tPA
tPa to the Rescue!
Tissue plasminogen activator (tPA) is a protein that will change your life. This protein is a natural enzyme in your body, but has now been made as a drug to breakdown blood clots featured in some particular diseases. tPA is used to treat myocardial infarction, pulmonary embolism, stroke, and even frostbite at times. "Tissue plasminogen activator improved tissue perfusion and reduced amputations when administered within 24 hours of injury... this modality represents the first clinically significant advancement in the treatment of frostbite in more than 25 years." (Bruen) Tissue plasminogen activator is a fibrin specific activator for the conversion of plasminogen to plasmin, causing thrombolysis and restoring blood flow. tPA works as a serine protease, increasing the plasmin to cause hyperfibrinolysis. The enzyme cleaves the zymogen plasminogen at Arg561 to Val 562 peptide bond into the serine protease plasmin, which lyses clots. Plasmin is an important enzyme in blood that breaks down blood plasma proteins and fibrin clots (fibrinolysis).
How it works:
Tissue plasminogen activator (tPA) is a protein that is a natural enzyme
in your body, but has now been made as a drug to breakdown blood clots featured
in some particular diseases. tPA works as a serine protease, increasing the
plasmin to cause hyperfibrinolysis. The enzyme cleaves the zymogen plasminogen
at Arg561 to Val 562 peptide bond into the serine protease plasmin, which lyses
clots. Plasmin is an important enzyme in blood that breaks down blood plasma
proteins and fibrin clots also known as fibrinolysis. tPA contains 527
amino acids with has four domains. The F domain is an N –terminal region with
47 residues (4-50) and corresponding to the finger domain which mediates the
fibrin to fibronectin affinity. E-domain consists of residues 50 to 87 and
corresponds to epidermal growth factor. Two kringle regions make up the
k-domain (87-176 for K1, 176-256 for K2) which
corresponds to kringles in plasminogen. The P-domain is a serine protease
region (276-527) with active-sites His322, Asp371, and Ser478 for inhibition.
The F-domain and K2-domain allow it to bind to fibrin while F-domain
and E-domain allow for rapid clearance(Pennica).The amino acid sequence of the first
domain of tPA has eight residues that are highly conserved in the type 1 finger
domains of human fibronectin. The overall fold of the t-PA finger domain
similarity to that of the seventh type 1 repeat of human fibronectin with the
side-chains of conserved residues lying in similar conformations is rather
striking. One significant difference between the two molecules is that
hydrophobic residues cover the exposed surface of the principal beta-sheet
region in the t-PA finger domain. One idea is that one face of this region may
interact with parts of the complete t-PA protein(Downing).
tPA also acts as a cytokine to activate receptor-mediated signaling events and has been shown to modulate inflammatory responses in tissue injury. This means that tPA could also be used to target autoimmune and inflammatory diseases. How it works in this situation, "A: tPA binds to LRP-1 and induces activation of NF-κB. B: tPA promotes the aggregation of annexin A2 and integrin CD11b, leading to activation of ILK, phosphorylation and degradation of IκB, and eventually activation of NF-κB. C: tPA forms complex with LRP-1, PAI-1, and Mac-1, which results in the increased macrophage migration." (Downing)
Pharmaceutical:
tPA is
known as a clot buster, and has a few different forms that are used in the
medical setting. There is alteplase which is the most common type of tPA and is
the glycosylated protein of 527 amino
acids with recombinant DNA. Alteplase is FDA-approved for acute ischemic
stroke, myocardial infarction, acute massive pulmonary embolism, and central
venous access devices. Reteplase (rPA) is the non-glycosylated shorter version
of native tPA, with only 355 amino acids. Reteplase is FDA-approved for
myocardial infarction as it administered more easily and quicker thrombolysis
than alteplase. Tenecteplase is the varied version of alteplase, with 6 mutant
amino acids. Tenecteplase is used to treat myocardial infarction, as it is
nearly the same as alteplase except it shows less bleeding complications(Rivera-Bou).
Physiology:
Damage to the wall of a blood vessel occurs, which exposes tissue factor
and collagen. Tissue factor will lead to a coagulation cascade which causes
thrombin formation that converts fibrinogen to fibrin, resulting in a clot,
reinforced platelet plug. When collagen is exposed, it can lead to the
coagulation cascade but it can also cause platelets to adhere and release
platelet factors that attract more platelets. Platelets aggregate into a loose
platelet plug or temporary homeostasis. Vasoconstriction is another part of
dealing with broken blood vessels. Once a clot is in place, it can allow cell
growth and tissue repair, leading to an intact blood vessel wall. Fibrin that
is slowly dissolved by plasmin will cause the clot to dissolve. As the damaged
tissue release thromboplastin which gets activated and leads a successive enzyme
cascade that causes prothrombin to turn into active thrombin which produces a
fibrin clot from fibrinogen that traps platelets and cells. The platelets
retract, drawing open edges closer, allowing healing to begin. Plasmin is
activated to fibrinolysis that dissolves the clot. tPA takes the same place of
thrombin as it causes plasminogen to turn into plasmin, causing fibrinolysis by
turning the fibrin polymer to fibrin fragments. Fibrinolysis
is the process that prevents blood clots from becoming too big and problematic.
Increased enzymatic activity of tPA results in hyperfibrinolysis, excessive
bleeding. Decreased activity leads to hypofibrinolysis, a very low rate of
fibrinolysis. (Silverthorn)
Acute Ischemic Stroke:
Approximately 80% of strokes are ischemic, where a blood clot stops the
blood supply and brain cells begin to die, causing brain damage and possibly
death (NHS). Stroke can occur from atherosclerosis,
embolism from the heart, small vessel disease, arterial dissection, patent
foramen ovale, and others. Risk factors include increased age, South Asian and
African Caribbean people, a family history of stroke, high blood pressure,
atrial fibrillation, high cholesterol, diabetes, smoking, obesity, excess
alcohol, and lack of exercise(Stroke assosciation). For acute Ischemic
stroke, tPA is the only FDA approved medication for treatment. tPA treatment
for stroke must be provided within the first 4.5 hours since the onset of
symptoms1. When tPA first came out in the U.S., it was only used
within 3 hours of the stroke, but studies have now shown that time after that
the 3 hours window is still effective. Angiographies, CT scans, and MRI scans
are often required to determine what type, if any, stroke has occurred so
treatment can be determined. If bleeding in the brain is already there, tPA
cannot be used. For stroke and other blood clot diseases, anticoagulants such
as Heparin and Warfarin (Coumadin) and antiplatelet agents such as Aspirin,
Dipyridamole, and Clopidogrel are often used as blood thinners or to decrease
platelet aggregation as to help ward off stroke in high-risk patients. The
desired outcome in acute ischemic stroke happens when there is the thrombus
embolism which leads to the cerebral vessel occlusion, where the stroke occurs,
and endogenous tPA goes to the clot. If the natural tPA causes spontaneous
resolution then all is good. However, if no resolution, then exogenous tPA is
to be used as treatment with the hope of a resolution. If no resolution from the tPA, then other
procedures might need to take place so as to have a resolution and stop injury
from increasing. One study in the UK has now found that thrombolysis with
alteplase (tPA) used for acute ischemic stroke within six hours were highly
significant clinically relevant improvements, outcomes, and health-related
quality of life for 18-month long-term outcomes.2 Barriers to using
tPA for in-hospital strokes were 70% of the patients were not viable due to
recent operations or contraindications to tPA, and of the 30% left, delay in
stroke discovery was the largest problem(Bunch).
The cost of rt-PA was estimated to be $2,750 and more expensive than standard care for ischemic stroke in the short term, owing to the cost of the drug and the need for additional resources. However, the drug and treatment is associated with lower costs in the long term, since it reduces the risk of subsequent disability. tPA use can help lower the estimated direct medical costs of stroke in the United States as in 2007 the costs were $25 billion, as tPA would help decrease health care costs long term. (Weschler)
Side effects:
tPA has been
shown to be potentially neurotoxic if it
reaches the extracellular space and can cause injury to the blood brain barrier
in severe cases(Kaur). Low blood pressure, fever, and bleeding are other possible side effects(Genentech).1-5% of patients have
angioedema of the face, lips, neck or tongue after treatment with tPA.
Exacerbated hemorrhaging can occur, as 1.7-8% of tPA treated patients have
symptomatic intracranial hemorrhage, with 0.4-1.5% of patients having extreme
systemic hemorrhage(Weschler). 3% of patients had fatal bleeding in the brain in the first week after
treatment.2 tPA does not decrease the chance of having another
stroke in the future. Despite these risks, nearly all providers find that tPA
is more effective than it is harmful when treating triage as long as the
patients fits in the viable category for treatment. The good effect most
definitely outweighs the bad effect for tPA treatment for nearly all the
patients.
The Gallery:
Icy Cool (a David Doud original)
The United Amino Acids of tPA (a David Doud original)
Citations:
Bruen, Kevin J., James R. Ballard, Stephen E. Morris, et al. Reduction of the incidence of amputation in frostbite injury with thrombolytic therapy. Archives of Surgery 142(6):546-553, 2007.
Bunch, Marjorie E., Edward C. Nunziato, and Daniel L. Labovitz. Barriers to the Use of Intravenous Tissue Plasminogen Activator for In-hospital Strokes. Journal of Stroke and Cerebrovascular Diseases 21(8):808-811, 2012.
Collen, D., and H. R. Lijnen. "The Tissue-Type Plasminogen Activator Story." The Tissue-Type Plasminogen Activator Story. 2009. American Heart Association. 08 Apr. 2014 <http://atvb.ahajournals.org/content/29/8/1151.full>.
Downing, A.K., P.C. Driscoll, T.S. Harvey, et al. Solution structure of the fibrin binding finger domain of tissue type plasminogen activator determined by 1H nuclear magnetic resonance. Journal of Molecular Biology 225:821-833, 1992.
Genentech. Activase alteplase prescribing. San Francisco, California, U.S.A. Genentech, Roche Group. (Accessed 15 April 2014, at http://www.gene.com/download/pdf/activase_prescribing.pdf.)
Kaur, J. Z. Zhao, G.M. Klein, et al. The neurotoxicity of tissue plasminogen activator. Journal of Cerebral Blood Flow Metabolism 24(9):945-63, 2004.
Lin, Ling, and Kebin Hu. "Abstract." National Center for Biotechnology Information. 27 Feb. 2014. U.S. National Library of Medicine. 08 Apr. 2014 <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3960759/>.
NHS: National Health Service. Stroke- Introduction. United Kingdom. 14 November 2013. Accessed 15 April 2014, at http://www.nhs.uk/conditions/Stroke/Pages/Introduction.aspx)
Pennica D, Holmes WE, Kohr WJ, Harkins RN, Vehar GA, Ward CA, Bennett WF, Yelverton E, Seeburg PH, Heyneker HL, Goeddel DV, Collen D. Cloning and expression of human tissue-type plasminogen activator cDNA in E. coli.Nature. 1983; 301: 214–221.
Rivera-Bou, Wanda L., Jose G. Cabanas, and Salvador E. Villanueva. Thrombolytic therapy. Medscape. 11 March 2014. (Accessed 15 April 2014, at http://emedicine.medscape.com/article/811234-overview.)
Silverthorn, Dee Unglaub, Bruce R. Johnson, William C. Ober, et al. Human Physiology: an integrated approach. Fifth edition. Pearson-Benjamin Cummings :558-567, 2010.
Stroke association. Ischaemic stroke- references used. (Accessed 15 April 2014, at http://www.stroke.org.uk/referenced/ischaemic-stroke.)
"Tissue plasminogen activator." Wikipedia. 04 May 2014. Wikimedia Foundation. 08 Apr. 2014 <http://en.wikipedia.org/wiki/Tissue_plasminogen_activator>.
tPA protein: Tissue Plasminogen Activator: 10.2210/pdb1tpm/pdb
Wechsler, Lawrence R. Intravenous Thrombolytic Therapy for Acute Ischemic Stroke. New England Journal of Medicine 364:2138-2146, 2011.
