By Shahid M. Nimjee, MD, Ph.D., FAANS, FAHA, The Ohio State University Medical Center and Basking Biosciences
Affecting about 800,000 people yearly and the leading cause of death and disability in the United States, acute ischemic stroke (AIS) accounts for almost 87% of all strokes and remains an area of high unmet medical need. While several therapeutic options are available to patients, each has its limitations, including narrow therapeutic windows, risk of bleeding, and accessibility issues.
Developing new treatments for acute ischemic stroke that can quickly and safely intervene at the right place and time has been a particularly challenging endeavor characterized by a graveyard of clinical trial failures since the early 2000s. However, today the situation is changing due to technological developments related to stroke and recent advances in RNA-based medicines.
The Limitations Of Current Treatments For Acute Ischemic Stroke
Tissue plasminogen activator (tPA) is a clot-dissolving medicine approved in 1996 by the U.S. FDA to treat acute ischemic stroke. Unfortunately, tPA has several drawbacks that limit its clinical usage. These include its narrow therapeutic window of 3 to 4.5 hours, potential complications with IV administration, and a high risk of bleeding into the brain (hemorrhages), which due to a lack of reversibility is the most severe complication of treatment, limiting its usefulness for the majority of stroke patients.
A newer treatment option for patients with acute ischemic stroke is endovascular thrombectomy (EVT), a minimally invasive surgical procedure that uses a mechanical device to remove an intra-arterial blood clot in patients who present with large vessel occlusion (LVO) stroke. This condition is where the blood clot is in one of the main arteries of the brain, including the internal carotid artery, middle cerebral artery, anterior cerebral artery, or basilar artery. During an endovascular thrombectomy, a CT angiogram scan confirms the location and size of the clot, which is then removed mechanically via an endovascular procedure using a catheter threaded through the arteries. Clinical studies show the method can significantly increase a stroke patient's return to independent life and drastically reduce mortality.
While EVT represents a significant advancement in AIS care, LVO stroke as described above represents only ~30% of all AIS, thereby leaving the majority of patients without acute treatment. Moreover, the medical infrastructure required to identify and treat a patient with an LVO is such that this therapy is limited to nations with comprehensive healthcare systems.
The limitations of both tPA and EVT treatments leave almost 85% of patients without acute intervention. Therefore, there is a significant unmet need for a widely accessible, off-the-shelf drug with a broad therapeutic window that is safe, effective, and reversible in the event of unwanted bleeding.
The Hunt For A New And Better Drug Target — Von Willebrand Factor (vWF)
Von Willebrand Factor (vWF) is a potential thrombolytic target that plays a role in blood clot formation during vascular damage. Expressed in platelets and endothelial cells, vWF is a critical protein for platelet function and participates in platelet adhesion, activation, and aggregation. vWF has been identified as a target ubiquitous to blood clots in the brain, acting as a critical structural component and driver of the clotting process.
Over the last two decades, scientists have studied vWF as a potential drug target for stroke. Research has shown that increased vWF levels in patients correlate to higher risk of stroke, myocardial infarction, and incidence of death. Moreover, diseases such as the rare blood disorders thrombotic thrombocytopenic purpura (TTP) and von Willebrand disease are characterized by dysfunctional expression of vWF. Patients with TTP have excess amounts of vWF and are predisposed to stroke and acute coronary thrombosis. Conversely, patients with von Willebrand disease have reduced amounts of vWF. Eighty percent of those individuals have the mild Type 1 form, where they do not have spontaneous bleeding episodes and do not experience a heart attack or stroke. Preclinical studies support this claim, demonstrating that genetic deficiency in vWF protects mice from ischemic stroke.
Applicability Of RNA Aptamers As Drug Candidates
While many RNA-based therapeutics have been discovered and developed, there have been few clinical successes. Recently, however, the industry has seen the effectiveness of RNA-based vaccines during the COVID-19 pandemic and siRNA medicines that provide significant amelioration of rare diseases. The industry is at an inflection point, and there continues to be an evolution in the life sciences community to uncover the power of RNA for treating a broad range of conditions -- from cancer to neurological disorders.
Aptamers are an emerging class of therapeutics. These single-stranded nucleic acid molecules (either DNA or RNA) directly bind and inhibit proteins through three-dimensional conformational folding. There are several aptamers in development as potential therapeutics, but only one aptamer-based drug is currently FDA-approved -- pegaptanib for treating macular degeneration in the eye.
Compared to antibodies, a significant benefit of aptamers is their high-affinity binding and specificity, allowing for differential targeting of proteins that share similar structural epitopes. Other advantages over antibodies include the ability to select aptamer candidates through in vitro methods, their uniform batch activity, the ability to quickly alter their structure for desired pharmacokinetic characteristics, their low inherent toxicity and low to no inherent immunogenicity, and their unlimited shelf life. Moreover, a reversal agent/antidote can be rationally designed against a given aptamer to halt its activity.
Aptamers with anti-thrombotic properties that can target extracellular proteins could offer patients a novel treatment option for thrombosis and vascular disease. One area where aptamers can provide utility is targeting blood-based proteins like vWF that play a role in cardiovascular diseases. Care must still be taken when developing aptamers against targets like vWF since bleeding into the brain is the most significant risk associated with developing an anti-thrombotic drug. However, teaming a therapeutic aptamer with a complementary reversal agent capable of halting its activity as needed could offer patients a novel and valuable treatment option for thrombosis and vascular disease.
BB-031: A Novel Aptamer Targeting vWF
Targeting vWF has the potential to significantly improve outcomes following acute ischemic stroke, and using RNA aptamers for therapeutic intervention could enable better targeting of the protein. To that end, scientists at Basking Biosciences are developing a novel anti-vWF RNA aptamer, BB-031, that binds and inhibits vWF-mediated platelet adhesion and arterial thrombosis. In tandem, they are also developing second aptamer, BB-025, that quickly neutralizes the effects of BB-031 in the event of bleeding, thus proving a potentially safer and more effective treatment option for patients following an acute ischemic stroke. Potential advantages of an anti-vWF RNA aptamer include a broader therapeutic window, applicability to a wide range of blood clots, and lack of disruption to normal hemostatic processes.
In vivo preclinical studies in a canine model of ischemic stroke demonstrated that BB-031 recanalized blocked blood vessels at rates of up to 60% when administered 6 hours after the formation of a large vessel occlusion compared to no revascularization in the control group. These results, along with data from additional animal studies, show that BB-031 can reduce infarct volume and prevent bleeding. Administration of the antidote drug BB-025 immediately reversed the effects of BB-031 within 5 minutes, essentially turning it off. This reversal strategy does not generate a procoagulant phenotype; instead, the reversal agent acts on the active drug. This approach differs from coagulation reversal currently used to reverse the anticoagulant warfarin by overwhelming the drug with clotting factors, which puts some patients at risk of thrombosis.
Results of a Phase 1 clinical study in healthy volunteers showed BB-031 to be safe and well tolerated with no significant or treatment-emergent adverse events reported over the 28-day study period. Additionally, platelet function analysis showed complete inhibition of clot formation at all doses tested, dose-dependent duration of clotting inhibition, and time to return to normal clotting. A Phase 2 clinical proof-of-concept study of BB-031 is planned in patients with acute ischemic stroke.
Summary And Conclusions
While tPA and EVT offer clinical benefits, the multiple factors limiting their effective use have left the majority of patients suffering acute ischemic stroke without efficacious treatment, and thus at risk of the significant morbidities and mortality associated with severe stroke events. Today, however, as our understanding of the molecular mechanisms behind stroke evolves, new therapeutic strategies for addressing acute ischemic stroke are emerging. RNA aptamers that directly bind to the blood-clotting extracellular target vWF demonstrate a desirable thrombolytic profile. Taken together with a second RNA molecule to bind to the first one to essentially turn it off, our safety and efficacy data suggest that this may offer the potential to widen the treatment window and expand the patient population for those who suffer from acute ischemic stroke, thus improving clinical outcomes and quality of life for patients.
About The Author:
Shahid M. Nimjee, MD, Ph.D., FAANS, FAHA is a professor of neurosurgery and a practicing neurosurgeon at The Ohio State University Wexner Medical Center (OSUWMC), where he treats stroke patients. He also leads a basic research laboratory at OSUWMC focused on evaluating anti-vWF aptamers and their antidotes to treat thromboembolic stroke. Nimjee is additionally the co-founder and interim chief medical officer at Basking Biosciences, where he is responsible for leading preclinical research activities at his laboratory at OSUWMC and designing the clinical trial plan for the company’s drug candidates, BB-031 and BB-025.