecaborane, obtaining the corresponding cyanides up to 89% isolated yield. The facile synthesis and evaluation of their spectroscopic properties reported herein will pave the way to exploring the chemistry and application of B-cyanodicarba-closo-dodecaborane clusters in more detail.

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Carbaboranes are increasingly studied as pharmacophores, particularly as replacements for aromatic systems. However, especially ortho-carbaborane is prone to degradation of the cluster, which hampers biological application. This study demonstrates that deboronation of the cluster may not only lead to a more active analogue, but can also improve the solubility and stability of a carbaborane-containing inhibitor. Notably, introduction of a nido-dicarbaborate cluster into the cyclooxygenase (COX) inhibitor indomethacin results in remarkably increased inhibitory potency and selectivity for COX-2 relative to the respective phenyl analogue. The first crystal structure of a carbaborane-containing inhibitor bound to COX-2 further reveals a novel binding mode for the inhibitor that is strikingly different from that of indomethacin. These results indicate that nido-dicarbaborate is a promising pharmacophore that exhibits properties which are also highly beneficial for its introduction into other inhibitor classes.

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Cyclooxygenase (COX) is an enzyme involved in tumorigenesis and is associated with tumor cell resistance against platinum-based antitumor drugs. Cisplatin analogues were conjugated with COX inhibitors (indomethacin, ibuprofen) to study the synergistic effects that were previously observed in combination treatments. The conjugates ensure concerted transport of both drugs into cells, and subsequent intracellular cleavage enables a dual-action mode. Whereas the platinum(II) complexes showed cytotoxicities similar to those of cisplatin, the platinum(IV) conjugates revealed highly increased cytotoxic activities and were able to completely overcome cisplatin-related resistance. Although some of the complexes are potent COX inhibitors, the conjugates appear to execute their cytotoxic action via COX-independent mechanisms. Instead, the increased lipophilicity and kinetic inertness of the conjugates seem to facilitate cellular accumulation of the platinum drugs and thus improve the efficacy of the antitumor agents. These conjugates are important tools for the elucidation of the direct influence of COX inhibitors on platinum-based anticancer drugs in tumor cells.

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