Potentiation of mitochondrial dysfunction in tumor cells by conjugates of metabolic modulator dichloroacetate with a Pt(IV) derivative of oxaliplatin

Dichloroacetate for Cancer Treatment: Some Facts and Many Doubts

Rarely has a chemical elicited as much controversy as dichloroacetate (DCA). DCA was initially considered a dangerous toxic industrial waste product, then a potential treatment for lactic acidosis. However, the main controversies started in 2008 when DCA was found to have anti-cancer effects on experimental animals. These publications showed contradictory results in vivo and in vitro such that a thorough consideration of this compound's in cancer is merited. Despite 50 years of experimentation, DCA's future in therapeutics is uncertain. Without adequate clinical trials and health authorities' approval, DCA has been introduced in off-label cancer treatments in alternative medicine clinics in Canada, Germany, and other European countries. The lack of well-planned clinical trials and its use by people without medical training has discouraged consideration by the scientific community. There are few thorough clinical studies of DCA, and many publications are individual case reports. Case reports of DCA's benefits against cancer have been increasing recently. Furthermore, it has been shown that DCA synergizes with conventional treatments and other repurposable drugs. Beyond the classic DCA target, pyruvate dehydrogenase kinase, new target molecules have also been recently discovered. These findings have renewed interest in DCA. This paper explores whether existing evidence justifies further research on DCA for cancer treatment and it explores the role DCA may play in it.

Pt(IV) Complexes in the Search for Novel Platinum Prodrugs with Promising Activity

The kinetically inert, six coordinated, octahedral Pt(IV) complexes are termed dual-, triple-, or multi-action prodrugs based on the nature of the axially substituted ligands. These ligands are either inert or biologically active, where the nature of these axial ligands provides additional stability, synergistic biological activity or cell-targeting ability. There are many literature reports from each of these classes, mentioning the varied nature of these axial ligands. The ligands comprise drug molecules such as chlorambucil, doxorubicin, valproic acid, ethacrynic acid, biologically active chalcone, coumarin, combretastatin, non-steroidal anti-inflammatory drugs (NSAIDs) and many more, potentiating the anti-proliferative profile or reducing the side effects associated with cisplatin therapy. The targeting and non-targeting nature of these moieties exert additive or synergistic effects on the anti-cancer activity of Pt(II) moieties. Herein, we discuss the effects of these axially oriented ligands and the changes in the non-leaving am(m)ine groups and in the leaving groups on the biological activity. In this review, we have presented the latest developments in the field of Pt(IV) complexes that display promising activity with a reduced resistance profile. We have discussed the structure activity relationship (SAR) and the effects of the ligands on the biological activity of Pt(IV) complexes with cisplatin, oxaliplatin, carboplatin and the Pt core other than approved drugs. This literature work will help researchers to get an idea about Pt(IV) complexes that have been classified based on the aspects of their biological activity.

Development of 5-fluorouracil-dichloroacetate mutual prodrugs as anticancer agents

5-Fluorouracil (5-FU) is one of the most widely applied chemotherapeutic agents with a broad spectrum of activity. However, despite this versatile activity, its use poses many limitations. Herein, novel derivatives of 5-FU and dichloroacetic acid have been designed and synthesized as a new type of codrugs, also known as mutual prodrugs, to overcome the drawbacks of 5-FU and enhance its therapeutic efficiency. The stability of the obtained compounds has been tested at various pH values using different analytical techniques, namely HPLC and potentiometry. The antiproliferative activity of the new 5-FU derivatives was assessed in vitro on SK-MEL-28 and WM793 human melanoma cell lines in 2D culture as well as on A549 human lung carcinoma, MDA-MB-231 breast adenocarcinoma, LL24 normal lung tissue, and HMF normal breast tissue as a multicellular 3D spheroid model cultured in standard (static) conditions and with the use of microfluidic systems, which to a great extent resembles the in vivo environment. In all cases, new mutual prodrugs showed a higher cytotoxic activity toward cancer models and lower to normal cell models than the parent 5-FU itself.

Study of the biological activity of photoactive bipyridyl-Ru(II) complexes containing 1,3,5-triaza-7-phosphaadamantane (PTA)

The water-soluble ruthenium complex cis-[Ru(dcbpyH)₂(PTAH)₂]Cl₂·3H₂O (1) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; PTA = 1,3,5-triaza-7-phosphaadamantane) has been synthesized and characterised by NMR, IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The optical properties of 1 were studied, including photoactivation under visible light, as well as its biological properties, together with those of the previously published Ru complexes cis-[Ru(bpy)₂(PTA)₂]Cl₂ (2), trans-[Ru(bpy)₂(PTA)₂](CF₃SO₃)₂ (3) and cis-[Ru(bpy)₂(H₂O)(PTA)](CF₃SO₃)₂ (4) (bpy = 2,2'-bipyridine). Anticancer activities of the complexes against human lung (A549), cervical (HeLa) and prostate (PC3) carcinoma cells were evaluated under dark conditions and upon photoactivation with visible light. None of the complexes exhibited cytotoxic activity in the absence of light irradiation (IC₅₀ > 100 μM). However, after photoactivation, the cytotoxicity of complexes 1, 2 and 3 against the three cell lines markedly increased, resulting in IC₅₀ values between 25.3 μM and 9.3 μM. Notably, these complexes did not show toxicity against red blood cells. These findings show the potential of complexes 1, 2 and, particularly, 3 for selective and controlled cancer photochemotherapy. The reactivity of the Ru complexes against DNA under UV-Vis irradiation was studied by analysing plasmid mobility. Experimental data shows that 4 unfolds supercoiled DNA (SC DNA) both in the dark and under visible irradiation, while 1 and 3 are only active under light, being 2 inactive in either case. The unfolding activities of complexes 3 and 4 were dependent on the air present in the reaction. The measured intracellular levels of reactive oxygen species (ROS) upon irradiation with complexes 1, 2 and 3 suggest that their mechanism of action is related to oxidative stress.

Versatile Platinum(IV) Prodrugs of Naproxen and Acemetacin as Chemo-Anti-Inflammatory Agents

Developing new and versatile platinum(IV) complexes that incorporate bioactive moieties is a rapidly evolving research strategy for cancer drug discovery. In this study, six platinum(IV) complexes (1-6) that are mono-substituted in the axial position with a non-steroidal anti-inflammatory molecule, naproxen or acemetacin, were synthesised. A combination of spectroscopic and spectrometric techniques confirmed the composition and homogeneity of 1-6. The antitumour potential of the resultant complexes was assessed on multiple cell lines and proved to be significantly improved compared with cisplatin, oxaliplatin and carboplatin. The platinum(IV) derivatives conjugated with acemetacin (5 and 6) were determined to be the most biologically potent, demonstrating GI₅₀ values ranging between 0.22 and 250 nM. Remarkably, in the Du145 prostate cell line, 6 elicited a GI₅₀ value of 0.22 nM, which is 5450-fold more potent than cisplatin. A progressive decrease in reactive oxygen species and mitochondrial activity was observed for 1-6 in the HT29 colon cell line, up to 72 h. The inhibition of the cyclooxygenase-2 enzyme was also demonstrated by the complexes, confirming that these platinum(IV) complexes may reduce COX-2-dependent inflammation and cancer cell resistance to chemotherapy.

Bioactive Platinum(IV) Complexes Incorporating Halogenated Phenylacetates

A new series of cytotoxic platinum(IV) complexes (1-8) incorporating halogenated phenylacetic acid derivatives (4-chlorophenylacetic acid, 4-fluorophenylacetic acid, 4-bromophenylacetic acid and 4-iodophenylacetic acid) were synthesised and characterised using spectroscopic and spectrometric techniques. Complexes 1-8 were assessed on a panel of cell lines including HT29 colon, U87 glioblastoma, MCF-7 breast, A2780 ovarian, H460 lung, A431 skin, Du145 prostate, BE2-C neuroblastoma, SJ-G2 glioblastoma, MIA pancreas, the ADDP-resistant ovarian variant, and the non-tumour-derived MCF10A breast line. The in vitro cytotoxicity results confirmed the superior biological activity of the studied complexes, especially those containing 4-fluorophenylacetic acid and 4-bromophenylacetic acid ligands, namely 4 and 6, eliciting an average GI₅₀ value of 20 nM over the range of cell lines tested. In the Du145 prostate cell line, 4 exhibited the highest degree of potency amongst the derivatives, displaying a GI₅₀ value of 0.7 nM, which makes it 1700-fold more potent than cisplatin (1200 nM) and nearly 7-fold more potent than our lead complex, 56MESS (4.6 nM) in this cell line. Notably, in the ADDP-resistant ovarian variant cell line, 4 (6 nM) was found to be almost 4700-fold more potent than cisplatin. Reduction reaction experiments were also undertaken, along with studies aimed at determining the complexes' solubility, stability, lipophilicity, and reactive oxygen species production.

Metal- and metalloid-based compounds to target and reverse cancer multidrug resistance

Drug resistance remains the major cause of cancer treatment failure especially at the late stage of the disease. However, based on their versatile chemistry, metal and metalloid compounds offer the possibility to design fine-tuned drugs to circumvent and even specifically target drug-resistant cancer cells. Based on the paramount importance of platinum drugs in the clinics, two main areas of drug resistance reversal strategies exist: overcoming resistance to platinum drugs as well as multidrug resistance based on ABC efflux pumps. The current review provides an overview of both aspects of drug design and discusses the open questions in the field. The areas of drug resistance covered in this article involve: 1) Altered expression of proteins involved in metal uptake, efflux or intracellular distribution, 2) Enhanced drug efflux via ABC transporters, 3) Altered metabolism in drug-resistant cancer cells, 4) Altered thiol or redox homeostasis, 5) Altered DNA damage recognition and enhanced DNA damage repair, 6) Impaired induction of apoptosis and 7) Altered interaction with the immune system. This review represents the first collection of metal (including platinum, ruthenium, iridium, gold, and copper) and metalloid drugs (e.g. arsenic and selenium) which demonstrated drug resistance reversal activity. A special focus is on compounds characterized by collateral sensitivity of ABC transporter-overexpressing cancer cells. Through this approach, we wish to draw the attention to open research questions in the field. Future investigations are warranted to obtain more insights into the mechanisms of action of the most potent compounds which target specific modalities of drug resistance.

Recent Progresses in Conjugation with Bioactive Ligands to Improve the Anticancer Activity of Platinum Compounds

Platinum (Pt) drugs, including cisplatin, are widely used for the treatment of solid tumors. Despite the clinical success, side effects and occurrence of resistance represent major limitations to the use of clinically available Pt drugs. To overcome these problems, a variety of derivatives have been designed and synthetized. Here, we summarize the recent progress in the development of Pt(II) and Pt(IV) complexes with bioactive ligands. The development of Pt(II) and Pt(IV) complexes with targeting molecules, clinically available agents, and other bioactive molecules is an active field of research. Even if none of the reported Pt derivatives has been yet approved for clinical use, many of these compounds exhibit promising anticancer activities with an improved pharmacological profile. Thus, planning hybrid compounds can be considered as a promising approach to improve the available Pt-based anticancer agents and to obtain new molecular tools to deepen the knowledge of cancer progression and drug resistance mechanisms.

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.

Hypoxia Active Platinum(IV) Prodrugs of Orotic Acid Selective to Liver Cancer Cells

Platinum(IV) complexes of orotic acid selectively target liver cancer cells displaying enhanced activity and higher uptake in Hep G2. The comparatively higher expression of Organic Anion Transporter 2 (OAT2) in Hep G2 and decrease in toxicity in the presence of OAT2 inhibitor suggest its involvement in the uptake of the complexes. They are resistant to sequestration by the copper transporter ATP7B, unlike cisplatin and oxaliplatin.

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.

Novel Pt(IV) Prodrugs Displaying Antimitochondrial Effects

The design, synthesis, characterization, and biological activity of a series of platinum(IV) prodrugs containing the axial ligand 3-(4-phenylquinazoline-2-carboxamido)propanoate (L3) are reported. L3 is a derivative of the quinazolinecarboxamide class of ligands that binds to the translocator protein (TSPO) at the outer mitochondrial membrane. The cytotoxicities of cis,cis,trans-[Pt(NH₃)₂Cl₂(L3)(OH)] (C-Pt1), cis,cis,trans-[Pt(NH₃)₂Cl₂(L3)(BZ)] (C-Pt2), trans-[Pt(DACH)(OX)(L3)(OH)] (C-Pt3), and trans-[Pt(DACH)(OX)(L3)(BZ)] (C-Pt4) (DACH: R,R-diaminocyclohexane, BZ: benzoate, OX: oxalate) in MCF-7 breast cancer and noncancerous MCF-10A epithelial cells were assessed and compared with those of cisplatin, oxaliplatin, and the free ligand L3. Moreover, the cellular uptake, ROS generation, DNA damage, and the effect on the mitochondrial function, mitochondrial membrane potential, and morphology were investigated. Molecular interactions of L3 in the TSPO binding site were studied using molecular docking. The results showed that complex C-Pt1 is the most effective Pt(IV) complex and exerts a multimodal mechanism involving DNA damage, potent ROS production, loss of the mitochondrial membrane potential, and mitochondrial damage.

Pt(IV) pro-drugs with an axial HDAC inhibitor demonstrate multimodal mechanisms involving DNA damage and apoptosis independent of cisplatin resistance in A2780/A2780cis cells

Epigenetic agents such as histone deacetylase (HDAC) inhibitors are widely investigated for use in combined anticancer therapy and the co-administration of Pt drugs with HDAC inhibitors has shown promise for the treatment of resistant cancers. Coordination of an HDAC inhibitor to an axial position of a Pt(IV) derivative of cisplatin allows the combination of the epigenetic drug and the Pt chemotherapeutic into a single molecule. In this work we carry out mechanistic studies on the known Pt(IV) complex cis,cis,trans-[Pt(NH₃)₂Cl₂(PBA)₂] (B) with the HDAC inhibitor 4-phenylbutyrate (PBA) and its derivatives cis,cis,trans-[Pt(NH₃)₂Cl₂(PBA)(OH)] (A), cis,cis,trans-[Pt(NH₃)₂Cl₂(PBA)(Bz)] (C), and cis,cis,trans-[Pt(NH₃)₂Cl₂(PBA)(Suc)] (D) (Bz = benzoate, Suc = succinate). The comparison of the cytotoxicity, effect on HDAC activity, reactive oxygen species (ROS) generation, γ-H2AX (histone 2A-family member X) foci generation and induction of apoptosis in cisplatin-sensitive and cisplatin-resistant ovarian cancer cells shows that A - C exhibit multimodal mechanisms involving DNA damage and apoptosis independent of cisplatin resistance.

Targeting drug delivery system for platinum(Ⅳ)-Based antitumor complexes

Classical platinum(II) anticancer agents are widely-used chemotherapeutic drugs in the clinic against a range of cancers. However, severe systemic toxicity and drug resistance have become the main obstacles which limit their application and effectiveness. Because divalent cisplatin analogues are easily destroyed in vivo, their bioavailability is low and no selective to tumor tissues. The platinum(IV) prodrugs are attractive compounds for cancer treatment because they have great advantages, e.g., higher stability in biological media, aqueous solubility and no cross-resistance with cisplatin, which may become the next generation of platinum anticancer drugs. In addition, platinum(IV) drugs could be taken orally, which could be more acceptable to cancer patients, breaking the current situation that platinum(II) drugs can only be given by injection. The coupling of platinum(IV) complexes with tumor targeting groups avoids the disadvantages such as instability in blood, irreversible binding to plasma proteins, rapid renal clearance, and non-specific distribution in normal tissues. Because of the above advantages, the combination of platinum complexes and tumor targeting groups has become the hottest field in the research and development of new platinum drugs. These approaches can be roughly categorized into two groups: active and passive targeted strategies. This review concentrates on various targeting and delivery strategies for platinum(IV) complexes to improve the efficacy and reduce the side effects of platinum-based anticancer drugs. We have made a summary of the related articles on platinum(IV) targeted delivery in recent years. We believe the results of the studies described in this review will provide new ideas and strategies for the development of platinum drugs.

Synthesis and Cytotoxicity of Water-Soluble Dual- and Triple-Action Satraplatin Derivatives: Replacement of Equatorial Chlorides of Satraplatin by Acetates

Pt(II) complexes, such as cisplatin and oxaliplatin, are in widespread use as anticancer drugs. Their use is limited by the toxic side effects and the ability of tumors to develop resistance to the drugs. A popular approach to overcome these drawbacks is to use their kinetically inert octahedral Pt(IV) derivatives that act as prodrugs. The most successful Pt(IV) complex in clinical trials to date is satraplatin, cct-[Pt(NH₃)(c-hexylamine)Cl₂(OAc)₂], that upon cellular reduction releases the cytotoxic cis-[Pt(NH₃)(c-hexylamine)Cl₂]. In an attempt to obtain water-soluble and more effective cytotoxic Pt(IV) complexes, we prepared a series of dual- and triple-action satraplatin analogues, where the equatorial chlorido ligands were replaced with acetates and the axial ligands include innocent and bioactive ligands. Replacement of the chlorides with acetates enhanced the water solubility of the compounds and, with one exception, all of the compounds were very stable in buffer. In general, compounds with one or two axial hydroxido ligands were reduced by ascorbate significantly more quickly than compounds with two axial carboxylates. While replacement of the chlorides with acetates in satraplatin led to a reduction in cytotoxicity, the dual- and triple-action analogues with equatorial acetates had low- to sub-micromolar IC₅₀ values in a panel of eight cancer cells. The triple-action compound cct-[Pt(NH₃)(c-hexylamine)(OAc)₂(PhB)(DCA)] was active in all cell lines, causing DNA damage that induced cell cycle inhibition and apoptosis. Its good activity against CT26 cells in vitro translated into good in vivo efficacy against the CT26 allograft, an in vivo model with intrinsic satraplatin resistance. This indicates that multiaction Pt(IV) derivatives of diamine dicarboxylates are interesting anticancer drug candidates.

Platinum(II)-oxalato complexes of seliciclib (CYC202) derivatives show different cellular effects and lesser adverse effects in mouse lymphoma model than cisplatin

This work presents a deeper pharmacological evaluation of two formerly prepared and characterized, and highly in vitro cytotoxic platinum(II) oxalato complexes [Pt(ox)(L₁)₂] (1) and [Pt(ox)(L₂)₂] (2), containing the derivatives of cyclin-dependent kinase inhibitor (CDKi) seliciclib ((R)-roscovitine, CYC202) coordinating as N-donor carrier ligands, i.e., 2-(1-ethyl-2-hydroxyethylamino)-N6-(4-methoxybenzyl)-9-isopropyladenine (L₁) and 2-chloro-N6-(2,4-dimethoxybenzyl)-9-isopropyladenine (L₂). The positive results of in vitro cytotoxicity screening on human cancer cell lines (HeLa, HOS, A2780, A2780R, G361 and MCF7 with IC₅₀ at low micromolar levels) published previously, motivated us to perform extended preclinical in vitro experiments to reveal the mechanisms associated with the induction of cancer cell death. In addition, the in vivo antitumor activity was evaluated using the mouse lymphocytic leukaemia L1210 model. The obtained results revealed that complex 1 exceeds the antitumor effect of cisplatin (as for the extension of life-span of mice) and shows far less adverse effects as compared to reference drug cisplatin. The in vitro and ex vivo studies of cellular effects and molecular mechanisms of cell death induction showed that the mechanism of action of complex 1 is essentially different from that of cisplatin. The obtained results showed a possible way how to obtain antitumor active platinum(II) oxalato complexes with better therapeutic profile than contemporary used platinum-based therapeutics.

A Subset of New Platinum Antitumor Agents Kills Cells by a Multimodal Mechanism of Action Also Involving Changes in the Organization of the Microtubule Cytoskeleton

The substitution inert platinum agent [Pt(1 S,2 S-diaminocyclohexane)(5,6-dimethyl-1,10-phenanthroline)]²⁺ (56MeSS, 5) is a potent cytotoxic metallodrug. In contrast to conventional cisplatin or oxaliplatin, the mechanism of action (MoA) of 5 is fundamentally different. However, details of the mechanism by which the 5,6-dimethyl-1,10-phenanthroline ligand contributes to the cytotoxicity of 5 and its derivatives have not been sufficiently clarified so far. Here, we show that 5 and its Pt(IV) derivatives exhibit an intriguing potency in the triple-negative breast cancer cells MDA-MB-231. Moreover, we show that the Pt(IV) derivatives of 5 act by multimodal MoA resulting in the global biological effects, that is, they damage nuclear DNA, reduce the mitochondrial membrane potential, induce the epigenetic processes, and last but not least, the data provide evidence that changes in the organization of cytoskeleton networks are functionally important for 5 and its derivatives, in contrast to clinically used platinum cytostatics, to kill cancer cells.

Synthesis and evaluation of bi-functional 7-hydroxycoumarin platinum(IV) complexes as antitumor agents

A series of bi-functional 7-hydroxycoumarin platinum(IV) complexes were synthesized, characterized, and evaluated for antitumor activities. The 7-hydroxycoumarin platinum(IV) complexes display moderate to effective antitumor activities toward the tested cell lines and show much potential in overcoming drug resistance of platinum(II) drugs. In reducing microenvironment, the title compounds could be reduced to platinum(II) complex accompanied with two equivalents of coumarin units. By a unique mechanism, the 7-hydroxycoumarin platinum(IV) complex attacks DNA via the released platinum(II) compound, meanwhile it also inhibits the activities of cyclooxygenase by coumarin fragment. This action mechanism might be of much benefit for reducing tumor-related inflammation in the progress of inhibiting tumor proliferation and overcoming cisplatin resistance. The incorporation of 7-hydroxycoumarin leads to significantly enhanced platinum accumulation in both whole tumor cells and DNA. The HSA interaction investigation reveals that the tested coumarin platinum(IV) compound could effectively combine with HSA via van der Waals force and hydrogen bond.

Mitochondria-Targeting Anticancer Metal Complexes

Background:

Since the serendipitous discovery of the antitumor activity of cisplatin there has been a continuous surge in studies aimed at the development of new cytotoxic metal complexes. While the majority of these complexes have been designed to interact with nuclear DNA, other targets for anticancer metallodrugs attract increasing interest. In cancer cells the mitochondrial metabolism is deregulated. Impaired apoptosis, insensitivity to antigrowth signals and unlimited proliferation have been linked to mitochondrial dysfunction. It is therefore not surprising that mitochondria have emerged as a major target for cancer therapy. Mitochondria-targeting agents are able to bypass resistance mechanisms and to (re-) activate cell-death programs.

Methods:

Web-based literature searching tools such as SciFinder were used to search for reports on cytotoxic metal complexes that are taken up by the mitochondria and interact with mitochondrial DNA or mitochondrial proteins, disrupt the mitochondrial membrane potential, facilitate mitochondrial membrane permeabilization or activate mitochondria-dependent celldeath signaling by unbalancing the cellular redox state. Included in the search were publications investigating strategies to selectively accumulate metallodrugs in the mitochondria.

Results:

This review includes 241 references on antimitochondrial metal complexes, the use of mitochondria-targeting carrier ligands and the formation of lipophilic cationic complexes.

Conclusion:

Recent developments in the design, cytotoxic potency, and mechanistic understanding of antimitochondrial metal complexes, in particular of cyclometalated Au, Ru, Ir and Pt complexes, Ru polypyridine complexes and Au-N-heterocyclic carbene and phosphine complexes are summarized and discussed.

Toward Multi-Targeted Platinum and Ruthenium Drugs-A New Paradigm in Cancer Drug Treatment Regimens?

While medicinal inorganic chemistry has been practised for over 5000 years, it was not until the late 1800s when Alfred Werner published his ground-breaking research on coordination chemistry that we began to truly understand the nature of the coordination bond and the structures and stereochemistries of metal complexes. We can now readily manipulate and fine-tune their properties. This had led to a multitude of complexes with wide-ranging biomedical applications. This review will focus on the use and potential of metal complexes as important therapeutic agents for the treatment of cancer. With major advances in technologies and a deeper understanding of the human genome, we are now in a strong position to more fully understand carcinogenesis at a molecular level. We can now also rationally design and develop drug molecules that can either selectively enhance or disrupt key biological processes and, in doing so, optimize their therapeutic potential. This has heralded a new era in drug design in which we are moving from a single- toward a multitargeted approach. This approach lies at the very heart of medicinal inorganic chemistry. In this review, we have endeavored to showcase how a "multitargeted" approach to drug design has led to new families of metallodrugs which may not only reduce systemic toxicities associated with modern day chemotherapeutics but also address resistance issues that are plaguing many chemotherapeutic regimens. We have focused our attention on metallodrugs incorporating platinum and ruthenium ions given that complexes containing these metal ions are already in clinical use or have advanced to clinical trials as anticancer agents. The "multitargeted" complexes described herein not only target DNA but also contain either vectors to enable them to target cancer cells selectively and/or moieties that target enzymes, peptides, and intracellular proteins. Multitargeted complexes which have been designed to target the mitochondria or complexes inspired by natural product activity are also described. A summary of advances in this field over the past decade or so will be provided.

Half-sandwich Os(ii) and Ru(ii) bathophenanthroline complexes: anticancer drug candidates with unusual potency and a cellular activity profile in highly invasive triple-negative breast cancer cells

There is an urgent need to discover new, selective compounds to add to the limited arsenal of chemotherapeutics displaying selective toxicity for aggressive triple-negative breast cancer (TNBC) cells. The effect of two, recently developed metal-based half-sandwich complexes [Os(η6-pcym)(bphen)(dca)]PF6 (Os-dca) and [Ru(η6-pcym)(bphen)(dca)]PF6 (Ru-dca) [pcym = 1-methyl-4-(propan-2-yl)benzene (p-cymene); bphen = 4,7-diphenyl-1,10-phenanthroline (bathophenanthroline); dca = dichloroacetate] on triple-negative breast cancer cells MDA-MB-231 is reported. The complexes display selective toxicity in several tumor cells (at submicromolar concentrations), and a prominent effect is observed against highly progressive triple negative breast cancer MDA-MB-231 cells for Os-dca. The lower potency of Ru-dca in comparison with Os-dca is apparently connected with a relatively quick release of the dca ligand due to the hydrolysis of Ru-dca before this complex enters the cells. Remarkably, both Os-dca and Ru-dca reduce successfully metastasis-related properties of the triple-negative breast cancer cells such as migration, invasion, and re-adhesion. The anti-metastatic effects of Os-dca and Ru-dca are associated with their ability to suppress matrix metalloproteinase activity and/or production and reduce the expression of aquaporins. Further detailed mechanistic studies reveal that Os-dca reverses Warburg's effect and oncosis seems to be a prominent mode of cell death that predominates over apoptosis. As such, Os-dca can efficiently overcome the resistance of cancer cells to clinically-used apoptotic inducers cisplatin and carboplatin. The cytostatic and anti-metastatic properties of Os-dca in MDA-MB-231 provide a strong impetus for the development of new metal-based compounds to target hardly treatable human TNBC cells and displaying different modes of action compared to the antitumor metallodrugs in clinical use.

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.

Current Developments in Pt(IV) Prodrugs Conjugated with Bioactive Ligands

To overcome the side effects of and resistance to cisplatin, a variety of Pt(IV) prodrugs were designed and synthesized via different modifications including combination with lipid chains to increase hydrophobicity, conjugation with short peptide chains or nanoparticles to improve drug delivery, or addition of bioactive ligands to the axial positions of Pt(IV) complexes to exert dual-function effects. This review summarizes the recent progress in the development of Pt(IV) prodrugs conjugated with bioactive-targeting ligands, including histone deacetylase inhibitors, p53 agonists, alkylating agents, and nonsteroidal anti-inflammatory agents. Although Pt(IV) complexes that conjugated with bioactive ligands show satisfactory anticancer effects, none has been approved for clinical use. Therefore, we hope that this review will contribute to further study and development of Pt(IV) complexes conjugated with bioactive and other ligands.

Surface-Functionalization of Zr-Fumarate MOF for Selective Cytotoxicity and Immune System Compatibility in Nanoscale Drug Delivery

Metal-organic frameworks (MOFs), network structures wherein metal ions or clusters link organic ligands into porous materials, are being actively researched as nanoscale drug delivery devices as they offer tunable structures with high cargo loading that can easily be further functionalized for targeting and enhanced physiological stability. The excellent biocompatibility of Zr has meant that its MOFs are among the most studied to date, in particular the archetypal Zr terephthalate UiO-66. In contrast, the isoreticular analog linked by fumarate (Zr-fum) has received little attention, despite the endogenous linker being part of the Krebs cycle. Herein, we report a comprehensive study of Zr-fum in the context of drug delivery. Reducing particle size is shown to increase uptake by cancer cells while reducing internalization by macrophages, immune system cells that remove foreign objects from the bloodstream. Zr-fum is compatible with defect loading of the drug dichloroacetate (DCA) as well as surface modification during synthesis, through coordination modulation and postsynthetically. DCA-loaded, PEGylated Zr-fum shows selective in vitro cytotoxicity toward HeLa and MCF-7 cancer cells, likely as a consequence of its enhanced caveolae-mediated endocytosis compared to uncoated precursors, and it is well tolerated by HEK293 kidney cells, J774 macrophages, and human peripheral blood lymphocytes. Compared to UiO-66, Zr-fum is more efficient at transporting the drug mimic calcein into HeLa cells, and DCA-loaded, PEGylated Zr-fum is more effective at reducing HeLa and MCF-7 cell proliferation than the analogous UiO-66 sample. In vitro examination of immune system response shows that Zr-fum samples induce less reactive oxygen species than UiO-66 analogs, possibly as a consequence of the linker being endogenous, and do not activate the C3 and C4 complement cascade pathways, suggesting that Zr-fum can avoid phagocytic activation. The results show that Zr-fum is an attractive alternative to UiO-66 for nanoscale drug delivery, and that a wide range of in vitro experiments is available to greatly inform the design of drug delivery systems prior to early stage animal studies.

Novel tacrine platinum(II) complexes display high anticancer activity via inhibition of telomerase activity, dysfunction of mitochondria, and activation of the p53 signaling pathway

In this work, we designed and synthesized tacrine platinum(II) complexes [PtClL(DMSO)]⋅CH₃OH (Pt1), [PtClL(DMP)] (Pt2), [PtClL(DPPTH)] (Pt3), [PtClL(PTH)] (Pt4), [PtClL(PIPTH)] (Pt5), [PtClL(PM)] (Pt6) and [PtClL(en)] (Pt7) with 4,4'-dimethyl-2,2'-bipyridine (DMP), 4,7-diphenyl-1,10-phenanthroline (DPPTH), 1,10-phenanthroline (PTH), 2-(1-pyrenecarboxaldehyde) imidazo [4,5-f]-[1,10] phenanthroline (PIPTH), 2-picolylamine (PM) and 1,2-ethylenediamine (en) as telomerase inhibitors and p53 activators. Biological evaluations demonstrated that Pt1Pt7 exhibited cytotoxic activity against the tested NCIH460, Hep-G2, SK-OV-3, SK-OV-3/DDP and MGC80-3 cancer cell lines, with Pt5 displaying the highest cytotoxicity. Pt5 exhibited an IC₅₀ value of 0.13 ± 0.16 μM against SK-OV-3/DDP cancer cells and significantly reduced tumor growth in a Hep-G2 xenograft mouse model (tumor growth inhibition (TGI) = 40.8%, p < 0.05) at a dose of 15.0 mg/kg. Interestingly, Pt1Pt7 displayed low cytotoxicity against normal HL-7702 cells. Mechanistic studies revealed that these compounds caused cell cycle arrest at the G2/M and S phases, and regulated the expression of CDK2, cyclin A, p21, p53 and p27. Further mechanistic studies showed that Pt5 induced SK-OV3/DDP cell apoptosis via dysfunction of mitochondria, inhibition of the telomerase activity by directly targeting the c-myc promoter, and activation of the p53 signaling pathway. Taken together, Pt5 has the potential to be further developed as a new antitumor drug.

Anticancer kiteplatin pyrophosphate derivatives show unexpected target selectivity for DNA

One of the promising new antitumor platinum complexes is a large-ring chelate complex [PtCl₂(cis-1,4-DACH)] (DACH = diaminocyclohexane) (kiteplatin). Recently, new platinum(ii) derivatives of kiteplatin with pyrophosphate as a carrier ligand have been synthesized and tested on a panel of human cancer cell lines. These derivatives of kiteplatin were found to be more effective than clinically used anticancer platinum drugs. The design of kiteplatin pyrophosphate derivatives was based on the concept of pyrophosphate coordinated platinum complexes, phosphaplatins. Phosphaplatins have been shown to function without binding to DNA and hence DNA has been excluded as the target of phosphaplatins in contrast to conventional antitumor platinum drugs. Cytotoxicity, major cellular targets and DNA interactions of the new anticancer platinum drug were characterized by standard biochemical methods and methods of molecular and cellular biology. We demonstrate that, in contrast to what has been reported on closely related phosphaplatins, the derivatives of kiteplatin with the pyrophosphate carrier ligand are activated in the cellular environment. This activation, which yields species capable of platination of DNA, very likely comprises the hydrolytic release of the pyrophosphate ligand that could be enzymatically catalyzed. Collectively, these data provide convincing evidence that unexpectedly DNA is an important target for the biological activity of the kiteplatin pyrophosphate derivatives, although the overall mechanism of action might be different from those of conventional platinum drugs.

Mechanistic Investigation into the Selective Anticancer Cytotoxicity and Immune System Response of Surface-Functionalized, Dichloroacetate-Loaded, UiO-66 Nanoparticles

The high drug-loading and excellent biocompatibilities of metal-organic frameworks (MOFs) have led to their application as drug-delivery systems (DDSs). Nanoparticle surface chemistry dominates both biostability and dispersion of DDSs while governing their interactions with biological systems, cellular and/or tissue targeting, and cellular internalization, leading to a requirement for versatile and reproducible surface functionalization protocols. Herein, we explore not only the effect of introducing different surface functionalities to the biocompatible Zr-MOF UiO-66 but also the efficacy of three surface modification protocols: (i) direct attachment of biomolecules [folic acid (FA) and biotin (Biot)] introduced as modulators for UiO-66 synthesis, (ii) our previously reported "click-modulation" approach to covalently attach polymers [poly(ethylene glycol) (PEG), poly-l-lactide, and poly-N-isopropylacrylamide] to the surface of UiO-66 through click chemistry, and (iii) surface ligand exchange to postsynthetically coordinate FA, Biot, and heparin to UiO-66. The innovative use of a small molecule with metabolic anticancer activity, dichloroacetate (DCA), as a modulator during synthesis is described, and it is found to be compatible with all three protocols, yielding surface-coated, DCA-loaded (10-20 w/w %) nano-MOFs (70-170 nm). External surface modification generally enhances the stability and colloidal dispersion of UiO-66. Cellular internalization routes and efficiencies of UiO-66 by HeLa cervical cancer cells can be tuned by surface chemistry, and anticancer cytotoxicity of DCA-loaded MOFs correlates with the endocytosis efficiency and mechanisms. The MOFs with the most promising coatings (FA, PEG, poly-l-lactide, and poly-N-isopropylacrylamide) were extensively tested for selectivity of anticancer cytotoxicity against MCF-7 breast cancer cells and HEK293 healthy kidney cells as well as for cell proliferation and reactive oxygen species production against J774 macrophages and peripheral blood lymphocytes isolated from the blood of human donors. DCA-loaded, FA-modified UiO-66 selectively kills cancer cells without harming healthy ones or provoking immune system response in vitro, suggesting a significant targeting effect and great potential in anticancer drug delivery. The results provide mechanistic insight into the design and functionalization of MOFs for drug delivery and underline the availability of various in vitro techniques to potentially minimize early-stage in vivo animal studies following the three Rs: reduction, refinement, and replacement.

Effects of 5-hydroxy-4'-nitro-7-propionyloxy-genistein on inhibiting proliferation and invasion via activating reactive oxygen species in human ovarian cancer A2780/DDP cells

5-hydroxy-4'-nitro-7-propionyloxy-genistein (HNPG), a novel synthetic isoflavone derivative, was demonstrated to possess antitumor activity in gastric cancer and breast cancer in vitro, but its antitumor effect and mechanism in ovarian cancer has not been characterized. The aim of the present study was to investigate the effects of HNPG on inhibiting the proliferation and invasion in human ovarian cancer A2780 cell lines of cisplatin resistance (A2780/DDP) and elucidate its underlying molecular mechanism. The results indicated that HNPG presented with marked antitumor activity against A2780/DDP cells in vitro, significantly inhibited the rates of proliferation, clone formation, invasion and metastasis, and markedly induced apoptosis in dose- and time-dependent manner. Simultaneously, levels of reactive oxygen species (ROS) were increased and mitochondrial membrane potential was decreased. In addition, Bcl-2 expression was downregulated, Bax expression was upregulated, and the ratio of Bcl-2/Bax was decreased. Concurrently, levels of Cyt-C were markedly enhanced and the caspase cascade was triggered. Taken together, the results suggested that HNPG exerted anticancer effects through promoting ROS accumulation in cells, triggering mitochondrial apoptotic pathways and ultimately resulting in cells apoptosis. Therefore, HNPG serves as a potential candidate in the chemoprevention and/or treatment of cisplatin-resistant human ovarian cancer.

Platinum(iv) anticancer prodrugs - hypotheses and facts

In this manuscript we focus on Pt(iv) anticancer prodrugs. We explore the main working hypotheses for the design of effective Pt(iv) prodrugs and note the exceptions to the common assumptions that are prevalent in the field. Special attention was devoted to the emerging class of "dual action" Pt(iv) prodrugs, where bioactive ligands are conjugated to the axial positions of platinum in order to obtain orthogonal or complementary effects that will increase the efficacy of killing the cancer cells. We discuss the rationale behind the design of the "dual action" prodrugs and the results of the pharmacological studies obtained. Simultaneous release of two bioactive moieties inside the cancer cells often triggers several processes that together determine the fate of the cell. Pt(iv) complexes provide many opportunities for applying new concepts in targeting, synergistic cell killing and exploiting novel nanodelivery systems.

Enhancing anticancer cytotoxicity through bimodal drug delivery from ultrasmall Zr MOF nanoparticles

Dual delivery of dichloroacetate and 5-fluorouracil from Zr MOFs into cancer cells is found to enhance in vitro cytotoxicity.

A potential method to improve the in vitro cytotoxicity of half-sandwich Os(ii) complexes against A2780 cells

[Os(η⁶-pcym)(dpa)(VP)]PF₆ (1-VP), containing the histone deacetylase inhibitor valproate, shows ca. 3-fold higher in vitro cytotoxicity against the A2780 human ovarian carcinoma cells than its chlorido analogue [Os(η⁶-pcym)(dpa)Cl]PF₆ (1-Cl).

Dual-acting antitumor Pt(iv) prodrugs of kiteplatin with dichloroacetate axial ligands

DNA and mitochondria of tumor cells are the targets of Pt(iv) complexes of kiteplatin with biologically active dichloroacetate as axial ligands.

Research progress in modern structure of platinum complexes

Since the antitumor activity of cisplatin was discovered in 1967 by Rosenberg, platinum-based anticancer drugs have played an important role in chemotherapy in clinic. Nevertheless, platinum anticancer drugs also have caused severe side effects and cross drug resistance which limited their applications. Therefore, a significant amount of efforts have been devoted to developing new platinum-based anticancer agents with equal or higher antitumor activity but lower toxicity. Until now, a large number of platinum-based complexes have been prepared and extensively investigated in vitro and in vivo. Among them, some platinum-based complexes revealing excellent anticancer activity showed the potential to be developed as novel type of anticancer agents. In this account, we present such platinum-based anticancer complexes which owning various types of ligands, such as, amine carrier ligands, leaving groups, reactive molecule, steric hindrance groups, non-covalently binding platinum (II) complexes, Platinum(IV) complexes and polynuclear platinum complexes. Overall, platinum-based anticancer complexes reported recently years upon modern structure are emphasized.

Antitumor platinum(IV) derivatives of carboplatin and the histone deacetylase inhibitor 4-phenylbutyric acid

Five new platinum(IV) derivatives of carboplatin each incorporating the histone deacetylase inhibitor 4-phenylbutyrate in axial position were synthesized and characterized by ¹H and ¹⁹⁵Pt NMR spectroscopy, electrospray ionization mass spectrometry and elemental analysis, namely cis,trans-[Pt(CBDCA)(NH₃)₂(PBA)(OH)] (1), cis,trans-[Pt(CBDCA)(NH₃)₂(PBA)₂] (2), cis,trans-[Pt(CBDCA)(NH₃)₂(PBA)(bz)] (3), cis,trans-[Pt(CBDCA)(NH₃)₂(PBA)(suc)] (4) and cis,trans-[Pt(CBDCA)(NH₃)₂)(PBA)(ac)] (5) (PBA=4-phenylbutyrate, CBDCA=1,1-cyclobutane dicarboxylate, bz=benzoate, suc=succinate and ac=acetate). The reduction behavior in the presence of ascorbic acid was studied by high performance liquid chromatography. The cytotoxicity against a panel of human tumor cell lines, histone deacetylase (HDAC) inhibitory activity, cellular accumulation and the ability to induce apoptosis were evaluated. The most effective complex, compound 3, was found to be up to ten times more effective than carboplatin and to decrease cellular basal HDAC activity by approximately 18% in A431 human cervical cancer cells.

Biotinylated platinum(IV) complexes designed to target cancer cells

Three biotinylated platinum(IV) complexes (1-3) were designed and synthesized. The resulting platinum(IV) complexes exhibited effective cytotoxicity against the tested cancer cell lines, especially complex 1, which was 2.0-9.6-fold more potent than cisplatin. These complexes were found to be rapidly reduced to their activated platinum(II) counterparts by glutathione or ascorbic acid under biologically relevant condition. Additional molecular docking studies revealed that the biotin moieties of all Pt(IV) complexes can effectively bind with the streptavidin through the noncovalent interactions. Besides, introduction of the biotin group can obviously promote the cancer cell uptake of platinum when treated with complex 1, particularly in cisplatin-resistant SGC-7901/Cis cancer cells. Further mechanistic studies on complex 1 indicated that it activated the expression of Bax, and induced cytochrome c release from the mitochondria, and finally activated caspase-3.

Exploring the Hydrolytic Behavior of the Platinum(IV) Complexes with Axial Acetato Ligands

Platinum(IV) complexes are generally thought to be kinetically inert, and are expected to be stable enough to resist premature aquation before entering the cancer cells. Nevertheless, in this work, complex 2 with axial acetato ligands can hydrolyze relatively quickly under biologically relevant conditions with a half-life of 91.7 min, resulting in the loss of the equatorial chlorido ligand. Further study indicated that the fast hydrolysis of complex 2 may be attributed to the strong σ-donor ability of N-isopropyl-1R,2R-diaminocyclohexane, and an increasing σ-donor ability of the amine group can promote the hydrolysis rate of the corresponding platinum(IV) complex. The experiment results were proven by the corresponding DFT calculation. Our study can help to re-evaluate the aqueous properties of the platinum(IV) complexes with axial acetate, which may be less inert to hydrolysis than expected under biologically relevant conditions.

Influence of the Number of Axial Bexarotene Ligands on the Cytotoxicity of Pt(IV) Analogs of Oxaliplatin

We present the synthesis and cytotoxic potencies of new Pt(IV) complexes with bexarotene, an anticancer drug that induces cell differentiation and apoptosis via selective activation of retinoid X receptors. In these complexes bexarotene is positioned as an axial ligand. The complex of one bexarotene ligand attached to Pt(IV) oxaliplatin moiety was potent whereas its counterpart carrying two bexarotene ligands was inactive.

New Acridine Thiourea Gold(I) Anticancer Agents: Targeting the Nucleus and Inhibiting Vasculogenic Mimicry

Two new 1-acridin-9-yl-3-methylthiourea Au(I) DNA intercalators [Au(ACRTU)₂]Cl (2) and [Au(ACRTU) (PPh₃)]PF₆ (3) have been prepared. Both complexes were highly active in the human ovarian carcinoma cisplatin-sensitive A2780 cell line, exhibiting IC₅₀ values in the submicromolar range. Compounds 2 and 3 are also cytotoxic toward different phenotypes of breast cancer cell lines MDA-MB-231 (triple negative), SK-BR-3 (HER2+, ERα-, and ERβ-), and MCF-7 (ER+). Both complexes induce apoptosis through activation of caspase-3 in vitro. While inhibition of some proteins (thiol-containing enzymes) seems to be the main mechanism of action for cytotoxic gold complexes, 2 and 3 present a DNA-dependent mechanism of action. They locate in the cell nucleus according to confocal microscopy and transmission electronic microscopy. The binding to DNA resulted to be via intercalation as shown by spectroscopic methods and viscometry, exhibiting a dose-dependent response on topoisomerase I mediated DNA unwinding. In addition, 2 and 3 exhibit potent antiangiogenic effects and are also able to inhibit vasculogenic mimicry of highly invasive MDA-MB-231 cells.

Oxidative Stress Induced by Pt(IV) Pro-drugs Based on the Cisplatin Scaffold and Indole Carboxylic Acids in Axial Position

The use of Pt(IV) complexes as pro-drugs that are activated by intracellular reduction is a widely investigated approach to overcome the limitations of Pt(II) anticancer agents. A series of ten mono- and bis-carboxylated Pt(IV) complexes with axial indole-3-acetic acid (IAA) and indole-3-propionic acid (IPA) ligands were synthesized and characterized by elemental analysis, ESI-MS, FT-IR, (1)H and (195)Pt NMR spectroscopy. Cellular uptake, DNA platination and cytotoxicity against a panel of human tumor cell lines were evaluated. All the complexes are able to overcome cisplatin-resistance and the most potent complex, cis,cis,trans-[Pt(NH3)2Cl2(IPA)(OH)] was on average three times more active than cisplatin. Mechanistic studies revealed that the trend in cytotoxicity of the Pt(IV) complexes is primarily consistent with their ability to accumulate into cancer cells and to increase intracellular basal reactive oxygen species levels, which in turn results in the loss of mitochondrial membrane potential and apoptosis induction. The role of the indole acid ligand as a redox modulator is discussed.