Reactive oxygen species are known to be potentially dangerous, but are also needed for signal-transduction pathways.
Tumor cells have relatively low amounts of
superoxide dismutase (SOD), which quenches
superoxide anion (O2(-*)), and as a result of a higher level of aerobic metabolism, higher concentrations of O2(-*) , compared to normal cells. But this may not be true of all
tumor cells. Some
tumor cells have relatively higher amounts of
vitamin E, a potent
anti-oxidant, and a higher level of anaerobic metabolism, resulting in a balance that is tilted more towards higher
anti-oxidant capacity. In both instances of higher aerobic and anaerobic metabolism methods designed to augment
free radical generation in
tumor cells can cause their death. It is suggested that
free radicals and
lipid peroxides suppress the expression of Bcl-2, activate
caspases and shorten telomere, and thus inducing apoptosis of
tumor cells. Ionizing radiation,
anthracyclines,
bleomycin and
cytokines produce
free radicals and thus are useful as anti-
cancer agents. But they also produce many side-effects.
2-methoxyoestradiol and
polyunsaturated fatty acids (PUFAs) inhibit SODs and cause an increase of O2(-*) in
tumor cells leading to their death. In addition, PUFAs (especially
gamma-linolenic acid),
2-methoxyoestradiol and
thalidomide may possess anti-angiogenic activity. This suggests that
free radicals can suppress angiogenesis. Limited clinical studies done with
gamma-linolenic acid showed that it can regress human brain
gliomas without any significant side-effects. Thus, PUFAs,
thalidomide and
2-methoxyoestradiol or their derivatives may offer a new radical approach to the treatment of
cancer.