Therapeutic potential of magnesium in the treatment of acute stroke

Therapeutic Potential of Magnesium in the Treatment of Acute Stroke Keith W. Muir, md, mrcp

Magnesium is a key cation in multiple biological processes, including membrane excitability, protein synthesis, and cellular bioenergetics. Parenterally administered magnesium crosses the blood-brain barrier, raises brain concentrations to supraphysiological levels, and is neuroprotective in preclinical models of cerebral and spinal cord ischemia, excitotoxic injury, and head trauma. Neuronal and vascular effects of therapeutic magnesium may be pertinent, including inhibition of presynaptic release of excitatory neurotransmitters, presynaptic potentiation of adenosine, blockade of the N-methyl d-aspartate receptor, block of voltage-gated calcium channels, relaxation of vascular smooth muscle with vasodilatation of large and small vessel vascular beds causing increased cerebral blood flow, antagonism of endothelin-1 and other vasoconstrictors, enhanced postischemic recovery of tissue adenosine triphosphate, buffering of intracellular calcium ions (especially antagonism of mitochondrial calcium entry), and inhibition of deleterious ion shifts in white matter. Wide clinical experience in obstetrics and myocardial infarction confirms safety and tolerability as a therapeutic agent. Clinical trials in stroke are ongoing following encouraging results of pilot studies. Key Words: Magnesium— Stroke—Cerebrovascular disease—Clinical trials—Pharmacology.

Multiple metabolic and neurochemical events influence the viability of ischemic neurones in experimental models of stroke, and intervention in many pathways is capable of tipping the balance in favor of neuronal survival. Although many agents that act by single, welldefined mechanisms have potent neuroprotective effects in animal models, these have not yet translated into benefits in clinical practice. Magnesium (Mg2⫹) is neuroprotective in preclinical models of cerebral ischemia both in vitro and in vivo. There are multiple potential sites at which Mg2⫹ may act, encompassing both neuronal and

From the Department of Neurology, Institute of Neurological Sciences, Southern General Hospital, Glasgow, Scotland. Received April 19, 2000; accepted September 30, 2000. K.W.M. is coprincipal investigator in the Intravenous Magnesium Efficacy in Stroke (IMAGES) trial, which is funded by the UK Medical Research Council. Address reprint requests to Keith W. Muir, MD, MRCP, Department of Neurology, Southern General Hospital, Glasgow G51 4TF, Scotland. Copyright © 2000 by National Stroke Association 1052-3057/00/0906-0008$3.00/0 doi:10.1053/jscd.2000.20669

vascular mechanisms of relevance to acute stroke. Unlike most synthetic neuroprotective compounds, parenteral magnesium has no major adverse effects in doses that achieve serum levels in the range of preclinical neuroprotective concentrations. Extensive clinical experience with magnesium sulfate (MgSO4) in obstetrics confirms central nervous system (CNS) penetrance, tolerability, safety, and clinical effects of supraphysiological serum concentrations of Mg2⫹. Clinical studies in myocardial infarction (MI) confirm the safety and tolerability of Mg2⫹ in therapeutic doses in populations with acute cardiovascular disease. This paper reviews the pharmacological properties of magnesium of potential relevance to its neuroprotective properties and summarizes the preclinical and early clinical evidence of its potential in acute stroke treatment.

Distribution and Biological Functions of Mg2ⴙ in the CNS Magnesium ions (Mg2⫹) are involved in many essential biochemical processes and act especially as cofactors in protein and energy synthesis.1 In the CNS, Mg2⫹ is

Journal of Stroke and Cerebrovascular Diseases, Vol. 9, No. 6 (November-December), 2000: pp 257-267

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predominantly intracellular, and approximately 80% bound to adenosine triphosphate (ATP). Neuronal free Mg2⫹ concentration is proportional to that of ATP and is influenced by intracellular pH, since hydrogen ions compete with Mg2⫹ for binding to phosphates. Magnesium may contribute to regulation of ATP concentrations. Active transport of Mg2⫹ across the blood-brain barrier2 maintains a concentration gradient between cerebrospinal fluid (CSF) and serum, such that [Mg2⫹] in CSF is approximately 1.1 mmol/L, and in serum is around 0.8 mmol/L, except in disease states that compromise the blood-brain barrier. In dietary magnesium deficiency, CSF [Mg2⫹] is maintained preferentially. Significant reductions in extracellular fluid and total brain Mg2⫹ have been found in animal models of traumatic brain injury and stroke.3,4 Phosphorus magnetic resonance spectroscopy in stroke patients indicates increased intracellular free [Mg2⫹] in the acute phase of stroke (up to day 5), correlating with the period of intracellular acidosis, and probably consequent to displacement of ATP-bound Mg2⫹ together with competition with hydrogen ions.5 Excitotoxicity Synaptic accumulation of neurotoxic glutamate concentrations and pathological stimulation of postsynaptic N-methyl d-aspartate (NMDA) and ␣-amino-3-hydroxy5-methylisoxazole-4-propionic acid (AMPA) receptors are thought to be key events in the evolution of ischemic neuronal cell death. The activated NMDA receptor ion channel conducts both sodium (Na⫹) and Ca2⫹ and NMDA stimulation is coupled to sustained intracellular free Ca2⫹ increases that trigger deleterious intracellular events. Pharmacological inhibition of glutamate release, or of NMDA or AMPA ion channel opening, is consistently neuroprotective in preclinical focal ischemia models, irrespective of mechanism. High concentrations of Mg2⫹ (around 10 mmol/L) block synaptic activity nonspecifically, thereby preventing death and functional impairment of neurones in primary culture or isolated brain slices,6-8 but are unattainable therapeutically. Physiological extracellular Mg2⫹ concentrations (250 ␮mol/L to 1 mmol/L) inhibit glutamate release.9 Adenosine-mediated inhibition of glutamate release via A1 receptors is potentiated by Mg2⫹ and may be magnesium-dependent.10 Physiological Mg2⫹ concentrations specifically antagonize NMDA receptor activation via a voltage-dependent ion channel block that must be overcome by membrane depolarization.11,12 At resting membrane potentials, current flow through the NMDA receptor is completely abolished by physiological Mg2⫹ concentrations. The behavior of the NMDA ionophore in patch-clamp indicates

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the likelihood of two distinct Mg binding sites within the ion channel,13 with the more external Mg2⫹ site overlapping the binding site for synthetic uncompetitive NMDA antagonists (ion channel blockers).14 In heteromeric receptors expressed in Xenopus oocytes, the susceptibility of NMDA receptor proteins to Mg2⫹ block is dependent on the presence of asparagine residues within the M2 (channel-lining) domain of the fundamental NR1 subunit, but especially of potentiating NR2 subunits.15-17 Additional amino acid groups further modulate Mg2⫹ sensitivity of different NR2 subtypes.18 There is some evidence that different modulatory sites on the NMDA receptor complex influence different intracellular signalling pathways: in cortical neuronal culture, extracellular Mg2⫹ (in micromolar concentrations) inhibited NMDAinduced Ca2⫹ increases but not cyclic guanosine monophosphate (cGMP).19 Receptor sensitivity to Mg2⫹ is also influenced by prior ischemic conditioning and neuronal maturity. Increasing extracellular [Mg2⫹] to supraphysiological concentrations noncompetitively antagonizes NMDA conductance.20 The toxicity of NMDA applied directly to neuronal culture is attenuated by physiological levels of extracellular Mg2⫹,19,21 whereas reduction of extracellular [Mg2⫹] enhances toxicity.22 Loss of NMDA receptor sensitivity to Mg2⫹ block may be a major factor in the delayed ischemic death of CA1 pyramidal cells.23 In addition to the inhibitory effect of extracellular Mg2⫹ on NMDA channel opening, intracellular Mg2⫹ also reduces NMDA ionic conductance at physiological concentrations.13,24 NMDA receptor activation causes a 20-fold increase in intracellular [Mg2⫹].25 The elevation of intracellular free Mg2⫹ is Ca2⫹-dependent and has two components, one consequent to shifts of intracellular Mg2⫹ (possibly due to release from Mg-ATP) and the other to entry of extracellular Mg2⫹. Changes in extracellular [Mg2⫹] may thus reduce NMDA channel opening by raising intracellular as well as extracellular [Mg2⫹]. Intracellular [Mg2⫹] increases are sufficiently large to block L-type voltage-gated calcium channels (VGCCs), as well as several voltage-gated potassium and Na⫹ channels.25 Systemic Mg2⫹ administration alters NMDA receptor affinity. In rats receiving MgSO4 270 mg/kg every 4 hours by intraperitoneal (IP) injection, radiolabeled CGP 39653 (an NMDA receptor glutamate recognition site ligand) binding was reduced by 50% compared with controls after 24 hours.26 The mechanism is unclear, but Mg2⫹ potentiates glycine binding to the NMDA receptor at depolarizing membrane potentials and inhibits binding at resting potentials,27 possibly through interactions with the polyamine modulatory site that are subtypespecific.

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Parenteral MgSO4 inhibits epileptic electroencephalographic activity induced by cortical penicillin in dogs, cats, and primates in a dose-dependent fashion.28 In rats, MgSO4 at 1 mmol/kg IP raised CSF [Mg2⫹] by 26% (from 0.99 to 1.25 mmol/L) and raised the threshold for seizures induced via stimulation of implanted hippocampal electrodes.29 The mechanism for changes in seizure threshold may involve modulation of NMDA-mediated events, since intravenous (IV) MgSO4 at 90 mg/kg significantly delayed and shortened seizures induced by direct intracerebroventricular injection of NMDA in rats30 (and also prevented mortality, which was 50% in controls). In a study of oral organic Mg2⫹ salts in mice, NMDA and strychnine-induced seizures were delayed by doses equivalent to 300 mg/kg Mg2⫹ (doubling serum [Mg2⫹]), results qualitatively similar to those obtained with the uncompetitive NMDA antagonist dizocilpine (MK 801).31 Non-Excitotoxic Neuronal Effects of Magnesium Increased free intracellular Ca2⫹ may result from extracellular calcium entry via VGCCs or receptor-operated Ca2⫹ channels (NMDA and AMPA receptors possessing the GluR2 protein subunit) and intracellular release from endoplasmic reticulum. Magnesium competes with Ca2⫹ at VGCCs on both intracellular and cell surface membranes and is an antagonist at N-, P-, and (at high concentrations) L-type VGCCs.32 It may thereby impede Ca2⫹-dependent presynaptic glutamate release and VGCC-mediated Ca2⫹ entry to ischemic neurones. Excessive Ca2⫹ entry to mitochondria appears to be a crucial event in late ischemic cell death, and Mg2⫹ enhances mitochondrial buffering of intracellular free Ca2⫹33 and is required for uptake of Ca2⫹ by endoplasmic reticulum (via membrane Mg2⫹-Ca2⫹ ATPase) in ischemic neurones.34 Recovery from white matter anoxic injury in an isolated rat optic nerve model is enhanced significantly by 10 mmol/L MgCl2 35 but not by other polyvalent cations or dihydropyridine calcium antagonists. Magnesium ions probably act by blocking voltage-sensitive Na⫹ channels and thereby preventing reversal of the Na⫹-Ca2⫹ exchanger. CNS Pharmacokinetics of Parenteral Magnesium Parenteral administration of magnesium raises serum, CSF, and brain [Mg2⫹] significantly, even in the presence of an intact blood-brain barrier; however, CSF [Mg2⫹] increases are less than those in serum. IV infusion of magnesium chloride (MgCl2) or MgSO4 to dogs increases plasma [Mg2⫹] to approximately 3 times baseline and increases CSF concentration by 21%,2 similar to increases in human subjects receiving Mg infusions. In animals,

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peripherally administered Mg appears within the CSF within minutes and peaks 2 to 3 hours after a bolus dose.2 Slower equilibration of CSF with brain Mg2⫹ suggests that further active transport processes govern distribution within the CNS.29 The half-life in CSF of intracisternally administered Mg2⫹ is around 70 minutes.2 Serum [Mg2⫹] correlates with CSF, cortical, and hippocampal [Mg2⫹]. Cardiovascular Effects of Magnesium Magnesium infusion causes vasodilatation while increasing cardiac output, with increased cardiac index despite transient lowering of blood pressure.36,37 Pressor effects of angiotensin II and norepinephrine are antagonized by MgSO4.38 Magnesium has direct effects on large, medium, and small vessels of the cerebral circulation. Carotid vasoconstriction caused by endothelin 1, neuropeptide Y, and angiotensin II in rats is reversed by IV MgSO4 or MgCl2 39,40 at plasma [Mg2⫹] of 2.4 to 2.7 mmol/L. Endothelin-1–mediated vasoconstriction is attenuated by Mg2⫹41 and is abolished at plasma concentrations of 3.8 to 4.6 mmol/L.39 Magnesium infusions cause cerebral arteriolar vasodilatation.42 Vasodilatation of mediumsized vessels may occur as a direct Ca2⫹ antagonist effect on vascular smooth muscle43 or may be mediated by prostacyclin.44 Physiological magnesium concentrations have significant anticonstrictor effects against prostaglandin F2␣ and serotonin in isolated postmortem human middle cerebral artery (MCA).45 Pial vessel vasoconstriction in response to excitatory amino acids is antagonized by IV or intra-arterial (IA) MgCl2 in rats.46 Basilar artery vasoconstriction was reversed by IV MgSO4 in a rat model of subarachnoid hemorrhage at mean plasma [Mg2⫹] of 4.32 mmol/L.47 Antiplatelet Activity Magnesium inhibits calcium-mediated ADP- and collagen-triggered platelet aggregation in vitro.48,49

Preclinical Studies in Cerebral Ischemia Focal Cerebral Ischemia Models Three full studies of magnesium in focal ischemia models have been published in full50-52 and one in abstract.53 These are summarized in Table 1. Magnesium was administered before onset of ischemia51,52 or immediately after MCA occlusion (MCAO),50,53 with repeat dosing at 1 hour50,53 or reperfusion.52 Additional unpublished studies by Wester and by Shuaib have found significant neuroprotection with systemically administered MgSO4 in mechanical MCAO and thromboembolic MCAO, respectively. Significant protection was seen

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MgCl2 Sprague-Dawley rats MCAO (90 min)-r Schmid-Elsaesser et al52

MgSO4 MgCl2 Sprague-Dawley rats Harland-Olac mice MCAO (90 or 120 min)-r pMCAO Marinov et al51 Roffe et al53

Significant infarct volume reductions are in bold. Abbreviations: MCAO, middle cerebral artery occlusion; pMCAO, permanent middle cerebral artery occlusion; MCAO-r, middle cerebral artery occlusion with reperfusion (occlusion time in parentheses); IP, intraperitoneal; IA, intra-arterial; IV, intravenous.

ⴚ37% Pre & 90 min post IV

ⴚ60% (90 mg/kg, 90 min ischemia) ⫹30% Pre Pre IA IP

Pre & 1 h post IP

1 mmol/kg (repeated) 30 or 90 mg/kg 1 mmol/kg (repeated) 1 mmol/kg (repeated) MgCl2 Fischer-344 rats pMCAO Izumi et al50

Route Dose Salt Species Model Study

Table 1. Magnesium in focal cerebral ischemia models

Timing

ⴚ26%

Infarct Volume

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with administration delayed for 2 hours after onset of ischemia. Wester also reported significant elevation of CSF and brain [Mg2⫹], especially within the penumbra.54 Additive neuroprotective effects of MgCl2 and the freeradical scavenging aminosteroid compound tirilazad, with or without hypothermia, have been reported.52,55 Clear evidence of dose-dependent neuroprotection was found by Marinov et al.51 The overall treatment effect in this study represents a conservative estimate, since rigorous statistical analysis excluded animals without established cortical infarcts: 7 of 8 animals so excluded were in magnesium-treated groups. Different magnesium salts have had differing effects on glycemic control in reported studies. Magnesium chloride has been reported consistently to cause hyperglycemia in focal ischemia models50,52,53 and in other models of ischemia.56 No such trend has been found for MgSO4.51 Hyperglycemia is recognized as exacerbating ischemic neuronal damage, and adjunctive insulin administration to maintain normoglycemia has resulted in enhanced neuroprotection in some studies, eg, Izumi et al50 found 26% infarct volume reduction with MgCl2 and 44% reduction with MgCl2 plus insulin. After permanent MCAO in rats, Chi et al57 reported abolition of significant reduction in cerebral blood flow (CBF) by IV MgSO4 (total dose of 1.95 mmol/kg over 30 minutes, serum [Mg2⫹] increased from 0.86 to 3.21 mmol/L), measuring CBF using the [14C]-iodoantipyrine method. However, another group reported no difference in cortical CBF during ischemia by laser Doppler flowmetry between MgCl2-treated and control rats subjected to transient MCAO.52

Other In Vivo Cerebral Ischemia and Excitotoxicity Models Global Cerebral Ischemia Models In rats subjected to 30 minutes of forebrain ischemia, intrastriatal infusion of 2.5 mmol/L MgCl2 inhibited striatal glutamate and aspartate release, as did dizocilpine (but not the AMPA antagonist NBQX).58 In a rat global cerebral ischemia model (20 minutes of forebrain ischemia followed by reperfusion), direct injection of 50 mmol/L MgCl2 to the hippocampus reduced death of CA1 neurones when administered 24 hours after onset of ischemia.59 Functional neurological outcome at day 7 was also improved by 2 mmol/kg MgSO4 in dogs subjected to 18 minutes of global ischemia followed by reperfusion.60 However, in another study, pretreatment with 5 mmol/kg MgCl2 (peak plasma [Mg2⫹], 5.22 to 6.29 mmol/L) did not prevent loss of hippocampal CA1 neurones assessed 7 days after 10 minutes of forebrain ischemia in Sprague-Dawley rats, despite correcting significant hyperglycemia with insulin.56

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Other Models In a pure excitotoxic brain injury model (stereotactic intrastriatal injection of NMDA to perinatal day 7 rats), there was dose-dependent reduction of infarcted brain tissue by MgSO4 up to 4 mmol/kg IP 15 minutes postNMDA injection.61 A cocktail of 0.3g/kg MgSO4, mannitol, and L-methionine IP after hypoxic and ischemic insults to perinatal day-8 rats reduced overall cerebral hemispheric damage.62 One hour postinjury, MgSO4 (600 mg/kg administered subcutaneously) improved evoked potentials and attenuated lipid peroxide products in rats.63 Intrathecal 1 mol/L MgSO4 improved functional recovery 1 week after spinal cord injury in rats without significant histological effects.64 Preischemic IV 5 mmol/kg MgCl2 increased the duration of recoverable ischemia in a rabbit spinal cord ischemia model but caused profound respiratory depression.65 In several models of traumatic brain injury, magnesium has been neuroprotective or facilitated recovery. McIntosh et al66 found improved neurological scores up to 4 weeks after focal fluid-percussion injury using 15minute duration IV infusions of 12.5 ␮mol or 125 ␮mol MgCl2 commenced 30 minutes after insult, improved performance of rats in memory testing paradigms with 125 ␮mol MgCl2,67 and reduced cerebral edema after 15-minute postinjury MgCl2 at 300 ␮mol/kg IV.68 Preinjury regimens of 1 mmol/kg MgCl2 IP daily enhanced neurological recovery, even when the final doses were administered 24 hours before electrolytic lesions to the sensorimotor cortex, and also reduced the volume of associated subcortical striatal degeneration 43 days postictus.69 One hour postinjury treatment with MgCl2 increased brain [Mg2⫹], reduced edema formation, and improved neurological recovery after focal contusional injury in rats.70 Enhanced motor recovery has been observed in a diffuse axonal injury model in rats receiving 100 ␮mol/kg of either MgCl2 or MgSO4 30 minutes after injury.71 Infusion of IV MgCl2 at 4 ␮mol/min (plasma [Mg2⫹] 3 times baseline) prevented intracerebral hemorrhage and death consequent to direct alcohol injection into rat brain.72 Several magnesium regimens prevent the vasospasm and microvascular hemorrhage induced by phencyclidine73 and cocaine74 in rats, and both IV and IA MgCl2 or Mg aspartate hydrochloride exhibit dose-dependent inhibition of phencyclidine-mediated pial arteriolar contraction at serum [Mg2⫹] between 0.99 and 2.72 mmol/L.73

Clinical Studies Clinical use of magnesium has been almost exclusively confined to the sulfate salt rather than the chloride. Mag-

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nesium sulfate has been used widely in the treatment of eclamptic seizures, prevention of preterm labor, acute MI, and cardiac dysrhythmias. Extensive data from randomized, controlled trials are available for MI and preeclampsia/eclampsia. Several smaller studies in acute stroke have been reported, and a large multicenter trial in stroke is ongoing. Magnesium is excreted renally, and dose adjustment is required in renal impairment. Toxicity is manifest as depression of deep tendon reflexes at serum levels of over 5 mmol/L, with respiratory depression, EKG changes (T wave flattening, PR interval increase, and diminished R wave amplitude), and neuromuscular weakness at concentrations of approximately 7 mmol/L.75 Calcium gluconate can be administered IV in emergencies to counteract these effects, but rapid renal clearance ensures that symptoms of hypermagnesemia are transient. Muscular strength returns rapidly and is normal 6 hours after terminating IV infusion.75 Cases of iatrogenic overdose can usually be managed with supportive care alone. Volunteers In patients undergoing craniotomy, a 15-minute IV infusion of MgSO4 at 60 mg/kg (peak serum [Mg2⫹], 1.24 mmol/L) raised CSF [Mg2⫹] from 0.95 to 1.13 mmol/L. CSF concentration peaked at 4 hours.76 In patients with hypertension or stable ischemic heart disease, IV MgSO4 infusions that produce serum concentrations of 1.5 to 2.0 mmol/L increase cardiac index,77 reduce systemic vascular resistance,77 increase arterial blood flow,78 cause flushing,36 inhibit aldosterone release,79 and potentiate insulin-mediated oxidative glucose metabolism.80 Blood pressure and heart rate effects are variable. Bleeding time was prolonged postinfusion in eclamptic patients receiving MgSO481 and also in healthy volunteers infused with 8 mmol MgSO4 over 15 minutes followed by 3 mmol/h (achieving mean serum [Mg2⫹], 1.50 mmol/L).82 Antiaggregant effects were independent of aspirin use in vitro.48,49 Acute Myocardial Infarction An overview of seven small trials of MgSO4 in MI, involving 1,301 patients, reported reduced mortality and incidence of ventricular dysrhythmias.83 Two large trials followed: the Second Leicester Intervention in Myocardial Infarction Trial (LIMIT-2),84 which included 2,316 patients and suggested a myocardial cytoprotective effect, and subsequently the fourth International Study of Infarct Survival (ISIS-4),85 which randomized 58,050 patients but showed no effect of treatment on any outcome. Both LIMIT-2 and ISIS-4 used similar dose regimens: IV

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boluses of 8 mmol being followed by IV infusions of 65 or 72 mmol, respectively over 24 hours. Both trials confirmed a low incidence of magnesium-related adverse effects. A small excess of flushing was noted in both trials, affecting 0.3% of patients in ISIS-4. In ISIS-4, there were small but statistically significant increases in hypotension (1.1%), cardiac failure (1.2%), and cardiogenic shock (0.5%). However, these may have been subject to pharmacological confounding—patients received concomitant captopril, nitrates, or streptokinase— or reporting bias, since the trial used an open rather than blinded control group. The LIMIT-2 study, in contrast, reported a significant reduction in the incidence of cardiac failure (3.7% absolute risk reduction) and nonsignificant reduction of cardiogenic shock (1.3%). Although ISIS-4 was much larger than LIMIT-2, concerns regarding differences in administration of magnesium and concomitant treatments have led to persistent debate. Pre-Eclampsia/Eclampsia Several large randomized, controlled trials involving over 2,000 women and a meta-analysis86 confirm the superiority of MgSO4 over placebo and conventional anticonvulsants (diazepam and phenytoin) for prevention of seizures in pre-eclamptic87 and eclamptic88,89 patients. Typical dose regimens involved initial IM MgSO4 at 10 g (with or without 4 g IV) and maintenance IM doses (4 to 5 g every 4 hours) or IV infusion (1 g/h for 24 hours). For heptahydrated MgSO4, 1 g approximates 4 mmol. IM regimens produce a high initial serum peak,90 with subsequent stabilization of serum levels at around 2 to 3 times physiological. Pregnancy-related changes in volume of distribution and renal excretion are further affected in pre-eclampsia and eclampsia, where edema and oliguria are features. These factors caution against direct extrapolation of these regimens to nonpregnant patients. In pre-eclamptic women, IV MgSO4 increases CSF concentration significantly (for example, from 1.05 to 1.25 mmol/L).91 The mechanism of action is uncertain. Transcranial and color-flow Doppler studies show changes consistent with vasodilatation of the maternal cerebral circulation distal to the MCA92 in pregnancy-induced hypertension and pre-eclampsia. Magnesium also reduces angiotensin-converting enzyme concentrations,93 restores urinary cGMP to that of normotensive pregnant women,94 and reduces endothelin-1 expression in vascular endothelium.95 Magnesium’s efficacy in retarding preterm labor is debatable, with several trials finding negative results. Perinatal Hypoxic Injury Magnesium use in mothers treated for pre-eclampsia or preterm labor has been associated in several studies

with reduced incidence of cerebral palsy in very low birth-weight babies.96,97 Others have reported neutral effects98,99 and the absence of an effect on neonatal white matter lesions on transcranial ultrasound.100 Reduced perinatal mortality with MgSO4 in preterm labor has been reported in a case-control study,101 but a recent randomized, controlled study was terminated after 3 deaths in 46 pregnancies treated with MgSO4.102 Nonsignificant reductions in perinatal deaths compared with phenytoin have been reported on post hoc analyses of pre-eclampsia/eclampsia trials.87,88 A study of MgSO4 in perinatal hypoxia was stopped early due to an excess of poor outcomes in neonates receiving magnesium. The trial remains unpublished to date. Subsequent analysis of the study has disclosed misunderstandings of the magnesium dose due to transAtlantic differences in pharmacy calculation of MgSO4 dose based on weight (differences arising between anhydrous and heptahydrated MgSO4). A proportion of neonates with adverse outcomes appears to have received an excessive dose of magnesium. Stroke Five small trials of magnesium in stroke have been reported, with three reported as abstracts only. A large multicenter trial is ongoing (the Intravenous Magnesium Efficacy in Stroke [IMAGES] trial). Wester et al reported open-label use of IV infusions of MgSO4 in 12 patients without cardiovascular or other tolerability problems and went on to conduct a doubleblind trial in 26 stroke patients103 receiving 4 mmol MgSO4 as a bolus followed by a 5 mmol/h infusion over 8 hours and then oral magnesium supplements for 5 days. Full details of these studies remain unpublished. Muir and Lees have reported two small randomized, double-blind, placebo-controlled studies of MgSO4.104,105 The first trial used a loading infusion of 8 mmol, followed by 65 mmol over 24 hours in 60 patients within 12 hours of stroke. No significant treatment-related adverse effects were reported. A mean serum [Mg2⫹] of 1.42 mmol/L was achieved, but maximal concentration was only obtained at the end of the 24 hours of infusion. A subsequent dose-optimization study105 used higher loading infusions of 8, 12, and 16 mmol, each followed by 65 mmol over 24 hours in 25 patients within 24 hours of stroke. No adverse cardiovacular effects or notable tolerability problems were encountered. The highest loading dose group elevated serum [Mg2⫹] to 1.84 mmol/L. Results of the IMAGES pilot trial have been reported in abstract, including 51 patients treated within 12 hours of stroke. The trial intends to recruit 2,700 patients, one third within 6 hours of stroke, and uses the 16 mmol loading plus 65 mmol over 24 hours regimen. No tolerability issues have been reported to date.

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A single-center Greek study has been reported in abstract in which 233 subjects received IV magnesium aspartate hydrochloride equivalent to 7.5 mmol of Mg2⫹ with improved outcome at days 30 and 90 compared with 277 controls. However, details of the study design and randomization procedures were not stated.106

Discussion There are multiple potentially beneficial pharmacological effects of supraphysiological magnesium concentrations in stroke. Peripherally administered magnesium crosses the intact blood-brain barrier and is therefore available in the acute phase of human stroke. Selective local increases in ischemic tissue are also reported. Systemically administered magnesium can block specific excitotoxic mechanisms of brain injury, and enhanced NMDA receptor block may be attainable with well-tolerated brain concentrations. Magnesium has also been found to reduce presynaptic glutamate release, block N-, P-, and L-type calcium channels; prevent intracellular free Ca2⫹ increase; enhance mitochondrial Ca2⫹ buffering; enhance recovery of neuronal ATP after ischemia; and antagonize the vasoconstrictor effects of several mediators, including endothelin-1. Magnesium reduces histological infarct volume in standard animal models of focal cerebral ischemia when administered before or up to 2 hours after onset of ischemia and also reduces neuronal loss in several other models of cerebral ischemia. Magnesium sulfate is neuroprotective in animal focal ischemia models and in several models of global ischemia and traumatic brain injury. Magnesium chloride is less consistently neuroprotective in animal models, probably by inducing hyperglycemia, but has been neuroprotective when normoglycemia is maintained. The mechanism of this apparently anion-specific effect is unknown, although physicochemical differences that may influence membrane transport of Mg2⫹ are recognized. Magnesium is excreted renally, and significant toxicity is unlikely in patients with normal renal function. The specific effects of hypermagnesemia are well-documented and can be reversed pharmacologically. Safety and tolerability of magnesium sulfate have been established in clinical trials in pre-eclampsia/eclampsia and acute MI. Trials in stroke have found no tolerability problems or evidence of potentially detrimental hypotension, bradycardia, or hyperglycemia. Preliminary data suggest the potential for improved clinical outcome in stroke patients, and the ongoing multicenter IMAGES trial is being conducted to establish efficacy. Several criticisms may be made of the existing clinical and preclinical data. Animal data for magnesium are less complete than is desirable, because, unlike pharmaceuti-

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cal compounds, it has never been subject to a comprehensive development program. There is only one study showing significant neuroprotection with delayed administration, most treating before or concurrently with ischemia. However, the consistency of reports of neuroprotection across a broad range of models, laboratories, and investigators is a strength. The magnitude of infarct reduction and maintenance of efficacy with 2 hours of delay is comparable to most NMDA antagonist drugs in focal cerebral ischemia models, and magnesium sulfate shares neither the toxicity nor the thermoregulatory problems associated with this drug class. However, in the absence of proof of clinical utility, animal models of neuroprotection continue to have a credibility gap. Trials in MI failed to establish therapeutic benefit for this indication, and although debate continues regarding the possibility that the lack of effect of magnesium in ISIS-4 may be attributable to delayed administration after reperfusion, subgroup analysis suggests that significant benefit is unlikely even in patients treated before thrombolysis. The negative MI trial results should probably not detract greatly from the potential for stroke treatment, since there are multiple additional CNS targets for Mg2⫹, and the weight of preclinical evidence is considerably greater than was the case for MI. Nevertheless, they emphasize the need for caution in interpretation of small trials and the necessity of confirmation by larger pragmatic studies. The failures of several moderate to large neuroprotective trials raise concerns about the viability of both the neuroprotection hypothesis and of current trial designs. Neuroprotective agents have failed for a variety of reasons. Trials of glutamate antagonists have been terminated prematurely due to concerns about toxicity or where interim analysis has suggested low probability of demonstrating significant benefit. Most of these drugs have had significant dose-limiting toxic effects (CNS or cardiac) that have prevented administration of doses likely to produce serum levels in the neuroprotective range derived from animal data being attained in humans (aptiganel HCl, lubeluzole, selfotel, and eliprodil are all examples). Calcium antagonists have similarly been ineffective in trials, but most studies (mainly of nimodipine) have used delayed oral dosing. Significant hypotensive effects of IV nimodipine were dose-limiting and detrimental. Magnesium has multiple relevant modes of action besides calcium-channel and glutamate antagonism and, unlike most agents in clinical trials to date, may achieve these pharmacological goals without major side-effects. Experience in pre-eclampsia suggests that magnesium, unlike most neuroprotective agents in trials to date, may exert significant CNS effects at very well tolerated serum concentrations. Nevertheless, in the absence of evidence of the relevance of neuroprotective

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mechanisms in human stroke, it remains possible that pharmacological targets simply cannot be extrapolated from animal models. Whether the neuroprotection hypothesis has yet been tested adequately is open to question.107 Clinical trial design has to date been predicated on the anticipation of large treatment effects, with most trials having been powered to detect effect sizes of approximately 10%. If full recruitment is achieved, IMAGES will have a sample size 50% greater than any individual trial to date and is powered to detect a 6% difference in outcome. However, it is compromised by the extended time window of 12 hours, which may be too long to expect benefit. As the time window lengthens, the proportion of patients with viable tissue decreases (possibly exponentially), and proven therapeutic benefit in stroke at present is limited to treatment within 3 hours. Nevertheless, the IMAGES sample size is sufficient to include in excess of 1,000 patients treated within 6 hours, which represents the standard window for neuroprotective trials. The IMAGES protocol is necessarily a compromise, since academic studies cannot be funded to a level that permits the intensive, short-window studies run by pharmaceutical industry sponsors: extension of the trial to centers not heavily involved in commercial trials has the advantage of improving the generalizability of results, but at the price of reduced data collection and an extended time window. Since even modest benefit in acute stroke would have a very substantial impact on stroke care due to the broad applicability of a safe, readily available, inexpensive, and familiar treatment, testing the efficacy of magnesium in a paradigm closely similar to that of existing neuroprotective trials is worthwhile.

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