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 potentials of cell and mitochondrial membranes, 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.

Metformin (Met) is an approved antidiabetic drug currently being explored for repurposing in cancer treatment based on recent evidence of its apparent chemopreventive properties. Met is weakly cationic and targets the mitochondria to induce cytotoxic effects in tumor cells, albeit not very effectively. We hypothesized that increasing its mitochondria-targeting potential by attaching a positively-charged lipophilic substituent would enhance the antitumor activity of Met. In pursuit of this question, we synthesized a set of mitochondria-targeted Met analogs (Mito-Mets) with varying alkyl chain lengths containing a triphenylphosphonium cation (TPP+). In particular, the analog Mito-Met10, synthesized by attaching TPP+ to Met via a 10-carbon aliphatic side chain, was nearly 1,000 times more efficacious than Met at inhibiting cell proliferation in pancreatic ductal adenocarcinoma (PDAC). Notably, in PDAC cells Mito-Met10 potently inhibited mitochondrial complex I, stimulating superoxide and AMPK activation, but had no effect in non-transformed control cells. Moreover, Mito-Met10 potently triggered G1 cell cycle phase arrest in PDAC cells, enhanced their radiosensitivity and more potently abrogated PDAC growth in preclinical mouse models, compared to Met. Collectively, our findings show how improving the mitochondrial targeting of Met enhances its anticancer activities, including in aggressive cancers like PDAC in great need of more effective therapeutic options.

Background: Recently, new "targeted" fluorescent probes that react selectively with reactive oxygen and nitrogen species to yield specific products have been discovered. Results: High-throughput fluorescence and HPLC-based methodology for global profiling of ROS/RNS is described. Conclusion: This methodology enables real-time monitoring of multiple oxidants in cellular systems. Significance: The global profiling approach using different ROS/RNS-specific fluorescent probes will help establish the identity of oxidants in redox regulation and signaling.

Boronates, a group of organic compounds, are emerging as one of the most effective probes for detecting and quantifying peroxynitrite, hypochlorous acid and hydrogen peroxide. Boronates react with peroxynitrite nearly a million times faster than with hydrogen peroxide. Boronate-containing fluorogenic compounds have been used to monitor real time generation of peroxynitrite in cells and for imaging hydrogen peroxide in living animals. This Perspective highlights potential applications of boronates and other fluorescent probes to high-throughput analyses of peroxynitrite and hydroperoxides in toxicological studies.

Lung cancer often has a poor prognosis, with brain metastases a major reason for mortality. We modified lonidamine (LND), an antiglycolytic drug with limited efficacy, to mitochondria-targeted mito-lonidamine (Mito-LND) which is 100-fold more potent. Mito-LND, a tumor-selective inhibitor of oxidative phosphorylation, inhibits mitochondrial bioenergetics in lung cancer cells and mitigates lung cancer cell viability, growth, progression, and metastasis of lung cancer xenografts in mice. Mito-LND blocks lung tumor development and brain metastasis by inhibiting mitochondrial bioenergetics, stimulating the formation of reactive oxygen species, oxidizing mitochondrial peroxiredoxin, inactivating AKT/mTOR/p70S6K signaling, and inducing autophagic cell death in lung cancer cells. Mito-LND causes no toxicity in mice even when administered for eight weeks at 50 times the effective cancer inhibitory dose. Collectively, these findings show that mitochondrial targeting of LND is a promising therapeutic approach for investigating the role of autophagy in mitigating lung cancer development and brain metastasis.

Methods for detection and quantitation of hydroethidine (HE) and its oxidation products by HPLC analysis are described. Synthetic methods for preparations of authentic standards (2-hydroxyethidium and diethidium) are provided. Potential applications of the HPLC methods to chemical and biological systems are discussed. Specific examples of chromatograms obtained using UV-Vis absorption, fluorescence, electrochemical and mass spectrometry detectors are provided. The development of a dual electrochemical and fluorescence detection methodology and its applications are described. The HPLC-based method enables analyses of HE and its oxidation products such as ethidium and the dimeric products of HE. Ramifications of HPLC measurements of HE and its oxidation products in the detection and quantitation of 2-hydroxyethidium, the diagnostic marker product of superoxide and HE, in the intracellular milieu are discussed. Similarly, mitochondria-targeted HE conjugated to a triphenylphosphonium group (Mito-HE or Mito-SOX) also forms oxidation products (dimers of Mito-HE and Mito-E+) that can affect the detection and quantitation of 2-hydroxy-mito-ethidium, the diagnostic marker product of Mito-HE and superoxide in mitochondria.

Here we report that ferricytochrome c (cyt c 3+ ) induces oxidation of hydroethidine (HE) and mitochondria-targeted hydroethidine (Mito-HE or MitoSOX ™ Red) forming highly characteristic homo-and heterodimeric products. Using a HPLC-electrochemical (EC) method, several products were detected from cyt c 3+ -catalyzed oxidation of HE and Mito-HE and characterized by mass spectrometry and NMR techniques as follows: homodimers (HE-HE, E + -E + ; Mito-HE-Mito-HE, Mito-E + -Mito-E + ) and heterodimers (HE-E + and Mito-HE-Mito-E + ), as well as the monomeric ethidium (E + ) and mito-ethidium (Mito-E + ). Similar products were detected when HE and Mito-HE were incubated with mitochondria. In contrast, mitochondria depleted of cyt c 3+ were much less effective in oxidizing HE or Mito-HE to corresponding dimeric products. Unlike E + or Mito-E + , the dimeric analogs (E + -E + and Mito-E + -Mito-E + ) were not fluorescent. Superoxide ( ) or Fremy's salt react with Mito-HE to form a product, 2-hydroxy-mito-ethidium (2-OH-Mito-E + ) that was detected by HPLC. We conclude that HPLC-EC but not the confocal and fluorescence microscopy is a viable technique for measuring superoxide and cyt c 3+ -dependent oxidation products of HE and Mito-HE in cells. Superoxide detection using HE and Mito-HE could be severely compromised due to their propensity to undergo oxidation.

The present review is a sequel to the previous review on cancer metabolism published in this journal. This review focuses on the selective antiproliferative and cytotoxic effects of mitochondria-targeted therapeutics (MTTs) in cancer cells. Emerging research reveals a key role of mitochondrial respiration on tumor proliferation. Previously, a mitochondria-targeted nitroxide was shown to selectively inhibit colon cancer cell proliferation at submicromolar levels. This review is centered on the therapeutic use of MTTs and their bioenergetic profiling in cancer cells. Triphenylphosphonium cation conjugated to a parent molecule (e.g., vitamin-E or chromanol, ubiquinone, and metformin) via a linker alkyl chain is considered an MTT. MTTs selectively and potently inhibit proliferation of cancer cells and, in some cases, induce cytotoxicity. MTTs inhibit mitochondrial complex I activity and induce mitochondrial stress in cancer cells through generation of reactive oxygen species. MTTs in combination with glycolytic inhibitors synergistically inhibit tumor cell proliferation. This review discusses how signaling molecules traditionally linked to tumor cell proliferation affect tumor metabolism and bioenergetics (glycolysis, TCA cycle, and glutaminolysis).