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.