Photochromic Indolyl Fulgimides as Chromo-pharmacophores Targeting Sirtuins

Aurones: Unexplored Visible-Light Photoswitches for Aqueous Medium

The development of synthetically accessible photoswitches showing an efficient performance in aqueous medium has recently become an urgent task due to the rapid progress of photopharmacology and novel biomedical applications. In response to this challenge, in this work, aurone derivatives are introduced as a novel class of efficient visible-light photoswitches for aqueous medium. In general, aurones exhibit superior performance in water, including significantly higher quantum yields, compared with other indigoid photoswitches (hemithioindigo and hemiindigo). Especially remarkable are the half-lives of the photoinduced E-isomers of aurones in water, reaching up to 7 years. Further modification of the aurone scaffold with substituents that increase water solubility does not affect most of the photoswitching characteristics and even improves some them. The highly advantageous property profile of the aurone photoswitches make them a perfect novel platform for the design of light-controllable systems in the areas requiring photoswitching in aqueous medium.

Inclusion-Activated Reversible E/Z Isomerization of a Cyanostilbene Derivative Based on Cucurbit[8]uril under 365 nm Ultraviolet Irradiation

The photoisomerization behavior of cyanostilbene molecules is a hotspot in supramolecular configuration transformation research. Here, we reported a cyanostilbene derivative that converted from the Z,Z-isomer to the E,E-isomer under UV light irradiation at 365 nm. This process can be reversibly converted only in the presence of cucurbit[8]uril under the same light source, accompanied by the reversible conversion of fluorescence from green to yellow. No effective configuration transformation occurred with guest molecules only or upon the addition of cucurbit[7]uril. The photoisomerization was fully characterized by UV-vis and fluorescence spectroscopy, NMR, high-resolution mass spectrometry, and transmission electron microscopy. This work provides a new method for the supramolecular macrocyclic-activated configuration transformation.

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.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Diarylethene-Based Photoswitchable Inhibitors of Serine Proteases

A bicyclic peptide scaffold was chemically adapted to generate diarylethene-based photoswitchable inhibitors of serine protease Bos taurus trypsin 1 (T1). Starting from a prototype molecule-sunflower trypsin inhibitor-1 (SFTI-1)-we obtained light-controllable inhibitors of T1 with Ki in the low nanomolar range, whose activity could be modulated over 20-fold by irradiation. The inhibitory potency as well as resistance to proteolytic degradation were systematically studied on a series of 17 SFTI-1 analogues. The hydrogen bond network that stabilizes the structure of inhibitors and possibly the enzyme-inhibitor binding dynamics were affected by isomerization of the photoswitch. The feasibility of manipulating enzyme activity in time and space was demonstrated by controlled digestion of gelatin-based hydrogel and an antimicrobial peptide BP100-RW. Finally, our design principles of diarylethene photoswitches are shown to apply also for the development of other serine protease inhibitors.

Photoresponsive molecular tools for emerging applications of light in medicine

Light-based therapeutic and imaging modalities, which emerge in clinical applications, rely on molecular tools, such as photocleavable protecting groups and photoswitches that respond to photonic stimulus and translate it into a biological effect. However, optimisation of their key parameters (activation wavelength, band separation, fatigue resistance and half-life) is necessary to enable application in the medical field. In this perspective, we describe the applications scenarios that can be envisioned in clinical practice and then we use those scenarios to explain the necessary properties that the photoresponsive tools used to control biological function should possess, highlighted by examples from medical imaging, drug delivery and photopharmacology. We then present how the (photo)chemical parameters are currently being optimized and an outlook is given on pharmacological aspects (toxicity, solubility, and stability) of light-responsive molecules. With these interdisciplinary insights, we aim to inspire the future directions for the development of photocontrolled tools that will empower clinical applications of light.

Optical Control of GABAA Receptors with a Fulgimide-Based Potentiator

Optogenetic and photopharmacological tools to manipulate neuronal inhibition have limited efficacy and reversibility. We report the design, synthesis, and biological evaluation of Fulgazepam, a fulgimide derivative of benzodiazepine that behaves as a pure potentiator of ionotropic γ‐aminobutyric acid receptors (GABA_ARs) and displays full and reversible photoswitching in vitro and in vivo. The compound enables high‐resolution studies of GABAergic neurotransmission, and phototherapies based on localized, acute, and reversible neuroinhibition.

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.

2-Azo-, 2-diazocine-thiazols and 2-azo-imidazoles as photoswitchable kinase inhibitors: limitations and pitfalls of the photoswitchable inhibitor approach

In photopharmacology, photoswitchable compounds including azobenzene or other diarylazo moieties exhibit bioactivity against a target protein typically in the slender E-configuration, whereas the rather bulky Z-configuration usually is pharmacologically less potent. Herein we report the design, synthesis and photochemical/inhibitory characterization of new photoswitchable kinase inhibitors targeting p38α MAPK and CK1δ. A well characterized inhibitor scaffold was used to attach arylazo- and diazocine moieties. When the isolated isomers, or the photostationary state (PSS) of isomers, were tested in commonly used in vitro kinase assays, however, only small differences in activity were observed. X-ray analyses of ligand-bound p38α MAPK and CK1δ complexes revealed dynamic conformational adaptations of the protein with respect to both isomers. More importantly, irreversible reduction of the azo group to the corresponding hydrazine was observed. Independent experiments revealed that reducing agents such as DTT (dithiothreitol) and GSH (glutathione) that are typically used for protein stabilization in biological assays were responsible. Two further sources of error are the concentration dependence of the E-Z-switching efficiency and artefacts due to incomplete exclusion of light during testing. Our findings may also apply to a number of previously investigated azobenzene-based photoswitchable inhibitors.

Development of photoswitchable inhibitors for β-galactosidase

Azobenzenes are of particular interest as a photochromic scaffold for biological applications because of their high fatigue resistance, their large geometrical change between extended (trans) and bent (cis) isomer, and their diverse synthetic accessibility. Despite their wide-spread use, there is no reported photochromic inhibitor of the well-investigated enzyme β-galactosidase, which plays an important role for biochemistry and single molecule studies. Herein, we report the synthesis of photochromic competitive β-galactosidase inhibitors based on the molecular structure of 2-phenylethyl β-d-thiogalactoside (PETG) and 1-amino-1-deoxy-β-d-galactose (β-d-galactosylamine). The thermally highly stable PETG-based azobenzenes show excellent photochromic properties in polar solvents and moderate to high photostationary states (PSS). The optimized compound 37 is a strong competitive inhibitior of β-galactosidase from Escherichia coli and its inhibition constant (Ki) changes between 60 nM and 290 nM upon irradiation with light. Additional docking experiments supported the observed structure-activity relationship.