Mitochondria are cellular organelles present in most eukaryotic cells. One of their primary functions is oxidative phosphorylation, a process through which energy derived from metabolism of fuels like glucose or fatty acids is converted to ATP, which is then used to drive various energy-requiring biosynthetic reactions and other metabolic activities. Mitochondria have their own genomes, separate from nuclear DNA, comprising rings of DNA with about 16,000 base pairs in human cells. Each mitochondrion may have multiple copies of its genome, and individual cells may have hundreds of mitochondria. In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth (McBride et al., Curr. Biol., 2006, 16 (14): R551).
As mitochondria produce ATP, they simultaneously yield reactive oxygen species (ROS), which are harmful free radicals that circulate throughout the cell, the mitochondria, and the body, causing more damage. The circulation of ROS leads to the activation of reactive nitrogen compounds, which in turn induce, or activate, genes in the DNA that are associated with many degenerative diseases. The DNA for each mitochondrion (mtDNA) remains unprotected within the membrane of the mitochondrion itself. In comparison to the DNA in the nucleus of the cell (nDNA), mtDNA is easily damaged by free radicals and the ROS that it produces. Freely floating mtDNA lacks protective measures associated with nDNA, and therefore suffers from multiple mutations. It has been estimated that the lack of protective measures results in mutations to mtDNA occurring 10 to 20 times more frequently than mutations to nDNA.
Mitochondrial damage and/or dysfunction contribute to various disease states. Some diseases are due to mutations or deletions in the mitochondrial genome. Mitochondria divide and proliferate with a faster turnover rate than their host cells, and their replication is under control of the nuclear genome. If a threshold proportion of mitochondria in a cell is defective, and if a threshold proportion of such cells within a tissue have defective mitochondria, symptoms of tissue or organ dysfunction can result. Practically any tissue can be affected, and a large variety of symptoms can be present, depending on the extent to which different tissues are involved.
A fertilized ovum might contain both normal and genetically defective mitochondria. The segregation of defective mitochondria into different tissues during division of this ovum is a stochastic process, as will be the ratio of defective to normal mitochondria within a given tissue or cell (although there can be positive or negative selection for defective mitochondrial genomes during mitochondrial turnover within cells). Thus, a variety of different pathologic phenotypes can emerge out of a particular point mutation in mitochondrial DNA. Conversely, similar phenotypes can emerge from mutations or deletions affecting different genes within mitochondrial DNA. Clinical symptoms in congenital mitochondrial diseases often manifest in postmitotic tissues with high energy demands like brain, muscle, optic nerve, and myocardium, but other tissues including endocrine glands, liver, gastrointestinal tract, kidney, and hematopoietic tissue are also involved, again depending in part on the segregation of mitochondria during development, and on the dynamics of mitochondrial turnover over time.
In addition to congenital disorders involving inherited defective mitochondria, acquired mitochondrial damage and/or dysfunction contribute to diseases, particularly neurodegenerative disorders associated with aging like Parkinson's, Alzheimer's, Huntington's Diseases. The incidence of somatic mutations in mitochondrial DNA rises exponentially with age; and diminished respiratory chain activity is found universally in aging people. Mitochondrial dysfunction is also implicated in excitotoxic neuronal injury, such as that associated with seizures or ischemia.
Other pathologies with etiology involving mitochondrial damage and/or dysfunction include schizophrenia, bipolar disorder, dementia, epilepsy, stroke, cardiovascular disease, retinal degenerative disease (e.g., age-related macular degeneration, Stargardt's disease, glaucoma, retinitis pigmentosa, and optic nerve degeneration), and diabetes mellitus. A common thread thought to link these seemingly-unrelated conditions is cellular damage causing oxidative stress. Oxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. All forms of life maintain a reducing environment within their cells. This reducing environment is preserved by enzymes that maintain the reduced state through a constant input of metabolic energy. Disturbances in this normal redox state can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins, lipids, and DNA.
Mitochondrial damage and/or dysfunction particularly contribute to degenerative diseases. Degenerative diseases are diseases in which the function or structure of the affected tissues or organs will progressively deteriorate over time. Some examples of degenerative diseases are retinal degenerative disease, e.g., age-related macular degeneration, Stargardt's disease, glaucoma, retinitis pigmentosa, and optic nerve degeneration; amyotrophic lateral sclerosis (ALS), e.g., Lou Gehrig's disease; Alzheimer's disease; Parkinson's Disease; multiple system atrophy; Niemann Pick disease; atherosclerosis; progressive supranuclear palsy; cancer; Tay-Sachs disease; diabetes; heart disease; keratoconus; inflammatory bowel disease (IBD); prostatitis; osteoarthritis; osteoporosis; rheumatoid arthritis; and Huntington's disease.
Treatment of degenerative diseases involving mitochondrial damage and/or dysfunction has heretofore involved administration of vitamins and cofactors used by particular elements of the mitochondrial respiratory chain. Coenzyme Q (ubiquinone), nicotinamide, riboflavin, carnitine, biotin, and lipoic acid are used in patients with occasional benefit, especially in disorders directly stemming from primary deficiencies of one of these cofactors. However, while useful in isolated cases, no such metabolic cofactors or vitamins have been shown to have general utility in clinical practice in treating degenerative diseases involving mitochondrial damage and/or dysfunction.
Therefore, a need exists for new drug therapies for the treatment of subjects suffering from or susceptible to the above disorders or conditions associated with mitochondrial damage and/or dysfunction. In particular, a need exists for new drugs having one or more improved properties (such as safety profile, efficacy or physical properties) relative to those currently available.
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