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W. Li et al.
criteria: highly selective for the therapeutic target, i.e., related coagulation factors
and receptors of platelets activation [17], rapid onset of action, predictable pharmacokinetic and pharmacodynamic profiles, short half-life and an antidote whenever
there is bleeding. Antidote reversal improves therapeutic safety and thus widens
clinical applications [18]. Finally, good antithrombotics should be user-friendly, for
example, no requirement for regular laboratory monitoring.
What is aptamer? Aptamer is derivative from the Latin ‘aptus’, with the meaning of
‘to fit’. In 1990, two groups of researchers independently demonstrated that specific
oligonucleotides targeting bacteriophage T4 DNA polymerase and organic dyes
could be screened out after iterative rounds of steps of incubation, isolation, amplification, and finally sequencing [19, 20]. The selection process was termed Systematic
Evolution of Ligands by Exponential enrichment (SELEX) and the screened RNA
or single-stranded DNA (ssDNA) oligonucleotides are named aptamer. Aptamers
can fold into 3D conformations to bind specific targets similar to protein–protein
interaction of monoclonal antibodies, and SELEX is a novel technique that is based
on affinity for target molecules and combines combinatorial chemistry with in vitro
evolution. Aptamers developed by the SELEX process and in vitro synthesized chemically combine the best characteristics of small molecule therapeutics with those
of monoclonal antibodies thereof. In addition, some unique characteristics such as
high specificity and affinity, modification flexibility, good stability, and lack of toxicity and immunogenicity, make aptamers hold great potential as a novel class of
oligonucleotide drugs and diagnostic reagents [9].
So far, most antithrombotic drugs targeting the two key factors in the coagulation
pathway, thrombin or FXa, as well as factor IX(FIX) and VII(FVII), factor XI(FXI)
and factor XII(FXII) have been under development. Since P-selectin is involved in
TF-mediated thrombosis and prothrombotic cell accumulation on the injury vascular
surface and von Willebrand Factor (vWF) is related to the initial platelet accumulation in thrombosis, the two anti-adhesion molecules aptamers together with abovementioned aptamers against coagulation factors have been developed and some of
them have entered into clinical trials [2].
10.2 Aptamers Against Thrombin
Thrombin, also named coagulation factor IIa (FIIa), is a serine protease and derivative
of activated prothrombin. Thrombin is an optimal anticoagulant target because it
plays a crucial role in physiological hemostasis and pathological thrombosis by
catalyzing the cleavage of fibrinogen, upstream coagulation cofactors, and platelet
receptors. In blood hemostasis and coagulation cascade, thrombin firstly activates
factors V, VII, VIII, XI, and XIII and then converts soluble fibrinogen into insoluble
strands of fibrin to forms fibrin clots at the end [21]. Thus thrombin can regulate
the intrinsic coagulation pathway. In addition, thrombin has some activities similar
to growth factors and cytokines involved in atherosclerotic plaque formation, injury
W. Li et al.
criteria: highly selective for the therapeutic target, i.e., related coagulation factors
and receptors of platelets activation [17], rapid onset of action, predictable pharmacokinetic and pharmacodynamic profiles, short half-life and an antidote whenever
there is bleeding. Antidote reversal improves therapeutic safety and thus widens
clinical applications [18]. Finally, good antithrombotics should be user-friendly, for
example, no requirement for regular laboratory monitoring.
What is aptamer? Aptamer is derivative from the Latin ‘aptus’, with the meaning of
‘to fit’. In 1990, two groups of researchers independently demonstrated that specific
oligonucleotides targeting bacteriophage T4 DNA polymerase and organic dyes
could be screened out after iterative rounds of steps of incubation, isolation, amplification, and finally sequencing [19, 20]. The selection process was termed Systematic
Evolution of Ligands by Exponential enrichment (SELEX) and the screened RNA
or single-stranded DNA (ssDNA) oligonucleotides are named aptamer. Aptamers
can fold into 3D conformations to bind specific targets similar to protein–protein
interaction of monoclonal antibodies, and SELEX is a novel technique that is based
on affinity for target molecules and combines combinatorial chemistry with in vitro
evolution. Aptamers developed by the SELEX process and in vitro synthesized chemically combine the best characteristics of small molecule therapeutics with those
of monoclonal antibodies thereof. In addition, some unique characteristics such as
high specificity and affinity, modification flexibility, good stability, and lack of toxicity and immunogenicity, make aptamers hold great potential as a novel class of
oligonucleotide drugs and diagnostic reagents [9].
So far, most antithrombotic drugs targeting the two key factors in the coagulation
pathway, thrombin or FXa, as well as factor IX(FIX) and VII(FVII), factor XI(FXI)
and factor XII(FXII) have been under development. Since P-selectin is involved in
TF-mediated thrombosis and prothrombotic cell accumulation on the injury vascular
surface and von Willebrand Factor (vWF) is related to the initial platelet accumulation in thrombosis, the two anti-adhesion molecules aptamers together with abovementioned aptamers against coagulation factors have been developed and some of
them have entered into clinical trials [2].
10.2 Aptamers Against Thrombin
Thrombin, also named coagulation factor IIa (FIIa), is a serine protease and derivative
of activated prothrombin. Thrombin is an optimal anticoagulant target because it
plays a crucial role in physiological hemostasis and pathological thrombosis by
catalyzing the cleavage of fibrinogen, upstream coagulation cofactors, and platelet
receptors. In blood hemostasis and coagulation cascade, thrombin firstly activates
factors V, VII, VIII, XI, and XIII and then converts soluble fibrinogen into insoluble
strands of fibrin to forms fibrin clots at the end [21]. Thus thrombin can regulate
the intrinsic coagulation pathway. In addition, thrombin has some activities similar
to growth factors and cytokines involved in atherosclerotic plaque formation, injury