Oxygen-derived free radicals
Oxygen sustains us but it also harms our cells via oxygen-derived free radicals, including ·O2– (superoxide radicals) and ·OH (hydroxyl radicals). Superoxide radicals and iron are key players in the production of ·OH. For example, in cancer treatment, myocardial ischemia-reperfusion and inflammation, radical production often exceeds the endogenous protective capability.
Knowledge about endogenous cellular protective systems has provided us with new clues to treat and prevent disease. Superoxide dismutase (SOD), discovered more than 40 years ago by James McCord and Irvine Fridovich, belong to the most powerful cellular protective systems. PledPharma looks to exploit this effect through the use of PLED-derivatives.
PyridoxyL EthylDiamine (PLED)-derivatives belong to a new class of SOD mimetics with metal chelating properties. By dismutating ·O2–, PLED-derivatives protect important cellular components, e.g., NO· (nitric oxide) from forming highly toxic ONOO- (peroxynitrate). For example, peroxynitrate turns off the enzymatic activity of endogenous cellular SOD enzymes. In addition, PLED-derivatives have an extremely high Fe3+-affinity. The SOD mimetic activity in combination with high iron chelating capacity make PLED-derivatives extremely effective in arresting production of the most toxic reactive oxygen species, namely the hydroxyl radical (·OH)
Scientific rationale for using PLED derivatives as therapeutics
PLED-derivatives mimic the mitochondrial enzyme manganese superoxide dismutase (MnSOD). MnSOD protects the mammalian cell from superoxide radicals (·O2–), which is produced in fairly high amounts during normal aerobic conditions – no mammalians survive without a functional MnSOD. MnSOD has the fastest turnover rate (reaction rate with its substrate) of any known enzyme (>109 M-1 s-1). SOD mimetics may have turnover rates close to that of native MnSOD.
Efficacious inactivation of superoxide is essential in preventing generation of devastating ·OH radicals and peroxynitrite. During pathological oxidative stress, the formation of superoxide radicals often exceeds the endogenous capacity for inactivation. Exogenous addition of PLED-derivatives may in such situations re-establish the protective potential. PLED-derivatives are in addition strong iron binders, and some PLED-derivatives may have catalase and glutathione reductase activities, which may further increase their antioxidant capacity.
The PLED-derivative mangafodipir has been shown to protect mice against serious side effects of several chemotherapy drugs (doxorubicin, oxaliplatin, 5-fluorouracil and paclitaxel), e.g., against the myelosuppressive effects of oxaliplatin and cardiotoxic effects of doxorubicin. The cytoprotective effects are obtained without interfering negatively with the anticancer effect of these drugs. Contrary, mangafodipir potentiates the anticancer effect of several cytotoxic/cytostatic drugs. Mangafodipir has been tested in one colorectal cancer patient going through palliative treatment with a combination of folinate, 5-fluorouracil and oxaliplatin (“Nordic FLOX”, similar to the FOLFOX regimen). The preclinical data and the results from this single patient were promising enough to start a clinical feasibility study in colorectal cancer patients in Sweden (MANFOL I), which has been completed.
Mangafodipir has also been shown to protect mice against acetaminophen (paracetamol)-induced acute liver failure in mice (ALF). ALF is characterized by massive hepatocyte cell death, a condition caused by glutathione depletion, oxygen-derived free radicals and mitochondrial damage.
Iron-induced oxidative stress
The main cellular amount of labile, low mass iron is localized inside lysosomes as a result of degradation of autophagocytosed iron-containing structures, such as mitochondria and ferritin. Lipofuscin (age pigment) accumulates in slowly- or non-dividing cells due to peroxidation of autophagocytosed material by lysosomal iron. Heavy lipofuscin accumulation prevents normal autophagic turnover of damaged cellular structures, a key feature of aged cells. Cellular and lysosomal sensitivity to oxidative stress is a function of labile, redox-active lysosomal iron and that in turn is regulated by autophagy of iron-binding molecules, such as metallothioneins, hsp70 and ferritin that temporarily bind iron. Cells with highly upregulated such proteins, e.g., retinal pigment epithelial cells, are much more resistant to oxidative stress than cells with low cytosolic amounts of these proteins. Lysosomal permeabilization has been shown to induce apoptosis by releasing proteolytic enzymes that activate several pro-apoptogenic molecules. Lysosomes have altogether been found to be much more implicated in important cellular regulatory mechanisms than was earlier assumed.
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