Glutathione-responsive PROTAC for targeted degradation of ERα in breast cancer cells.

ERα (estrogen receptor-α)-targeting PROTACs (PROteolysis TArgeting Chimeras) have emerged as a novel and promising modality for breast cancer therapeutics. However, ERα PROTACs-induced degradation in normal tissues raises concerns about potential off-tissue toxicity. Tumor microenvironment-responsive strategy provides potential for specific control of the PROTAC's on-target degradation activity. The glutathione (GSH) level has been reported to be significantly increased in tumor cells. Here, we designed a GSH-responsive ERα PROTAC, which is generated by conjugating an o-nitrobenzenesulfonyl group to the hydroxyl group of VHL-based ERα PROTAC through a nucleophilic substitution reaction. The o-nitrobenzenesulfonyl group as a protecting group blocks the bioactivity of ERα PROTAC (ER-P1), and that can be specifically recognized and removed by highly abundant GSH in cancer cells. Consequently, the GSH-responsive ERα PROTAC (GSH-ER-P1) exhibits significantly enhanced degradation of ERα in cancer cells compared to that in normal cells, leading to a remarkable inhibition of breast cancer cell proliferation and less toxic effects on normal cells. This study provides a potentially valuable strategy for breast cancer treatment using tumor microenvironment-responsive PROTACs.

Dual-responsive probe and DNA interstrand crosslink precursor target the unique redox status of cancer cells.

Elevated GSH and H2O2 in cancer cells is sometimes doubted due to their contrary reactivities. Here, we construct a dual-responsive fluorescent probe to confirm the conclusion, and employ this to exploit a redox-inducible DNA interstrand crosslink (ICL) precursor. It crosslinks DNA upon activation by GSH and H2O2, affording an alternative dual-responsive strategy.

Selective degradation of BRD4 suppresses lung cancer cell proliferation using GSH-responsive PROTAC precursors.

BRD4,as a transcriptional and epigenetic regulator to mediate cellular functions, plays an important role in cancer development.Targeting BRD4 with conventional inhibitors in cancer therapy requires high doses, which often leads to off-target and adverse effects. BRD4-targeted proteolysis-targeting chimeras (PROTACs) can catalytically degrade BRD4 utilizing the endogenous proteasome system, and exhibit promising anti-tumor activity. However, most of the developed PROTACs are non-cancer specific and relatively toxic towards normal cells, limiting their practical applications in cancer treatment. By taking advantage of higher glutathione (GSH) levels in cancer cells than that in normal cells, we developed several GSH-responsive PROTAC precursors 1a-c via the attachment of a GSH-trigger unit on the hydroxyl group of the VHL (von Hippel-Lindau) ligand for the recruitment of E3 ligase. Among the precursors, 1a can be efficiently activated by the innately higher concentrations of GSH in lung cancer cells (A549 and H1299) to release active PROTAC 1, degrading intracellular BRD4 and resulting in cytotoxicity, which is confirmed by mechanistic investigation. On the other hand, 1a cannot be efficiently triggered in normal lung cells (WI38 and HULEC-5a) containing lower levels of GSH, therefore reducing the adverse effects on normal cells. This work provides an alternative proof of concept approach for developing stimuli-responsive PROTAC precursors, and affords a novel insight to improve the selectivity and minimize the adverse effects of current PROTACs, hence enhancing their clinical potential.

Hydrogen Peroxide-Inducible PROTACs for Targeted Protein Degradation in Cancer Cells.

Proteolysis-targeting chimeras (PROTACs) provide a powerful technique to degrade targeted proteins utilizing the cellular ubiquitin-proteasome system. The major concern is the host toxicity resulting from their poor selectivity. Inducible PROTACs responding to exogenous stimulus, such as light, improve their specificity, but it is difficult for photo-activation in deep tissues. Herein, we develop H2 O2 -inducible PROTAC precursors 2/5, which can be activated by endogenous H2 O2 in cancer cells to release the active PROTACs 1/4 to effectively degrade targeted proteins. This results in the intended cytotoxicity towards cancer cells while targeted protein in normal cells remains almost unaffected. The higher Bromodomain-containing protein 4 (BRD4) degradation activity and cytotoxicity of 2 towards cancer cells is mainly due to the higher endogenous concentration of H2 O2 in cancer cells (A549 and H1299), characterized by H2 O2 -responsive fluorescence probe 3. Western blot assays and cytotoxicity experiments demonstrate that 2 degrades BRD4 more effectively and is more cytotoxic in H2 O2 -rich cancer cells than in H2 O2 -deficient normal cells. This method is also extended to estrogen receptor (ER)-PROTAC precursor 5, showing H2 O2 -dependent ER degradation ability. Thus, we establish a novel strategy to induce targeted protein degradation in a H2 O2 -dependent way, which has the potential to improve the selectivity of PROTACs.

Light and hydrogen peroxide dual-responsive DNA interstrand crosslink precursors with potent cytotoxicity.

Arylboronic acid/esters and phenyl selenides-based quinone methide (QM) precursors were reported to induce DNA interstrand crosslink (ICL) formation upon reaction with the inherently high concentrations of H2O2 in cancer cells. However, some normal cells (such as macrophages) also contain high-levels of H2O2, which may interfere with precursors' selectivity. In order to enhance the spatiotemporal specificity by the photolysis, we developed photo- and H2O2- dual-responsive DNA ICL precursors 1-3, bearing a photo-responsive coumarin moiety and a H2O2 inducible phenyl selenide group. Precursors 1-3 are efficiently activated by photoirradiation and H2O2 to generate reactive QMs crosslinking DNA. Moreover, the reactivity of precursors can be modulated by the introduction of aromatic substituents (OMe, F), and the electron donating group (OMe) displays a more pronounced promoting effect on DNA ICL formation. A subsequent piperidine heat stability study confirmed that the formed QMs primarily alkylate dAs, dGs and dCs in DNA. Furthermore, 1-3 inhibit lung cancer cell (H1299) growth by inducing DNA damage and producing toxic reactive oxygen species (ROS) upon photolysis of released coumarin. This study illustrates the potent cytotoxicity achieved by novel photo/H2O2 dual-responsive QM precursors 1-3, affording a novel strategy for the development of inducible DNA interstrand cross-linkers.

Rapid GSH detection and versatile peptide/protein labelling to track cell penetration using coumarin-based probes.

Biothiols play essential roles in balancing the redox state and modulating cellular functions. Fluorescent probes for monitoring/labelling biothiols often suffer from slow reaction rates, strong background fluorescence and cytotoxic byproduct release. Thus, developing facile and versatile probes to overcome the challenges is still in high demand. Here, we report four coumarin-maleimides as fast responding and fluorogenic probes to detect GSH or label peptides/proteins. The probes quantitatively and selectively react with GSH via Michael addition within 1-2 min, achieving an 11-196-fold increase in fluorescence quantum yield via blockage of the photoinduced electron transfer (PET) process. Optimized probe 4 is applied for the detection of GSH in vitro (A549 cells) and in vivo (zebrafish embryos). Taking advantage of the fast Michael addition between the maleimide moiety and the sulfhydryl group, we expand the application of our method for fluorescent labelling of peptides/proteins and for tracking their cellular uptake process. The labelling strategy works for both Cys-bearing and Cys-free proteins after the introduction of a sulfhydryl group using Traut's reagent. Fluorescence assay reveals that the TAT-peptide can efficiently enter cells, but H3 protein, part of nucleosomes, prefers to bind on the cell membrane by electrostatic interactions, shedding light on the cellular uptake activity of nucleosomes and affording a potential membrane staining strategy. Overall, our study illustrates the broad potential of coumarin-maleimide based dual-functional probes for GSH detection and versatile protein labelling in biochemical research.

Selective Antitumor Activity and Photocytotoxicity of Glutathione-Activated Abasic Site Trapping Agents.

Abasic (AP) sites are one of the most common DNA lesions in cells. Aldehyde-reactive alkoxyamines capture AP sites and block the activity of APE1, the enzyme responsible for initiating their repair. Blocking the APE1 repair of AP sites leads to cell death, and it is an actively investigated approach for treating cancer. However, unselective AP site capture in different cells produces side effects and limits the application of alkoxyamines in chemotherapy. Herein we take advantage of the higher glutathione (GSH) concentration in cancer cells over normal cells to develop GSH-inducible agents that selectively kill cancer cells. 2,4-Dinitrobenzenesulfonamide caged coumarin-based alkoxyamines 1 and 2 are selectively revealed by GSH to release SO2 and fluorescent coumarin-based alkoxyamines 3 and 4 that trap AP sites in cells. GSH-directed AP site trapping and SO2 release result in selective cytotoxicity (defined as IC50WI38/IC50H1299) against H1299 lung cancer cells over normal WI38 lung cells, ranging from 1.8 to 2.8 for 1 and 2. The alkylating agent methylmethanesulfonate (MMS) promotes the formation of AP sites in cells and enhances the cytotoxicity of agent 1 in a dose-dependent way. Moreover, the comet assay and γH2AX assay suggest that AP adducts form a highly toxic DNA interstrand cross-link (ICL) upon photolysis, leading to further cell death. DNA flow cytometric analysis showed that 1 promoted cell apoptosis in the early stage and induced G2/M phase cell-cycle arrest. The 2,4-dinitrobenzenesulfonamide-caged alkoxyamines exhibited selective antitumor activity and photocytotoxicity in cancer cells, illuminating their potential as GSH-directed chemotherapeutic agents.

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 strand separation. Most ICL agents aiming to kill cancer cells, also generate adverse side effects to normal cells. H2 O2 -inducible DNA ICL agents are highly selective for targeting cancer cells, as the concentration of H2 O2 is higher in cancer cells than normal cells. Previous studies have focused on arylboronate-based precursors, reacting with H2 O2 to generate reactive quinone methides (QMs) crosslinking DNA. Here we explore phenyl selenide-based precursors 1-3 as H2 O2 -inducible DNA ICL agents. The precursors 1-3 can be activated by H2 O2 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 H2 O2 -inducible DNA interstrand cross-linkers.

Assessment of Phenylboronic Acid Nitrogen Mustards as Potent and Selective Drug Candidates for Triple-Negative Breast Cancer.

Triple-negative breast cancer (TNBC) has limited treatment options and the worst prognosis among all types of breast cancer. We describe two prodrugs, namely, CWB-20145 (1) and its methyl analogue FAN-NM-CH3 (2) that reduced the size of TNBC-derived tumors. The DNA cross-linking of nitrogen mustard prodrugs 1 and 2 was superior to that of chlorambucil and melphalan once activated in the presence of H2O2. The cellular toxicity of 1 and 2 was demonstrated in seven human cancer cell lines. The TNBC cell line MDA-MB-468 was particularly sensitive toward 1 and 2. Compound 2 was 10 times more cytotoxic than chlorambucil and 16 times more active than melphalan. An evaluation of the gene expression demonstrated an upregulation of the tumor suppressor genes p53 and p21 supporting a transcriptional mechanism of a reduced tumor growth. Pharmacokinetic studies with 1 showed a rapid conversion of the prodrug. The introduction of a methyl group generated 2 with an increased half-life. An in vivo toxicity study in mice demonstrated that both prodrugs were less toxic than chlorambucil. Compounds 1 and 2 reduced tumor growth with an inhibition rate of more than 90% in athymic nude mice xenografted with MDA-MB-468 cells. Together, the in vivo investigations demonstrated that treatment with 1 and 2 suppressed tumor growth without affecting normal tissues in mice. These phenylboronic acid nitrogen mustard prodrugs represent promising drug candidates for the treatment of TNBC. However, the mechanisms underlying their superior in vivo activity and selectivity as well as the correlation between H2O2 level and in vivo efficacy are not yet fully understood.

Photoinduced DNA Interstrand Cross-Linking by 1,1'-Biphenyl Analogues: Substituents and Leaving Groups Combine to Determine the Efficiency of Cross-Linker.

Two series of 1,1'-biphenyl analogues with various leaving groups (L=OAc, OCH3 , OCHCH=CH2 , OCH2 Ph, SPh, SePh, and Ph3 P+ ) were synthesized. Their reactivity towards DNA and the reaction mechanism were investigated by determining DNA interstrand cross-link (ICL) efficiency, radical and carbocation formation, and the cross-linking reaction sites. All compounds induced DNA ICL formation upon 350 nm irradiation via a carbocation that was generated from oxidation of the corresponding free radicals. The ICL efficiency and the reaction rate strongly depended on the combined effect of the leaving group and the substituent. Among all compounds tested, the high ICL efficiency (30-43 %) and fast reaction rate were observed with compounds carrying a nitrophenyl group and acetate (2 a), ether (2 b and 2 c), or triphenylphosphonium salt (2 g) as leaving groups. Most compounds with a 4-methoxybenzene group showed similar DNA ICL efficiency (≈30 %) with a slow DNA cross-linking reaction rate. Both cation trapping and free radical trapping adducts were detected in the photo activation process of these compounds, which provided direct evidence for the proposed mechanism. Heat stability study in combination with sequence study suggested that these photo-generated benzyl cations alkylate DNA at dG, dA, and dC sites.

Photoinduced DNA Interstrand Cross-Linking by Benzene Derivatives: Leaving Groups Determine the Efficiency of the Cross-Linker.

We have synthesized and characterized two small libraries of 2-OMe or 2-NO2-benzene analogues 2a-i and 3a-i containing a wide variety of leaving groups. Irradiation of these compounds at 350 nm generated benzyl radicals that were spontaneously oxidized to benzyl cations directly producing DNA interstrand cross-links (ICLs). Compounds with a 2-methoxy substituent showed a faster cross-linking reaction rate and higher ICL efficiency than the corresponding 2-nitro analogues. Apart from the aromatic substituent, the benzylic leaving groups greatly affected DNA cross-linking efficiency. Higher ICL yields were observed for compounds with OCH3 (3b), OCH2Ph (3d), or Ph3P+ (3i) as leaving groups than those containing OAc (3a), NMe2 (3e), morpholine (3f), OCH2CH═CH2 (3c), SPh (3g), or SePh (3h). The heat stability study of the isolated ICL products indicated that dGs were the preferred alkylation sites in DNA for the benzyl cations produced from 2a-i, 3c, and 3e-i while 3a (L = OAc), 3b (L = OMe), and 3d (L = OCH2Ph) showed a similar photoreactivity toward dGs and dAs. Although the photogenerated benzyl cations alkylated dG, dC, and dA, ICL assay with variation of DNA sequences showed that the ICL reaction occurred with opposing dG/dC but not with staggered dA/dA.

Discovery and Optimization of Novel Hydrogen Peroxide Activated Aromatic Nitrogen Mustard Derivatives as Highly Potent Anticancer Agents.

We describe several new aromatic nitrogen mustards with various aromatic substituents and boronic esters that can be activated with H2O2 to efficiently cross-link DNA. In vitro studies demonstrated the anticancer potential of these compounds at lower concentrations than those of other clinically used chemotherapeutics, such as melphalan and chlorambucil. In particular, compound 10, bearing an amino acid ester chain, is selectively cytotoxic toward breast cancer and leukemia cells that have inherently high levels of reactive oxygen species. Importantly, 10 was 10-14-fold more efficacious than melphalan and chlorambucil for triple-negative breast-cancer (TNBC) cells. Similarly, 10 is more toxic toward primary chronic-lymphocytic-leukemia cells than either chlorambucil or the lead compound, 9. The introduction of an amino acid side chain improved the solubility and permeability of 10. Furthermore, 10 inhibited the growth of TNBC tumors in xenografted mice without obvious signs of general toxicity, making this compound an ideal drug candidate for clinical development.

Design, Synthesis, and Characterization of Binaphthalene Precursors as Photoactivated DNA Interstrand Cross-Linkers.

Most recently, alkylation via photogenerated carbocations has been identified as a novel mechanism for photoinduced DNA interstrand cross-link (ICL) formation by bifunctional aryl compounds. However, most compounds showed a low efficiency for DNA cross-linking. Here, we have developed a series of new 1,1'-binaphthalene analogues that efficiently form DNA ICLs upon 350 nm irradiation via generated 2-naphthalenylmethyl cations. The DNA cross-linking efficiency depends on the substituents at position 4 of the naphthalene moiety as well as the leaving groups. Compounds with NO2, Ph, H, Br, or OMe substituents led to 2-4 times higher DNA ICL yields than those with a boronate ester group. Compounds with trimethylammonium salt as a leaving group showed slightly better cross-linking efficiency than those with bromo as a leaving group. Some of these compounds showed a better cross-linking efficiency than that of traditional alkylating agents, such as nitrogen mustard analogues or quinone methide precursors. These highly efficient photoactivated carbocation precursors allow determination and characterization of the adducts formed between the photogenerated naphthalenyl cations and four natural nucleosides, indicating that the alkylation sites for these naphthalene analogues are dG, dA, and dC.

Substituents Have a Large Effect on Photochemical Generation of Benzyl Cations and DNA Cross-Linking.

Photoactivated DNA interstrand cross-linking agents have a wide range of biological applications. Recently, several aryl boronates have been reported to induce DNA interstrand cross-link (ICL) formation via carbocations upon photoirradiation. Herein, we synthesized a series of new bifunctional phenyl compounds to test the generality of such a mechanism, and to understand how the chemical structure influences carbocation formation and the DNA cross-linking process. These compounds efficiently form DNA ICLs via generated benzyl cations upon 350 nm irradiation. The DNA cross-linking efficiency and the pathway for carbocation generation depend on both the aromatic substituents and the leaving groups. Bromine as a leaving group facilitates the DNA cross-linking process in comparison with trimethyl ammonium salt. Both electron-donating and -withdrawing substituents induce bathochromic shifts, which favor photoinduced DNA ICL formation. For the bromides, the benzyl cation intermediates were generated through oxidation of the corresponding benzyl radicals. However, for the ammonia salts, the benzyl cations were formed through two pathways: either through oxidation of the benzyl radicals or by direct heterolysis of the C-N bond. Photoinduced C-N homolysis to form benzyl radicals occurred with compounds having donating substituents, whereas direct heterolysis of the C-N bond occurred with those bearing withdrawing substituents. The adducts formed between 1 a and four natural nucleosides were characterized, indicating that the alkylation sites for the photogenerated benzyl cations are dG, dA, and dC.

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.

Coumarin-Induced DNA Ligation, Rearrangement to DNA Interstrand Crosslinks, and Photorelease of Coumarin Moiety.

Coumarin moieties react with thymine and cytosine in DNA by photoinduced [2+2] cycloaddition, which allows quantitative DNA interstrand crosslink (ICL) formation. Here, we report the application of coumarin analogues for DNA photoligation and the rearrangement of coumarin-induced ligation to ICL products. Both DNA sequences and the linker units at position 4 of the coumarin moieties affected coumarin-induced DNA photoligation. A flexible linker unit favored DNA ICL formation but led to inefficient photoligation, whereas coumarins without linker units greatly increased DNA photoligation efficiency. DNA photoligation induced by the coumarin moiety was photoswitchable. Ligation products were formed between coumarin and dT or dC upon 350 nm irradiation but reverted to the original single-stranded oligodeoxyribonucleotides (ODNs) upon 254 nm irradiation. Rearrangement of ligated ODNs into ICL products occurred during the switchable (350 nm/254 nm) processes. Additionally, photoinduced cleavage of coumarin 3 occurred with dC-3 cycloadducts upon 254 nm irradiation, which was confirmed by mass spectrometry analysis.

Photoinduced DNA Interstrand Cross-Link Formation by Naphthalene Boronates via a Carbocation.

Most photoinduced DNA cross-link formation by a bifunctional aryl derivative is through a bisquinone methide. DNA cross-linking via a bisarylcarbocation remains a less explored area. We designed and synthesized a series of naphthalene boronates that produce DNA interstrand cross-links via a carbocation upon UV irradiation. A free radical was generated from the naphthalene boronates with 350 nm irradiation and further converted to a carbocation by electron transfer. The activation mechanism was determined using the orthogonal traps, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and methoxyamine that react with either the free radical or the carbocation but not both. This represents a novel example of photoinduced DNA cross-link formation via carbocations generated from a bisaryl derivative. This work provides information useful for the design of novel photoactivated DNA cross-linking agents.

Quantitative DNA interstrand cross-link formation by coumarin and thymine: structure determination, sequence effect, and fluorescence detection.

The coumarin analogues have been widely utilized in medicine, biology, biochemistry, and material sciences. Here, we report a detailed study on the reactivity of coumarins toward DNA. A series of coumarin analogues were synthesized and incorporated into oligodeoxynucleotides. A photoinduced [2 + 2] cycloaddition occurs between the coumarin moiety and the thymidine upon 350 nm irradiation forming both syn- and anti-cyclobutane adducts (17 and 18), which are photoreversible by 254/350 nm irradiation in DNA. Quantitative DNA interstrand cross-link (ICL) formation was observed with the coumarin moieties containing a flexible two-carbon or longer chain. DNA cross-linking by coumarins shows a kinetic preference when flanked by an A:T base pair as opposed to a G:C pair. An efficient photoinduced electron transfer between coumarin and dG slows down ICL formation. ICL formation quenches the fluorescence of coumarin, which, for the first time, enables fast, easy, and real-time monitoring of DNA cross-linking and photoreversibility via fluorescence spectroscopy. It can be used to detect the transversion mutation between pyrimidines and purines. Overall, this work provides new insights into the biochemical properties and possible toxicity of coumarins. A quantitative, fluorescence-detectable, and photoswitchable DNA cross-linking reaction of the coumarin moieties can potentially serve as mechanistic probes and tools for bioresearch without disrupting native biological environment.