Processing and Bypass of a Site-Specific DNA Adduct of the Cytotoxic Platinum-Acridinylthiourea Conjugate by Polymerases Involved in DNA Repair: Biochemical and Thermodynamic Aspects

DNA-dependent DNA and RNA polymerases are important modulators of biological functions such as replication, transcription, recombination, or repair. In this work performed in cell-free media, we studied the ability of selected DNA polymerases to overcome a monofunctional adduct of the cytotoxic/antitumor platinum–acridinylthiourea conjugate [PtCl(en)(L)](NO₃)₂ (en = ethane-1,2-diamine, L = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea) (ACR) in its favored 5′-CG sequence. We focused on how a single site-specific ACR adduct with intercalation potency affects the processivity and fidelity of DNA-dependent DNA polymerases involved in translesion synthesis (TLS) and repair. The ability of the G(N7) hybrid ACR adduct formed in the 5′-TCGT sequence of a 24-mer DNA template to inhibit the synthesis of a complementary DNA strand by the exonuclease-deficient Klenow fragment of DNA polymerase I (KF^exo−) and human polymerases eta, kappa, and iota was supplemented by thermodynamic analysis of the polymerization process. Thermodynamic parameters of a simulated translesion synthesis across the ACR adduct were obtained by using microscale thermophoresis (MST). Our results show a strong inhibitory effect of an ACR adduct on enzymatic TLS: there was only small synthesis of a full-length product (less than 10%) except polymerase eta (~20%). Polymerase eta was able to most efficiently bypass the ACR hybrid adduct. Incorporation of a correct dCMP opposite the modified G residue is preferred by all the four polymerases tested. On the other hand, the frequency of misinsertions increased. The relative efficiency of misinsertions is higher than that of matched cytidine monophosphate but still lower than for the nonmodified control duplex. Thermodynamic inspection of the simulated TLS revealed a significant stabilization of successively extended primer/template duplexes containing an ACR adduct. Moreover, no significant decrease of dissociation enthalpy change behind the position of the modification can contribute to the enzymatic TLS observed with the DNA-dependent, repair-involved polymerases. This TLS could lead to a higher tolerance of cancer cells to the ACR conjugate compared to its enhanced analog, where thiourea is replaced by an amidine group: [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine).

Are Pt(IV) Prodrugs That Release Combretastatin A4 True Multi-action Prodrugs?

"Multi-action" Pt(IV) derivatives of cisplatin with combretastatin A4 (CA4) bioactive ligands that are conjugated to Pt(IV) by carbonate are unique because the ligand (IC₅₀ < 10 nM) is dramatically 1000-folds more cytotoxic than cisplatin in vitro. The Pt(IV)-CA4 prodrugs were as cytotoxic as CA4 itself, indicating that the platinum moiety probably plays an insignificant role in triggering cytotoxicity, suggesting that the Pt(IV)-CA4 complexes act as prodrugs for CA4 rather than as true multi-action prodrugs. In vivo tests (Lewis lung carcinoma) show that ctc-[Pt(NH₃)₂(PhB)(CA4)Cl₂] inhibited tumor growth by 93% compared to CA4 (67%), cisplatin (84%), and 1:1:1 cisplatin/CA4/PhB (85%) while displaying <5% body weight loss compared to cisplatin (20%) or CA4 (10%). In this case, and perhaps with other extremely potent bioactive ligands, platinum(IV) acts merely as a self-immolative carrier triggered by reduction in the cancer cell with only a minor contribution to cytotoxicity.

On the Biology of Werner's Complex

Werner's Complex, as a cationic coordination complex (CCC), has hitherto unappreciated biological properties derived from its binding affinity to highly anionic biomolecules such as glycosaminoglycans (GAGs) and nucleic acids. Competitive inhibitor and spectroscopic assays confirm the high affinity to GAGs heparin, heparan sulfate (HS), and its pentasaccharide mimetic Fondaparinux (FPX). Functional consequences of this affinity include inhibition of FPX cleavage by bacterial heparinase and mammalian heparanase enzymes with inhibition of cellular invasion and migration. Werner's Complex is a very efficient condensing agent for DNA and tRNA. In proof-of-principle for translational implications, it is demonstrated to display antiviral activity against human cytomegalovirus (HCMV) at micromolar concentrations with promising selectivity. Exploitation of non-covalent hydrogen-bonding and electrostatic interactions has motivated the unprecedented discovery of these properties, opening new avenues of research for this iconic compound.

DNA-Intercalative Platinum Anticancer Complexes Photoactivated by Visible Light

Photoactivatable agents offer the prospect of highly selective cancer therapy with low side effects and novel mechanisms of action that can combat current drug resistance. 1,8-Naphthalimides with their extended π system can behave as light-harvesting groups, fluorescent probes and DNA intercalators. We conjugated N-(carboxymethyl)-1,8-naphthalimide (gly-R-Nap) with an R substituent on the naphthyl group to photoactive diazido PtIV complexes to form t,t,t-[Pt(py)2 (N3 )2 (OH)(gly-R-Nap)], R=H (1), 3-NO2 (2) or 4-NMe2 (3). They show enhanced photo-oxidation, cellular accumulation and promising photo-cytotoxicity in human A2780 ovarian, A549 lung and PC3 prostate cancer cells with visible light activation, and low dark cytotoxicity. Complexes 1 and 2 exhibit pre-intercalation into DNA, resulting in enhanced photo-induced DNA crosslinking. Complex 3 has a red-shifted absorption band at 450 nm, allowing photoactivation and photo-cytotoxicity with green light.

FeII Metallohelices Stabilize DNA G-Quadruplexes and Downregulate the Expression of G-Quadruplex-Regulated Oncogenes

DNA G-quadruplexes (G4s) have been identified within the promoter regions of many proto-oncogenes. Thus, G4s represent attractive targets for cancer therapy, and the design and development of new drugs as G4 binders is a very active field of medicinal chemistry. Here, molecular biophysics and biology methods were employed to investigate the interaction of chiral metallohelices with a series of four DNA G4s (hTelo, c-myc, c-kit1, c-kit2) that are formed by the human telomeric sequence (hTelo) and in the promoter regions of c-MYC and c-KIT proto-oncogenes. We show that the investigated water-compatible, optically pure metallohelices, which are made by self-assembly of simple nonpeptidic organic components around Fe^II ions and exhibit bioactivity emulating the natural systems, bind with high affinity to G4 DNA and much lower affinity to duplex DNA. Notably, both enantiomers of a metallohelix containing a m-xylenyl bridge (5 b) were found to effectively inhibit primer elongation catalyzed by Taq DNA polymerase by stabilizing G4 structures formed in the template strands containing c-myc and c-kit2 G4-forming sequences. Moreover, both enantiomers of 5 b downregulated the expression of c-MYC and c-KIT oncogenes in human embryonic kidney cells at mRNA and protein levels. As metallohelices also bind alternative nucleic acid structures, they hold promise as potential multitargeted drugs.

Unexpected solution behaviour of ester-functionalized half-sandwich Ru(II) and Ir(III) complexes

Complexes [Ru(η6-pcym)(bpydca)Cl]PF6 (Rudca) and [Ir(η5-Cp*)(bpydca)Cl]PF6 (Irdca) were developed as model compounds for the investigation of multi-targeted ester-functionalized half-sandwich ruthenium(ii) and iridium(iii) complexes; pcym = 1-methyl-4-(propan-2-yl)benzene (p-cymene), bpydca = 2,2'-bipyridine-4,4'-diyldimethanediyl bis(dichloroacetate), Cp* = pentamethylcyclopentadienyl. Aiming to understand the in-solution behaviour of these first-in-class complexes containing the pyruvate dehydrogenase kinase inhibitor dichloroacetate (dca) as the terminal bioactive substituent, several experiments were performed under aqueous conditions for Rudca and Irdca, as well as for compounds [Ru(η6-pcym)(bpyOH)Cl]PF6 (RuOH) and [Ir(η5-Cp*)(bpyOH)Cl]PF6 (IrOH), and acetyl analogues [Ru(η6-pcym)(bpyac)Cl]PF6 (Ruac) and [Ir(η5-Cp*)(bpyac)Cl]PF6 (Irac) bearing a different (biologically inactive) terminal substituent; bpyOH = 2,2'-bipyridine-4,4'-diyldimethanol, bpyac = 2,2'-bipyridine-4,4'-diyldimethanediyl diacetate. The experiments were also conducted in the presence of porcine liver esterase (PLE). All the six complexes were characterized by relevant techniques (e.g., NMR and mass spectrometry), including a single-crystal X-ray analysis of complexes Rudca, Ruac, RuOH and IrOH. Although designed as model compounds, Rudca, Irdca, RuOH and IrOH were also screened for their antiproliferative activity in four human cancer cell lines (HCT116 colon carcinoma, MDA-MB-231 and MCF-7 breast adenocarcinomas, DU145 prostate carcinoma), where the tested complexes did not show any effect (IC50 > 100 μM).

Hetero-Bis-Conjugation of Bioactive Molecules to Half-Sandwich Ruthenium(II) and Iridium(III) Complexes Provides Synergic Effects in Cancer Cell Cytotoxicity

Four bipyridine-type ligands variably derivatized with two bioactive groups (taken from ethacrynic acid, flurbiprofen, biotin, and benzylpenicillin) were prepared via sequential esterification steps from commercial 2,2'-bipyridine-4,4'-dicarboxylic acid and subsequently coordinated to ruthenium(II) p-cymene and iridium(III) pentamethylcyclopentadienyl scaffolds. The resulting complexes were isolated as nitrate salts in high yields and fully characterized by analytical and spectroscopic methods. NMR and MS studies in aqueous solution and in cell culture medium highlighted a substantial stability of ligand coordination and a slow release of the bioactive fragments in the latter case. The complexes were assessed for their antiproliferative activity on four cancer cell lines, showing cytotoxicity to the low micromolar level (equipotent with cisplatin). Additional biological experiments revealed a multimodal mechanism of action of the investigated compounds, involving DNA metalation and enzyme inhibition. Synergic effects provided by specific combinations of metal and bioactive fragments were identified, pointing toward an optimal ethacrynic acid/flurbiprofen combination for both Ru(II) and Ir(III) complexes.

A Cyclometalated IrIII Complex Conjugated to a Coumarin Derivative Is a Potent Photodynamic Agent against Prostate Differentiated and Tumorigenic Cancer Stem Cells

A cyclometalated Ir^III complex conjugated to a far-red-emitting coumarin, Ir^III -COUPY (3), was recently shown as a very promising photosensitizer suitable for photodynamic therapy of cancer. Therefore, the primary goal of this work was to deepen knowledge on the mechanism of its photoactivated antitumor action so that this information could be used to propose a new class of compounds as drug candidates for curing very hardly treatable human tumors, such as androgen resistant prostatic tumors of metastatic origin. Conventional anticancer chemotherapies exhibit several disadvantages, such as limited efficiency to target cancer stem cells (CSCs), which are considered the main reason for chemotherapy resistance, relapse, and metastasis. Herein, we show, using DU145 tumor cells, taken as the model of hormone-refractory and aggressive prostate cancer cells resistant to conventional antineoplastic drugs, that the photoactivated conjugate 3 very efficiently eliminates both prostate bulk (differentiated) and prostate hardly treatable CSCs simultaneously and with a similar efficiency. Notably, the very low toxicity of Ir^III -COUPY conjugate in the prostate DU145 cells in the dark and its pronounced selectivity for tumor cells compared with noncancerous cells could result in low side effects and reduced damage of healthy cells during the photoactivated therapy by this agent. Moreover, the experiments performed with the 3D spheroids formed from DU145 CSCs showed that conjugate 3 can penetrate the inner layers of tumor spheres, which might markedly increase its therapeutic effect. Also interestingly, this conjugate induces apoptotic cell death in prostate cancer DU145 cells associated with calcium signaling flux in these cells and autophagy. To the best of our knowledge, this is the first study demonstrating that a photoactivatable metal-based compound is an efficient agent capable of killing even hardly treatable CSCs.

Triazole-based, optically-pure metallosupramolecules; highly potent and selective anticancer compounds

Functionalised triazole aldehydes are used in the highly selective self-assembly of water-compatible, optically pure, low symmetry Fe(ii)- and Zn(ii)-based metallohelices. Sub-micromolar antiproliferative activity is observed against various cancerous cell lines, accompanied by excellent selectivity versus non-cancerous cells and potential for synergistic combinatorial therapy with cisplatin.

Antitumor substitution-inert polynuclear platinum complexes stabilize G-quadruplex DNA and suppress G-quadruplex-mediated gene expression

Anticancer substitution-inert polynuclear platinum(ii) complexes (SI-PPCs) effectively stabilize DNA G-quadruplexes (G4) and terminate DNA polymerization on templates containing G4-forming sequences.

A photoactivated Ir(iii) complex targets cancer stem cells and induces secretion of damage-associated molecular patterns in melanoma cells characteristic of immunogenic cell death

The new iridium complex selectively targets cancer stem cells and has potential to induce immunogenic cell death in cancer cells.

Platinum(IV)-Estramustine Multiaction Prodrugs Are Effective Antiproliferative Agents against Prostate Cancer Cells

Herein, we describe the synthesis, characterization, and biological properties of Pt(IV) derivatives of cisplatin with estramustine at the first axial position, which is known to disrupt the microtubule assembly and act as an androgen antagonist, and varying the second axial position using an innocent ligand (acetate or hydroxyl) to prepare dual-action and triple-action prodrugs with known inhibitors of histone deacetylase, cyclooxygenase, and pyruvate dehydrogenase kinase. We demonstrate superior antiproliferative activity at submicromolar concentrations of the prodrugs against a panel of cancer cell lines, particularly against prostate cancer cell lines. The results obtained in this study exemplify the complex mode of action of "multiaction" Pt(IV) prodrugs. Interestingly, changing the second axial ligand in the Pt-estramustine complex has a significant effect on the mode of action, suggesting that all three components of the Pt(IV) prodrugs (platinum moiety and axial ligands) contribute to the killing of cells and not just one dominant component.

Thermodynamic Insights by Microscale Thermophoresis into Translesion DNA Synthesis Catalyzed by DNA Polymerases Across a Lesion of Antitumor Platinum-Acridine Complex

Translesion synthesis (TLS) through DNA adducts of antitumor platinum complexes has been an interesting aspect of DNA synthesis in cells treated with these metal-based drugs because of its correlation to drug sensitivity. We utilized model systems employing a DNA lesion derived from a site-specific monofunctional adduct formed by antitumor [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine) at a unique G residue. The catalytic efficiency of TLS DNA polymerases, which differ in their processivity and fidelity for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the adduct of AMD, was investigated. For a deeper understanding of the factors that control the bypass of the site-specific adducts of AMD catalyzed by DNA polymerases, we also used microscale thermophoresis (MST) to measure the thermodynamic changes associated with TLS across a single, site-specific adduct formed in DNA by AMD. The relative catalytic efficiency of the investigated DNA polymerases for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the AMD adduct, was reduced. Nevertheless, incorporation of the correct C opposite the G modified by AMD of the template strand was promoted by an increasing thermodynamic stability of the resulting duplex. The reduced relative efficiency of the investigated DNA polymerases may be a consequence of the DNA intercalation of the acridine moiety of AMD and the size of the adduct. The products of the bypass of this monofunctional lesion produced by AMD and DNA polymerases also resulted from the misincorporation of dNTPs opposite the platinated G residues. The MST analysis suggested that thermodynamic factors may contribute to the forces that governed enhanced incorporation of the incorrect dNTPs by DNA polymerases.

Substitution-Inert Polynuclear Platinum Complexes Inhibit Reverse Transcription Preferentially in RNA Triplex-Forming Templates

RNA triplexes are significant tertiary structure motifs that are found in many functional RNAs. Hence, small molecules capable of recognition, binding, and stabilization of the triple-helical RNA structures are emerging as attractive potential molecular biology tools and therapeutic agents. Here, we utilize methods of molecular biology and biophysics to study the interactions of a series of antitumor substitution-inert polynuclear platinum complexes (SI-PPCs) with triple-helical RNA structures. We show that SI-PPCs recognize and stabilize RNA triplexes and inhibit reverse transcription preferentially in the RNA template prone to the triplex formation. These so far unexplored properties of SI-PPCs suggest that the targeting of triple-stranded regions in RNA might contribute to the biological effects of SI-PPCs.

A Multi-action PtIV Conjugate with Oleate and Cinnamate Ligands Targets Human Epithelial Growth Factor Receptor HER2 in Aggressive Breast Cancer Cells

HER2-positive breast cancer is an aggressive subtype that typically responds poorly to standard chemotherapy. To design an anticancer drug selective for HER2-expressing breast cancer, a Pt^IV prodrug with axial oleate and cinnamate ligands was synthesized. We demonstrate its superior antiproliferative activity in monolayer and 3D spheroid models; the antiproliferative efficiency increases gradually with increasing expression of HER2. The results also suggest that the released Pt^II compound inhibits the proliferation of cancer cells by a DNA-damage-mediated mechanism. Simultaneously, the released oleic and cinnamic acid can effectively inhibit HER2 expression. To our knowledge, this is the first platinum-based complex inhibiting HER2 expression that does not contain protein or peptide. Moreover, this Pt^IV prodrug is capable of overcoming the resistance of cancer stem cells (CSCs), inducing death in both CSCs and differentiated cancer cells. Thus, the results substantiate our design strategy and demonstrate the potential of this approach for the development of new, therapeutically relevant compounds.

Stabilization of human telomeric RNA G-quadruplex by the water-compatible optically pure and biologically-active metallohelices

RNA G-quadruplexes have been suggested to play key roles in fundamental biological processes and are linked to human diseases. Thus, they also represent good potential therapeutic targets. Here, we describe, using the methods of molecular biophysics, interactions of a series of biologically-active supramolecular cationic metallohelices with human telomeric RNA G-quadruplex. We demonstrate that the investigated metallohelices bind with a high affinity to human telomeric RNA G-quadruplex and that their binding selectivity considerably differs depending on the dimensions and overall shape of the metallohelices. Additionally, the investigated metallohelices inhibit DNA synthesis on the RNA template containing four repeats of the human telomeric sequence by stabilizing the RNA G-quadruplex structure. Collectively, the results of this study suggest that stabilization of RNA sequences capable of G-quadruplex formation by metallohelices investigated in this work might contribute to the mechanism of their biological activity.

Synthesis, antiproliferative activity in cancer cells and DNA interaction studies of [Pt(cis-1,3-diaminocycloalkane)Cl2] analogs

The search for more effective platinum anticancer drugs has led to the design, synthesis, and preclinical testing of hundreds of new platinum complexes. This search resulted in the recognition and subsequent FDA approval of the third-generation Pt(II) anticancer drug, [Pt(1,2-diaminocyclohexane)(oxalate)], oxaliplatin, as an effective agent in treating colorectal and gastrointestinal cancers. Another promising example of the class of anticancer platinum(II) complexes incorporating the Pt(1,n-diaminocycloalkane) moiety is kiteplatin ([Pt(cis-1,4-DACH)Cl₂], DACH = diaminocyclohexane). We report here our progress in evaluating the role of the cycloalkyl moiety in these complexes focusing on the synthesis, characterization, evaluation of the antiproliferative activity in tumor cells and studies of the mechanism of action of new [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes wherein the cis-1,3-diaminocycloalkane group contains the cyclobutyl, cyclopentyl, and cyclohexyl moieties. We demonstrate that [Pt(cis-1,3-DACH)Cl₂] destroys cancer cells with greater efficacy than the other two investigated 1,3-diamminocycloalkane derivatives, or cisplatin. Moreover, the investigated [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes show selectivity toward tumor cells relative to non-tumorigenic normal cells. We also performed several mechanistic studies in cell-free media focused on understanding some early steps in the mechanism of antitumor activity of bifunctional platinum(II) complexes. Our data indicate that reactivities of the investigated [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes and cisplatin with glutathione and DNA binding do not correlate with antiproliferative activity of these platinum(II) complexes in cancer cells. In contrast, we show that the higher antiproliferative activity in cancer cells of [Pt(cis-1,3-DACH)Cl₂] originates from its highest hydrophobicity and most efficient cellular uptake.

Glycoconjugated Metallohelices have Improved Nuclear Delivery and Suppress Tumour Growth In Vivo

Monosaccharides are added to the hydrophilic face of a self-assembled asymmetric FeII metallohelix, using CuAAC chemistry. The sixteen resulting architectures are water-stable and optically pure, and exhibit improved antiproliferative selectivity against colon cancer cells (HCT116 p53+/+ ) with respect to the non-cancerous ARPE-19 cell line. While the most selective compound is a glucose-appended enantiomer, its cellular entry is not mainly glucose transporter-mediated. Glucose conjugation nevertheless increases nuclear delivery ca 2.5-fold, and a non-destructive interaction with DNA is indicated. Addition of the glucose units affects the binding orientation of the metallohelix to naked DNA, but does not substantially alter the overall affinity. In a mouse model, the glucose conjugated compound was far better tolerated, and tumour growth delays for the parent compound (2.6 d) were improved to 4.3 d; performance as good as cisplatin but with the advantage of no weight loss in the subjects.

Oxidation of cis-Diamminediacetato PtII with Hydrogen Peroxide Can Give Rise to Two Isomeric PtIV Products

The oxidation of cis-[Pt(NH₃ )₂ (OAc)₂ ] with H₂ O₂ yields a mixture of two isomers: ctc-[Pt(NH₃ )₂ (OH)₂ (OAc)₂ ] and ctc-[Pt(NH₃ )₂ (OH)(OAc)(OH)(OAc)]. Following modification with 4-phenylbutyric (PhB) anhydride, two isomers were separated and characterized; the symmetric ctc-[Pt(NH₃ )₂ (PhB)₂ (OAc)₂ ] (1) and the nonsymmetric ctc-[Pt(NH₃ )₂ (PhB)(OAc)(PhB)(OAc)] (2). They differ in their log P values and despite having similar cellular uptake and similar DNA platination levels, the symmetric ctc-[Pt(NH₃ )₂ (OH)₂ (OAc)₂ ] is more than 4-fold more potent than the nonsymmetric isomer in a panel of 4 cancer cell lines.