A Radical Approach to Stroke Therapy

Commentary

A radical approach to stroke therapy

James McCulloch* and Deborah Dewar

Wellcome Surgical Institute and Hugh Fraser Neuroscience Laboratories, University of Glasgow, Glasgow G61 1QH, United Kingdom

troke is a major cause of death and readily crosses the blood–brain barrier, (mitogen-activated protein kinases͞ Sdisability throughout the developed to augment endogenous brain ascorbic ERK1͞2). During ischemia, ERK1͞2 are world. Cerebrovascular disease ranks acid levels by up to 2 mM. However, in dephosphorylated, and there is a signif- third after cancer and heart disease as a the brain, ascorbic acid levels are heter- icant increase in ERK1͞2 phosphoryla- cause of death in the European Union ogeneous and highly compartmental- tion during reperfusion after forebrain and the U.S. The economic and social ized. In the rat, normal neuronal, glial, ischemia. Neurons and oligodendrocytes burdens of stroke are not consequences and cerebrospinal fluid levels of ascorbic at the margin of a focal ischemic lesion of mortality; they are imposed by the acid are Ϸ10 mM, 1 mM, and 0.5 mM, display increased MEK1͞2, indicating large majority of stroke patients who respectively (3). The antioxidant effects that this signaling pathway is activated survive but are physically and mentally of ascorbic acid at all of these sites within after ischemia and reperfusion in vivo disabled by stroke-induced brain dam- the brain could contribute to the efficacy (8). If MEK1͞2 is inhibited by the novel age. In the U.S., less than 2% of stroke of this agent in ischemia. Brain ascorbic agent, U0126, the extent of brain damage patients benefit from access to early acid levels are highly dynamic (although is reduced after either forebrain or focal thrombolysis, which removes the primary under rigorous homeostatic control), ischemia (2). MEK1͞2 are activated in cause of blood flow reduction. However, display circadian variations, and, impor- vitro by various factors (glutamate, inter- no treatment is presently available that tantly, decline markedly and rapidly in leukin-1, and tumor necrosis factor), protects brain tissue from the multiple the intracellular compartment at the on- which are elevated in the brain by isch- neurochemical cascades initiated by both set of ischemia (3). Any therapeutic use emia and inhibitors of MEK1͞2ordrugs ischemia and reperfusion, and it is these of DHA in stroke patients will need to that prevent phosphorylation of MEK1 cascades that ultimately cause irrevers- ensure that adequate levels of ascorbic protect neuronal cell cultures from oxi- ible brain damage. acid can be directed to the appropriate dative damage. Mitogen-activated pro- Over the last decade, remarkable in- subcellular sites of oxidative damage at tein kinases are an attractive target for sight has been gained into the neuro- crucial time points after the onset of drug development because of their chemical mechanisms that contribute to ischemia, particularly as the most impor- multiplicity of actions, which influence COMMENTARY ischemic brain damage, and there is com- tant cellular site of action of ascorbic not only cell survival and apoptosis pelling evidence that oxidative damage acid in the brain is not but also inflamma- plays an important role. Two papers in known. Although sys- tory mechanisms this issue of PNAS test the therapeutic temic ascorbic acid (9). The develop- potential of reducing oxidative damage was ineffective in Oxidative damage does not ment of drugs such in animal models of ischemic brain dam- mice (2), ischemic occur in isolation but as U0126, which are age (1, 2). However, the two studies use brain damage can be participates in the complex systemically active quite different conceptual and pharma- markedly reduced by and display selectiv- cological approaches. The study of i.v. ascorbic acid in interplay between ity of action (e.g., Huang et al. (1) is primarily concerned various animal mod- excitotoxicity, apoptosis, p38 MAPK, JNK, with the anti-ischemic efficacy of the els of ischemia, in- and p70s6 kinase are and inﬂammation in long-recognized antioxidant ascorbic cluding those in non- not affected in acid and uses a prodrug treatment strat- human primates (4, ischemia and reperfusion. vitro), suggests that egy with dehydroascorbic acid to deliver 5). These data suggest intracellular signal- ascorbic acid to the central nervous sys- that the crucial sites ing may provide a tem. The study of Namura et al. (2) of action of ascorbic acid may be located new therapeutic approach to reducing focuses on establishing the participation outside the brain, e.g., in endothelial oxidative damage during postischemic of specific mitogen-activated protein ki- cells. The spin trap agent, NXY059, reperfusion. nases and, after characterizing their in- which has poor brain penetration, pro- There is already compelling evidence volvement in ischemia, examines duces large reductions in ischemic brain that a variety of pharmacological and ge- whether pharmacological inhibition of damage in animals (6) and is well toler- netic strategies that attenuate oxidative the kinases alters the outcome after isch- ated in human patients after stroke (7). damage also reduce ischemic brain dam- emia. By using pharmacological agents The crucial issues for drugs such as DHA age (10). Oxidative damage in ischemia is that act at quite different points in the and NXY059 remain those of establish- an exceedingly complex therapeutic target oxidative damage cascade (Fig. 1), both ing appropriate dosing regimens after (Fig. 1). Free radicals are a diverse group studies report success of the treatment stroke onset. strategies in reducing the extent of isch- The strategy followed by Namura et al. emic damage. (2) focuses on a well-defined cell signal- See companion articles on pages 11569 and 11720. Huang et al. (1) have focused on the ing mechanism that is activated by reac- *To whom reprint requests should be addressed at: Well- classic antioxidant ascorbic acid (vitamin tive oxygen and nitrogen species; extra- come Surgical Institute and Hugh Fraser Neuroscience Lab- ͞ oratories, University of Glasgow, Garscube Estate, Bears- C) and have used the oxidized form, cellular signal regulated kinases (ERK1 den Road, Glasgow G61 1QH, United Kingdom. E-mail: dehydroascorbic acid (DHA), which 2), which are activated by MEK1͞2 [email protected].

www.pnas.org͞cgi͞doi͞10.1073͞pnas.211430898 PNAS ͉ September 25, 2001 ͉ vol. 98 ͉ no. 20 ͉ 10989–10991 Downloaded by guest on October 2, 2021 (11, 14). The anatomical and temporal extent of the ischemic penumbra in stroke patients is neither as consistent nor as large as in animals, even when assessed with similar technology, for example, diffusion-weighted imaging (15). There has been concern that the patient entry window in clinical trials (typically within6hofstroke onset) has been too long in view of the few agents with proven efficacy in animal models at this time point. The successful recombinant tissue plasminogen activator thrombolysis trial in human stroke had an entry window of 3 h (16). The long therapeutic window displayed by DHA, U0126 (1, 2), and other antioxidants (6), with protection when treatment is initiated at 3 h after onset of ischemia, is encouraging. The failure of drugs such as N-methyl- D-aspartate (NMDA) antagonists to protect cerebral white matter from ischemic damage may have contributed to the ab- sence of functional improvement in clinical trials of these agents, despite their proven ability to protect neuronal perikarya in animal models (17, 18). Un- like NMDA receptor antagonists, brain- penetrating drugs with antioxidant affects protect not only neuronal perikarya but also axons and oligodendrocytes as well as Fig. 1. Oxidative damage is mediated through attack by multiple radicals with differing reactivity, improving neurological outcome after fo- path length, and half-life. Radical attack takes place at many different sites within the brain, and there cal ischemia in rats (19). The view that is interplay between mechanisms. For example, peroxidation of lipid membranes generates toxic aldehydes such as 4-hydroxynonenal (4-HNE), which themselves damage a variety of ion channel, oxidative damage is a crucial target for transporter, and cytoskeletal proteins. Free radicals also activate speciﬁc cell signaling pathways, such therapeutic intervention is further sup- as MEK͞ERK, which contribute to damage. The promiscuity of radical attack means that all cellular ported by evidence that oxidative stress types and elements in the brain are susceptible to oxidative damage, although the mechanisms and lipid peroxidation by-products are mediating this damage differ. For example, DNA damage occurs in the nucleus, whereas lipid toxic to axons and oligodendrocytes as peroxidation and damage to structural proteins mediate damage in axons and myelin. Cell signaling- well as to neuronal perikarya (20, 21). The mediated damage will occur only in locations where the pathways are located. The pharmacological nonselective nature of oxidative damage agents used by Huang et al. (1) and Namura et al. (2) target distinct points in the oxidative damage means that it has the potential to involve cascade, but both reduce ischemic brain damage. all cellular types and cellular components of normally functioning brain tissue: neuronal perikarya, axons, oligodendrocytes, of agents, are generated at diverse sites, zole, Elipordil, D-CPPene, Lubeluzole, astrocytes, microglia, and endothelial cells and have diverse reactive distances in tis- Gavestinel, BMS 204352) were sus- (Fig. 1). By attenuating oxidative damage, sue. In addition, toxic by-products such as pended before completion of patient re- protection of all cellular elements rather peroxynitrite or 4-hydroxynonenal are cruitment or failed to provide compel- than selective neuronal protection may be generated during oxidative damage, and ling evidence of clinical efficacy (12). possible. ERK1͞2 is activated in oligoden- these also contribute to evolving brain The failure to translate mechanistic in- drocytes after cerebral ischemia (8), and injury. Oxidative damage does not occur sight into ischemic damage to effective ERK activation is involved in oxidative in isolation but participates in the complex clinical treatment is due to multiple fac- damage to cultured oligodendrocytes interplay between excitotoxicity, apopto- tors. Definition of the effective dose in (22). However, it is not clear whether sis, and inflammation in ischemia and clinical trials has been a recurring issue activation of this signaling pathway is in- reperfusion (9, 10). in anti-ischemic drug development be- volved in ischemic damage to other glia, A huge number of pharmacological cause of concerns about adverse effects axons, or endothelial cells. In contrast, the agents targeted at neurotransmitter re- and, for some agents (e.g., Gavestinel), widespread distribution of ascorbic acid in ceptors and ion channels have been about kinetics across the blood–brain the central nervous system suggests that shown to reduce ischemic brain damage barrier (13). Both of these issues are the nonselective approach of DHA treat- in animal models (11). However, no clear pertinent to targeting oxidative mecha- ment to scavenge free radicals could have evidence of clinical improvement with nisms. The adverse effects of MEK1͞2 the potential to protect all cellular types any neuroprotective drug has yet been inhibition remain to be established, and and elements. obtained in clinical trials of human the dose setting with DHA will be diffi- Anti-ischemic drug development is at a stroke. Numerous human trials of neu- cult because of the tissue compartmen- crossroads. In excess of $1 billion has roprotective drugs have not advanced to talization of ascorbic acid. In animal already been expended in evaluating neu- a definitive assessment of clinical effi- models, neuroprotective efficacy is con- roprotective drugs targeted at neurotrans- cacy in a Phase 3 trial, and those that fined to the ischemic penumbra with its mitter receptors or ion channels in stroke have (Aptiganel, Selfotel, Clomethia- special neurochemical characteristics patients, without success. A re-evaluation

10990 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.211430898 McCulloch and Dewar Downloaded by guest on October 2, 2021 of appropriate pharmacological and cel- most promising avenues. If mechanistic drug efficacy (17, 23) and clinical trial lular targets for therapeutic intervention insight is to be translated into effective design (including the use of magnetic res- is taking place. Pharmacological modifi- therapy, the advances that have been onance imaging technology) (24) need to cation of oxidative damage is one of the made in both preclinical evaluation of be harnessed.

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