By David N. Leff
Of all the 107 elements in the periodic table, the most two-faced is oxygen - number 8. It keeps us alive, one breath at a time, but makes sure that we die of old age - if not before.
That, at least, is the theory that's been kicking around for the past half century or so. "The oxidative-stress theory of aging has been around for nearly 50 years," observed molecular gerontologist Simon Melov, a faculty member of the Buck Institute for Age Research, not-for-profit foundation in Novato, Calif.
"A tremendous body of evidence has accumulated in that period - largely correlative," Melov continued. That is, oxidative damage occurs with aging, but it's only over the last five years or so that people have begun to make much more rigorous tests of this hypothesis - primarily through genetic approaches. They had demonstrated for instance that overexpression of certain antioxidant enzymes in model systems, such as the fruit fly, Drosophila, actually increases life span, and thereby directly implicates the production of reactive oxygen species in limiting life span.
However, he pointed out, when it comes to people, antioxidants have their own limitations. "There have been less-than-interesting results with traditional antioxidants and aging, such as vitamin E, which, generally speaking, doesn't show much of an effect. People have done that for the last 15 or 20 years," Melov went on, "tried supplementing diets in various animal models - and a whole range of human studies as well with specific diseases have been tried with vitamin E and vitamin C. Really, nothing much has come of those studies."
Melov is lead author of a paper in today's issue of Science, dated Sept. 1, 2000, titled: "Extension of life-span with superoxide dismutase/catalase mimetics." Its co-authors include biochemist Susan Doctrow, vice president of research at Eukarion Inc. in Bedford, Mass. "Oxidative damage," she told BioWorld Today, "includes oxidation of DNA bases, lipid peroxidation, and protein nitration, among other forms of oxygen free-radical stress. Another example, in addition to aging, is in inflammation - the respiratory burst - a huge outpouring of superoxide, released by the host's immune system phagocytes.
"The useful or beneficial functions of oxygen radicals in the body," Doctrow continued, "are a fairly new area of research. There is some evidence of signaling effects. Superoxide, for example, is a mitogenic signal. The ras oncogene uses superoxide as a signaling agent. Mainly, free radicals are considered a harmful byproduct of oxidative metabolism, which is incomplete when the cell's mitochondrion is malfunctioning. When components of the mitochondrial electron transport chain are impaired, you can see increased free-radical production."
Double-Edged SOD Goes Toxic
The enzyme superoxide dismutase (SOD) is the body's own antioxidant, curbing the excesses of oxidative damage. "SOD catalyzes the production of toxic hydrogen peroxide [H2O2]," Doctrow explained. "That's the enzyme's product. It's pretty damaging itself, and is longer lived than that superoxide. Reason enough why SOD - despite early clinical trials - won't do as a therapeutic against the oxidative stress of aging."
Eukarion has developed a pair of SOD analogues - EUK-134 and EUK-8 - which aim to fill that gap. "What our compounds do," Doctrow explained, "is they not only have the superoxide dismutase function, but also are catalases. They destroy H2O2, converting it to oxygen and water - pretty innocuous substances."
The Buck Institute and Eukarion are close collaborators. Their joint article in Science reports on a series of in vivo experiments that test the life-prolonging properties of EUK-134 and EUK-8 in the nematode Caenorhabditis elegans. This millimeter-long roundworm, with 959 cells in its body, has a mean life span of 21 to 24 days "Its whole genome is known," Melov observed, "and C. elegans is used as a model system for the Human Genome Project. It's primarily a tool of developmental biologists, heavily employed in aging research."
When the co-authors added EUK-134 to the worms' drinking water, at dose-ranging concentrations, the critters survived an extra 10 to 13 days - their lives prolonged by 44 percent.
There's a mutant variant of C. elegans that dies early in life of premature aging. Their typical life span is 14 or 15 days. But treatment with EUK-134 raised that ante to 27 days - a fourth longer than the wild types' 21.
What sets Eukarion's small-molecule compounds apart from their SOD chemical progenitors, Melov pointed out, "is, for one thing, that they penetrate cells very easily. If you give the actual SOD protein to any animal, it's going to have difficulty getting into the cells. You also will engender a certain immune response. These EUK compounds are too small to provoke any sort of immune reaction. They penetrate cells much more effectively, and do get into the mitochondria. We have some evidence of that from studies we have done here at the institute.
"We are now carrying out studies in mice here," Melov said, "using this approach with EUK compounds, to see if the results in C. elegans translate into mammals. This is a great start," he added. "It's very encouraging; we're all very excited by the action of these compounds in this simple system. We've got those studies in mice under way, and expect the results very shortly."
Aim: Not Aging Per Se But Its Diseases
"Our company is not developing these compounds as anti-aging medicines," Eukarion's Doctrow stated. "What we're planning to do is develop them for diseases of aging. So how they would be used in higher organisms, including humans, depends on what medical indication we're after.
"Our first clinical indications," she went on, "will be dermatologic, and the one we're pursuing right now is a radiation protection agent in breast cancer patients. When these or other cancer patients receive radiotherapy, there are a variety of inflammatory tissue effects, and a lot of these are attributed to oxidative stress.
"So we are pursuing a dermatological treatment that can protect or prevent this skin toxicity occurring in breast cancer patients after radiation therapy. The implications are quite straightforward, and it's a nice quality-of-life issue for the patients.
"We're doing our preclinical work now," Doctrow said, "supporting an IND filing. So that of course depends on the FDA. We're hoping to file the Phase I trial application certainly within the year - or sooner." n