New England Journal of Medicine JUNE 23, 2016

Low-Dose versus Standard-Dose Intravenous Alteplase in Acute Ischemic Stroke

Craig S. Anderson, et al. (ENCHANTED Investigators)

Bottom Line

The ENCHANTED trial found that low-dose intravenous alteplase (0.6 mg/kg) was not non-inferior to standard-dose (0.9 mg/kg) regarding 90-day death or disability, although it resulted in fewer cases of symptomatic intracranial hemorrhage.

Key Findings

1. The primary outcome of death or disability (mRS scores 2-6) at 90 days occurred in 53.2% of the low-dose group and 51.1% of the standard-dose group (OR, 1.09; 95% CI, 0.95-1.25), failing to meet the predefined non-inferiority margin.

2. Low-dose alteplase was associated with a significantly reduced risk of symptomatic intracerebral hemorrhage compared to standard-dose (1.0% vs. 2.1%; OR, 0.48; 95% CI, 0.27-0.87; P=0.01).

3. Mortality at 90 days did not differ significantly between the low-dose (8.5%) and standard-dose (10.3%) groups (OR, 0.81; 95% CI, 0.63-1.04; P=0.07).

Study Design

Design

RCT

Open-Label, Blinded-Endpoint

Sample

3,310

Patients

Duration

90 days

Median

Setting

Multicenter, International

Population Adult patients with acute ischemic stroke eligible for intravenous thrombolysis within 4.5 hours of symptom onset.

Intervention Low-dose intravenous alteplase (0.6 mg/kg: 15% as bolus, 85% as 1-hour infusion).

Comparator Standard-dose intravenous alteplase (0.9 mg/kg: 10% as bolus, 90% as 1-hour infusion).

Outcome Death or disability at 90 days, defined as a score of 2 to 6 on the modified Rankin scale.

Study Limitations

• The trial was open-label, which may have introduced assessment bias, although the primary outcome was evaluated by blinded investigators.

• The majority of the population (63%) was of Asian ethnicity, which may limit the generalizability of findings to other global populations.

• The non-inferiority margin of 1.14 was not met, and the study did not demonstrate superiority of the low dose in any key efficacy outcome.

Clinical Significance

The study suggests that while low-dose alteplase (0.6 mg/kg) offers a favorable safety profile regarding reduced intracranial hemorrhage, it does not achieve the same efficacy as the standard 0.9 mg/kg dose in preventing death or disability. Consequently, standard-dose alteplase remains the guideline-recommended treatment for eligible patients with acute ischemic stroke.

Historical Context

The standard 0.9 mg/kg dose of alteplase was established based on early North American (NINDS) trials. Because of observed higher rates of symptomatic intracerebral hemorrhage, particularly in Asian populations, a lower dose (0.6 mg/kg) had been adopted in some regions, notably Japan, prompting the ENCHANTED trial to formally evaluate the non-inferiority of the lower dose.

Guided Discussion

High-yield insights from every perspective

Med Student

Medical Student

What is the primary mechanism of action of alteplase in acute ischemic stroke, and why does increasing the dose potentially lead to the safety concern of symptomatic intracranial hemorrhage (sICH)?

Key Response

Alteplase is a recombinant tissue plasminogen activator (rtPA) that converts plasminogen to plasmin, which then degrades fibrin in a blood clot. While higher doses (0.9 mg/kg) are more effective at recanalization, they also cause more systemic fibrinolysis and disruption of the blood-brain barrier in ischemic tissue, increasing the risk of blood leaking into the brain parenchyma (sICH).

Resident

Given that the ENCHANTED trial failed to prove non-inferiority for the 0.6 mg/kg dose regarding functional recovery, under what specific clinical circumstances might a resident still encounter the use of low-dose alteplase in practice?

Key Response

While 0.9 mg/kg remains the standard, low-dose alteplase (0.6 mg/kg) is often considered for patients at a perceived higher risk of hemorrhage, such as the very elderly, those with severe leukoaraiosis, or in certain East Asian populations where smaller studies originally suggested 0.6 mg/kg might be safer and equally effective.

Fellow

The ENCHANTED trial population was predominantly Asian (approx. 63%). How does this demographic distribution affect the interpretation of the secondary safety endpoints compared to the primary efficacy endpoint in a global context?

Key Response

Asian populations may have a higher baseline risk for ICH and different pharmacokinetics for rtPA. While the lower dose significantly reduced sICH (p=0.01) across the board, the failure to meet non-inferiority for functional recovery suggests that the standard 0.9 mg/kg dose is necessary for optimal clot lysis regardless of ethnicity, challenging prior regional preferences for lower dosing.

Attending

In the era of mechanical thrombectomy (MT), does the ENCHANTED finding that 0.6 mg/kg reduces sICH risk warrant a re-evaluation of the 'drip-and-ship' dose for patients who are definitely headed for the angiosuite?

Key Response

This is a key area of ongoing debate. If a patient is guaranteed to receive MT, a lower dose of rtPA might theoretically reduce the risk of peri-procedural bleeding while still providing some fibrinolysis. However, ENCHANTED suggests that lowering the dose might compromise early recanalization before the patient reaches the suite, potentially worsening the final outcome.

Scholarly Review

Critical appraisal through the lens of expert reviewers and guideline development

PhD

The ENCHANTED trial utilized a non-inferiority margin of 1.14 for the odds ratio of a poor outcome. Critique the selection of this margin and its impact on the study's power to detect subtle differences in functional recovery.

Key Response

A non-inferiority margin of 1.14 is relatively wide, meaning the study allowed for a potential 14% increase in the odds of death or disability while still calling the low-dose 'non-inferior.' Despite this lenient margin, the lower dose still failed to meet the criteria, reinforcing the superiority of the 0.9 mg/kg dose for efficacy.

Journal Editor

The primary outcome was analyzed using a binary mRS (2-6 vs 0-1). How might an ordinal 'shift analysis' have potentially altered the conclusions regarding the clinical utility of the 0.6 mg/kg dose?

Key Response

Binary outcomes (dead/disabled vs. independent) can hide movements between mRS levels (e.g., from mRS 4 to 3). An ordinal analysis often provides more statistical power in stroke trials. If a shift analysis had been the primary endpoint, it might have more clearly quantified exactly how much functional recovery was sacrificed for the safety benefit of fewer hemorrhages.

Guideline Committee

Based on the ENCHANTED trial results, should the AHA/ASA guidelines maintain 0.9 mg/kg as the sole Class I recommendation, or is there sufficient evidence to create a Class IIb recommendation for 0.6 mg/kg in high-bleeding-risk subsets?

Key Response

Current AHA/ASA guidelines (2019 update) maintain 0.9 mg/kg (Class I, LOE A) because ENCHANTED failed its primary non-inferiority endpoint. While 0.6 mg/kg showed a safety signal (fewer sICH), the risk of permanent disability was higher. Until a subgroup analysis identifies a population where the benefit of reduced sICH clearly outweighs the loss of efficacy, the standard dose remains the benchmark.

Clinical Landscape

Noteworthy Related Trials

1995

NINDS rt-PA Stroke Study

n = 624 · NEJM

Tested

Intravenous alteplase (0.9 mg/kg)

Population

Patients with acute ischemic stroke

Comparator

Placebo

Endpoint

Disability at 3 months

Key result: Treatment with intravenous t-PA within 3 hours of the onset of ischemic stroke improved clinical outcomes at 3 months.

2008

ECASS III Trial

n = 821 · NEJM

Tested

Intravenous alteplase (0.9 mg/kg)

Population

Patients with acute ischemic stroke

Comparator

Placebo

Endpoint

Disability at 90 days

Key result: Intravenous alteplase administered between 3 and 4.5 hours after the onset of symptoms significantly improved clinical outcomes.

2012

IST-3 Trial

n = 3035 · Lancet

Tested

Intravenous alteplase (0.9 mg/kg)

Population

Patients with acute ischemic stroke

Comparator

Control group (no thrombolysis)

Endpoint

Oxford Handicap Score at 6 months

Key result: Although there was no significant difference in the primary endpoint, the study suggested benefit in patients treated within 6 hours of stroke onset.

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