Keys for Unlocking Photolabile Metal-Containing Cages

A cascade nanosystem with "Triple-Linkage" effect for enhanced photothermal and activatable metal ion therapy for hepatocellular carcinoma

Due to the limitations of single-model tumor therapeutic strategies, multimodal combination therapy have become a more favorable option to enhance efficacy by compensating for its deficiencies. However, in nanomaterial-based multimodal therapeutics for tumors, exploiting synergistic interactions and cascade relationships of materials to achieve more effective treatments is still a great challenge. Based on this, we constructed a nanoplatform with a "triple-linkage" effect by cleverly integrating polydopamine (PDA), silver nanoparticles (AgNPs), and glucose oxidase (GOx) to realize enhanced photothermal therapy (PTT) and activatable metal ion therapy (MIT) for hepatocellular carcinoma (HCC) treatment. First, the non-radiative conversion of PDA under light conditions was enhanced by AgNPs, which directly enhanced the photothermal conversion efficiency of PDA. In addition, GOx reduced the synthesis of cellular heat shock proteins by interfering with cellular energy metabolism, thereby enhancing cellular sensitivity to PTT. On the other hand, H2O2, a by-product of GOx-catalyzed glucose, could be used as an activation source to activate non-toxic AgNPs to release cytotoxic Ag+, achieving activatable Ag+-mediated MIT. In conclusion, this nanosystem achieved efficient PTT and MIT for HCC by exploiting the cascade effect among PDA, AgNPs, and GOx, providing a novel idea for the design of multimodal tumor therapeutic systems with cascade regulation.

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.

Targeting drug delivery with light: A highly focused approach

Light is a uniquely powerful tool for controlling molecular events in biology. No other external input (e.g., heat, ultrasound, magnetic field) can be so tightly focused or so highly regulated as a clinical laser. Drug delivery vehicles that can be photonically activated have been developed across many platforms, from the simplest "caging" of therapeutics in a prodrug form, to more complex micelles and circulating liposomes that improve drug uptake and efficacy, to large-scale hydrogel platforms that can be used to protect and deliver macromolecular agents including full-length proteins. In this Review, we discuss recent innovations in photosensitive drug delivery and highlight future opportunities to engineer and exploit such light-responsive technologies in the clinical setting.

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.

Photocontrolled activation of small molecule cancer therapeutics

Cancer remains one of the leading causes of death worldwide. Conventional treatment of the disease is comprised of chemotherapy, radiation and surgery among other treatment approaches. Chemotherapy is plagued by multiple side-effects caused due to non-specific drug action. Light-based therapies offer an alternative treatment approach that can be fine tuned to achieve the desired effect to treat the disease and address challenges posed by chemotherapeutic side-effects. Photodynamic therapy (PDT) is one of the light mediated treatment modalities that has been successfully applied to treat superficial malignancies with high-efficiency, although its dependence on normoxic conditions limits its efficiency to treat deep-seated tumors. On the other hand, light-sensitive drug-mimetics and drug-release platforms have been deemed efficient in preclinical settings to induce cancer cell death with minimal collateral damage. Drawing from about a decade's worth of examples, we highlight the application of photosensitive molecules as an alternative therapeutic option to PDT and describe their designs that influence the biology of the cancer cells, in turn affecting their viability with high spatio-temporal control.

Understanding the Role of Aromaticity and Conformational Changes in Bond Dissociation Processes of Photo-Protecting Groups

Photoremovable protecting groups (PPGs) provide spatial and temporal control over the release of various chemicals. Using surface hopping studies with multireference electronic structure methods we have unravelled the nuclear and the electronic events at play. Furthermore, the electronic changes along the reaction path were probed using excited state aromaticity quantifiers and orbital analysis. We find that upon irradiation with light of appropriate wavelength on the substituted coumarin system a π-π* electronic excitation occurs which is followed by an electron loss from the aromatic ring on gaining proper alignment between the π* and the C-LG (LG = leaving group) σ*. This alignment is brought about by a critical dihedral angle change in the molecule, which subsequently triggers C-LG bond cleavage. The sequence of events is indicative of an intramolecular electron catalyzed process which is established through investigations of changes in aromaticity of the phenyl ring which acts as an electron reservoir.

Near infrared light-triggered metal ion and photodynamic therapy based on AgNPs/porphyrinic MOFs for tumors and pathogens elimination

Although metal-based agents are widely used in disease treatment, precisely controlled metal ions release is still a challenge. Here, we demonstrated a nanoplatform (PAM) to achieve on-demand activation and release of metal ions via controlling oxidation condition by near infrared (NIR) light-inducted photodynamic therapy (PDT). PAM was constructed by decorating silver nanoparticles (AgNPs) onto the porphyrinic porous coordination network (PCN) and further camouflaging with the neutrophil membrane (NM) with inflammatory targeting ability. PAM was inactive without irradiation, causing no damage to normal tissues. However, under NIR irradiation at tumor or infected tissues, PCN locally generated singlet oxygen (¹O₂), enabling AgNPs to be partly degraded to release cytotoxic Ag⁺ for metal ions therapy (MIT). Simultaneously, the incorporated AgNPs promoted the ¹O₂ yield of PCN due to the localized electric field effect. Consequently, the NIR light-controlled interlocking interactions between AgNPs and PCN might offer a great potential for achieving controlled, precise and efficient disease treatment with reduced side-effect.

Photo-controlled release of metal ions using triazoline-containing amphiphilic copolymers

Photo-controlled release of metal ions can be achieved by denitrogenation of triazoline from the micelles of amphiphilic copolymer, and has potential applications for biomedicines.

Radical-Mediated Thiol-Ene Strategy: Photoactivation of Thiol-Containing Drugs in Cancer Cells

Photoactivated drugs provide an opportunity to improve efficacy alongside reducing side-effects in the treatment of severe diseases such as cancer. Described herein is a photoactivation decaging method of isobutylene-caged thiols through a UV-initiated thiol-ene reaction. The method was demonstrated with an isobutylene-caged cysteine, cyclic disulfide-peptide, and thiol-containing drug, all of which were rapidly and efficiently released under mild UV irradiation in the presence of thiol sources and a photoinitiator. Importantly, it is shown that the activity of histone deacetylase inhibitor largazole can be switched off when stapled, but selectively switched on within cancer cells when irradiated with non-phototoxic light.

Ruthenium-containing P450 inhibitors for dual enzyme inhibition and DNA damage

Cytochrome P450s are key players in drug metabolism, and overexpression in tumors is associated with significant resistance to many medicinal agents. Consequently, inhibition of P450s could serve as a strategy to restore drug efficacy. However, the widespread expression of P450s throughout the human body and the critical roles they play in various biosynthetic pathways motivates the development of P450 inhibitors capable of controlled local administration. Ruthenium complexes containing P450 inhibitors as ligands were synthesized in order to develop pro-drugs that can be triggered to release the inhibitors in a spatially and temporally controlled fashion. Upon light activation the compounds release ligands that directly bind and inhibit P450 enzymes, while the ruthenium center is able to directly damage DNA.

Ru(II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging

Metal complexes have many proven applications in the caging and photochemical release of biologically active compounds. Photocaging groups derived from Ru(II) traditionally have been composed of ancillary ligands that are planar and bi- or tridentate, such as 2,2'-bipyridine (bpy), 2,2':6',2″-terpyridine (tpy), and 1,10-phenanthroline (phen). Complexes bearing ancillary ligands with denticities higher than three represent a new class of Ru(II)-based photocaging groups that are grossly underdeveloped. Because high-denticity ancillary ligands provide the ability to increase the structural rigidity and control the stereochemistry, our groups initiated a research program to explore the applications of such ligands in Ru(II)-based photocaging. Ru(TPA), bearing the tetradentate ancillary ligand tris(2-pyridylmethyl)amine (TPA), has been successfully utilized to effectively cage nitriles and aromatic heterocycles. Nitriles and aromatic heterocycles caged by the Ru(TPA) group show excellent stability in aqueous solutions in the dark, and the complexes can selectively release the caged molecules upon irradiation with light. Ru(TPA) is applicable as a photochemical agent to offer precise spatiotemporal control over biological activity without undesired toxicity. In addition, Ru(II) polypyridyl complexes with desired photochemical properties can be synthesized and identified by solid-phase synthesis, and the resulting complexes show properties to similar to those of complexes obtained by solution-phase synthesis. Density functional theory (DFT) calculations reveal that orbital mixing between the π* orbitals of the ancillary ligand and the Ru-N dσ* orbital is essential for ligand photodissociation in these complexes. Furthermore, the introduction of steric bulk enhances the photoliability of the caged molecules, validating that steric effects can largely influence the quantum efficiency of photoinduced ligand exchange in Ru(II) polypyridyl complexes. Recently, two new photocaging groups, Ru(cyTPA) and Ru(1-isocyTPQA), have been designed and synthesized for caging of nitriles and aromatic heterocycles, and these complexes exhibit unique photochemical properties distinct from those derived from Ru(TPA). Notably, the unusually greater quantum efficiency for the ligand exchange in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂, Φ₄₀₀ = 0.033(3), uncovers a trans-type effect in the triplet metal-to-ligand charge transfer (³MLCT) state that enhances photoinduced ligand exchange in a new manner. DFT calculations and ultrafast transient spectroscopy reveal that the lowest-energy triplet state in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is a highly mixed ³MLCT/³ππ* excited state rather than a triplet metal-centered ligand-field (³LF) excited state; the latter is generally accepted for ligand photodissociation. In addition, Mulliken spin density calculations indicate that a majority of the spin density in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is localized on the isoquinoline arm, which is opposite to the cis MeCN, rather than on the ruthenium center. This significantly weakens the Ru-N6 ( cis MeCN) bond, which then promotes the ligand photodissociation. This newly discovered effect gives a clearer perception of the interplay between the ³MLCT and ³LF excited states of Ru(II) polypyridyl complexes, which may be useful in the design and applications of ruthenium complexes in the areas of photoactivated drug delivery and photosensitizers.

Cargo delivery on demand from photodegradable MOF nano-cages

We report the photo-induced degradation of and cargo release from a nanoscale metal-organic framework (nMOF) incorporating photo-isomerizable 4,4'-azobenzenedicarboxylate (AZB) linkers. The structure matches a UiO-type framework where 12 4,4'-azobenzenedicarboxylate moieties are connected to a Zr₆O₄(OH)₄ cluster, referred to as UiO-AZB. Due to the incorporation of photo-isomerizable struts, the degradation of UiO-AZB is accelerated by irradiation with white light (1.3 ± 0.1% h^-1 under dark conditions vs. 8.4 ± 0.4% h^-1 when irradiated). Additionally, we show slow release of Nile Red (NR) which is triggered by irradiation (0.04 ± 0.01% h^-1 under dark conditions vs. 0.36 ± 0.02% h^-1 when irradiated).

Ru(ii) polypyridyl complexes as photocages for bioactive compounds containing nitriles and aromatic heterocycles

Photocaging allows for precise spatiotemporal control over the release of biologically active compounds with light. Most photocaged molecules employ organic photolabile protecting groups; however, biologically active compounds often contain functionalities such as nitriles and aromatic heterocycles that cannot be caged with organic groups. Despite their prevalence, only a few studies have reported successful caging of nitriles and aromatic heterocycles. Recently, Ru(ii)-based photocaging has emerged as a powerful method for the release of bioactive molecules containing these functional groups, in many cases providing high levels of spatial and temporal control over biological activity. This Feature Article discusses recent developments in applying Ru(ii)-based photocaging towards biological problems. Our groups designed and synthesized Ru(ii)-based platforms for the photoinduced delivery of cysteine protease and cytochrome P450 inhibitors in order to achieve selective control over enzyme inhibition. We also reported Ru(ii) photocaging groups derived from higher-denticity ancillary ligands that possess photophysical and photochemical properties distinct from more traditional Ru(ii)-based caging groups. In addition, for the first time, we are able to rapidly synthesize and screen Ru(ii) polypyridyl complexes that elicit desired properties by solid-phase synthesis. Finally, our work also defined steric and orbital mixing effects that are important factors in controlling photoinduced ligand exchange.

Effects of Methyl Substitution in Ruthenium Tris(2-pyridylmethyl)amine Photocaging Groups for Nitriles

Four complexes of the general formula [Ru(L)(CH3CN)2](PF6)2, [L = TPA (5), MeTPA (6), Me2TPA (7), and Me3TPA (8)] [TPA = tris[(pyridin-2-yl)methyl]amine, where methyl groups were introduced consecutively onto the 6-position of py donors of TPA, were prepared and characterized by various spectroscopic techniques and mass spectrometry. While 5 and 8 were isolated as single stereoisomers, 6 and 7 were isolated as mixtures of stereoisomers in 2:1 and 1.5:1 ratios, respectively. Steric effects on ground state stability and thermal and photochemical reactivities were studied for all four complexes using (1)H NMR and electronic absorption spectroscopies and computational studies. These studies confirmed that the addition of steric bulk accelerates photochemical and thermal nitrile release.

Photoactivation of Diiodido-Pt(IV) Complexes Coupled to Upconverting Nanoparticles

The preparation, characterization, and surface modification of upconverting lanthanide-doped hexagonal NaGdF4 nanocrystals attached to light sensitive diiodido-Pt(IV) complexes is presented. The evaluation for photoactivation and cytotoxicity of the novel carboxylated diiodido-Pt(IV) cytotoxic prodrugs by near-infrared (NIR) light (λ = 980 nm) is also reported. We attempted two different strategies for attachment of light-sensitive diiodido-Pt(IV) complexes to Yb,Er- and Yb,Tm-doped β-NaGdF4 upconverting nanoparticles (UCNPs) in order to provide nanohybrids, which offer unique opportunities for selective drug activation within the tumor cells and subsequent spatiotemporal controlled drug release by NIR-to-visible light-upconversion: (A) covalent attachment of the Pt(IV) complex via amide bond formation and (B) carboxylate exchange of oleate on the surface of the UCNPs with diiodido-Pt(IV) carboxylato complexes. Initial feasibility studies showed that NIR applied by a 980 nm laser had only a slight effect on the stability of the various diiodido-Pt(IV) complexes, but when UCNPs were present more rapid loss of the ligand-metal-charge transfer (LMCT) bands of the diiodido-Pt(IV) complexes was observed. Furthermore, Pt released from the Pt(IV) complexes platinated calf-thymus DNA (ct-DNA) more rapidly when NIR was applied compared to dark controls. Of the two attachment strategies, method A with the covalently attached diiodido-Pt(IV) carboxylates via amide bond formation proved to be the most effective method for generating UCNPs that release Pt when irradiated with NIR; the released Pt was also able to bind irreversibly to calf thymus DNA. Nonetheless, only ca. 20% of the Pt on the surface of the UCNPs was in the Pt(IV) oxidation state, the rest was Pt(II), indicating chemical reduction of the diiodido-Pt(IV) prodrug by the UCNPs. Cytotoxicity studies with the various UCNP-Pt conjugates and constructs, tested on human leukemia HL60 cells in culture, indicated a substantial increase in cytotoxicity when modified UCNPs were combined with five rounds of 30 min irradiation with NIR compared to dark controls, but NIR alone also had a significant cytotoxic effect at this duration.

Photoactivated inhibition of cathepsin K in a 3D tumor model

Collagenolytic activity of cathepsin K is important for many physiological and pathological processes including osteoclast-mediated bone degradation, macrophage function and fibroblast-mediated matrix remodeling. Here, we report application of a light-activated inhibitor for controlling activity of cathepsin K in a 3D functional imaging assay. Using prostate carcinoma cell line engineered to overexpress cathepsin K, we demonstrate the utility of the proteolytic assay in living tumor spheroids for the evaluation and quantification of the inhibitor effects on cathepsin K-mediated collagen I degradation. Importantly, we also show that utilizing the ruthenium-caged version of a potent nitrile cathepsin K inhibitor (4), cis-[Ru(bpy)2(4)2](BF4)2 (5), offers significant advantage in terms of effective concentration of the inhibitor and especially its light-activated control in the 3D assay. Our results suggest that light activation provides a suitable, attractive approach for spatial and temporal control of proteolytic activity, which remains a critical, unmet need in treatment of human diseases, especially cancer.

Excited-State Dynamics of a Two-Photon-Activatable Ruthenium Prodrug

We present a new approach to investigate how the photodynamics of an octahedral ruthenium(II) complex activated through two-photon absorption (TPA) differ from the equivalent complex activated through one-photon absorption (OPA). We photoactivated a Ru(II) polypyridyl complex containing bioactive monodentate ligands in the photodynamic therapy window (620-1000 nm) by using TPA and used transient UV/Vis absorption spectroscopy to elucidate its reaction pathways. Density functional calculations allowed us to identify the nature of the initially populated states and kinetic analysis recovers a photoactivation lifetime of approximately 100 ps. The dynamics displayed following TPA or OPA are identical, showing that TPA prodrug design may use knowledge gathered from the more numerous and easily conducted OPA studies.

Photoactivatable cytotoxic agents derived from mitochondria-targeting luminescent iridium(iii) poly(ethylene glycol) complexes modified with a nitrobenzyl linkage

Novel photoactivatable luminescent iridium(iii) complexes were designed to show minimal cytotoxic activity in the dark and become significantly cytotoxic upon irradiation.

Selective Photodissociation of Acetonitrile Ligands in Ruthenium Polypyridyl Complexes Studied by Density Functional Theory

Metal complexes that release ligands upon photoexcitation are important tools for biological research and show great potential as highly specific therapeutics. Upon excitation with visible light, [Ru(TQA)(MeCN)2](2+) [TQA = tris(2-quinolinylmethyl)amine] exchanges one of the two acetonitriles (MeCNs), whereas [Ru(DPAbpy)MeCN](2+) [DPAbpy = N-(2,2'-bipyridin-6-yl)-N,N-bis(pyridin-2-ylmethyl)amine] does not release MeCN. Furthermore, [Ru(TQA)(MeCN)2](2+) is highly selective for release of the MeCN that is perpendicular to the plane of the two axial quinolines. Density functional theory calculations provide a clear explanation for the photodissociation behavior of these two complexes. Excitation by visible light and intersystem crossing leads to a six-coordinate (3)MLCT state. Dissociation of acetonitrile can occur after internal conversion to a dissociative (3)MC state, which has an occupied dσ* orbital that interacts in an antibonding fashion with acetonitrile. For [Ru(TQA)(MeCN)2](2+), the dissociative (3)MC state is lower than the (3)MLCT state. In contrast, the (3)MC state of [Ru(DPAbpy)MeCN](2+) that releases acetonitrile has an energy higher than that of the (3)MLCT state, indicating dissociation is unfavorable. These results are consistent with the experimental observations that efficient photodissociation of acetonitrile occurs for [Ru(TQA)(MeCN)2](2+) but not for [Ru(DPAbpy)MeCN](2+). For the release of the MeCN ligand in [Ru(TQA)(MeCN)2](2+) that is perpendicular to the axial quinoline rings, the (3)MLCT state has an occupied quinoline π* orbital that can interact with a dσ* Ru-NCCH3 antibonding orbital as the Ru-NCCH3 bond is stretched and the quinolines bend toward the departing acetonitrile. This reduces the barrier for the formation of the dissociative (3)MC state, leading to the selective photodissociation of this acetonitrile. By contrast, when the acetonitrile is in the plane of the quinolines or bpy, no interaction occurs between the ligand π* orbital and the dσ* Ru-NCCH3 orbital, resulting in high barriers for conversion to the corresponding (3)MC structures and no release of acetonitrile.

Solid-phase synthesis as a platform for the discovery of new ruthenium complexes for efficient release of photocaged ligands with visible light

Ruthenium-based photocaging groups have important applications as biological tools and show great potential as therapeutics. A method was developed to rapidly synthesize, screen, and identify ruthenium-based caging groups that release nitriles upon irradiation with visible light. A diverse library of tetra- and pentadentate ligands was synthesized on polystyrene resin. Ruthenium complexes of the general formula [Ru(L)(MeCN)n](m+) (n = 1-3, m = 1-2) were generated from these ligands on solid phase and then cleaved from resin for photochemical analysis. Data indicate a wide range of spectral tuning and reactivity with visible light. Three complexes that showed strong absorbance in the visible range were synthesized by solution phase for comparison. Photochemical behavior of solution- and solid-phase complexes was in good agreement, confirming that the library approach is useful in identifying candidates with desired photoreactivity in short order, avoiding time-consuming chromatography and compound purification.

Caging metal ions with visible light-responsive nanopolymersomes

Polymersomes are bilayer vesicles that self-assemble from amphiphilic diblock copolymers, and provide an attractive system for the delivery of biological and nonbiological molecules due to their environmental compatibility, mechanical stability, synthetic tunability, large aqueous core, and hyperthick hydrophobic membrane. Herein, we report a nanoscale photoresponsive polymersome system featuring a meso-to-meso ethyne-bridged bis[(porphinato)zinc] (PZn2) fluorophore hydrophobic membrane solute and dextran in the aqueous core. Upon 488 nm irradiation in solution or in microinjected zebrafish embryos, the polymersomes underwent deformation, as monitored by a characteristic red-shifted PZn2 emission spectrum and confirmed by cryo-TEM. The versatility of this system was demonstrated through the encapsulation and photorelease of a fluorophore (FITC), as well as two different metal ions, Zn(2+) and Ca(2+).

A photoswitchable supramolecular complex with release-and-report capabilities

A supramolecular platform with an inherent report function has been designed for photoreversible control of guest concentration in solution.

Ruthenium tris(2-pyridylmethyl)amine as an effective photocaging group for nitriles

Ruthenium(II) tris(2-pyridylmethyl)amine (TPA) is an effective caging group for nitriles that provides high levels of control over the enzyme activity with light. Two caged nitriles were prepared, [Ru(TPA)(MeCN)₂](PF₆)₂ (1) and [Ru(TPA)(3)₂](PF₆)₂ (2), where 3 is the cathepsin K inhibitor Cbz-Leu-NHCH₂CN, and characterized by various spectroscopic techniques and mass spectrometry. Both 1 and 2 show the release of a single nitrile within 20 min of irradiation with 365 nm light. Complex 2 acts as a potent, photoactivated inhibitor of human cathepsin K. IC₅₀ values were determined for 2 and 3. Enzyme inhibition for 2 was enhanced by a factor of 89 upon exposure to light, with IC₅₀ values of 63 nM (light) and 5.6 μM (dark).

The photochemical release of nitriles from the caging fragment ruthenium tris(2-pyridylmethyl)amine (TPA) was studied. Caged complexes of the general formula [Ru(TPA)(RCN)₂]²⁺ are stable in the dark but release a single nitrile upon irradiation with 365 nm light. Photoactivated inhibition of cathepsin K was demonstrated with a caged inhibitor complex.

A bis(dipyridophenazine)(2-(2-pyridyl)pyrimidine-4-carboxylic acid)ruthenium(II) complex with anticancer action upon photodeprotection

Improving the selectivity of anticancer drugs towards cancer cells is one of the main goals of drug optimization; the prodrug strategy has been one of the most promising. A light-triggered prodrug strategy is presented as an efficient approach for controlling cytotoxicity of the substitutionally inert cytotoxic complex [Ru(dppz)2(CppH)](PF6)2(C1; CppH=2-(2-pyridyl)pyrimidine-4-carboxylic acid; dppz=dipyrido[3,2-a:2',3'-c]phenazine). Attachment of a photolabile 3-(4,5-dimethoxy-2-nitrophenyl)-2-butyl (DMNPB) ester ("photocaging") makes the otherwise active complex C1 innocuous to both cancerous (HeLa and U2OS) and non-cancerous (MRC-5) cells. The cytotoxic action can be successfully unleashed in living cells upon light illumination (350 nm), reaching similar level of activity as the parent cytotoxic compound C1. This is the first substitutionally inert cytotoxic metal complex to be used as a light-triggered prodrug candidate.

Trans,trans,trans-[PtIV(N3)2(OH)2(py)(NH3)]: a light-activated antitumor platinum complex that kills human cancer cells by an apoptosis-independent mechanism

Photoactivatable Pt(IV) diazido complexes have unusual photobiologic properties. We show here that trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(py)(NH(3))] complex 3 is a potent photoactivated cytotoxin toward human cancer cells in culture, with an average IC(50) value in 13 cell lines of 55 ± 28 μmol/L after 30 minutes (0.12 mW/cm(2)) photoactivation with UVA, although visible light was also effective. Photoactivated complex 3 was noncross-resistant to cisplatin in 3 of 4 resistant cell lines. Cell swelling but very little blebbing was seen for HL60 cells treated with irradiated complex 3. Unlike cisplatin and etoposide, both of which cause apoptosis in HL60 cells, no apoptosis was observed for UVA-activated complex 3 by the Annexin V/propidium iodide flow cytotometry assay. Changes in the levels of the autophagic proteins LC3B-II and p62 in HL60 cells treated with UVA-activated complex 3 indicate autophagy is active during cell death. In a clonogenic assay with the SISO human cervix cancer cell line, 3 inhibited colony formation when activated by UVA irradiation. Antitumor activity of complex 3 in mice bearing xenografted OE19 esophageal carcinoma tumors was photoaugmented by visible light. Insights into the novel reaction pathways of complex 3 have been obtained from (14)N{(1)H} nuclear magnetic resonance studies, which show that photoactivation pathways can involve release of free azide in buffered solution. Density functional theory (DFT) and time-dependent DFT calculations revealed the dissociative character of singlet and triplet excited states of complex 3, which gives rise to reactive, possibly cytotoxic azidyl radicals.

Traceless cross-linker for photocleavable bioconjugation

Photoresponsive bioconjugation empowers the development of novel methods for drug discovery, disease diagnosis, and high-throughput screening, among others. In this paper, we report on the characteristics of a traceless photocleavable cross-linker, di-6-(3-succinimidyl carbonyloxymethyl-4-nitro-phenoxy)-hexanoic acid disulfide diethanol ester (SCNE). The traceless feature and the biocompatibility of this photocleavable cross-linking reagent were corroborated. Consequently, we demonstrated its application in reversible phage particle immobilization that could provide a platform for direct single-phage screening. We also applied it in protein-photoprinting, where SCNE acts as a "photo-eraser" to remove the cross-linked protein molecules at a desired region in a simple, clean, and light-controllable fashion. We further demonstrated the two-tier atomic force microscopic (AFM) method that uses SCNE to carry out two subsequent AFM tasks in situ. The approach allows guided protein delivery and subsequent high-resolution imaging at the same local area, thus opening up the possibility of monitoring protein functions in live cells. The results imply that SCNE is a versatile cross-linker that can be used for a wide range of applications where photocleavage ensures clean and remote-controllable release of biological molecules from a substrate.