Inhibition of DNA Synthesis by a Platinum-Acridine Hybrid Agent Leads to Potent Cell Kill in Non-Small Cell Lung Cancer

The Influence of the Auxiliary Ligand in Monofunctional Pt(II) Anticancer Complexes on the DNA Backbone

Monofunctional platinum complexes offer a promising alternative to cisplatin in cancer chemotherapy, showing a unique mechanism of action. Their ability to induce minor helix distortions effectively inhibits DNA transcription. In our study, we synthesized and characterized three monofunctional Pt(II) complexes with the general formula [Pt(en)(L)Cl]NO3, where en = ethylenediamine, and L = pyridine (py), 2-methylpyridine (2-mepy), and 2-phenylpyridine (2-phpy). The hydrolysis rates of [Pt(en)(py)Cl]NO3 (1) and [Pt(en)(2-mepy)Cl]NO3 (2) decrease with the bulkiness of the auxiliary ligand with k(1) = 2.28 ± 0.15 × 10-4 s-1 and k(2) = 8.69 ± 0.98 × 10-5 s-1 at 298 K. The complex [Pt(en)(2-phpy)Cl]Cl (3) demonstrated distinct behavior. Upon hydrolysis, an equilibrium (Keq = 0.385 mM) between the complexes [Pt(en)(2-phpy)Cl]+ and [Pt(en)(2-phpy-H+)]+ was observed with no evidence (NMR or HR-ESI-MS) for the presence of the aquated complex [Pt(en)(2-phpy)(H2O)]2+. Despite the kinetic similarities between phenanthriplatin and (2), complexes (1) and (2) exhibit minimal activity against A549 lung cancer cell line (IC50 > 100 μΜ), whereas complex (3) exhibits notable cytotoxicity (IC50 = 41.11 ± 2.1 μΜ). In examining the DNA binding of (1) and (2) to the DNA model guanosine (guo), we validated their binding through guoN7, which led to an increased population of the C3'-endo sugar conformation, as expected. However, we observed that the rapid transition 2E (C2'-endo) ↔ 3E (C3'-endo), in the case of [Pt(en)(py)(guo)](NO3)2 ([1-guo]), slows down in the case of [Pt(en)(2-mepy)(guo)](NO3)2 ([2-guo]), resulting in separate signals for the two conformers in the 1H NMR spectra. This phenomenon arises from the steric hindrance between the methyl group of pyridine and the sugar moiety of guanosine. Notably, this hindrance is absent in [2-(9-MeG)] (9-MeG = 9-methylguanine), probably due to the absence of a bulky sugar unit in 9-MeG. In the case of (3), where the bulkiness of the substitution on the pyridine is further increased by a phenyl group, we observed a notable proximity between 9-MeGH8 and the phenyl ring of 2-phpy. Considering that only (3) exhibited good cytotoxicity against the A549 cancer cell line, it is suggested that auxiliary ligands, L, with an extended aromatic system and proper orientation in complexes of the type cis-[Pt(en)(L)Cl]NO3, may enhance the cytotoxic activity of such complexes.

Acridine as an Anti-Tumour Agent: A Critical Review

This review summarized the current breakthroughs in the chemistry of acridines as anti-cancer agents, including new structural and biologically active acridine attributes. Acridine derivatives are a class of compounds that are being extensively researched as potential anti-cancer drugs. Acridines are well-known for their high cytotoxic activity; however, their clinical application is restricted or even excluded as a result of side effects. The photocytotoxicity of propyl acridine acts against leukaemia cell lines, with C1748 being a promising anti-tumour drug against UDP-UGT's. CK0403 is reported in breast cancer treatment and is more potent than CK0402 against estrogen receptor-negative HER2. Acridine platinum (Pt) complexes have shown specificity on the evaluated DNA sequences; 9-anilinoacridine core, which intercalates DNA, and a methyl triazene DNA-methylating moiety were also studied. Acridine thiourea gold and acridinone derivatives act against cell lines such as MDA-MB-231, SK-BR-3, and MCF-7. Benzimidazole acridine compounds demonstrated cytotoxic activity against Dual Topo and PARP-1. Quinacrine, thiazacridine, and azacridine are reported as anti-cancer agents, which have been reported in the previous decade and were addressed in this review article.

Chemosensitivity-Gene Expression Correlations and Functional Enrichment Analysis Provide Insight into the Mechanism of Action of a Platinum-Acridine Anticancer Agent

NCI-60 growth inhibition and gene expression profiles were analyzed using Pearson correlation and functional enrichment computational tools to demonstrate critical mechanistic differences between a nucleolus-targeting platinum-acridine anticancer agent (PA) and other DNA-directed chemotherapies.  The results support prior experimental data and are consistent with DNA being a major target of the hybrid agent based on the negative correlations observed between its potency and expression levels of genes implicated in DNA double-strand break (DSB) repair.  Gene ontology terms related to RNA processing, including ribosome biogenesis, are also negatively enriched, suggesting a mechanism by which these processes render cancer cells more resistant to the highly cytotoxic agent.  The opposite trend is observed for oxaliplatin and other DNA-targeted drugs.  Significant functional interactions exist between genes/gene products involved in ribosome biogenesis and DSB repair, including the ribosomal protein (RPL5)-MDM2-p53 surveillance pathway, as a response to the nucleolar stress produced by PAs.

Platinum binding preferences dominate the binding of novel polyamide amidine anthraquinone platinum(II) complexes to DNA

Complexes incorporating a threading anthraquinone intercalator with pyrrole lexitropsin and platinum(II) moieties attached were developed with the goal of generating novel DNA binding modes, including the targeting of AT-rich regions in order to have high cytotoxicities. The binding of the complexes to DNA has been investigated and profiles surprisingly similar to that for cisplatin were observed; the profiles were different to those for a complex lacking the pyrrole lexitropsin component. The lack of selective binding to AT-rich regions suggests the platinum binding was dominating the sequence selectivity, and is consistent with the pyrrole lexitropsin slowing intercalation. The DNA unwinding profiles following platinum binding were evaluated by gel electrophoresis and suggested that intercalation and platinum binding were both occurring.

Covalent and noncovalent interactions of coordination compounds with DNA: An overview

Deoxyribonucleic acid plays a central role in crucial cellular processes, and many drugs exert their effects through binding to DNA. Since the discovery of cisplatin and its derivatives considerable attention of researchers has been focused on the development of novel anticancer metal-based drugs. Transition metal complexes, due to their great diversity in size and structure, have a big potential to modify DNA through diverse types of interactions, making them the prominent class of compounds for DNA targeted therapy. In this review we describe various binding modes of metal complexes to duplex DNA based on covalent and noncovalent interactions or combination of both. Specific examples of each binding mode as well as possible cytotoxic effects of metal complexes in tumor cells are presented.

DNA Adduct Detection after Post-Labeling Technique with PCR Amplification (DNA-ADAPT-qPCR) Identifies the Pre-Ribosomal RNA Gene as a Direct Target of Platinum-Acridine Anticancer Agents

To study the DNA damage caused by a potent platinum-acridine anticancer agent (PA) in cancer cells, an assay based on biorthogonal post-labeling using a click chemistry-enabled, azide-modified derivative (APA) was developed. The method involves biotinylation, affinity capture, and bead-based enrichment of APA-modified genomic DNA. The key steps of the assay were validated and optimized in model duplexes, including full-length plasmids, restriction fragments, and a DNA ladder. Native DNA treated with APA and subsequently subjected to post-labeling with a biotin affinity tag was enzymatically digested and fragments were analyzed by in-line LC-MS and MS/MS. The monofunctional-intercalative adducts formed by APA in 5´-pyrimidine/guanine sequences in double-stranded DNA are quantitatively biotinylated by strain-promoted 1,3-dipolar cycloaddition chemistry. When applied to DNA extracted from A549 lung cancer cells, the assay in combination with qPCR amplification demonstrates that platinum-acridines form adducts in the gene sequences encoding pre-ribosomal RNA, a potential pharmacological target of these agents.

Novel polyamide amidine anthraquinone platinum(II) complexes: cytotoxicity, cellular accumulation, and fluorescence distributions in 2D and 3D cell culture models

1- and 1,5-Aminoalkylamine substituted anthraquinones (AAQs, 1C3 and 1,5C3) were peptide coupled to 1-, 2-, and 3-pyrrole lexitropsins to generate compounds that incorporated both DNA minor groove and intercalating moieties. The corresponding platinum(II) amidine complexes were synthesized through a synthetically facile amine-to-platinum mediated nitrile 'Click' reaction. The precursors as well as the corresponding platinum(II) complexes were biologically evaluated in 2D monolayer cells and 3D tumour cell models. Despite having cellular accumulation levels that were up to five-fold lower than that of cisplatin, the platinum complexes had cytotoxicities that were only three-fold lower. Accumulation was lowest for the complexes with two or three pyrrole groups, but the latter was the most active of the complexes exceeding the activity of cisplatin in the MDA-MB-231 cell line. All compounds showed moderate to good penetration into spheroids of DLD-1 cells with the distributions being consistent with active uptake of the pyrrole containing complexes in regions of the spheroids starved of nutrients.

Monofunctional Platinum(II) Anticancer Agents

Platinum-based anticancer drugs represented by cisplatin play important roles in the treatment of various solid tumors. However, their applications are largely compromised by drug resistance and side effects. Much effort has been made to circumvent the drug resistance and general toxicity of these drugs. Among multifarious designs, monofunctional platinum(II) complexes with a general formula of [Pt(3A)Cl]⁺ (A: Ammonia or amine) stand out as a class of "non-traditional" anticancer agents hopeful to overcome the defects of current platinum drugs. This review aims to summarize the development of monofunctional platinum(II) complexes in recent years. They are classified into four categories: fluorescent complexes, photoactive complexes, targeted complexes, and miscellaneous complexes. The intention behind the designs is either to visualize the cellular distribution, or to reduce the side effects, or to improve the tumor selectivity, or inhibit the cancer cells through non-DNA targets. The information provided by this review may inspire researchers to conceive more innovative complexes with potent efficacy to shake off the drawbacks of platinum anticancer drugs.

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.

Acridine-decorated cyclometallated gold(iii) complexes: synthesis and anti-tumour investigations

(C^N) and (C^N^C) cyclometalated Au(iii) represent a highly promising class of potential anticancer agents. We report here the synthesis of seven new cyclometalated Au(iii) complexes with five of them bearing an acridine moiety attached via (N^O) or (N^N) chelates, acyclic amino carbenes (AAC) and N-heterocyclic carbenes (NHC). The antiproliferative properties of the different complexes were evaluated in vitro on a panel of cancer cells including leukaemia, lung and breast cancer cells. We observed a trend between the cytotoxicity and the intracellular gold uptake of some representative compounds of the series. Some of the acridine-decorated complexes were demonstrated to interact with ds-DNA using FRET-melting techniques.

Medicinal chemistry of acridine and its analogues

'Acridine' along with its functional analogue 'Acridone' is the most privileged pharmacophore in medicinal chemistry with diverse applications ranging from DNA intercalators, endonuclease mimics, ratiometric selective ion sensors, and P-glycoprotein inhibitors in countering the multi-drug resistance, enzyme inhibitors, and reversals of neurodegenerative disorders. Their interaction with DNA and ability of selectively identifying numerous biologically useful ions has cemented exploitability of the acridone nucleus in modern day therapeutics. Additionally, most derivatives and salts of acridine are planar, crystalline, and stable displaying a strong fluorescence which, when coupled with their marked bio selectivity and low cytotoxicity, enables the studying and monitoring of several biochemical, metabolic, and pharmacological processes. In this review, a detailed picture covering the important therapeutic aspects of the acridone nucleus and its functional analogues is discussed.

Piperlongumine Induces Apoptosis in Human Melanoma Cells Via Reactive Oxygen Species Mediated Mitochondria Disruption

Malignant melanoma is a devastating skin cancer due to its severe drug resistance and prompt metastasis. Piperlongumine is an anti-inflammation and tumor-suppressing natural product with defined structure. While numerous studies revealed exceptional inhibitory effects of piperlongumine on several carcinomas, few investigations were performed on melanoma. Therefore, the present study investigated the anti-tumor effects of piperlongumine on human melanoma cells in vitro, and explored the mechanisms of action. Results from cytotoxicity and proliferation studies demonstrated that piperlongumine inhibited cell growth in melanoma cell lines A375, A875, and B16-F10 in a dose- and time-dependent manner. Flow cytometric analysis showed that piperlongumine obstructed cell cycle progression at G2/M phase and induced apoptosis in A375 cells. Mechanistic investigations illustrated that piperlongumine promoted reactive oxygen species production and decreased mitochondrial membrane potential. In addition, piperlongumine was reported to interfere with the expression of p21, p27, cleaved caspases-3, Bax, Bcl-2, and p-Jun N-terminal kinase (JNK), which are typical regulators associated with cell proliferation, intrinsic apoptosis, and JNKs pathway. Taken together, these results strongly suggested that piperlongumine inhibits cell growth and induces apoptosis in human melanoma cells via ROS mediated mitochondria disruption and JNKs pathway, and piperlongumine may exert promising potential for patients suffering from malignant melanoma.

The potential of combi-molecules with DNA-damaging function as anticancer agents

DNA-damaging agents, such as methylating agents, chloroethylating agents and platinum-based agents, have been extensively used as anticancer drugs. However, the side effects, high toxicity, lack of selectivity and resistance severely limit their clinical applications. In recent years, a strategy combining a DNA-damaging agent with a bioactive molecule (e.g., enzyme inhibitors) or carrier (e.g., steroid hormone and DNA intercalators) to produce a new 'combi-molecule' with improved efficacy or selectivity has been attempted to overcome these drawbacks. The combi-molecule simultaneously acts on two targets and is expected to possess better potency than the parent compounds. Many studies have shown DNA-damaging combi-molecules exhibiting excellent anticancer activity in vitro and in vivo. This review focuses on the development of combi-molecules, which possess increased DNA-damaging potency, anticancer efficacy and tumor selectivity and reduced side reactions than the parent compounds. The future opportunities and challenges in the discovery of combi-molecules were also discussed.

Large-Pore Functionalized Mesoporous Silica Nanoparticles as Drug Delivery Vector for a Highly Cytotoxic Hybrid Platinum-Acridine Anticancer Agent

Large-pore mesoporous silica nanoparticles (MSN) were prepared and functionalized to serve as a highly robust and biocompatible delivery platform for platinum-acridine (PA) anticancer agents. The material showed a high loading capacity for the dicationic, hydrophilic hybrid agent [PtCl(en)(N-[acridin-9-ylaminoethyl]-N-methylpropionamidine)] dinitrate salt (P1A1) and virtually complete retention of payload at neutral pH in a high-chloride buffer. In acidic media mimicking the pH inside the cell lysosomes, rapid, burst-like release of P1A1 from the nanoparticles is observed. Coating of the materials in phospholipid bilayers resulted in nanoparticles with greatly improved colloidal stability. The lipid and carboxylate-modified nanoparticles containing 40 wt % drug caused S-phase arrest and inhibited cell proliferation in pancreatic cancer cells at submicromolar concentrations similar to carrier-free P1A1. The most striking feature of nanoparticle-delivered P1A1 was that the payload did not escape from the acidified lysosomal vesicles into the cytoplasm, but was shuttled to the nuclear membrane and released into the nucleus.

Enhanced anti-cancer efficacy to cancer cells by a novel monofunctional mononuclear platinum(ii) complex containing a mixed S,N,S-donor ligand

A novel platinum–intercalator hybrid complex (1) exhibits a cytotoxicity comparable to that of cisplatin against MCF-7 cell lines, and more potent activities against HeLa and A-549 cell lines, especially against the former.

Design and cellular studies of a carbon nanotube-based delivery system for a hybrid platinum-acridine anticancer agent

A three-component drug-delivery system has been developed consisting of multi-walled carbon nanotubes (MWCNTs) coated with a non-classical platinum chemotherapeutic agent ([PtCl(NH3)2(L)]Cl (P3A1; L=N-(2-(acridin-9-ylamino)ethyl)-N-methylproprionimidamide) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-5000] (DSPE-mPEG). The optimized P3A1-MWCNTs are colloidally stable in physiological solution and deliver more P3A1 into breast cancer cells than treatment with the free drug. Furthermore, P3A1-MWCNTs are cytotoxic to several cell models of breast cancer and induce S-phase cell cycle arrest and non-apoptotic cell death in breast cancer cells. By contrast, free P3A1 induces apoptosis and allows progression to G2/M phase. Photothermal activation of P3A1-MWCNTs to generate mild hyperthermia potentiates their cytotoxicity. These findings suggest that delivery of P3A1 to cancer cells using MWCNTs as a drug carrier may be beneficial for combination cancer chemotherapy and photothermal therapy.

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.

Platinum-based drugs: past, present and future

Platinum-based drugs cisplatin, carboplatin and oxaliplatin are widely used in the therapy of human neoplasms. Their clinical success is, however, limited due to severe side effects and intrinsic or acquired resistance to the treatment. Much effort has been put into the development of new platinum anticancer complexes, but none of them has reached worldwide clinical application so far. Nedaplatin, lobaplatin and heptaplatin received only regional approval. Some new platinum complexes and platinum drug formulations are undergoing clinical trials. Here, we review the main classes of new platinum drug candidates, such as sterically hindered complexes, monofunctional platinum drugs, complexes with biologically active ligands, trans-configured and polynuclear platinum complexes, platinum(IV) prodrugs and platinum-based drug delivery systems. For each class of compounds, a detailed overview of the mechanism of action is given, the cytotoxicity is compared to that of the clinically used platinum drugs, and the clinical perspectives are discussed. A critical analysis of lessons to be learned is presented. Finally, a general outlook regarding future directions in the field of new platinum drugs is given.

Absence of Activation of DNA Repair Genes and Excellent Efficacy of Phosphaplatins against Human Ovarian Cancers: Implications To Treat Resistant Cancers

Phosphaplatins, platinum(II) and platinum(IV) complexes coordinated to a pyrophosphate moiety, exhibit excellent antitumor activities against a variety of cancers. To determine whether phosphaplatins trigger resistance to treatment by engaging DNA damage repair genes, a yeast genome-wide fitness assay was used. Treatment of yeast cells with pyrodach-2 (D2) or pyrodach-4 (D4) revealed no particular sensitivity to nucleotide excision repair, homologous recombination repair, or postreplication repair when compared with platin control compounds. Also, TNF receptor superfamily member 6 (FAS) protein was overexpressed in phosphaplatin-treated ovarian tumor cells, and platinum colocalized with FAS protein in lipid rafts. An overactivation of sphingomyelinase (ASMase) was noted in the treated cells, indicating participation of an extrinsic apoptotic mechanism due to increased ceramide release. Our results indicate that DNA is not the target of phosphaplatins and accordingly, that phosphaplatins might not cause resistance to treatment. Activation of ASMase and FAS along with the colocalization of platinum with FAS in lipid rafts support an extrinsic apoptotic signaling mechanism that is mediated by phosphaplatins.

Cellular Recognition and Repair of Monofunctional-Intercalative Platinum--DNA Adducts

The cellular recognition and processing of monofunctional-intercalative DNA adducts formed by [PtCl(en)(L)](NO3)2 (P1-A1; en = ethane-1,2-diamine; L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine, acridinium cation), a cytotoxic hybrid agent with potent anticancer activity, was studied. Excision of these adducts and subsequent DNA repair synthesis were monitored in plasmids modified with platinum using incubations with mammalian cell-free extract. On the basis of the levels of [α-(32)P]-dCTP incorporation, P1-A1-DNA adducts were rapidly repaired with a rate approximately 8 times faster (t1/2 ≈ 18 min at 30 °C) than the adducts (cross-links) formed by the drug cisplatin. Cellular responses to P1-A1 and cisplatin were also studied in NCI-H460 lung cancer cells using immunocytochemistry in conjunction with confocal fluorescence microscopy. At the same dose, P1-A1, but not cisplatin, elicited a distinct requirement for DNA double-strand break repair and stalled replication fork repair, which caused nuclear fluorescent staining related to high levels of MUS81, a specialized repair endonuclease, and phosphorylated histone protein γ-H2AX. The results confirm previous observations in yeast-based chemical genomics assays. γ-H2AX fluorescence is observed as a large number of discrete foci signaling DNA double-strand breaks, pan-nuclear preapoptotic staining, and unique circularly shaped staining around the nucleoli and nuclear rim. DNA cleavage assays indicate that P1-A1 does not act as a typical topoisomerase poison, suggesting the high level of DNA double-strand breaks in cells is more likely a result of topoisomerase-independent replication fork collapse. Overall, the cellular response to platinum-acridines shares striking similarities with that reported for DNA adduct-forming derivatives of the drug doxorubicin. The results of this study are discussed in light of the cellular mechanism of action of platinum-acridines and their ability to overcome resistance to cisplatin.

Redesigning the DNA-targeted chromophore in platinum-acridine anticancer agents: a structure-activity relationship study

Platinum-acridine hybrid agents show low-nanomolar potency in chemoresistant non-small cell lung cancer (NSCLC), but high systemic toxicity in vivo. To reduce the promiscuous genotoxicity of these agents and improve their pharmacological properties, a modular build-click-screen approach was used to evaluate a small library of twenty hybrid agents containing truncated and extended chromophores of varying basicities. Selected derivatives were resynthesized and tested in five NSCLC cell lines representing large cell, squamous cell, and adenocarcinomas. 7-Aminobenz[c]acridine was identified as a promising scaffold in a hybrid agent (P1-B1) that maintained submicromolar activity in several of the DNA-repair proficient and p53-mutant cancer models, while showing improved tolerability in mice by 32-fold compared to the parent platinum-acridine (P1-A1). The distribution and DNA/RNA adduct levels produced by the acridine- and benz[c]acridine-based analogues in NCI-H460 cells (confocal microscopy, ICP-MS), and their ability to bind G-quadruplex forming DNA sequences (CD spectroscopy, HR-ESMS) were studied. P1-B1 emerges as a less genotoxic, more tolerable, and potentially more target-selective hybrid agent than P1-A1.

Design of enzymatically cleavable prodrugs of a potent platinum-containing anticancer agent

Using a versatile synthetic approach, a new class of potential ester prodrugs of highly potent, but systemically too toxic, platinum-acridine anticancer agents was generated. The new hybrids contain a hydroxyl group, which has been masked with a cleavable lipophilic acyl moiety. Both butanoic (butyric) and bulkier 2-propanepentanoic (valproic) esters were introduced. The goals of this design were to improve the drug-like properties (e.g., logD) and to reduce the systemic toxicity of the pharmacophore. Two distinct pathways by which the target compounds undergo effective ester hydrolysis, the proposed activating step, have been confirmed: platinum-assisted, self-immolative ester cleavage in a low-chloride environment (LC-ESMS, NMR spectroscopy) and enzymatic cleavage by human carboxylesterase-2 (hCES-2) (LC-ESMS). The valproic acid ester derivatives are the first example of a metal-containing agent cleavable by the prodrug-converting enzyme. They show excellent chemical stability and reduced systemic toxicity. Preliminary results from screening in lung adenocarcinoma cell lines (A549, NCI-H1435) suggest that the mechanism of the valproic esters may involve intracellular deesterification.

Enhanced drug toxicity by conjugation of platinum drugs to polymers with guanidine containing zwitterionic functional groups that mimic cell-penetrating peptides

Inspired by the Ringsdorf model, statistical copolymers with solubility enhancers, platinum drugs and groove binders were compared.

Design of a platinum-acridine-endoxifen conjugate targeted at hormone-dependent breast cancer

The synthesis of a novel pharmacophore comprising a DNA-targeted platinum-acridine hybrid agent and estrogen receptor-targeted 4-hydroxy-N-desmethyltamoxifen (endoxifen) using carbamate coupling chemistry and its evaluation in breast cancer cell lines are described.

PT-ACRAMTU, a platinum-acridine anticancer agent, lengthens and aggregates, but does not stiffen or soften DNA

We used atomic force microscopy (AFM) to study the dose-dependent change in conformational and mechanical properties of DNA treated with PT-ACRAMTU ([PtCl(en)(ACRAMTU-S)](NO3)2, (en = ethane-1,2-diamine, ACRAMTU = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea. PT-ACRAMTU is the parent drug of a family of non-classical platinum-based agents that show potent activity in non-small cell lung cancer in vitro and in vivo. Its acridine moiety intercalates between DNA bases, while the platinum group forms mono-adducts with DNA bases. AFM images show that PT-ACRAMTU causes some DNA looping and aggregation at drug-to-base pair ratio (r b) of 0.1 and higher. Very significant lengthening of the DNA was observed with increasing doses of PT-ACRAMTU, and reached saturation at an r b of 0.15. At r b of 0.1, lengthening was 0.6 nm per drug molecule, which is more than one fully stretched base pair stack can accommodate, indicating that ACRAMTU also disturbs the stacking of neighboring base pair stacks. Analysis of the AFM images based on the worm-like chain (WLC) model showed that PT-ACRAMTU did not change the flexibility of (non-aggregated) DNA, despite the extreme lengthening. The persistence length of untreated DNA and DNA treated with PT-ACRAMTU was in the range of 49-65 nm. Potential consequences of the perturbations caused by this agent for the recognition and processing of the DNA adducts it forms are discussed.

Comparative chemogenomics to examine the mechanism of action of dna-targeted platinum-acridine anticancer agents

Platinum-based drugs have been used to successfully treat diverse cancers for several decades. Cisplatin, the original compound of this class, cross-links DNA, resulting in cell cycle arrest and cell death via apoptosis. Cisplatin is effective against several tumor types, yet it exhibits toxic side effects and tumors often develop resistance. To mitigate these liabilities while maintaining potency, we generated a library of non-classical platinum-acridine hybrid agents and assessed their mechanisms of action using a validated genome-wide screening approach in Saccharomyces cerevisiae and in the distantly related yeast Schizosaccharomyces pombe. Chemogenomic profiles from both S. cerevisiae and S. pombe demonstrate that several of the platinum-acridines damage DNA differently than cisplatin based on their requirement for distinct modules of DNA repair.

Using a build-and-click approach for producing structural and functional diversity in DNA-targeted hybrid anticancer agents

An efficient screening method was developed for functionalized DNA-targeted platinum-containing hybrid anticancer agents based on metal-mediated amine-to-nitrile addition, a form of "click" chemistry. The goal of the study was to generate platinum-acridine agents for their use as cytotoxic "warheads" in targeted and multifunctional therapies. This was achieved by introducing hydroxyl, carboxylic acid, and azide functionalities in the acridine linker moiety and by varying the nonleaving groups attached to platinum. The assay, which was based on microscale reactions between 6 platinum-nitrile complexes and 10 acridine derivatives, yielded a small library of 60 platinum-acridines. Reactions were monitored, and product mixtures were quantitatively analyzed by automated in-line high-performance liquid chromatography-electrospray mass spectrometry (LC-ESMS) analysis and subjected to cell viability screening using a nonradioactive cell proliferation assay. The new prescreening methodology proves to be a powerful tool for establishing structure-activity relationships and for identifying target compounds.

Exploring the DNA binding/cleavage, cellular accumulation and topoisomerase inhibition of 2-hydroxy-3-(aminomethyl)-1,4-naphthoquinone Mannich bases and their platinum(II) complexes

Several chlorido and amino Pt(2+) complexes of 2-hydroxy-3-(aminomethyl)-1,4-naphthoquinone Mannich bases HL exhibiting moderate to high cytotoxicity against cancer cell lines were studied in order to investigate their modes of DNA binding, in vitro DNA strand breaks, mechanism of topoisomerase (Topo I) inhibition and cellular accumulation. DNA model base studies have shown that complex 1a [Pt(HL1)Cl(2)] was capable of binding covalently to 9-ethylguanine (9-EtG) and 5'-GMP. (1)H NMR and mass spectrometry studies have shown that both chlorides were substituted by 9-EtG ligands, whereas 5'-GMP was able to replace only one chlorido ligand, due to steric hindrance. The chlorido Pt(2+) complexes [Pt(HL)Cl(2)] highly accumulate in prostate (PC-3) and melanoma (MDA-MB-435) cell lines, being able to induce DNA strand breaks in vitro and inhibit Topo I by a catalytic mode. On the other hand, the free 2-hydroxy-3-(aminomethyl)-1,4-naphthoquinones HL and the amino Pt(2+) complexes [Pt(L(-))(NH(3))(2)]NO(3) neither cause DNA strand breakage nor exhibit strong DNA interaction, nevertheless the latter were also found to be catalytic inhibitors of Topo I at 100μM. Thus, coordination of the Mannich bases HL to the "PtCl(2)" fragment substantially affects the chemical and biophysical properties of the pro-ligands, leading to an improvement of their DNA binding properties and generating compounds that cleave DNA and catalytically inhibit Topo I. Finally, the high cytotoxicity exhibited by the free (uncomplexed) 2-hydroxy-3-(aminomethyl)-1,4-naphthoquinones might be associated with their decomposition in solution, which is not observed for the Pt(2+) complexes.

Synthesis, aqueous reactivity, and biological evaluation of carboxylic acid ester-functionalized platinum-acridine hybrid anticancer agents

The synthesis of platinum-acridine hybrid agents containing carboxylic acid ester groups is described. The most active derivatives and the unmodified parent compounds showed up to 6-fold higher activity in ovarian cancer (OVCAR-3) and breast cancer (MCF-7, MDA-MB-231) cell lines than cisplatin. Inhibition of cell proliferation at nanomolar concentrations was observed in pancreatic (PANC-1) and nonsmall cell lung cancer cells (NSCLC, NCI-H460) of 80- and 150-fold, respectively. Introduction of the ester groups did not affect the cytotoxic properties of the hybrids, which form the same monofunctional-intercalative DNA adducts as the parent compounds, as demonstrated in a plasmid unwinding assay. In-line high-performance liquid chromatography and electrospray mass spectrometry (LC-ESMS) shows that the ester moieties undergo platinum-mediated hydrolysis in a chloride concentration-dependent manner to form carboxylate chelates. Potential applications of the chloride-sensitive ester hydrolysis as a self-immolative release mechanism for tumor-selective delivery of platinum-acridines are discussed.

Analysis of the DNA damage produced by a platinum-acridine antitumor agent and its effects in NCI-H460 lung cancer cells

High-performance liquid chromatography in conjunction with electrospray mass spectrometry (LC-ESMS) was used to structurally characterize the adducts formed by the platinum-acridine agent [PtCl(en)(N-(2-(acridin-9-ylamino)ethyl)-N-methylpropionimidamide)](NO(3))(2) (compound 1) in cell-free DNA. Compound 1 forms monofunctional adducts exclusively with guanine, based on the fragments identified in enzymatic digests (dG*, dGMP*, dApG*, and dTpG*, where the asterisk denotes bound drug). The time course of accumulation and DNA adduct formation of compound 1 and the clinical drug cisplatin in NCI-H460 lung cancer cells at physiologically relevant drug concentrations (0.1 μM) was studied by inductively-coupled plasma mass spectrometry (ICP-MS). Compound 1 accumulates rapidly in cells and reaches intracellular levels of up to 60-fold higher than those determined for cisplatin. The hybrid agent shows unusually high DNA binding levels: while cisplatin adducts form at a maximum frequency of 5 adducts per 10(6) nucleotides, compound 1 produces 25 adducts per 10(6) nucleotides after only 3 h of continuous incubation with the lung cancer cells. The high overall levels of compound 1 in the cells and in cellular DNA over the entire 12-h treatment period translate into a rapid decrease in cell viability. Possible implications of these findings for the mechanism of action of compound 1 and the agent's potential to overcome tumor resistance to cisplatin are discussed.