Photosensitive Ru(II) Complexes as Inhibitors of the Major Human Drug Metabolizing Enzyme CYP3A4

Leveraging the Photofunctions of Transition Metal Complexes for the Design of Innovative Phototherapeutics

Despite the advent of various medical interventions for cancer treatment, the disease continues to pose a formidable global health challenge, necessitating the development of new therapeutic approaches for more effective treatment outcomes. Photodynamic therapy (PDT), which utilizes light to activate a photosensitizer to produce cytotoxic reactive oxygen species (ROS) for eradicating cancer cells, has emerged as a promising approach for cancer treatment due to its high spatiotemporal precision and minimal invasiveness. However, the widespread clinical use of PDT faces several challenges, including the inefficient production of ROS in the hypoxic tumor microenvironment, the limited penetration depth of light in biological tissues, and the inadequate accumulation of photosensitizers at the tumor site. Over the past decade, there has been increasing interest in the utilization of photofunctional transition metal complexes as photosensitizers for PDT applications due to their intriguing photophysical and photochemical properties. This review provides an overview of the current design strategies used in the development of transition metal complexes as innovative phototherapeutics, aiming to address the limitations associated with PDT and achieve more effective treatment outcomes. The current challenges and future perspectives on the clinical translation of transition metal complexes are also discussed.

Mixed Ru(II)-Ir(III) Complexes as Photoactive Inhibitors of the Major Human Drug Metabolizing Enzyme CYP3A4

Cytochrome P450 3A4 (CYP3A4) is a crucial enzyme in human drug metabolism. To garner photochemical control over the inhibition of CYP3A4, a potent Ir(III)-based inhibitor of CYP3A4 was complexed with two Ru(II)-based photocaging groups. Chemical, photochemical, and biological properties of the photocaged inhibitors were characterized. Importantly, mixed Ru(II)-Ir(III) complexes strongly absorb green light, which facilitates the photochemical release of the Ir(III) inhibitor from the Ru(II) caging fragment [Ru(tpy)(Me2bpy)]2+, where tpy = 2,2':6',2″-terpyridine and Me2bpy = 6,6'-dimethyl-2,2'-bipyridine. Emission turn on, type II heme binding, and more potent inhibition under light vs dark conditions were observed. The study also demonstrated that a Ru(II)-Ir(III) conjugate can be photoactivated to exert cytotoxic effects on MCF-7 breast cancer cells upon green light exposure. Additionally, a synthesized analogue with one [Ru(TPA)]2+ fragment (TPA = tris(pyridin-2-ylmethyl)amine) and two Ir(III) centers, although resistant to photochemical release, showed strong inhibition of CYP3A4 both in purified form and in CYP3A4-overexpressing HepG2 cells, with nanomolar potency. These mixed Ru(II)-Ir(III) compounds can permeate cell membranes and inhibit CYP3A4, presenting a new class of bioactive compounds.

Computational Exploration of the Mechanism of Action of a Sorafenib-Containing Ruthenium Complex as an Anticancer Agent for Photoactivated Chemotherapy

Ruthenium(II) polypyridyl complexes are being tested as potential anticancer agents in different therapies, which include conventional chemotherapy and light-activated approaches. A mechanistic study on a recently synthesized dual-action Ru(II) complex [Ru(bpy)2(sora)Cl]+ is described here. It is characterized by two mono-dentate leaving ligands, namely, chloride and sorafenib ligands, which make it possible to form a di-aquo complex able to bind DNA. At the same time, while the released sorafenib can induce ferroptosis, the complex is also able to act as a photosensitizer according to type II photodynamic therapy processes, thus generating one of the most harmful cytotoxic species, 1O2. In order to clarify the mechanism of action of the drug, computational strategies based on density functional theory are exploited. The photophysical properties of the complex, which include the absorption spectrum, the kinetics of ISC, and the character of all the excited states potentially involved in 1O2 generation, as well as the pathway providing the di-aquo complex, are fully explored. Interestingly, the outcomes show that light is needed to form the mono-aquo complex, after releasing both chloride and sorafenib ligands, while the second solvent molecule enters the coordination sphere of the metal once the system has come back to the ground-state potential energy surface. In order to simulate the interaction with canonical DNA, the di-aquo complex interaction with a guanine nucleobase as a model has also been studied. The whole study aims to elucidate the intricate details of the photodissociation process, which could help with designing tailored metal complexes as potential anticancer agents.

Ru(II) Complexes with Absorption in the Photodynamic Therapy Window: 1O2 Sensitization, DNA Binding, and Plasmid DNA Photocleavage

Two Ru(II) complexes, [Ru(pydppn)(bim)(py)]2+ [2; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene; bim = 2,2'-bisimidazole; py = pyridine] and [Ru(pydppn)(Me4bim)(py)]2+ [3; Me4bim = 2,2'-bis(4,5-dimethylimidazole)], were synthesized and characterized, and their photophysical properties, DNA binding, and photocleavage were evaluated and compared to [Ru(pydppn)(bpy)(py)]2+ (1; bpy = 2,2'-bipyridine). Complexes 2 and 3 exhibit broad 1MLCT (metal-to-ligand charge transfer) transitions with maxima at ∼470 nm and shoulders at ∼525 and ∼600 nm that extend to ∼800 nm. These bands are red-shifted relative to those of 1, attributed to the π-donating ability of the bim and Me4bim ligands. A strong signal at 550 nm is observed in the transient absorption spectra of 1-3, previously assigned as arising from a pydppn-centered 3ππ* state, with lifetimes of ∼19 μs for 1 and 2 and ∼270 ns for 3. A number of methods were used to characterize the mode of binding of 1-3 to DNA, including absorption titrations, thermal denaturation, relative viscosity changes, and circular dichroism, all of which point to the intercalation of the pydpppn ligand between the nucleobases. The photocleavage of plasmid pUC19 DNA was observed upon the irradiation of 1-3 with visible and red light, attributed to the sensitized generation of 1O2 by the complexes. These findings indicate that the bim ligand, together with pydppn, serves to shift the absorption of Ru(II) complexes to the photodynamic therapy window, 600-900 nm, and also extend the excited state lifetimes for the efficient production of cytotoxic singlet oxygen.

Modulating Excited State Properties and Ligand Ejection Kinetics in Ruthenium Polypyridyl Complexes Designed to Mimic Photochemotherapeutics

Ruthenium(II) polypyridyl complexes have gained significant interest as photochemotherapeutics (PCTs) due to their synthetic viability, strong light absorption, well understood excited state properties, and high phototoxicity indexes. Herein, we report the synthesis, characterization, electrochemical, spectrochemical, and preliminary cytotoxicity analyses of three series of ruthenium(II) polypyridyl complexes designed to mimic PCTs. The three series have the general structure of [Ru(bpy)2(N-N)]2+ (Series 1), [Ru(bpy)(dmb)(N-N)]2+ (Series 2), and [Ru(dmb)2(N-N)]2+ (Series 3, where N-N is a bidentate polypyridyl ligand, bpy = 2,2'-bipyridine, and dmb = 6,6'-dimethyl-2,2'-bipyridine). In the three series, the N-N ligand was systematically modified to incorporate increased conjugation and/or electronegative heteroatoms to increase dπ-π* backbonding, red-shifting the lowest energy metal-to-ligand charge transfer (MLCT) absorptions from λmax = 454 to λmax = 580 nm, nearing the therapeutic window for PCTs (600-1100 nm). In addition, steric bulk was systematically introduced through the series, distorting the Ru(II) octahedra, making the dissociative 3dd* state thermally accessible at room and body temperatures. This resulted in a 4 orders of magnitude increase in photoinduced ligand ejection kinetics, and demonstrates the ability to modulate both the MLCT* and dd* manifolds in the complexes, which is critical in PCT drug design. Preliminary cell viability assays suggest that the increased steric bulk to lower the 3dd* states may interfere with the cytotoxicity mechanism, limiting photoinitiated toxicity of the complexes. This work demonstrates the importance of understanding both the MLCT* and dd* manifolds and how they impact the ability of a complex to act as a PCT agent.

A 700 nm LED Light Activated Ru(II) Complex Destroys Tumor Cytoskeleton via Photosensitization and Photocatalysis

Photoactivable chemotherapy (PACT) using metallic complexes provides spatiotemporal selectivity over drug activation for targeted anticancer therapy. However, the poor absorption in near-infrared (NIR) light region of most metallic complexes renders tissue penetration challenging. Herein, an NIR light triggered dinuclear photoactivable Ru(II) complex (Ru2) is presented and the antitumor mechanism is comprehensively investigated. The introduction of a donor-acceptor-donor (D-A-D) linker greatly enhances the intramolecular charge transition, resulting in a high molar extinction coefficient in the NIR region with an extended triplet excited state lifetime. Most importantly, when activated by 700 nm NIR light, Ru2 exhibits unique slow photodissociation kinetics that facilitates synergistic photosensitization and photocatalytic activity to destroy diverse intracellular biomolecules. In vitro and in vivo experiments show that when activated by 700 nm NIR light, Ru2 exhibits nanomolar photocytotoxicity toward 4T1 cancer cells via the induction of calcium overload and endoplasmic reticulum (ER) stress. These findings provide a robust foundation for the development of NIR-activated Ru(II) PACT complexes for phototherapeutic application.

Photoactivatable Ruthenium Complexes Containing Minimal Straining Benzothiazolyl-1,2,3-triazole Chelators for Cancer Treatment

Ruthenium(II) complexes containing diimine ligands have contributed to the development of agents for photoactivated chemotherapy. Several approaches have been used to obtain photolabile Ru(II) complexes. The two most explored have been the use of monodentate ligands and the incorporation of steric effects between the bidentate ligands and the Ru(II). However, the introduction of electronic effects in the ligands has been less explored. Herein, we report a systematic experimental, theoretical, and photocytotoxicity study of a novel series of Ru(II) complexes Ru1-Ru5 of general formula [Ru(phen)2(N∧N')]2+, where N∧N' are different minimal strained ligands based on the 1-aryl-4-benzothiazolyl-1,2,3-triazole (BTAT) scaffold, being CH3 (Ru1), F (Ru2), CF3 (Ru3), NO2 (Ru4), and N(CH3)2 (Ru5) substituents in the R4 of the phenyl ring. The complexes are stable in solution in the dark, but upon irradiation in water with blue light (λex = 465 nm, 4 mW/cm2) photoejection of the ligand BTAT was observed by HPLC-MS spectrometry and UV-vis spectroscopy, with t1/2 ranging from 4.5 to 14.15 min depending of the electronic properties of the corresponding BTAT, being Ru4 the less photolabile (the one containing the more electron withdrawing substituent, NO2). The properties of the ground state singlet and excited state triplet of Ru1-Ru5 have been explored using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. A mechanism for the photoejection of the BTAT ligand from the Ru complexes, in H2O, is proposed. Phototoxicity studies in A375 and HeLa human cancer cell lines showed that the new Ru BTAT complexes were strongly phototoxic. An enhancement of the emission intensity of HeLa cells treated with Ru5 was observed in response to increasing doses of light due to the photoejection of the BTAT ligand. These studies suggest that BTAT could serve as a photocleavable protecting group for the cytotoxic bis-aqua ruthenium warhead [Ru(phen)2(OH2)2]2+.

The Impact of Inorganic Systems and Photoactive Metal Compounds on Cytochrome P450 Enzymes and Metabolism: From Induction to Inhibition

While cytochrome P450 (CYP; P450) enzymes are commonly associated with the metabolism of organic xenobiotics and drugs or the biosynthesis of organic signaling molecules, they are also impacted by a variety of inorganic species. Metallic nanoparticles, clusters, ions, and complexes can alter CYP expression, modify enzyme interactions with reductase partners, and serve as direct inhibitors. This commonly overlooked topic is reviewed here, with an emphasis on understanding the structural and physiochemical basis for these interactions. Intriguingly, while both organometallic and coordination compounds can act as potent CYP inhibitors, there is little evidence for the metabolism of inorganic compounds by CYPs, suggesting a potential alternative approach to evading issues associated with rapid modification and elimination of medically useful compounds.

Ion Pairing Enables Targeted Prodrug Activation via Red Light Photocatalysis: A Proof-of-Concept Study with Anticancer Gold Complexes

Photocatalysis has found increasing applications in biological systems, for example, in localized prodrug activation; however, high-energy light is usually required without giving sufficient efficiency and target selectivity. In this work, we report that ion pairing between photocatalysts and prodrugs can significantly improve the photoactivation efficiency and enable tumor-targeted activation by red light. This is exemplified by a gold-based prodrug (1d) functionalized with a morpholine moiety. Such a modification causes 1d to hydrolyze in aqueous solution, forming a cationic species that tightly interacts with anionic photosensitizers including Eosin Y (EY) and Rose Bengal (RB), along with a significant bathochromic shift of absorption tailing to the far-red region. As a result, a high photoactivation efficiency of 1d by EY or RB under low-energy light was found, leading to an effective release of active gold species in living cells, as monitored by a gold-specific biosensor (GolS-mCherry). Importantly, the morpholine moiety, with pKa ∼6.9, in 1d brings in a highly pH-sensitive and preferential ionic interaction under a slightly acidic condition over the normal physiological pH, enabling tumor-targeted prodrug activation by red light irradiation in vitro and in vivo. Since a similar absorption change was found in other morpholine/amine-containing clinic drugs, photocages, and precursors of reactive labeling intermediates, it is believed that the ion-pairing strategy could be extended for targeted activation of different prodrugs and for mapping of an acidic microenvironment by low-energy light.

Red-Light Activation of a Microtubule Polymerization Inhibitor via Amide Functionalization of the Ruthenium Photocage

Photoactivated chemotherapy (PACT) is a promising cancer treatment modality that kills cancer cells via photochemical uncaging of a cytotoxic drug. Most ruthenium-based photocages used for PACT are activated with blue or green light, which penetrates sub-optimally into tumor tissues. Here, we report amide functionalization as a tool to fine-tune the toxicity and excited states of a terpyridine-based ruthenium photocage. Due to conjugation of the amide group with the terpyridine π system in the excited state, the absorption of red light (630 nm) increased 8-fold, and the photosubstitution rate rose 5-fold. In vitro, red light activation triggered inhibition of tubulin polymerization, which led to apoptotic cell death both in normoxic (21 % O2 ) and hypoxic (1 % O2 ) cancer cells. In vivo, red light irradiation of tumor-bearing mice demonstrated significant tumor volume reduction (45 %) with improved biosafety, thereby demonstrating the clinical potential of this compound.

MetalDock: An Open Access Docking Tool for Easy and Reproducible Docking of Metal Complexes

Despite the proven potential of metal complexes as therapeutics, the lack of computational tools available for the high-throughput screening of their interactions with proteins is a limiting factor toward clinical developments. To address this challenge, we introduce MetalDock, an easy-to-use, open access docking software for docking metal complexes to proteins. Our tool integrates the AutoDock docking engine with three well-known quantum software packages to automate the docking of metal-organic complexes to proteins. We used a Monte Carlo sampling scheme to obtain the missing Lennard-Jones parameters for 12 metal atom types and demonstrated that these parameters generalize exceptionally well. Our results show that the poses obtained by MetalDock are highly accurate, as they predict the binding geometries experimentally determined by crystal structures with high spatial reproducibility. Three different case studies are presented that demonstrate the versatility of MetalDock for the docking of diverse metal-organic compounds to different biomacromolecules, including nucleic acids.

Carbon network-hosted porphyrin as a highly biocompatible nanophotosensitizer for enhanced photodynamic therapy

Photodynamic therapy (PDT) has the characteristics of being simple and non-invasive, and with on-demand light control. However, most photosensitizers exhibit strong hydrophobicity, low quantum yields in water and low tumor selectivity. In this study, carbon network-hosted porphyrins (CPs) with high biocompatibility and efficient singlet oxygen (1O2) generation were developed to reduce the biotoxicity of photosensitizers and avoid quenching caused by hydrophobic aggregation for enhanced PDT. The CPs were prepared by a simple solid-phase synthesis method using porphyrin, green non-toxic citric acid and urea as the raw materials. The CPs exhibited excellent water solubility and high biocompatibility. Even when the concentration reached 1.5 mg mL-1, cells still had good biological activity. By separately fixing the porphyrins in the carbon network, the CPs avoided aggregation-induced inactivation and had high generation efficiency of 1O2. Furthermore, in order to improve the PDT effect, the CPs were modified with the upper nuclear targeting peptide TAT (T-CPs), which was used to target the nucleus and generate 1O2in situ to directly destroy genetic material. The proposed strategy provides a simple and green path to prepare nanophotosensitizers with high biocompatibility and efficient 1O2 generation for PDT.

Ruthenium-Based Photoactivated Chemotherapy

Ruthenium(II) polypyridyl complexes form a vast family of molecules characterized by their finely tuned photochemical and photophysical properties. Their ability to undergo excited-state deactivation via photosubstitution reactions makes them quite unique in inorganic photochemistry. As a consequence, they have been used, in general, for building dynamic molecular systems responsive to light but, more particularly, in the field of oncology, as prodrugs for a new cancer treatment modality called photoactivated chemotherapy (PACT). Indeed, the ability of a coordination bond to be selectively broken under visible light irradiation offers fascinating perspectives in oncology: it is possible to make poorly toxic agents in the dark that become activated toward cancer cell killing by simple visible light irradiation of the compound inside a tumor. In this Perspective, we review the most important concepts behind the PACT idea, the relationship between ruthenium compounds used for PACT and those used for a related phototherapeutic approach called photodynamic therapy (PDT), and we discuss important questions about real-life applications of PACT in the clinic. We conclude this Perspective with important challenges in the field and an outlook.

Antitumor studies evaluation of triphenylphosphine ruthenium complexes with 5,7-dihalo-substituted-8-quinolinoline targeting mitophagy pathways

Both ruthenium-containing complexes and 8-quinolinoline compounds have emerged as a potential novel agent for malignant tumor therapy. Here, three triphenylphosphine ruthenium complexes, [Ru(ZW1)(PPh3)2Cl2] (PPh3 = triphenylphosphine) (RuZ1), [Ru(ZW2)(PPh3)2Cl2] (RuZ2) and [Ru(ZW2)2(PPh3)Cl2]·CH2Cl2 (RuZ3) bearing 5,7-dichloro-8-quinolinol (H-ZW1) and 5,7-dichloro-8-hydroxyquinaldine (H-ZW2), have been synthesized, characterized and tested for their anticancer potential. We showed that triphenylphosphine ruthenium complexes RuZ1-RuZ3 impaired the cell viability of ovarian adenocarcinoma cisplatin-resistant SK-OV-3/DDP (SKO3CR) and SK-OV-3 (SKO3) cancer cells with greater selectivity and specificity than cisplatin. In addition, RuZ1-RuZ3 show higher excellent cytotoxicity than cisplatin towards SKO3CR cells, with IC50 values of 9.66 ± 1.08, 4.05 ± 0.67 and 7.18 ± 0.40 μM, respectively, in which the SKO3CR cells was the most sensitive to RuZ1-RuZ3. Depending on the substituent type, the antiproliferative ability of RuZ1-RuZ3 followed the trend: -CH3 > -H. However, RuZ1-RuZ3 have no obvious toxicity to normal cell HL-7702. Besides, RuZ1 and RuZ2 could induce mitophagy related-apoptosis pathways through suppression of mitochondrial membrane potential (ΔΨm), accumulation of [Ca2+] and reactive oxygen species (ROS), and regulation of LC3 II/LC3 I, Beclin-1, P62, FUNDC1, PINK1, Parkin, cleaved-caspase-3, caspase-9 and cytochrome c signaling pathway, and hindering the preparation of mitochondrial respiration complexes I and IV and ATP levels. Mechanistic study revealed that RuZ1 and RuZ2 induce apoptosis in SKO3CR cells via mitophagy related-apoptosis pathways induction and energy (ATP) generation disturbance. Taken together, the studied triphenylphosphine ruthenium complexes RuZ1-RuZ3 are promising chemotherapeutic agents with high effectiveness and low toxicity.

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2':6',2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT-3MC Intersystem Crossing Pathway

The synthesis, electrochemistry, and photophysical characterization of five 2,2':6',2″-terpyridine ruthenium complexes (Ru-tpy complexes) is reported. The electrochemical and photophysical behavior varied depending on the ligands, i.e., amine (NH₃), acetonitrile (AN), and bis(pyrazolyl)methane (bpm), for this series of Ru-tpy complexes. The target [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ complexes were found to have low-emission quantum yields in low-temperature observations. To better understand this phenomenon, density functional theory (DFT) calculations were performed to simulate the singlet ground state (S₀), T_e, and metal-centered excited states (³MC) of these complexes. The calculated energy barriers between T_e and the low-lying ³MC state for [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ provided clear evidence in support of their emitting state decay behavior. Developing a knowledge of the underlying photophysics of these Ru-tpy complexes will allow new complexes to be designed for use in photophysical and photochemical applications in the future.

Dynamic Ir(III) Photosensors for the Major Human Drug-Metabolizing Enzyme Cytochrome P450 3A4

Probing the activity of cytochrome P450 3A4 (CYP3A4) is critical for monitoring the metabolism of pharmaceuticals and identifying drug-drug interactions. A library of Ir(III) probes that detect occupancy of the CYP3A4 active site were synthesized and characterized. These probes show selectivity for CYP3A4 inhibition, low cellular toxicity, Kd values as low as 9 nM, and are highly emissive with lifetimes up to 3.8 μs in cell growth media under aerobic conditions. These long emission lifetimes allow for time-resolved gating to distinguish probe from background autofluorescence from growth media and live cells. X-ray crystallographic analysis revealed structure-activity relationships and the preference or indifference of CYP3A4 toward resolved stereoisomers. Ir(III)-based probes show emission quenching upon CYP3A4 binding, then emission increases following displacement with CYP3A4 inhibitors or substrates. Importantly, the lead probes inhibit the activity of CYP3A4 at concentrations as low as 300 nM in CYP3A4-overexpressing HepG2 cells that accurately mimic human hepatic drug metabolism. Thus, the Ir(III)-based agents show promise as novel chemical tools for monitoring CYP3A4 active site occupancy in a high-throughput manner to gain insight into drug metabolism and drug-drug interactions.

Shine, Shine, Ruthenium Caged Drug

This is a highlight on the paper by Bonnet et al.: A Lock-and-Kill Anticancer Photoactivated Chemotherapy Agent. which constitutes an important step toward establishing photoactivated chemotherapy (PACT) as a widespread tool to treat different health issues, specially tumors. PACT can be a useful technique to deliver already tested drugs, where the effect of the desired molecule is directed only to its target after light irradiation, even in the cases in which it is difficult to achieve a precise delivery in the desired organ or tissue. Ruthenium-polipyridyl caged-compounds are near ideal devices to deliver a drug in that precise fashion, albeit they usually fail in revealing their actual location due to their weak light emission properties. The mentioned work introduces a simple and clever idea: the use of a covalently linked fluorophore to map the caged-compounds in-vivo distribution prior to the eventual irradiation to activate the chemotherapy.

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)₂(N^∧N)]²⁺ (bpy = 2,2'-bipyridyl; N^∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N^∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^∧N ligand. Complexes 2 to 6 all release the ligand N^∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)₂(NCMe)₂]²⁺, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N^∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Organogold(III) Complexes Display Conditional Photoactivities: Evolving From Photodynamic into Photoactivated Chemotherapy in Response to O2 Consumption for Robust Cancer Therapy

Photodynamic therapy (PDT) is a spatiotemporally controllable, powerful approach in combating cancers but suffers from low activity under hypoxia, whereas photoactivated chemotherapy (PACT) operates in an O₂ -independent manner but compromises the ability to harness O₂ for potent photosensitization. Herein we report that cyclometalated gold(III)-alkyne complexes display a PDT-to-PACT evolving photoactivity for efficient cancer treatment. On the one hand, the gold(III) complexes can act as dual photosensitizers and substrates, leading to conditional PDT activity in oxygenated condition that progresses to highly efficient PACT (ϕ up to 0.63) when O₂ is depleted in solution and under cellular environment. On the other hand, the conditional PDT-to-PACT reactivity can be triggered by external photosensitizers in a similar manner in vitro and in vivo, giving additional tumor-selectivity and/or deep tissue penetration by red-light irradiation that leads to robust anticancer efficacy.

A Photoactivated Sorafenib-Ruthenium(II) Prodrug for Resistant Hepatocellular Carcinoma Therapy through Ferroptosis and Purine Metabolism Disruption

The curative effect of sorafenib in hepatocellular carcinoma (HCC) is limited and sorafenib resistance remains a major obstacle for HCC. To overcome this obstacle, a new photoactive sorafenib-Ru(II) complex Ru-Sora has been designed. Upon irradiation (λ = 465 nm), Ru-Sora rapidly releases sorafenib and generates reactive oxygen species, which can oxidize intracellular substances such as GSH. Cellular experiments show that irradiated Ru-Sora is highly cytotoxic toward Hep-G2 cells, including sorafenib-resistant Hep-G2-SR cells. Compared to sorafenib, Ru-Sora has a significant photoactivated chemotherapeutic effect against Hep-G2-SR cancer cells and 3D Hep-G2 multicellular tumor spheroids. Furthermore, Ru-Sora inducing apoptosis and ferroptosis is proved by GSH depletion, GPX4 downregulation, and lipid peroxide accumulation. Metabolomics results suggest that Ru-Sora exerts photocytotoxicity by disrupting the purine metabolism, which is expected to inhibit tumor development. This study provides a promising strategy for enhancing chemotherapy and combating drug-resistant HCC disease.

Ir(III)-Based Agents for Monitoring the Cytochrome P450 3A4 Active Site Occupancy

Cytochromes P450 (CYPs) are a superfamily of enzymes responsible for biosynthesis and drug metabolism. Monitoring the activity of CYP3A4, the major human drug-metabolizing enzyme, is vital for assessing the metabolism of pharmaceuticals and identifying harmful drug-drug interactions. Existing probes for CYP3A4 are irreversible turn-on substrates that monitor activity at specific time points in end-point assays. To provide a more dynamic approach, we designed, synthesized, and characterized emissive Ir(III) and Ru(II) complexes that allow monitoring of the CYP3A4 active-site occupancy in real time. In the bound state, probe emission is quenched by the active-site heme. Upon displacement from the active site by CYP3A4-specific inhibitors or substrates, these probes show high emission turn-on. Direct probe binding to the CYP3A4 active site was confirmed by X-ray crystallography. The lead Ir(III)-based probe has nanomolar Kd and high selectivity for CYP3A4, efficient cellular uptake, and low toxicity in CYP3A4-overexpressing HepG2 cells.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

Ru(II) photocages enable precise control over enzyme activity with red light

The cytochrome P450 family of enzymes (CYPs) are important targets for medicinal chemistry. Recently, CYP1B1 has emerged as a key player in chemotherapy resistance in the treatment of cancer. This enzyme is overexpressed in a variety of tumors, and is correlated with poor treatment outcomes; thus, it is desirable to develop CYP1B1 inhibitors to restore chemotherapy efficacy. However, possible off-target effects, such as inhibition of liver CYPs responsible for first pass metabolism, make selective inhibition a high priority to avoid possible drug-drug interactions and toxicity. Here we describe the creation of light-triggered CYP1B1 inhibitors as "prodrugs", and achieve >6000-fold improvement in potency upon activation with low energy (660 nm) light. These systems provide a selectivity index of 4,000-100,000 over other off-target CYPs. One key to the design was the development of coordinating CYP1B1 inhibitors, which suppress enzyme activity at pM concentrations in live cells. The metal binding group enforces inhibitor orientation in the active site by anchoring to the iron. The second essential component was the biologically compatible Ru(II) scaffold that cages the inhibitors before photochemical release. These Ru(II) photocages are anticipated to provide similar selectivity and control for any coordinating CYP inhibitors.

Radiotherapy Reduces N-Oxides for Prodrug Activation in Tumors

Precisely activating chemotherapeutic prodrugs in a tumor-selective manner is an ideal way to cure cancers without causing systemic toxicities. Although many efforts have been made, developing spatiotemporally controllable activation methods is still an unmet challenge. Here, we report a novel prodrug activation strategy using radiotherapy (X-ray). Due to its precision and deep tissue penetration, X-ray matches the need for altering molecules in tumors through water radiolysis. We first demonstrated that N-oxides can be effectively reduced by hydrated electrons (e^-_aq) generated from radiation both in tubes and living cells. A screening is performed to investigate the structure-reduction relationship and mechanism of the e^-_aq-mediated reductions. We then apply the strategy to activate N-oxide prodrugs. The anticancer drug camptothecin (CPT)-based N-oxide prodrug shows a remarkable anticancer effect upon activation by radiotherapy. This radiation-induced in vivo chemistry may enable versatile designs of radiotherapy-activated prodrugs, which are of remarkable clinical relevance, as over 50% of cancer patients take radiotherapy.

A Comprehensive Analysis of Human CYP3A4 Crystal Structures as a Potential Tool for Molecular Docking-Based Site of Metabolism and Enzyme Inhibition Studies

The notable ability of human liver cytochrome P450 3A4 (CYP3A4) to metabolize diverse xenobiotics encourages researchers to explore in-depth the mechanism of enzyme action. Numerous CYP3A4 protein crystal structures have been deposited in protein data bank (PDB) and are majorly used in molecular docking analysis. The quality of the molecular docking results depends on the three-dimensional CYP3A4 protein crystal structures from the PDB. Present review endeavors to provide a brief outline of some technical parameters of CYP3A4 PDB entries as valuable information for molecular docking research. PDB entries between 22 April 2004 and 2 June 2021 were compiled and the active sites were thoroughly observed. The present review identified 76 deposited PDB entries and described basic information that includes CYP3A4 from human genetic, Escherichia coli (E. coli) use for protein expression, crystal structure obtained from X-ray diffraction method, taxonomy ID 9606, Uniprot ID P08684, ligand–protein structure description, co-crystal ligand, protein site deposit and resolution ranges between 1.7[Formula: see text]Å and 2.95[Formula: see text]Å. The observation of protein–ligand interactions showed the various residues on the active site depending on the ligand. The residues Ala305, Ser119, Ala370, Phe304, Phe108, Phe213 and Phe215 have been found to frequently interact with ligands from CYP3A4 PDB. Literature surveys of 17 co-crystal ligands reveal multiple mechanisms that include competitive inhibition, noncompetitive inhibition, mixed-mode inhibition, mechanism-based inhibition, substrate with metabolite, inducer, or combination modes of action. This overview may help researchers choose a trustworthy CYP3A4 protein structure from the PDB database to apply the protein in molecular docking analysis for drug discovery.

Metalloimmunotherapy with Rhodium and Ruthenium Complexes: Targeting Tumor-Associated Macrophages

Tumor associated macrophages (TAMs) suppress the cancer immune response and are a key target for immunotherapy. The effects of ruthenium and rhodium complexes on TAMs have not been well characterized. To address this gap in the field, a panel of 22 dirhodium and ruthenium complexes were screened against three subtypes of macrophages, triple-negative breast cancer and normal breast tissue cells. Experiments were carried out in 2D and biomimetic 3D co-culture experiments with and without irradiation with blue light. Leads were identified with cell-type-specific toxicity toward macrophage subtypes, cancer cells, or both. Experiments with 3D spheroids revealed complexes that sensitized the tumor models to the chemotherapeutic doxorubicin. Cell surface exposure of calreticulin, a known facilitator of immunogenic cell death (ICD), was increased upon treatment, along with a concomitant reduction in the M2-subtype classifier arginase. Our findings lay a strong foundation for the future development of ruthenium- and rhodium-based chemotherapies targeting TAMs.

Alkylgold(III) Complexes Undergo Unprecedented Photo-Induced β-Hydride Elimination and Reduction for Targeted Cancer Therapy

Spatiotemporally controllable activation of prodrugs within tumors is highly desirable for cancer therapy to minimize toxic side effects. Herein we report that stable alkylgold(III) complexes can undergo unprecedented photo-induced β-hydride elimination, releasing alkyl ligands and forming gold(III)-hydride intermediates that could be quickly converted into bioactive [Au^III -S] adducts; meanwhile, the remaining alkylgold(III) complexes can photo-catalytically reduce [Au^III -S] into more bioactive Au^I species. Such photo-reactivities make it possible to functionalize gold complexes on the auxiliary alkyl ligands without attenuating the metal-biomacromolecule interactions. As a result, the gold(III) complexes containing glucose-functionalized alkyl ligands displayed efficient and tumor-selective uptake; notably, after one- or two-photon activation, the complexes exhibited high thioredoxin reductase (TrxR) inhibition, potent cytotoxicity, and strong antiangiogenesis and antitumor activities in vivo.

Biodegradable Ru-Containing Polycarbonate Micelles for Photoinduced Anticancer Multitherapeutic Agent Delivery and Phototherapy Enhancement

The lack of selectivity between tumor and healthy cells, along with inefficient reactive oxygen species production in solid tumors, are two major impediments to the development of anticancer Ru complexes. The development of photoinduced combination therapy based on biodegradable polymers that can be light activated in the "therapeutic window" would be beneficial for enhancing the therapeutic efficacy of Ru complexes. Herein, a biodegradable Ru-containing polymer (poly(DCARu)) is developed, in which two different therapeutics (the drug and the Ru complex) are rationally integrated and then conjugated to a diblock copolymer (MPEG-b-PMCC) containing hydrophilic poly(ethylene glycol) and cyano-functionalized polycarbonate with good degradability and biocompatibility. The polymer self-assembles into micelles with high drug loading capacity, which can be efficiently internalized into tumor cells. Red light induces the generation of singlet oxygen and the release of anticancer drug-Ru complex conjugates from poly(DCARu) micelles, hence inhibiting tumor cell growth. Furthermore, the phototherapy of polymer micelles demonstrates remarkable inhibition of tumor growth in vivo. Meanwhile, polymer micelles exhibit good biocompatibility with blood and healthy tissues, which opens up opportunities for multitherapeutic agent delivery and enhanced phototherapy.

Unlocking the Potential of Ru(II) Dual-action Compounds with the Power of the Heavy-atom Effect

We report the synthesis, photochemical and biological characterization of two new Ru(II) photoactivated complexes based on [Ru(tpy)(Me₂ bpy)(L)]²⁺ (tpy = 2,2':6',2''-terpyridine, Me₂ bpy = 6,6'-dimethyl-2,2'-bipyridine), where L = pyridyl-BODIPY (pyBOD). Two pyBOD ligands were prepared bearing flanking hydrogen or iodine atoms. Ru(II)-bound BODIPY dyes show a red-shift of absorption maxima relative to the free dyes and undergo photodissociation of BODIPY ligands with green light irradiation. Addition of iodine into the BODIPY ligand facilitates intersystem crossing, which leads to efficient singlet oxygen production in the free dye, but also enhances quantum yield of release of the BODIPY ligand from Ru(II). This represents the first report of a strategy to enhance photodissociation quantum yields through the heavy-atom effect in Ru(II) complexes. Furthermore, Ru(II)-bound BODIPY dyes display fluorescence turn-on once released, with a lead analog showing nanomolar EC₅₀ values against triple negative breast cancer cells, >100-fold phototherapeutic indexes under green light irradiation, and higher selectivity toward cancer cells as compared to normal cells than the corresponding free BODIPY photosensitizer. Conventional Ru(II) photoactivated complexes require nonbiorthogonal blue light for activation and rarely show submicromolar potency to achieve cell death. Our study represents an avenue for the improved photochemistry and potency of future Ru(II) complexes.

Unraveling the Structural Basis of Selective Inhibition of Human Cytochrome P450 3A5

The human cytochrome P450 (CYP) CYP3A4 and CYP3A5 enzymes metabolize more than one-half of marketed drugs. They share high structural and substrate similarity and are often studied together as CYP3A4/5. However, CYP3A5 preferentially metabolizes several clinically prescribed drugs, such as tacrolimus. Genetic polymorphism in CYP3A5 makes race-based dosing adjustment of tacrolimus necessary to minimize acute rejection after organ transplantation. Moreover, the differential tissue distribution and expression levels of CYP3A4 and CYP3A5 can aggravate toxicity during treatment. Therefore, selective inhibitors of CYP3A5 are needed to distinguish the role of CYP3A5 from that of CYP3A4 and serve as starting points for potential therapeutic development. To this end, we report the crystal structure of CYP3A5 in complex with a previously reported selective inhibitor, clobetasol propionate (CBZ). This is the first CYP3A5 structure with a type I inhibitor, which along with the previously reported substrate-free and type II inhibitor-bound structures, constitute the main CYP3A5 structural modalities. Supported by structure-guided mutagenesis analyses, the CYP3A5-CBZ structure showed that a unique conformation of the F-F' loop in CYP3A5 enables selective binding of CBZ to CYP3A5. Several polar interactions, including hydrogen bonds, stabilize the position of CBZ to interact with this unique F-F' loop conformation. In addition, functional and biophysical assays using CBZ analogs highlight the importance of heme-adjacent moieties for selective CYP3A5 inhibition. Our findings can be used to guide further development of more potent and selective CYP3A5 inhibitors.