Exploration of the medical periodic table: towards new targets

Exploration of the medical periodic table: towards new targets†

Nicolas P. E. Barry and Peter J. Sadler*

University of Warwick, Department of Chemistry, Gibbet Hill Road, Warwick, UK. E-mail: N.Barry@warwick.ac.uk; P.J.Sadler@warwick.ac.uk

Received 11th February 2013, Accepted 3rd April 2013

First published on the web 1st May 2013

Chem. Commun., 2013, 49, 5106--5131

26 pages

ABSTRACT AND FULL LENGTH OF ARTICLE: http://pubs.rsc.org/en/content/articlehtml/2013/cc/c3cc41143e

This article is a review of current research and interests in metallodrugs and their interactions with (proposed) target sites.  While DNA was commonly used as a target site for anticancer drugs, for instance, this has changed due to the challenge of isolating cytotoxicity to cancerous cells and sparing normal cells.  Besides learning about specific developments in metallodrugs, this article also gives a good synopsis of various targets, mode of delivery, activation, and mechanism of action for metallodrugs currently used as anticancer, antimicrobial, antivirus, antidiabetic, and anti-inflammatory agents and diagnostic and therapeutic radiopharmaceuticals. More than the specific findings, it is probably this overall review that has proven useful for me in reading this article.

In the introduction section, the author presents a medical periodic table showing the 24 essential metals, radioisotopes, and elements currently used in therapy and diagnosis.  It also provides Table 1 listing metallodrugs or chelating agents in development, clinical trial, or approved for clinical use.

The authors note that all essential metals should be investigated for therapeutic functions but there are some whose biochemistry is still unknown. They also propose a strong focus on characterizing target sites and the molecular mechanism.  Research in metallodrugs is also involved in investigating a range of target sites as described in this paper.

1. METAL-BASED ANTICANCER DRUGS

DNA TARGETING

Direct Coordinative DNA Binding

PLATINUM: See Chart 1 for a list of platinum-based anticancer drugs that are approved or undergoing trials.  The target site for cis-platin is nuclear DNA.  DNA adduct formation is important for the mechanism of these metallodrugs.

There has been interest in developing similar platinum-based compounds that are structurally different from cis-platin in the hope that they would form different DNA adducts with different biological activity.  Polyplatinum complexes have been developed with a highly positive charge leading to a stronger electrostatic recognition of DNA; they are also thought to be more flexible.  These drugs passed Phase I trials but not Phase II trials.

Low spin octahedral PtIV complexes have the advantage over PtII square complexes because they are less prone to substitution reaction and therefore more likely to reach the tumor site unchanged.  They also are more water-soluble.

In general, the new PtIV complexes being developed are unpredictable in terms of their rate and extent of localization in vivo compared to active PtII species.

RUTHENIUM: Because of the toxicity of platinum-based drugs, other active metals are being considered.  One example is ruthenium.  New drugs being developed use RuIII complexes but RuII organic arene complexes also show promise. One of these RuIII anticancer drugs called NAMI-A has antimetastatic activity that is thought to be due to the combined effects of on the control of angiogenesis and anti-invasive properties towards tumor cells and blood vessels and not due to interactions with DNA.  Another RuIII complex drug is known to target DNA and cause apoptosis.  RuIII is thought to be activated by reduction to RuII in vivo. The paper describes current thinking on the biochemical basis for the anticancer activity of RuII arene complexes.

OSMIUM: A heavy congener of ruthenium, osmium is considered relatively inert compared to ruthenium because of its slower kinetics.  The paper describes the potential of altering the biochemical activity of osmium based on its aqueous properties, e.g. the rate of hydrolysis by using the right chelating ligand and can be made active in vivo.  The paper notes that injected OsO4 has been used to treat chronic synovitis.

IRIDIUM: IrIII anticancer complexes bind to DNA but are also known to react with NADH producing hydrogen catalytically.  This finding presents a new potential target site to explore.

Non-covalent DNA Binders

-Instead of forming covalently bonded adducts, metals can intercalate between DNA base pairs to induce mutagenesis due to structural and functional changes in duplex DNA.  These changes can lead to inhibition of transcription, replication, and repair.  These intercalators have potentials as antibiotics, antibacterials, antitumor agents, etc.  Table 3 in the article gives a list of metallo-intercalator complexes (zinc, gold, silver, copper, lead, platinum, nickel, iridium, rhenium, cobalt, and ruthenium).

Coordination plus Intercalation: Dual Mode DNA Binders

This dual capability is present in metal complexes with sigma bonded aromatic side arms or organometallic complexes with pi-bonded arenes. These complexes are capable of binding to DNA through direct metal coordination to a DNA base and an attached aromatic ligand intercalating between bases. The paper gives examples Pt complexes with planar aromatic ligands that can exhibit this dual function (ethidium bromide is one). A biphenyl RuII complex given as an example binds strongly to G bases in DNA but also its phenyl substituent intercalates.  Examples of RhII dual function complexes are also given. In the case of some IrIII complexes, the ability to intercalate into DNA increases their anticancer potency.  Combining two metals in one complex also appears to facilitate both coordinative binding and intercalation ability.

G-Quadruplex Binders

This binding action occurs in the G-rich areas of telomeres in human genes that contain many repeats of the sequence d(GGTTAG). A Ni-Cu complex was prepared in the presence of telomeric human DNA and was observed to effectively bind to telomeric DNA by end-stacking.  Targeting telomerase is an effective anticancer activity.  Table 4 in the article provides a list of complexes with this function that have been investigated.

SELECTIVE DNA BINDING

Metallodrugs with sequence specificity

The selectivity of the binding to DNA can be enhanced by ligands that have sequence specificity. A PtII complex with linear and hairpin polyamide ligands has been shown to recognize 7-base pair sequences.  Specific selectivity can also help reduce side effects and resistance to platinum-based drugs.  A nucleotide can also be incorporated into the complex that can complementary bind to a DNA strand to form a triple helices.

Protein mediated DNA recognition

Another method for enhancing DNA recognition is the formation of a ternary complex that has a presenter protein and a small molecule that can bind to both protein and target.  The antibiotic rapamycin has such a structure.

PROTEIN TARGETING

Examples of protein targets include kinases and bacterial Zn enzymes.  Investigations of protein targets must include correlations of activity with protein binding.  In this section, the authors describe recent work investigating interactions of potential protein targets with metallodrugs.

(Targeting) Hormone Receptors

Prostate cancer is initially androgen-dependent and may be treated using hormone therapy which aims to “block androgen receptors by binding antagonists (antiandrogens) to these receptors instead of the endogenous testosterone”.  Prostate cancer is currently clinically treated using hormone therapy with the following drugs: flutamide, nilutamide, bicalutimide (see structures in Chart 6 of paper).  Hormone therapy is prone to resistance acquired by cancer cells (possibly due to androgen receptor mutations, according to the paper).

Some metallodrugs in development or in use that target proteins are described below:

A ferrocenyl organometallic complex with steroidal androgens testosterone and dihydrotesterone have been shown to exhibit a strong anti-proliferative effect on hormone-independent prostate cancer cells.

In ferrocenyl tamoxifen derivatives, a phenyl ring has been replaced by a ferrocenyl derivative (see structure in paper). Tamoxifen is a commonly prescribed drug for hormone-dependent breast cancers.  The active form is hydroxytamoxifen which is a competitive inhibitor to estrogen (it competitively binds to estrogen receptors).  It has critical limitations: it is ineffective against a third of hormone dependent breast cancers, it is prone to developing resistance in a third of these cancers, and increases endometrial and uterine cancer risks.  “The replacement of the phenyl group by ferrocene reduces receptor affinity by about 40%, whilst the increase in length of the dimethylaminoalkyl chain has an adverse effect on receptor binding.”  A ruthenocenyl tamoxifen analogues behave as antiestrogen [an antiestrogen is an agent the blocks the production or utilization of estrogen or inhibits its effects].  “Therefore, the redox activity of the ferrocenyl group is of central importance, along with the antiestrogenic properties of the hydroxytamoxifen derivative, for providing a unique dual mechanism of action of these ferrocenyl tamoxifen derivatives.“  Titanocenyl tamoxifen derivatives have also been shown to have anticancer activities but their effectiveness and degree of toxicity are not well-resolved issues at this point.

A carborane derivative of tamoxifen was also discussed in the paper.

Mitochondrial Protein Targeting

Mitochondria in the cells regulate energy production, modulate redox potentials, and generate reactive oxygen species (ROS) (radical oxygen and hydrogen peroxide). In cancerous cells, the mitochondrial membrane becomes hyperpolarized and ROS is overproduced.  These defective functions make mitochondria a prime target for anticancer drugs.

Arsenic compounds have been shown to be effective in increasing the production of ROS and inducing apoptopic signaling pathways because they can strongly bind to thiol groups in mitochondrial proteins. Many studies have been done on potential of arsenic metallodrugs for cancer therapy.  The paper points out that arsenic trioxide has been used for therapy for 2000 years.  Arsenic-based salvarsan is used to treat syphilis while arsenic trioxide is the most effective single agent treatment for a form of acute leukemia.  GSAO (see full name in the article) is an arsenic-based compound that has been shown to inhibit adenine nucleotide translocase in the mitochondria. The GSAO needs to be metabolized and converted to the biologically active form; see detailed mechanism in the article.  In the last paragraph or two, the authors note that in general any lipophilic complexes are taken up by mitochondria.  A recent combinatorial parallel coordination chemistry study on 500 ruthenium-based monocationic polypyridyl complexes has revealed anticancer activity of a lead-ruthenium complex against lymphoma cells, strongly reducing the “mitochondrial membrane potential, suggesting the involvement of the intrinsic pathway of programmed cell death”. These drugs have been prepared and tested as an enantiomeric or diastereomeric mixture.

(Targeting) Kinases, TNF-a and thioredoxin

Kinase targets are a complex study because of the more than 500 different proteins belonging to this family of enzymes.  Studies have found that inert octahedral metal complexes “are emerging as promising scaffolds for targeting kinase active sites, thanks to their rigid globular shapes, and high synthetic versatility”.  One study has shown that the most promising metals due to their high potency at the micromolar down to the nanomolar range and selectivity as kinase inhibitors are RuII, OsII, RhIII, and IrIII.  Of note, by the authors, “An interesting strategy is the combination of kinase inhibition and photoactivity demonstrated for an angiogenic octahedral organoiridium complex. This can undergo substitution of a selenocyanate ligand on irradiation with visible light and induces apoptosis in cancer cells.”

TNF-a is a protein target of interest because aberrant activity of this factor has been implicated in diabetes, tumorigenesis, and autoinflammatory diseases.  An iridium(III) inert octahedral metal complex has shown potential as TNF-a inhibitor.

Thioredoxin is another target of interest resulting in the development of Motefacin gadolinium that has shown inhibitory activity against thioredoxin reductase and ribonucleotide reductase for the treatment of lung cancers with brain metastases.  Gold complexes are also being investigated that target thioredoxin reductase.  See other metal examples in Table 5 in the paper.

METALLODRUG DELIVERY AND ACTIVATION

Light Activation of PtIV Prodrugs

In light activation of PtIV prodrugs, a specific wavelength of light is used to excite the species of interest to the excited singlet or triplet states with an electron distribution different from the ground state resulting in different geometries and reactivities and, perhaps, different modes of action.  The wavelength of light required is an important consideration because longer wavelengths (red) penetrate tissues more deeply than shorter wavelengths (blue).  Light activation has the advantage of controlling when a particular agent is activated thereby increasing the efficiency, avoiding unnecessary damage, and delivering the active drug to the tumor itself. For example, a particular type of PtIV complex drug (see paper for exact name) reacts more rapidly with DNA bases such as guanine and becomes more cytotoxic after short treatment and short irradiation times.

Strained complexes like the octahedral tris-bipyridyl ruthenium(II) complex is inert until activated by visible light: “Under irradiation, a light-activated ligand release mechanism occurs, leading to DNA binding, and to an increase in cytotoxicity of 2 orders of magnitude in cancer cells (with potencies superior to cisplatin against 3D tumour spheroids).”  The authors conclude that, “The use of intramolecular strain is a promising strategy for developing light-activated Ru complexes for PDT applications.”

Photorelease of biologically-active small molecules

NO: Photoactivation is a way to control the release and scavenging of NO by metal complexes.  An example is a Ru complex that can release NO when irradiated with visible light.  NO is an important signaling molecule that have varied functions in the cardiovascular, nervous, and immune systems. NO interacts with metal complexes like heme and non-heme Fe as an integral part of its functions. 

CO is another signaling molecule whose scavenging and release by metal complexes can be controlled by photoactivation.  CO may act as a messenger, has anti-inflammatory properties, and can suppress organ graft rejection. The ability to control the release of CO provides a way to deliver this toxic molecule safely to where it is needed.  An example of a metal complex involved in CO delivery is a Mn-based complex:  “the CO-releasing compound [Mn(CO)3(tpm)]PF6 (tpm = tris-(pyrazolyl)methane) have revealed a significant photoinduced cytotoxicity, comparable to that of the established anticancer agent 5-fluorouracil.”

Anticancer platinum drug delivery

Several delivery methods are being investigated and developed that take advantage of the more flexible and permeable cell membranes of cancer cells and the dysfunction of the lymphatic drainage system leading which retains large molecules and lipids.  Some of the carrier and delivery systems being investigated are: “functionalised carbon nanotubes, nanorods, metal–organic frameworks, metalla-cages, nanoparticles, liposomes, nanogels, proteins, and polymers”.  Some examples of this include:

A PtIV complex tethered to gold nanoparticles via amide linkages showing a 12-fold increase in activity compared to free cisplatin towards a form of lung cancer cells

PtIV prodrug encapsulated in prostate-specific membrane nanoparticles to deliver a lethal dose of cisplatin

In both of these cases, the delivered PtIV is reduced to PtII which can form intrastrand cross-linked bonds to nuclear DNA.

Square planar acetylacetonato PdII and PtII complexes encapsulated in ruthenium metallacages (see Chart 9 in paper) which exhibits an order of magnitude higher activity than empty metallacages.

2. ANTI-VIRAL, ANTI-MICROBIAL, AND ANTI-DIABETIC METALLODRUGS

ANTI-VIRAL METALLODRUGS

Hepatitis

One promising copper- and nickel-based complex metallodrugs has been shown to catalytically and irreversibly inhibit the replication of the Hep C virus.  The method aims to target the critical RNA sequences present in Hep C virus but rare in the genome of infected cells.  The turnover rate for efficacy is low but the approach is promising.

Human immunodeficiency virus

The integration of viral DNA into the host cell DNA is facilitated by the enzyme integrase.  This enzyme has MgII metal cofactors and is a prime target candidate.  Raltegavir is a drug designed to chelate to the MgII ions and inactivate the enzyme (see Figure 8).  At this point, however, the paper points out that the drug is limited by an “overall dose burden and a lack of potency” despite its clinical success.  More work is being done to optimize the activity of this drug.  Carbamoyl pyridine scaffolds have also been shown to chelate the two MgII ions in the active site with nanomolar affinity.

Another promising HIV target is one of its membrane protein used to infect a type of T cells.  A bicyclam metallodrug complexed with either a CuII, ZnII, or NiII ion has shown 7, 36, and 50 times more affinity, respectively for the membrane protein receptor.  Studies mentioned in the paper “illustrate that direct metal coordination, H-bonding and hydrophobic interactions with the ligands can all play important roles in metallodrug-protein target recognition.”

ANTIMICROBIAL AGENTS

Tuberculosis

Compared to other metallodrugs uses, there have been fewer studies on their use as antituberculosis drugs even though the last drug development against this disease is 40 years old.  One metallodrugs of interest from recent studies however is the redox-activated isoniazid FeII complex.  Isoniazid has been used as an antituberculosis agent but has proven to be limited by development of resistant strains.  This resistance is thought to be due to deletions or mutations in the catalase-peroxidase enzyme (KatG) that catalyzes the required electron transfer reaction for activating isoniazid.  A study has shown that coordination of an FeII metal (e.g. cyanoferrates) can induce this electron transfer reaction intramolecularly obviating the dependence on the KatG enzyme. See Figure 10 for a possible mechanism.  “This metal complex inhibits both wild-type InhA and its isoniazid-resistant mutant InhA I21V, even in the absence of KatG and NADH, bypassing the enzymatic activation.” InhA is the target enzyme of mycobacterium tuberculosis agents.  The authors note also that a ruthenium analogue showed no inhibitory effect on InhA.

Other microbial agents

Compounds containing silver, bismuth, mercury, arsenic, and antimony are well-established as antimicrobial agents.  Salvarsan (a syphilis drug), for example, has been shown to be activated by in vivo oxidation of the AsI to AsIII.  In addition to this, the authors note the following summary list of metallodrugs that have been investigated as antimicrobial agents:  “a wide range of inorganic compounds from organometallic complexes to metal organic frameworks, nanoparticles or metallic surfaces have also been recently investigated for killing or inhibiting microbial growth, including mercury, silver, gold wires and nanoparticles, copper, cobalt, platinum, palladium, ruthenium, iron (chelation therapy), manganese complexes, titanium dioxide nanoparticles, noble metal nanoparticles, and nanostructures as antibacterial drug delivery systems.”.

ANTIDIABETIC METALLODRUGS

This section begins with a brief statement of diabetes and its importance in drug-development: “Diabetes mellitus (DM) is a group of chronic metabolic diseases characterised by persistent hyperglycaemia associated with absolute or relative deficiency in insulin secretion from the beta cells of pancreas. The dysfunction of insulin receptors might also be associated with diabetes. The current (285 million patients worldwide) and dramatically-increasing incidence of this disease is stimulating the search for new anti-diabetic agents”.  Zinc and vanadium complexes have been an important focus of metallodrug studies targeting diabetes. 

Metals of interest for antidiabetic metallodrugs include zinc, vanadium, and chromium.  Zinc is involved in the structure, physiology, and function of insulin; vanadium also plays a role.  A vanadium complex metallodrug has recently shown efficacy in reducing hyperglycemia in diabetic rats.  See chart 11 for structures.  Table 6 in the paper shows a list of chromium-based antidiabetic drugs although the paper states that “there is now no good evidence” that chromium is an essential metal. 

In addition, metal complexes such as vanadate, molybdate, and tungstate complexes can compete with phosphate substrates (e.g. glucose-1-phosphate) for binding to phosphatases and are potent inhibitors of muscle glycogen phosphorylases.  It has been shown that WO₄^2- in Na₂WO₄ has been able to restore hepatic glucose metabolism and “mimics the metabolic effects of insulin and stimulate insulin output”. Na₂MoO₄ has shown similar treatment efficacy in the early stages of diabetes mellitus.  Neither of these drugs have received clinical approval yet.

ANTI-PARASITIC, ANTI-INFLAMMATORY AND, ANTI-NEURODEGENERATIVE METALLODRUGS

ANTI-PARASITIC METALLODRUGS

Malaria

The common treatment for malaria today consists of chloroquine and antifolate drugs or other artemisinin derivatives.  The lack of a vaccine and emergence of resistant plasmodium strains have prompted interest in developing other drugs including metallodrugs.

Plasmodium falciparum is the parasite species that causes the most lethal form of malaria. Auranofin, a gold-based complex, is being investigated for its inhibitory effect on plasmodium falciparum growth.  Molecular docking experiments have shown that the auranofin can bind to the N of the histidine on the active site of thioredoxin reductase in plasmodium falciparum. 

Ferroquine (see chart 12), a chloroquine coordinated to a metal, has shown advantages over just chloroquine: “the weaker base properties and higher lipophilicity at physiological pH of ferroquine compared to chloroquine, as well as intramolecular H-bonding with the lateral side chain of ferroquine (in non polar conditions) leads to the improved ability of ferroquine to cross membranes and a higher accumulation in the digestive vacuole.”  Chloroquine is thought to disrupt the digestion of hemoglobin in the blood stages of the malaria life cycle.  Ferrocene is also thought to undergo oxidation that can lead to the production of hydroxyl radicals damaging to the DNA and the cell membrane.  Along with ferroquine, ruthenoquine has also shown activity against drug-susceptible and drug-resistant forms of plasmodium falciparum.  An interesting side note is the finding that intramolecular H-bonding in some ruthenocenic derivatives of chloroquine has improved the permeability and delivery of the drug to the target.

Amoebiasis

“Amoebiasis is an infection of the intestine caused by the parasite Entamoeba histolytica, responsible for about 70 000 deaths a year.” Metallodrug complexes of AuI, RuII, and CuII of the current medication for amoebiasis, metronidazole, have been shown to have higher activity than the uncomplexed form.  Auranofin, mentioned above as a possible antimalarial, has also shown ten times higher potency than metronidazole.

ANTI-INFLAMMATORY METALLODRUGS

Arthritis

The authors give a very brief history of some of the gold compounds that have been used to treat certain conditions (chrysotherapy).  Injectable AuI compounds have been used to treat rheumatoid arthritis as early as 1929; the auranofin AuI triethylphosphine was approved in 1985 as an oral antiarthritic agent although the mechanism is still not well-understood.  Auranofins have also been shown to be cytotoxic to cancer cells.

Ulcers

Bismuth has been found to be an active inhibitor of the bacterium that prevents the healing of ulcers, heliobacter pylori.  Bismuth has been used against dyspepsia, syphilis, colitis, wound infections, and quartan malaria.  Possible mechanism: “Time-resolved ICP-MS studies of the uptake of bismuth-based

drugs suggest a competitive FeIII/BiIII transport pathway into H. pylori”.

Neurodegenerative diseases

Neurodegenerative diseases are characterized by the “progressive loss of structure or function of neurons”.  Metals biochemistry has been shown to be involved in the pathogenesis of Alzheimer’s disease: Zn is released in millimolar amounts during neuronal activation while Cu is involved in the regulation of synaptic functions. A recent paper reported “the post-hoc analysis of a Phase IIa double-blind, randomised, placebo-controlled clinical trial for a copper/zinc ionophore, PBT2,397 an hydroxyquinoline derivative that facilitates the clearance of Ab aggregates in the cortex by targeting the zinc and copper ions that mediate the assembly of these aggregates in amyloid and diffuse deposits, effectively detoxifying the Ab. The output of this study shows clear improvement for treated patients compared to placebo group.” Ab is amyloid-beta peptide.  A promising approach for treatment is inhibiting the interaction between metals and amyloid-beta peptide by using a competing metal. Chelation is also a potential mode of inhibition by extraction and excretion of the required metals.

DIAGNOSTIC AND THERAPEUTIC RADIOPHARMACEUTICALS

The very small doses required reducing the risk to a negligible toxicity hazard of radiopharmaceuticals have led to a more rapid transition from lab to clinic in the development of these drugs.  Most of the therapeutic nuclides that deliver cytotoxic doses of ionizing radiation are beta emitters (se article for a list) although some alpha emitters are used as well. An alpha-emitting radioactive actinium and bismuth (emission type not mentioned in the article) have been the focus of recent clinical trials (see article).

SPECT (single photon emission computed tomography is a type of radioimaging that makes use of gamma-emitting radionuclides of Tc, Ga, In, and Tl.  PET (positron emission tomography) radioimaging uses beta-emitting radionuclides of Co, Cu, Ga, Rb, and Y.  See article for the specific radioisotopes.   A particular focus of new imaging methods involved multimodal imaging like the PET/MR (MR is magnetic resonance) which uses the Ga-DOTE-TATE molecule shown in Chart 13 of the paper.

CONCLUSION

The conclusion section reiterates the primary motivation of this review in terms of the importance of metallodrugs as exhibiting drug action mechanisms distinct from organic molecules.  In principle, the 13 essential metals in mammals can all be exploited in terms of their therapeutic (or diagnostic) efficacies.  In addition, however, non-essential metals also play a role as has been demonstrated through competitive inhibition treatment approaches, for example.   Current approval process emphasizes the need for understanding not just the efficacy but the targets and mechanisms of action as well.  This is especially important in developing individual treatments for certain diseases and conditions. Equally important is an understanding of control of delivery and activation to prevent unwanted side effects. The authors point out, however, than only very few of the metallodrug agents discussed in this paper and in literature elsewhere have been validated in vivo.