Spontaneous Reduction of Mixed 2,2′-Bipyridine/Methylamine/Chloro Complexes of PtIV in Water in the Presence of Light Is Accompanied by Complex Isomerization, Loss of Methylamine, and Formation of a Strong Oxidant, Presumably HOCl

Ligand Evolution in the Photoactivatable Platinum(IV) Anticancer Prodrugs

Photoactivatable Pt(IV) anticancer prodrugs with the structure of [Pt^IV(N₁)(N₂)(L₁)(L₂)(A₁)(A₂)], where N₁ and N₂ are non-leaving nitrogen donor ligands, L₁ and L₂ are leaving ligands, and A₁ and A₂ are axial ligands, have attracted increasing attention due to their promising photo-cytotoxicity even to cisplatin-resistant cancer cells. These photochemotherapeutic prodrugs have high dark-stability under physiological conditions, while they can be activated by visible light restrained at the disease areas, as a consequence showing higher spatial and temporal controllability and much more safety than conventional chemotherapy. The coordinated ligands to the Pt center have been proved to be pivotal in determining the function and activity of the photoactivatable Pt(IV) prodrugs. In this review, we will focus on the development of the coordinated ligands in such Pt(IV) prodrugs and discuss the effects of diverse ligands on their photochemistry and photoactivity as well as the future evolution directions of the ligands. We hope this review can help to facilitate the design and development of novel photoactivatable Pt(IV) anticancer prodrugs.

Reactions of a photoactivatable diazido Pt(iv) anticancer complex with a single-stranded oligodeoxynucleotide

Platinum based anticancer agents are widely applied in clinic and their major target is believed to be DNA. Herein, the interaction of a photoactivatable diazido Pt(iv) anticancer prodrug trans,trans,trans-[Pt(N3)2(OH)2(py)2] (py = pyridine; 1) with a 15-mer single-G-containing oligodeoxynucleotide (ODN I: 5'-CT2CTCTTG8T9CT11TCTC-3') was investigated by mass spectrometric methods. Up to penta-platinated ODN I adducts were identified from primary mass spectra while the mono- and di-platinated adducts had the highest intensity. Fragmentation of mono-, di- and tri-platinated I adducts in tandem MS revealed that T2, G8, T11 and T9 are binding sites. No cytosine sites were identified which may be due to the facile loss of Pt adducts from cytosine during CID. The intensity of {Pt(py)2}-bound adducts was comparable to that of {Pt(N3)(py)2}-bound adducts, indicating that the photo-reduction pathway of complex 1 from Pt(iv) to Pt(ii) through two one-electron donations from two azides was substantial. Moreover, no transformation of N3 to NH3 on the {Pt(N3)(py)2}-bound adducts was observed, whereas it is very popular during the reactions of complexes with short ODNs or mono-nucleotides. The oxidation on I induced by the reactive oxygen species (ROS) formed by the photodecomposition of complex 1 was significant, and the oxidation of G8 to 8-hydroxyguanine (8-OH-G), spiroiminodihydantoin (Sp) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) was discovered. These results unambiguously revealed a sequence-length-dependent photochemical reactivity of complex 1 when it interacted with different ODNs, providing deeper understanding in the reactivity of photoactivatable diazido anticancer Pt(iv) prodrugs to DNA.

Photoactivatable diazido Pt(IV) anticancer complex can bind to and oxidize all four nucleosides

Photoactivatable diazidodihydroxido Pt(iv) complex trans,trans,trans-[Pt(N3)2(OH)2(py)2] (1; py = pyridine) is a promising anticancer agent which can be activated by visible light to induce cancer cell death. DNA has been thought to be involved in the mechanism of action of this kind of Pt(iv) prodrug. However, the detailed photodecomposition pathways of complex 1 and its interaction modes with DNA are complex. Herein we report that upon light irradiation complex 1 can bind to all four nucleosides covalently with the reduced Pt(ii) species. Moreover, apart from the covalent coordination, various oxidation adducts of these four nucleosides induced by the reactive oxidative species (ROS) generated during the photoactivation of the complex 1 have also been identified, especially the induced oxidation of adenosine and cytidine which was firstly reported for this kind of photoactivatable Pt(iv) prodrug. Such dual-action may contribute to the highly potent photo-antiproliferativity of complex 1 towards cancer cells, which may account for the unique mechanism of action of the photoactivatable diazido Pt(iv) anticancer complexes.

Photoactivatable Platinum-Based Anticancer Drugs: Mode of Photoactivation and Mechanism of Action

Platinum-based anticancer drugs are a class of widely used agents in clinical cancer treatment. However, their efficacy was greatly limited by their severe side effects and the arising drug resistance. The selective activation of inert platinum-based drugs in the tumor site by light irradiation is able to reduce side effects, and the novel mechanism of action of photoactivatable platinum drugs might also conquer the resistance. In this review, the recent advances in the design of photoactivatable platinum-based drugs were summarized. The complexes are classified according to their mode of action, including photoreduction, photo-uncaging, and photodissociation. The rationale of drug design, dark stability, photoactivation process, cytotoxicity, and mechanism of action of typical photoactivatable platinum drugs were reviewed. Finally, the challenges and opportunities for designing more potent photoactivatable platinum drugs were discussed.

Unexpected Thymine Oxidation and Collision-Induced Thymine-Pt-guanine Cross-Linking on 5'-TpG and 5'-GpT by a Photoactivatable Diazido Pt(IV) Anticancer Complex

The photochemical products of dinucleotides 5'-TpG/5'-GpT with a photoactivatable anticancer Pt(IV) complex (trans,trans,trans-[Pt(N₃)₂(OH)₂(py)₂], py = pyridine; 1) were characterized by electrospray ionization mass spectrometry. The primary MS showed the main products were monoplatinated and diplatinated adducts for both the dinucleotides accompanied by the formation of minor triplatinated dinucleotides, indicating that T-N3 and G-N1 may be platination sites additional to the well-known G-N7 site. Surprisingly, a series of minor platinated adducts with oxidation of guanine and/or thymine were observed. Although guanine is more sensitive to oxidation than thymine, thymine can compete with guanine for complex 1-induced oxidation, of which the oxidation adducts were identified as cis- and trans-diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine (cis,trans-ThdGly), 5-formyl-2'-deoxyuridine (5-FormdUrd), and 5-(hydroxymethyl)-2'-deoxyuridine (5-HMdUrd), respectively. While for guanine, apart from 8-hydroxyguanine (8-OH-G) and N-formylamidoiminohydantoin (RedSp), other guanine oxidized adducts such as spiroiminodihydantoin (Sp), dehydroguanidinohydantoin (DGh), and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) were also identified. MS/MS analysis showed that unique fragments with a Pt moiety [Pt(N₃)(py)] cross-linking the G and T bases were formed during the fragmentation of monoplatinated dinucleotides. Such binding mode to and oxidative damages on DNA bases imposed by the diazido Pt(IV) complex are apparently distinct from those of cisplatin, perhaps accounting for its unique mechanism of action.

Stable mixed-valence diphenylphosphanido bridged platinum(ii)-platinum(iv) complexes

The reaction between [NnBu4][(C6F5)2PtII(μ-PPh2)2PtIV(C^N)(I)2] (C^N = κ2-N,C-benzoquinolinate, 1) and (i) bidentate S^S, N^S and O^O anionic ligands or (ii) monodentate S- N- or O-based anionic ligands was studied in order to investigate the factors that may guarantee the stability of Pt(ii),Pt(iv) mixed-valence dinuclear phosphanido complexes. While reactions of 1 with S^S or N^S ligands afforded stable Pt(ii),Pt(iv) species of general formula [(C6F5)2PtII(μ-PPh2)2PtIV(C^N)(L^S)]x- [(L^S)(x-1) = 2-mercaptopyrimidinate (pymS-), 2-mercaptopyridinate (pyS-), dimethyldithiocarbamate (Me2NCS2-), ethyl xanthogenate (EtOCS2-) and 1,2-benzenedithiolate (PhS22-)], the reaction of 1 with the O^O ligand sodium acetylacetonate gave several products, and no pure Pt(ii),Pt(iv) complex could be isolated. The reaction of monodentate ligands such as PhS-, OH- or N3- with 1 led to a stable Pt(ii),Pt(iv) complex only in the case of N3-. The reaction with OH- afforded the Pt(ii),Pt(ii) complex [(C6F5)2PtII(μ-PPh2)(κ2-O,P-μ-O-PPh2)PtII(C^N)]- (8) deriving from reductive coupling of a diphenylphosphanide and an O-donor ligand coordinated to the Pt(iv) centre, while the reaction with PhS- produced the unstable Pt(ii),Pt(iv) complex [NnBu4][(C6F5)2PtII(μ-PPh2)2PtIV(C^N)(PhS)2] (11) that evolved in solution to the Pt(ii),Pt(ii) species [NnBu4][(C6F5)2PtII(μ-PPh2)2PtII(C^N)] (9) by elimination of diphenyldisulfide. Thus, the stability of mixed valence Pt(ii),Pt(iv) phosphanide complexes is affected by several concurrent factors, including the chelating effect of the ligands and the type of ligating atoms.

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.

Stimuli-Responsive Therapeutic Metallodrugs

The success of platinum-based anticancer agents has motivated the exploration of novel metal-based drugs for several decades, whereas problems such as drug-resistance and systemic toxicity hampered their clinical applications and efficacy. Stimuli-responsiveness of some metal complexes offers a good opportunity for designing site-specific prodrugs to maximize the therapeutic efficacy and minimize the side effect of metallodrugs. This review presents a comprehensive and up-to-date overview on the therapeutic stimuli-responsive metallodrugs that have appeared in the past two decades, where stimuli such as redox, pH, enzyme, light, temperature, and so forth were involved. The compounds are classified into three major categories based on the nature of stimuli, that is, endo-stimuli-responsive metallodrugs, exo-stimuli-responsive metallodrugs, and dual-stimuli-responsive metallodrugs. Representative examples of each type are discussed in terms of structure, response mechanism, and potential medical applications. In the end, future opportunities and challenges in this field are tentatively proposed. With diverse metal complexes being introduced, the foci of this review are pointed to platinum and ruthenium complexes.

Probing the Interactions of Cytotoxic [Pt(1S,2S-DACH)(5,6-dimethyl-1,10-phenanthroline)] and Its PtIV Derivatives with Human Serum

The discrepancy between the in vitro cytotoxic results and the in vivo performance of Pt56MeSS prompted us to look into its interactions and those of its Pt^IV derivatives with human serum (HS), human serum albumin (HSA), lipoproteins, and serum-supplemented cell culture media. The Pt^II complex, Pt56MeSS, binds noncovalently and reversibly to slow-tumbling proteins in HS and in cell culture media and interacts through the phenanthroline group with HSA, with a K_d value of ∼1.5×10^-6  m. All Pt^IV complexes were found to be stable toward reduction in HS, but those with axial carboxylate ligands, cct-[Pt(1S,2S-DACH)(5,6-dimethyl-1,10-phenantroline)(acetato)₂ ](TFA)₂ (Pt56MeSS(OAc)₂ ) and cct-[Pt(1S,2S-DACH)(5,6-dimehtyl-1,10-phenantroline)(phenylbutyrato)₂ ](TFA)₂ (Pt56MeSS(PhB)₂ ), were spontaneously reduced at pH 7 or higher in phosphate buffer, but not in Tris buffer (pH 8). HS also decreased the rate of reduction by ascorbate of the Pt^IV complexes relative to the reduction rates in phosphate buffer, suggesting that for this compound class, phosphate buffer is not a good model for HS.

DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks

Ultrawideline dynamic nuclear polarization (DNP)-enhanced (195)Pt solid-state NMR (SSNMR) spectroscopy and theoretical calculations are used to determine the coordination of atomic Pt species supported within the pores of metal-organic frameworks (MOFs). The (195)Pt SSNMR spectra, with breadths reaching 10 000 ppm, were obtained by combining DNP with broadbanded cross-polarization and CPMG acquisition. Although the DNP enhancements in static samples are lower than those typically observed under magic-angle spinning conditions, the presented measurements would be very challenging using the conventional SSNMR methods. The DNP-enhanced ultrawideline NMR spectra served to separate signals from cis- and trans-coordinated atomic Pt(2+) species supported on the UiO-66-NH2 MOF. Additionally, the data revealed a dominance of kinetic effects in the formation of Pt(2+) complexes and the thermodynamic effects in their reduction to nanoparticles. A single cis-coordinated Pt(2+) complex was confirmed in MOF-253.

Synthesis and Analysis of the Structure, Diffusion and Cytotoxicity of Heterocyclic Platinum(IV) Complexes

We have developed six dihydroxidoplatinum(IV) compounds with cytotoxic potential. Each derived from active platinum(II) species, these complexes consist of a heterocyclic ligand (HL) and ancillary ligand (AL) in the form [Pt(HL)(AL)(OH)2](2+), where HL is a methyl-functionalised variant of 1,10-phenanthroline and AL is the S,S or R,R isomer of 1,2-diaminocyclohexane. NMR characterisation and X-ray diffraction studies clearly confirmed the coordination geometry of the octahedral platinum(IV) complexes. The self-stacking of these complexes was determined using pulsed gradient stimulated echo nuclear magnetic resonance. The self-association behaviour of square planar platinum(II) complexes is largely dependent on concentration, whereas platinum(IV) complexes do not aggregate under the same conditions, possibly due to the presence of axial ligands. The cytotoxicity of the most active complex, exhibited in several cell lines, has been retained in the platinum(IV) form.

A conformationally flexible dinuclear Pt(II) complex with differential behavior of its two states toward quadruplex DNA

The reaction of tetrakis(pyridine-2-yl)pyrazine (tppz) with 2 equiv of (2,2'-bpy)Pt(II) in water yields two isomeric dinuclear cations, [{Pt(2,2'-bpy)}2 (tppz)](4+) , in which Pt coordination exclusively takes place through the two pairs of pyridine-2-yl nitrogen atoms. The two conformational isomers differ in their overall shape, with the formation of "Z" and "U" shapes, which are formed at 40 °C (Z isomer, 1) and under reflux conditions (U isomer, 2), respectively. X-ray crystal-structure analyses of the Z isomer, [{Pt(2,2'-bpy)}2 (tppz)](PF6 )4 ⋅3 CHCl3 ⋅4 H2 O (1 a), and of the U isomer, [{Pt(2,2'-bpy)}2 ](PF6 )4 ⋅2 CH3 CN⋅1.5 H2 O (2 a), were carried out. Co-crystallization of compound 2 with PtCl2 (2,2'-bpy) yielded [{Pt(2,2'-bpy)}2 (tppz)](BF4 )4 ⋅[PtCl2 (2,2'-bpy)]⋅4.5 H2 O (3), in which the PtCl2 (2,2'-bpy) entity was sandwiched between the two 2,2'-bpy faces of the U-shaped cation (2). Quantum chemical calculations revealed that the U isomer was more stable than the Z isomer, both in the gas phase and in an aqueous environment. These two isomers display different affinities toward duplex DNA and human telomeric quadruplex DNA (Htelo), as concluded from CD spectroscopy and FID assays. Thus, the U isomer binds significantly more strongly to quadruplex DNA (DC50 =0.38 μM) than the Z isomer (DC50 =8.50 μM).

Studies on the photoactivation of two cytotoxic trans,trans,trans-diazidodiaminodihydroxo-Pt(IV) complexes

Light-activation of metal ion complexes to cytotoxic species is of interest due to the potential use in anticancer therapy. Two platinum complexes, trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)] (3) and trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(py)(NH(3))] (4) were irradiated with either UV (λ = 366 nm) or white fluorescent light and the various photochemical and photobiological phenomena were characterized. HPLC coupled to UV/Vis and MS detection was used to identify photochemical species resulting from irradiation of 4 with UV and white light. These studies showed that various Pt(IV) and Pt(II) products formed during the photolysis. The mass spectra of Pt(IV) complexes showed Pt ions in both the positive as well as the negative mode while Pt(II) complexes resulted in only positively charged Pt(III) ions. Since cellular DNA is considered to be a key target for platinum antitumor drugs, the irreversible platination of calf thymus DNA by the photoactivated Pt(IV) complexes was followed by Atomic Adsorption spectrometry (AAS). The effect of adding chloride or biological reducing agents glutathione (GSH) and ascorbic acid on the rates of DNA platination where also studied. Upon activation by light, both compounds show similar binding behaviour to DNA, but the rates of DNA platination for 3 were faster than for 4. Both chloride and GSH protected DNA from platination by the photoactivated compounds; consistent with the trapping of reactive aqua-Pt species. The presence of ascorbate increased the level of platinum bound to DNA for photoactivated 4 but not for 3. Without photoactivation, little or no DNA platination was observed, either with or without ascorbate or GSH. Cytotoxicity studies with two human cancer cell lines underline the photochemotherapeutic potential of these compounds. Striking is the increase in cytotoxic potency with the replacement of an ammine by a pyridine ligand.

Effective photoreduction of a Pt(IV) complex with quantum dots: a feasible new light-induced method of releasing anticancer Pt(II) drugs

Irradiation of CdSe-ZnS quantum dots (QDs) with visible light in the presence of [PtCl(4)(bpy)] (1) (bpy = 2,2'-bipyridine) produced with high efficiency [PtCl(2)(bpy)] (2) by photoinduced electron transfer; a reaction and strategy which opens up new opportunities for cancer therapy.

Synthesis, characterisation and photochemistry of Pt(IV) pyridyl azido acetato complexes

Pt(II) azido complexes [Pt(bpy)(N(3))(2)] (1), [Pt(phen)(N(3))(2)] (2) and trans-[Pt(N(3))(2)(py)(2)] (3) incorporating the bidentate diimine ligands 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or the monodentate pyridine (py) respectively, have been synthesised from their chlorido precursors and characterised by X-ray crystallography; complex 3 shows significant deviation from square-planar geometry (N(3)-Pt-N(3) angle 146.7 degrees ) as a result of steric congestion at the Pt centre. The novel Pt(IV) complexes trans, cis-[Pt(bpy)(OAc)(2)(N(3))(2)] (), trans, cis-[Pt(phen)(OAc)(2)(N(3))(2)] (), trans, trans, trans-[Pt(OAc)(2)(N(3))(2)(py)(2)] (), were obtained from via oxidation with H(2)O(2) in acetic acid followed by reaction of the intermediate with acetic anhydride. Complexes 4-6 exhibit interesting structural and photochemical properties that were studied by X-ray, NMR and UV-vis spectroscopy and TD-DFT (time-dependent density functional theory). These Pt(IV) complexes exhibit greater absorption at longer wavelengths (epsilon = 9756 M(-1) cm(-1) at 315 nm for 4 ; epsilon = 796 M(-1) cm(-1) at 352 nm for 5 ; epsilon = 16900 M(-1) cm(-1) at 307 nm for 6 , in aqueous solution) than previously reported Pt(IV) azide complexes, due to the presence of aromatic amines, and 4-6 undergo photoactivation with both UVA (365 nm) and visible green light (514 nm). The UV-vis spectra of complexes 4-6 were calculated using TD-DFT; the nature of the transitions contributing to the UV-vis bands provide insight into the mechanism of production of the observed photoproducts. The UV-vis spectra of 1-3 were also simulated by computational methods and comparison between Pt(II) and Pt(IV) electronic and structural properties allowed further elucidation of the photochemistry of 4-6.

Photoinduced reactions of cis,trans,cis-[Pt(IV)(N3)2(OH)2(NH3)2) with 1-methylimidazole

The photodecomposition of cis,trans,cis-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)] in phosphate buffered saline (PBS), as well as in the presence of 1-methylimidazole (1-MeIm), induced by UVA light (centred at lambda=365 nm) has been studied by multinuclear NMR spectroscopy. We show that photoreduction, photoisomerisation and trans-labilisation pathways are involved. The photodecomposition pathway in PBS, which involves azide release, as detected by (14)N NMR spectroscopy, appears to differ from that in acidic aqueous conditions, under which N(2) is a product. A number of trans-{N-Pt(II)-NH(3)} species were also observed as photoproducts, as well as the release of free ammonia with a corresponding increase in pH. Oxygen was also detected as a product in solution. In the presence of 1-methylimidazole, surprisingly the major photoproduct was the tetra-substituted Pt(II) complex [Pt(II)(1-MeIm-N(3))(4)](2+) (structure confirmed by crystallography), even at a Pt/1-MeIm molar ratio of 1:1, together with cis- and trans-[Pt(II)(NH(3))(2)(1-MeIm-N(3))(2)](2+) as minor products. In these photoinduced 1-MeIm reactions, free ammonia, azide and oxygen were also detected. The results from this study illustrate that photoinduced reactions of platinum complexes can lead to novel reaction pathways, and therefore to new cytotoxic mechanisms in cancer cells.