Control and utilization of ruthenium and rhodium metal complex excited states for photoactivated cancer therapy

Vitamin B12: a tunable, long wavelength, light-responsive platform for launching therapeutic agents

Light-responsive agents offer the promise of targeted therapy, whose benefits include (i) prolonged action at the target site, (ii) overall reduced systemic dosage, (iii) reduced adverse effects, and (iv) localized delivery of multiple agents. Although photoactivated prodrugs have been reported, these species generally require short wavelengths (<450 nm) for activation. However, maximal tissue penetrance by light occurs within the "optical window of tissue" (600-900 nm), well beyond the wavelength range of most existing photocleavable functional groups. Furthermore, since multidrug therapy holds promise for the treatment of complex diseases, from cancer to neurological disorders, controlling the action of multiple drugs via wavelength modulation would take advantage of a property that is unique to light. However, discrimination between existing photoresponsive moieties has thus far proven to be limited. We have developed a vitamin B12/light-facilitated strategy for controlling drug action using red, far-red, and NIR light. The technology is based on a light-triggered reaction displayed by a subset of B12 derivatives: alkyl-cob(III)alamins suffer photohomolysis of the C-Co(III) bond. The C-Co(III) bond is weak (<30 kcal/mol), and therefore all wavelengths absorbed by the corrin ring (330-580 nm) induce photocleavage. In addition, by appending fluorophores to the corrin ring, long wavelength light (>600 nm) is readily captured and used to separate the Co-appended ligand (e.g., a drug) from B12. Consequently, it is now feasible to preassign the wavelength of homolysis by simply installing a fluorescent antenna with the desired photophysical properties. The wavelength malleability inherent within this strategy has been used to construct photoresponsive compounds that launch different drugs by simply modulating the wavelength of illumination. In addition, these phototherapeutics have been installed on the surface and interior of cells, such as erythrocytes or neural stem cells, and released upon expoure to the appropriate wavelength. We have shown that cytotoxic agents, such as doxorubicin, anti-inflammatories, such as dexamethasone, and anti- and pro-vascular agents are readily released from cellular vehicles as biologically active agents. We have also demonstrated that the concept of "optical window of tissue" phototherapeutics is not just limited to prodrugs. For example, stem cells have received considerable attention in the area of regenerative medicine. Hydrogels serve as scaffolds for stem cell growth and differentiation. We have shown that the formation of hydrogels can be triggered, in the presence of cells, using appropriately designed alkyl-cob(III)alamins and long wavelength light. The potential applications of phototherapeutics are broad and include drug delivery for a variety of indications, tissue engineering, and surgery.

5-Arylvinyl-2,2'-bipyridyls: Bright "push-pull" dyes as components in fluorescent indicators for zinc ions

This article reviews the zinc(II)-dependent photophysical properties of arylvinylbipyridines (AVBs), a class of fluoroionophores in which 2,2'-bipyridyl and an aryl moiety are electronically conjugated. Zinc(II) binding of an AVB may lead to an emission bathochromic shift of the fluoroionophore without diminishing its fluorescence quantum yield. This observation can be explained using the excited state model of electron donor-π bridge-electron acceptor "push-pull" fluorophores, in which the bipy moiety acts as an electron acceptor, and zinc(II)-coordination strengthens its electron affinity. The spectral sensitivity of bipy-containing fluoroionophores, such as AVBs, to zinc(II) can be exploited to prepare fluorescent indicators for this ion. In several cases, AVB moieties are incorporated in fluorescent heteroditopic ligands, so that the variation of zinc(II) concentration over a relatively large range can be correlated to fluorescence changes in either intensity or color. AVB fluoroionophores are also used to introduce an intramolecular Förster resonance energy transfer (FRET) strategy for creating zinc(II) indicators with high photostability and a narrow emission band, two desired characteristics of dyes used in fluorescence microscopy.

Identification of an iridium(III) complex with anti-bacterial and anti-cancer activity

Group 9 transition metal complexes have been widely explored as therapeutic agents due to their unique geometry, their propensity to undergo ligand exchanges with biomolecules and their diverse steric and electronic properties. These metal complexes can offer distinct modes of action in living organisms compared to carbon-based molecules. In this study, we investigated the antimicrobial and anti-proliferative abilities of a series of cyclometallated iridium(III) complexes. The iridium(III) complex 1 inhibited the growth of S. aureus with MIC and MBC values of 3.60 and 7.19 μM, respectively, indicating its potent bactericidal activity. Moreover, complex 1 also exhibited cytotoxicity against a number of cancer cell lines, with particular potency against ovarian, cervical and melanoma cells. This cyclometallated iridium(III) complex is the first example of a substitutionally-inert, Group 9 organometallic compound utilized as a direct and selective inhibitor of S. aureus.

Synthesis, characterization and biological evaluation of mixed-ligand ruthenium(II) complexes for photodynamic therapy

This study investigated the photodynamic therapy (PDT) and anticancer activity of mixed ligand Ru(ii) terpyridyl complexes (Ru1-Ru3). The photophysical and photochemical properties, hydrophobic properties, DNA binding and DNA transcription inhibition abilities, cell uptake efficiency, cellular localization and photo-cytotoxicity were investigated. Ru1-Ru3 exhibited red luminescence between 670-710 nm and functioned as photo-sensitizers (PSs) by generating both singlet oxygen and radical ions. Without light activation, Ru1-Ru3 were located at the cytoplasm and were nontoxic to cells. However, upon light activation, Ru1-Ru3 exhibited significant photocytotoxicity. After PDT treatment, mitochondria alteration and nuclear membrane disruption occurred, which resulted in relocalization of the complexes from the cytoplasm to the nucleus. Moreover, high cellular oxidative stress caused cell necrocytosis after PDT treatment.

New Ru(II) complexes for dual photoreactivity: ligand exchange and (1)O2 generation

Uncovering the factors that govern the electronic structure of Ru(II)-polypyridyl complexes is critical in designing new compounds for desired photochemical reactions, and strategies to tune excited states for ligand dissociation and (1)O2 production are discussed herein. The generally accepted mechanism for photoinduced ligand dissociation proposes that population of the dissociative triplet ligand field ((3)LF) state proceeds through thermal population from the vibrationally cooled triplet metal-to-ligand charge transfer ((3)MLCT) state; however, temperature-dependent emission spectroscopy provides varied activation energies using the emission and ligand exchange quantum yields for [Ru(bpy)2(L)2](2+) (bpy = 2,2'-bipyridine; L = CH3CN or py). This suggests that population of the (3)LF state proceeds from the vibrationally excited (3)MLCT state. Because the quantum yield of ligand dissociation for nitriles is much more efficient than that for py, steric bulk was introduced into the ligand set to distort the pseudo-octahedral geometry and lower the energy of the (3)LF state. The py dissociation quantum yield with 500 nm irradiation in a series of [Ru(tpy)(NN)(py)](2+) complexes (tpy = 2,2':6',2″-terpyridine; NN = bpy, 6,6'-dimethyl-2,2'-bipyridine (Me2bpy), 2,2'-biquinoline (biq)) increases by 2-3 orders of magnitude with the sterically bulky Me2bpy and biq ligands relative to bpy. Ultrafast transient absorption spectroscopy reveals population of the (3)LF state within 3-7 ps when NN is bulky, and density functional theory calculations support stabilized (3)LF states. Dual activity via ligand dissociation and (1)O2 production can be achieved by careful selection of the ligand set to tune the excited-state dynamics. Incorporation of an extended π system in Ru(II) complexes such as [Ru(bpy)(dppn)(CH3CN)2](2+) (dppn = benzo[i]dipyrido[3,2-a:2',3'-c]phenazine) and [Ru(tpy)(Me2dppn)(py)](2+) (Me2dppn = 3,6-dimethylbenzo[i]dipyrido[3,2-a:2',3'-c]phenazine) introduces low-lying, long-lived dppn/Me2dppn (3)ππ* excited states that generate (1)O2. Similar to [Ru(bpy)2(CH3CN)2](2+), photodissociation of CH3CN occurs upon irradiation of [Ru(bpy)(dppn)(CH3CN)2](2+), although with lower efficiency because of the presence of the (3)ππ* state. The steric bulk in [Ru(tpy)(Me2dppn)(py)](2+) is critical in facilitating the photoinduced py dissociation, as the analogous complex [Ru(tpy)(dppn)(py)](2+) produces (1)O2 with near-unit efficiency. The ability to tune the relative energies of the excited states provides a means to design potentially more active drugs for photochemotherapy because the photorelease of drugs can be coupled to the therapeutic action of reactive oxygen species, effecting cell death via two different mechanisms. The lessons learned about tuning of the excited-state properties can be applied to the use of Ru(II)-polypyridyl compounds in a variety of applications, such as solar energy conversion, sensors and switches, and molecular machines.

Metal complexes of curcumin for cellular imaging, targeting, and photoinduced anticancer activity

Curcumin is a polyphenolic species. As an active ingredient of turmeric, it is well-known for its traditional medicinal properties. The therapeutic values include antioxidant, anti-inflammatory, antiseptic, and anticancer activity with the last being primarily due to inhibition of the transcription factor NF-κB besides affecting several biological pathways to arrest tumor growth and its progression. Curcumin with all these positive qualities has only remained a potential candidate for cancer treatment over the years without seeing any proper usage because of its hydrolytic instability involving the diketo moiety in a cellular medium and its poor bioavailability. The situation has changed considerably in recent years with the observation that curcumin in monoanionic form could be stabilized on binding to a metal ion. The reports from our group and other groups have shown that curcumin in the metal-bound form retains its therapeutic potential. This has opened up new avenues to develop curcumin-based metal complexes as anticancer agents. Zinc(II) complexes of curcumin are shown to be stable in a cellular medium. They display moderate cytotoxicity against prostate cancer and neuroblastoma cell lines. A similar stabilization and cytotoxic effect is reported for (arene)ruthenium(II) complexes of curcumin against a variety of cell lines. The half-sandwich 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1]decane (RAPTA)-type ruthenium(II) complexes of curcumin are shown to be promising cytotoxic agents with low micromolar concentrations for a series of cancer cell lines. In a different approach, cobalt(III) complexes of curcumin are used for its cellular delivery in hypoxic tumor cells using intracellular agents that reduce the metal and release curcumin as a cytotoxin. Utilizing the photophysical and photochemical properties of the curcumin dye, we have designed and synthesized photoactive curcumin metal complexes that are used for cellular imaging by fluorescence microscopy and damaging the cancer cells on photoactivation in visible light while being minimally toxic in darkness. In this Account, we have made an attempt to review the current status of the chemistry of metal curcumin complexes and present results from our recent studies on curcumin complexes showing remarkable in vitro photocytotoxicity. The undesirable dark toxicity of the complexes can be reduced with suitable choice of the metal and the ancillary ligands in a ternary structure. The complexes can be directed to specific subcellular organelles. Selectivity by targeting cancer cells over normal cells can be achieved with suitable ligand design. We expect that this methodology is likely to provide an impetus toward developing curcumin-based photochemotherapeutics for anticancer treatment and cure.

Light-activated protein inhibition through photoinduced electron transfer of a ruthenium(II)-cobalt(III) bimetallic complex

We describe a mechanism of light activation that initiates protein inhibitory action of a biologically inert Co(III) Schiff base (Co(III)-sb) complex. Photoinduced electron transfer (PET) occurs from a Ru(II) bipyridal complex to a covalently attached Co(III) complex and is gated by conformational changes that occur in tens of nanoseconds. Reduction of the Co(III)-sb by PET initiates displacement of the inert axial imidazole ligands, promoting coordination to active site histidines of α-thrombin. Upon exposure to 455 nm light, the rate of ligand exchange with 4-methylimidazole, a histidine mimic, increases by approximately 5-fold, as observed by NMR spectroscopy. Similarly, the rate of α-thrombin inhibition increases over 5-fold upon irradiation. These results convey a strategy for light activation of inorganic therapeutic agents through PET utilizing redox-active metal centers.

Hyaluronan/Ru(ii)-cyclodextrin supramolecular assemblies for colorimetric sensor of hyaluronidase activity

A hyaluronidase-induced colorimetric change was found in a hyaluronan/Ru(ii)-cyclodextrin supramolecular assembly under a laser (532 nm) irradiation.

A dinuclear Ru(ii) complex capable of photoinduced ligand exchange at both metal centers

{[Ru(CH₃CN)₃]₂(tppz)}⁴⁺ (tppz = tetra-2-pyridylpyrazine) undergoes photoinduced CH₃CN exchange with λ_irr ≥ 610 nm in H₂O from both metal centers.