Advances in Acute Ischemic Stroke Therapy

Intravenous thrombolysis (IVT) is the mainstay of acute ischemic stroke therapy. Over the past 5 years, IVT has been moving from a purely time window to include a tissue clock approach, the development of new thrombolytic drugs and to determine whether IVT should be skipped or used as bridging therapy before thrombectomy for patients with anterior circulation LVO.

Staphylokinase was first isolated and purified in 1948. It has high fibrin-selectivity, but early experience in patients with acute myocardial infarction showed that many patients developed neutralizing antibodies, preventing further research on this agent.³⁹ A new thrombolytic drug, nonimmunogenic staphylokinase is a modified recombinant staphylokinase with low immunogenicity, high thrombolytic activity, and selectivity to fibrin. Recently, the FRIDA trial (Nonimmunogenic Recombinant Staphylokinase Versus Alteplase for Patients With Acute Ischemic Stroke 4.5 Hours After Symptom Onset in Russia) in 385 patients revealed that nonimmunogenic staphylokinase was noninferior to alteplase on favorable functional outcome (OR, 1.47 [95% CI, 0.93–2.32) with comparable sICH rates.⁴⁰ This phase 3, randomized, multicenter, parallel-group, open-label, noninferiority trial was challenged because of its relatively wide noninferiority margin (16%), small sample size, and lack of phase 2 trial data in acute ischemic stroke.⁴¹ Further trials are needed to investigate the superiority of nonimmunogenic staphylokinase compared with alteplase.

A meta-analysis of 5 major RCTs (MR CLEAN [Multicenter Randomized Clinical Trial of Endovascular Treatment for Acute Ischemic Stroke in the Netherlands],⁴⁵ ESCAPE [Endovascular Treatment for Small Core and Anterior Circulation Proximal Occlusion With Emphasis on Minimizing CT to Recanalization Times],⁴⁶ REVASCAT [Randomized Trial of Revascularization With Solitaire FR Device Versus Best Medical Therapy in the Treatment of Acute Stroke Due to Anterior Circulation Large Vessel Occlusion Presenting Within Eight Hours of Symptom Onset],⁴⁷ SWIFT PRIME [Solitaire With the Intention for Thrombectomy as Primary Endovascular Treatment],⁴⁸ and EXTEND-IA [Extending the Time for Thrombolysis in Emergency Neurological Deficits—Intra-Arterial]⁴⁹) showed that EVT was superior to medical management of acute ischemic stroke associated with LVO of the middle cerebral artery (MCA) and internal carotid artery (ICA) primarily within 6 hours of symptom onset as seen in Figure 4.⁹ EVT led to significantly reduced disability at 90 days compared with control (adjusted common OR, 2·49 [95% CI, 1·76–3·53]; P<0.0001). The number of patients needed to treat to reduce disability by at least one level on the mRS for one patient was 2.6. EVT was beneficial over medical management in several prespecified subgroups, including in subjects ≥80 years (common OR, 3.68 [95% CI, 1.95–6.92]), patients randomized >5 hours after symptom onset (1·76, 1·05–2·97), and those not qualifying for intravenous tPA (2·43, 1·30–4·55; mixed-effects ordinal logistic regression models). Mortality at 90 days and the risk of sICH did not differ between studied populations.

Subsequent EVT studies, the DAWN (Diffusion-Weighted Imaging or Computerized Tomography Perfusion Assessment with Clinical Mismatch in the Triage of Wake-Up and Late Presenting Strokes Undergoing Neurointervention With Trevo)⁵⁰ and DEFUSE 3 trials (Endovascular Therapy Following Imaging Evaluation for Ischemic Stroke)⁵¹ extended the time window for revascularization up to 24 hours after symptom onset. DAWN compared the use of a specific stent-retriever (Trevo, Stryker NV, Fremont, CA) for a 6- to 24-hour time window.⁵⁰ Patients had to have a mismatch between the severity of the clinical deficit and the infarct volume associated with an ICA or M1/MCA occlusion. Infarct volume was determined with diffusion-weighted MRI or perfusion CT (automated software RAPID, iSchemaView). A total of 107 subjects were randomized to EVT and 99 to the control group. Median infarct volume was small in both subgroups (7.6 mL for EVT and 8.9 mL for control group; interquartile range, 2.0–18.0 and 3.0–18.1). The rate of functional independence at 90 days was 49% in the thrombectomy group as compared to 13% in the control group (adjusted difference, 33% [95% CI, 21–44]; posterior probability of superiority, >0.999). The rate of neurological deterioration was lower in the EVT group than in the control group (14% versus 26%; absolute difference, −12 percentage points; 95% CI, −23 to −1; P=0.04). Serious adverse event rates including stroke-related death at 90 days and sICH did not differ between the two subgroups. The DEFUSE 3 trial enrolled patients within the 6- to 16-hour time window and an infarct size <70 mL who had a ratio of volume of ischemic tissue to initial infarct volume of 1.8 or more on CT perfusion or MR diffusion and perfusion imaging (RAPID software, iSchemaView).⁵¹ If needed, carotid angioplasty with or without stenting was permitted as part of the intervention. Based on an interim analysis, the trial was halted prematurely because the prespecified efficacy boundaries were reached. The rate of complete recanalization at 24 hours in the EVT group was significantly higher (78% versus 18% for medical therapy). A higher percentage of patients were functionally independent (0–2 mRS) following EVT as compared to the medical arm (45% versus 17%, risk ratio, 2.67 [95% CI, 1.60–4.48]; P<0.001). The 90-day mortality rate was 14% in the EVT group versus 26% in the control arm (P=0.05), the frequency of sICH (7% and 4%, P=0.75), and serious adverse events (43% and 53%, P=0.18). Interestingly, despite the clinical benefit of EVT, there was no difference in median infarct growth and volume at 24-hour as compared to medical therapy. The investigators suggest that infarct growth may develop over several days in patients who do not experience reperfusion. Both trials expanded the time window for EVT up to 24 h based on CT/MR-perfusion imaging and a mismatch in a selected patient population. In 2019, the American Heart Association/American Stroke Association acute stroke guidelines expanded the class I level A recommendation for LVO of MCA and ICA up to 16 hours and class IIA level B-R recommendation for patients presenting between 16- and 24-hour time window and a NIHSS score of ≥6.³⁸ However, the recently published AURORA (Analysis of pooled data from randomized studies of thrombectomy more than 6 hours after last known well) meta-analysis of randomized controlled EVT trials >6 hours after last-well-known, suggests that the American Heart Association guideline distinction between DEFUSE 3 (6–16 hours) and DAWN (6–24 hours) selection criteria is unnecessary.⁵² Thrombectomy was beneficial over best medical management with an unadjusted common OR of 2.42 (95% CI, 1·76–3.33]; P<0·0001) and higher rates of independent living (mRS score ≤2; 122 [45.9%] of 266 versus 46 [19.3%] of 238; P<0.0001). No heterogeneity of treatment effect was found across subgroups characterized by age, gender, baseline stroke severity, arterial occlusion site, baseline Alberta Stroke Program Early CT Score, and mode of presentation. The EVT treatment effect was stronger in patients randomly assigned within 12 to 24 hours (common OR, 5.86 [95% CI, 3.14–10.94]) than those randomized between 6 and 12 hours (OR, 1.76 [95% CI, 1.18–2.62]; P for interaction=0.0087). At 90-day no significant difference in mortality or symptomatic ICH was found between the intervention and control groups (mortality 44 [16·5%] of 266 versus 46 [19·3%] of 238; sICH 14 [5·3%] of 266 versus 8 [3·3%] of 239). Pooled mixed-effects modeling with fixed effects for parameters of interest showed that EVT was associated with a reduction in disability at 90 days in the clinical mismatch (OR, 3.57 [95% CI, 2.29–5.57]; P<0.001) and target perfusion mismatch (OR, 3.13 [95% CI, 2.10–4.66]; P=0.001) subgroups as compared with standard medical management in patients presenting 6- to 24-hour last known well.⁵³ The best OR calculated was for the subgroup between 12 and 24 hours after last known well (clinical mismatch subgroup, OR, 4.95 [95% CI, 2.20–11.16]; P<0.001; target perfusion mismatch subgroup, OR, 5.01 [95% CI, 2.37–10.60]; P<0.001).

A recently published multinational cohort study showed that in the extended time window (within 6 to 24 hours last known well) there were no significant differences in 90-day clinical outcomes (ordinal mRS shift) when patients for EVT were selected based on a noncontrast CT compared with those selected with CTP or MRI (CT versus CTP, adjusted OR, 0.95 [95% CI, 0.77–1.17]; P=0.64), CT versus MRI adjusted OR, 0.95 [95% CI, 0.8–1.13]; P=0.55) but lower in patients selected by MRI than CT (adjusted OR, 0.79 [95% CI, 0.64–0.98]; P=0.03). Successful reperfusion was more common in the CT and CTP groups compared with the MRI group (474 [88.9%] and 670 [89.5%] versus 250 [78.9%]; P<0.001). No significant differences in symptomatic ICH (CT, 42 [8.1%]; CTP, 43 [5.8%]; MRI, 15 [4.7%]; P=0.11) or 90-day mortality (CT, 125 [23.4%]; CTP, 159 [21.1%]; MRI, 62 [19.5%]; P=0.38) were found.⁵⁴ Thus, in this patient population the use of noncontrast CT, readily available in most centers, may suffice to make the decision for an EVT treatment.

EVT trials (DIRECT ANGIO [Effect of Direct Transfer to Angiosuite on Functional Outcome in Patient With Severe Acute Stroke Treated With Thrombectomy, https://www.clinicaltrials.gov; Unique identifier: NCT03969511],⁵⁵ WE-TRUST [Workflow Optimization to Reduce Time to Endovascular Reperfusion for Ultra-fast Stroke Treatment, https://www.clinicaltrials.gov; Unique identifier: NCT04701684]) are underway to further improve the workflow and reduce time to treatment by direct patient transfer to catheter labs, bypassing the Emergency Department (ED) and conventional CT scanners. Newer angiography systems used for EVT have either integrated CT scanning capabilities using flat-panel detectors (Cone-beam CT) or provide a hybrid system composed of a CT scanner and an angiography system. Early but growing experience in randomized clinical trials has shown a 50%-time reduction in symptom-to-recanalization with direct transfer to the angiography suite and significant improvement in clinical outcomes.^56–58 Several comprehensive stroke centers have already adopted this workflow successfully.

Alternative thrombectomy strategies have been developed as the first pass technique with direct aspiration being one of the most promising. In the ASTER trial (Contact Aspiration versus Stent Retriever for Successful Revascularization), first-line thrombectomy with contact aspiration was compared with stent-retriever. Contact aspiration did not result in an increased successful revascularization rate.⁵⁹ For the primary outcome, successful revascularization was achieved in 85.4% in the contact aspiration group versus 83.1% in the stent-retriever group (OR, 1.20 [95% CI, 0.68–2.10]; P=0.53; difference 2.4% [95% CI, −5.4% to 9.7%]). There were no differences in clinical efficacy and serious adverse events between both groups. Similarly in the COMPASS trial (Aspiration Thrombectomy Versus Stent Retriever Thrombectomy as First-Line Approach for Large Vessel Occlusion) trial, although cost-effective, direct aspiration as first-pass thrombectomy showed a noninferior functional outcome at 90 days compared with stent-retriever first-line thrombectomy.⁶⁰ A modified Rankin score of 0–2 at 90 days was achieved in 52% (95% CI, 43.8–60.3) in the aspiration group and versus 50% (95% CI, 41.6–57.4]) in the stent-retriever group, showing that aspiration as first pass was noninferior to stent-retriever first-line (noninferiority P=0.0014). There were no differences in ICH or all-cause mortality at 3 months between both groups. Of note, in 21% of the aspiration first-pass thrombectomy group, a stent-retriever was used. Distal aspiration during stent-retriever thrombectomy was used in 85% of the first-line stent-retriever group.

Critical for a good functional outcome (mRS score of 0–1) as adjudicated at 90 days, remains an early and complete revascularization (grade 3 on the Treatment in Cerebral Ischemia Score 3) achieved with a single pass.⁶¹ Clot characteristics, that is, density, prone to fracture, amount of calcification, perviousness, length, location, and vessel wall interaction may all play a role in achieving a first pass effect and is being studied using CT imaging and various prediction algorithms based on imaging.^62,63

Use of balloon guide catheters to induce flow arrest and reversal during EVT has proven to reduce the time to revascularization, increase the first pass effect during thrombectomy and improve clinical outcome and was described initially based on the NASA (North American Solitaire Acute Stroke) registry.⁶⁴ The EVT–balloon guide catheter group had a higher rate of first-pass effect (48%) versus EVT with use of a conventional guide catheter (26%; P=0.001) or EVT with a distal access catheter (35%; P=0.002).⁶⁵ The balloon guide catheter group achieved the highest rate of functional independence (61%) versus conventional guide catheter (42%; P=0.007) and distal access catheter (52%; P=0.027). Final revascularization and mortality rates did not differ across the groups.

To broaden the indication for EVT, numerous ongoing trials are investigating the value of thrombectomy alone or in combination with thrombolysis for patients presenting within 4.5-hour time window, with a large core volume infarct >70 mL, an Alberta Stroke Program Early CT Score <6, mild strokes NIHSS score <6 and subjects presenting with distal anterior (A1/A2) and middle cerebral artery (M2/M3) occlusion. Based on a pooled meta-analysis of several EVT studies, the 2019 American Heart Association/American Stroke Association guidelines have issued a class IIB recommendation for EVT in patients with a symptomatic occlusion of M2 or M3/MCA segments.^38,66 Limitations of these studies include small sample sizes, selection bias, definition and size of treated M2 segments, lack of control groups, and various EVT modalities used. Nevertheless, the 2019 Society for Neuro-Interventional Surgery guidelines have issued a class I, level A recommendation for thrombectomy in occlusions of the ICA (including intracranial, cervical segments, or tandem occlusion), and M1/M2-MCA that replaces the 2015 Society for Neuro-Interventional Surgery guidelines in which thrombectomy recommendations were limited to ICA and MCA M1 LVO locations.⁶⁷ Although single-center case series have demonstrated the safety and feasibility of EVT for patients with a low NIHSS <6, a recent multicenter study of 251 subjects with LVO did not show any significant difference between the EVT and the medical arm.⁶⁸ Other future studies will have to clarify the role of EVT in the growing population of nonagenarians presenting with acute ischemic stroke, subjects <18 years of age, patients with a ≥3 mRS at baseline, subjects with a large core stroke volume with a low Alberta Stroke Program Early CT Score, and impact of associated co-morbidities, for example, cancer on functional outcome.

Use of conscious sedation versus general anesthesia (GA) for EVT has been debated over the past decade but has not shown significant difference in clinical outcomes between both groups.⁶⁹ A total of 150 subjects were studied. The mean NIHSS score for the GA group was 16.8 at admission versus 13.6 after 24 hours; difference, −3.2 points (95% CI −5.6 to −0.8), and for the conscious sedation group, the mean NIHSS score was 17.2 at admission versus 13.6 after 24 hours; difference, −3.6 points (95% CI −5.5 to −1.7); mean difference between groups, −0.4 (95% CI −3.4 to 2.7; P=0.82). However, in the GA versus conscious sedation group, a higher rate of hypothermia (32.9% versus 9.1%; P<0.001) and pneumonia (13.7% versus 3.9%; P=0.03) were encountered. There were no differences in mortality at 3 months (24.7% in both groups). With the exception of a recently published meta-analysis including randomized data from 3 single-center trials,⁷⁰ other randomized studies have reached similar conclusions that GA did not impact clinical outcome as compared to conscious sedation.⁷¹ This study showed an increased risk for hypotension during GA (decline in systolic blood pressure of >20% from baseline (80.8% versus 53.1% for the conscious sedation subgroup; OR, 4.26 [95% CI, 2.55–7.09]; P<0.001) and blood pressure variability (systolic blood pressure >180 mm Hg or <120 mm Hg; 79.7 versus 62.3%; OR, 2.42 [95% CI, 1.49–3.93]; P<0.001) impacting the mean 90-day mRS score (2.8 [95% CI, 2.5–3.1]) in the general anesthesia group versus (3.2 [95% CI, 3.0–3.5]) in the sedation group (difference, 0.43 [95% CI, 0.03–0.83]; common OR, 1.58 [95% CI, 1.09–2.29]; P=0.02).⁷⁰

Although effective in the anterior circulation, the value of EVT in basilar artery occlusion has not been established in RCTs. Two recently published studies (BEST [Basilar Artery Occlusion Endovascular Intervention Versus Standard Medical Treatment]⁷² and BASICS [Basilar Artery International Cooperation Study]⁷³ trials) showed no benefit of EVT for basilar artery occlusion. The BEST trial after enrolling 131 patients in China was prematurely terminated because of a high crossover rate and poor recruitment. In the small sample size studied 42% of patients in the EVT and 32% in the medical arm had an mRS of 0 to 3 at 90 days (statistically not significant). However, patients treated with EVT per-protocol had significantly better outcomes for all the primary and secondary functional end points on adjusted analysis compared with those of patients receiving standard medical care. In the BASICS trial, EVT was initiated at a median of 4.4 hours after stroke onset, and 154 subjects were enrolled in the EVT group and 146 in the medical arm. There was no difference in the use of intravenous thrombolysis in both groups (78.6% in EVT and 79.5% in the medical group). A favorable functional outcome occurred in 44.2% of the EVT population versus 37.7% in the medical arm (risk ratio, 1.18 [95% CI, 0.92–1.50]). sICH occurred in 4.5% of the patients after endovascular therapy and in 0.7% of those after medical therapy (risk ratio, 6.9 [95% CI, 0.9–53.0]); mortality at 90 days was 38.3% and 43.2%, respectively (risk ratio, 0.87 [95% CI, 0.68–1.12]). The study has several limitations including that a large percentage of patients received EVT outside the trial thus causing selection bias.⁷⁴ A beneficial impact of EVT in these subjects was not reported. Patients would have likely benefited from EVT with appropriate use of imaging, such as assessment of the collateral circulation on CT angiography and size of core infarct on MRI diffusion.⁷⁵ As proposed by the editorial, a sensitive outcome measures of functional disability in patients with strokes in the basilar artery territory should be developed.⁷⁴

The plethora of prior therapeutic approaches targeted at the cellular consequences of focal ischemic brain injury were all unsuccessful. There are multiple reasons for these prior failures and they include both preclinical testing in animal stroke models and clinical trial design and implementation as outlined in Table 3.⁴ Regarding preclinical modeling deficiencies, the lessons learned from past failures have led to strengthening of the rigor of current preclinical evaluation programs of purported cytoprotective molecules as reviewed in Table 4.⁹² In the future, most cytoprotective drugs will be developed in conjunction with reperfusion therapy, as an adjunct, so the preclinical modeling assessment should focus on temporary occlusion models. Evaluation in permanent occlusion stroke models should also be performed, especially when the cytoprotective drug under development will be evaluated in clinical trials that focus on slowing down the evolution of the ischemic penumbra into ischemic core. For drugs that will target reperfusion injury, preclinical testing should only be done in temporary occlusion stroke models with the initiation of the drug after reperfusion to anticipate how the clinical trials will likely be conducted. A recent advance in the approach to the preclinical assessment of purported cytoprotective drugs has been the implementation of laboratory networks at multiple sites akin to what is done in later-stage clinical trials. In the United States, the National Institutes of Health–funded SPAN (Stroke Preclinical Assessment Network) which is currently evaluating five cytoprotective drugs in six laboratories that were selected by peer review.⁹³ A similar pilot approach was done previously in Europe. The rationale for this multicenter evaluation approach is that before cytoprotective drug candidates advance to clinical trials they should demonstrate favorable outcomes in stroke models in several laboratories that perform rigorous and careful stroke modeling experiments, that is, the beneficial effects of treatment are reproducible and robust.

In the past, cytoprotective drugs under development typically targeted one aspect of the ischemic cascade, and drugs that had multiple targets were considered to be flawed. With our enhanced understanding of the ischemic cascade in neurons and its complexity, as well as the roles of the other components of the neurovascular unit, drugs that affect multiple aspects of the ischemic cascade and the neurovascular unit actually appears to be more attractive because they may have a better chance to impede focal ischemic brain injury in several different ways. Previous clinical trials of potential cytoprotective drugs predominantly included patients who did not receive thrombolytic therapy so most of the patients included had an occluded blood vessel at the time the drug was initially given. With reperfusion, drug delivery will be enhanced, so in the future, if a candidate cytoprotective drug is given after imaging confirmed reperfusion, then drug delivery to the target brain tissue should be enhanced. Another potential explanation for the plethora of unsuccessful cytoprotective drug development programs is that patients were typically included in clinical trials at late time points after stroke onset.⁹⁴ Advanced imaging was not used to determine if patients had persistence of a substantial amount of ischemic penumbra as a mechanism to identify patients likely to benefit, nor were slow progressors distinguished from more rapid progressors. In some prior trials, safety concerns reduced the dose that could be given and likely affected the chance of a beneficial treatment effect.

The future development of cytoprotective drugs for acute ischemic stroke in the era of highly effective reperfusion therapy can be envisioned in several ways. The first possibility is to administer the cytoprotective drug before thrombectomy. Many patients with LVO related ischemic stroke who are candidates for thrombectomy initially present at smaller hospitals where the procedure is not currently available. If their clinical and imaging evaluation at the outlying hospital identifies them as potential thrombectomy candidates, they must be transferred to a thrombectomy-capable center. This interfacility transfer can take several hours or longer depending upon the distance between the two facilities and also on traffic patterns. In patients with a substantial amount of ischemic penumbra, progression to irreversible injury occurs at variable rates and is problematic in fast progressors. A potential role for cytoprotection would be to slow down this progression during transport. A clinical trial can be envisioned using a hub and spoke model.¹¹ Patients presenting to the smaller hospital would be enrolled there where they are initially evaluated. If they demonstrate appropriate clinical deficits and their imaging studies confirm an LVO and significant mismatch between the ischemic core and penumbra, they would then be randomized to the study cytoprotective drug versus placebo. After transport to the thrombectomy center, they would undergo repeat imaging to assess whether the ischemic core has enlarged. The primary end point of this type of phase 2 trial would be ischemic core growth on advanced imaging with the drug versus placebo. A secondary end point could be the percentage of patients in the two groups who remain thrombectomy candidates, if ongoing studies determine that thrombectomy is futile when the ischemic core exceeds a certain threshold. Typical clinical outcome data would also be collected to help inform the design and adequate sample size estimates for a phase 3 clinical efficacy trial. For such a trial, the candidate cytoprotective drug should have demonstrated a reduction of infarct size and improved functional outcome in permanent occlusion animal stroke models and ideally a beneficial effect on ischemic core growth in vivo with the use of advanced imaging techniques should also be identified.

A trial with initiation of treatment in the ambulance is another potential approach to clinical trial design before thrombectomy. The FAST-MAG trial (Field Administration of Stroke Therapy-/Magnesium) of magnesium was the first phase 3 clinical trial to demonstrate that an acute stroke trial could be performed in the prehospital setting.⁹⁵ With the advent of mobile stroke unit ambulances equipped with CT scanners, appropriately trained personnel and telemedicine capability, an ambulance-based trial in thrombectomy candidates is now feasible in some locations. MSUs have demonstrated improved outcomes for onboard thrombolysis as compared to transporting patients to hospital for initiation of treatment.^18,19 A trial can be designed in a similar manner to a facility to facility transfer trial with the help of a vascular neurologist at the thrombectomy-capable center via telemedicine who can evaluate the patient in conjunction with the ambulance staff and also help to interpret the head CT and CT angiogram obtained in the ambulance. Appropriate patients can then be consented and randomized while being transported to the thrombectomy center and the primary outcome would again likely be growth of ischemic core after initiation of the study drug versus placebo.

Another type of clinical trial for cytoprotection initiated before thrombectomy would be randomization of thrombectomy eligible patients at the thrombectomy-capable center before the procedure. If the study drug can be infused rapidly, its administration would be completed before thrombectomy and if a more prolonged administration is needed then it can be continued during and after the procedure. This type of trial could also include patients who are being transferred from outlying hospitals with the capability to perform the necessary imaging studies required for enrollment. Such a trial would have a clinical efficacy primary outcome measure such as the 90-day mRS that is typically used for determining efficacy in acute stroke therapy trials. Such a trial would evaluate if the cytoprotective drug conferred additional benefit to that observed with thrombectomy alone and would probably require a substantial number of patients to be adequately powered.

The recently completed phase 3 trial of Nerinetide, a PSD-95 (post-synaptic density protein-95) inhibitor, was the first such trial.⁹⁶ In the ESCAPE-NA1 trial (Safety and efficacy of nerinetide [NA-1] in subjects undergoing endovascular thrombectomy for stroke) thrombectomy candidates without an extensive ischemic core and moderate to good collaterals were randomized to intravenous Nerinetide or placebo before thrombectomy at an outlying hospital or the thrombectomy-capable center. At 90 days, a favorable mRS outcome of 0 to 2 was achieved in 59.2% of the placebo group and 61.4% in the Nerinetide group, a nonstatistically significant difference. However, in a prespecified subgroup analysis of patients who did not receive tPA before randomization, a 9.5% absolute difference of favorable outcome with Nerinetide was observed that was statistically significant. This benefit of Nerinetide in patients who did not receive tPA is plausible because the plasma levels of Nerinetide were substantially lower in patients who had received tPA, and it is now known that the peptide plasmin produced by the interaction of tPA and plasminogen cleaves Nerinetide and inactivates it. In a comprehensive series of subsequent experiments, it was determined that concomitant administration of Nerinetide and tPA significantly reduced the plasma levels of Nerinetide, as in the clinical trials.⁹⁷ However, if Nerinetide was given 30 minutes before tPA, no such reduction in plasma concentration was seen and a significant reduction of infarct volume was detected that was not observed when the 2 drugs were given at the same time. Another phase 3 clinical trial of Nerinetide in thrombectomy patients is currently underway in patients who have not received tPA to determine if the results observed in the initial trial can be replicated. If the trial demonstrates a significant benefit on 90-day clinical outcome, we can anticipate that Nerinetide will become the first cytoprotective drug to do so and will likely be approved by regulatory authorities.

Lastly, clinical trials with initiation of treatment after documentation of successful reperfusion can be considered because not all of the effects of the restoration of blood flow are beneficial and during the ischemic period detrimental processes may have been initiated.⁹⁸ Damage to the endothelium may occur during ischemia as well as to the blood-brain barrier. These effects could enhance the risk of hemorrhage into the region that was previously ischemic. Drugs directed at ameliorating these effects could be evaluated in clinical trials where blood-brain barrier compromise has been identified.⁹⁹ After reperfusion the generation of reactive oxygen molecules and activation of the immune system with the recruitment of inflammatory cells and activation of the microglia may lead to further brain tissue injury despite reperfusion.¹⁰⁰ This can occur with the release of cytokines, chemokines, and complement. Neutrophils recruited as part of the inflammatory response release matrix metalloproteinases which can degrade the extracellular matrix and tight cellular junctions, leading to breakdown of the blood-brain barrier.¹⁰¹ Apoptosis or programmed cell death initiated during ischemia may also occur at late time points after reperfusion.¹⁰² All of these mechanisms of tissue injury that occur after reperfusion are potential therapeutic targets. In the past, they were of theoretical concern, but now that we have entered the era of highly effective reperfusion with thrombectomy, they may potentially affect the ultimate clinical outcome of ischemic stroke patients. In designing clinical trials targeting the various contributors to reperfusion injury, drugs should be chosen that affect one or more of the described mechanisms. Patients should be included in the trials who have substantially reperfused as documented by a perfusion imaging study after thrombectomy and the patients should also not have an extensive ischemic core because such patients are unlikely to benefit from additional therapy. Since all of the patients included in the trial will have undergone thrombectomy, the control group of thrombectomy alone will demonstrate a substantial rate of favorable 90-day outcome. Thus, to detect a significant benefit of the study drug in addition to thrombectomy will likely require a large sample size in phase 3 trials. The possibility of delivering drugs via an intraarterial catheter after thrombectomy with direct targeting of the ischemic region may help to reduce the sample size requirements as might the careful imaging-based selection of patients to include in the trial. The design and implementation of such post-thrombectomy trials will evolve as knowledge increases and lessons are learned from the initial trials.

In conclusion, despite the dismal past of cytoprotection as a therapy for acute ischemic stroke, it may have a role in the future. Many different types of clinical trials can be envisioned and some are already underway.¹ Some will be successful, but many will not be successful. All of the types of trials discussed and others not yet envisioned will inform future investigators and likely lead to a future where cytoprotection serves as an important adjunctive therapy to reperfusion to maximize favorable outcomes after acute ischemic stroke.