Recent advances in the development of multifactorial ...

Review Monthly Focus: Central & Peripheral Nervous System

1. Introduction 2. Secondary injury 3. Attenuating secondary injury

Expert Opin. Investig. Drugs Downloaded from informahealthcare.com by University of Adelaide on 08/19/13 For personal use only.

4. Conclusion

Recent advances in the development of multifactorial therapies for the treatment of traumatic brain injury Robert Vink† & Corinna Van Den Heuvel †Department

of Pathology, University of Adelaide, Adelaide, South Australia

5. Expert opinion

Traumatic brain injury (TBI) is one of the leading causes of death and disability in the industrialised world and remains a major health problem with serious socioeconomic consequences. So far, despite encouraging preclinical results, almost all neuroprotection trials have failed to show any significant efficacy in the treatment of clinical TBI. This may be due, in part, to the fact that most of the therapies investigated have targeted an individual injury factor. It is now recognised that TBI is a very heterogeneous type of injury that varies widely in its aetiology, clinical presentation, severity and pathophysiology. The pathophysiological sequelae of TBI are mediated by an interaction of acute and delayed molecular, biochemical and physiological events that are both complex and multifaceted. Accordingly, a successful TBI treatment may have to simultaneously attenuate many injury factors. Recent efforts in experimental TBI have, therefore, focused on the development of neuropharmacotherapies that target multiple injury factors and thus improve the likelihood of a successful outcome. This review will focus on three such novel compounds that are currently being assessed in clinical trials; progesterone, dexanabinol and dexamethasone, and provide an update on the progress of both magnesium and cyclosporin A. Keywords: cyclosporin A, dexamethasone, dexanabinol, magnesium, neuroprotection, neurotrauma, progesterone, traumatic brain injury Expert Opin. Investig. Drugs (2004) 13(10):1263-1274 1. Introduction

For reprint orders, please contact: [email protected]

Ashley Publications www.ashley-pub.com

Traumatic brain injury (TBI) is the leading cause of death and disability in people under the age of 45 in industrialised countries. In Australia, the US, France and Spain the incidence of death from TBI is 20 – 30 per 100,000 [1]. Motor vehicle accidents account for the majority of fatal head injuries [2]. Those individuals who survive TBI are often left with permanent neurological deficits, which adversely affect their quality of life and as a result, the social and economic cost of TBI is substantial. It is now recognised that neuronal cell death resulting from TBI is caused by both primary and secondary injury mechanisms. Primary injury encompasses the mechanical forces at the time of the injury that damage blood vessels, axons, glia and neurons through shearing, tearing and stretching. It includes contusions, lacerations, diffuse axonal injury and haemorrhage. The resultant injury is not reversible and the clinical effects are produced immediately [3]. In an attempt to prevent primary injury from occurring safety devices including seat belts, airbags and helmets have been introduced. Secondary injury has been identified as an evolving sequence of cellular, neurochemical and metabolic alterations that are initiated by the primary traumatic 2004 © Ashley Publications Ltd ISSN 1354-3784

1263


Recent advances in the development of multifactorial therapies for the treatment of traumatic brain injury

insult [4]. For example, the shearing forces applied to neurons in response to injury causes massive ion fluxes across neuronal membranes, widespread loss of membrane potential and the rapid release of neurotransmitters from damaged cells. These events overwhelm ion channel receptors leading to a massive influx of calcium ions into cells, which then triggers a host of biochemical events that generate large amounts of toxic and pro-inflammatory molecules such as nitric oxide, prostaglandins, free radicals and inflammatory cytokines. The resulting inflammatory response leads to a breakdown of the blood– brain barrier (BBB), the development of oedema and increases in intracranial pressure that may cause local hypoxia and ischaemia, with subsequent neuronal cell death via necrosis and apoptosis. Typical secondary injury cascades such as this are thought to be associated with the development of many of the neurological deficits observed after TBI [4]. Moreover, the fact that these cascades occur over minutes to days following the initial trauma suggests that a therapeutic window exists for treatment to prevent, attenuate or, at least, delay the resultant neurological deficits. Experimental models of TBI have been developed to facilitate the study of the mechanisms following trauma and assess the effectiveness of potential therapeutic interventions. A number of potentially successful pharmacological treatments for TBI have been identified in these preclinical studies and have progressed to clinical trial. Specifically, > 20 compounds and therapeutic interventions have been described and subjected to > 50 clinical trials in the last three decades [5-7]. However, despite the encouraging preclinical results, almost all of these clinical trials have failed to show any consistent improvement in outcome. A number of reasons have been proposed to account for these failures. For example, time to treatment intervention is clearly a problem given that most experimental studies use either pretreatment or very early (< 30 min) treatment protocols despite the recognition that metabolic and biochemical changes occur many hours after the initial trauma. Moreover, such early intervention is not always possible in clinical TBI. Drug-dosing schedules also often differ between the preclinical and clinical trials, with the latter frequently utilising lower doses to avoid potential toxicity, or more frequent dosages (continuous infusions) that have not been supported by the preclinical data. Thus, the dose could either be too low or too high reflecting the inverted U characteristics typical of many pharmacological compounds. Clinical trials also incorporate mixed injury severities as opposed to the preclinical screening that generally uses a well-defined and highly controlled animal model. Moreover, the use of healthy young animals of the same sex results in high injury reproducibility that is not apparent in clinical trials. Secondary insults, such as CNS hypoxia and systemic injuries to the whole organism, are also generally avoided in these animal TBI models but are commonplace in clinical TBI. This is especially the case for victims of motor vehicle accidents and may contribute to the failure of targeted interventions. For a critical appraisal of these factors and others, interested readers are referred to 1264

some excellent reviews that have been published elsewhere [6,7]. Whereas many of these shortcomings can be easily addressed, the most significant hurdle to trial success is the complexity and heterogeneity of clinical TBI, and in particular the multifactorial nature of the secondary injury process [7,8]. It is unlikely that targeting a single factor will result in any significant improvement in outcome. Conversely, simultaneously targeting several injury factors may be an effective therapeutic strategy likely to improve outcome. This review will firstly summarise some of the major secondary injury processes associated with cell death following TBI and then focus on progesterone, dexanabinol and dexamethasone, which are the subjects of three ongoing clinical trails. Discussion will then briefly address two of the putative multifactorial therapies previously discussed (magnesium and cyclosporin A [CyA]) [9] and summarise any new developments that have arisen with these compounds over the last two years. 2. Secondary

injury

2.1 Excitotoxicity Excitotoxicity is widely accepted as an important process in secondary damage and cell death following TBI [4]. It is produced by excessive activation of the excitatory amino acid (EAA) receptors of which the NMDA receptor complex seems to play the most prominent role [10]. Following injury, EAAs (primarily glutamate) are released into the synapse in high concentrations and overstimulate the NMDA receptor [11]. Ionic imbalance occurs with Na+ influx and K+ efflux leading to further depolarisation, which can overcome the Mg2+ blockade of the NMDA receptor [12]. Moreover, the ionic imbalance results in a reduction in glutamate re-uptake, further increasing its concentration. The high quantities of glutamate binding to the NMDA receptor promotes substantial Ca2+ influx resulting in Ca2+ overload. The increase in intracellular Ca2+ concentration is known to activate a plethora of calcium-dependent enzymes including proteases, lipases and endonucleases leading to neuronal destruction. Given the detrimental effects of glutamate excitotoxicity, it is not surprising that there have been numerous clinical trials assessing the efficacy of EAA inhibitors on patient outcome following TBI. A detailed review of these trials is covered by Willis [13]. Briefly, nine trials of compounds that attenuate excitotoxicity were identified, only two of which are still ongoing (dexanabinol and magnesium salts, which are discussed in Sections 3.2 and 3.4.1, respectively). Of the Phase III trials, three were completed and two were terminated early. Unfortunately, data from most of these studies have not yet been released but it is known that three trials were stopped due to lack of efficacy. Perhaps one of the reasons for the lack of efficacy of the NMDA antagonists is the finding that non-NMDA glutamate receptors have also been shown to play a role in post-traumatic glutamate excitotoxicity [14]. Thus, glutamate toxicity is more complex than originally thought and may in fact be mediated by a number of different receptors.

Expert Opin. Investig. Drugs (2004) 13(10)


Vink & Van Den Heuvel

2.2 Oxidative stress Reactive oxygen species (ROS), or free radicals, are highly reactive molecules that contain an unpaired electron in the outermost orbit, increasing their potential for chemical reactivity [15,16]. As normal by-products of oxidative metabolism, their concentration is usually tightly controlled by endogenous antioxidant mechanisms. However, TBI dramatically increases their production [15,16]. The result is oxidative stress, which can be defined as damage inflicted via processes involving the production of ROS and their detrimental reactions with proteins, lipids and DNA [17]. The brain tissue is extremely vulnerable to oxidative damage because of its high rate of oxidative metabolic activity, production of reactive oxygen metabolites, relatively low antioxidant capacity, low repair mechanism activity, the nonreplicating nature of its neuronal cells, and the high membrane surface to cytoplasm ratio [17,18]. The ROS can be generated via arachidonic acid cascade activity, catecholamine oxidation, mitochondrial leak, oxidation of extravasated haemoglobin and by neutrophils [16,19,20]. They initiate tissue damage through complex mechanisms including excitotoxicity, metabolic failure and the disturbance of intracellular calcium homeostasis [15,19,20]. Oxidative damage also frequently involves lipid peroxidation of neuronal, glial and vascular cell membranes and myelin, resulting in the decomposition of polyunsaturated fatty acids in lipid membranes, the disruption of ionic gradients and, if severe enough, membrane lysis [19]. It is tightly linked to other pathological mechanisms such as Ca2+ overload, mitochondrial cytochrome c release, caspase activation and apoptosis [16]. The administration of antioxidants has been shown to be effective in experimental models of TBI [19]. However, on their own, they have not shown efficacy in clinical trials of TBI. 2.3 Mitochondria The mitochondrion is a key participant in TBI-induced neuropathology [21] and its dysfunction has serious implications for outcome following head injury. Following oxidative injury this organelle has been shown to undergo a mitochondrial permeability transition (MPT) [22]. This permeabilisation of the inner mitochondrial membrane often results from the accumulation of excessive quantities of calcium [23,24] and results in a loss of matrix components, impairment of mitochondrial function and swelling of the organelle with outer membrane rupture [25]. The MPT is integrally involved in apoptotic cell death as well as with uncoupling and inhibition of oxidative phosphorylation and stimulating the generation of mitochondrial ROS [24,26]. Moreover, the propensity of mitochondria to undergo MPT has recently been suggested to account for the selective vulnerability of different brain regions to an ischaemic insult [27]. Any mitochondrial dysfunction would lead to energy depletion, free radical release and further cell death pathway activation [28,29]. Currently, Phase III trials are underway using CyA in an attempt to block the MPT and thus inhibit the destructive cellular cascades that follow permeability. This will be discussed in Section 3.4.2.

2.4 Oedema Cerebral oedema is a deleterious secondary injury factor that occurs following TBI, and can manifest either locally or diffusely throughout the brain. It is broadly defined as a volumetric enlargement of the brain tissue due to an abnormal accumulation of fluid [30]. Harmful consequences of cerebral oedema include raised intracranial pressure (ICP), dangerously reduced cerebral blood flow (CBF), reduced cerebrospinal fluid (CSF) and deformation and shifting of brain tissue, all of which contribute substantially to increased morbidity and mortality following TBI [30-32]. Although pure types of oedema rarely exist [30], cerebral oedema is generally classified primarily as vasogenic or cytotoxic depending on the underlying mechanisms associated with the oedema formation. Vasogenic oedema results from increased BBB permeability causing a disruption to the balance between oncotic and hydrostatic pressures that govern the movement of fluid between blood plasma and brain interstitial fluid. The compromised BBB allows water and solutes such as protein exudates to escape from the cerebral vasculature and enter the interstitium of brain parenchyma, resulting in a net gain of interstitial fluid and subsequent fluid retention [31,33]. Due to the limited lymphatic system in the brain, resorption of exudate from the extracellular space is greatly impaired. Vasogenic oedema then spreads throughout the extracellular space as a result of pressure gradients involving the least tissue resistance. This mechanism of movement explains why oedema is seen primarily in the structurally ordered cerebral white matter rather than in the more densely organised grey matter [30,31,34]. In contrast, cytotoxic oedema is characterised by intracellular swelling in neuronal, glial and endothelial cells in the absence of any measurable breakdown of the BBB [31]. As cells swell there is a concurrent reduction in extracellular space [34]. Glutamate-mediated excitotoxicity is thought to contribute to cytotoxic oedema by causing intracellular accumulation of sodium [34,35]. Water then follows by osmosis increasing intracellular fluid volumes. Cytotoxic oedema occurs primarily in the grey matter and is commonly associated with ischaemia and energy depletion [31,34]. Despite the serious consequences associated with oedema formation, there is currently no effective treatment in clinical practice. Treatments to date, which include mannitol, glucocorticoids, hypothermia, barbiturates and drainage of CSF, have had either limited success or been completely ineffective [36]. Nonetheless, recent studies with progesterone and glucocorticoids have been encouraging.

3. Attenuating

secondary injury

3.1 Progesterone Aside from its well-described influence on sexual and reproductive behaviour, a growing body of evidence demonstrates that this hormone, which is also synthesised de novo by glial cells in the nervous system, influences several brain functions


1265



via steroidal (genomic), neuroactive (non-genomic) and neurosteroidal actions [37]. Progesterone and its three reduced metabolites, including allopregnanolone, modulate neuronal excitability by interacting with the inhibitory GABA type A (GABAA) receptors. Other neurotransmitter receptors modulated by progesterone’s actions include 5-hydroxytryptamine, glycine, nicotinic acetylcholine and kainate receptors [37]. Progesterone’s attenuation of excitatory amino acid responses has also been demonstrated [38]. The hormone’s modulating effects are thought to involve anaesthetic [39], anxiolytic [40] and analgesic and anticonvulsant properties [41]. Effects on sleep patterns, memory [37] and depression [41] have also been reported. In the peripheral nervous system, progesterone has been shown to modulate myelin protein synthesis in Schwann cells of the rat sciatic nerve [42], possibly by stimulating the synthesis of specific myelin proteins or lipids. The increase in myelin basic protein has since been associated with an increased rate of remyelination of axons in both central and peripheral nerve preparations, suggesting potential neuroprotective effects in multiple sclerosis [43]. Neurotrophic and neuroprotective effects of progesterone have now been demonstrated both in vitro and in vivo in a number of models of CNS injury including spinal cord injury [44], stroke [45,46], neurodegeneration [47] and TBI [48]. In spinal cord injury, progesterone has been shown to protect against glutamate toxicity in vitro [49], possibly by modulation of inhibitory (GABAA) and excitatory (EAA) neurotransmitter receptors [50]. Similarly, following in vivo acute spinal cord transection injury in rats, progesterone restored choline acetyltransferase (ChAT) immunoreactivity, mRNA for neuronal Na+/K+-ATPase and enhanced growth-associated phosphoprotein (GAP)-43 mRNA expression [51]. Following incomplete paraplegic spinal cord injury, rats treated with progesterone showed less tissue and white-matter damage at the epicentre of the injury and recorded a better functional activity as assessed using the Basso-Beattie-Bresnehen locomotor rating scale [52]. This was very similar to the results in experimental stroke where progesterone administration also reduced infarct volume and improved functional outcome in rats [46]. The improvement in functional outcome is consistent with the observation that progesterone reduces neuronal cell loss in the hippocampus when administered after global ischaemia [53]. In TBI, Roof et al. [54] used a cortical lesion model to demonstrate that females performed better than males after injury in the Morris water maze. Following the observation that progesterone-treated male rats were less impaired on the Morris water maze task than vehicle-treated animals [55], they attributed this protective effect to progesterone. This protective effect of progesterone on post-traumatic performance in the Morris water maze was subsequently confirmed in studies using an air-driven cortical contusion model of TBI in rats [56] and the positive effects of progesterone on cognitive outcome were linked to a reduction in neuronal cell loss [57]. Although the mechanisms by which functional outcome are 1266

improved after TBI are unknown, progesterone has the ability to reduce membrane lipid peroxidation after TBI [58], indicative of an effect on oxidative stress. This effect on oxidative stress has been confirmed in tissue culture as well as in an in vitro stretch model of TBI. In both cases, progesterone reduced oxidative stress as reflected by 2-thiobarbituric acid, cytochrome oxidase or manganese superoxide dismutase levels [59]. It has also been shown to inhibit cell death, particularly caspase-3 activation and subsequent apoptosis [60]. One of the first noted beneficial effects of progesterone on secondary injury was with respect to oedema. Administration of progesterone after cortical contusion brain injury attenuated oedema in both female and male rats [61], with this effect being independent of oestrogen. Moreover, the reduction of oedema was still apparent even when progesterone treatment was delayed for ≤ 24 h after injury [62]. The observation has since been confirmed in an alternative model of injury, the bilateral medial frontal cortex injury [63] as well as in ischaemic injury [45]. The underlying mechanisms through which progesterone may reduce oedema have not been fully elucidated. However, several possible mechanisms have been proposed including the ability of the hormone to inhibit active ion uptake through Na+/K+-ATPase, to inhibit vessel growth associated with leaky BBB function after TBI, to modulate levels of vasopressin, and finally its actions as a free radical scavenger mediating lipid peroxidation [48]. Finally, there are systemic benefits of progesterone administration on soft tissue injury that must be considered. Subcutaneous progesterone administration following trauma– haemorrhage by midline laparotomy has been shown to ameliorate the pro-inflammatory response and reduce hepatocellular injury in ovariectomised female rats [64]. Related studies using the same model demonstrated that progesterone attenuated the cardiovascular depression evident in ovariectomised female rats and significantly improved cardiac output, heart performance and increased circulating blood volume [65]. The combined systemic effects of progesterone may be particularly beneficial in TBI victims with multi-system trauma, a common occurrence in motor vehicle accidents, and frequently unaccounted for in clinical trails. The multifactorial nature of progesterone has seen it incorporated into several clinical trials investigating disorders of the CNS including TBI (Progesterone for Traumatic Brain Injury: Experimental Clinical Treatment [ProTECT]), epilepsy (Progesterone Versus Placebo Therapy in Women with Epilepsy), and several in mood/depression (The effects of Hormones on Postpartum Mood Disorders; The effects of Reproductive Hormones on Brain Function; Combined Hormone Replacement in Menstrually Related Mood Disorders). A number of these trials are due to be completed later in 2004. 3.2 Dexanabinol Dexanabinol (HU-211) is a non-psychotropic analogue of tetrahydrocannabinol developed by the Hebrew University




of Jerusalem and subsequently licensed to Pharmos [66]. The compound, which does not bind to the cannabinoid receptor, has been shown to confer significant neuroprotection in animal models of severe closed head injury, hypoxaemia/ ischaemia, neurotoxin exposure and nerve crush injury [67-69]. It is thought that the compound has a number of properties that may account for these neuroprotective effects. The molecule readily crosses the BBB and combines NMDA-blocking activity, free radical scavenging and antiinflammatory properties. It weakly blocks NMDA receptors by interacting with a site close to but distinct from that of uncompetitive NMDA antagonists [70]. Accordingly, it is able to provide the therapeutic benefits of uncompetitive NMDA-receptor antagonists without the adverse psychotropic effects associated with this class of compounds [67]. By blocking the NMDA receptor, it attenuates calcium influx and thus reduces the likelihood of calcium-triggered autodestruction. This reduction in calcium entry would also contribute to the antioxidant properties of the compound, although not completely account for them as the antioxidant potential of dexanabinol is more pronounced than that of MK-801 [66,71]. What contributes to this additional antioxidant effect is unclear but it is likely to involve direct scavenging of free radicals or the ability to increase endogenous antioxidant ability. Whatever the antioxidant mechanism, it is known that dexanabinol protects cultured neurons from the toxic effects of ROS [72]. Finally, its ability to inhibit TNF-α synthesis and other inflammatory cytokines confirms its anti-inflammatory potential, which has now been demonstrated both in vitro and in vivo [18]. This anti-inflammatory effect would contribute to the observed attenuation of BBB permeability after injury, with a consequent reduction in oedema formation. By having properties that attenuate three of the major known secondary injury factors in TBI, it is not surprising that dexanabinol was one of the first multifactorial drugs that was entered into clinical trials. Phase II clinical trials of dexanabinol concluded that it was safe and well-tolerated by TBI patients, with few adverse consequences. The long half-life of the drug meant that a single intravenous dose was all that was required. As predicted, dexanabinol did not produce psychotropic effects that have prematurely curtailed some of the clinical trails with other such compounds. Interestingly, intracranial pressure was better controlled in dexanabinol-treated patients than in the control group [73], supporting a pronounced effect on oedema development. This is consistent with the preclinical tests that were generally performed in severe TBI containing an ischaemic component. The effects of the compound on mild-to-moderate TBI remain to be elucidated. Phase III trials have commenced (Efficacy Assessment of Dexanabinol Treatment of TBI Victims) with ∼ 40 medical centres in Europe and 29 in the US expected to participate. With 1000 patients targeted for enrolment, results are expected towards the end of 2004.

3.3 Dexamethasone While not considered a multifactorial drug, interest in the glucocorticoid dexamethasone has continued despite conflicting results. This is somewhat puzzling given that there have been several failed clinical trials in both head injury and spinal cord injury, and that dexamethasone is a less effective inhibitor of free radical-induced changes than methylprednisolone [74]. Nonetheless, the use of glucocorticoid therapy after TBI has persisted to the current day, perhaps in part because some success has been achieved with glucocorticoids in traumatic spinal cord injury, and in the absence of an effective alternative in TBI, hope for any beneficial effect in TBI persists. Interest in dexamethasone began in 1961 when Galicich and French [75] reported a rapid and significant improvement in response to dexamethasone in 28 of 34 people with cerebral oedema either postoperatively or due to the presence of a brain tumour. Subsequent clinical trials conducted two decades later failed to show any significant positive effect of dexamethasone on intracranial pressure and neurological outcome following TBI. Indeed, these studies concluded that dexamethasone had no effect on morbidity and mortality following severe head injury [76,77]. Another clinical study in the early 1990s administered ultra-high intravenous dexamethasone in patients with moderate and severe head injury and also found no significant difference between the treatment group and controls [78]. However, experimental studies with dexamethasone and other glucocorticoids persisted, with a focus on their potential role in attenuating oedema. In mice subjected to moderate-to-severe blunt head injury, dexamethasone was of benefit in decreasing oedema and improving neurological recovery [79], presumably by reducing lipid peroxidation [19]. As with the clinical data, however, contradictory reports also appeared indicating that the compound does not improve outcome and may even have deleterious effects [80]. However, Barks and colleagues [81] demonstrated in a model of hypoxia–ischaemia that the neuroprotective effects of dexamethasone were dose and time dependent, with even low doses being beneficial to outcome. This was contrary to the high-dose philosophy that was pursued at the time in an effort to prevent oxidative damage, and perhaps reflects the fact that the antioxidant and the glucocorticoid effects of these compounds are independent and can be more appropriately capitalised upon using different compounds [19]. The glucocorticoid effects of dexamethasone included anti-inflammatory properties that resulted in a decrease in monocyte/ macrophage infiltration and major histocompatibility complex (MHC) molecule expression following contusion injury [82]. Dexamethasone also reduces neuropeptide expression [83], which may attenuate the neurogenic component of inflammation that has been reported after TBI [84]. In addition, dexamethasone has been reported to prevent the induction of both nitric oxide synthase and the related NADPH diaphorase [85]. Finally, lower doses of dexamethasone have recently been shown to reduce K+-evoked glutamate release in hippocampal slices, whereas higher doses exacerbated release [86]. These


1267



properties are, therefore, consistent with the observation that lower doses of dexamethasone can inhibit inflammation and prevent oedema formation following experimental TBI [82,87], whereas higher doses of dexamethasone may have no significant protective effect [88]. Thus, the conflicting data for glucocorticoids in TBI [89] highlight the need for a thorough understanding of dose/response effects prior to the commencement of clinical trials. While the evidence-based Guidelines for the Management of Severe Traumatic Brain Injury state “The use of steroids is not recommended for improving outcome or reducing ICP in patients with severe head injury” [89], dexamethasone is currently undergoing a multinational Phase III clinical trial in TBI that has enrolled > 10,000 patients to date (Corticosteroid Randomisation After Significant Head Injury [CRASH]). 3.4 Other compounds:

updates

3.4.1 Magnesium

Magnesium is perhaps one of the most intriguing of all the putative therapies currently under investigation because it is a readily available endogenous ion with modulatory effects on a number of secondary injury processes. The role of magnesium in TBI, and its potential as a neuroprotective therapy, has been previously discussed in detail in an earlier issue of this journal [9]. Briefly, brain magnesium decline is a ubiquitous feature of TBI and is associated with the development of neurological deficits. Experimentally, parenteral administration of magnesium ≤ 12 h post-trauma restores brain magnesium homeostasis and profoundly improves both motor and cognitive outcome. While the mechanism of action is unclear, magnesium has been shown to attenuate a variety of secondary injury factors including brain oedema, cerebral vasospasms, glutamate excitotoxicity, calcium-mediated events, lipid peroxidation, MPT and apoptosis [9]. It is thus a truly multifactorial pharmacological intervention with a proven safety record in previous clinical studies. Over the last 2 years a number of additional studies have been published further supporting the potential for magnesium as a neuroprotective compound in TBI, and providing additional insight into its mechanism of action. Similar to declines in free magnesium observed in brain and blood after TBI in earlier studies [17,90,91], an injury-dependent decline in serum ionised magnesium has been noted in clinical TBI [92] and anuerysmal subarachnoid haemorrhage [93]. Interestingly, bolus administration of magnesium was insufficient to restore and maintain serum free-magnesium concentrations in the latter condition [94], consistent with the finding in experimental TBI that a single bolus of magnesium does not maintain brain free-magnesium concentration in the presence of cerebral haemorrhage [95]. These findings suggest that a continuous infusion of magnesium is required to maintain therapeutic levels when haemorrhage is present [94]. The caveat is that any ongoing haemorrhage could potentially be exacerbated by the vasodilator effects of magnesium. The importance of restoring low serum magnesium levels to 1268

normal levels was highlighted in a study examining the neurological events associated with low serum magnesium in patients with advanced atherosclerosis [96,97]. Low serum magnesium levels are associated with a 3.29-fold increased risk of adverse neurological events, including stroke. Protective effects of magnesium administration on functional outcome have now also been reported in acoustic trauma [98] and traumatic cortical lesions [99], although results in hypoxia/ischaemia have been mixed [100-102]. The fact that preinjury treatment is protective in this condition while postinjury treatment is less effective suggests that intracellular energy depletion may attenuate the neuroprotective effects of magnesium administration, perhaps by restricting the action of the ion to extracellular secondary injury factors. For example, at the intracellular level, magnesium has been shown to reduce apoptosis by both decreasing the expression of apoptosis inducing p53-related factors [103] and by decreasing caspase-3 expression [101]. However, this effect on apoptosis is reduced in the presence of ATP depletion [100,101]. Similarly, magnesium decreases mitochondrial ultrastructural damage and improves respiratory function after TBI [104] but may exacerbate the rate of ATP depletion and acidosis in hypoxia/ ischaemia [102]. On the other hand, extracellular events such as the ability of extracellular magnesium to block the glutamate NMDA channel or Na+ influx [105,106] would be independent of the intracellular ATP concentration. Thus, the efficacy of magnesium as a neuroprotective agent may depend somewhat on the energy status of the condition under study. Clinical trials of magnesium in TBI are now in Phase III (Magnesium Sulfate for Neuroprotection after Brain Trauma), while trials have recently commenced examining the efficacy of magnesium treatment in paediatric TBI and in neurocognitive function following coronary artery bypass surgery (Perioperative Interventional Neuroprotection Trial [POINT]). 3.4.2 Cyclosporin A

Another potentially multifactorial compound that has been proposed as a neuroprotective agent following TBI is CyA. CyA is a short polypeptide that has been shown to exert neuroprotective and neurotrophic effects in TBI, sciatic nerve injury and focal and global ischaemia [107]. Recent studies have now shown that in addition to the well-described improvement in motor outcome observed with CyA administration after TBI [108], the compound also improves learning and memory performance [109]. This effect was dose dependent and was correlated to an improvement in brain oxygen consumption after trauma. The preserved brain oxygen consumption after TBI was thought to reflect improved mitochondrial function, which is consistent with the ability of CyA to inhibit the MPT [23,110,111], and in so doing, prevent mitochondrial swelling and improve energy recovery [112]. Although CyA also inhibits calcineurin [113], it is thought that the compound’s effects on mitochondrial activity are pivotal to its neuroprotective action [114]. Indeed, when compared with FK-506, a related immunosuppressant that inhibits



Table 1. Beneficial effects of progesterone, dexanabinol and dexamethasone on various secondary injury factors and injury outcome. Compound

Injury factors


Excitotoxicity

Oxidative Stress

Mitochondrial events Oedema

Functional outcome

Lesion volume

Progesterone

[38,49]

[58,59]

[44]

[45,61-63]

[46,52,53,57]

[46,52,55,56,60]

Dexanabinol

[70]

[66,71,72]

-

[68,69]

[69]

[68,69,72]

Dexamethasone

[86]

[19]

-

[79,82,87]

[81]

[79]

calcineurin as effectively as CyA [113,115], CyA was superior in protecting against ischaemic damage [114-116] and hypoglycaemic brain injury [117]. Thus, the superior ability of CyA to inhibit the MPT may be a major factor in its neuroprotective ability [116]. This ability to inhibit the MPT also explains the recent observation that CyA blocks free radical production after trauma [118]. Recent reports suggest that the compound also completely inhibits excitotoxin-induced seizures, and in so doing, virtually eliminated neuronal cell death [119]. There is also a dose-dependent inhibition of traumatic axonal injury after TBI [120], which is consistent with the observation that CyA reduces the number of disconnected and dysfunctional axons following impact acceleration TBI in rodents [121-124]. Finally, dose/response studies have now established that the drug can be administered intravenously as opposed to intrathecally, thereby avoiding any exacerbation of oedema [125]. Phase III clinical trials of CyA treatment of TBI are continuing as a collaboration between the University of Florida and the Medical College of Virginia.

effects on a variety of physiological and biochemical processes. Being multifactorial in nature, these compounds are more likely to be successful in such a heterogeneous type of injury. This review has discussed three of these compounds that are currently in an advanced phase of clinical TBI trials (progesterone, dexanabinol, progesterone), and has briefly summarised the progress made in another two that have been previously reviewed (magnesium, CyA) [9]. It should be noted that the three drugs emphasised in this review all attenuate multiple injury factors (summarised in Table 1) and have had their neuroprotective potential illustrated in different models of brain injury. All three have been tested in a diversity of experimental injury models incorporating aspects of trauma, ischaemia, hypoxia and haemorrhage. Finally, all three have demonstrable benefit on neuronal survival, lesion volume and, perhaps most importantly, functional outcome. It is this multifactorial approach that will lead to the development of a successful therapy for TBI. 5. Expert

opinion

4. Conclusion

TBI is a heterogeneous form of brain injury with possible contributions from trauma, ischaemia, haemorrhage and hypoxia. With such heterogeneity, there is a large diversity of secondary injury factors whose identification is essential if a successful pharmacological intervention is to be developed. Unfortunately, the identification of these secondary injury factors is complicated by the fact that many of them interact resulting in cell death, and not all of them occur at the same point in time relative to the traumatic event. For example, while excitatory amino acids are released very early after the traumatic event, apoptosis is known to occur at delayed time points [4]. Moreover, attenuating one injury factor is known to exacerbate another. Adding further complexity to the development of appropriate therapies after TBI is the fact that not all injury factors are present at all levels of injury. For example, energy deficits are common at more severe levels of injury that incorporate hypoxic and ischaemic events, whereas such energy deficits do not occur in mild-to-moderate injury. Accordingly, a therapeutic intervention that successfully targets a particular injury factor at one injury level may not be successful at another. Attention has, therefore, turned to compounds that are mutifactorial in nature, and thus have potentially beneficial

A large number of pharmacological substances have been experimentally tested for their ability to ameliorate secondary injury after TBI, or are currently under clinical trial. To date, however, despite encouraging preclinical and experimental results, almost all of those studies that have gone on to Phase II and III clinical trials have failed to show any consistent improvement in outcome for TBI patients. While a number of reasons can be shown to contribute to these failures, it is the heterogeneity and multifactorial nature of TBI that will present the greatest challenge. Accordingly, there has been a recent shift away from the selective inhibitors of secondary injury factors to therapies that simultaneously target a multitude of secondary factors. Accordingly, these interventions protect against a broader spectrum of secondary injury factors irrespective of their temporal contribution to tissue injury. Despite the success of dexamethasone and dexanabinol in some preclinical studies, there remain lingering doubts as to their potential effectiveness in clinical TBI. Dexamethasone has been unsuccessfully used in several earlier clinical trials in brain and spinal cord injury, and a limited number of laboratories have since examined its use in multiple models of traumatic injury. The commencement of Phase III clinical trials, therefore, seems premature and susceptible to limitations


1269



imposed by the controversy surrounding its dose, timing of therapy and its potential efficacy across a broad spectrum of injury. Dexanabinol has been extensively used in studies of experimental ischaemia, and a limited number of experimental studies of severe trauma likely to include an ischaemic component. It is, therefore, unclear whether the drug will be efficacious in the mild-to-moderate TBI that makes up most clinical trauma cases. Nonetheless, it is hopeful that the drug may be of benefit to severely injured patients. What is encouraging about dexanabinol is its ability to significantly reduce inflammation. It shares this property with the other multifactorial drugs progesterone, magnesium and CyA, which have been shown to be effective across a wide range of injury models and severities, have been tested in different laboratories and have therapeutic windows that extend to hours after injury. All of these compounds are potent anti-inflammatories that significantly reduce posttraumatic BBB dysfunction and oedema formation. Whether this emphasis on anti-inflammatory effects reflects

1.

FINFER SR, COHEN J: Severe traumatic brain injury. Resuscitation (2001) 48:77-90.

2.

KRAUS JF: Epidemiology of head injury. In: Head Injury (3rd Edition). PR Cooper (Ed.), Williams and Wilkins, Baltimore, USA (1993):1-25.

3.

MENDELOW AD, CRAWFORD PJ: Primary and secondary brain injury. In: Head Injury: Pathophysiology and Management of Severe Closed Injury.. Chapman and Hall, London (1997):72-88.

4.

McINTOSH TK, SMITH DH, MEANEY DF, KOTAPKA MJ, GENNARELLI TA, GRAHAM DI: Neuropathological sequelae of traumatic brain injury: relationship to neurochemical and biomechanical mechanisms. Lab. Invest. (1996) 74:315-342.

5.

MAAS AIR: Neuroprotective agents in traumatic brain injury. Expert Opin. Investig. Drugs (2001) 10:753-767.

6.

NARAYAN RK, MICHEL ME, ANSELL B et al.: Clinical trials in head injury. J. Neurotrauma (2002) 19:503-571.

7.

••

TOLIAS CM, BULLOCK RM: Critical appraisal of neuroprotection trials in head injury: what have we learned? NeuroRx (2004) 1:71-79. Excellent critical review of recent clinical trials in head injury examining the

1270

Acknowledgement Robert Vink is supported, in part, by the Australian National Health and Medical Research Council.

limitations inherent in translational research.

Bibliography Papers of special note have been highlighted as either of interest (•) or of considerable interest (••) to readers.

the central role that inflammation plays in the determination of outcome following TBI, or whether it simply reflects that contemporary patient management has a focus on oedema, will be clear upon completion of the trials. In any case, the emphasis is clearly upon compounds that have beneficial effects on multiple injury factors, and in particular, antiinflammatory properties. Despite the fact that no drug has been successful in clinical TBI trials so far, there are some promising candidates that are currently undergoing such trials. Whereas previous failures in TBI clinical trials led to scepticism that any drug could offer therapeutic benefits to victims of brain trauma, the new multipotential drug therapies bring with them a renewed optimism in the field.

8.

9.

FADEN AI: Neuroprotection and traumatic brain injury: Theoretical option or realistic proposition? Curr. Opin. Neurol. (2002) 15:707-712. VINK R, NIMMO AJ: Novel therapies in development for the treatment of traumatic brain injury. Expert Opin. Invest. Drugs (2002) 11:1-12.

10.

ROTHMAN SM, OLNEY JW: Excitoxicity and the NMDA receptor. Trends Neurol. Sci. (1987) 10:299-302.

11.

FADEN AI, DEMEDIUK P, PANTER SS, VINK R: Excitatory amino acids, N-methyl-D-asparate receptors and traumatic brain injury. Science (1989) 244:798-800.

12.

13.

•

14.

STRECKER GL: Blockade of NMDA activated channels by magnesium in the immature rat hippocampus. J. Neurophysiol. (1994) 4:1538-1545. WILLIS C, LYBRAND S, BELLAMY N: Excitatory amino acid inhibitors for traumatic brain injury (Cochrane Review). In: The Cochrane Library, (Volume 2). John Wiley & Sons, Ltd, Chichester, UK (2004) Very good review of previous and ongoing clinical trials examining the efficacy of excitatory amino acid antagonists in TBI. LEA PMT, FADEN AI: Traumatic brain injury: developmental differences in glutamate receptor response and the impact on treatment. Ment. Retard. Dev. Disabil. Res. Rev. (2001) 7:235-248.


15.

IKEDA Y, LONG DM: The molecular basis of brain injury and brain edema. The role of oxygen free radicals. Neurosurgery (1990) 27:1-11.

16.

LEWEN A, MATZ P, CHAN PH: Free radical pathways in CNS injury. J. Neurotrauma (2000) 17:871-890.

17.

CERNAK I, SAVIC VJ, KOTUR J, PROKIC V, VELJOVIC M, GRBOVIC D: Characterization of plasma magnesium concentration and oxidative stress following graded traumatic brain injury in humans. J. Neurotrauma (2000) 17:53-68.

18.

SHOHAMI E, BEITYANNAI E, HOROWITZ M, KOHEN R: Oxidative stress in closed-head injury – brain antioxidant capacity as an indicator of functional outcome. J. Cereb. Blood Flow Metab. (1997) 17:1007-1019.

19.

HALL ED, YONKERS PA, ANDRUS PK, COX JW, ANDERSON DK: Biochemistry and pharmacology of lipid antioxidants in acute brain and spinal cord injury. J. Neurotrauma (1992) 9:S425-S442.

20.

MARKLUND N, CLAUSEN F, LEWANDER T, HILLERED L: Monitoring of reactive oxygen species production after traumatic brain injury in rats with microdialysis and the 4-hydroxybenzoic acid trapping method. J. Neurotrauma (2001) 18:1217-1223.

21.

HAEBERLEIN SL: Mitochondrial function in apoptotic neuronal cell death. Neurochem. Res. (2004) 29:521-530.


22.


23.

24.

•

25.

26.

27.

•

28.

29.

30.

31.

PETRONILLI V, PENZO D, SCORRANO L, BERNARDI P, DI LISA F: The mitochondrial permeability transition, release of cytochrome c and cell death. Correlation with the duration of pore openings in situ. J. Biol. Chem. (2001) 276:12030-12034. HANSSON MJ, MANSSON R, MATTIASSON G et al.: Brain-derived respiring mitochondria exhibit homogeneous, complete and cyclosporinsensitive permeability transition. J. Neurochem. (2004) 89:715-729. FISKUM G: Mitochondrial participation in ischemic and traumatic neural cell death. J. Neurotrauma (2000) 17:843-855. Detailed review of how mitochondria may be involved in the development of neuronal cell death. LEMASTERS JL, NIEMINEN A-L, QIAN T et al.: The mitochondrial permeability transition in cell death: a common mechanism in necrosis, apoptosis and autophagy. Biochim. Biophys. Acta (1998) 1366:177-196. FISKUM G: Mechanisms of neuronal death and neuroprotection. J. Neurosurg. Anesthesiol. (2004) 16:108-110. MATTIASSON G, FRIBERG H, HANSSON M, ELMER E, WIELOCH T: Flow cytometric analysis of mitochondria from CA1 and CA3 regions of rat hippocampus reveals differences in permeability transition pore activation. J. Neurochem. (2003) 87:532-544. Examines the role of the MPT and how it may explain selective vulnerability of different neurons. XIONG Y, GU Q, PETERSON PL, MUIZELAAR JP, LEE CP: Mitochondrial dysfunction and calcium pertubation induced by traumatic brain injury. J. Neurotrauma (1997) 14:23-34. CLAUSEN T, ZAUNER A, LEVASSEUR JE, RICE AC, BULLOCK R: Induced mitochondrial failure in the feline brain: implications for understanding acute post-traumatic metabolic events. Brain Res. (2001) 908:35-48. KLATZO I: Brain Edema. In: Central Nervous System Trauma Research Status Report. Gl Odom (Ed.), NIH, Bethesda, USA (1979):110-112. KIMELBERG HK: Current concepts of brain edema. Review of laboratory investigations. J. Neurosurg. (1995) 83:1051-1059.

••

Excellent review summarising brain oedema development with an emphasis on experimental investigations.

43.

GRUBER CJ, HUBER JC: Differential effects of progestins on the brain. Maturitas (2003) 46(Suppl.):S71-S75.

32.

GRAHAM DI: Neuropathology of head injury. In: Neurotrauma. RK Narayan, JE Wilberger Jr, JT Povlishock (Eds), McGraw-Hill, New York, USA (1996).

44.

33.

BASKAYA MK, DOGAN A, RAO AM, DEMPSEY RJ: Neuroprotective effects of citicoline on brain edema and blood–brain barrier breakdown after traumatic brain injury. J. Neurosurg. (2000) 92:448-452.

GONZALEZ DENISELLE MC, COSTA JJL, GONZALEZ SL et al.: Basis of progesterone protection in spinal cord neurodegeneration. J. Steroid Biochem. Mol. Biol. (2002) 83:199-209.

45.

KUMON Y, SOON CK, TOMPKINS P, STEVENS AL, SAKAKI S, LOFTUS M: Neuroprotective effect of postischemic administration of progesterone in spontaneously hypertensive rats with focal cerebral ischemia. J. Neurosurg. (2000) 92:848-852.

46.

JIANG N, CHOPP M, STEIN DG, FEIT H: Progesterone is neuroprotective after transient middle cerebral occlusion in male rats. Brain Res. (1996) 735:101-107.

47.

VONGHER J, FRYE C: Progesterone in conjunction with estradiol has neuroprotective effects in an animal model of neurodegeneration. Pharmacol. Biochem. Behav. (1999) 64:777-785.

48.

ROOF RL, HALL ED: Gender differences in acute CNS trauma and stroke: neuroprotective effects of estrogen and progesterone. J. Neurotrauma (2000) 17:367-388. Comprehensive review of the roles of gonadal hormones in brain injury, with an emphasis on oestrogen.

34.

MARMAROU A: Pathophysiology of traumatic brain edema: current concepts. Acta Neurochir. Suppl. (2003) 86:7-10.

35.

STOVER JF, UNTERBERG AW: Increased cerebrospinal fluid glutamate and taurine concentrations are associated with traumatic brain edema formation in rats. Brain Res. (2000) 875:51-55.

36.

37.

38.

39.

REILLY PL: Management of intracranial pressure and cerebral perfusion. In: Head Injury: Pathophysiology and Management of Severe Closed Head Injury. PL Reilly, R Bullock (Eds), Chapman and Hall, Sydney, Australia (1997):385-406. RUPPRECHT R, HOLSBOER F: Neuroactive steroids: mechanisms of action and neuropsychopharmacological perspectives. Trends Neurosci. (1999) 22:410-416. SMITH SS: Progesterone administration attenuates excitatory amino acid responses of cerebellar Purkinje Cells. Neuroscience (1991) 42:309-320. KORNEYEV A, COSTA E: Allopregnanolone (THP) mediates anesthetic effects of progesterone in rat brain. Horm. Behav. (1996) 30:37-43.

40.

RODGERS RJ, JOHNSON NJ: Behaviorally selective effects of neuroactive steroids on plus-maze anxiety in mice. Pharmacol. Biochem. Behav. (1998) 59:221-232.

41.

FRYE CA, WALF AA, RHODES ME, HARNEY JP: Progesterone enhances motor, anxiolytic, analgesic, and antidepressive behavior of wild-type mice, but not those deficient in Type 1 5α-reductase. Brain Res. (2004) 1004:116-124.

42.

KOENIG HL, GONG WH, PELISSIER P: Role of progesterone in peripheral nerve repair. Rev. Reprod. (2000) 5:189-199.


••

49.

OGATA T, NAKAMURA Y, TSUJI K, SHIBATA T, KATAOKA K: Steroid hormones protect spinal cord neurons from glutamate toxicity. Neuroscience (1993) 55:445-449.

50.

PAUL SM, PURDY RH: Neuroactive steroids. FASEB J. (1992) 6:2311-2322.

51.

LABOMBARDA F, GONZALEZ SL, GONZALEZ DM, GUENNOUN R, SCHUMACHER M,DE NICOLA AF: Cellular basis for progesterone neuroprotection in the injured spinal cord. J. Neurotrauma (2002) 19:343-355.

52.

THOMAS AJ, NOCKLES RP, HIU QP, SHAFFREY CI, CHOPP M: Progesterone is neuroportective after acute experimental spinal cord trauma in rats. Spine (1999) 24:2134-2138.

53.

GONZALEZ-VIDAL MD, CERVERA-GAVIRIA M et al.: Progesterone: protective effects on the cat hippocampal neuronal damage due to acute global cerebral ischemia. Arch. Med. Res. (1998) 28:117-124.

1271


54.

ROOF RL, ZHANG Q, GLASIER MM, STEIN DG: Gender-specific impairment on Morris water maze task after entorhinal cortex lesion. Behav. Brain Res. (1993) 57:47-51.

64.

KUEBLER JF, YOKOYAMA Y, JARRAR D et al.: Administration of progesterone after trauma and hemorrhagic shock prevents hepatocellular injury. Arch. Surg. (2003) 138:727-734.

55.

ROOF RL, DUVDEVANI R, BRASWELL L, STEIN DG: Progesterone facilitates cognitive recovery and reduces secondary neuronal loss caused by cortical contusion injury in male rats. Exp. Neurol. (1994) 129:64-69.

65.

KUEBLER JF, JARRAR D, BLAND KI, RUE L 3rd, WANG P, CHAUDRY IH: Progesterone administration after trauma and hemorrhagic shock improves cardiovascular responses. Crit. Care Med. (2003) 31:1786-1793.

SHEAR DA, GALANI R, HOFFMAN SW, STEIN DG: Progesterone protects against necrotic damage and behavioral abnomalities caused by traumatic brain injury. Exp. Neurol. (2002) 178:59-67.

66.

POP E: Dexanabinol Pharmos. Curr. Opin. Invest. Drugs. (2000) 1:494-503.

67.


56.

57.

HE J, HOFFMAN SW, STEIN DG: Allopregnanolone, a progesterone metabolite, enhances behavioral recovery and decreases neuronal loss after traumatic brain injury. Restor. Neurol. Neurosci. (2004) 22:19-31.

58.

ROOF RL, HOFFMAN SW, STEIN DG: Progesterone protects against lipid peroxidation following traumatic brain injury in rats. Mol. Chem. Neuropathol. (1997) 31:1-11.

59.

GOODMAN Y, BRUCE AJ, CHENG B, MATTSON MP: Estrogens attenuate and corticosterone exacerbates excitotoxicity, oxidative injury, and amyloid beta-peptide toxicity in hippocampal neurones. J. Neurochem. (1996) 66:1836-1844.

60.

• 61.

62.

63.

DJEBAILI M, HOFFMAN SW, STEIN DG: Allopregnanolone and progesterone decrease cell death and cognitive deficits after a contusion of the rat pre-frontal cortex. Neuroscience (2004) 123:349-359. Description of progesterone’s effects on apoptosis following TBI. ROOF RL, DUVDEVANI R, STEIN DG: Progesterone treatment attenuates brain edema following contusion injury in male and female rats. Rest. Neurol. Neurosci. (1992) 4:425-427. ROOF RL, DUVDEVANI R, HEYBURN JW, STEIN DG: Progesterone rapidly decreases brain edema: treatment delayed up to 24 hours is still effective. Exp. Neurol. (1996) 138:246-251. WRIGHT DW, BAUER ME, HOFFMAN SW, STEIN DG: Serum progesterone levels correlate with decreased cerebral edema after traumatic brain injury in male rats. J. Neurotrauma (2001) 18:901-909.

1272

• 68.

69.

70.

71.

72.

73.

•

74.

A nonglucocorticoid steroid analog of methylprednisolone duplicates its high-dose pharmacology in models of central nervous system trauma and neuronal membrane damage. J. Pharmacol. Exp. Ther. (1987) 242:137-142. 75.

GALICICH JH, FRENCH LA: Use of dexamethasone in the treatment of cerebral edema resulting from brain tumours and brain surgery. Am. Pract. Dig. Treat. (1961) 12:169-174.

76.

DARLINGTON CL: Dexanabinol: a novel cannabinoid with neuroprotective properties. IDrugs (2003) 6:976-979. Comprehensive review of the neuroprotective properties of dexanabinol.

COOPER PR, MOODY S, CLARK WK et al.: Dexamethasone and severe head injury. A prospective double-blind study. J. Neurosurg. (1979) 51:307-316.

77.

SHOHAMI E, NOVIKOV M, MECHOULAM R: A nonpsychotropic cannabinoid, HU-211, has cerebrovascular effects after closed head injury in the rat. J. Neurotrauma (1993) 10:109-119.

DEARDEN NM, GIBSON JS, MCDOWELL DG, GIBSON RM, CAMERON MM: Effect of high-dose dexamethasone on outcome from severe head injury. J. Neurosurg. (1986) 64:81-88.

78.

GAAB MR, TROST HA, ALCANTARA A et al.: ‘Ultrahigh’ dexamethasone in acute brain injury. Results from a prospective randomized double-blind multicenter trial (GUDHIS). German Ultrahigh Dexamethasone Head Injury Study Group. Zentralbl. Neurochir. (1994) 55:135-143.

79.

HALL ED: High-dose glucocorticoid treatment improves neurological recovery in head-injured mice. J. Neurosurg. (1985) 62:882-887.

80.

MAINIG G, DEISENROTH K: Doseresponse relation for dexamethasone in cold lesion-induced brain edema in rats. Adv. Neurol. (1990) 52:295-300.

81.

BARKS JD, POST M, TOUR UI: Dexamethasone prevents hypoxic-ischemic brain damage in the neonatal rat. Pediatr. Res. (1991) 29:558-563. Report illustrating that the neuroprotective effects of dexamethasone are both dose and time dependent.

BELAYEV L, BUSTO R, ZHAO W, GINSBERG MD: HU-211, a novel noncompetitive N-methyl-D-aspartate antagonist, improves neurological deficit and reduces infarct volume after reversible focal cerebral ischemia in the rat. Stroke (1995) 26:2313-2319. FEIGENBAUM JJ, BERGMANN F, RICHMOND SA et al.: Nonpsychotropic cannabinoid acts as a functional N-methylD-aspartate receptor blocker. Proc. Natl. Acad. Sci. USA (1989) 86:9584-9587. ESHHAR N, STRIEM S, KOHEN R, TIROSH O, BIEGON A: Neuroprotective and antioxidant activities of HU-211, a novel NMDA receptor antagonist. Eur. J. Pharmacol. (1995) 283:19-29. MECHOULAM R, PANIKASHVILI D, SHOHAMI E: Cannabinoids and brain injury: therapeutic implications. Trends Mol. Med. (2002) 8:58-61. KNOLLER N, LEVI L, SHOHAMI E et al.: Dexanabinol (HU-211) in the treatment of severe closed head injury: a randomised, placebo-controlled, Phase II clinical trial. Crit. Care Med. (2002) 30:548-554. The first demonstration of the neuroprotective potential of dexanabinol in a Phase II trial, illustrating the beneficial effects of the compound on ICP control. HALL ED, McCALL JM, CHASE RL, YONKERS PA, BRAUGHLER JM:


•

82.

HOLMIN S, MATHIESEN T: Dexamethasone and colchicine reduce inflammation and delayed oedema following experimental brain contusion. Acta. Neurochir. (1996) 138:418-424.

83.

ZHANG Y, TANG D, CHEN L, ZHANG Y, XU L, ZENG J: Dexamethasone regulates the neuropeptide expression in rabbit brain injury induced by endotoxin. Zhonghua Yi Xue Za Zhi (2002) 82:1641-1644.

84.

NIMMO AJ, CERNAK I, HEATH DL, HU X, BENNETT CJ, VINK R: Neurogenic inflammation is associated with development of edema and functional


deficits following traumatic brain injury in rats. Neuropeptides (2004) 38:40-47. 85.


86.

•

REGIDOR J, MONTESDEOCA J, RAMERIZ-GONZALEZ JA et al.: Antiinflammatory drugs suppress injury induced NADPH-d activity in CA1 pyramidal neurones. Neuroreport (1994) 8:1766-1768. IOANNOU N, LIAPI C, SEKERIS CE, PALAIOLOGOS G: Effects of dexamethasone on (K) +-evoked glutamate release from rat hippocampal slices. Neurochem. Res. (2003) 28:875-881. This manuscript demonstrates that lower doses of dexamethasone were protective whereas higher doses exacerbated injury.

87.

HORTOBAGYI T, HORTOBAGYI S, GORLACH C et al.: A novel brain trauma model in the mouse: effects of dexamethasone treatment. Plugers Arch. (2000) 441:409-415.

88.

VACHON P, MOREAU JP: Low doses of dexamethasone decrease brain water content of collagenase-induced cerebral hematoma. Can. J. Vet. Res. (2003) 68:157-159.

89.

90.

91.

92.

93.

94.

ALDERSON P, ROBERTS I: Corticosteroids for acute traumatic brain injury (Cochrane Review). In: The Cochrane Library, (Volume 2), John Wiley & Sons Ltd., Chichester, UK (2004) CD000196. HEATH DL, VINK R: Traumatic brain axonal injury produces sustained decline in intracellular free magnesium concentration. Brain Res. (1996) 738:150-153. HEATH DL, VINK R: Blood free magnesium concentration declines following graded experimental traumatic brain injury. Scand. J. Clin. Lab. Invest. (1998) 58:161-166. KAHRAMAN S, OZGURTAS T, KAYALI H, ATABEY C, KUTLUAY T, TIMURKAYNAK E: Monitoring of serum ionized magnesium in neurosurgical intensive care unit: preliminary results. Clin. Chem. Acta (2003) 334:211-215. VAN DEN BURGH WM, ALGRA A, VAN DER SPRENKEL JW, TULLENKEN CA, RINKEL GJ: Hypomagnesia after aneurysmal subarachnoid hemorrhage. Neurosurgery (2003) 52:276-281. VAN DEN BURGH WM, ALBRECHT KW, BERKELBACH VAN DER SPRENKEL JW, RINKEL GJ: Magnesium therapy after aneurysmal subarachnoid haemorrhage a dose-finding study for long term treatment. Acta. Neurochir. (2003) 145:195-199.

•

95.

96.

97.

98.

99.

This manuscript demonstrates that in the presence of cerebral haemorrhage a continuous infusion of magnesium is required over 14 days to maintain serum levels of the cation.

104. XU M, DAI W, DENG X: Effects on

HEATH DL, VINK R: Subdural hematoma following traumatic brain injury causes a secondary decline in brain free magnesium concentration. J. Neurotrauma (2001) 18:465-469.

105. SANG N, MENG Z: Blockade by

AMIGHI J, SABETI S, SCHLAGER O et al.: Low serum magnesium predicts neurological events in patients with advanced atherosclerosis. Stroke (2004) 35:22-27. ALTURA BM, KOSTELLOW AB, ZHANG A et al.: Expression of the nuclear factor-κ B and proto-oncogenes c-fos and cjun are induced by low extracellular Mg2+ in aortic and cerebral vascular smooth muscle cells: possible links to hypertension, atherogenesis and stroke. Am. J. Hypertension (2003) 16:701-707. HAUPT H, SCHEIBE F, MAZUREK B: Therapeutic efficacy of magnesium in acoustic trauma in guinea pigs. J. Otorhinolaryngol. Relat. Spec. (2003) 65:134-139. HOANE MR, KNOTTS AA, AKSTULEWICZ SL, AQUILANO M, MEANS LW: The behavioral effects of magnesium therapy on recovery of function following bilateral anterior medial cortex lesions in the rat. Brain Res. Bull. (2003) 15:105-114.

brain injury in rats. J. Neurotrauma (2004) 21:549-561. magnesium sulfate on brain mitochondrial respiratory function in rats after experimental traumatic brain injury. Chin. J. Traumatol. (2002) 5:361-364. magnesium of sodium currents in accutely isolated hippocampal CA1 neurons of rat. Brain Res. (2002) 18:218-221. 106. OKIYAMA K, SMITH DH,

GENNARELLI TA, SIMON RP, LEACH M, McINTOSH TK: The sodium channel blocker and glutamate release inhibitor BW1003C87 and magnesium attenuate regional cerebral edema following experimental brain injury in the rat. J. Neurochem. (1995) 64:802-809. 107. KAMINSKA B,

GAWEDA-WALERYCH K, ZAWADZKA M: Molecular mechanisms of neuroprotective action of immunosuppressants – facts and hypotheses. J. Cell Mol. Med. (2004) 8:45-48. 108. RIESS P, BAREYRE FM, SAATMAN KE

et al.: Effects of chronic, post-injury Cyclosporin A administration on motor and sensorimotor function following severe, experimental traumatic brain injury. Restor. Neurol. Neurosci. (2001) 18:1-8. 109. ALESSANDRI B, RICE AC,

100. TURKYILMAZ C, TURKYILMAZ Z,

ATALAY Y, SOYLEMEZOGLU F, CELASUN B: Magnesium pre-treatment reduces neuronal apoptosis in newborn rats in hypoxia-ischemia. Brain Res. (2002) 955:133-137. 101. TANG YN, ZHAO FL, YE HM:

Expression of caspase-3 mRNA in the hippocampus of seven-day-old hypoxicischemic rats and the mechanism of neural protection with magnesium sulfate. Zhonghua Yi Xue Za Zhi (2003) 41:212-214. 102. GEE II JB, CORBETT RJ, PERLMAN J,

LAPTOOK AR: The effects of systemic magnesium sulfate infusion on brain magnesium concentrations and energy state during hypoxia-ischemia in newborn miniswine. Pediatr. Res. (2004) 55:93-100. 103. LEE JS, HAN YM, YOO DO S et al.:

A molecular basis for the efficacy of magnesium treatment following traumatic


•

LEVASSEUR J, DEFORD M, HAMM RJ, BULLOCK RM: Cyclosporin A improves brain tissue oxygen consumption and learning/memory performance after lateral fluid percussion injury in rats. J. Neurotrauma (2002) 19:829-841. The first demonstration that CyA improves learning and memory performance following TBI.

110. SZABO I, ZORATTI M: The giant

channel of the inner mitochondrial membrane is inhibited by cyclosporin A. J. Biol. Chem. (1991) 266:3376-3379. 111. SULLIVAN PG, THOMPSON MB,

SCHEFF SW: Cyclosporin A attenuates acute mitochondrial dysfunction following traumatic brain injury. Exp. Neurol. (1999) 160:226-234. 112. NAKAI A, SHIBAZAKI Y, TANIUCHI Y,

MIYAKE H, OYA A, TAKESHITA T: Role of mitochondrial permeability transition in fetal brain damage in rats. Pediatr. Neurol. (2004) 30:247-253.

1273


113. KAMINSKA B,

GAWEDA-WALERYCH K, ZAWADZKA M: Molecular mechanisms of neuroprotective action of immunosuppressants-facts and hypotheses. J. Cell. Mol. Med. (2004) 8:45-58. 114. UCHINO H, ISHII N, SHIBASAKI F:


Calcineurin and cyclophilin D are differential targets of neuroprotection by immunosuppressants CsA and FK506 in ischemic brain damage. Acta Neurochir. Suppl. (2003) 86:105-111. 115. UCHINO H,

MINAMIKAWA-TACHINO R, KRISTIAN T et al.: Differential neuroprotection by cyclosporin A and FK506 following ischemia corresponds with differing abilities to inhibit calcineurin and the mitochondrial permeability transition. Neurobiol. Dis. (2002) 10:219-233. 116. DOMANSKA-JANIK K, BUZANSKA L,

DLUZNIEWSKA J, KOZLOWSKA H, SARNOWSKA A, ZABLOCKA B: Neuroprotection by cyclosporin A following transient brain ischemia correlates with the inhibition of the early efflux of cytochrome C to cytoplasm. Brain Res. Mol. Brain Res. (2004) 121:50-59. 117. FERRAND-DRAKE M, ZHU C,

GIDO G et al.: Cyclosporin A prevents calpain activation despite increased intracellular calcium concentrations, as well

1274

as translocation of apoptosis-inducing factor, cytochrome c and caspase-3 activation in neurons exposed to transient hypoglycemia. J. Neurochem. (2003) 85:1431-1442. 118. PANICKAR KS, JAYAKUMAR AR,

123. OKONKWO DO, BUKI A, SIMAN R,

POVLISHOCK JT: Cyclosporin A limits calcium-induced axonal damage following traumatic brain injury. NeuroReport (1999) 10:353-358. 124. SUEHIRO E, SINGLETON RH,

NORENBERG MD: Differential response of neural cells to trauma-induced free radical production in vitro. Neurochem. Res. (2002) 27:161-166.

STONE JR, POVLISHOCK JT: The immunophilin ligand FK506 attenuated the axonal damage associated with rapid rewarming following posttraumatic hypothermia. Exp. Neurol. (2001) 172:199-210.

119. SANTOS JB, SCHAUWECKER PE:

Protection propided by cyclosporin A against excitotoxic neuronal death is genotype specific. Epilepsia (2003) 44:995-1002.

125. FUKUI S, SIGNORETTI S,

120. OKONKWO DO, MELON DE,

PELLICANE AJ et al.: Dose-response of cyclosporin A in attenuating traumatic axonal injury in rat. Neuroreport (2003) 14:463-466. 121. BUKI A, OKONKWO DO,

POVLISHOCK JT: Postinjury cyclosporin A administration limits axonal damage and disconnection in traumatic brain injury. J. Neurotrauma (1999) 16:511-521. 122. OKONKWO DO, POVLISHOCK JT: An

intrathecal bolus of cyclosporin A before injury preserves mitochondrial intergrity and attenuates axonal disruption in traumatic brain injury. J. Cerebral. Blood Flow Metab. (1999) 19:443-451.


•

DUNBAR JG, MARMAROU A: The effect of cyclosporin A on brain edema formation following cortical contusion. Acta Neurochir. (2003) 86:301-303. A demonstration that CyA can be administered intravenously and still have beneficial effects on outcome.

Affiliation Robert Vink PhD† & Corinna Van Den Heuvel PhD †Author for correspondence Department of Pathology, Level 3, Medical School North, The University of Adelaide, Adelaide, SA 5005, Australia Tel: +61 (08) 8222 3092; Fax: +61 (08) 8222 3093; E-mail: [email protected]