Re-directing an alkylating agent to mitochondria alters drug target and cell death mechanism

Mitochondria-Targeted Temozolomide Probe for Overcoming MGMT-Mediated Resistance in Glioblastoma

Temozolomide (Tmz) is a DNA methylating agent used for the treatment of glioblastoma multiforme (GBM). Resistance to Tmz in GBM is caused by the DNA direct repair enzyme O6-methylguanine DNA methyltransferase (MGMT), which is expressed in ~50 % of GBM tumours. It has yet to be confirmed that MGMT acts within mitochondria to repair mitochondrial DNA (mtDNA), and in this report we discuss the development of a novel mitochondria-targeted temozolomide probe (mtTmz) for evading MGMT-mediated resistance. Through conjugation of Tmz to a mitochondria-penetrating peptide (MPP), exclusive mitochondrial localization was achieved, and the probe retained alkylation activity demonstrated by chemical and DNA-based assays. Absence of nuclear DNA damage was assessed by detecting γH2AX foci. mtTmz demonstrated efficient cell killing capabilities independent of MGMT status in GBM cells as determined by cell viability assays. It was determined using a Proteinase K digestion assay that MGMT does not translocate to mitochondria in response to mtTmz treatment, and RT-qPCR analysis demonstrated that mtTmz does not induce MGMT gene expression compared to Tmz. The results reported highlight both the potential of mitochondrial targeting of Tmz and mitochondria as a therapeutic target in MGMT-expressing GBM.

Nanomaterials in Medicine: Understanding Cellular Uptake, Localization, and Retention for Enhanced Disease Diagnosis and Therapy

Nanomaterials (NMs) have emerged as promising tools for disease diagnosis and therapy due to their unique physicochemical properties. To maximize the effectiveness and design of NMs-based medical applications, it is essential to comprehend the complex mechanisms of cellular uptake, subcellular localization, and cellular retention. This review illuminates the various pathways that NMs take to get from the extracellular environment to certain intracellular compartments by investigating the various mechanisms that underlie their interaction with cells. The cellular uptake of NMs involves complex interactions with cell membranes, encompassing endocytosis, phagocytosis, and other active transport mechanisms. Unique uptake patterns across cell types highlight the necessity for customized NMs designs. After internalization, NMs move through a variety of intracellular routes that affect where they are located subcellularly. Understanding these pathways is pivotal for enhancing the targeted delivery of therapeutic agents and imaging probes. Furthermore, the cellular retention of NMs plays a critical role in sustained therapeutic efficacy and long-term imaging capabilities. Factors influencing cellular retention include nanoparticle size, surface chemistry, and the cellular microenvironment. Strategies for prolonging cellular retention are discussed, including surface modifications and encapsulation techniques. In conclusion, a comprehensive understanding of the mechanisms governing cellular uptake, subcellular localization, and cellular retention of NMs is essential for advancing their application in disease diagnosis and therapy. This review provides insights into the intricate interplay between NMs and biological systems, offering a foundation for the rational design of next-generation nanomedicines.

Chimeric Small Molecules for Detouring Drugs into Mitochondria to Engender Apoptosis in Cancer Cells

Mitochondrion has appeared as one of the important targets for anti-cancer therapy. Subsequently, small molecule anti-cancer drugs are directed to the mitochondria for improved therapeutic efficacy. However, simultaneous imaging and impairing mitochondria by a single probe remained a major challenge. To address this, herein Chimeric Small Molecules (CSMs) encompassing drugs, fluorophore and mitochondria homing moiety were designed and synthesized through a concise strategy. Screening of the CSMs in a panel of cancer cell lines (HeLa, MCF7, A549, and HCT-116) revealed that one of the CSMs comprising Indomethacin V exhibited remarkable cervical cancer cell (HeLa) killing (IC50 =0.97 μM). This lead CSM homed into the mitochondria of HeLa cells within 1 h followed by mitochondrial damage and reactive oxygen species (ROS) generation. This novel Indomethacin V-based CSM-mediated mitochondrial damage induced programmed cell death (apoptosis). We anticipate these CSMs can be used as tools to understand the drug effects in organelle chemical biology in diseased states.

A new mitochondrial probe combining pyrene and a triphenylphosphonium salt for cellular oxygen and free radical detection via fluorescence lifetime measurements

To improve and diversify the quantification of reactive oxygen species (ROS) in mitochondria of single cells, we connected pyrene derivatives (PB) to a triphenylphosphonium salt (TPP⁺) as a mitochondrial vector forming PB-TPP⁺ probes. Two pyrene isomers with the n-butyltriphenylphosphonium moieties attached at their 1- or 2- positions were synthesized and characterized. Using the long fluorescence lifetime of pyrene, it was possible to monitor the variation of cellular free radicals and oxygen and to follow the reversibility of both quenchers in real-time. We compared the behavior of these new probes to the previously published pyrene-probes, functionalized by a mitochondrial-penetrating peptide, allowing their transfer to the mitochondria (Mito-PB) or to the cytosolic membrane for pyrene butyric acid (PBA). The high cellular uptake of the new probes allows cells to be loaded with an initial concentration 40 times lower than that for Mito-PB probes, without inducing perturbations in cell growth. The variation in free radicals and oxygen levels was monitored within cells under different stress conditions through the fluorescence lifetime of the new TPP⁺-based probes giving comparable results to those obtained for MPP-based probes. However, at a loading concentration as low as 25 nM, our technique allows the detection of increased production of free radicals in the mitochondria in the presence of the TPP⁺ vector, a warning to the user of this well-known vector.

Recent Advancements in Mitochondria-Targeted Nanoparticle Drug Delivery for Cancer Therapy

Mitochondria are critical subcellular organelles that produce most of the adenosine triphosphate (ATP) as the energy source for most eukaryotic cells. Moreover, recent findings show that mitochondria are not only the "powerhouse" inside cells, but also excellent targets for inducing cell death via apoptosis that is mitochondria-centered. For several decades, cancer nanotherapeutics have been designed to specifically target mitochondria with several targeting moieties, and cause mitochondrial dysfunction via photodynamic, photothermal, or/and chemo therapies. These strategies have been shown to augment the killing of cancer cells in a tumor while reducing damage to its surrounding healthy tissues. Furthermore, mitochondria-targeting nanotechnologies have been demonstrated to be highly efficacious compared to non-mitochondria-targeting platforms both in vitro and in vivo for cancer therapies. Moreover, mitochondria-targeting nanotechnologies have been intelligently designed and tailored to the hypoxic and slightly acidic tumor microenvironment for improved cancer therapies. Collectively, mitochondria-targeting may be a promising strategy for the engineering of nanoparticles for drug delivery to combat cancer.

Targeted elimination of mutated mitochondrial DNA by a multi-functional conjugate capable of sequence-specific adenine alkylation

Mutations in mitochondrial DNA (mtDNA) cause mitochondrial diseases, characterized by abnormal mitochondrial function. Although eliminating mutated mtDNA has potential to cure mitochondrial diseases, no chemical-based drugs in clinical trials are capable of selective modulation of mtDNA mutations. Here, we construct a class of compounds encompassing pyrrole-imidazole polyamides (PIPs), mitochondria-penetrating peptide, and chlorambucil, an adenine-specific DNA-alkylating reagent. The sequence-selective DNA binding of PIPs allows chlorambucil to alkylate mutant adenine more efficiently than other sites in mtDNA. In vitro DNA alkylation assay shows that our compound 8950A-Chb(Cl/OH) targeting a nonpathogenic point mutation in HeLa S3 cells (m.8950G>A) can specifically alkylate the mutant adenine. Furthermore, the compound reduces the mtDNA possessing the target mutation in cultured HeLa S3 cells. The programmability of PIPs to target different sequences could allow this class of compounds to be developed as designer drugs targeting pathogenic mutations associated with mitochondrial diseases in future studies.

Strategies to target bioactive molecules to subcellular compartments. Focus on natural compounds

Many potential pharmacological targets are present in multiple subcellular compartments and have different pathophysiological roles depending on location. In these cases, selective targeting of a drug to the relevant subcellular domain(s) may help to sharpen its impact by providing topological specificity, thus limiting side effects, and to concentrate the compound where needed, thus increasing its effectiveness. We review here the state of the art in precision subcellular delivery. The major approaches confer "homing" properties to the active principle via permanent or reversible (in pro-drug fashion) modifications, or through the use of special-design nanoparticles or liposomes to ferry a drug(s) cargo to its desired destination. An assortment of peptides, substituents with delocalized positive charges, custom-blended lipid mixtures, pH- or enzyme-sensitive groups provide the main tools of the trade. Mitochondria, lysosomes and the cell membrane may be mentioned as the fronts on which the most significant advances have been made. Most of the examples presented here have to do with targeting natural compounds - in particular polyphenols, known as pleiotropic agents - to one or the other subcellular compartment.

Mitochondria-targeting drug conjugates for cytotoxic, anti-oxidizing and sensing purposes: current strategies and future perspectives

Mitochondrial targeting is a promising approach for solving current issues in clinical application of chemotherapy and diagnosis of several disorders. Here, we discuss direct conjugation of mitochondrial-targeting moieties to anticancer drugs, antioxidants and sensor molecules. Among them, the most widely applied mitochondrial targeting moiety is triphenylphosphonium (TPP), which is a delocalized cationic lipid that readily accumulates and penetrates through the mitochondrial membrane due to the highly negative mitochondrial membrane potential. Other moieties, including short peptides, dequalinium, guanidine, rhodamine, and F16, are also known to be promising mitochondrial targeting agents. Direct conjugation of mitochondrial targeting moieties to anticancer drugs, antioxidants and sensors results in increased cytotoxicity, anti-oxidizing activity and sensing activity, respectively, compared with their non-targeting counterparts, especially in drug-resistant cells. Although many mitochondria-targeted anticancer drug conjugates have been investigated in vitro and in vivo, further clinical studies are still needed. On the other hand, several mitochondria-targeting antioxidants have been analyzed in clinical phases I, II and III trials, and one conjugate has been approved for treating eye disease in Russia. There are numerous ongoing studies of mitochondria-targeted sensors.

pH-Sensitive micelles with mitochondria-targeted and aggregation-induced emission characterization: synthesis, cytotoxicity and biological applications

Subcellular organelle-specific reagents for simultaneous tumor targeting, imaging, and treatment are of enormous interest in cancer therapy. Herein, we present a mitochondria targeting micelle (PEG-AIE-TPP) by conjugating a triphenylphosphonium (TPP) with a fluorogen which can undergo aggregation-induced emission (AIE). At first, the in vitro and in vivo properties of the PEG-AIE-TPP micelle were characterized in detail. It was found that the micelle was reasonably stable at physiological pH and highly sensitive to mildly acidic pH stimuli. Importantly, this micelle could selectively localize and accumulate in the mitochondria, thus generating an aggregation-induced emission (AIE) effect as confirmed by the green fluorescence. Additionally, the micelle exhibited selective cytotoxicity to cancer cells and negligible toxicity to normal cells in vitro. The in vivo imaging and ex vivo imaging results showed that the accumulation tendency of the micelle at the tumor region was obvious. We also further proved the biocompatible, tumor targeting ability and antitumor activity of the PEG-AIE-TPP micelle in MCF-7 tumor-bearing mice. Accordingly, this mitochondria-targeted therapeutic micelle with good stability, biocompatibility, and tumor-targeting and antitumor activity provides a potentially unique tumor-targeted system for cancer therapy.

Mitochondrial-Targeting Anticancer Agent Conjugates and Nanocarrier Systems for Cancer Treatment

The mitochondrion is an important intracellular organelle for drug targeting due to its key roles and functions in cellular proliferation and death. In the last few decades, several studies have revealed mitochondrial functions, attracting the focus of many researchers to work in this field over nuclear targeting. Mitochondrial targeting was initiated in 1995 with a triphenylphosphonium-thiobutyl conjugate as an antioxidant agent. The major driving force for mitochondrial targeting in cancer cells is the higher mitochondrial membrane potential compared with that of the cytosol, which allows some molecules to selectively target mitochondria. In this review, we discuss mitochondria-targeting ligand-conjugated anticancer agents and their in vitro and in vivo behaviors. In addition, we describe a mitochondria-targeting nanocarrier system for anticancer drug delivery. As previously reported, several agents have been known to have mitochondrial targeting potential; however, they are not sufficient for direct application for cancer therapy. Thus, many studies have focused on direct conjugation of targeting ligands to therapeutic agents to improve their efficacy. There are many variables for optimal mitochondria-targeted agent development, such as choosing a correct targeting ligand and linker. However, using the nanocarrier system could solve some issues related to solubility and selectivity. Thus, this review focuses on mitochondria-targeting drug conjugates and mitochondria-targeted nanocarrier systems for anticancer agent delivery.

Lipid-based DNA/siRNA transfection agents disrupt neuronal bioenergetics and mitophagy

A multitude of natural and artificial compounds have been recognized to modulate autophagy, providing direct or, through associated pathways, indirect entry points to activation and inhibition. While these pharmacological tools are extremely useful in the study of autophagy, their abundance also suggests the potential presence of unidentified autophagic modulators that may interfere with experimental designs if applied unknowingly. Here, we report unanticipated effects on autophagy and bioenergetics in neuronal progenitor cells (NPCs) incubated with the widely used lipid-based transfection reagent lipofectamine (LF), which induced mitochondria depolarization followed by disruption of electron transport. When NPCs were exposed to LF for 5 h followed by 24, 48, and 72 h in LF-free media, an immediate increase in mitochondrial ROS production and nitrotyrosine formation was observed. These events were accompanied by disrupted mitophagy (accumulation of dysfunctional and damaged mitochondria, and of LC3II and p62), in an mTOR- and AMPK-independent manner, and despite the increased mitochondrial PINK1 (PTEN-inducible kinase 1) localization. Evidence supported a role for a p53-mediated abrogation of parkin translocation and/or abrogation of membrane fusion between autophagosome and lysosomes. While most of the outcomes were LF-specific, only two were shared by OptiMEM exposure (with no serum and reduced glucose levels) albeit at lower extents. Taken together, our findings show that the use of transfection reagents requires critical evaluation with respect to consequences for overall cellular health, particularly in experiments designed to address autophagy-inducing effects and/or energy stress.

Mitochondria-Targeted Triphenylphosphonium-Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and Diagnostic Applications

Mitochondria are recognized as one of the most important targets for new drug design in cancer, cardiovascular, and neurological diseases. Currently, the most effective way to deliver drugs specifically to mitochondria is by covalent linking a lipophilic cation such as an alkyltriphenylphosphonium moiety to a pharmacophore of interest. Other delocalized lipophilic cations, such as rhodamine, natural and synthetic mitochondria-targeting peptides, and nanoparticle vehicles, have also been used for mitochondrial delivery of small molecules. Depending on the approach used, and the cell and mitochondrial membrane potentials, more than 1000-fold higher mitochondrial concentration can be achieved. Mitochondrial targeting has been developed to study mitochondrial physiology and dysfunction and the interaction between mitochondria and other subcellular organelles and for treatment of a variety of diseases such as neurodegeneration and cancer. In this Review, we discuss efforts to target small-molecule compounds to mitochondria for probing mitochondria function, as diagnostic tools and potential therapeutics. We describe the physicochemical basis for mitochondrial accumulation of lipophilic cations, synthetic chemistry strategies to target compounds to mitochondria, mitochondrial probes, and sensors, and examples of mitochondrial targeting of bioactive compounds. Finally, we review published attempts to apply mitochondria-targeted agents for the treatment of cancer and neurodegenerative diseases.

Triphenylphosphonium-desferrioxamine as a candidate mitochondrial iron chelator

Cell-impermeant iron chelator desferrioxamine (DFO) can have access to organelles if appended to suitable vectors. Mitochondria are important targets for the treatment of iron overload-related neurodegenerative diseases. Triphenylphosphonium (TPP) is a delocalized lipophilic cation used to ferry molecules to mitochondria. Here we report the synthesis and characterization of the conjugate TPP-DFO as a mitochondrial iron chelator. TPP-DFO maintained both a high affinity for iron and the antioxidant activity when compared to parent DFO. TPP-DFO was less toxic than TPP alone to A2780 cells (IC₅₀ = 135.60 ± 1.08 and 4.34 ± 1.06 μmol L^-1, respectively) and its native fluorescence was used to assess its mitochondrial localization (Rr = +0.56). These results suggest that TPP-DFO could be an interesting alternative for the treatment of mitochondrial iron overload e.g. in Friedreich's ataxia.

Mitochondria-penetrating peptides conjugated to desferrioxamine as chelators for mitochondrial labile iron

Desferrioxamine (DFO) is a bacterial siderophore with a high affinity for iron, but low cell penetration. As part of our ongoing project focused on DFO-conjugates, we synthesized, purified, characterized and studied new mtDFOs (DFO conjugated to the Mitochondria Penetrating Peptides TAT_49-57, 1A, SS02 and SS20) using a succinic linker. These new conjugates retained their strong iron binding ability and antioxidant capacity. They were relatively non toxic to A2780 cells (IC50 40–100 μM) and had good mitochondrial localization (Rr +0.45 –+0.68) as observed when labeled with carboxy-tetramethylrhodamine (TAMRA) In general, mtDFO caused only modest levels of mitochondrial DNA (mtDNA) damage. DFO-SS02 retained the antioxidant ability of the parent peptide, shown by the inhibition of mitochondrial superoxide formation. None of the compounds displayed cell cycle arrest or enhanced apoptosis. Taken together, these results indicate that mtDFO could be promising compounds for amelioration of the disease symptoms of iron overload in mitochondria.

Synthesis, photophysical properties and in vitro evaluation of a chlorambucil conjugated ruthenium(ii) complex for combined chemo-photodynamic therapy against HeLa cells

We designed a new heteroleptic Ru(ii) complex CHL-RuL as an imaging-guided chemotherapy/PDT agent, which shows a moderate antiproliferative effect in dark and strong photodynamic inhibitory effect against HeLa cells.

Peptide-Mediated Delivery of Chemical Probes and Therapeutics to Mitochondria

Mitochondria are organelles with critical roles in key processes within eukaryotic cells, and their dysfunction is linked with numerous diseases including neurodegenerative disorders and cancer. Pharmacological manipulation of mitochondrial function is therefore important both for basic science research and eventually, clinical medicine. However, in comparison to other organelles, mitochondria are difficult to access due to their hydrophobic and dense double membrane system as well as their negative membrane potential. To tackle the challenge of targeting these important subcellular compartments, significant effort has been put forward to develop mitochondria-targeted systems capable of transporting bioactive cargo into the mitochondrial interior. Systems now exist that utilize small molecule, peptide, liposome, and nanoparticle-based transport. The vectors available vary in size and structure and can facilitate transport of a variety of compounds for mitochondrial delivery. Notably, peptide-based delivery scaffolds offer attractive features such as ease of synthesis, tunability, biocompatibility, and high uptake both in cellulo and in vivo. Owing to their simple and modular synthesis, these peptides are highly adaptable for delivering chemically diverse cargo. Key design features of mitochondria-targeted peptides include cationic charge, which allows them to harness the negative membrane potential of mitochondria, and lipophilicity, which permits favorable interaction with hydrophobic membranes of mitochondria. These peptides have been covalently tethered to target therapeutic agents, including anticancer drugs, to enhance their drug properties, and to provide probes for mitochondrial biology. Interestingly, mitochondria-targeted DNA damaging agents demonstrate high potency and the ability to evade resistance mechanisms and off-target effects. Moreover, a combination of mitochondria-targeted DNA damaging agents was applied to an siRNA screen for the elucidation of poorly understood mitochondrial DNA repair and replication pathways. In this work, a variety of novel proteins were identified that are essential for the maintenance of mitochondrial nucleic acids. Mitochondria-targeted peptides have also been used to increase the therapeutic window of antibacterial drugs with significant mammalian toxicity. Given the evolutionary similarity of mitochondria and bacteria, peptides are effective transporters that can target both of these entities. These antimicrobial peptides are highly effective even in difficult to target intracellular bacteria which reside within host cells. This peptide-based approach to targeting mitochondria has provided a variety of insights into the "druggability" of mitochondria and new biological processes that could be future drug targets. Nevertheless, the mitochondrial-targeting field is quite nascent and many exciting applications of organelle-specific conjugates remain to be explored. In this Account, we highlight the development and optimization of the mitochondria-penetrating peptides that our laboratory has developed, the unique applications of mitochondria-targeted bioactive cargo, and offer a perspective on important directions for the field.

Massive Bioaccumulation and Self-Assembly of Phenazine Compounds in Live Cells

Clofazimine is an orally administered, FDA-approved drug that massively bioaccumulates in macrophages, forming membrane-bound intracellular structures possessing nanoscale supramolecular features. Here, a library of phenazine compounds derived from clofazimine was synthesized and tested for their ability to accumulate and form ordered molecular aggregates inside cells. Regardless of chemical structure or physicochemical properties, bioaccumulation was consistently greater in macrophages than in epithelial cells. Microscopically, some self-assembled structures exhibited a pronounced, diattenuation anisotropy signal, evident by the differential absorption of linearly polarized light, at the peak absorbance wavelength of the phenazine core. The measured anisotropy was well above the background anisotropy of endogenous cellular components, reflecting the self-assembly of condensed, insoluble complexes of ordered phenazine molecules. Chemical variations introduced at the R-imino position of the phenazine core led to idiosyncratic effects on the compounds' bioaccumulation behavior, as well as on the morphology and organization of the resulting intracellular structures. Beyond clofazimine, these results demonstrate how the self-assembly of membrane-permeant, orally-bioavailable small molecule building blocks can endow cells with unnatural structural elements possessing chemical, physical and functional characteristics unlike those of other natural cellular components.

Development of a mitochondria-targeted Hsp90 inhibitor based on the crystal structures of human TRAP1

The mitochondrial pool of Hsp90 and its mitochondrial paralogue, TRAP1, suppresses cell death and reprograms energy metabolism in cancer cells; therefore, Hsp90 and TRAP1 have been suggested as target proteins for anticancer drug development. Here, we report that the actual target protein in cancer cell mitochondria is TRAP1, and current Hsp90 inhibitors cannot effectively inactivate TRAP1 because of their insufficient accumulation in the mitochondria. To develop mitochondrial TRAP1 inhibitors, we determined the crystal structures of human TRAP1 complexed with Hsp90 inhibitors. The isopropyl amine of the Hsp90 inhibitor PU-H71 was replaced with the mitochondria-targeting moiety triphenylphosphonium to produce SMTIN-P01. SMTIN-P01 showed a different mode of action from the nontargeted PU-H71, as well as much improved cytotoxicity to cancer cells. In addition, we determined the structure of a TRAP1-adenylyl-imidodiphosphate (AMP-PNP) complex. On the basis of comparative analysis of TRAP1 structures, we propose a molecular mechanism of ATP hydrolysis that is crucial for chaperone function.

Highly sensitive cell imaging "Off-On" fluorescent probe for mitochondria and ATP

A smart Off-On molecular scaffold/fluorescent probe 1 has been designed and synthesized. The probe has shown considerable photostability, cell permeability, organelle specificity and selectivity for ATP. The multicolor live cell imaging experiments in HeLa cells showed high selectivity of probe 1 for mitochondria with fluorescence "turn-on" response. As a proof of concept and promising prospects for application in biological sciences probe 1 has been utilized to detect ATP sensitively in a partial aqueous medium and intracellularly in HeLa cells. The favorable interaction between triphosphate unit of ATP and piperazine N atoms of probe 1 is attributed to synergistic effects of H-bonding and electrostatic interactions that encouraged the CH-π and π→π stacking between anthracene and purine rings. Consequently, the observed enhanced "turn-on" emission and a naked-eye sensitive blue-green color in the medium is attributable to arrest in photoinduced electron transfer (PET) process.

Mitochondria-targeted cancer therapy using a light-up probe with aggregation-induced-emission characteristics

Subcellular organelle-specific reagents for simultaneous tumor targeting, imaging, and treatment are of enormous interest in cancer therapy. Herein, we present a mitochondria-targeting probe (AIE-mito-TPP) by conjugating a triphenylphosphine (TPP) with a fluorogen which can undergo aggregation-induced emission (AIE). Owing to the more negative mitochondrial membrane potential of cancer cells than normal cells, the AIE-mito-TPP probe can selectively accumulate in cancer-cell mitochondria and light up its fluorescence. More importantly, the probe exhibits selective cytotoxicity for studied cancer cells over normal cells. The high potency of AIE-mito-TPP correlates with its strong ability to aggregate in mitochondria, which can efficiently decrease the mitochondria membrane potential and increase the level of intracellular reactive oxygen species (ROS) in cancer cells. The mitochondrial light-up probe provides a unique strategy for potential image-guided therapy of cancer cells.

Structural modifications of mitochondria-targeted chlorambucil alter cell death mechanism but preserve MDR evasion

Multidrug resistance (MDR) remains one of the major obstacles in chemotherapy, potentially rendering a multitude of drugs ineffective. Previously, we have demonstrated that mitochondrial targeting of DNA damaging agents is a promising tool for evading a number of common resistance factors that are present in the nucleus or cytosol. In particular, mitochondria-targeted chlorambucil (mt-Cbl) has increased potency and activity against resistant cancer cells compared to the parent compound chlorambucil (Cbl). However, it was found that, due to its high reactivity, mt-Cbl induces a necrotic type of cell death via rapid nonspecific alkylation of mitochondrial proteins. Here, we demonstrate that by tuning the alkylating activity of mt-Cbl via chemical modification, the rate of generation of protein adducts can be reduced, resulting in a shift of the cell death mechanism from necrosis to a more controlled apoptotic pathway. Moreover, we demonstrate that all of the modified mt-Cbl compounds effectively evade MDR resulting from cytosolic GST-μ upregulation by rapidly accumulating in mitochondria, inducing cell death directly from within. In this study, we systematically elucidated the advantages and limitations of targeting alkylating agents with varying reactivity to mitochondria.

Molecular vehicles for mitochondrial chemical biology and drug delivery

The mitochondria within human cells play a major role in a variety of critical processes involved in cell survival and death. An understanding of mitochondrial involvement in various human diseases has generated an appreciable amount of interest in exploring this organelle as a potential drug target. As a result, a number of strategies to probe and combat mitochondria-associated diseases have emerged. Access to mitochondria-specific delivery vectors has allowed the study of biological processes within this intracellular compartment with a heightened level of specificity. In this review, we summarize the features of existing delivery vectors developed for targeting probes and therapeutics to this highly impermeable organelle. We also discuss the major applications of mitochondrial targeting of bioactive molecules, which include the detection and treatment of oxidative damage, combating bacterial infections, and the development of new therapeutic approaches for cancer. Future directions include the assessment of the therapeutic benefit achieved by mitochondrial targeting for treatment of disease in vivo. In addition, the availability of mitochondria-specific chemical probes will allow the elucidation of the details of biological processes that occur within this cellular compartment.

Targeting mitochondrial DNA with a platinum-based anticancer agent

An analog of the anticancer drug cisplatin (mtPt) was delivered to mitochondria of human cells using a peptide specifically targeting this organelle. mtPt induces apoptosis without damaging nuclear DNA, indicating that mtDNA damage is sufficient to mediate the activity of a platinum-based chemotherapeutic. This study demonstrates the specific delivery of a platinum drug to mitochondria and investigates the effects of directing this agent outside the nucleus.

A selective mitochondrial-targeted chlorambucil with remarkable cytotoxicity in breast and pancreatic cancers

Nitrogen mustards, widely used as chemotherapeutics, have limited safety and efficacy. Mitochondria lack a functional nucleotide excision repair mechanism to repair DNA adducts and are sensitive to alkylating agents. Importantly, cancer cells have higher intrinsic mitochondrial membrane potential (Δψmt) than normal cells. Therefore, selectively targeting nitrogen mustards to cancer cell mitochondria based on Δψmt could overcome those limitations. Herein, we describe the design, synthesis, and evaluation of Mito-Chlor, a triphenylphosphonium derivative of the nitrogen mustard chlorambucil. We show that Mito-Chlor localizes to cancer cell mitochondria where it acts on mtDNA to arrest cell cycle and induce cell death, resulting in a 80-fold enhancement of cell kill in a panel of breast and pancreatic cancer cell lines that are insensitive to the parent drug. Significantly, Mito-Chlor delayed tumor progression in a mouse xenograft model of human pancreatic cancer. This is a first example of repurposing chlorambucil, a drug not used in breast and pancreatic cancer treatment, as a novel drug candidate for these diseases.

Targeted delivery of doxorubicin to mitochondria

Several families of highly effective anticancer drugs are selectively toxic to cancer cells because they disrupt nucleic acid synthesis in the nucleus. Much less is known, however, about whether interfering with nucleic acid synthesis in the mitochondria would have significant cellular effects. In this study, we explore this with a mitochondrially targeted form of the anticancer drug doxorubicin, which inhibits DNA topoisomerase II, an enzyme that is both in mitochondria and nuclei of human cells. When doxorubicin is attached to a peptide that targets mitochondria, it exhibits significant toxicity. However, when challenged with a cell line that overexpresses a common efflux pump, it does not exhibit the reduced activity of the nuclear-localized parent drug and resists being removed from the cell. These results indicate that targeting drugs to the mitochondria provides a means to limit drug efflux and provide evidence that a mitochondrially targeted DNA topoisomerase poison is active within the organelle.