Mitochondria as a target of third row transition metal-based anticancer complexes

Abstract

In pursuit of better treatment options for malignant tumors, metal-based complexes continue to show promise as attractive chemotherapeutics due to tunability, novel mechanisms, and potency exemplified by platinum agents. The metabolic character of tumors renders the mitochondria and other metabolism pathways fruitful targets for medicinal inorganic chemistry. Cumulative understanding of the role of mitochondria in tumorigenesis has ignited research in mitochondrial targeting metal-based complexes to overcome resistance and inhibit tumor growth with high potency and selectivity. Here, we discuss recent progress made in third row transition metal-based mitochondrial targeting agents with the goal of stimulating an active field of research toward new clinical anticancer agents and the elucidation of novel mechanisms of action.

1.0. Introduction

Mitochondria are organelles prominent for their major function, which is ATP production[1]. However, the mitochondria also play a major role in many important cellular processes including anabolic (synthesis of biomolecules) and catabolic processes, regulated cell death processes, cell signaling, and redox homeostasis[1,2]. For many years, following Otto Warburg’s discovery of aerobic glycolysis in cancer cells[3], it was widely accepted that the mitochondria are defective in cancer cells despite Weinhouse’s seminal report on active mitochondria in tumors[4]. Increasing extensive investigation of mitochondria processes in cancer over the past decades have highlighted the crucial role of functional mitochondria in cancer cell growth and survival. Thus, creating impetus to develop strategies aimed at targeting the mitochondria for effective cancer treatment[5–8]. The field of medicinal inorganic chemistry has seen significant advancement since the discovery of the anticancer activity of the platinum-based metal complex, cisplatin[9–11]. The success of cisplatin and its analogs in the clinic has been hampered by adverse effects in patients[12,13], acquired tumor resistance, and recurrence in many aggressive tumors including lung, bladder, ovarian, and breast tumors. Therefore, efficacious metal-based complexes with high potency and enhanced selectivity for tumor cells over normal cells remain an unmet need. Metabolic heterogeneity and prominent dependence of some aggressive cancers such as PDAC, and glioblastoma on mitochondrial metabolism present liabilities that can be exploited by mitochondrial targeting agents. The redox active character of transition metals provides opportunities for optimization to target the inherently active mitochondria organelle. Additionally, mitochondria-targeted metallodrugs generally exhibit a differentiated cytotoxic mechanism of action from the FDA approved platinum drugs, with implications to circumvent the problem of tumor resistance associated with platinum drugs. To this end, several novel platinum and non-platinum complexes such as gold[14,15], ruthenium[16–18], rhenium[19], iridium[20–22] osmium[23] complexes with mitochondria targeting ability have been developed over the years. In this review, we provide an up-to-date development of metallo-agents targeting the mitochondria with translational potential and with primary focus on third-row transition metals (Figure 1). We regret to have omitted closely related work in the field due to limited word constraints for this review.

Figure 1.

Summary of mechanism-of-action of selected metal-based complexes in this review.

2.0. Mitochondria-targeted platinum complexes

The impact of the FDA approved platinum drugs for cancer treatment has been revolutionary. To overcome limiting side effects such as drug resistance and debilitating side effects, different structural scaffolds that include monofunctional Pt(II)[24,25] and dual-threat Pt(IV) have been developed[26,27]. Whereas pyriplatin and phenanthriplatin bind DNA nucleobases in a monodentate fashion, chemical modification of pyriplatin to incorporate delocalized positive charges in triphenylphosphonium (Ph₃P⁺, TPP) moieties on the pyridine coordinating ligand (OPT, MPT, PPT, TTP, Mon-Pt) leads to inhibition of mitochondrial bioenergetics, mtDNA, and the perturbation of glucose metabolism[28] (Figure 2). Similarly, as with mitaplatin[29], and Platin-M[30], the axial functionalization of cisplatin with TPP, dichloroacetates, luminogens, (PMT[31], DPB[26], and ACPt[32]), etcetera to form Pt(IV) prodrugs facilitates mitochondria delivery and subsequent modulation of oxidative phosphorylation (OXPHOS) and mtDNA inhibition towards cell death[26,32]. The ability of these agents to inhibit Pt-resistant cells and tumor growth in vivo demonstrates their clinical relevance.

Figure 2.

Chemical structures of mitochondria targeting platinum complexes.

3.0. Mitochondria-targeted gold complexes

Gold-based therapeutic agents have emerged among the leading class of metal-based drugs that are being developed as potential alternatives to platinum-based drugs[14,33–35]. This is energized by the several repurposing and current phase II clinical trials of the FDA approved gold drug, auranofin in ovarian and non-small cell lung cancer. Different structural variations of gold-based complexes exist, including cyclometalated gold complexes, gold thiolates, gold porphyrins, gold NHCs, and gold-phosphine complexes[14]. Although their molecular mechanism of action is a subject of intense investigation, the mitochondrion is implicated as a potential target responsible for the observed cytotoxic effect of some of these complexes[36–40]. Conceivably, cationic gold complexes with lipophilic character are susceptible to mitochondrial localization due to the negative mitochondrial membrane potential in cancer cells that drive selectivity. Recent target identification strategies such as chemical proteomics, thermal proteome profiling, and click chemistry have accelerated the identification of novel targets including mitochondrial proteins in metallodrug discovery. Early work by Berners-Price uncovered mitochondria-targeting gold chemotherapeutics[15,41,42]. Che and colleagues reported the development of gold(III) complexes that interact with mitochondrial proteins. Notably, a mesoporphyrin complex (AuMesoIX) exerts its anticancer activity via a unique mechanism that involves gold(III) ion activation of the porphyrin ring towards adduct formation with cysteine thiols found in proteins important for cancer survival including mitochondrial peroxiredoxin (PRDX3) and HSP90, thioredoxin (TrxR) and protein deubiquitinases (DUBs) (Figure 3a)[43]. AuMesoIX inhibits cancer cell growth in the nanomolar and submicromolar range, distorts mitochondria morphology, and inhibits tumor growth in ovarian and lung xenograft mouse models.

Figure 3.

Chemical structures of mitochondria targeting gold complexes.

Cyclometalated gold complexes have become an attractive structural class for anticancer action due to the strong sigma donation that impart enhanced stability [36,40,44–46]. In addition to electrochemical stability often derived from cyclometalation, the strong Au-C covalent bond character promotes solution stability even under physiological conditions to overcome premature deactivation[14]. Recently, the development of cyclometalated Au(III) complexes bearing metformin or phenformin biguanides (Figure 3c) [40,47] demonstrates significant anticancer potential in vitro and in vivo by altering mitochondrial bioenergetics. Specifically, Auraformin significantly accumulates in the mitochondria and efficiently impair mitochondria respiration in triple negative breast cancer (TNBC) MDA-MB-468 cells as well as depolarize the mitochondria membrane in the cells and 3met severely disrupts mitochondria metabolism in MDA-MB-231 cells leading to ER stress and autophagy. 3met demonstrates in vivo efficacy in a MDA-MB-231 orthotopic xenograft mouse model.

Analogs of a unique cyclometalated gold(III) complex bearing bisphosphine ligands known as the AuPhos class of compounds interfere with the mitochondrial electron transport chain (ETC) and mitochondria biogenesis [36,37]. Lead complexes, AuPhos-89 and AuPhos-19 (Figure 3b and 3d) display potent inhibitory effects in a panel of cancer cell lines including the recalcitrant TNBC cell lines and inhibits tumor growth in syngeneic and xenograft models of TNBC in mice. The novel mechanism of AuPhos-89 to promote mitochondria biogenesis has facilitated its use in the treatment of the inflammatory disease, ulcerative colitis[48–50].

The Awuah lab developed new tricoordinate gold(I) complexes (AuTri-9) (Figure 3f) capable of disrupting mitochondrial structure. In addition to transmission emission microscopy images and quantification of cristae length, immunoblotting showed depletion of key mitochondrial fusion and fission proteins such as OPA1, MFN, MFF and TOM20 which are involved in the maintenance of mitochondrial structure and dynamics[51]. Taken together, ligand tuning at the gold center and oxidation state contribute to modulating distinct pathways of the mitochondria for therapeutic gain. We posit that this exciting area of research will generate tool compounds to study mitochondrial metabolism towards uncovering elusive targets and develop therapeutic agents to treat metabolic and non-metabolic diseases.

4.0. Mitochondria-targeted iridium complexes

Third row iridium-based complexes possess peculiar triplet excited state characteristics that make them useful for phosphorescence applications including bioimaging and photosensitization, especially in photodynamic therapy (PDT). Organelle selective iridium complexes that interact with the ER, lysosome, and the mitochondria hold tremendous promise as anticancer agents than conventional treatments [52–57]. The development of cell-permeable cyclometalated Ir(III) complexes as photosensitizers for PDT is attractive due to their efficient singlet oxygen (¹O₂) production required for tumor damage and luminescent character for theranostics (Figure 4a and 4c) [58,59]. Furthermore, the potential for structural optimization via rational ligand design and tuning provides opportunities to expand the diversity of iridium-based complexes to achieve more photostable and better phototoxic complexes. Recent work by Zhuezhao et. al. described the development of new dinuclear Ir^III-based metallohelices that target mitochondria DNA with enhanced PDT tunability in cancer cells[58]. The synthesis of these Ir^III-based triple stranded metallohelices was achieved by reacting the aldehyde functionalized fac-Ir(ppy)₃ (ppy=2-phenylpyridine) and linear alkanediamine spacers via imine-coupling to afford stereochemically distinct helicates (H4b and H2b) or mesocates (M5b and M3b) (Figure 4a). These structures localize predominantly in the mitochondria, interact with mitochondria DNA, and generate cytotoxic ¹O₂ upon irradiation. This work highlights the potential for mitochondria-DNA targeted Ir^III-based metallohelices for PDT.

Figure 4:

Chemical structures of mitochondria targeting iridium complexes. (a) Ir(III)-based metallohelices. Reproduced with permission from ref.[58]. Copyright 2020 John Wiley and Sons. (b) Analogs of Ir complexes targeting mitochondrial topoisomerase. (c) Iridium (III) based complex for photodynamic therapy/photoactivated therapy. Reproduced with permission from ref.[59]. Copyright 2022 American Chemical Society. (d) Chemical structures of mitochondria targeting rhenium complexes.

Phototoxic octahedral Ir(III) complexes undergo Type I photochemical mechanism under hypoxic conditions to induce cancer cell death in 2D culture and 3D spheroids[60]. A derivative of the cationic Ir(III) thienyl benzimidazole of the formula [Ir(C^N)₂(dppz)][PF₆], where C^N = 1-methyl-2-(2′-thienyl)benzimidazole, significantly inhibits the growth of the aggressive rhabdomyosarcoma (RD) cells, a rare form of pediatric cancer and cancer stem cells[61]. The complex accumulates in the mitochondria and induces depolarization of the mitochondria membrane potential (MMP), which may be a premise for the selective cancer stem cell targeting reported[62]. Despite the promising effect of the complex in bulk and CSCs, more work is required to delineate the mechanism of action, which implicates the mitochondria.

Beyond PDT, Liting et. al. developed two iridium complexes (Ir1 and Ir2) that target mitochondria topoisomerase (Figure 4b) [63]. The concentration dependent inhibitory effect of the Ir(III) complex on mitochondrial topoisomerase I (TOP1mt) is comparative to the clinically used anticancer drug, camptothecin (CPT). The synthetic design of these complexes leverages the topoisomerase I inhibiting ability of indenoisoquinoline moieties and the mitochondria-targeting Ir(III) core to develop novel mitochondrial topoisomerase I targeting Ir(III) complexes. These complexes demonstrate high efficacy against cancer cells (40-fold > than cisplatin) with no cross-resistance in cisplatin resistant cells (A549R).

5.0. Mitochondria-targeted Rhenium complexes

The upward trajectory in the development of rhenium organometallics continues with recent reports of new potent anticancer rhenium complexes highlighting the progress made[64–68]. The design of mitochondria targeting rhenium(I) core to incorporate the FDA approved iron chelator, deferasirox affords the complex, DFX-Re3, which is capable of relocating intracellular iron into the mitochondria and disrupt mitochondria metabolism (Figure 4d)[67]. Acting at the interface between mitochondria process and epigenetic modification, DFX-Re3 transfer of iron downregulates Fe(II)/ α-ketoglutarate -dependent demethylases and causes a marked increase in DNA, RNA and histone methylation. DFX-Re3 modulates levels of metabolites such as α-ketoglutarate, succinate, and fumarate associated with epigenetic regulation. Moreover, DFX-Re3 inhibit TNBC tumor growth in 4T1 tumor-bearing mice at a dose of 5 mg/kg over a 14-day treatment period. Given that there was no observed toxicity on organs after the treatment duration, it ignites enthusiasm in overcoming deferasirox-associated nephrotoxicity and the advancement of this complex for TNBC chemotherapy[69].

Dinuclear rhenium(I) tricarbonyl complexes linked via a diazo (ReN) or disulfide (ReS) bridge targets the mitochondria and causes oxidative stress induced cell death in cancer cells (Figure 4d) [68]. The complexes display cytotoxic action against different cancer cell lines including the cisplatin resistant A549R. ReN and ReS inhibit mitochondria metabolism and decreased mtDNA copy number. An examination of the effect of these complexes on the gene expression profile showed downregulation of 13 mtDNA encoded genes after treatment with the complexes. The encouraging in vivo efficacy of these complexes with minimal toxicity highlights the excitement for rhenium organometallics. Other Re(I) complexes (Re1)[70] including those bearing artesunate ligands (Re-ART-1 and Re-ART-2) (Figure 4d)[71] have been shown to modulate mitochondria function. Opportunities remain for the development of mitochondrial targeting Re complexes.

6.0. Conclusion & Outlook

Mitochondria-inspired metallodrug discovery is a fruitful arena for the development of new medicines. Transition metal complexes possess inherent redox properties that resonate with the redox-active mitochondria organelle. We posit that tuning electrochemical behavior, lipophilicity, and cationic character of metal-complexes are crucial descriptors that influence mitochondria localization and modulation. The geometry, oxidation states, coordination sites, ligand effects, kinetic and thermodynamic characteristics of metal complexes provide an enormous framework for structural diversity that can target distinct mitochondrial targets and pathways. The unique spatial and functional complexes have the potential to unravel new mitochondrial-related molecular targets and mechanisms critical for the treatment of cancer and overcome chemotherapeutic drug resistance, especially platinum drug resistance. To accelerate mitochondrial targeting agents as selective chemical probes or clinically relevant drugs, complexes amenable to optimization and compound libraries for assay screening are needed. One major structural feature depending on the metal core will be to conduct a robust structure activity study that explores the degree of lipophilicity of cationic metal complexes and selective cancer mitochondria accumulation, given the highly negative mitochondria membrane potential of cancer cells. Importantly, relevant biological and disease models are crucial for evaluating compounds. These studies must incorporate selectivity, absorption, distribution, metabolism, excretion, and toxicity parameters in 2D and where possible 3D spheroids or organoids. Employing rigorous mechanism of action studies that factors target identification and validation strategies will be poignant in the development of mitochondria selective anticancer agents. This minireview highlights the potential for selective, metal-based mitochondrial targeting agents using representative examples to ignite excitement and further development.