Thrombolytic Agents: Nanocarriers in Controlled Release

Abstract

Thrombosis is a life-threatening pathological condition in which blood clots form in blood vessels, obstructing or interfering with blood flow. Thrombolytic agents (TAs) are enzymes that can catalyze the conversion of plasminogen to plasmin to dissolve blood clots. The plasmin formed by TAs breaks down fibrin clots into soluble fibrin that finally dissolves thrombi. Several TAs have been developed to treat various thromboembolic diseases, such as pulmonary embolisms, acute myocardial infarction, deep vein thrombosis, and extensive coronary emboli. However, systemic TA administration can trigger non-specific activation that can increase the incidence of bleeding. Moreover, protein-based TAs are rapidly inactivated upon injection resulting in the need for large doses. To overcome these limitations, various types of nanocarriers have been introduced that enhance the pharmacokinetic effects by protecting the TA from the biological environment and targeting the release into coagulation. The nanocarriers show increasing half-life, reducing side effects, and improving overall TA efficacy. In this work, the recent advances in various types of TAs and nanocarriers are thoroughly reviewed. Various types of nanocarriers, including lipid-based, polymer-based, and metal-based nanoparticles are described, for the targeted delivery of TAs. This work also provides insights into issues related to the future of TA development and successful clinical translation.

Keywords: drug delivery; nanoparticles; plasminogen activators; thrombolytic agents; thrombosis.

© 2020 Wiley-VCH GmbH.

Figures

Figure 1.

Illustration of the principles of thrombolysis in a fibrin surface and circulating blood environment. This figure describes the catalytic principle of the conversion of plasminogen to plasmin according to the binding method of the plasminogen activator (e.g., tissue-type plasminogen activator [t-PA], urokinase [UK], and streptokinase [SK]). Plasminogen specifically binds to the surface of the fibrin blood clot. In direct activation, t-PA preferentially attaches to plasminogen, resulting in the formation of a ternary complex. On the other hand, in indirect activation, SK cannot directly bind to the plasminogen but induce conformational changes of the plasminogen to form a streptokinase-plasminogen complex. Subsequently, these complexes form plasmin through cleavage of the fibrin-associated plasminogen. Plasmin formed by direct/indirect activation breaks down fibrin into fibrin breakdown products (FDP), which eventually dissolves blood clots. The thrombolytic process in circulating blood is triggered by non-fibrin-specific or less fibrin-specific plasminogen activators. Plasminogen activators such as the UK and SK induce plasmin production by cleavage of circulating plasminogen. Subsequently, plasmin degrades fibrinogen factor VIII instead of fibrinogen. Plasmin activator inhibitor-1 acts on plasminogen, blocking cleavage into plasmin, causing blood clot formation. α2-antiplasmin acts only on circulating blood, can inhibit thrombolysis by interfering with plasmin binding sites with fibrinogen factor VIII.

Figure 2.

Fucoidan functionalized core-shell polymeric NPs targeted to P-selectin to improve the specific aggregation of loaded recombinant tissue PA at the thrombus Redrawn from Ref [49].

Figure 3.

a) Formation of camouflaged tissue PA composed of tissue-PA-low molecular-weight heparin (t-PA-LMWH) and HSA protamine-targeting peptide. b) The linkage of albumin with tissue plasminogen activator serves as a steric barrier to blood plasma protein. c) Binding of peptide-GP IIb/IIIa (expressed on the activated platelets surface) to the thrombus leads to complex deposition on the activated platelet surface. d) Administering heparin after the complex has accumulated at the thrombus region. e) Provoked release of t-PA and fibrinolysis. Redrawn from Ref [52].

Figure 4.

Schematic illustration of an albumin-camouflaged/thrombin-provoked system for targeted delivery of t-PA. A) Formation of camouflaged t-PA. Albumins linked with thrombin-cleavable peptides mask the activity of the t-PA. B) Complex accumulated on the surface of a thrombus through a targeting peptide. This leads to the targeted release of camouflaged t-PA. Redrawn from Ref. [53].

Figure 5.

A) A schematic representation of targeted thrombolytic therapy via cyclic RGD functionalized liposomes loaded with UK. B) Flow cytometry analysis demonstrating that cRGD-liposomes only slightly interact with fresh platelets, C) while they interact well with activated platelets. D) The thrombolysis consequences of the cyclic RGD liposomes loaded with UK in the mesenteric vessels of a mouse. The images were taken after treatment with 100 U/g of cyclic RGD-UK-liposomes at 0 min, 5 min, 10 min, 20 min, 30 min, 40min to demonstrate the change in thrombus size over time. Redrawn from Ref. [58].

Figure 6.

A) A schema illustrating surface entities of PMP (platelet-derived microparticles). B) Schematic illustration of PMIN (platelet microparticle-inspired nanovesicles), B1) cryo-TEM image of platelet microparticle-inspired nanovesicles. C) Representation of the PMIN mechanism of targeting thrombi, C1) PMINs actively connect to thrombi through P-selectin and GPIIb-IIIa on activated platelets, C2) PMIN-bound clot is targeted by secreted phospholipase-A2 produced from activated platelets and leukocytes in the thrombus, C3) Degraded PMINs release SK. D) In vivo examination set-up. A FeCl3 (ferric chloride)- induced mouse model of carotid artery thrombosis was utilized to assess the linking of PMINs, E) Representative image of thrombosed carotid as observed by (E1) intravital microscopy and (E2) ex vivo immunofluorescence. Redrawn from Ref. [59].

Figure 7.

Schematic illustration of the preparation and effect of t-PA loaded micro-bubble in thrombolysis. Reproduced from Ref. [68] with permission from Elsevier.

Figure 8.

A schematic illustration of chitosan/glycol chitosan-based nanocarriers. Redrawn from Ref. [83].