Photochemical Activation of Enediyne Warheads: A Potential Tool for Targeted Antitumor Therapy

Tunable and Photoactivatable Mimics of Calicheamicin γ1 for DNA Cleavage

Calicheamicin γ1 and related natural products are renowned for their potency in DNA cleavage, serving as the warheads in commercial ADCs used for treating leukemia. Their mechanism of action involves the formation of aryl radicals, which abstract hydrogen atoms from nucleic acids. However, the complex strained enediyne structure of calicheamicin γ1 presents significant challenges in synthesis, resulting in high production costs and limited structural and activity modularity for tuning the therapeutic window. This report describes the development of simple molecular mimics based on diazonium salts, synthesized in fewer than 3 steps, capable of generating aryl radicals upon green or red light irradiation. SAR studies conducted on over 30 analogues reveal a wide range of potencies in DNA cleavage, with EC50 values ranging from low nanomolar to micromolar. Forming benzenoid diradicals does not appear to be necessary for potent DNA cleavage; instead, DNA cleavage can be achieved with radicals distributed among different arenes when connected with proper linkages. The potency is influenced by electronic effects, stereochemistry, orbital orientations, the distance between multiradicals, and the number of diazonium motifs within the molecule. In addition to providing a more cost-effective, efficient, and modular alternative to calicheamicin γ1, this technology offers the potential for enhanced specificity through spatiotemporal control.

Triggered Crosslinking of Main-Chain Enediyne Polyurethanes via Bergman Cyclization

Crosslinking chemistries occupy an important position in polymer modification with a particular importance when triggered in response to external stimuli. Enediyne (EDY) moieties are used as functional entities in this work, known to undergo a pericyclic Bergman cyclization (BC) to induce a triggered crosslinking of polyurethanes (PU) via the intermediately formed diradicals. Diamino-EDYs, where the distance between the enyne-moieties is known to be critical to induce a BC, are placed repetitively as main-chain structural elements in isophorone-based PUs to induce reinforcement upon heating, compression, or stretching. A 7-day compression under room temperature results in a ≈69% activation of the BC, together with the observation of an increase in tensile strength by 62% after 25 stretching cycles. The occurrence of BC is further proven by the decreased exothermic values in differential scanning calorimetry, together with characteristic peaks of the formed benzene moieties via IR spectroscopy. Purely heat-induced crosslinking contributes to 191% of the maximum tensile strength in comparison to the virgin PU. The BC herein forms an excellent crosslinking strategy, triggered by heat or force in PU materials.

Functionalized 10-Membered Aza- and Oxaenediynes through the Nicholas Reaction

The scope and limitations of the Nicholas-type cyclization for the synthesis of 10-membered benzothiophene-fused heterocyclic enediynes with different functionalities were investigated. Although the Nicholas cyclization through oxygen could be carried out in the presence of an ester group, the final oxaenediyne was unstable under storage. Among the N-type Nicholas reactions, cyclization via an arenesulfonamide functional group followed by mild Co-deprotection was found to be the most promising, yielding 10-membered azaendiynes in high overall yields. By contrast, the Nicholas cyclization through the acylated nitrogen atom did not give the desired 10-membered cycle. It resulted in the formation of a pyrroline ring, whereas cyclization via an alkylated amino group resulted in a poor yield of the target 10-membered enediyne. The acylated 4-aminobenzenesulfonamide nucleophilic group was found to be the most convenient for the synthesis of functionalized 10-membered enediynes bearing a clickable function, such as a terminal triple bond. All the synthesized cyclic enediynes exhibited moderate activity against lung carcinoma NCI-H460 cells and had a minimal effect on lung epithelial-like WI-26 VA4 cells and are therefore promising compounds in the search for novel antitumor agents that can be converted into conjugates with tumor-targeting ligands.

Synthesis and biological properties of maleimide-based macrocyclic lactone enediynes

Natural enediyne antibiotics are powerful DNA-cleavage agents due to the presence of the highly reactive hex-3-ene-1,5-diyne units. However, the complicated chemical structure and thermal instability make their synthesis, derivatization, and storage challenging. Heterocycle-fused enediynes, which exhibit strong antineoplastic activity, are promising analogues of natural enediynes for medicinal applications. To this end, a series of maleimide-based enediynes with macrocyclic lactone moieties were synthesized through the Sonagashira coupling reaction. Differential scanning calorimetry and electron paramagnetic resonance results showed that these macrocyclic enediynes exhibited a rather low onset temperature and the ability to generate radicals at physiological temperature. In addition, the structure-activity relationship of enediynes was analyzed by changing the ring size and the substituents on the propargyl group. Cellular experiments indicated that the diradicals produced by these enediynes efficiently cleaved DNA and disrupted the cell cycle distribution, and consequently induced tumor cell death via an apoptosis pathway at low half inhibitory concentrations. Computational studies suggested that the maleimide moiety promoted the propargyl-allenyl rearrangement of the cyclic enediyne, enabling the generation of diradical species through the Myers-Saito cyclization, and then abstracted hydrogen atoms from the H-donors.

Synthesis of 14-membered enediyne-embedded macrocycles

A concise and practical strategy towards a novel class of 14-membered macrocycles containing an enediyne (Z-3-ene-1,5-diyne) structural unit is described. A highly modular assembly of various precursors via sequential Ugi/Sonogashira reactions allowed the preparation of hybrid enediyne-peptide macrocycles in most cases as single diastereoisomers. Selected macrocyclic compounds showed moderate antiproliferative activity, and can be considered as templates suitable for further diversification in terms of ring size, shape, and stereochemistry.

Synthesis and Biological Evaluation of Taurine-Derived Diarylmethane and Dibenzoxanthene Derivatives as Possible Cytotoxic and Antimicrobial Agents

The series of novel taurine-derived diarylmethanes and dibenzoxanthenes was synthesized starting from simple commercially available precursors via modular three-stage approach. All the newly synthesized compounds were screened for in vitro antibacterial and antifungal activity, as well as cytotoxicity towards normal and cancer cell lines. Some of the synthesized compounds exhibited 2-4-fold higher activity against S. aureus, E. faecalis and B. cereus compared with Chloramphenicol. In contrast to Chloramphenicol, the tested compounds also showed bactericidal, rather than bacteriostatic effect, which makes them promising candidates for further studies.

Nuclear Ceramide Is Associated with Ataxia Telangiectasia Mutated Activation in the Neocarzinostatin-Induced Apoptosis of Lymphoblastoid Cells

Ceramide is a bioactive sphingolipid that mediates ionizing radiation- and chemotherapy-induced apoptosis. Neocarzinostatin (NCS) is a genotoxic anti-cancer drug that induces apoptosis in response to DNA double-strand breaks (DSBs) through ataxia telangiectasia mutated (ATM) activation. However, the involvement of ceramide in NCS-evoked nuclear events such as DSB-activated ATM has not been clarified. Here, we found that nuclear ceramide increased by NCS-mediated apoptosis through the enhanced assembly of ATM and the meiotic recombination 11/double-strand break repair/Nijmengen breakage syndrome 1 (MRN) complex proteins in human lymphoblastoid L-39 cells. NCS induced an increase of ceramide production through activation of neutral sphingomyelinase (nSMase) and suppression of sphingomyelin synthase (SMS) upstream of DSB-mediated ATM activation. In ATM-deficient lymphoblastoid AT-59 cells compared with L-39 cells, NCS treatment showed a decrease of apoptosis even though ceramide increase and DSBs were observed. Expression of wild-type ATM, but not the kinase-dead mutant ATM, in AT-59 cells increased NCS-induced apoptosis despite similar ceramide accumulation. Interestingly, NCS increased ceramide content in the nucleus through nSMase activation and SMS suppression and promoted colocalization of ceramide with phosphorylated ATM and foci of MRN complex. Inhibition of ceramide generation by the overexpression of SMS suppressed NCS-induced apoptosis through the inhibition of ATM activation and assembly of the MRN complex. In addition, inhibition of ceramide increased by the nSMase inhibitor GW4869 prevented NCS-mediated activation of the ATM. Therefore, our findings suggest the involvement of the nuclear ceramide with ATM activation in NCS-mediated apoptosis. SIGNIFICANCE STATEMENT: This study demonstrates that regulation of ceramide with neutral sphingomyelinase and sphingomyelin synthase in the nucleus in double-strand break-mimetic agent neocarzinostatin (NCS)-induced apoptosis. This study also showed that ceramide increase in the nucleus plays a role in NCS-induced apoptosis through activation of the ataxia telangiectasia mutated/meiotic recombination 11/double-strand break repair/Nijmengen breakage syndrome 1 complex in human lymphoblastoid cells.

Bergman cyclization of main-chain enediyne polymers for enhanced DNA cleavage

The design of novel polymers bearing multiple ene-diynes in the main chain are reported, allowing to tune activity of DNA cleavage via the Bergman cyclization.

An Insight into FDA Approved Antibody-Drug Conjugates for Cancer Therapy

The large number of emerging antibody-drug conjugates (ADCs) for cancer therapy has resulted in a significant market 'boom', garnering worldwide attention. Despite ADCs presenting huge challenges to researchers, particularly regarding the identification of a suitable combination of antibody, linker, and payload, as of September 2021, 11 ADCs have been granted FDA approval, with eight of these approved since 2017 alone. Optimism for this therapeutic approach is clear, despite the COVID-19 pandemic, 2020 was a landmark year for deals and partnerships in the ADC arena, suggesting that there remains significant interest from Big Pharma. Herein we review the enthusiasm for ADCs by focusing on the features of those approved by the FDA, and offer some thoughts as to where the field is headed.

Cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene: A Convenient Enediyne

Enediynes are widely studied to understand their cycloaromatization and the trapping of the resulting p-dehydrobenzene diradical. However, few model substrates are known, and they are hard to synthesize and difficult to handle. Herein we report cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene as a convenient model for studying the reactivity of enediynes. It can be easily synthesized from 1,2-diethynylcyclohexene and 1,4-diiodobutane. It is a solid that is stable at room temperature. In solution the p-dehydrobenzene diradical derived from its cycloaromatization can be trapped by nucleophiles. The rate-limiting step is the cyclization, which is slightly slower than that of the parent cyclodec-1,5-diyne-3-ene but faster than that of its benzo analogue, consistent with the distances between the reacting carbon atoms.

Intramolecular Nicholas Reactions in the Synthesis of Heteroenediynes Fused to Indole, Triazole, and Isocoumarin

The applicability of an intramolecular Nicholas reaction for the preparation of 10-membered O- and N-enediynes fused to indole, 1,2,3-triazole, and isocoumarin was investigated. The general approach to acyclic enediyne precursors fused to heterocycles includes inter- and intramolecular buta-1,3-diyne cyclizations with the formation of iodoethynylheterocycles, followed by Sonogashira coupling. The nature of both a heterocycle and a nucleophilic group affects the possibility of a 10-membered ring closure by the Nicholas reaction. Among oxacycles, an isocoumarin-fused enediyne was obtained. In the case of O-enediyne annulated with indole, instead of the formation of a 10-membered cycle, BF₃-promoted addition of an OH-group to the proximal triple bond at the C3 position afforded dihydrofuryl-substituted indole. For 1,2,3-triazole-fused analogues, using NH-Ts as a nucleophilic functional group allowed obtaining 10-membered azaenediyne, while the substrate with a hydroxyl group gave only traces of the desired 10-membered oxacycle. An improved method for the deprotection of Co-complexes of cyclic enediynes using tetrabutylammonium fluoride in an acetone/water mixture and the investigation of the 10-membered enediynes' reactivity in the Bergman cyclization are also reported. In the solid state, all synthesized iodoethynylheterocycles were found to be involved in halogen bond (XB) formation with either O or N atoms as XB acceptors.

Regioselective Synthesis of Benzo-Fused Tetrahydroisoquinoline-Based Biaryls through a Tandem One-Pot Halogenation of p-Benzynes from Enediynes and Suzuki-Miyaura Coupling

A regioselective halogenation of p-benzyne derived from a nonaromatic enediyne core via Bergman cyclization and Suzuki-Miyaura coupling of the resulting haloarene in one-pot is disclosed. For the one-pot protocol to work, the reaction conditions were modified compared to an earlier reported procedure ( J. Org. Chem. 2019 , 84 , 2911 - 2921 ) by reducing the amount of lithium iodide and exclusion of pivalic acid. Under these modified conditions, the products, benzo-fused tetrahydroisoquinoline-based biaryl derivatives were obtained in overall high to excellent yields (71-90%).

The Outliers: Metal-Mediated Radical Reagents for Biological Substrate Degradation

The predictable and controllable interaction of small organic or peptidic molecules with biological substrates is the primary reason most pharmaceuticals are narrowly decorated carbon frameworks. The inhibition or activation binding models are measurable and without side reactions that can cause pathological angst. Yet many diseases, especially those involving rapid proliferation of cells (i.e., cancer) or aggregation of peptides (e.g., heart disease, Alzheimer's disease) have not yet been cured by inhibition therapeutics. Additionally, interventional medicine is often required to alleviate such maladies by physical removal first, followed by molecular-level therapy as a second stage. Thus, there appears to be a niche for more aggressive therapeutics that may employ harsher chemical processes to realize clinical efficacy, albeit without causing catastrophic side effects. Molecules that may be considered for this challenge are not typically biomimetic, nor do they fit the traditional pharmaceutical paradigm. They may have unusual modes of action or undesired reactivity that can be lethal if not controlled. These are the outliers; potential pharmacophores that biology does not know how to manage or adapt to. This is why they may be an intriguing class of agents that needs continuous development. In this Account, we connect the under-developed enediyne family of compounds and our metalloenediyne derivatives to existing radical-based therapeutics such as bleomycin and doxorubicin to illustrate that controlled diradical reactivity, although an outlier mechanism, has a place in the therapeutic portfolio. This is self-evident in that of the 11 natural product enediynes known, 2 have clinical impact, a strong ratio. We expand on the chemical diversity of potential enediyne constructs and focus on the accessible trigger mechanisms to activate diradical formation as a method to control toxicity. Moreover, we further illustrate how electromagnetic fields can be employed to activate both molecular and larger nanomaterial constructs that carry highly concentrated payloads of reactive reagent. Finally, we describe how controlled diradical reactivity can reach beyond traditional therapeutic targets such as DNA, to peptide aggregates found in blood clots, neural fibrils, and membrane scaffolds. It is our belief that cleverly constructed frameworks with well-designed and controlled activation/reaction schemes can lead to novel therapeutics that can challenge evolving viral and bacterial invaders. From this evangelical perspective, our hope is that the conceptual framework, if not the specific designs in this Account, stimulate the readership to develop out-of-the-box therapeutic designs that may combat resistant disease targets.

Photochemical properties of enediyne-cored dendrimers bearing naphthalenes at the periphery

A novel series of trans and cis enediyne-cored dendrimers bearing naphthalenes at the periphery were synthesized and their photochemical properties were examined. The trans/cis isomer ratio in the photostationary state was dependent on the excitation site in the dendrimers. When the enediyne core was selectively excited, the trans/cis isomer ratio in the photostationary state was either around 50/50 or a cis-rich mixture in all dendrimers due to the larger molar extinction coefficient of the trans-enediynes. On the other hand, when naphthalene was excited, a trans-rich mixture was unexpectedly obtained in higher generation dendrimers even though the energy transfer efficiency was almost quantitative in the trans dendrimers. These results could be explained by the energy transfer process, which was different depending on the geometric isomerism of the enediyne core.

Alkyne Origami: Folding Oligoalkynes into Polyaromatics

Do not bend the triple bonds! This familiar undergraduate mantra must be disobeyed if the alkyne group is used as a building block in molecular construction. This Account will describe our exploits in "alkyne origami", that is, folding oligoalkynes into new shapes via cyclization cascades. This research stems from a set of guidelines for the cyclizations of alkynes that we suggested in 2011 ( Gilmore Chem. Rev. 2011 , 111 , 6513 ; Alabugin J. Am. Chem. Soc. 2011 , 133 , 12608 ). The guidelines blended critical analysis of ∼40 years of experimental research with computations into the comprehensive predictions of the relative favorability of dig-cyclizations of anions and radicals. In this Account, we will show how this new understanding has been instrumental in building polyaromatics. In particular, we illustrate the utility of these stereoelectronic models by developing a toolbox of practical, selective, and efficient synthetic transformations. The high energy and high carbon content render alkynes the perfect precursors for the preparation of polyaromatic ribbons and other carbon-rich materials with precisely controlled structure and reactivity. Still, the paradox of alkyne reactivity (alkynes store a lot of energy but are protected kinetically by their relatively strong π-bonds) requires precise use of stereoelectronic factors for lowering the activation barriers for alkyne cyclizations. These factors are drastically different in the "all-exo" and the "all-endo" cyclization cascades of oligoynes. This Account will highlight the interplay between the stereoelectronics of bond formation and topology of acyclic precursor "folding" into a polycyclic ribbon. The topology of folding is simpler for the endo cascades, which are compatible with initiation either at the edge or at the center. In contrast, the exo cascades require precise folding of an oligoalkyne ribbon by starting the cascade exactly at the center of the chain. These differences define the key challenges in the design of these two types of alkyne cyclization cascades. For the endo processes, the folding is simple, but these processes require a strategy ("LUMO Umpolung") for inverting the usual stereoelectronic requirements of alkyne cyclizations. We also show how alkenes can be used as alkyne equivalents in cyclizations coupled with fragmentations and how one can make endo cyclization products without ever going through an endo cyclization. In contrast, each elementary step of the exo cascades benefits from the inherent exo preference for the radical attack, but these cascades require precise initiation by starting exactly at the central alkyne unit of the oligoyne. This strict selectivity requirement led to the development of traceless directing groups capable of supramolecular assistance to the initiation step and self-terminating departure at the end of the cascade. With attention to electronic effects that can stop radical cascades, oligoalkynes can be selectively converted into precisely shaped and functionalized polyaromatic products. The generality of these concepts is further illustrated by the development of radical "peri annulations" at the zigzag edge of acenes.

An aqueous medium-controlled stereospecific oxidative iodination of alkynes: efficient access to (E)-diiodoalkene derivatives

A new and versatile approach for the stereospecific iodination of alkynes has been developed in aqueous media. Scale-up reactions (up to 5 g) established the proficiency of this protocol and highlight the feasibility of large scale reactions.

Porphyrin–ferrocene conjugates for photodynamic and chemodynamic therapy

Porphyrin–ferrocene conjugates were designed and synthesized for photodynamic and chemodynamic therapy.