Neue Ziele für die Photopharmakologie

Increased Dentate Gyrus Excitability in the Intrahippocampal Kainic Acid Mouse Model for Temporal Lobe Epilepsy

The intrahippocampal kainic acid (IHKA) mouse model is an extensively used in vivo model to investigate the pathophysiology of mesial temporal lobe epilepsy (mTLE) and to develop novel therapies for drug-resistant epilepsy. It is characterized by profound hippocampal sclerosis and spontaneously occurring seizures with a major role for the injected damaged hippocampus, but little is known about the excitability of specific subregions. The purpose of this study was to electrophysiologically characterize the excitability of hippocampal subregions in the chronic phase of the induced epilepsy in the IHKA mouse model. We recorded field postsynaptic potentials (fPSPs) after electrical stimulation in the CA1 region and in the dentate gyrus (DG) of hippocampal slices of IHKA and healthy mice using a multielectrode array (MEA). In the DG, a significantly steeper fPSP slope was found, reflecting higher synaptic strength. Population spikes were more prevalent with a larger spatial distribution in the IHKA group, reflecting a higher degree of granule cell output. Only minor differences were found in the CA1 region. These results point to increased neuronal excitability in the DG but not in the CA1 region of the hippocampus of IHKA mice. This method, in which the excitability of hippocampal slices from IHKA mice is investigated using a MEA, can now be further explored as a potential new model to screen for new interventions that can restore DG function and potentially lead to novel therapies for mTLE.

A Photoisomerizable Zinc (II) Complex Inhibits Microtubule Polymerization for Photoactive Therapy

The photoisomerization-induced cytotoxicity in photopharmacology provides a unique pathway for phototherapy because it is independent of endogenous oxygen. In this study, we developed a biosafe photoisomerizable zinc(II) complex (Zn1), which releases its trans ligand (trans-L1) after being irradiated with blue light. This causes the complex to undergo photoisomerization and produce the toxic cis product (cis-L1) and generate singlet oxygen (¹ O₂ ). The resulting series of events caused impressive phototoxicity in hypoxic A431 skin cancer cells, as well as in a tumor model in vivo. Interestingly, Zn1 was able to inhibit tumor microtubule polymerization, while still showing good biocompatibility and biosafety in vivo. This photoisomerizable zinc(II) complex provides a novel strategy for addressing the oxygen-dependent limitation of traditional photodynamic therapy.

Next Generation Opto-Jasplakinolides Enable Local Remodeling of Actin Networks

The natural product jasplakinolide is widely used to stabilize F-actin. Based on extensive structure-activity relationship studies, we have developed a new generation of photoswitchable jasplakinolides that feature rationally designed red-shifted azobenzene photoswitches. Our lead compound, nOJ, can be activated with longer wavelengths in the visible range (e.g. 440-475 nm) and rapidly returns to its inactive state through thermal relaxation. nOJ enables the reversible control of F-actin dynamics, as shown through live-cell imaging, cell migration, and cell proliferation assays. Short, local irradiation with blue light resulted in highly localized and reversible actin aggregation with subcellular precision. Our optical tool can be useful in diverse fields to study actin dynamics with excellent spatiotemporal resolution.

Photochromic Fentanyl Derivatives for Controlled μ-Opioid Receptor Activation

Photoswitchable ligands as biological tools provide an opportunity to explore the kinetics and dynamics of the clinically relevant μ-opioid receptor. These ligands can potentially activate or deactivate the receptor when desired by using light. Spatial and temporal control of biological activity allows for application in a diverse range of biological investigations. Photoswitchable ligands have been developed in this work, modelled on the known agonist fentanyl, with the aim of expanding the current "toolbox" of fentanyl photoswitchable ligands. In doing so, ligands have been developed that change geometry (isomerize) upon exposure to light, with varying photophysical and biochemical properties. This variation in properties could be valuable in further studying the functional significance of the μ-opioid receptor.

Development of an Indole-Amide-Based Photoswitchable Cannabinoid Receptor Subtype 1 (CB1R) "Cis-On" Agonist

Activation of the human cannabinoid receptor type 1 (hCB₁R) with high spatiotemporal control is useful to study processes involved in different pathologies related to nociception, metabolic alterations, and neurological disorders. To synthesize new agonist ligands for hCB₁R, we have designed different classes of photoswitchable molecules based on an indole core. The modifications made to the central core have allowed us to understand the molecular characteristics necessary to design an agonist with optimal pharmacological properties. Compound 27a shows high affinity for CB₁R (K_i (cis-form) = 0.18 μM), with a marked difference in affinity with respect to its inactive "trans-off" form (CB₁R K_i trans/cis ratio = 5.4). The novel compounds were evaluated by radioligand binding studies, receptor internalization, sensor receptor activation (GRABeCB2.0), Western blots for analysis of ERK1/2 activation, NanoBiT βarr2 recruitment, and calcium mobilization assays, respectively. The data show that the novel agonist 27a is a candidate for studying the optical modulation of cannabinoid receptors (CBRs), serving as a new molecular tool for investigating the involvement of hCB₁R in disorders associated with the endocannabinoid system.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Advanced Strategies in Enzyme Activity Regulation for Biomedical Applications

Enzymes are important macromolecular biocatalysts that accelerate chemical and biochemical reactions in living organisms. Most human diseases are related to alterations in enzyme activity. Moreover, enzymes are potential therapeutic tools for treating different diseases, such as cancer, infections, and cardiovascular and cerebrovascular diseases. Precise remote enzyme activity regulation provides new opportunities to combat diseases. This review summarizes recent advances in the field of enzyme activity regulation, including reversible and irreversible regulation. It also discusses the mechanisms and approaches for on-demand control of these activities. Furthermore, a range of stimulus-responsive inhibitors, polymers, and nanoparticles for regulating enzyme activity and their prospective biomedical applications are summarized. Finally, the current challenges and future perspectives on enzyme activity regulation are discussed.

A Photoswitchable Ligand Targeting the β 1 ‐Adrenoceptor Enables Light‐Control of the Cardiac Rhythm**

Catecholamine‐triggered β‐adrenoceptor (β‐AR) signaling is essential for the correct functioning of the heart. Although both β₁‐ and β₂‐AR subtypes are expressed in cardiomyocytes, drugs selectively targeting β₁‐AR have proven this receptor as the main target for the therapeutic effects of beta blockers in the heart. Here, we report a new strategy for the light‐control of β₁‐AR activation by means of photoswitchable drugs with a high level of β₁‐/β₂‐AR selectivity. All reported molecules allow for an efficient real‐time optical control of receptor function in vitro. Moreover, using confocal microscopy we demonstrate that the binding of our best hit, pAzo‐2, can be reversibly photocontrolled. Strikingly, pAzo‐2 also enables a dynamic cardiac rhythm management on living zebrafish larvae using light, thus highlighting the therapeutic and research potential of the developed photoswitches. Overall, this work provides the first proof of precise control of the therapeutic target β₁‐AR in native environments using light.

Computational Design, Synthesis, and Photochemistry of Cy7-PPG, an Efficient NIR-Activated Photolabile Protecting Group for Therapeutic Applications

Photolabile Protecting Groups (PPGs) are molecular tools used, for example, in photopharmacology for the activation of drugs with light, enabling spatiotemporal control over their potency. Yet, red-shifting of PPG activation wavelengths into the NIR range, which penetrates the deepest in tissue, has often yielded inefficient or insoluble molecules, hindering the use of PPGs in the clinic. To solve this problem, we report herein a novel concept in PPG design, by transforming clinically-applied NIR-dyes with suitable molecular orbital configurations into new NIR-PPGs using computational approaches. Using this method, we demonstrate how Cy7, a class of NIR dyes possessing ideal properties (NIR-absorption, high molecular absorptivity, excellent aqueous solubility) can be successfully converted into Cy7-PPG. We report a facile synthesis towards Cy7-PPG from accessible precursors and confirm its excellent properties as the most redshifted oxygen-independent NIR-PPG to date (λmax =746 nm).

Photoresponsive Small Molecule Inhibitors for the Remote Control of Enzyme Activity

The development of new and effective therapeutics is reliant on the ability to study the underlying mechanisms of potential drug targets in live cells and multicellular systems. A persistent challenge in many drug development programmes is poor selectivity, which can obscure the mechanisms involved and lead to poorly understood modes of action. In efforts to improve our understanding of these complex processes, small molecule inhibitors have been developed in which their OFF/ON therapeutic activity can be toggled using light. Photopharmacology is devoted to using light to modulate drugs. Herein, we highlight the recent progress made towards the development of light-responsive small molecule inhibitors of selected enzymatic targets. Given the size of this field, literature from 2015 onwards has been reviewed.

Mechanistic Insights into the Photoisomerization of N,N'-Disubstituted Indigos

N,N'-disubstituted indigos are photoswitchable molecules that have recently caught the attention due to their addressability by red-light. When alkyl and aryl groups are utilized as the N-substituents, the thermal half-lives of Z isomers can be tuned independently while maintaining the advantageous red-shifted absorption spectra. To utilize these molecules in real-world applications, it is of immense importance to understand how their molecular structures as well as the environment influence their switching properties. To this end, we probed their photoisomerization mechanism by carrying out photophysical and computational studies in solvents of different polarities. The fluorescence and transient absorption experiments suggest for more polar excited and transition states, which explains the bathochromic shifts of absorption spectra and shorter thermal half-lives. On the other hand, the quantum chemical calculations reveal that in contrast to N-carbonyl groups, N-alkyl and N-aryl substituents are not strongly conjugated with the indigo chromophore and can thus serve as a tool for tuning the thermal stability of Z isomers. Both approaches are combined to provide in-depth understandings of how indigos undergo photoswitching as well as how they are influenced by N-substituent and the chemical surroundings. These mechanistic insights will serve as guiding principles for designing molecules eyeing broader applications.

Water-Involved Ring-Opening of 4-Phenyl-1,2,4-triazoline-3,5-dione for "Photo-Clicked" Access to Carbamoyl Formazan Photoswitches In Situ

Cyclic azodicarbonyl derivatives, particularly 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), commonly serve as arenophile, dienophile, enophile and electrophile. Perplexed by its instability in aqueous environment, there are few studies focused on the transient intermediate produced by hydrolysis of PTAD to achieve synthetic significance. Herein, we describe a "photo-click" method that involves nitrile imine (NI) from diarylsydnone to capture the diazenecarbonyl-phenyl-carbamic acid (DACPA) generated by water-promoted ring-opening of PTAD. DFT calculation reveal that H-bonding interactions between PTAD and water are vital to form DACPA which exhibited an umpolung effect during ligation by nature bond orbit (NBO) analysis. The ultra-fast ligation resulted in carbamoyl formazans, as a unique Z↔E photo-switchable linker on target molecules, including peptide and drugs, with excellent anti-fatigue performance. This strategy is showcased to construct highly functionalized carbamoyl formazans in situ for photo-pharmacology and material studies, which also expands the chemistry of PTAD in aqueous media.

Targeted optical fluorescence imaging: a meta-narrative review and future perspectives

Purpose

The aim of this review is to give an overview of the current status of targeted optical fluorescence imaging in the field of oncology, cardiovascular, infectious and inflammatory diseases to further promote clinical translation.

Methods

A meta-narrative approach was taken to systematically describe the relevant literature. Consecutively, each field was assigned a developmental stage regarding the clinical implementation of optical fluorescence imaging.

Results

Optical fluorescence imaging is leaning towards clinical implementation in gastrointestinal and head and neck cancers, closely followed by pulmonary, neuro, breast and gynaecological oncology. In cardiovascular and infectious disease, optical imaging is in a less advanced/proof of concept stage.

Conclusion

Targeted optical fluorescence imaging is rapidly evolving and expanding into the clinic, especially in the field of oncology. However, the imaging modality still has to overcome some major challenges before it can be part of the standard of care in the clinic, such as the provision of pivotal trial data. Intensive multidisciplinary (pre-)clinical joined forces are essential to overcome the delivery of such compelling phase III registration trial data and subsequent regulatory approval and reimbursement hurdles to advance clinical implementation of targeted optical fluorescence imaging as part of standard practice.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05504-y.

Light-driven photoswitching of quinazoline analogues of combretastatin A-4 as an effective approach for targeting skin cancer cells

A novel quinazoline series of photoswitchable combretastatin A-4 (CA-4) analogues were synthesized and their photochemical properties and antiproliferative activity against A431 epidermoid carcinoma cells were studied. It was found that quinazoline analogues, in contrast to the majority of the known CA-4, exhibit high antiproliferative activity in the E-form as well. Photoswitching of the E-form to the Z-form resulted in a multiple (9-fold) increase in antiproliferative activity. 1H NMR monitoring showed that these compounds are very resistant to UV (λ = 365 nm) or sunlight irradiation and do not undergo photodegradation with a loss of antiproliferative activity that is inherent in heterocyclic analogues of CA-4. Similar photoswitching and an increase in antiproliferative activity are observed on exposure to sunlight. A selected compound (1a-Z51) in sub-micromolar concentrations induced apoptosis in A431 cells, while rad50/ATM/p53 were not involved in cell death. The growth of A431 cells was significantly inhibited after combination treatment with compound 1a-Z51 and chemotherapy drugs (cisplatin or 5-fluorouracil). In summary, the quinazoline analogues of CA-4 represent a promising strategy to achieve a photoswitchable potency for the treatment of cancers, including the development of combination therapies.

Controlling Antibacterial Activity Exclusively with Visible Light: Introducing a Tetra-ortho-Chloro-Azobenzene Amino Acid

The introduction of a novel tetra-ortho-chloroazobenzene amino acid (CEBA) has enabled photoswitching of the antimicrobial activity of tyrocidine A analogues by using exclusively visible light, granting spatiotemporal control under benign conditions. Compounds bearing this photoswitchable amino acid become active upon irradiation with red light, but quickly turn-off upon exposure to other visible light wavelengths. Critically, sunlight quickly triggers isomerisation of the red light-activated compounds into their original trans form, offering an ideal platform for self-deactivation upon release into the environment. Linear analogues of tyrocidine A were found to provide the best photocontrol of their antimicrobial activity, leading to compounds active against Acinetobacter baumannii upon isomerisation. Exploration of their N- and C-termini has provided insights into key elements of their structure and has allowed obtaining new antimicrobials displaying excellent strain selectivity and photocontrol.

A Photo- and Redox-Driven Two-Directional Terthiazole-Based Switch: A Combined Experimental and Computational Investigation

Terthiazoles with redox-active substituents like an N-methyl pyridinium group and ferrocene have been synthesized and their photo- and electro-chromic behaviors were investigated. The presence of two lateral N-methyl pyridinium substituents in the structure of terthiazole proved to be effective in inducing not only the reductive ring-closure of the terthiazole core but also its oxidative ring-opening reaction, leading to the first terarylene-based switch able to fully operate both photochemically and electrochemically. Moreover, the large increase in the redox potential between its open and closed form (700 mV) means that a part of the photon energy necessary to trigger the cyclization is stored in the form of chemical potential available for other works. Introduction of a second redox-active unit such as ferrocene onto the central thiazolyl moiety is found to inhibit the photochromism of the switch but not its redox switchability, which, instead, got improved for the ring-opening reaction via the redox properties of the ferrocenyl unit. The optical and redox properties of the switch in its different oxidation states are analyzed with the aid of DFT calculations in order to rationalize different switching processes.

Photo-Responsive Behavior of Azobenzene Based Polar Hockey-Stick-Shaped Liquid Crystals

A study on the photoswitching behavior of azobenzene-based polar hockey-stick-shaped liquid crystals (HSLCs) has been presented. Two new series of five phenyl rings based polar HSLCs have been designed and synthesized. Solution state photoisomerization of the synthesized materials was investigated thoroughly via UV-visible and ¹ H NMR spectroscopic techniques, whereas solid-state photochromic behavior was elucidated via physical color change of the materials, solid-state UV-visible study, powder XRD, and FE-SEM techniques. The materials exhibited decent photochromic behavior for different potential applications. The thermal phase behavior of the superstructural assembly has been characterized via polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and temperature-dependent small and wide-angle X-ray scattering (SAXS/WAXS) studies. Depending upon the length of the terminal alkyl chain, nematic (N) and partially bilayer smectic A (SmA_d ) phases were observed. DFT calculations revealed the favorable anti-parallel arrangement of the polar molecules that substantiate the formation of SmA_d phase.

Fluorinated Azobenzenes Switchable with Red Light

Molecular photoswitches triggered with red or NIR light are optimal for photomodulation of complex biological systems, including efficient penetration of the human body for therapeutic purposes ("therapeutic window"). Yet, they are rarely reported, and even more rarely functional under aqueous conditions. In this work, fluorinated azobenzenes are shown to exhibit efficient E→Z photoisomerization with red light (PSS660nm >75 % Z) upon conjugation with unsaturated substituents. Initially demonstrated for aldehyde groups, this effect was also observed in a more complex structure by incorporating the chromophore into a cyclic dipeptide with propensity for self-assembly. Under physiological conditions, the latter molecule formed a supramolecular material that reversibly changed its viscosity upon irradiation with red light. Our observation can lead to design of new photopharmacology agents or phototriggered materials for in vivo use.

"Photo-Rimonabant": Synthesis and Biological Evaluation of Novel Photoswitchable Molecules Derived from Rimonabant Lead to a Highly Selective and Nanomolar "Cis-On" CB1R Antagonist

Human cannabinoid receptor type 1 (hCB₁R) plays important roles in the regulation of appetite and development of addictive behaviors. Herein, we describe the design, synthesis, photocharacterization, molecular docking, and in vitro characterization of "photo-rimonabant", i.e., azo-derivatives of the selective hCB₁R antagonist SR1411716A (rimonabant). By applying azo-extension strategies, we yielded compound 16a, which shows marked affinity for CB₁R (K_{i (cis form)} = 29 nM), whose potency increases by illumination with ultraviolet light (CB₁R K_itrans/cis ratio = 15.3). Through radioligand binding, calcium mobilization, and cell luminescence assays, we established that 16a is highly selective for hCB₁R over hCB₂R. These selective antagonists can be valuable molecular tools for optical modulation of CBRs and better understanding of disorders associated with the endocannabinoid system.

Chemoproteomics-Enabled De Novo Discovery of Photoswitchable Carboxylesterase Inhibitors for Optically Controlled Drug Metabolism

Herein, we report arylazopyrazole ureas and sulfones as a novel class of photoswitchable serine hydrolase inhibitors and present a chemoproteomic platform for rapid discovery of optically controlled serine hydrolase targets in complex proteomes. Specifically, we identify highly potent and selective photoswitchable inhibitors of the drug-metabolizing enzymes carboxylesterases 1 and 2 and demonstrate their pharmacological application by optically controlling the metabolism of the immunosuppressant drug mycophenolate mofetil. Collectively, this proof-of-concept study provides a first example of photopharmacological tools to optically control drug metabolism by modulating the activity of a metabolizing enzyme. Our arylazopyrazole ureas and sulfones offer synthetically accessible scaffolds that can be expanded to identify specific photoswitchable inhibitors for other serine hydrolases, including lipases, peptidases, and proteases. Our chemoproteomic platform can be applied to other photoswitches and scaffolds to achieve optical control over diverse protein classes.

Towards Photochromic Azobenzene-Based Inhibitors for Tryptophan Synthase

Light regulation of drug molecules has gained growing interest in biochemical and pharmacological research in recent years. In addition, a serious need for novel molecular targets of antibiotics has emerged presently. Herein, the development of a photocontrollable, azobenzene-based antibiotic precursor towards tryptophan synthase (TS), an essential metabolic multienzyme complex in bacteria, is presented. The compound exhibited moderately strong inhibition of TS in its E configuration and five times lower inhibition strength in its Z configuration. A combination of biochemical, crystallographic, and computational analyses was used to characterize the inhibition mode of this compound. Remarkably, binding of the inhibitor to a hitherto-unconsidered cavity results in an unproductive conformation of TS leading to noncompetitive inhibition of tryptophan production. In conclusion, we created a promising lead compound for combatting bacterial diseases, which targets an essential metabolic enzyme, and whose inhibition strength can be controlled with light.

Light-Controlled Cell-Cycle Arrest and Apoptosis

Cell‐cycle interference by small molecules has widely been used to study fundamental biological mechanisms and to treat a great variety of diseases, most notably cancer. However, at present only limited possibilities exist for spatio‐temporal control of the cell cycle. Here we report on a photocaging strategy to reversibly arrest the cell cycle at metaphase or induce apoptosis using blue‐light irradiation. The versatile proteasome inhibitor MG132 is photocaged directly at the reactive aldehyde function effectively masking its biological activity. Upon irradiation reversible cell‐cycle arrest in the metaphase is demonstrated to take place in vivo. Similarly, apoptosis can efficiently be induced by irradiation of human cancer cells. With the developed photopharmacological approach spatio‐temporal control of the cell cycle is thus enabled with very high modulation, as caged MG132 shows no effect on proliferation in the dark. In addition, full compatibility of photo‐controlled uncaging with dynamic microscopy techniques in vivo is demonstrated. This visible‐light responsive tool should be of great value for biological as well as medicinal approaches in need of high‐precision targeting of the proteasome and thereby the cell cycle and apoptosis.

General Principles for the Design of Visible-Light-Responsive Photoswitches: Tetra-ortho-Chloro-Azobenzenes

Molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. Their development is driven by the need for low energy (green-red-NIR) light switching, to allow non-invasive operation with deep tissue penetration. The lack of clear design principles for the adaptation and optimization of such systems limits further applications. Here we provide a design rulebook for tetra-ortho-chloroazobenzenes, an emerging class of visible-light-responsive photochromes, by elucidating the role that substituents play in defining their key characteristics: absorption spectra, band overlap, photoswitching efficiencies, and half-lives of the unstable cis isomers. This is achieved through joint photochemical and theoretical analyses of a representative library of molecules featuring substituents of varying electronic nature. A set of guidelines is presented that enables tuning of properties to the desired application through informed photochrome engineering.

Photoswitchable Lipids

Photoswitchable lipids are emerging tools for the precise manipulation and study of lipid function. They can modulate many aspects of membrane biophysics, including permeability, fluidity, lipid mobility and domain formation. They are also very useful in lipid physiology and enable optical control of a wide array of lipid receptors, such as ion channels, G protein-coupled receptors, nuclear hormone receptors, and enzymes that translocate to membranes. Enzymes involved in lipid metabolism often process them in a light-dependent fashion. Photoswitchable lipids complement other functionalized lipids widely used in lipid chemical biology, including isotope-labeled lipids (lipidomics), fluorescent lipids (imaging), bifunctional lipids (lipid-protein crosslinking), photocaged lipids (photopharmacology), and other labeled variants.

Bioorthogonal Turn-On BODIPY-Peptide Photosensitizers for Tailored Photodynamic Therapy

Photodynamic therapy (PDT) leads to cancer remission via the production of cytotoxic species under photosensitizer (PS) irradiation. However, concomitant damage and dark toxicity can both hinder its use. With this in mind, we have implemented a versatile peptide-based platform of bioorthogonally activatable BODIPY-tetrazine PSs. Confocal microscopy and phototoxicity studies demonstrated that the incorporation of the PS, as a bifunctional module, into a peptide enabled spatial and conditional control of singlet oxygen (1 O2 ) generation. Comparing subcellular distribution, PS confined in the cytoplasmic membrane achieved the highest toxicities (IC50 =0.096±0.003 μm) after activation and without apparent dark toxicity. Our tunable approach will inspire novel probes towards smart PDT.

Activation of Sirtuin 2 Inhibitors Employing Photoswitchable Geometry and Aqueous Solubility

Because isoenzymes of the experimentally and therapeutically extremely relevant sirtuin family show high similarity, addressing the unique selectivity pocket of sirtuin 2 is a promising strategy towards selective inhibitors. An unrelated approach towards selective inhibition of isoenzymes with varied tissue distribution is targeted drug delivery or spatiotemporal activation by photochemical activation. Azologization of two nicotinamide‐mimicking lead structures was undertaken to combine both approaches and yielded a set of 33 azobenzenes and azopyridines that have been evaluated for their photochemical behaviour and bioactivity. For some compounds, inhibitory activity reached the sub‐micromolar range in their thermodynamically favoured E form and could be decreased by photoisomerization to the metastable Z form. Besides, derivatization with long‐chain fatty acids yielded potent sirtuin 2 inhibitors, featuring another intriguing aspect of azo‐based photoswitches. In these compounds, switching to the Z isomer increased aqueous solubility and thereby enhanced biological activity by up to a factor of 21. The biological activity of two compounds was confirmed by hyperacetylation of sirtuin specific histone proteins in a cell‐based activity assay.

Functionalized Cannabinoid Subtype 2 Receptor Ligands: Fluorescent, PET, Photochromic and Covalent Molecular Probes

Cannabinoid subtype 2 receptors (CB2 Rs) are G protein-coupled receptors (GPCRs) belonging to the endocannabinoid system, a complex network of signalling pathways leading to the regulation of key physiological processes. Interestingly, CB2 Rs are strongly up-regulated in pathological conditions correlated with the onset of inflammatory events like cancer and neurodegenerative diseases. Therefore, CB2 Rs represent an important biological target for therapeutic as well as diagnostic purposes. No CB2 R-selective drugs are yet on the market, thus underlining a that deeper comprehension of CB2 Rs' complex activation pathways and their role in the regulation of diseases is needed. Herein, we report an overview of pharmacological and imaging tools such as fluorescent, positron emission tomography (PET), photochromic and covalent selective CB2 R ligands. These molecular probes can be used in vitro as well as in vivo to investigate and explore the unravelled role(s) of CB2 Rs, and they can help to design suitable CB2 R-targeted drugs.

Stiff-Stilbene Photoswitches: From Fundamental Studies to Emergent Applications

Stiff-stilbene, a sterically restricted fused ring analogue of stilbene, has been regularly used as a model compound in theoretical studies of stilbene photoisomerization. Lately, owing to its excellent photoswitching properties, it is increasingly being applied to reversibly control the properties and function of chemical as well as biological systems. Stiff-stilbene photoswitches possess a number of advantageous properties including a high quantum yield for photoisomerization and a high thermal stability. Furthermore, they undergo a large geometrical change upon isomerization and their synthesis is straightforward. Herein, we provide an overview of the basic properties of stiff-stilbene and of recent applications in supramolecular chemistry, catalysis, and biological systems.

Optical Control of Cytokine Production Using Photoswitchable Galactosylceramides

α-Galactosylceramides are glycosphingolipids that show promise in cancer immunotherapy. After presentation by CD1d, they activate natural killer T cells (NKT), which results in the production of a variety of pro-inflammatory and immunomodulatory cytokines. Herein, we report the synthesis and biological evaluation of photochromic derivatives of KRN-7000, the activity of which can be modulated with light. Based on established structure-activity relationships, we designed photoswitchable analogues of this glycolipid that control the production of pro-inflammatory cytokines, such as IFN-γ. The azobenzene derivative α-GalACer-4 proved to be more potent than KRN-7000 itself when activated with 370 nm light. Photolipids of this type could improve our mechanistic understanding of cytokine production and could open new directions in photoimmunotherapy.

Planarization-Induced Activation Wavelength Red-Shift and Thermal Half-Life Acceleration in Hydrazone Photoswitches

The optimization and modulation of the properties of photochromic compounds, such as their activation wavelengths and thermal relaxation half-lives (τ1/2), are essential for their adaptation in various applications. In this work, we studied the effect of co-planarization of the rotary fragment of two photochromic hydrazones with the core of the molecule on their switching properties. The Z and E isomers of both compounds exhibit red-shifted absorption bands relative to their twisted versions, allowing for their photoswitching using longer wavelengths of light. Additionally, the thermal half-lives of both hydrazones are drastically shortened from hundreds of years to days.

Photoswitchable Macroscopic Solid Surfaces Based On Azobenzene-Functionalized Polydopamine/Gold Nanoparticle Composite Materials: Formation, Isomerization and Ligand Exchange

The facile preparation of dynamic interfaces is presented based on the combination of photoisomerizable azobenzenes and polydopamine (PDA)/Au nanoparticle composite materials. Azobenzenes with different spacer lengths (C₃ , C₆ ) and surface-binding groups (SH, NH₂ ) were synthesized. The polymer layer on macroscopic quartz surface was prepared by the facile aerobic autopolymerisation of dopamine hydrochloride under basic conditions. The presence of redox-active catechol moieties meant that gold nanoparticles were formed on the polymer surface. The obtained UV-Vis spectroscopic results confirmed that following their successful assembly, the switching of azobenzenes on PDA/Au was not affected by the surface binding group and the spacer length of the azobenzene molecules under the measurement conditions. Furthermore, facilitated by the curved nature of the Au particles, the surface-bound azobenzene layer could be reconstructed by ligand-exchange processes, and the photochemical characterization of the mixed layer was performed.

Photobodies: Light-Activatable Single-Domain Antibody Fragments

Photocaged antibody fragments, termed photobodies, have been developed that are impaired in their antigen-binding capacity and can be activated by irradiation with UV light (365 nm). This rational design concept builds on the selective photocaging of a single tyrosine in a nanobody (a single-domain antibody fragment). Tyrosine is a frequently occurring residue in central positions of the paratope region. o-Nitrobenzyl-protected tyrosine variants were incorporated into four nanobodies, including examples directed against EGFR and HER2, and photodeprotection restores the native sequence. An anti-GFP photobody exhibited an at least 10 000-fold impaired binding affinity before photodeprotection compared with the parent nanobody. A bispecific nanobody-photobody fusion protein was generated to trigger protein heterodimerization by light. Photoactivatable antibodies are expected to become versatile protein reagents and to enable novel approaches in diagnostic and therapeutic applications.

Tunable Aggregation-Induced Emission of Imidazole Hydrazones by pH and Anions

Aggregation-induced emission (AIE) materials have drawn great attention for applications as organic light-emitting diodes (OLED) and probes. The applications are, however, restricted by the complex syntheses and hydrophobic properties. Herein, a one-step synthesis of an AIE material based on imidazole hydrazone is assessed. Protonation of the imidazole-H leads to emission color change from yellow to green in the solid state. The emission color is recovered upon imidazole-H⁺ deprotonation. Moreover, the emission wavelength shifts from 532 to 572 nm by anion exchange. In addition, an enhanced emission (Φ_F up to 22.6 %) was obtained with the Br^- anion compared with NTf₂ ^- , SbCl₅ ^- , PF₆ ^- , and OTf^- anions. X-ray crystallography studies together with theoretical calculations show that the enhanced emission of hydrazone salts arises from strong hydrogen bonding between the hydrazone proton and the halide ion (Cl^- or Br^- ).

Optical Control of the GTP Affinity of K-Ras(G12C) by a Photoswitchable Inhibitor

Photocontrol of protein activity is an emerging field in biomedicine. For optical control of a mutant small GTPase K-Ras(G12C), we developed small-molecule inhibitors with photoswitchable efficacy, where one configuration binds the target protein and exert different pharmacological effects upon light irradiation. The compound design was based on the structure feature of a previously identified allosteric pocket of K-Ras(G12C) and the chemical structure of covalent inhibitors, and resulted in the synthesis and characterization of two representative azobenzene-containing compounds. Nucleotide exchange assays demonstrated the different efficacy to control the GTP affinity by photoswitching of one potent compound PS-C2, which would be a useful tool to probe the conformation of mutational K-Ras. Our study demonstrated the feasibility of designing photoswitchable modulators from allosteric covalent inhibitor of small GTPases.