BODIPY-based photocages: rational design and their biomedical application

Photocages, also known as photoactivated protective groups (PPGs), have been utilized to achieve controlled release of target molecules in a non-invasive and spatiotemporal manner. In the past decade, BODIPY fluorophores, a well-established class of fluorescent dyes, have emerged as a novel type of photoactivated protective group capable of efficiently releasing cargo species upon irradiation. This is due to their exceptional properties, including high molar absorption coefficients, resistance to photochemical and thermal degradation, multiple modification sites, favorable uncaging quantum yields, and highly adjustable spectral properties. Compared to traditional photocages that mainly absorb UV light, BODIPY-based photocages that absorb visible/near-infrared (Vis/NIR) light offer advantages such as deeper tissue penetration and reduced bio-autofluorescence, making them highly suitable for various biomedical applications. Consequently, different types of photoactivated protective groups based on the BODIPY skeleton have been established. This highlight provides a comprehensive overview of the strategies employed to construct BODIPY photocages by substituting leaving groups at different positions within the BODIPY fluorophore, including the meso-methyl position, boron position, 2,6-position, and 3,5-position. Furthermore, the application of these BODIPY photocages in biomedical fields, such as fluorescence imaging and controlled release of active species, is discussed.

Enlightening epigenetics: optochemical tools illuminate the path

Optochemical tools have become potent instruments for understanding biological processes at the molecular level, and the past decade has witnessed their use in epigenetics and epitranscriptomics (also known as RNA epigenetics) for deciphering gene expression regulation. By using photoresponsive molecules such as photoswitches and photocages, researchers can achieve precise control over when and where specific events occur. Therefore, these are invaluable for studying both histone and nucleotide modifications and exploring disease-related mechanisms. We systematically report and assess current examples in the field, and identify open challenges and future directions. These outstanding proof-of-concept investigations will inspire other chemical biologists to participate in these emerging fields given the potential of photochromic molecules in research and biomedicine.

pH-Sensitive and Lysosome Targetable Photosensitizers Based on BODIPYs

Photodynamic therapy (PDT) is an effective and U.S. Food and Drug Administration (FDA) approved treatment for cancer and other diseases. Photosensitizer is one of the three key components that harvest the energy of light at a certain wavelength. Compared to the conventional fluorophores used as photosensitizers, boron dipyrromethene (BODIPY) derivatives have grown fast in recent years due to their low dark toxicity, versatile tunable sites, and easiness of being paired with other treatments. In this paper, two pH-sensitive BODIPY-based photosensitizers (BDC and BDBrC) were synthesized by adding carbazole moieties onto the BODIPY cores (BD and BDBr) through condensation reactions. BDBrC has two Br atoms at the BODIPY core that promote singlet oxygen generation and further red-shift the absorption maximum peak. Both compounds showed sensitivity toward pH change and generated more singlet oxygen under acidic conditions. The cellular uptake and cell imaging experiments showed that BDBrC can selectively target the lysosome organelle. The further dark cell viability and light cytotoxicity indicate the light triggered PDT treatment can be accomplished with BDBrC.

Far-red BODIPY-based oxime esters: photo-uncaging and drug delivery

Far-red BODIPY-based oxime esters for photo-uncaging were designed to release molecules of interest with carboxylic acids. The low power red LED light breaks the N-O oxime ester bond and frees the caged molecules. We studied the mechanism and kinetics of the uncaging procedure using a 1H NMR spectrometer. Moreover, the drug delivery strategy to release valproic acid (VPA) on demand was tested in vitro using this far-red BODIPY photo-uncaging strategy to induce apoptosis in tumor cells.

Novel BODIPY-Based Photobase Generators for Photoinduced Polymerization

Photobase generators (PBGs) are compounds that utilize light-sensitive chemical-protecting groups to offer spatiotemporal control of releasing organic bases upon targeted light irradiation. PBGs can be implemented as an external control to initiate anionic polymerizations such as thiol-ene Michael addition reactions. However, there are limitations for common PBGs, including a short absorption wavelength and weak base release that lead to poor efficiency in photopolymerization. Therefore, there is a great need for visible-light-triggered PBGs that are capable of releasing strong bases efficiently. Here, we report two novel BODIPY-based visible-light-sensitive PBGs for light-induced activation of the thiol-ene Michael "click" reaction and polymerization. These PBGs were designed by connecting the BODIPY-based light-sensitive protecting group with tetramethylguanidine (TMG), a strong base. Moreover, we exploited the heavy atom effect to increase the efficiency of releasing TMG and the polymerization rate. These BODIPY-based PBGs exhibit extraordinary activity toward thiol-ene Michael addition-based polymerization, and they can be used in surface coating and polymer network formation of different thiol and vinyl monomers.

Stimuli-Responsive Prodrug Chemistries for Cancer Therapy

Prodrugs are pharmacologically inactive, chemically modified derivatives of active drugs, which, following in vivo administration, are converted to the parent drugs through chemical or enzymatic cleavage. The prodrug approach holds tremendous potential to create the enhanced version of an existing pharmacological agent and leverage those improvements to augment the drug molecules' bioavailability, targeting ability, therapeutic efficacy, safety, and marketability. Especially in cancer therapy, prodrug application has received substantial attention. A prodrug can effectively broaden the therapeutic window of its parent drug by enhancing its release at targeted tumor sites while reducing its access to healthy cells. The spatiotemporally controlled release can be achieved by manipulating the chemical, physical, or biological stimuli present at the targeted tumor site. The critical strategy comprises drug-carrier linkages that respond to physiological or biochemical stimuli in the tumor milieu to yield the active drug form. This review will focus on the recent advancements in the development of various fluorophore-drug conjugates that are widely used for real-time monitoring of drug delivery. The use of different stimuli-cleavable linkers and the mechanisms of linker cleavage will be discussed. Finally, the review will conclude with a critical discussion of the prospects and challenges that might impede the future development of such prodrugs.

meso-Methyl BODIPY Photocages: Mechanisms, Photochemical Properties, and Applications

meso-methyl BODIPY photocages have recently emerged as an exciting new class of photoremovable protecting groups (PPGs) that release leaving groups upon absorption of visible to near-infrared light. In this Perspective, we summarize the development of these PPGs and highlight their critical photochemical properties and applications. We discuss the absorption properties of the BODIPY PPGs, structure-photoreactivity studies, insights into the photoreaction mechanism, the scope of functional groups that can be caged, the chemical synthesis of these structures, and how substituents can alter the water solubility of the PPG and direct the PPG into specific subcellular compartments. Applications that exploit the unique optical and photochemical properties of BODIPY PPGs are also discussed, from wavelength-selective photoactivation to biological studies to photoresponsive organic materials and photomedicine.

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.

A BODIPY-Based Far-Red-Absorbing Fluorescent Probe for Hypochlorous Acid Imaging

Hypochlorous acid (HClO) is produced by white blood cells to defend against injury and bacteria. However, as one of the reactive oxygen species, high intracellular HClO concentration could lead to chronic diseases that affect the cardiovascular and nervous systems. To monitor HClO concentrations in bio-samples, the fluorescent probe is preferred to have: a) absorbability in the far-red window with reduced light-toxicity and improved tissue penetration depth, b) ratiometric feature for accurate analysis. In this study, we reported a far-red ratiometric HClO fluorescence probe based on BODIPY chromophore and aldoxime sensing group. Not only the color change of the probe solution can be detected by naked eyes, but also the emission ratios (I₆₄₅/I₆₇₀) showed a significant increase upon the introduction of HClO. More importantly, the feasibility of HClO monitoring in bio-samples was demonstrated in vitro using a confocal microscope.

Green Light-Activated Single-Component Organic Fluorescence-Based Nano-Drug Delivery System for Dual Uncaging of Anticancer Drugs

Developing green or red light-activated drug delivery systems (DDSs) for cancer treatment is highly desirable. Herein, we have reported a green light-responsive single component-based organic fluorescence nano-DDS by simply anchoring 2-hydroxy-6-naphthacyl (phototrigger) on both sides of the 1,5-diaminonaphthalene (DAN) chromophore. This green light (λ ≥ 500 nm)-activated DDS released two equivalents of the anticancer drug (valproic acid) in a spatio-temporally controlled manner. Our photoresponsive DDS [DAN-bis(HO-Naph-VPA)] exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) accompanied with aggregation-induced emission (AIE) phenomena. AIE initiated the photorelease, and ESIPT enhanced the rate of the photorelease. Further, in vitro studies revealed that our green light-activated nano-DDS exhibited good cytocompatibility, excellent cellular internalization, and effective cancer cell killing ability.

Molecular platforms based on biocompatible photoreactions for photomodulation of biological targets

Photoirradiation provides a convenient and biocompatible approach for spatiotemporal modulation of biological systems with photoresponsive components. The construction of molecular platforms with a photoresponse to be integrated into biomolecules for photomodulation has been of great research interest in optochemical biology. In this review, we summarize typical molecular platforms that are integratable with biomolecules for photomodulation purposes. We categorize these molecular platforms according to their excitation light source, namely ultraviolet (UV), visible (Vis) or near-infrared (NIR) light. The protype chemistry of these molecular platforms is introduced along with an overview of their most recent applications for spatiotemporal regulation of biomolecular function in living cells or mice models. Challenges and the outlook are also presented. We hope this review paper will contribute to further progress in the development of molecular platforms and their biomedical use.

C=C Bond Oxidative Cleavage of BODIPY Photocages by Visible Light

Photocages for protection and the controlled release of bioactive compounds have been widely investigated. However, the vast majority of these photocages employ the cleavage of single bonds and high-energy ultraviolet light. The construction of a photoactivation system that uses visible light to cleave unsaturated bonds still remains a challenge. Herein, we report a regioselective oxidative cleavage of C=C bonds from a boron-dipyrrolemethene (BODIPY)-based photocage by illumination at 630 nm, resulting in a free aldehyde and a thiol fluorescent probe. This strategy was demonstrated in live HeLa cells, and the generated α-formyl-BODIPY allowed real-time monitoring of aldehyde release in the cells. In particular, it is shown that a mannose-functionalized photocage can target HepG2 cells.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Boron dipyrromethene (BODIPY) in polymer chemistry

The present review provides both a summary and outlook on the exciting field of BODIPYs in polymer chemistry.

Stimulus-cleavable chemistry in the field of controlled drug delivery

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

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

The therapeutic and neuroprotective effects of an antiepileptic drug valproic acid in glioma patients

Glioma is the most common primary malignant brain tumor in adults and the patients have poor prognosis despite treatment with surgery, radiotherapy and chemotherapy. The anti-epileptic drug, valproic acid (VPA) as a HDAC inhibitors is often used in glioma patients even if the patients don't have brain tumors associated epilepsy (BAE). Some previous studies have found that VPA not only has anti-epileptic effect, but also has anti-glioma growth effect through enhance radiotherapy sensitivity or other mechanism. Then VPA is reported to improve the survival of glioma patients receiving chemoradiation therapy. In addition, there are limited researches have shown that VPA has a neuroprotective effect in protect normal cells and tissues from the deleterious effects of treatment of glioma, especially radiotherapy. We'll give a brief overview of these effects of VPA in glioma patients.

p-Pyridine BODIPY-based fluorescence probe for highly sensitive and selective detection of picric acid

A novel fluorescence probe p-PBP for PA was synthesized based on a basic N atom as the electronic donor. The probe could detect PA over TNT, CE, PETN, RDX, HMX, NB, NT, DNT, NP, DNP, and common inorganic explosive ions (K⁺, Ba²⁺, NH₄⁺, NO₃^-, ClO₃^-, and ClO₄^-), and common ions (Na⁺, Ca²⁺, and Mg²⁺) with high selectivity. The fluorescence quenching was attributed to the photo-induced electron transfer (PET) processes from the excited state of p-PBP to the ground state PA. The detection limit of probe p-PBP for PA was as low as 13.06 nmol/L, which is far lower than the concentration stipulated by the Environmental quality standards for surface water. The response time was less than 30 s. Hence, the fluorescence probe p-PBP was successfully developed to detect the concentration level of PA in real samples, which would provide a novel quantitative analysis method of PA in forensic science.