Luca Salassa

Photoactivated chemotherapy (PACT): the potential of excited-state d-block metals in medicine

The fields of phototherapy and of inorganic chemotherapy both have long histories. Inorganic photoactivated chemotherapy (PACT) offers both temporal and spatial control over drug activation and has remarkable potential for the treatment of cancer. Following photoexcitation, a number of different decay pathways (both photophysical and photochemical) are available to a metal complex. These pathways can result in radiative energy release, loss of ligands or transfer of energy to another species, such as triplet oxygen. We discuss the features which need to be considered when developing a metal-based anticancer drug, and the common mechanisms by which the current complexes are believed to operate. We then provide a comprehensive overview of PACT developments for complexes of the different d-block metals for the treatment of cancer, detailing the more established areas concerning Ti, V, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Pt, and Cu and also highlighting areas where there is potential for greater exploration. Nanoparticles (Ag, Au) and quantum dots (Cd) are also discussed for their photothermal destructive potential. We also discuss the potential held in particular by mixed-metal systems and Ru complexes.

A Potent Trans‐Diimine Platinum Anticancer Complex Photoactivated by Visible Light

Activating platinum with light: An inert platinum(IV) diazido complex trans, trans,trans-[Pt(N3)2(OH)2(py)2] becomes potently cytotoxic to cancer cells when activated by low doses of visible light.

The contrasting activity of iodido versus chlorido ruthenium and osmium arene azo- and imino-pyridine anticancer complexes: control of cell selectivity, cross-resistance, p53 dependence, and apoptosis pathway.

Organometallic half-sandwich complexes [M(p-cymene)(azo/imino-pyridine)X](+) where M = Ru(II) or Os(II) and X ═ Cl or I, exhibit potent antiproliferative activity toward a range of cancer cells. Not only are the iodido complexes more potent than the chlorido analogues, but they are not cross-resistant with the clinical platinum drugs cisplatin and oxaliplatin. They are also more selective for cancer cells versus normal cells (fibroblasts) and show high accumulation in cell membranes. They arrest cell growth in G1 phase in contrast to cisplatin (S phase) with a high incidence of late-stage apoptosis. The iodido complexes retain potency in p53 mutant colon cells. All complexes activate caspase 3. In general, antiproliferative activity is greatly enhanced by low levels of the glutathione synthase inhibitor l-buthionine sulfoxime. The work illustrates how subtle changes to the design of low-spin d(6) metal complexes can lead to major changes in cellular metabolism and to potent complexes with novel mechanisms of anticancer activity.

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 PtIV 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 1 p and 1 q. 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 PtII 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.

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 PtIV diazido complexes have unusual photobiologic properties. We show here that trans,trans,trans-[PtIV(N3)2(OH)2(py)(NH3)] complex 3 is a potent photoactivated cytotoxin toward human cancer cells in culture, with an average IC50 value in 13 cell lines of 55 ± 28 μmol/L after 30 minutes (0.12 mW/cm2) 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 14N{1H} 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. Mol Cancer Ther; 11(9); 1894–904. ©2012 AACR.

Cationic Heteroleptic Cyclometalated Iridium Complexes with 1‐Pyridylimidazo[1,5‐α]pyridine Ligands: Exploitation of an Efficient Intersystem Crossing

Luminescent ligands in Ir(III) cyclometalated complexes. The photophysical and photochemical properties of Ir-cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir-cyclometalated complexes containing N(amido) groups directly bound to Ir.A series of phosphorescent cyclometalated heteroleptic iridi um(III) phenylpyridinato (ppy) complexes containing luminescent 1-pyridylimidazo[1,5-alpha]pyridine (pip) ligands, namely [Ir(ppy)(2)(pip)](+), have been synthesised, characterised and their electrochemical, photophysical and electronic properties studied. Seven X-ray structures have been resolved. Excitation of [Ir(ppy)(2)(pip)](+) in acetonitrile at room temperature results in a dual luminescence, strongly quenched by O(2). Four complexes show, in absence of O(2), a high-energy emission (assigned to a (3)MLLCT transition) with two maxima in the blue region of the visible spectra, and a second structured emission (assigned largely to a (3)LC pi-pi* transition) centred around lambda=555 nm. Lifetimes of high-energy emissions are between 0.6 and 1.3 mus. Time-dependent density functional calculations combined with the conductor-like polarisable continuum model method, with acetonitrile as solvent, have been used to calculate a series of ground and excited states of the derivatives under investigation, and the transitions compared with the experimental UV/Vis absorption spectra. A quick and efficient photochemical reaction has been observed for these iridium derivatives that leads to the formation of a new class of cyclometalated iridium complexes containing a stable deprotonated amide unusually coordinated to the metal through a nitrogen bond. The synthesis of a (15)N enriched selected ligand has been performed to investigate, by means of NMR, the particular facile route to these new set of derivatives. The electrochemical behaviour of all complexes is also reported.

Decomposition pathways for the photoactivated anticancer complex cis,trans,cis-[Pt(N3)2(OH)2(NH3)2]: insights from DFT calculations

Density functional theory (DFT) and time-dependent DFT (TD-DFT) provide new insights into the photodegradation pathways of the cytotoxic complex cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))(2)] allowing assignment of (1)LMCT transitions in the visible region of the UV-Vis spectrum; upon excitation to these low-energy (1)LMCT states, release of one N(3)(-) ligand is facilitated, and on triplet formation, the dissociation of both NH(3) and N(3)(-) groups trans to each other is promoted with no apparent reduction of the Pt(IV) centre.

Cytotoxic Gold(I) N-heterocyclic Carbene Complexes with Phosphane Ligands as Potent Enzyme Inhibitors

Organometallic gold complexes with N‐heterocyclic carbene (NHC) ligands have been demonstrating promising properties as novel anticancer agents. Gold(I) NHC complexes containing different phosphanes as secondary ligands were shown to trigger strong cytotoxic effects in cancer cells, and their effective uptake into the cells was quantified by atomic absorption spectroscopy. Moreover, the new compounds strongly inhibited the activity of the seleno‐enzyme thioredoxin reductase (TrxR) and of the zinc‐finger enzyme poly(ADP‐ribose) polymerase 1 (PARP‐1). In the case of TrxR inhibition, their activity depended clearly on the size of the alkyl/aryl residues of phosphorus atoms. Density functional theory (DFT) calculations showed that the AuP bond of the triphenylphosphane complex [AuI(NHC)(PPh3)]I had a lower bond dissociation energy compared to trialkylphosphane complexes [AuI(NHC)(PR3)]I, indicating a higher kinetic reactivity of this particular compound. In fact, [AuI(NHC)(PPh3)]I triggered an enhanced inhibitory activity against PARP‐1.

Mirror-Image Organometallic Osmium Arene Iminopyridine Halido Complexes Exhibit Similar Potent Anticancer Activity

Four chiral OsII arene anticancer complexes have been isolated by fractional crystallization. The two iodido complexes, (SOs,SC)-[Os(η6-p-cym)(ImpyMe)I]PF6 (complex 2, (S)-ImpyMe: N-(2-pyridylmethylene)-(S)-1-phenylethylamine) and (ROs,RC)-[Os(η6-p-cym)(ImpyMe)I]PF6 (complex 4, (R)-ImpyMe: N-(2-pyridylmethylene)-(R)-1-phenylethylamine), showed higher anticancer activity (lower IC50 values) towards A2780 human ovarian cancer cells than cisplatin and were more active than the two chlorido derivatives, (SOs,SC)-[Os(η6-p-cym)(ImpyMe)Cl]PF6, 1, and (ROs,RC)-[Os(η6-p-cym)(ImpyMe)Cl]PF6, 3. The two iodido complexes were evaluated in the National Cancer Institute 60-cell-line screen, by using the COMPARE algorithm. This showed that the two potent iodido complexes, 2 (NSC: D-758116/1) and 4 (NSC: D-758118/1), share surprisingly similar cancer cell selectivity patterns with the anti-microtubule drug, vinblastine sulfate. However, no direct effect on tubulin polymerization was found for 2 and 4, an observation that appears to indicate a novel mechanism of action. In addition, complexes 2 and 4 demonstrated potential as transfer-hydrogenation catalysts for imine reduction.

Photo-induced pyridine substitution in cis-[Ru(bpy)(2)(py)(2)]Cl(2): a snapshot by time-resolved X-ray solution scattering.

Determination of transient structures in light-induced processes is a challenging goal for time-resolved techniques. Such techniques are becoming successful in detecting ultrafast structural changes in molecules and do not require the presence of probe-like groups. Here, we demonstrate that TR-WAXS (Time-Resolved Wide Angle X-ray Scattering) can be successfully employed to study the photochemistry of cis-[Ru(bpy)(2)(py)(2)]Cl(2), a mononuclear ruthenium complex of interest in the field of photoactivatable anticancer agents. TR-WAXS is able to detect the release of a pyridine ligand and the coordination of a solvent molecule on a faster timescale than 800 ns of laser excitation. The direct measurement of the photodissociation of pyridine is a major advance in the field of time-resolved techniques allowing detection, for the first time, of the release of a multiatomic ligand formed by low Z atoms. These data demonstrate that TR-WAXS is a powerful technique for studying rapid ligand substitution processes involving photoactive metal complexes of biological interest.