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.

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.

Novel Pt(IV) complexes to overcome multidrug resistance in gastric cancer by targeting P-glycoprotein

Systematic toxicity and drug resistance significantly limited FDA-approved platinum drugs for further clinical applications. In order to reverse the resistance (MDR) and enhance their anticancer efficiency, four Pt(IV) complexes (12-15) conjugating with P-glycoprotein (P-gp) inhibitors were designed and synthesized. Among them, complex 14 (IC₅₀ = 3.37 μM) efficiently reversed cisplatin resistance in SGC-7901/CDDP cell line and increased selectivity index (6.9) against normal HL-7702 cell line. Detailed mechanisms in SGC-7901/CDDP cells assays revealed that complex 14 efficiently induced apoptosis via down-regulating expression of P-gp for enhanced intracellular uptake of platinum, arrested cells at G2/M phase, induced DNA damage and initiated mitochondrial apoptosis pathway. Further in vivo studies demonstrated that the enhanced accumulation of complex 14 contributed to tumor inhibition of 75.6% in SGC-7901/CDDP xenografts, which was much higher than cisplatin (25.9%) and oxaliplatin (43%). Moreover, the low systematic toxicity made 14 a potential novel P-gp-mediated MDR modulator.

Synthesis, structural characterization and electrochemical and magnetic studies of M(hfac)2 (M = CuII, CoII) and Nd(hfac)3 complexes of 4-amino-TEMPO

Three mononuclear complexes [M(hfac)x(ATEMPO)y], where M = Cu (11) and Co (12), x = y = 2; M = Nd (13), x = 4, y = 1, and two polynuclear complexes [{Cu(hfac)2(ATEMPO)}n], where n = 2 (14) and 4 (15), were obtained by the reaction of M(hfac)x (M = CuII, CoII, NdIII; x = 2, 3) with 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-N-oxyl) in good yields and their structural, electrochemical and magnetic properties were examined. In all cases, the radical is coordinated to the metal through the amino group, except 15, and the metal ions have an octahedral geometry, except 13. Different coordination architectures of the copper complexes were obtained as a function of the stoichiometry and solvents used. In complexes 11 and 12 the radicals show an equatorial-equatorial and axial-equatorial arrangement, respectively, giving rise to two distinct 2D supramolecular systems through intermolecular interactions. Compound 13 is the first example of a lanthanide complex of the ATEMPO radical. The NdIII ion adopts a rare nine-coordination via binding to four hfac ligands and the radical. The dinuclear complex 14 shows a (Cu-O)2 core in which the CuII ions are bridged by the oxygen atoms from the hfac ligands. In compound 15 the ATEMPO radical acts as a bidentate ligand through the amino and nitroxyl groups leading to an unprecedented tetranuclear square-shaped framework. Cyclic voltammetry showed redox processes associated with the copper and TEMPO moieties. Electrochemical impedance spectroscopy revealed the temperature dependence of the conductivity for compound 15 with a maximum of 2.09 × 10-5 S cm-1 at 408 K. The magnetic behavior of complexes 11-15 is determined by metal-radical interactions. Ferromagnetic interaction has been observed for complex 11 due to the existence of two different exchange pathways arising from the conformational arrangement of the radicals around the metal center, whereas the single conformation of the radical in complex 14 resulted in a weak antiferromagnetic coupling. In complex 15 both O-Cu and N-Cu contacts are present giving rise to ferromagnetic and antiferromagnetic interactions, respectively.

Metallocomplex-Peptide Interactions Studied by Ultrahigh Resolution Mass Spectrometry

The Os^II arene anticancer complex [(η⁶-bip)Os(en)Cl]⁺ (Os1-Cl; where bip = biphenyl and en = ethylenediamine) binds strongly to DNA¹ and biomolecules. Here we investigate the interaction between Os1-Cl and the model protein, BSA, using ultrahigh resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). The specific binding location of Os1 on BSA was investigated with the use of collisionally activated dissociation (CAD) and electron capture dissociation (ECD). CAD MS/MS was found to dissociate the osmium complex from the metallo-peptide complex readily producing unmodified fragments and losing location information. ECD MS/MS, however, successfully retains the osmium modification on the peptides upon fragmentation allowing localization of metallocomplex binding. This study reveals that lysine is a possible binding location for Os1-Cl, apart from the expected binding sites at methionine, histidine, and cysteine. Using a nano liquid chromatography (nLC)-FT-ICR ECD MS/MS study, multiple binding locations, including the N-terminus and C-terminus of digested peptides, glutamic acid, and lysine were also revealed. These results show the multitargeting binding ability of the organo-osmium compound and can be used as a standard workflow for more complex systems, e.g., metallocomplex-cell MS analysis, to evaluate their behavior toward commonly encountered biomolecules.

Novel tetranuclear PdII and PtII anticancer complexes derived from pyrene thiosemicarbazones

Cyclometallated palladium(ii) and platinum(ii) pyrenyl-derived thiosemicarbazone (H₂PrR) complexes of the type [M₄(μ-S-PrR-κ³-C,N,S)₄] (M = Pd^II, Pt^II; R = ethyl, cyclohexyl) have been synthesised in good yields and fully characterised.

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.

Photoactive platinum(iv) complex conjugated to a cancer-cell-targeting cyclic peptide

Conjugate of a cancer-cell targeting cyclic peptide with a photoactive platinum(iv) complex exhibits enhanced photocytotoxicity and cell accumulation.

Photoactivatable Cell-Selective Dinuclear trans-Diazidoplatinum(IV) Anticancer Prodrugs

A series of dinuclear octahedral Pt^IV complexes trans,trans,trans-[{Pt(N₃)₂(py)₂(OH)(OC(O)CH₂CH₂C(O)NH)}₂R] containing pyridine (py) and bridging dicarboxylate [R = −CH₂CH₂– (1), trans-1,2-C₆H₁₀– (2), p-C₆H₄– (3), −CH₂CH₂CH₂CH₂– (4)] ligands have been synthesized and characterized, including the X-ray crystal structures of complexes 1·2MeOH and 4, the first photoactivatable dinuclear Pt^IV complexes with azido ligands. The complexes are highly stable in the dark, but upon photoactivation with blue light (420 nm), they release the bridging ligand and mononuclear photoproducts. Upon irradiation with blue light (465 nm), they generate azidyl and hydroxyl radicals, detected using a 5,5-dimethyl-1-pyrroline N-oxide electron paramagnetic resonance spin trap, accompanied by the disappearance of the ligand-to-metal charge-transfer (N₃ → Pt) band at ca. 300 nm. The dinuclear complexes are photocytotoxic to human cancer cells (465 nm, 4.8 mW/cm², 1 h), including A2780 human ovarian and esophageal OE19 cells with IC₅₀ values of 8.8–78.3 μM, whereas cisplatin is inactive under these conditions. Complexes 1, 3, and 4 are notably more photoactive toward cisplatin-resistant ovarian A2780cis compared to A2780 cells. Remarkably, all of the complexes were relatively nontoxic toward normal cells (MRC5 lung fibroblasts), with IC₅₀ values >100 μM, even after irradiation. The introduction of an aromatic bridging ligand (3) significantly enhanced cellular uptake. The populations in the stages of the cell cycle remained unchanged upon treatment with complexes in the dark, while the population of the G2/M phase increased upon irradiation, suggesting that DNA is a target for these photoactivated dinuclear Pt^IV complexes. Liquid chromatography–mass spectrometry data show that the photodecomposition pathway of the dinuclear complexes results in the release of two molecules of mononuclear platinum(II) species. As a consequence, DNA binding of the dinuclear complexes after photoactivation in cell-free media is, in several respects, qualitatively similar to that of the photoactivated mononuclear complex FM-190. After photoactivation, they were 2-fold more effective in quenching the fluorescence of EtBr bound to DNA, forming DNA interstrand cross-links and unwinding DNA compared to the photoactivated FM-190.

Novel all-trans dinuclear Pt^IV complexes bridged by a dicarboxylate linker, highly stable in the dark, generate azidyl and hydroxyl radicals upon irradiation with blue light. They are photocytotoxic to human cancer cells, whereas cisplatin was inactive under these conditions and more photoactive toward cisplatin-resistant ovarian cancer cells compared to wild-type cells. Remarkably, the dinuclear complexes were relatively nontoxic toward normal human cells. Cell cycle and DNA binding experiments suggested that DNA is a target.

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.

Mitochondria-targeted spin-labelled luminescent iridium anticancer complexes

Mitochondria generate energy but malfunction in many cancer cells, hence targeting mitochondrial metabolism is a promising approach for cancer therapy. Here we have designed cyclometallated iridium(iii) complexes, containing one TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) spin label [C43H43N6O2Ir1·PF6]˙ (Ir-TEMPO1) and two TEMPO spin labels [C52H58N8O4Ir1·PF6]˙ (Ir-TEMPO2). Electron paramagnetic resonance (EPR) spectroscopy revealed spin-spin interactions between the TEMPO units in Ir-TEMPO2. Both Ir-TEMPO1 and Ir-TEMPO2 showed bright luminescence with long lifetimes (ca. 35-160 ns); while Ir-TEMPO1 displayed monoexponential decay kinetics, the biexponential decays measured for Ir-TEMPO2 indicated the presence of more than one energetically-accessible conformation. This observation was further supported by density functional theory (DFT) calculations. The antiproliferative activity of Ir-TEMPO2 towards a range of cancer cells was much greater than that of Ir-TEMPO1, and also the antioxidant activity of Ir-TEMPO2 is much higher against A2780 ovarian cancer cells when compared with Ir-TEMPO1. Most notably Ir-TEMPO2 was particularly potent towards PC3 human prostate cancer cells (IC50 = 0.53 μM), being ca. 8× more active than the clinical drug cisplatin, and ca. 15× more selective towards cancer cells versus normal cells. Confocal microscopy showed that both Ir-TEMPO1 and Ir-TEMPO2 localise in the mitochondria of cancer cells.

Theranostic TEMPO-functionalized Ru(ii) complexes as photosensitizers and oxidative stress indicators

New TEMPO-functionalized Ru(ii) polypyridyl complexes displayed greatly improved PDT efficacy, capable of simultaneously monitoring cellular oxidative stress during photodynamic therapy.