Photoreaction pathways for the anticancer complex trans,trans,trans-[Pt(N3)2(OH)2(NH3)2]

Photosubstitution and photoreduction of a diazido platinum(IV) anticancer complex

The hyphenation of HPLC with its high separation ability and ICP-MS with its excellent sensitivity, allows the analysis of Pt drugs in biological samples at the low nanomolar concentration levels. On the other hand, LC-MS provides molecular structural confirmation for each species. Using a combination of these methods, we have investigated the speciation of the photoactive anticancer complex diazido Pt(IV) complex trans, trans, trans-[Pt(N3)2(OH)2(py)2] (FM-190) in aqueous solution and biofluids at single-digit nanomolar concentrations before and after irradiation. FM-190 displays high stability in human blood plasma in the dark at 37 °C. Interestingly, the polyhydroxido species [{PtIV(py)2(OH)4} + Na]+ and [{PtIV(py)2(N3)(OH)3} + Na]+ resulting from the replacement of azido ligands, as determined by LC-MS, were the major products after photoirradiation of FM-190 with blue light (463 nm). This finding suggests that such photosubstituted Pt(IV) tri- and tetra-hydroxido species could play important roles in the biological activity of this anticancer complex. Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations show that these Pt(IV) species arising from FM-190 in aqueous media can be formed directly from a singlet excited state. The results highlight how speciation analysis (metallomics) can shed light on photoactivation pathways for FM-190 and formation of potential excited-state pharmacophores. The ability to detect and identify photoproducts at physiologically-relevant concentrations in cells and tissues will be important for preclinical development studies of this class of photoactivatable platinum drugs.

Reaction of human telomeric unit TTAGGG and a photoactivatable Pt(IV) anticancer prodrug

The interaction of a photoactivatable diazidodihydroxido Pt(IV) prodrug, trans,trans,trans-[Pt(N3)2(OH)2(py)2] (py = pyridine; 1), with a hexamer straight human telomeric DNA unit sequence (5'-T1T2A3G4G5G6-3', I) upon light irradiation was investigated by electrospray ionization mass spectroscopy (ESI-MS). In the primary mass spectrum, two major mono-platinated I adducts with the bound Pt moieties, trans-[PtII(N3)(py)2]+ (1') and trans-[PtII(py)2]2+ (1''), respectively, were detected. It is rare to observe such high abundance and nearly equal intensity platinated DNA adducts formed by these two PtII species because 1' is usually the only major reduced Pt(II) species produced by the photodecomposition of complex 1 in the presence of DNA while 1'' was rarely detected as the major reduced PtII species reported previously. Subsequent tandem mass spectrometric analysis by collision-induced dissociation (CID) showed that in the former adduct {I + 1'}2+, G6 and A3 were the platination sites. While in the latter adduct {I + 1''}2+, a potential intrastrand crosslink was speculated after G4 and G6 sites were identified. Additionally, other minor platinated adducts like di-platinated I adduct by 1' with platination sites at G4 and G6 and mono-platinated I adducts containing base oxidation were also detected by mass spectrometry. Due to the rich guanines and their sensitivity to oxidation, the oxidation induced by 1 most probably occurred at guanine. The oxidation adducts were proposed as 8-hydroxyl guanine, spiroiminodihydantoin (Sp), 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG), 5-guanidinohydantoin (Gh), and/or dehydroguanidinohydantoin (DGh) referring to previous reports. The obtained results provide useful chemical information about the photoreaction between photoactivatable Pt(IV) anticancer prodrugs and human telomeric DNA. Such special damages of Pt(IV) prodrugs on human telomeric DNA implicate its active role in the mechanism of Pt(IV) prodrugs and further support the unique sequence-dependent photointeraction profile of complex 1 reacting with DNA.

A method to capture the large relativistic and solvent effects on the UV-vis spectra of photo-activated metal complexes

We have recently developed a method based on relativistic time-dependent density functional theory (TD-DFT) that allows the calculation of electronic spectra in solution (Creutzberg, Hedegård, J. Chem. Theory Comput.18, 2022, 3671). This method treats the solvent explicitly with a classical, polarizable embedding (PE) description. Furthermore, it employs the complex polarization propagator (CPP) formalism which allows calculations on complexes with a dense population of electronic states (such complexes are known to be problematic for conventional TD-DFT). Here, we employ this method to investigate both the dynamic and electronic effects of the solvent for the excited electronic states of trans-trans-trans-[Pt(N₃)₂(OH)₂(NH₃)₂] in aqueous solution. This complex decomposes into species harmful to cancer cells under light irradiation. Thus, understanding its photo-physical properties may lead to a more efficient method to battle cancer. We quantify the effect of the underlying structure and dynamics by classical molecular mechanics simulations, refined with a subsequent DFT or semi-empirical optimization on a cluster. Moreover, we quantify the effect of employing different methods to set up the solvated system, e.g., how sensitive the results are to the method used for the refinement, and how large a solvent shell that is required. The electronic solvent effect is always included through a PE potential.

Overview of Nanoparticle-Based Approaches for the Combination of Photodynamic Therapy (PDT) and Chemotherapy at the Preclinical Stage

Simple Summary

The present review represents the outstanding and promising recent literature reports (2017–2022) on nanoparticle-based formulations developed for anticancer therapy with photodynamic therapy (PDT), photosensitizers, and chemotherapeutics. Besides brief descriptions of chemotherapeutics’ classification and of PDT mechanisms and limitations, several examples of nanosystems endowed with different responsiveness (e.g., acidic pH and reactive oxygen species) and peculiarity (e.g., tumor oxygenation capacity, active tumor targeting, and biomimetic features) are described, and for each drug combination, in vitro and in vivo results on preclinical cancer models are reported.

Abstract

The widespread diffusion of photodynamic therapy (PDT) as a clinical treatment for solid tumors is mainly limited by the patient’s adverse reaction (skin photosensivity), insufficient light penetration in deeply seated neoplastic lesions, unfavorable photosensitizers (PSs) biodistribution, and photokilling efficiency due to PS aggregation in biological environments. Despite this, recent preclinical studies reported on successful combinatorial regimes of PSs with chemotherapeutics obtained through the drugs encapsulation in multifunctional nanometric delivery systems. The aim of the present review deals with the punctual description of several nanosystems designed not only with the objective of co-transporting a PS and a chemodrug for combination therapy, but also with the goal of improving the therapeutic efficacy by facing the main critical issues of both therapies (side effects, scarce tumor oxygenation and light penetration, premature drug clearance, unspecific biodistribution, etc.). Therefore, particular attention is paid to the description of bio-responsive drugs and nanoparticles (NPs), targeted nanosystems, biomimetic approaches, and upconverting NPs, including analyzing the therapeutic efficacy of the proposed photo-chemotherapeutic regimens in in vitro and in vivo cancer models.

Dual-platination and induced oxidation of uridine by a photoactivatable diazido Pt(IV) anticancer prodrug

Photoactivatable Pt(IV) anticancer prodrug trans,trans,trans-[Pt(N₃)₂(OH)₂(pyridine)₂] (1) has been shown to bind to and induce oxidation of all four DNA nucleobases. Herein, to further render the binding spectrum of complex 1 to nucleic acids, the interaction between complex 1 and uridine, an exclusive RNA component, was investigated by electrospray ionization mass spectrometry (ESI-MS) and NMR spectroscopy. The results showed that complex 1 can bind to uridine through the N3 (major) and O4 (minor) sites upon light irradiation to form the major mono-platinated uridine adduct and the minor di-platinated uridine adduct. Moreover, mono-platinated uridine associated with the oxidation of uridine to 5-hydroxyuridine and 6-hydroxyuridine was also observed. This is the first report that the photoactivatable Pt(IV) prodrug binds to and induces the oxidation of uridine, and also the last piece of the puzzle for the interactions of complex 1 with nucleobases. Combined with our previous results about the interactions between complex 1 and DNA bases, these data showed a wide interaction spectrum of this kind of photoactivatable diazido Pt(IV) prodrugs with nucleobases through such dual-action modes, strongly suggesting that RNA may be a potential target of Pt(IV) prodrugs.

DFT Study on the Substituent Effect of Anticancer Picoline-Diazido-Pt(IV) Compounds

The geometric structure of azido Pt(IV) compounds containing picoline was calculated by using density functional theory(DFT) at the LSDA/SDD level. The ESP distribution shows the possible reaction sites of the compounds. In addition, the frequency calculation results assigned the infrared spectra of these compounds, and specified important stretching and bending vibrations. The HOMO-LUMO energy gaps of these compounds are also calculated to explain the charge transfer of the molecules. The distribution of Mulliken charges and natural atomic charges of these atoms is also calculated. Natural bond orbital(NBO) analysis explains the intramolecular interactions and their electron density.

Simplified State Interaction for Matrix Product State Wave Functions

We present an approximation to the state-interaction approach for matrix product state (MPS) wave functions (MPSSI) in a nonorthogonal molecular orbital basis, first presented by Knecht et al. [J. Chem. Theory Comput.,2016, 28, 5881], that allows for a significant reduction of the computational cost without significantly compromising its accuracy. The approximation is well-suited if the molecular orbital basis is close to orthogonality, and its reliability may be estimated a priori with a single numerical parameter. For an example of a platinum azide complex, our approximation offers up to 63-fold reduction in computational time compared to the original method for wave function overlaps and spin–orbit couplings, while still maintaining numerical accuracy.

Density functional study of trans,trans,trans-[Pt(N3)2(OH)2(Py)2] on molecular structure and vibrational spectroscopy

This study records the selection of the best combination of geometric optimization and frequency calculation methods of the trans,trans,trans-[Pt(N₃)₂(OH)₂(py)₂](FM-190) by using different density functional calculation methods and basis sets. The results show that the CAM-B3LYP/SDD method has the best fit to the experimental data for geometric optimization, while the LSDA/SDD method has better performance in frequency calculation, and the infrared vibration peak is assigned. In addition, the calculated HOMO and LUMO show the energy gap and the internal charge transfer of this complex. The hyperconjugation of the pyridine ring have been explained by NBO analysis.

Research progress of azido-containing Pt(IV) antitumor compounds

Cancer is a long-known incurable disease, and the medical use of cisplatin has been a significant discovery. However, the side-effects of cisplatin necessitate the development of new and improved drug. Therefore, in this study, we focused on the photoactivatable Pt(IV) compounds Pt[(X₁)(X₂)(Y₁)(Y₂)(N₃)₂], which have a completely novel mechanism of action. Pt(IV) can efficiently overcome the side-effects of cisplatin and other drugs. Here, we have demonstrated, summarized and discussed the effects and mechanism of these compounds. Compared to the relevant articles in the literature, we have provided a more detailed introduction and a made comprehensive classification of these compounds. We believe that our results can effectively provide a reference for the development of these drugs.

Photoresponsive metallopolymer nanoparticles for cancer theranostics

Over the past decades, transition metal complexes have been successfully used in anticancer phototherapies. They have shown promising properties in many different areas including photo-induced ligand exchange or release, rich excited state behavior, and versatile biochemical properties. When encorporated into polymeric frameworks and become part of nanostructures, photoresponsive metallopolymer nanoparticles (MPNs) show enhanced water solubility, extended blood circulation and increased tumor-specific accumulation, which greatly improves the tumor therapeutic effects compared to low-molecule-weight metal complexes. In this review, we aim to present the recent development of photoresponsive MPNs as therapeutic nanomedicines. This review will summarize four major areas separately, namely platinum-containing polymers, zinc-containing polymers, iridium-containing polymers and ruthenium-containing polymers. Representative MPNs of each type are discussed in terms of their design strategies, fabrication methods, and working mechanisms. Current challenges and future perspectives in this field are also highlighted.

The Role of Triplet States in the Photodissociation of a Platinum Azide Complex by a Density Matrix Renormalization Group Method

Platinum azide complexes are appealing anticancer photochemotherapy drug candidates because they release cytotoxic azide radicals upon light irradiation. Here we present a density matrix renormalization group self-consistent field (DMRG-SCF) study of the azide photodissociation mechanism of trans,trans,trans-[Pt(N₃)₂(OH)₂(NH₃)₂], including spin-orbit coupling. We find a complex interplay of singlet and triplet electronic excited states that falls into three different dissociation channels at well-separated energies. These channels can be accessed either via direct excitation into barrierless dissociative states or via intermediate doorway states from which the system undergoes non-radiative internal conversion and intersystem crossing. The high density of states, particularly of spin-mixed states, is key to aid non-radiative population transfer and enhance photodissociation along the lowest electronic excited states.

Investigating the influence of relativistic effects on absorption spectra for platinum complexes with light-activated activity against cancer cells

We report the first systematic investigation of relativistic effects on the UV-vis spectra of two prototype complexes for so-called photo-activated chemotherapy (PACT), trans-trans-trans-[Pt(N3)2(OH)2(NH3)2] and cis-trans-cis-[Pt(N3)2(OH)2(NH3)2]. In PACT, design of new drugs requires in-depth understanding of the photo-activation mechanisms. A first step is usually to rationalize their UV-vis spectra for which time-dependent density functional theory (TD-DFT) is an indispensable tool. We carried out TD-DFT calculations with a systematic series of non-relativistic (NR), scalar-relativistic (SR), and four-component (4c) Hamiltonians. As expected, large differences are found between spectra calculated within 4c and NR frameworks, while the most intense features (found at higher energies below 300 nm) can be reasonably well reproduced within a SR framework. It is also shown that effective core potentials (ECPs) yield essentially similar results as all-electron SR calculations. Yet the underlying transitions can be strongly influenced by spin-orbit coupling, which is only present in the 4c framework: while this can affect both intense and less intense transitions in the spectra, the effect is most pronounced for weaker transitions at lower energies, above 300 nm. Since the investigated complexes are activated with light of wavelengths above 300 nm, employing a method with explicit inclusion of spin-orbit coupling may be crucial to rationalize the activation mechanism.

New Designs for Phototherapeutic Transition Metal Complexes

In this Minireview, we highlight recent advances in the design of transition metal complexes for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT), and discuss the challenges and opportunities for the translation of such agents into clinical use. New designs for light-activated transition metal complexes offer photoactivatable prodrugs with novel targeted mechanisms of action. Light irradiation can provide spatial and temporal control of drug activation, increasing selectivity and reducing side-effects. The photophysical and photochemical properties of transition metal complexes can be controlled by the appropriate choice of the metal, its oxidation state, the number and types of ligands, and the coordination geometry.

Diazido platinum(iv) complexes for photoactivated anticancer chemotherapy

Diazido Pt(iv) complexes with a general formula [Pt(N₃)₂(L)(L′)(OR)(OR′)] are a new generation of anticancer prodrugs designed for use in photoactivated chemotherapy.

Oxygen and Pt(II) self-generating conjugate for synergistic photo-chemo therapy of hypoxic tumor

Cancer cells in hypoxic tumors are remarkably resistant to photodynamic therapy. Here, we hypothesize that an oxygen and Pt(II) self-generating multifunctional nanocomposite could reverse the hypoxia-triggered PDT resistance. The nanocomposite contains Pt(IV) and chlorin e6, in which upconversion nanoparticles are loaded to convert 980 nm near-infrared light into 365 nm and 660 nm emissions. Upon accumulation at the tumor site, a 980 nm laser is used to trigger the nanocomposite to generate O₂ for consumption in the PDT process and to produce cytotoxic reactive oxygen species. The composite also releases active Pt(II) for synergistic photo-chemo therapy to enhance antitumor efficiency. The oxygen and Pt(II) self-generating prodrug is shown to have high potential to inhibit tumors out of the range of UV light, to overcome the hypoxia-triggered PDT resistance and significantly improve anticancer efficacy by the synergistic PDT-chemotherapy.

Photodynamic therapy has attracted interest in treating cancer but it is limited in hypoxic tumors due to a lack of oxygen. Here, the authors describe a nano-composite capable of generating oxygen under near-infrared light for improved photodynamic and chemotherapy for treating cancer.

Solvent-dependent structural dynamics of an azido-platinum complex revealed by linear and nonlinear infrared spectroscopy

The vibrational and anisotropic relaxation dynamics and structural dynamics of a potential anticancer prodrug, trans,trans,trans-[Pt(N3)2(OH)2(py)2], were investigated using time-resolved infrared pump-probe spectroscopy and ultrafast two-dimensional infrared (2D IR) spectroscopy. Herein, two representative bio-friendly solvents, H2O and DMSO, were used, in which the local structural and dynamical variations were monitored using the antisymmetric linear combination of the two N3 stretching vibrational modes as an infrared probe. It was found that the vibrational relaxation process of the N3 antisymmetric stretching (as) mode in H2O is two to three times faster than that in DMSO. The anisotropic relaxation process of the anticancer prodrug was observed to be hindered in DMSO; this indicated a tighter solvent environment around the sample molecule in this solvent. The vibrational frequency time correlation of the N3 antisymmetric stretching mode in H2O decays with a time constant of 1.94 ps, in agreement with the hydrogen bond formation and breaking times of water. In DMSO, the frequency time correlation of the N3 as mode decays on a much longer time scale; this further indicates its sensitivity to the out-layer DMSO structural dynamics, which are relatively static in the experimental time window.

Platinum complexes as light promoted anticancer agents: a redefined strategy for controlled activation

Site-specific delivery and amenable activation of prodrugs are indispensible criteria for designing novel anticancer agents. Platinum based drugs vanguard the chemotherapeutic regimes and over the years significant attention has been paid to achieve more efficacious drugs with fewer adverse effects. The switch from platinum(ii) drugs to the inert platinum(iv) analogues proved advantageous but the new prodrugs still suffered from unspecific cytotoxic actions. Thus the photoactivation of an inert platinum prodrug specifically within neoplastic cells provided the desired spatio-temporal control over drug activation by means of illumination, thereby limiting the cytotoxic events to only at the targeted tumors. This article collates research on platinum complexes which exhibit potential light mediated anticancer effects and provides insights into the underlying mechanisms of activation. Fine tuning of the coordination sphere results in dramatic alteration of the redox and spectral properties of both ground and excited states and the cellular properties of the molecules. This concise article highlights the various light promoted strategies employed to attain a controlled release of active platinum(ii) and/or reactive oxygen species such as photoreduction, photocaging, photodissociation and photosensitization. Such dual action photoactive metal complexes with improved aqueous solubility and versatility are promising candidates for combination therapy which is likely to be the future of anticancer research.

Primary photochemical processes for Pt(iv) diazido complexes prospective in photodynamic therapy of tumors

A case study of chain photoaquation of mixed-ligand Pt(iv) diazido complexes tested in PDT of tumors is performed.

Nanoparticle delivery of sterically hindered platinum(IV) prodrugs shows 100 times higher potency than that of cisplatin upon light activation

The introduction of a sterically hindered pyridine ligand to a photosensitive platinum(iv) drug for drug delivery resulted in a >100 fold increase in effectiveness compared to the gold standard, cisplatin.

The Next Generation of Platinum Drugs: Targeted Pt(II) Agents, Nanoparticle Delivery, and Pt(IV) Prodrugs

The platinum drugs, cisplatin, carboplatin, and oxaliplatin, prevail in the treatment of cancer, but new platinum agents have been very slow to enter the clinic. Recently, however, there has been a surge of activity, based on a great deal of mechanistic information, aimed at developing nonclassical platinum complexes that operate via mechanisms of action distinct from those of the approved drugs. The use of nanodelivery devices has also grown, and many different strategies have been explored to incorporate platinum warheads into nanomedicine constructs. In this Review, we discuss these efforts to create the next generation of platinum anticancer drugs. The introduction provides the reader with a brief overview of the use, development, and mechanism of action of the approved platinum drugs to provide the context in which more recent research has flourished. We then describe approaches that explore nonclassical platinum(II) complexes with trans geometry or with a monofunctional coordination mode, polynuclear platinum(II) compounds, platinum(IV) prodrugs, dual-threat agents, and photoactivatable platinum(IV) complexes. Nanoparticles designed to deliver platinum(IV) complexes will also be discussed, including carbon nanotubes, carbon nanoparticles, gold nanoparticles, quantum dots, upconversion nanoparticles, and polymeric micelles. Additional nanoformulations, including supramolecular self-assembled structures, proteins, peptides, metal-organic frameworks, and coordination polymers, will then be described. Finally, the significant clinical progress made by nanoparticle formulations of platinum(II) agents will be reviewed. We anticipate that such a synthesis of disparate research efforts will not only help to generate new drug development ideas and strategies, but also will reflect our optimism that the next generation of approved platinum cancer drugs is about to arrive.

Exploring the photochemosensitivity by novel cysteine-based mixed ligand complexes

A new series of cysteine-based metal(II) complexes with 2,2'-bipyridine or 1,10-phenanthroline as co-ligand have been prepared and characterized. Their DNA binding and cleavage properties have been studied. The analytical and spectroscopic data of complexes 1-18 reveal that the complexes adopt an octahedral geometry around the central metal ion in which the cysteine is coordinated through NS and NN atoms, respectively. Spectroscopic titration and viscosity measurements reveal that the complexes bind to DNA through an intercalative mode. Electrophoresis measurements exhibit that they cleave pBR322 DNA efficiently in the presence of 3-mercaptopropionic acid (MPA), probably via hydrolytic mechanism with the involvement of (•)OH. The in vitro anticancer activities indicate that the Cu(II) complexes are active against four selected human tumor cell lines. Furthermore, it is remarkable that all the complexes exhibit significant photocytotoxicity against human breast cancer cell lines (MCF-7) with a potency more than the widely used drugs photofrin and cisplatin indicating that they have the potential to act as effective anticancer drugs in a dose-dependent manner.

Delivering a photosensitive transplatin prodrug to overcome cisplatin drug resistance

Clinically ineffective transplatin was converted into a photosensitive prodrug for drug delivery and triggered release to overcome cisplatin resistance.

Nanoparticle delivery of photosensitive Pt(IV) drugs for circumventing cisplatin cellular pathway and on-demand drug release

A photosensitive platinum(IV) prodrug (UVA-Pt2) was attached to a biodegradable polymer (PE, methoxyl-poly(ethylene glycol)-block-poly(lactide-co-2-methyl-2-carboxyl-propylene carbonate-ethanol amine)) and then the conjugate was self-assembled to micelles (NP-UVA-Pt2). In vitro MTT assay of NP-UVA-Pt2 demonstrated an improved cytotoxicity against SKOV-3 cells than that of cisplatin. Confocal laser scanning microscopy (CLSM) indicated that NP-UVA-Pt2 were endocytosed rather than internalized by passive diffusion, and thus, this process has nothing to do with copper transporter protein (Ctr1) as reported for cisplatin, which is closely related to drug resistance of Pt based drugs. Intracellular platinum content measured by ICP-MS result suggested that NP-UVA-Pt2 expressed higher platinum intracellular uptake than cisplatin. NP-UVA-Pt2 demonstrated fast and robust response to photo irradiation while the nanoparticles were stable in PBS at PH7.4 in the dark. The great drug efficacy of NP-UVA-Pt2 under UVA irradiation and the ineffectiveness in the dark makes NP-UVA-Pt2 an ideal light responsive on-demand drug delivery system. Hence, NP-UVA-Pt2 will be a promising platinum based drug in the near future.

Platinum anticancer drugs and photochemotherapeutic agents: recent advances and future developments

Platinum-based chemotherapeutic drugs such as cisplatin, carboplatin and oxaliplatin are widely applied for the treatment of various types of tumours. Over the last few decades, a large variety of Pt(II) and Pt(IV) complexes have been developed to improve the applicability in a wider spectrum of cancers, increase their therapeutic window and reduce the dose-limiting side effects. Photodynamic therapy (PDT), which is the administration of a photosensitiser followed by visible light activation, is a promising route to avoid damage to healthy cells and the surrounding tissue. Transition metal complexes as photochemotherapeutic agents are an attractive option for further development in the field of photoactivated chemotherapy (PACT). These complexes exhibit different numbers and types of excited states which are easily accessible upon light irradiation, subsequently giving rise to the formation of various photoproducts that can enable a distinct mode of action. Platinum-diazido complexes are promising candidates for PACT due to the low cytotoxicity when irradiated with visible light. This review summarises the mode of action of current platinum anticancer drugs with cisplatin as a lead example and the development of non-conventional Pt(II) complexes. Background information regarding PDT the photophysical and photochemical properties of metal complexes is provided, as well as notable examples of photoactivated metal complexes with biological activity. Particular emphasis is placed on recent developments on platinum photoactivated drugs.

De novo generation of singlet oxygen and ammine ligands by photoactivation of a platinum anticancer complex

Worth the excitement: Highly reactive oxygen and nitrogen species are generated by photoactivation of the anticancer platinum(IV) complex trans,trans,trans-[Pt(N3 )2 (OH)2 (MA)(Py)] (MA=methylamine, Py=pyridine). Singlet oxygen is formed from the hydroxido ligands and not from dissolved oxygen, and ammine ligands are products from the conversion of azido ligands to nitrenes. Both processes can induce oxidation of guanine.

Effects of light-activated diazido-PtIV complexes on cancer cells in vitro

Various Pt(IV) diazides have been investigated over the years as light-activatable prodrugs that interfere with cell proliferation, accumulate in cancer cells and cause cell death. The potencies of the complexes vary depending on the substituted amines (pyridine=piperidine>ammine) as well as the coordination geometry (trans diazide>cis). Light-activated Pt(IV) diazides tend to be less specific than cisplatin at inhibiting cancer cell growth, but cells resistant to cisplatin show little cross-resistance to Pt(IV) diazides. Platinum is accumulated in the cancer cells to a similar level as cisplatin, but only when activated by light, indicating that reactive Pt species form photolytically. Studies show that Pt also becomes attached to cellular DNA upon the light activation of various Pt(IV) diazides. Structures of some of the photolysis products were elucidated by LC-MS/MS; monoaqua- and diaqua-Pt(II) complexes form that are reactive towards biomolecules such as calf thymus DNA. Platination of calf thymus DNA can be blocked by the addition of nucleophiles such as glutathione and chloride, further evidence that aqua-Pt(II) species form upon irradiation. Evidence is presented that reactive oxygen species may be generated in the first hours following photoactivation. Cell death does not take the usual apoptotic pathways seen with cisplatin, but appears to involve autophagy. Thus, photoactivated diazido-Pt(IV) complexes represent an interesting class of potential anti-cancer agents that can be selectively activated by light and kill cells by a mechanism different to the anti-cancer drug cisplatin.

Photochemistry and DNA photocleavage by a new unsupported dirhodium(II,II) complex

The new complex [Rh2(phen)2(CH3CN)6](BF4)4 (1) was synthesized and characterized in solution and its crystal structure was determined. Irradiation of 1 with visible light (λirr>590 nm) in water results in the release of two equatorial CH3CN ligands, CH3CNeq, as well as in the formation of mononuclear radical Rh(II) fragments stemming from the homolytic photocleavage of the metal-metal bond. The photoproducts, identified by electrospray ionization mass spectrometry, include [Rh(phen)(CH3CN)(OH)](+) and [Rh(phen)(CH3CN)(H2O)3(BF4)](+). The quantum yield for the photochemical transformation of 1 in H2O exceeds unity (Φ550 nm=1.38) indicative of dark reactions following the initial photoprocess. DNA photocleavage was observed for 1 (λirr>590 nm), whereas the complex is unreactive in the dark. This feature makes 1 a promising photodynamic therapy agent that does not operate via the production of singlet oxygen, 1O2.

Diazido mixed-amine platinum(IV) anticancer complexes activatable by visible-light form novel DNA adducts

Platinum diam(m)ine complexes, such as cisplatin, are successful anticancer drugs, but suffer from problems of resistance and side-effects. Photoactivatable Pt(IV) prodrugs offer the potential of targeted drug release and new mechanisms of action. We report the synthesis, X-ray crystallographic and spectroscopic properties of photoactivatable diazido complexes trans,trans,trans-[Pt(N3)2(OH)2(MA)(Py)] (1; MA=methylamine, Py=pyridine) and trans,trans,trans-[Pt(N3)2(OH)2(MA)(Tz)] (2; Tz=thiazole), and interpret their photophysical properties by TD-DFT modelling. The orientation of the azido groups is highly dependent on H bonding and crystal packing, as shown by polymorphs 1p and 1q. Complexes 1 and 2 are stable in the dark towards hydrolysis and glutathione reduction, but undergo rapid photoreduction with UVA or blue light with minimal amine photodissociation. They are over an order of magnitude more potent towards HaCaT keratinocytes, A2780 ovarian, and OE19 oesophageal carcinoma cells than cisplatin and show particular potency towards cisplatin-resistant human ovarian cancer cells (A2780cis). Analysis of binding to calf-thymus (CT), plasmids, oligonucleotide DNA and individual nucleotides reveals that photoactivated 1 and 2 form both mono- and bifunctional DNA lesions, with preference for G and C, similar to transplatin, but with significantly larger unwinding angles and a higher percentage of interstrand cross-links, with evidence for DNA strand cross-linking further supported by a comet assay. DNA lesions of 1 and 2 on a 50 bp duplex were not recognised by HMGB1 protein, in contrast to cisplatin-type lesions. The photo-induced platination reactions of DNA by 1 and 2 show similarities with the products of the dark reactions of the Pt(II) compounds trans-[PtCl2(MA)(Py)] (5) and trans-[PtCl2(MA)(Tz)] (6). Following photoactivation, complex 2 reacted most rapidly with CT DNA, followed by 1, whereas the dark reactions of 5 and 6 with DNA were comparatively slow. Complexes 1 and 2 can therefore give rapid potent photocytotoxicity and novel DNA lesions in cancer cells, with no activity in the absence of irradiation.

Acetate-bridged platinum(III) complexes derived from cisplatin

Oxidation of the acetate-bridged half-lantern platinum(II) complex cis-[Pt(II)(NH(3))(2)(μ-OAc)(2)Pt(II)(NH(3))(2)](NO(3))(2), [1](NO(3))(2), with iodobenzene dichloride or bromine generates the halide-capped platinum(III) species cis-[XPt(III)(NH(3))(2)(μ-OAc)(2)Pt(III)(NH(3))(2)X](NO(3))(2), where X is Cl in [2](NO(3))(2) or Br in [3](NO(3))(2), respectively. These three complexes, characterized structurally by X-ray crystallography, feature short (≈2.6 Å) Pt-Pt separations, consistent with formation of a formal metal-metal bond upon oxidation. Elongated axial Pt-X distances occur, reflecting the strong trans influence of the metal-metal bond. The three structures are compared to those of other known dinuclear platinum complexes. A combination of (1)H, (13)C, (14)N, and (195)Pt NMR spectroscopy was used to characterize [1](2+)-[3](2+) in solution. All resonances shift downfield upon oxidation of [1](2+) to [2](2+) and [3](2+). For the platinum(III) complexes, the (14)N and (195)Pt resonances exhibit decreased line widths by comparison to those of [1](2+). Density functional theory calculations suggest that the decrease in the (14)N line width arises from a diminished electric field gradient at the (14)N nuclei in the higher valent compounds. The oxidation of [1](NO(3))(2) with the alternative oxidizing agent bis(trifluoroacetoxy)iodobenzene affords the novel tetranuclear complex cis-[(O(2)CCF(3))Pt(III)(NH(3))(2)(μ-OAc)(2)Pt(III)(NH(3))(μ-NH(2))](2)(NO(3))(4), [4](NO(3))(4), also characterized structurally by X-ray crystallography. In solution, this complex exists as a mixture of species, the identities of which are proposed.

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.

Photodissociation of a ruthenium(II) arene complex and its subsequent interactions with biomolecules: a density functional theory study

The piano-stool Ru(II) arene complex [(η⁶-benz)Ru(bpm)(py)]²⁺ (benz = benzene, bpm = 2,2'-bipyrimidine, and py = pyridine), which is conventionally nonlabile (on a timescale and under conditions relevant for biological reactivity), can be activated by visible light to selectively photodissociate the monodentate ligand (py). In the present study, the aquation and binding of the photocontrolled ruthenium(II) arene complex [(η⁶-benz)Ru(bpm)(py)]²⁺ to various biomolecules are studied by density functional theory (DFT) and time-dependent DFT (TDDFT). Potential energy curves (PECs) calculated for the Ru-N (py) bonds in [(η⁶-benz)Ru(bpm)(py)]²⁺ in the singlet and triplet state give useful insights into the photodissociation mechanism of py. The binding energies of the various biomolecules are calculated, which allows the order of binding affinities among the considered nuleic-acid- or protein-binding sites to be discerned. The kinetics for the replacement of water in the aqua complex with biomolecules is also considered, and the results demonstrate that guanine is superior to other biomolecules in terms of coordinating with the Ru(II) aqua adduct, which is in reasonable agreement with experimental observations.

Insights into the acid-base properties of Pt(IV)-diazidodiam(m)inedihyroxido complexes from multinuclear NMR spectroscopy

Platinum(IV) am(m)ine complexes are of interest as potential anticancer pro-drugs, but there are few reports of their acid-base properties. We have studied the acid-base properties of three photoactivatable anticancer platinum(IV)-diazidodiam(m)ine complexes (cis,trans,cis-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], and cis,trans-[Pt(IV)(N(3))(2)(OH)(2)(en)]) using multinuclear NMR methods and potentiometry. In particular, the combination of both direct and indirect techniques for the detection of (15)N signals has allowed changes of the chemical shifts to be followed over the pH range 1-11; complementary (14)N NMR studies have been also carried out. A distinct pK(a) value of approximately 3.4 was determined for all the investigated complexes, involving protonation/deprotonation reactions of one of the axial hydroxido groups, whereas a second pH-dependent change for the three complexes at approximately pH 7.5 appears not to be associated with a loss of an am(m)ine or hydroxido proton from the complex. Our findings are discussed in comparison with the limited data available in the literature on related complexes.

[Ru(bpy)2(5-cyanouracil)2]2+ as a potential light-activated dual-action therapeutic agent

The cation cis-[Ru(bpy)(2)(5CNU)(2)](2+) (bpy = 2,2'-bipyridine; 5CNU = 5-cyanouracil) was synthesized and investigated for use as a potential light-activated dual-action therapeutic agent. The complex undergoes efficient photoinduced 5CNU ligand exchange for solvent water molecules, thus simultaneously releasing biologically active 5CNU and generating [Ru(bpy)(2)(H(2)O)(2)](2+). The latter binds covalently to ds-DNA, such that photolysis results in the generation of 3 equiv of potential therapeutic agents from a single molecule.

Probing platinum azido complexes by 14N and 15N NMR spectroscopy

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) azido complexes is extremely broad (W(1/2)≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic<py<NH(3)<Cl(-)<N(3)(-)<NO(2)(-)<SCN(-) (2-pic=2-methylpyridine). In addition, an axial Cl(-) appears to weaken an equatorial Pt(IV)-NH(3) bond to a greater extent than an axial OH(-) ligand.