Antioxidants: the basics--what they are and how to evaluate them

Oxidative stress induced-neurodegenerative diseases: the need for antioxidants that penetrate the blood brain barrier

Oxidative stress (OS) has been implicated in the pathophysiology of many neurological, particularly neurodegenerative diseases. OS can cause cellular damage and subsequent cell death because the reactive oxygen species (ROS) oxidize vital cellular components such as lipids, proteins, and DNA. Moreover, the brain is exposed throughout life to excitatory amino acids (such as glutamate), whose metabolism produces ROS, thereby promoting excitotoxicity. Antioxidant defense mechanisms include removal of O(2), scavenging of reactive oxygen/nitrogen species or their precursors, inhibition of ROS formation, binding of metal ions needed for the catalysis of ROS generation and up-regulation of endogenous antioxidant defenses. However, since our endogenous antioxidant defenses are not always completely effective, and since exposure to damaging environmental factors is increasing, it seems reasonable to propose that exogenous antioxidants could be very effective in diminishing the cumulative effects of oxidative damage. Antioxidants of widely varying chemical structures have been investigated as potential therapeutic agents. However, the therapeutic use of most of these compounds is limited since they do not cross the blood brain barrier (BBB). Although a few of them have shown limited efficiency in animal models or in small clinical studies, none of the currently available antioxidants have proven efficacious in a large-scale controlled study. Therefore, any novel antioxidant molecules designed as potential neuroprotective treatment in acute or chronic neurological disorders should have the mandatory prerequisite that they can cross the BBB after systemic administration.

Nicaraven for the treatment of cerebral vasospasm in subarachnoid haemorrhage

Cerebral vasospasm is a complication of subarachnoid haemorrhage and can cause cerebral ischaemia. Antivasospastic agents are used to relieve vasospasm after subarachnoid haemorrhage. A large number of agents with varying modes of action currently being investigated are reviewed. Pharmacology and clinical trials of nicaraven are discussed. The drug has been found to have both antivasospastic as well as neuroprotective effects. Clinically, the most documented efficacy of nicaraven is in the management of vasospasm associated with subarachnoid haemorrhage based on its free radical scavenging effect. Other potential areas for application are cerebral oedema associated with intracerebral haemorrhage and for neuroprotection in cerebral infarction. Nicaraven is in pre-registration by Chugai Pharma Ltd. in Japan for the treatment of vasospasm following subarachnoid haemorrhage. The regulatory atmosphere in Japan regarding the approval of neuroprotectives is reviewed and nicaraven is likely to be approved by the year 2001 when the patent on it expires.

Antioxidants in the prevention of renal disease

In view of the role of oxidative processes in inflicting damage that leads to glomerulosclerosis and renal medullary interstitial fibrosis, more attention could be paid to the use of antioxidant food constituents and the usage of drugs with recognized antioxidant potential. In any case atherosclerosis is an important component of chronic renal diseases. There is a wide choice of foods and drugs that could confer benefit. Supplementation with vitamins E and C, use of soy protein diets and drinking green tea could be sufficient to confer remarkable improvements.

Modulation of peroxynitrite- and hypochlorous acid-induced inactivation of alpha1-antiproteinase by mercaptoethylguanidine

1. Peroxynitrite is a cytotoxic species that can be formed, among other mechanisms, by the rapid reaction of superoxide with nitric oxide. Peroxynitrite formation has been implicated in a wide range of neurodegenerative and chronic inflammatory diseases, as has the formation of hypochlorous acid by myeloperoxidase. 2. There is considerable interest in the development of peroxynitrite scavengers as therapeutic agents. The thiol compound mercaptoethylguanidine has been suggested to fulfil this role since it has recently been shown to be not only a potent inhibitor of inducible nitric oxide synthase but also a scavenger of peroxynitrite. Indeed, it has been shown to be protective in some experimental models of circulatory shock and inflammation at plasma levels in the approximate range 100-300 microM. 3. One protein inactivated by peroxynitrite is the major inhibitor of serine proteinases in human body fluids, alpha1-antiproteinase. At high (250-1000 microM) concentrations, mercaptoethylguanidine was found to be effective in preventing peroxynitrite-mediated tyrosine nitration and alpha1-AP inactivation. 4. By contrast, lower concentrations of mercaptoethylguanidine (1-60 microM) enhanced the inactivation of alpha1-antiproteinase by peroxynitrite. 5. At all concentrations tested (1-1000 microM), mercaptoethylguanidine decreased the inactivation of alpha1-antiproteinase by hypochlorous acid. 6. We suggest that products of reaction of mercaptoethylguanidine with peroxynitrite or peroxynitrite-derived products could cause damage to alpha1-antiproteinase, and possibly other proteins in vivo, whereas scavenging of hypochlorous acid by mercaptoethylguanidine could contribute to its anti-inflammatory action in vivo.

Reevaluation of cyclosporine induced hepatotoxicity in the isolated perfused rat liver

Livers of male rats were perfused for 120 min in a recirculating hemoglobin-free system with different concentrations of cyclosporine (CS 2, 10, 50, 150 and 200 mg/l). CS produced damage to the livers in a dose dependent manner. The first sign of hepatotoxicity was a reduction of bile flow amounting to 50% already at 50 mg/l CS. At concentrations of 150 mg/l and 200 mg/l, CS lead to a nearly complete suppression of bile flow, furthermore to a release of cytosolic (GPT, glutamate-pyruvate transaminase, LDH, lactate dehydrogenase) and mitochondrial (GLDH, glutamate dehydrogenase) enzymes into the perfusate and to a decrease in hepatic oxygen consumption (30% at 200 mg/l CS). As a consequence of the reduced aerobic energy supply, hepatic ATP concentration declined (70% at 200 mg/l CS). The hepatic concentrations of reduced glutathione (GSH) were not changed but those of oxidized glutathione (GSSG) increased up to 5-fold by CS. Malondialdehyde (MDA) concentrations in the liver and in the perfusate were not affected consistently by CS. The toxic actions of CS in the isolated rat liver were not influenced (a) by the feeding status of the rats (fed or fasted before surgery) or (b) by addition of superoxide dismutase (SOD, 20 mg/l) and catalase (20 mg/l) to the perfusate 30 min before CS. On the other hand, CS-induced hepatic injury could be attenuated or inhibited completely by addition to the perfusate of (1) 2 mmol/l GSH; (2) 12 mmol/l serine; (3) 12 mmol/l glycine; (4) 0.09 mmol/l deferoxamine (DFO).

CONCLUSIONS

CS induces cholestasis at lower concentrations, probably by another mechanism(s) than the other signs of hepatotoxicity (enzyme release, ATP depletion). Several lines of evidence indicate a probable participation of reactive oxygen species in CS-induced hepatotoxicity. GSH, DFO, glycine and serine could provide therapeutic opportunities to prevent CS-induced hepatotoxicity in patients treated with high doses of CS.