Substituent effects on oxidation-induced formation of quinone methides from arylboronic ester precursors

Coumarin Based Fluorescent Probe for Detecting Heavy Metal Ions

Heavy metals such as Iron, Copper, and Zinc are micro-essential trace metal and involve animportant biological role, but it quickly turns toxic at exceeding the permissible limit, causing gastrointestinal irritation, liver, bone, and kidney damage, as well as disorders including Wilson's, Parkinson's, and Alzheimer's. It is important to detect the metal ions as well as their concentration quickly and affordable cost using organic probes. Among the organic probes,the coumarin fluorescent probe shows a very prominent candidate with heavy metal ions. Therefore, in the present review, we reviewed the very recent literature the identify the heavy metals using modified coumarin fluorescent probes. Readers will get information quickly about the method of preparation of modified coumarin core and their use as fluorescent probes with heavy metals using absorption and emission spectroscopic methods along with the probable mechanistic pathway of detection.

Latest advances in glucose-responsive microneedle-based systems for transdermal insulin delivery

The development of a self-regulated minimally invasive system for insulin delivery can be considered as the holy grail in the field of diabetes mellitus. A delivery system capable of releasing insulin in response to blood glucose levels would significantly improve the quality of life of diabetic patients, eliminating the need for frequent finger-prick tests and providing better glycaemic control with lower risk of hypoglycaemia. In this context, the latest advances in glucose-responsive microneedle-based transdermal insulin delivery are here compiled with a thorough analysis of the delivery mechanisms and challenges lying ahead in their clinical translation. Two main groups of microneedle-based systems have been developed so far: glucose oxidase-containing and phenylboronic acid-containing systems. Both strategies in combination have also been tested and two other novel strategies are under development, namely electronic closed-loop and glucose transporter-based systems. Results from preclinical studies conducted using these different types of glucose-triggered release systems are comprehensively discussed. Altogether, this analysis from both a mechanistic and translational perspective will provide rationale and/or guidance for future trends in the research hotspot of glucose-responsive microneedle-based insulin delivery systems.

H2 O2 -Inducible DNA Cross-linking Agents Capable of Releasing Multiple DNA Alkylators as Anticancer Prodrugs

Three compounds with arylboronate esters conjugated with two equivalent nitrogen mustards [bis(2-chloroethyl)methylamine, HN2] have been synthesized and characterized. These inactive small molecules selectively react with H2 O2 to produce multiple DNA cross-linkers, such as two HN2 molecules alongside a bisquinone methide (bisQM), leading to efficient DNA ICL formation. In comparison to other amine functional groups, using HN2 as a leaving group greatly improves the DNA cross-linking efficiency of these arylboronate esters as well as cellular activity. The introduction of HN2 in these arylboronate ester analogues favored the generation of bisQM that can directly cross-link DNA. Two equivalents of HN2 are also generated from these compounds upon treatment with H2 O2 , which directly produces DNA ICL products. The cumulative effects of HN2 and bisQM on DNA cross-linking makes these molecules highly effective H2 O2 -inducible DNA ICL agents. The three compounds with HN2 as a leaving group showed greatly enhanced cytotoxicity towards cancer cells in comparison to those containing trimethyl amine as a leaving group. This provides an effective strategy for further design of novel potential ROS-activated anticancer prodrugs.

Regulating the activity of boronate moiety to construct fluorescent probes for the detection of ONOO-in vitro and in vivo

Abnormal intracellular peroxynitrite (ONOO^-) concentration is related to oxidative damage, which is correlated with many pathological consequences, such as local inflammation and other diseases. In this work, a series of resorufin benzyl ether-based fluorescent probes were designed using boronate as a recognizing moiety installed on a phenyl moiety for ONOO^- detection via a self-immolation mechanism. The location of the boronate as well as the substitution patterns on the phenyl moiety were investigated and the responding behaviors of the designed probes to ONOO^-, other reactive oxygen species, and biothiols were examined. It was found that all the immolative probes were inevitably dominated by ONOO^-. Compared with other probes, p-Borate possessed favorable selectivity and high sensitivity to ONOO^-. Moreover, p-Borate was successfully used to detect ONOO^- in cells and inflamed mice.

Flucytosine-based prodrug activation by cold physical plasma

Reactive oxygen species (ROS) are known to trigger drug release from arylboronate-containing ROS-responsive prodrugs. In cancer cells, elevated levels of ROS can be exploited for the selective activation of prodrugs via Baeyer-Villiger type oxidation rearrangement sequences. Here, we report a proof of concept to demonstrate that these cascades can as well be initiated by cold physical plasma (CPP). An analog of a recently reported fluorouracil prodrug based on the less toxic drug 5-fluorocytosine (5-FC) was synthesized with a view to laboratory safety reasons and used as a model compound to prove our hypothesis that CPP is suitable as a trigger for the prodrug activation. Although the envisioned oxidation and rearrangement with successive loss of boronic acid species could be achieved by plasma treatment, the anticipated spontaneous liberation of 5-FC was inefficient in the model case. However, the obtained results suggest that custom-tailored CPP-responsive prodrugs might become an evolving research field.

Phenyl Selenide-Based Precursors as Hydrogen Peroxide Inducible DNA Interstrand Cross-Linkers

DNA interstrand crosslinks (ICLs) are highly toxic DNA lesions, and induce cell death by blocking DNA strands separation. Most developed ICL agents, aiming to kill cancer cells, also generate adverse side effects to normal cells. H2O2-inducible DNA ICL agents are highly selective to target cancer cells, as the concentration of H2O2 is higher in cancer cells than normal cells. Previous studies focus on arylboronate-based precursors, reacting with H2O2 to generate reactive quinone methides (QMs) crosslinking DNA. Here we explore phenyl selenide-based precursors 1-3 as H2O2-inducible DNA ICL agents. The precursors 1-3 can be activated by H2O2 to generate the good benzylic leaving group and promote production of reactive QMs to crosslink DNA. Moreover, the DNA cross-linking ability is enhanced by the introduction of substituents in the para position of the phenolic hydroxyl group. From the substituents explored (H, OMe, F), the introduction of electron donating group (OMe) shows a pronounced elevating effect. Further mechanistic studies at the molecular and DNA levels confirm alkylation sites located mainly at dAs, dCs and dGs in DNA. Additionally, cellular experiments reveal that agents 1-3 exhibit higher cytotoxicity toward H1299 human lung cancer cells compared to clinically used drugs, by inducing cellular DNA damage, apoptosis and G0/G1 cell cycle arrest. This study provides a strategy to develop H2O2-inducible DNA interstrand cross-linkers.

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

Recent Advances in Hydrogen Peroxide Responsive Organoborons for Biological and Biomedical Applications

Hydrogen peroxide is the most stable reactive oxygen species generated endogenously, participating in numerous physiological processes and abnormal pathological conditions. Mounting evidence suggests that a higher level of H₂ O₂ exists in various disease conditions. Thus, H₂ O₂ functions as an ideal target for site-specific bioimaging and therapeutic targeting. The unique reactivity of organoborons with H₂ O₂ provides a method for developing chemoselective molecules for biological and biomedical applications. This review highlights the design and application of boron-derived molecules for H₂ O₂ detection, and the utility of boron moieties toward masking reactive compounds leading to the development of metal prochelators and prodrugs for selectively delivering an active species at the target sites with elevated H₂ O₂ levels. Additionally, the emergence of H₂ O₂ -responsive theranostic agents consisting of both therapeutic and diagnostic moieties in one integrated system are discussed. The purpose of this review is to provide a better understanding of the role of boron-derived molecules toward biological and pharmacological applications.

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

Hydroboration of amides is a useful synthetic strategy to access the corresponding amines. In this contribution, it was found that the supported lanthanum benzyl material La(CH₂C₆H₄NMe₂-o)₃@SBA-15 was highly active for the hydroboration of primary, secondary, and tertiary amides to amines with pinacolborane. These reactions selectively produced target amines and showed good tolerance for functional groups such as -NO₂, -halogen, and -CN, as well as heteroatoms such as S and O. This reduction procedure exhibited the recyclable and reusable property of heterogeneous catalysts and was applicable to gram-scale synthesis. The reaction mechanisms were proposed based on some control experiments and the previous literature. This is the first example of hydroborative reduction of amides to amines mediated by heterogeneous catalysts.

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Design Strategies and Applications of ROS-Responsive Phenylborate Ester-Based Nanomedicine

Reactive oxygen species (ROS)-responsive nanomedicine has been extensively developed to improve the therapeutic effects while reducing the systemic toxicity. ROS, as important biological metabolites and signaling molecules, are known to overexpress in most of tumors and inflammations. Among various ROS-sensitive moieties, phenylborate ester (PBAE) with easy modifiable structure and excellent biocompatibility, represents one of the most ROS-sensitive structures. To harness it as a switch, the past several years had witnessed a booming of ROS-sensitive PBAE-based nanomedicine for various medical purposes. Much of the efforts were devoted to exploiting the potential in the management of antitumor and anti-inflammation. This review first summarizes the design strategies of PBAE in the construction of nanomedicine, with PBAE acting as not only the ROS-responsive unit, but also the roles of hydrophobic backbone or bridging segment in the macromolecular structures. The ROS-responsive mechanisms are then briefly discussed. Afterward, we focus on the introduction of the state-of-the-art research on ROS-responsive PBAE-based nanomedicine for antitumor and anti-inflammation applications. The conclusion and future perspectives of ROS-responsive nanomedicine are also provided.

Electrochemiluminescent detection of glucose in human serum by BODIPY-based chemodosimeters for hydrogen peroxide using accelerated self-immolation of boronates

BODIPY-based ECL chemodosimeters were developed for the detection of hydrogen peroxide. The reactivity of boronate towards hydrogen peroxide was enhanced by adjacent fluorine atoms. In combination with glucose oxidase, a fluorine-substituted probe successfully quantified the glucose level in human serum, providing its potential as a versatile tool in point-of-care testing applications.

Directing Quinone Methide-Dependent Alkylation and Cross-Linking of Nucleic Acids with Quaternary Amines

Polyamine and polyammonium ion conjugates are often used to direct reagents to nucleic acids based on their strong electrostatic attraction to the phosphoribose backbone. Such nonspecific interactions do not typically alter the specificity of the attached reagent, but polyammonium ions dramatically redirected the specificity of a series of quinone methide precursors. Replacement of a relatively nonspecific intercalator based on acridine with a series of polyammonium ions resulted in a surprising change of DNA products. Piperidine stable adducts were generated in duplex DNA that lacked the ability to support a dynamic cross-linking observed previously with acridine conjugates. Minor reaction at guanine N7, the site of reversible reaction, was retained by a monofunctional quinone methide-polyammonium ion conjugate, but a bisfunctional analogue designed for tandem quinone methide formation modified guanine N7 in only single-stranded DNA. The resulting intrastrand cross-links were sufficiently dynamic to rearrange to interstrand cross-links. However, no further transfer of adducts was observed in duplex DNA. An alternative design that spatially and temporally decoupled the two quinone methide equivalents neither restored the dynamic reaction nor cross-linked DNA efficiently. While di- and triammonium ion conjugates successfully enhanced the yields of cross-linking by a bisquinone methide relative to a monoammonium equivalent, alternative ligands will be necessary to facilitate the migration of cross-linking and its potential application to disrupt DNA repair.

Migratory ability of quinone methide-generating acridine conjugates in DNA

Conversion of a bisquinone methide–acridine conjugate to its monofunctional analogue releases the constraints that limit migration of its reversible adducts within DNA.

Arylboronate prodrugs of doxorubicin as promising chemotherapy for pancreatic cancer

This study describes the synthesis of arylboronate-based ROS-responsive prodrugs of doxorubicin and their biological evaluation as anticancer agents. The determination of the most sensitive cancer type toward arylboronate prodrugs is crucial for further consideration of these molecules in clinical phase. To address this goal, an arylboronate-based profluorescent probe was used to compare the capacity of various cancer cell lines to efficiently convert the precursor into the free fluorophore. On the selected MiaPaCa-2 pancreatic cancer cells, a benzeneboronate prodrug exhibited 67% of the cytotoxicity obtained with the free doxorubicin. The prodrug was also able to induce tumor regression on MiaPaCa-2 pancreatic tumor model in ovo. Using this model, the amount of free doxorubicin liberated from this prodrug into the tumor was equivalent to the quantity measured after direct intratumoral injection of the same concentration of doxorubicin.

Influence of fluorine substituents on the properties of phenylboronic compounds

Rapid development of research on the chemistry of boronic acids is connected with their applications in organic synthesis, analytical chemistry, materials’ chemistry, biology and medicine. In many applications Lewis acidity of boron atoms plays an important role. Special group of arylboronic acids are fluoro-substituted compounds, in which the electron withdrawing character of fluorine atoms influences their properties. The present paper deals with fluoro-substituted boronic acids and their derivatives: esters, benzoxaboroles and boroxines. Properties of these compounds, i.e. acidity, hydrolytic stability, structures in crystals and in solution as well as spectroscopic properties are discussed. In the next part examples of important applications are given.

Hydrogen peroxide activated quinone methide precursors with enhanced DNA cross-linking capability and cytotoxicity towards cancer cells

Quinone methide (QM) formation induced by endogenously generated H₂O₂ is attractive for biological and biomedical applications. To overcome current limitations due to low biological activity of H₂O₂-activated QM precursors, we are introducing herein several new arylboronates with electron donating substituents at different positions of benzene ring and/or different neutral leaving groups. The reaction rate of the arylboronate esters with H₂O₂ and subsequent bisquinone methides formation and DNA cross-linking was accelerated with the application of Br as a leaving group instead of acetoxy groups. Additionally, a donating group placed meta to the nascent exo-methylene group of the quinone methide greatly improves H₂O₂-induced DNA interstrand cross-link formation as well as enhances the cellular activity. Multiple donating groups decrease the stability and DNA cross-linking capability, which lead to low cellular activity. A cell-based screen demonstrated that compounds 2a and 5a with a OMe or OH group dramatically inhibited the growth of various tissue-derived cancer cells while normal cells were less affected. Induction of H2AX phosphorylation by these compounds in CLL lymphocytes provide evidence for a correlation between cell death and DNA damage. The compounds presented herein showed potent anticancer activities and selectivity, which represent a novel scaffold for anticancer drug development.

Targeting duplex DNA with the reversible reactivity of quinone methides

DNA alkylation and crosslinking remains a common and effective strategy for anticancer chemotherapy despite its infamous lack of specificity. Coupling a reactive group to a sequence-directing component has the potential to enhance target selectivity but may suffer from premature degradation or the need for an external signal for activation. Alternatively, quinone methide conjugates may be employed if they form covalent but reversible adducts with their sequence directing component. The resulting self-adducts transfer their quinone methide to a chosen target without an external signal and avoid off-target reactions by alternative intramolecular self-trapping. Efficient transfer is shown to depend on the nature of the quinone methide and the sequence-directing ligand in applications involving alkylation of duplex DNA through a triplex recognition motif. Success required an electron-rich derivative that enhanced the stability of the transient quinone methide intermediate and a polypyrimidine strand of DNA to associate with its cognate polypurine/polypyrimidine target. Related quinone methide conjugates with peptide nucleic acids were capable of quinone methide transfer from their initial precursor but not from their corresponding self-adduct. The active peptide nucleic acid derivatives were highly selective for their complementary target.

Photochemical Generation of Benzyl Cations That Selectively Cross-Link Guanine and Cytosine in DNA

UV irradiation of several aryl boronates efficiently produced bifunctional benzyl cations that selectively form guanine-cytosine cross-links in DNA. Photoinduced homolysis of the C-Br bond took place with the aryl boronate bromides 3a and 4a, generating free radicals that were oxidized to benzyl cations via electron transfer. However, photoirradiation of the quaternary ammonium salts 3b and 4b led to heterolysis of C-N bond, directly producing benzyl cations. The electron-donating group in the aromatic ring greatly enhanced cross-linking efficiency.

Photodeamination Reaction Mechanism in Aminomethyl p-Cresol Derivatives: Different Reactivity of Amines and Ammonium Salts

Derivatives of p-cresol 1-4 were synthesized, and their photochemical reactivity, acid-base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9-11) or diamine is in the monocationic form (pH 7-9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.

Novel delivery approaches for cancer therapeutics

Currently, a majority of cancer treatment strategies are based on the removal of tumor mass mainly by surgery. Chemical and physical treatments such as chemo- and radiotherapies have also made a major contribution in inhibiting rapid growth of malignant cells. Furthermore, these approaches are often combined to enhance therapeutic indices. It is widely known that surgery, chemo- and radiotherapy also inhibit normal cells growth. In addition, these treatment modalities are associated with severe side effects and high toxicity which in turn lead to low quality of life. This review encompasses novel strategies for more effective chemotherapeutic delivery aiming to generate better prognosis. Currently, cancer treatment is a highly dynamic field and significant advances are being made in the development of novel cancer treatment strategies. In contrast to conventional cancer therapeutics, novel approaches such as ligand or receptor based targeting, triggered release, intracellular drug targeting, gene delivery, cancer stem cell therapy, magnetic drug targeting and ultrasound-mediated drug delivery, have added new modalities for cancer treatment. These approaches have led to selective detection of malignant cells leading to their eradication with minimal side effects. Lowering multi-drug resistance and involving influx transportation in targeted drug delivery to cancer cells can also contribute significantly in the therapeutic interventions in cancer.

Self-immolative spacers: kinetic aspects, structure-property relationships, and applications

Self-immolative spacers are covalent assemblies tailored to correlate the cleavage of two chemical bonds after activation of a protective part in a precursor: Upon stimulation, the protective moiety is removed, which generates a cascade of disassembling reactions leading to the temporally sequential release of smaller molecules. Originally introduced to overcome limitations for drug delivery, self-immolative spacers have gained wide interest in medicinal chemistry, analytical chemistry, and material science. For most applications, the kinetics of the disassembly of the activated self-immolative spacer governs functional properties. This Review addresses kinetic aspects of self-immolation. It provides information for selecting a particular self-immolative motif for a specific demand. Moreover, it should help researchers design kinetic experiments and fully exploit the rich perspectives of self-immolative spacers.

UV-Induced DNA Interstrand Cross-Linking and Direct Strand Breaks from a New Type of Binitroimidazole Analogue

Four novel photoactivated binitroimidazole prodrugs were synthesized. These agents produced DNA interstrand cross-links (ICLs) and direct strand breaks (DSB) upon UV irradiation, whereas no or very few DNA ICLs and DSBs were observed without UV treatment. Although these four molecules (1-4) contain the same binitroimidazole moiety, they bear four different leaving groups, which resulted in their producing different yields of DNA damage. Compound 4, with nitrogen mustard as a leaving group, showed the highest ICL yield. Surprisingly, compounds 1-3, without any alkylating functional group, also induced DNA ICL formation, although they did so with lower yields, which suggested that the binitroimidazole moiety released from UV irradiation of 1-3 is capable of cross-linking DNA. The DNA cross-linked products induced by these compounds were completely destroyed upon 1.0 M piperidine treatment at 90 °C (leading to cleavage at dG sites), which revealed that DNA cross-linking mainly occurred via alkylation of dGs. We proposed a possible mechanism by which alkylating agents were released from these compounds. HRMS and NMR analysis confirmed that free nitrogen mustards were generated by UV irradiation of 4. Suppression of DNA ICL and DSB formation by a radical trap, TEMPO, indicated the involvement of free radicals in the photo reactions of 3 and 4 with DNA. On the basis of these data, we propose that UV irradiation of compounds 1-4 generated a binitroimidazole intermediate that cross-links DNA. The higher ICL yield observed with 4 resulted from the amine effector nitrogen mustard released from UV irradiation.

The leaving group strongly affects H₂O₂-induced DNA cross-linking by arylboronates

We evaluated the effects of the benzylic leaving group and core structure of arylboronates on H2O2-induced formation of bisquinone methides for DNA interstrand cross-linking. The mechanism of DNA cross-linking induced by these arylboronates involves generation of phenol intermediates followed by departure of benzylic leaving groups leading to QMs which directly cross-link DNA via alkylation. The QM formation is the rate-determining step for DNA cross-linking. A better leaving group (Br) and stepwise bisquinone methide formation increased interstrand cross-linking efficiency. These findings provide essential guidelines for designing novel anticancer prodrugs.