To further increase the therapeutic activity of drugs known to act on intracellular target sites, in vivo drug delivery approaches must actively mediate the specific delivery of drug molecules to the subcellular site of action. We show here that surface modification of nanocarriers with mitochondriotropic triphenylphosphonium cations facilitates the efficient subcellular delivery of a model drug to mitochondria of mammalian cells and improves its activity in vitro and in vivo.

Mitochondrial DNA mutations are the direct cause of several physiological disorders and are also associated with the aging process. The modest progress made over the past two decades towards manipulating the mitochondrial genome and understanding its function within living mammalian cells means that cures for mitochondrial DNA mutations are still elusive. Here, we report that transformed mammalian cells internalize exogenous isolated mitochondria upon simple co-incubation. We first demonstrate the physical presence of internalized mitochondria within recipient cells using fluorescence microscopy. Second, we show that xenogenic transfer of murine mitochondria into human cells lacking functional mitochondria can functionally restore respiration in cells lacking mtDNA. Third, utilizing the natural competence of isolated mitochondria to take up linear DNA molecules, we demonstrate the feasibility of using cellular internalization of isolated exogenous mitochondria as a potential tool for studying mitochondrial genetics in living mammalian cells.

Mitochondrial dysfunction contributes to a large variety of human disorders, ranging from neurodegenerative and neuromuscular diseases, obesity, and diabetes to ischemia-reperfusion injury and cancer. Increasing pharmacological efforts toward therapeutic interventions have been made leading to the emergence of "Mitochondrial Medicine" as a new field of biomedical research. The identification of molecular mitochondrial drug targets in combination with the development of methods for selectively delivering biologically active molecules to the site of mitochondria will eventually launch a multitude of new therapies for the treatment of mitochondria-related diseases, which are based either on the selective protection, repair, or eradication of cells. Yet, while tremendous efforts are being undertaken to identify new mitochondrial drugs and drug targets, the development of mitochondria-specific drug carrier systems is lagging behind. To ensure a high efficiency of current and future mitochondrial therapeutics, delivery systems need to be developed, which are able to selectively transport biologically active molecules to and into mitochondria within living human cells. In this study we present the first data demonstrating that conventional liposomes can be rendered mitochondria-specific via the attachment of known mitochondriotropic residues to the liposomal surface.

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Mitochondrial research is presently one of the fastest growing disciplines in biomedicine. Since the early 1990s, it has become increasingly evident that mitochondrial dysfunction contributes to a large variety of human disorders, ranging from neurodegenerative and neuromuscular diseases, obesity, and diabetes to ischemia-reperfusion injury and cancer. Most remarkably, mitochondria, the "power house" of the cell, have also become accepted as the "motor of cell death" reflecting their recognized key role during apoptosis. Based on these recent exciting developments in mitochondrial research, increasing pharmacological efforts have been made leading to the emergence of "Mitochondrial Medicine" as a whole new field of biomedical research. The identification of molecular mitochondrial drug targets in combination with the development of methods for selectively delivering biologically active molecules to the site of mitochondria will eventually launch a multitude of new therapies for the treatment of mitochondria-related diseases, which are based either on the selective protection, repair, or eradication of cells. Yet, while tremendous efforts are being undertaken to identify new mitochondrial drugs and drug targets, the development of mitochondria-specific drug carrier systems is lagging behind. To ensure a high efficiency of current and future mitochondrial therapeutics, colloidal vectors, i.e., delivery systems, need to be developed able to selectively transport biologically active molecules to and into mitochondria within living human cells. Here we review ongoing efforts in our laboratory directed toward the development of different phospholipid- and non-phospholipid-based mitochondriotropic drug carrier systems.

Many drug molecules exert their biological action on intracellular molecular targets present on or inside various cellular organelles. Consequently, it has become more evident that the efficiency and efficacy of drug action is dependent largely on how well an unaided drug molecule is able to reach its intracellular target. We hypothesized that the biological action of such drug molecules might be improved by specific delivery to the appropriate sub-cellular site by a pharmaceutical carrier designed for the purpose. To test our hypothesis, we used paclitaxel, a molecule that has recently been shown to have pro-apoptotic biological targets on the mitochondria but has a quantitative structure-activity relationship-predicted cytosolic accumulation and no affinity for mitochondria. Using a mitochondria-specific nanocarrier system (DQAsomes) prepared from the amphiphilic quinolinium derivative dequalinium chloride to deliver paclitaxel to mitochondria in cells, we report that it is possible to improve the pro-apoptotic action of paclitaxel.

Mitochondrial research has made an enormous leap since mitochondrial DNA mutations were identified as a primary cause for human diseases in 1988 and the organelle's crucial role in apoptosis was identified during the 1990s. Considerable progress has been made in identifying the molecular components of the mitochondrial machinery responsible for life and cell death; however, effective therapies for diseases caused by mitochondrial dysfunction remain elusive. An impediment to manipulating, probing and assessing the functional components of mammalian mitochondria within living cells is their limited accessibility to direct physical, biochemical and pharmacological manipulation. Recent advances in nanotechnology hold the promise of helping to overcome these obstacles. New tools will undoubtedly emerge, creating new avenues for the diagnosis and therapy of mitochondrial disorders. This review briefly discusses current efforts to merge nanobiotechnology with mitochondrial medicine.

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