Combination treatment for acute ischemic stroke: A ray of Hope?

BACKGROUND: With the implementation of thrombolysis, a large number of distinct pharmacological agents are now under consideration for the treatment of acute ischemic stroke, with disappoiting early results. Because the processes that ultimately lead to ischemic cell death involve a variety of pathophysiologic pathways, it is likely that combinations of agents may be necessary to positively affect neurological outcome. We review the general strategies under consideration for reduction of ischemic injury in the central nervous system, the types of possible interactions between compounds, and the experimental evidence showing effective combination therapies. SUMMARY OF REVIEW: Reduction of ischemic injury has been attempted by the following pharmacologic mechanisms: thrombolysis, neuroprotection, and perfusion/reperfusion enhancers. There is experimental evidence that the combination of thrombolytic therapy with a neuroprotective agent is additive in some ischemic models, as is the combination of a thrombolytic with an agent that facilitates reperfusion (thromboxane A(2) receptor antagonist and neutrophil adhesion/activation inhibition). Combinations of neuroprotective agents such as glutamate antagonists and calcium channel antagonists may be additive, and other combinations of neuroprotective agents, such as a glutamate antagonist with a gamma-aminobutyric acid (GABA) agonist, have even shown synergism in a rat stroke model. It has also been suggested that lower doses of toxic drugs may be used together to yield a positive neurologic outcome. Successful demonstration of additive or synergistic effects of pharmacologic agents in ischemia will depend on (1) the model used (well below a maximal "ceiling effect"); (2) the timing of drug administration; (3) the doses of the drugs used; and (4) the primary neurologic endpoint used. (Infarction size requires prolonged survival.) CONCLUSIONS: It appears from preclinical studies that some combinations of pharmacotherapeutic agents may be beneficial in cerebral ischemia, but rigorous evaluation is needed before initiating clinical trials.

Acute hypertension promotes hemorrhagic transformation in a rabbit embolic stroke model: effect of labetalol.

We examined the relationship between acute hypertension following cerebral embolization and subsequent hemorrhagic transformation (HT) in a rabbit embolic stroke model. We have shown previously that the likelihood and severity of hemorrhage were significantly correlated with the magnitude of an acute hypertensive response to embolization. It was not clear, however, whether hypertension actually caused hemorrhage or was merely a marker of more severe stroke. In the current studies, we attempted to clarify the relationship between acute hypertension and HT by either pharmacologically inducing or attenuating the brief hypertensive response to embolization in rabbits. Under halothane anesthesia, two catheters were implanted in the right carotid arteries of male New Zealand white rabbits, one oriented toward the heart and one toward the brain. The animals were allowed to awaken and were embolized using blood clot emboli injected into the middle cerebral artery. Blood pressure was monitored via the second carotid catheter. In the first experiment, hypertension was induced with angiotensin II, administered at the time of embolization or 1 h later. In the second experiment, we attempted to attenuate the hypertensive response using intravenous labetalol. The animals were sacrificed 18 h after embolization and the brains evaluated for hemorrhage. In the first experiment, administration of angiotensin II immediately after embolization did not increase the hypertensive response to embolization further than that spontaneously occurring, and no angiotensin II-related HT was observed. In contrast, an additional angiotensin-II-induced hypertensive episode 1 h after embolization significantly increased the number of 5-mm serial brain sections displaying HT, from 3.0 +/- .3 (mean +/- SE) in Controls to 5.4 +/- .8 in treated animals. In the second experiment, administration of labetalol (15 mg/kg) significantly reduced the number of brain sections with visible HT, from 3.2 +/- .5 in controls to 1.6 +/- .4 in treated animals. Acute hypertension during the first hour after cerebral embolization promotes HT in this rabbit embolic stroke model. Labetalol prevents blood pressure elevation and reduces the extent of HT in the same model.

Acute hypertension, but not thrombolysis, increases the incidence and severity of hemorrhagic transformation following experimental stroke in rabbits.

Hemorrhagic transformation (HT) is a poorly understood yet frequent complication of stroke. A transient increase in blood pressure (BP) occurs immediately after experimental embolization in rabbits and we evaluated the relationship between this acute hypertensive response and subsequent hemorrhagic transformation, as well as the attenuation of this hypertensive response with an anesthetic dose of halothane. We also examined embolism-induced HT during infusion of the thrombolytic agents tissue plasminogen activator and streptokinase. A blood clot embolus was injected into the internal carotid artery and flushed into the middle cerebral artery. In the first experiment, BP was monitored in anesthetized or unanesthetized rabbits for 20 min prior to and up to 1 h after embolization. In the second experiment, animals were embolized half-way through an infusion of tPA (3.0 mg/kg; 20% administered as an iv bolus, with the remainder infused over 30 min) or streptokinase (30,000 U/kg iv infused over 30 min). In unanesthetized animals, the HT score (number of brain sections displaying visible HT) was significantly correlated with the peak mean arterial pressure recorded at embolization (r = 0.60, n = 24, P < 0.01). No relationship was observed between BP and HT score in animals anesthetized with halothane. Although HT incidence and extent were significantly related to elevated BP in the unanesthetized animals, halothane administration actually increased HT incidence. Embolization during thrombolytic infusion did not increase the occurrence or severity of HT. These data suggest that acute hypertension, but not ongoing thrombolysis, is a significant risk factor for HT following cerebral embolization.

The AMPA antagonist LY293558 improves functional neurological outcome following reversible spinal cord ischemia in rabbits.

Glutamate (Glu) neurotoxicity is an important element of a number of neurological disorders including central nervous system (CNS) ischemia. We evaluated the effects of the novel AMPA Glu antagonist LY293558 on functional neurological outcome in two rabbit stroke models. In the reversible spinal cord ischemia model, ischemia of the caudal lumbar spinal cord was produced by temporary occlusion of the abdominal aorta. LY293558 was administered 5 min after recirculation as a 16 mg/kg i.v. bolus followed by 2.2 mg/kg infused over 1 h. Control animals received saline. LY293558 significantly increased the duration of ischemia required to produce paraplegia, from 30.5 +/- 15.8 min (mean +/- SD) controls to 50.1 +/- 11.5 in treated animals (p < 0.01). In an irreversible model of cerebral ischemia, 50 microns plastic microspheres were injected into the carotid artery and lodged in the cerebral microvasculature. LY293558 did not significantly reduce neurological damage in this model. These data suggest that LY293558 may have therapeutic benefit following some types of ischemic injury.

Monoclonal antibodies preventing leukocyte activation reduce experimental neurologic injury and enhance efficacy of thrombolytic therapy.

We evaluated the ability of monoclonal antibodies directed against leukocyte adhesion molecules (intercellular adhesion molecule-1 [ICAM-1], CD18) to enhance the efficacy of thrombolysis in a rabbit cerebral embolism stroke model. Both tissue-type plasminogen activator (tPA) and anti-CD18 (alpha-CD18) monoclonal antibody administered 5 minutes after embolization increased the quantity of clots required to produce neurologic damage, although the combination was no more effective than either substance alone. Neither alpha-CD18 nor anti-ICAM-1 (alpha-ICAM-1) improved neurologic outcome at postischemic delays of 15 or 30 minutes. However, the combination of alpha-ICAM-1 (15 minutes after embolization) and tPA (2 hours after embolization) significantly improved neurologic outcome even though neither substance was effective alone at these postembolization delays. These findings suggest that prevention of leukocyte adhesion increases the postischemic duration at which thrombolytic therapy remains effective.

Diaspirin cross-linked hemoglobin improves neurological outcome following reversible but not irreversible CNS ischemia in rabbits.

BACKGROUND AND PURPOSE: Hemodilution using modified hemoglobin solutions may reduce ischemic central nervous system injury. Purified diaspirin cross-linked hemoglobin (DCLHb) is a cell-free hemoglobin that is intramolecularly cross-linked between the two alpha subunits, resulting in enhanced oxygen offloading to tissues and increased half-life. In the present experiments, we evaluated the ability of DCLHb to reduce neurological damage in two rabbit stroke models. METHODS: In a reversible spinal cord ischemia model, ischemia of the caudal lumbar spinal cord was produced by temporary occlusion of the abdominal aorta. In an irreversible model of cerebral ischemia, plastic microspheres (50 microns) were injected into the internal carotid artery and lodged in the cerebral microvasculature. DCLHb was administered 5 minutes after initiation of ischemia as either a 10-mL/kg infusion, 10-mL/kg exchange transfusion, or a 20-mL/kg infusion. Control animals received human serum albumin that was oncotically matched to the DCLHb. RESULTS: In the spinal cord model, DCLHb significantly increased the duration of ischemia required to produce permanent paralysis from 27.33 +/- 8.71 minutes (mean +/- SD) in controls to 42.59 +/- 10.10 minutes in the 10-mL/kg exchange transfusion group and to 40.82 +/- 18.16 minutes in the 20-mL/kg infusion condition (P < .05). DCLHb did not significantly reduce neurological damage in the microsphere embolization model. CONCLUSIONS: These data suggest that cross-linked hemoglobin reduces neurological damage after reversible central nervous system ischemia and that this is not attributable to hemodilution or hypervolemia only.

Reduction of neurological damage by a peptide segment of the amyloid beta/A4 protein precursor in a rabbit spinal cord ischemia model.

Although found as a precursor of Alzheimer amyloid, substantial evidence suggests that beta/A4 protein precursor (APP) is involved in regulation of neuronal growth and survival. Recently, we have obtained evidence that the trophic properties of APP are fully preserved in a 17-amino acid sequence. If APP is neurotrophic, then it would be anticipated that administration of the growth-promoting segment of the APP 17-mer peptide might attenuate the neuronal dysfunction or loss or behavioral deficits associated with neuronal injury, such as that accompanying central nervous system ischemia. We evaluated this 17-mer peptide in a rabbit spinal cord ischemia model and found that this peptide alleviates paraplegia resulting from ischemia/reperfusion. Ischemia of the distal lumbar cord was produced by temporary occlusion of the abdominal aorta. Saline, 17-mer APP peptide, or a control peptide (200, 500, or 1000 nM) was administered intrathecally 20 min prior to ischemia and once daily for 3 days thereafter. The neurologic and morphologic outcomes were evaluated after 4 days. Durations of ischemia encompassing all grades of neurologic function were included. The 500 nM dose of 17-mer APP peptide significantly reduced neurologic damage. The average ischemia duration necessary to produce permanent neurologic damage increased from 27.9 +/- 1.9 min in saline-injected controls and 27.7 +/- 2.0 in scrambled sequence peptide-injected controls to 40.2 +/- 4.0 min in the 500 nM 17-mer APP-injected group. The 200 nM dose produced a nonsignificant trend toward reduced neurologic damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibody to the ICAM-1 adhesion site reduces neurological damage in a rabbit cerebral embolism stroke model.

We evaluated the effect of antibodies directed against a leukocyte adhesion molecule (ICAM-1) in an embolic model of stroke followed by thrombolysis with tissue plasminogen activator (tPA). To test whether reperfusion injury after ischemia was related to white cell adhesion, microclots were injected into carotid circulation. The conditions examined were control, alpha-ICAM 5 min following ischemia, tPA 30 min after ischemia, and the combination of alpha-ICAM and tPA. alpha-ICAM and tPA both increased the amount of clot necessary to produce permanent neurological damage. Combined therapy was no more effective than either substance alone. A similar outcome was obtained when tPA was delayed until 90 min postischemia. When thrombolysis was delayed 3 h following embolism, neither tPA nor the tPA/alpha-ICAM combination reduced neurological damage. Thus, the alpha-ICAM antibody and tPA each effectively reduced neurological damage but the interaction was not significant.

Reproducibility of time structure in motor activity of rats under nocturnal conditions.

Computer pattern recognition systems for the study of spontaneous rat behavior have introduced a new analytical technique, the K functions, that expands the definition of experimentally induced changes in behavior. Such studies normally evaluate three measures, a measure of the number of initiations of specific behavioral acts, a measure of the total time of each act, and a measure of behavioral time structure, the K functions. Such measures have been shown to be very stable and reproducible among control rats observed under normal light conditions. This study examines the stability of results from these three measures as applied to nine different groups of control Sprague-Dawley male rats observed under red light during their normal nocturnal hours. All three measures provided stable and reproducible results, but the measure of time structure, the K function analysis, provided the greatest consistency, producing values that vary by only a few percent.

Effects of yohimbine and idazoxan on motor behaviors in male rats.

Yohimbine, an alpha 2 adrenergic antagonist, facilitates copulatory behaviors in male rats. This facilitation may reflect nonspecific activation of behavior rather than a more selective activation of copulatory behaviors. The present experiments assessed the effects of yohimbine on locomotor behaviors at a dose (2.0 mg/kg) known to facilitate sexual behaviors. Experiment 1 used a computer pattern-recognition system to classify motor behaviors into specific acts and act groups. Male albino rats were tested in three conspecific conditions: estrous female, anestrous female, or no conspecific. Yohimbine decreased locomotor activity in all three conspecific conditions. Experiment 2 examined the effects of yohimbine (2.0 mg/kg) and amphetamine (1.0 mg/kg) on locomotor behavior in a photocell-equipped activity measurement system. Amphetamine increased and yohimbine decreased locomotor activity. Experiment 3 used the computer pattern-recognition system to compare the effects of yohimbine and idazoxan, another alpha 2 adrenergic antagonist, on motor behaviors. Yohimbine and idazoxan both decreased activity but produced different patterns of behavioral change. The facilitatory effects of yohimbine on copulatory behaviors at a dose of 2.0 mg/kg are apparently not mediated by nonspecific activation of behavior.