There is significant debate regarding recombinant tPA's effectiveness in ischemic stroke. There have been twelve relevant, large scale, high-quality trials of rtPA in acute ischemic stroke. One common complaint about these studies is that they all have authors with significant conflicts of interest such as financial ties to Genentech, which manufactures tPA. A prominent meta-analysis of these trials concluded that rtPA given within 6 hours of a stroke significantly increased the odds of being alive and independent at final follow-up, particularly in patients treated within 3 hours. However, there was an excess of mortality in treated patients in the first week after the event, mostly from intracranial haemorrhage.
The NNT Group on evidence-based medicine concluded that it was inappropriate to combine these twelve trials into a single analysis, because of substantial clinical heterogeneity (i.e., variations in study design, setting, and population characteristics). Examining each study individually, the NNT group noted that two of these studies showed benefit to patients given tPA (and that, using analytical methods that they think flawed); four studies showed harm and had to be stopped before completion; and the remaining studies showed neither benefit nor harm. On the basis of this evidence, the NNT Group recommended against the use of tPA in acute ischaemic stroke.
It has been suggested that if tPA is effective in ischemic stroke, it must be administered as early as possible after the onset of stroke symptoms. Indeed, tPA has become widely considered standard of care in acute ischemic stroke, so long as the patient presents soon after the onset of stroke symptoms. Many national guidelines including the AHA have interpreted this cohort of studies as suggesting that there are specific subgroups who may benefit from tPA and thus recommend its use within a limited time window after the event. Protocol guidelines require its use intravenously within the first three hours of the event, after which its detriments may outweigh its benefits. For example, the Canadian Stroke Network guideline states "All patients with disabling acute ischemic stroke who can be treated within 4.5 hours of symptom onset should be evaluated without delay to determine their eligibility for treatment" with tPA. Because of this, only about 3% of people qualify for this treatment, since most patients do not seek medical assistance quickly enough. Similarly in the United States, the window of administration used to be 3 hours from onset of symptoms, but the newer guidelines also recommend use up to 4.5 hours after symptom onset. tPA appears to show benefit not only for large artery occlusions but also for lacunar strokes. Since tPA dissolves blood clots, there is risk of hemorrhage with its use.
However, the NNT Group notes that the case for this time window arises largely from analysis of two trials: NINDS-2 and subgroup results from IST-3. "However, presuming that early (0-3h) administration is better than later administration (3-4.5h or 4.5-6h) the subgroup results of IST-3 suggest an implausible biological effect in which early administration is beneficial, 3-4.5h administration is harmful, and 4.5-6h administration is again beneficial." Indeed, even the original publication of the IST-3 trial found that time-window effects were not significant predictors of outcome (p=0.61).
Use of tPA in the United States in treatment of patients who are eligible for its use, no contra-indications and arrival at the treating facility less than 3 hours after onset of symptoms, is reported to have doubled from 2003 to 2011. Use on patients with mild deficits, of nonwhite race/ethnicity, and oldest old age increased. However, many patients who were eligible for treatment were not treated.
tPA has also been given to patients with acute ischemic stroke above age 90 years old. Although a small fraction of patients 90 years and above treated with tPA for acute ischemic stroke recover, most patients have a poor 30-day functional outcome or die. Nonagenarians may do as well as octogenarians following treatment with IV-tPA for acute ischemic stroke. In addition, people with frostbite treated with tPA had fewer amputations than those not treated with tPA.
A simplified illustration demonstrates clot breakdown (fibrinolysis), with blue arrows denoting stimulation, and red arrows inhibition.
tPA and plasmin are the key enzymes of the fibrinolytic pathway in which tPA mediated plasmin generation occurs. To be specific, tPA cleaves the zymogen plasminogen at its Arg561 - Val562 peptide bond, into the serine protease plasmin.
Increased enzymatic activity causes hyperfibrinolysis, which manifests as excessive bleeding. Decreased activity leads to hypofibrinolysis which can result in thrombosis or embolism.
Tissue plasminogen activator also plays a role in cell migration and tissue remodeling.
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