Targeting Cerebral Oedema and Elevated Intracranial Pressure in a Preclinical Model of Ischaemic Stroke

Abstract

Stroke is a devastating condition, affecting over 17 million people worldwide annually. Interruption of cerebral arterial blood flow in ischaemic stroke initiates a cascade of deleterious events, leading to irreversible cell damage and secondary injury. Specifically, the development of cerebral oedema, whereby disruption to the blood–brain barrier (BBB) precipitates abnormal fluid accumulation within the brain parenchyma, leads to a consequent rise in intracranial pressure (ICP). Persistently elevated ICP results in loss of cerebral autoregulation and brain herniation, making it a leading cause of death and disability poststroke. Despite this, current treatments fail to prevent the pathophysiological development of oedema, rather targeting the symptoms once established. As such, understanding the mechanisms underlying cerebral oedema and elevated ICP is essential in order to develop more targeted and effective treatments to prevent oedema genesis and improve patient outcomes. Neurogenic inflammation, mediated by substance P (SP), has been linked to profound BBB disruption, oedema development and poor functional outcome post-stroke. SP binds to the neurokinin 1 tachykinin receptor (NK1-R), with administration of an NK1-R antagonist shown to ameliorate BBB dysfunction and cerebral oedema following stroke in rodent models. However, more clinically-relevant animal models are required to validate efficacy of novel stroke therapeutics to improve clinical translation. Thus, this thesis sought to determine the efficacy of NK1-R antagonist treatment in reducing cerebral oedema and ICP in a clinically-relevant ovine stroke model. Merino sheep (Ovis aries; n=125) were used across the five thesis studies. For Aim 1 (n=34F), a non-survival permanent middle cerebral artery occlusion (MCAo) model was used to determine the efficacy of various NK1-R receptor antagonist regimens or decompressive craniectomy in reducing ICP post-stroke. Once an optimum dosage was determined, Aims 2 and 3 (n=23F;24M) involved development of a transient MCAo survival model and characterisation of the temporal profile of oedema and ICP. Finally, Aim 4 and 5 (n=9F;9M), sought to determine the ideal time-course of treatment, and investigate the efficacy of the NK1-R antagonist following transient stroke via assessment of neurological and functional outcomes. ICP was assessed via invasive monitoring, with arterial blood pressure, temperature and blood gases also measured. Magnetic resonance imaging (MRI) and immunohistochemistry (IHC) was also performed. Motion capture and a modified neuroscore was used to assess changes in motor function and demeanour. This thesis identified that two doses of the NK1-R antagonist were efficacious in reducing ICP following permanent MCAo. A transient stroke model was successfully developed and characterised, with ICP peaking at 5-6 days post-stroke. NK1-R antagonist treatment, both acutely (1-3 days), and in a delayed fashion (5-day), significantly reduced ICP post-stroke. Finally, a motion capture tool was successfully established and validated as a quantitative method to assess motor function following transient ovine stroke, with animal gait significantly impaired post-stroke. The work presented in this thesis encompasses a comprehensive evaluation of the efficacy of NK1-R antagonist treatment in a clinically-relevant ovine model, providing strong preclinical evidence for further investigation. Accordingly, Phase II trials are currently underway in Australia, the United Kingdom and the United States of America.