Serial changes in cerebral blood flow and flow-metabolism uncoupling in primates with acute thromboembolic stroke

Mechanism of Action and Translational Potential of (S)-Meclizine in Preemptive Prophylaxis Against Stroke

BACKGROUND

Mild chemical inhibition of mitochondrial respiration can confer resilience against a subsequent stroke or myocardial infarction, also known as preconditioning. However, the lack of chemicals that can safely inhibit mitochondrial respiration has impeded the clinical translation of the preconditioning concept. We previously showed that meclizine, an over-the-counter antivertigo drug, can toggle metabolism from mitochondrial respiration toward glycolysis and protect against ischemia-reperfusion injury in the brain, heart, and kidney. Here, we examine the mechanism of action of meclizine and report the efficacy and improved safety of the (S) enantiomer.

METHODS

We determined the anoxic depolarization latency, tissue and neurological outcomes, and glucose uptake using micro-positron emission tomography after transient middle cerebral artery occlusion in mice pretreated (-17 and -3 hours) with either vehicle or meclizine. To exclude a direct effect on tissue excitability, we also examined spreading depression susceptibility. Furthermore, we accomplished the chiral synthesis of (R)- and (S)-meclizine and compared their effects on oxygen consumption and histamine H1 receptor binding along with their brain concentrations.

RESULTS

Micro-positron emission tomography showed meclizine increases glucose uptake in the ischemic penumbra, providing the first in vivo evidence that the neuroprotective effect of meclizine indeed stems from its ability to toggle metabolism toward glycolysis. Consistent with reduced reliance on oxidative phosphorylation to sustain the metabolism, meclizine delayed anoxic depolarization onset after middle cerebral artery occlusion. Moreover, the (S) enantiomer showed reduced H1 receptor binding, a dose-limiting side effect for the racemate, but retained its effect on mitochondrial respiration. (S)-meclizine was at least as efficacious as the racemate in delaying anoxic depolarization onset and decreasing infarct volumes after middle cerebral artery occlusion.

CONCLUSIONS

Our data identify (S)-meclizine as a promising new drug candidate with high translational potential as a chemical preconditioning agent for preemptive prophylaxis in patients with high imminent stroke or myocardial infarction risk.

A clinically relevant model of focal embolic cerebral ischemia by thrombus and thrombolysis in rhesus monkeys

Over decades of research into the treatment of stroke, nearly all attempts to translate experimental treatments from discovery in cells and rodents to use in humans have failed. The prevailing belief is that it might be necessary to pretest pharmacological neuroprotection in higher-order brains, especially those of nonhuman primates (NHPs). Over the past few years, chemical thrombolysis and mechanical thrombectomy have been established as the standard of care for ischemic stroke in patients. The spotlight is now shifting towards emphasizing both focal ischemia and subsequent reperfusion in developing a clinically relevant stroke model in NHPs. This protocol describes an embolic model of middle cerebral artery occlusion in adult rhesus monkeys. An autologous clot is combined with a microcatheter or microwire through endovascular procedures, and reperfusion is achieved through local intra-artery thrombolysis with tissue plasminogen activator. These NHP models formed relatively stable infarct sizes, delivered predictable reperfusion and survival outcomes, and recapitulated key characteristics of patients with ischemic stroke as observed on MRI images and behavioral assays. Importantly, treated animals could survive 30 d after the surgery for post-stroke neurologic deficit analyses. Thus far, this model has been used in several translational studies. Here we describe in detail the teamwork necessary for developing stroke models of NHPs, including the preoperation preparations, endovascular surgery, postoperation management and histopathological analysis. The model can be established by the following procedures over a 45-d period, including preparation steps (14 d), endovascular operation (1 d) and evaluation steps (30 d).

Development and Testing of Thrombolytics in Stroke

Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo.

Large animal ischemic stroke models: replicating human stroke pathophysiology

The high morbidity and mortality rate of ischemic stroke in humans has led to the development of numerous animal models that replicate human stroke to further understand the underlying pathophysiology and to explore potential therapeutic interventions. Although promising therapeutics have been identified using these animal models, with most undergoing significant testing in rodent models, the vast majority of these interventions have failed in human clinical trials. This failure of preclinical translation highlights the critical need for better therapeutic assessment in more clinically relevant ischemic stroke animal models. Large animal models such as non-human primates, sheep, pigs, and dogs are likely more predictive of human responses and outcomes due to brain anatomy and physiology that are more similar to humans-potentially making large animal testing a key step in the stroke therapy translational pipeline. The objective of this review is to highlight key characteristics that potentially make these gyrencephalic, large animal ischemic stroke models more predictive by comparing pathophysiological responses, tissue-level changes, and model limitations.

Significantly Increased Accumulation of 18F-FDG Throughout the Left Middle Cerebral Artery Territory Corresponding to Acute-Phase Infarction

A 70-year-old woman had spontaneous resolution of an embolism in her right middle cerebral artery (MCA) (day 1); another embolism occurred in her left MCA (day 3), which was promptly removed. On day 5, F-FDG PET/CT performed for staging mediastinal lymphoma showed marked FDG accumulation in the left MCA territory, whereas a defect was seen in the right insular region. Eventually, bilateral lesions developed irreversible infarction. Anaerobic metabolism and/or inflammation in acute-phase infarction were the supposed mechanism for the increased accumulation of FDG in her left MCA territory.

Determining the Temporal Profile of Intracranial Pressure Changes Following Transient Stroke in an Ovine Model

BACKGROUND AND PURPOSE

Cerebral edema and elevated intracranial pressure (ICP) are the leading cause of death in the first week following stroke. Despite this, current treatments are limited and fail to address the underlying mechanisms of swelling, highlighting the need for targeted treatments. When screening promising novel agents, it is essential to use clinically relevant large animal models to increase the likelihood of successful clinical translation. As such, we sought to develop a survival model of transient middle cerebral artery occlusion (tMCAO) in the sheep and subsequently characterize the temporal profile of cerebral edema and elevated ICP following stroke in this novel, clinically relevant model.

METHODS

Merino-sheep (27M;31F) were anesthetized and subject to 2 h tMCAO with reperfusion or sham surgery. Following surgery, animals were allowed to recover and returned to their home pens. At preselected times points ranging from 1 to 7 days post-stroke, animals were re-anesthetized, ICP measured for 4 h, followed by imaging with MRI to determine cerebral edema, midline shift and infarct volume (FLAIR, T2 and DWI). Animals were subsequently euthanized and their brain removed for immunohistochemical analysis. Serum and cerebrospinal fluid samples were also collected and analyzed for substance P (SP) using ELISA.

RESULTS

Intracranial pressure and MRI scans were normal in sham animals. Following stroke, ICP rose gradually over time and by 5 days was significantly (p < 0.0001) elevated above sham levels. Profound cerebral edema was observed as early as 2 days post-stroke and continued to evolve out to 6 days, resulting in significant midline shift which was most prominent at 5 days post-stroke (p < 0.01), in keeping with increasing ICP. Serum SP levels were significantly elevated (p < 0.01) by 7 days post-tMCAO.

CONCLUSION

We have successfully developed a survival model of ovine tMCAO and characterized the temporal profile of ICP. Peak ICP elevation, cerebral edema and midline shift occurred at days 5-6 following stroke, accompanied by an elevation in serum SP. Our findings suggest that novel therapeutic agents screened in this model targeting cerebral edema and elevated ICP would most likely be effective when administered prior to 5 days, or as early as possible following stroke onset.

Large animals in neurointerventional research: A systematic review on models, techniques and their application in endovascular procedures for stroke, aneurysms and vascular malformations

Neuroendovascular procedures have led to breakthroughs in the treatment of ischemic stroke, intracranial aneurysms, and intracranial arteriovenous malformations. Due to these substantial successes, there is continuous development of novel and refined therapeutic approaches. Large animal models feature various conceptual advantages in translational research, which makes them appealing for the development of novel endovascular treatments. However, the availability and role of large animal models have not been systematically described so far. Based on comprehensive research in two databases, this systematic review describes current large animal models in neuroendovascular research including their primary use. It may therefore serve as a compact compendium for researchers entering the field or looking for opportunities to refine study concepts. It also describes particular applications for ischemic stroke and aneurysm therapy, as well as for the treatment of arteriovenous malformations. It focuses on most promising study designs and readout parameters, as well as on important pitfalls in endovascular translational research including ways to circumvent them.

Oxygen metabolism in acute ischemic stroke

Gaining insights into brain oxygen metabolism has been one of the key areas of research in neurosciences. Extensive efforts have been devoted to developing approaches capable of providing measures of brain oxygen metabolism not only under normal physiological conditions but, more importantly, in various pathophysiological conditions such as cerebral ischemia. In particular, quantitative measures of cerebral metabolic rate of oxygen using positron emission tomography (PET) have been shown to be capable of discerning brain tissue viability during ischemic insults. However, the complex logistics associated with oxygen-15 PET have substantially hampered its wide clinical applicability. In contrast, magnetic resonance imaging (MRI)-based approaches have provided quantitative measures of cerebral oxygen metabolism similar to that obtained using PET. Given the wide availability, MRI-based approaches may have broader clinical impacts, particularly in cerebral ischemia, when time is a critical factor in deciding treatment selection. In this article, we review the pathophysiological basis of altered cerebral hemodynamics and oxygen metabolism in cerebral ischemia, how quantitative measures of cerebral metabolism were obtained using the Kety-Schmidt approach, the physical concepts of non-invasive oxygen metabolism imaging approaches, and, finally, clinical applications of the discussed imaging approaches.

Large animal models of stroke and traumatic brain injury as translational tools

Acute central nervous system injury, encompassing traumatic brain injury (TBI) and stroke, accounts for a significant burden of morbidity and mortality worldwide. Studies in animal models have greatly enhanced our understanding of the complex pathophysiology that underlies TBI and stroke and enabled the preclinical screening of over 1,000 novel therapeutic agents. Despite this, the translation of novel therapeutics from experimental models to clinical therapies has been extremely poor. One potential explanation for this poor clinical translation is the choice of experimental model, given that the majority of preclinical TBI and ischemic stroke studies have been conducted in small animals, such as rodents, which have small lissencephalic brains. However, the use of large animal species such as nonhuman primates, sheep, and pigs, which have large gyrencephalic human-like brains, may provide an avenue to improve clinical translation due to similarities in neuroanatomical structure when compared with widely adopted rodent models. This purpose of this review is to provide an overview of large animal models of TBI and ischemic stroke, including the surgical considerations, key benefits, and limitations of each approach.

Endovascular Ischemic Stroke Models in Nonhuman Primates

To bridge the gap between rodent and human studies, the Stroke Therapy Academic Industry Roundtable committee suggests that nonhuman primates (NHPs) be used for preclinical, translational stroke studies. Owing to the fact that vast majority of ischemic strokes are caused by transient or permanent occlusion of a cerebral blood vessel eventually leading to brain infarction, ischemia induced by endovascular methods closely mimics thromboembolic or thrombotic cerebrovascular occlusion in patients. This review will make a thorough summary of transient or permanent occlusions of a cerebral blood vessel in NHPs using endovascular methods. Then, advantages and disadvantages, and potential applications will be analyzed for each kind of models. Additionally, we also make a further analysis based on different kinds of emboli, various occlusion sites, infract size, abnormal hemodynamics, and potential dysfunctions. Experimental models of ischemic stroke in NHPs are valuable tools to analyze specific facets of stroke in patients, especially those induced by endovascular methods.

Nonhuman primate model in clinical modeling of diseases for stem cell therapy

Nonhuman primates (NHPs) are alike humans in size, behavior, physiology, biochemistry, and immunology. Given close similarities to humans, the NHP model offers exceptional opportunities to understand the biological mechanisms and translational applications with direct relevance to human conditions. Here, we evaluate the opportunities and limitations of NHPs as animal models for translational regenerative medicine. NHP models of human disease propose exceptional opportunities to advance stem cell-based therapy by addressing pertinent translational concerns related to this research. Nonetheless, the value of these primates must be carefully assessed, taking into account the expense of specialized equipment and requirement of highly specialized staff. Well-designed initial fundamental studies in small animal models are essential before translating research into NHP models and eventually into human trials. In addition, we suggest that applying a directed and collaborative approach, as seen in the evolution of stroke NHP models, will greatly benefit the translation of stem cell therapy in other NHP disease models.

Reproducibility of a Parkinsonism-related metabolic brain network in non-human primates: A descriptive pilot study with FDG PET

BACKGROUND

We have previously defined a parkinsonism-related metabolic brain network in rhesus macaques using a high-resolution research positron emission tomography camera. This brief article reports a descriptive pilot study to assess the reproducibility of network activity and regional glucose metabolism in independent parkinsonian macaques using a clinical positron emission tomography/CT camera.

METHODS

[(18)F]fluorodeoxyglucose PET scans were acquired longitudinally over 3 months in three drug-naïve parkinsonian and three healthy control cynomolgus macaques. Group difference and test-retest stability in network activity and regional glucose metabolism were evaluated graphically, using all brain images from these macaques.

RESULTS

Comparing the parkinsonian macaques with the controls, network activity was elevated and remained stable over 3 months. Normalized glucose metabolism increased in putamen/globus pallidus and sensorimotor regions but decreased in posterior parietal cortices.

CONCLUSIONS

Parkinsonism-related network activity can be reliably quantified in different macaques with a clinical positron emission tomography/CT scanner and is reproducible over a period typically employed in preclinical intervention studies. This measure can be a useful biomarker of disease process or drug effects in primate models of Parkinson's disease.

Impaired Arm Function and Finger Dexterity in a Nonhuman Primate Model of Stroke

Background and Purpose—

Ischemic stroke is the leading cause of upper extremity motor impairments. Although several well-characterized experimental stroke models exist, modeling of upper extremity motor impairments, which are unique to primates, is not well established. Cortical representation of dexterous movements in nonhuman primates is functionally and topographically similar to that in humans. In this study, we characterize the African green monkey model of focal ischemia reperfusion with a defined syndrome, impaired dexterous movements.

Methods—

Cerebral ischemia was induced by transient occlusion of the M3 segment of the left middle cerebral artery. Motor and cognitive functions after stroke were evaluated using the object retrieval task with barrier-detour. Postmortem magnetic resonance imaging and histopathology were performed to map and characterize the infarct.

Results—

The middle cerebral artery occlusion consistently produced a necrotic infarct localized in the sensorimotor cortex in the middle cerebral artery territory. The infarction was reproducible and resulted in significant loss of fine motor function characterized by impaired dexterity. No significant cognitive impairment was detected. Magnetic resonance imaging and histopathology demonstrated consistent and significant loss of tissue on the left parietal cortex by the central sulcus covering the sensorimotor area. The results suggest that this species has less collateralization, which closely resembles humans.

Conclusions—

The reported nonhuman primate model produces a defined and reproducible syndrome relevant to our understanding of ischemic stroke, cortical representation, and sensorimotor integration controlling dexterous movements. This model will be useful in basic and translational research addressing loss of arm function and dexterity.

Nonhuman primate models in translational regenerative medicine

Humans and nonhuman primates (NHPs) are similar in size, behavior, physiology, biochemistry, structure and function of organs, and complexity of the immune system. Research on NHPs generates complementary data that bridge translational research from small animal models to humans. NHP models of human disease offer unique opportunities to develop stem cell-based therapeutic interventions that directly address relevant and challenging translational aspects of cell transplantation therapy. These include the use of autologous induced pluripotent stem cell-derived cellular products, issues related to the immune response in autologous and allogeneic setting, pros and cons of delivery techniques in a clinical setting, as well as the safety and efficacy of candidate cell lines. The NHP model allows the assessment of complex physiological, biochemical, behavioral, and imaging end points, with direct relevance to human conditions. At the same time, the value of using primates in scientific research must be carefully evaluated and timed due to expense and the necessity for specialized equipment and highly trained personnel. Often it is more efficient and useful to perform initial proof-of-concept studies for new therapeutics in rodents and/or other species before the pivotal studies in NHPs that may eventually lead to first-in-human trials. In this report, we present how the Southwest National Primate Research Center, one of seven NIH-funded National Primate Research Centers, may help the global community in translating promising technologies to the clinical arena.

Imaging of a clinically relevant stroke model: glucose hypermetabolism revisited

BACKGROUND AND PURPOSE

Ischemic stroke has been shown to cause hypermetabolism of glucose in the ischemic penumbra. Experimental and clinical data indicate that infarct-related systemic hyperglycemia is a potential therapeutic target in acute stroke. However, clinical studies aiming for glucose control in acute stroke have neither improved functional outcome nor reduced mortality. Thus, further studies on glucose metabolism in the ischemic brain are warranted.

METHODS

We used a rat model of stroke that preserves collateral flow. The animals were analyzed by [2-(18)F]-2-fluoro-2-deoxy-d-glucose positron emission tomography or magnetic resonance imaging during 90-minute occlusion of the middle cerebral artery and during 60 minutes after reperfusion. Results were correlated to magnetic resonance imaging of cerebral blood flow, diffusion of water, lactate formation, and histological data on cell death and blood-brain barrier breakdown.

RESULTS

We detected an increased [2-(18)F]-2-fluoro-2-deoxy-d-glucose uptake within ischemic regions succumbing to infarction and in the peri-infarct region. Magnetic resonance imaging revealed impairment of blood flow to ischemic levels in the infarct and a reduction of cerebral blood flow in the peri-infarct region. Magnetic resonance spectroscopy revealed lactate in the ischemic region and absence of lactate in the peri-infarct region. Immunohistochemical analyses revealed apoptosis and blood-brain barrier breakdown within the infarct.

CONCLUSIONS

The increased uptake of [2-(18)F]-2-fluoro-2-deoxy-d-glucose in cerebral ischemia most likely reflects hypermetabolism of glucose meeting increased energy needs of ischemic and hypoperfused brain tissue, and it occurs under both anaerobic and aerobic conditions measured by local lactate production. Infarct-related systemic hyperglycemia could serve to facilitate glucose supply to the ischemic brain. Glycemic control by insulin treatment could negatively influence this mechanism.

The potential roles of 18F-FDG-PET in management of acute stroke patients

Extensive efforts have recently been devoted to developing noninvasive imaging tools capable of delineating brain tissue viability (penumbra) during acute ischemic stroke. These efforts could have profound clinical implications for identifying patients who may benefit from tPA beyond the currently approved therapeutic time window and/or patients undergoing neuroendovascular treatments. To date, the DWI/PWI MRI and perfusion CT have received the most attention for identifying ischemic penumbra. However, their routine use in clinical settings remains limited. Preclinical and clinical PET studies with [¹⁸F]-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) have consistently revealed a decreased ¹⁸F-FDG uptake in regions of presumed ischemic core. More importantly, an elevated ¹⁸F-FDG uptake in the peri-ischemic regions has been reported, potentially reflecting viable tissues. To this end, this paper provides a comprehensive review of the literature on the utilization of ¹⁴C-2-DG and ¹⁸F-FDG-PET in experimental as well as human stroke studies. Possible cellular mechanisms and physiological underpinnings attributed to the reported temporal and spatial uptake patterns of ¹⁸F-FDG are addressed. Given the wide availability of ¹⁸F-FDG in routine clinical settings, ¹⁸F-FDG PET may serve as an alternative, non-invasive tool to MRI and CT for the management of acute stroke patients.

Nonhuman primate models of stroke for translational neuroprotection research

Despite the discovery of several promising neuroprotective therapies in rodent models of stroke, no therapy other than the fibrinolytics has been found to be effective in human clinical trials. To address potential discrepancies between rodent and human studies, the Stroke Therapy Academic Industry Roundtable (STAIR) committee suggested that nonhuman primates (NHPs) be used for preclinical, translational stroke studies. Due to the paucity of stroke studies in NHPs, few experimental models have been described. Critical factors in designing NHP stroke models include the choice of species, the method of inducing the stroke and the choice of outcome measures. In this review, we describe established NHP models of stroke and discuss factors that may influence model development with a focus on models that may be useful in preclinical studies for neuroprotective drug screening prior to clinical trials.

Thrombotic stroke in the anesthetized monkey (Macaca mulatta): characterization by MRI--a pilot study

BACKGROUND

The lack of a relevant stroke model in large nonhuman primates hinders the development of innovative diagnostic/therapeutic approaches concerned with this cerebrovascular disease. Our objective was to develop a novel and clinically relevant model of embolic stroke in the anesthetized monkey that incorporates readily available clinical imaging techniques and that would allow the possibility of drug delivery including strategies of reperfusion.

METHODS

Thrombin was injected into the lumen of the middle cerebral artery (MCA) in 12 anesthetized (sevoflurane) male rhesus macaques (Macaca mulatta). Sequential MRI studies (including angiography, FLAIR, PWI, DWI, and gadolinium-enhanced T1W imaging) were performed in a 3T clinical MRI. Physiological and biochemical parameters were monitored throughout the investigations.

RESULTS

Once standardized, the surgical procedure induced transient occlusion of the middle cerebral artery in all operated animals. All animals studied showed spontaneous reperfusion, which occurred some time between 2 h and 7 days post-ictus. Eighty percent of the studied animals showed diffusion/perfusion mismatch. The ischemic lesions at 24 h spared both superficial and profound territories of the MCA. Some animals presented hemorrhagic transformation at 7 days post-ictus.

CONCLUSION

In this study, we developed a pre-clinically relevant model of embolic stroke in the anesthetized nonhuman primate.

Imaging Stroke Evolution after Middle Cerebral Artery Occlusion in Non-human Primates

This article reviews imaging approaches applied to the study of stroke in nonhuman primates. We briefly survey the various surgical and minimally invasive experimental stroke models in nonhuman primates, followed by a summary of studies using computed tomography, positron emission tomography and magnetic resonance imaging and spectroscopy to monitor stroke from the hyperacute phase (within minutes of the onset of cerebral ischemia) to the chronic phase (1 month and beyond).

Translating promising preclinical neuroprotective therapies to human stroke trials

Stroke is the third leading cause of mortality and carries the greatest socioeconomic burden of disease in North America. Despite several promising therapies discovered in the preclinical setting, there have been no positive results in human stroke clinical trials to date. In this article, we review the potential causes for failure and discuss strategies that have been proposed to overcome the barrier to translation of stroke therapies. To improve the chance of success in future human stroke trials, we propose that therapies be tested in stroke models that closely resemble the human condition with molecular, imaging and functional outcomes that relate to outcomes utilized in clinical trials. These strategies include higher-order, old-world, nonhuman primate models of stroke with clinically relevant outcome measures. Although stroke neuroprotection has been looked upon pessimistically given the many failures in clinical trials to date, we propose that neuroprotection in humans is feasible and will be realized with rigorous translational science.

Longitudinal studies of ischemic penumbra by using 18F-FDG PET and MRI techniques in permanent and transient focal cerebral ischemia in rats

At present, the goal of stroke research is the identification of a potential recoverable tissue surrounding the ischemic core, suggested as ischemic penumbra, with the aim of applying a treatment that attenuates the growth of this area. Our purpose was to determine whether a combination of imaging techniques, including (18)F-FDG PET and MRI could identify the penumbra area. Longitudinal studies of (18)F-FDG PET and MRI were performed in rats 3 h, 24 h and 48 h after the onset of ischemia. A transient and a permanent model of focal cerebral ischemia were performed. Regions of interest were located, covering the ischemic core, the border that progresses to infarction (recruited tissue), and the border that recovers (recoverable tissue) with early reperfusion. Analyses show that permanent ischemia produces severe damage, whereas the transient ischemia model does not produce clear damage in ADC maps at the earliest time studied. The only significant differences between values for recoverable tissue, (18)F-FDG (84±2%), ADC (108±5%) and PWI (70±8%), and recruited tissue, (18)F-FDG (77±3%), ADC (109±4%) and PWI (77±4%), are shown in (18)F-FDG ratios. We also show that recoverable tissue values are different from those in non-infarcted tissue. The combination of (18)F-FDG PET, ADC and PWI MRI is useful for identification of ischemic penumbra, with (18)F-FDG PET being the most sensitive approach to its study at early times after stroke, when a clear DWI deficit is not observed.

Performance characterization of the Inveon preclinical small-animal PET/SPECT/CT system for multimodality imaging

PURPOSE

We investigated the performance of the Inveon small-animal PET/SPECT/CT system and compared the imaging capabilities of the SPECT and PET components.

METHODS

For SPECT, the energy resolution, tomographic spatial resolution and system sensitivity were evaluated with a (99m)Tc solution using a single pinhole collimator. For PET, the spatial resolution, absolute sensitivity, scatter fraction and peak noise equivalent count were evaluated. Phantoms and a normal rat were scanned to compare the imaging capabilities of SPECT and PET.

RESULTS

The SPECT spatial resolution was 0.84 mm full-width at half-maximum (FWHM) at a radius of rotation of 25 mm using a 0.5-mm pinhole aperture collimator, while the PET spatial resolution was 1.63 mm FWHM at the centre. The SPECT system sensitivity at a radius of rotation of 25 mm was 35.3 cps/MBq (4 × 10(-3)%) using the 0.5-mm pinhole aperture, while the PET absolute sensitivity was 3.2% for 350-650 keV and 3.432 ns. Accordingly, the volume sensitivity of PET was three orders of magnitude higher than that of SPECT.

CONCLUSION

This integrated PET/SPECT/CT system showed high performance with excellent spatial resolution for SPECT and sensitivity for PET. Based on the tracer availability and system performance, SPECT and PET have complementary roles in multimodality small-animal imaging.

PET in Cerebrovascular Disease

Investigation of the interplay between the cerebral circulation and brain cellular function is fundamental to understanding both the pathophysiology and treatment of stroke. Currently, PET is the only technique that provides accurate, quantitative in vivo regional measurements of both cerebral circulation and cellular metabolism in human subjects. We review normal human cerebral blood flow and metabolism and human PET studies of ischemic stroke, carotid artery disease, vascular dementia, intracerebral hemorrhage and aneurysmal subarachnoid hemorrhage and discuss how these studies have added to our understanding of the pathophysiology of human cerebrovascular disease.

Targeting ischemic penumbra: part I - from pathophysiology to therapeutic strategy

Penumbra is the viable tissue around the irreversibly damaged ischemic core. The purpose of acute stroke treatment is to salvage penumbral tissue and to improve brain function. However, the majority of acute stroke patients who have treatable penumbra are left untreated. Therefore, developing an effective non-recanalizational therapeutics, such as neuroprotective agents, has significant clinical applications. Part I of this serial review on "targeting penumbra" puts special emphases on penumbral pathophysiology and the development of therapeutic strategies. Bioenergetic intervention by massive metabolic suppression and direct energy delivery would be a promising future direction. An effective drug delivery system for this purpose should be able to penetrate BBB and achieve high local tissue drug levels while non-ischemic region being largely unaffected. Selective drug delivery to ischemic stroke penumbra is feasible and deserves intensive research.

Effects of Intra-arterial Urokinase on a Non-human Primate Thromboembolic Stroke Model

One of the most important prognostic factors in the thrombolytic treatment of acute ischemic stroke is to re-canalize. The purpose of this study was to evaluate the effectiveness and safety of urokinase in a primate thromboembolic stroke model. Thromboembolic stroke was accomplished via occlusion of the middle cerebral artery (MCA) obtained by injecting an autologous blood clot into the left internal carotid artery in 21 male cynomolgus monkeys. Animals were randomly assigned to the following treatment groups: Group 1: vehicle (saline), Group 2: urokinase (40,000 IU), Group 3: urokinase (120,000 IU,) over 2 or 6 h via intra-internal carotid catheter starting 1 h after embolization, respectively. In the urokinase-treated groups, neurologic deficits were improved in consciousness and skeletal muscle coordination, but not sensory and motor systems. The infarction size in Group 2 (11.9 +/- 3.9% of the hemisphere) and 3 (7.6 +/- 2.5%) were significantly smaller than that (24.7 +/- 3.5%) in Group 1. However, 2 of 5 animals in Group 3 died. In conclusion, urokinase improved neurologic deficits and reduced cerebral infarction on thromboembolic stroke in the cynomolgus monkey.

Acute studies of a new primate model of reversible middle cerebral artery occlusion

The recent failure of many clinical trials of neuroprotective compounds may be due in part to poor animal models of human stroke. We have developed an endovascular stroke model in nonhuman primates that is compatible with serial magnetic resonance imaging (MRI) monitoring. Using cynomologous macaques (n = 4), a microcatheter was navigated transarterially (under fluoroscopic guidance) from the femoral artery to the middle cerebral artery (MCA). The microcatheter was wedged in a branch of the MCA for 3 hours to cause focal cerebral ischemia, as verified angiographically. During occlusion and/or reperfusion, animals were scanned with MRI, and imaging findings were compared with the stained brain sections. All animals demonstrated small stroke lesions in the expected vascular territory, as seen on diffusion-weighted MRI and confirmed by postmortem examination. Reperfusion after 3 hours was confirmed angiographically (n = 2) and also by MRI (n = 4). The mean initial lesion volume, measured on the postreperfusion MRI scans, was 2.3 +/- 1.3 mL (n = 4). There was good agreement between anatomic location of the lesion on MRI and postmortem histological staining (n = 3). A "minimally invasive" primate model of focal cerebral ischemia was developed that is ideally suited to MRI studies of both acute and chronic stroke. By using serial MRI scans to measure changes in lesion size over time, we will be able to control for variability in lesion size/location. This model should prove useful as a test bed for new stroke therapies, in which noninvasive imaging findings are readily comparable to human stroke.

Clinical perspectives in perfusion: neuroradiologic applications

Imaging of cerebral perfusion, particularly by the dynamic tracking of a bolus of gadolinium-based contrast agent, has emerged from the experimental laboratory and become a routine aspect of neuroradiologic practice. This article discusses the practical implementation of "perfusion" protocols into neuroradiologic examinations, as well as discussing the role of postprocessing and quantitative interpretation in terms of vascular physiology and function. Several key clinical indications are introduced, such as acute cerebral ischemia, chronic vascular disease, and tumors.

Interstitial pO2 in ischemic penumbra and core are differentially affected following transient focal cerebral ischemia in rats

Stroke causes heterogeneous changes in tissue oxygenation, with a region of decreased blood flow, the penumbra, surrounding a severely damaged ischemic core. Treatment of acute ischemic stroke aims to save this penumbra before its irreversible damage by continued ischemia. However, effective treatment remains elusive due to incomplete understanding of processes leading to penumbral death. While oxygenation is central in ischemic neuronal death, it is unclear exactly what actual changes occur in interstitial oxygen tension (pO2) in ischemic regions during stroke, particularly the penumbra. Using the unique capability of in vivo electron paramagnetic resonance (EPR) oximetry to measure localized interstitial pO2, we measured both absolute values, and temporal changes of pO2 in ischemic penumbra and core during ischemia and reperfusion in a rat model. Ischemia rapidly decreased interstitial pO2 to 32% +/- 7.6% and 4% +/- 0.6% of pre-ischemic values in penumbra and core, respectively 1 hour after ischemia. Importantly, whilst reperfusion restored core pO2 close to its pre-ischemic value, penumbral pO2 only partially recovered. Hyperoxic treatment significantly increased penumbral pO2 during ischemia, but not in the core, and also increased penumbral pO2 during reperfusion. These divergent, important changes in pO2 in penumbra and core were explained by combined differences in cellular oxygen consumption rates and microcirculation conditions. We therefore demonstrate that interstitial pO2 in penumbra and core is differentially affected during ischemia and reperfusion, providing new insights to the pathophysiology of stroke. The results support normobaric hyperoxia as a potential early intervention to save penumbral tissue in acute ischemic stroke.

A new primate model of focal stroke: endothelin-1-induced middle cerebral artery occlusion and reperfusion in the common marmoset

The purpose of the present set of studies was to develop a new primate model of focal ischemia with reperfusion for long-term functional assessment in the common marmoset. Initially, the cerebral vascular anatomy of the marmoset was interrogated by Araldite-cast and ink-perfusion methods to determine the feasibility of an intravascular surgical approach. The methods showed that the internal carotid artery was highly tortuous in its passage, precluding the development of an extracranial method of inducing temporary middle cerebral artery occlusion in the marmoset. A pilot dose-response study investigated an intracranial approach of topically applying endothelin-1 (ET-1) to the M2 portion of the middle cerebral artery in a small sample of marmosets for up to 6 hours (n = 2 or 3 per group). Dose-dependent reductions in middle cerebral artery vessel caliber followed by gradual reperfusion were inversely related to increases in corrected lesion volume after ET-1 treatment, relative to vehicle control application. Finally, the functional consequences of ET-1-induced lesions to the M2 vascular territory were assessed up to 24 hours after surgery using the optimal dose established in the pilot study (2.5 nmol/25 microL). ET-1-treated marmosets (n = 4) showed marked contralateral motor deficits in grip strength and retrieval of food rewards and contralateral sensory/motor neglect towards tactile stimulation, relative to their ipsilateral side and vehicle-treated marmosets (n = 4). Strong correlations were shown between contralateral impairments and histopathologic parameters, which revealed unilateral putamen and cortical damage to the middle cerebral artery territory. No deficits were shown on general mobility, and self-care was promptly resumed in ET-1 marmosets after surgery. These results show that this novel model of ischemia with reperfusion in the marmoset has the potential to assess long-term function and to gauge the efficacy of novel therapeutic strategies targeted for clinical stroke.

Post-ischemic cyclooxygenase-2 expression is regulated by the extent of cerebral blood flow reduction in non-human primates

We determined whether up to 24 h of ischemia could induce the expression of cyclooxygenase-2 (COX-2) in the brain of nonhuman primates. Randomized animals were subjected to either a 2 h ischemia (group II; n=3) or a 24 h ischemia (group III; n=3). Three animals in group I served as controls. In group III, regional cerebral blood flow (CBF) and the cerebral glucose metabolic rate (CMRglc) were evaluated using positron emission tomography. Upregulation of COX-2 mRNA expression was observed after 2 h of ischemia, but disappeared by 24 h in the ischemic temporal cortex, in which both CMRglc and CBF were markedly reduced. In the ischemic parietal cortex, where CMRglc was preserved, COX-2 expression persisted even 24 h after ischemia. This study is the first to demonstrate neuronal COX-2 induction within potentially viable hypoperfused brain areas in nonhuman primates.

Unique profile of spreading depression in a primate model

Spreading depression (SD) is considered to play a role in pathologic conditions of humans such as in the evolution of ischemic brain injury and migraine aura. Because many studies have demonstrated spreading hypoperfusion in patients with migraine and persistent hypoperfusion in nonprimate animal models of SD, these changes in cerebral blood flow (CBF) were regarded as an epiphenomenon of SD. However, there is no direct evidence of the occurrence of SD in primates. The authors attempted to elicit SD by applying 3.3 mol/L potassium chloride to the cerebral cortex of nine male cynomolgus monkeys. The CBF was monitored by positron emission tomography in five animals. Propagated direct-current shifts were found by the two neighboring microelectrodes only in one animal. The direct-current wave propagated at a speed of 4 mm/min and its amplitude was 20 mV, being consistent with the SD findings. Except in one animal with 6 SD episodes, SD waves were recorded infrequently at the rostral site (none in three animals, once in three, and twice in two). Focal hyperemia accompanied SD. Neither spreading hypoperfusion nor persistent hypoperfusion was found. These unique features of SD in primates raise a doubt as to whether the role of SD in nonprimate animals is the same as that in stroke and migraine in humans.