294
W. Li et al.
2
-amino pyrimidine modified RNA aptamer significantly prolonged the PT values
in human plasma in a dose-dependent manner, with an in vitro half-life of approximately 15 h in human plasma. The RNA aptamer is potentially a novel anticoagulant
agent against FVIIa [64].
10.4 Aptamers Against FIX
Presently, antiplatelet and anticoagulation is the mainstay therapy for ACS. In particular, it is necessary for patients with ACS during coronary revascularization procedures, either with PCI or through CABG, to receive periprocedural anticoagulation
to eliminate ischemic events [65]. According to recommendations of the American
College of Cardiology/American Heart Association guidelines, the anticoagulants
of ACS and PCI currently include UFH, LMWH, indirect FXa inhibitors such as
fondaparinux, and the direct thrombin inhibitors (DTI) like bivalirudin [66–68].
However, there are several shortcomings for each of these anticoagulants. On one
hand, bleeding involved in ACS treatment or coronary revascularization procedures
directly contributes to increased morbidity and mortality, and further blood transfusions may worsen the prognosis [69, 70]. On the other hand, current anticoagulants are
associated with adverse events due to unpredictable pharmacodynamics, immunogenicity, toxicity, and limited reversibility [69]. In addition, warfarin is a routine
agent for prevention and treatment of prothrombotic events, and during initiation of
warfarin therapy, serum levels of FVII and protein C decrease more rapidly than that
of prothrombin, FIX, and X. Thus warfarin propagation of thrombin generation keeps
unaffected much longer, which lead to increased risk of inadequate anticoagulation
[71].
FIX is a vitamin K-dependent serine protease involved in the activation of intrinsic
coagulation pathway in the coagulation system [72]. In the coagulation cascade,
FIX is firstly activated into FIXa by FXIa-VIIa/TF complex after vascular injury
and then FVIII is activated. In the presence of Ca
2+ and membrane phospholipids,
the complex of FIXa-VIIIa-Ca
2+ -PF 3 activates FX into Xa, and FXa-Va-Ca
2+ -
PF 3 converts prothrombin into thrombin [73]. Finally, thrombin converts soluble
fibrinogen into soluble fibrin, and FXIIIa crosslinks soluble fibrin into insoluble
fibrin, and thus forms stable fibrin clots [21]. So FIX plays a vital role in an intermediate stage of the blood coagulation cascade and elevated FIX level is believed
to be correlated to venous thromboembolism [73]. Since FIXa activates FXa and
thrombin during clot propagation, FIXa is a more potent initiator of coagulation
compared with FXa or thrombin. FIX is a promising drug target for the development
of a novel anticoagulant.
Rusconi et al. selected a specific FIXa inhibiting aptamer (9.3t) from a pool of
more than 10
14 random nucleic acid sequences by using SELEX technology, and then
designed a complementary RNA antidote to reverse anti-FIXa activity of aptamer
[74, 75]. To increase the in vivo half-life and limit its volume of distribution, the initial
aptamer (9.3t) was linked to a 40 kDa PEG moiety. In vitro binding studies showed
W. Li et al.
2
-amino pyrimidine modified RNA aptamer significantly prolonged the PT values
in human plasma in a dose-dependent manner, with an in vitro half-life of approximately 15 h in human plasma. The RNA aptamer is potentially a novel anticoagulant
agent against FVIIa [64].
10.4 Aptamers Against FIX
Presently, antiplatelet and anticoagulation is the mainstay therapy for ACS. In particular, it is necessary for patients with ACS during coronary revascularization procedures, either with PCI or through CABG, to receive periprocedural anticoagulation
to eliminate ischemic events [65]. According to recommendations of the American
College of Cardiology/American Heart Association guidelines, the anticoagulants
of ACS and PCI currently include UFH, LMWH, indirect FXa inhibitors such as
fondaparinux, and the direct thrombin inhibitors (DTI) like bivalirudin [66–68].
However, there are several shortcomings for each of these anticoagulants. On one
hand, bleeding involved in ACS treatment or coronary revascularization procedures
directly contributes to increased morbidity and mortality, and further blood transfusions may worsen the prognosis [69, 70]. On the other hand, current anticoagulants are
associated with adverse events due to unpredictable pharmacodynamics, immunogenicity, toxicity, and limited reversibility [69]. In addition, warfarin is a routine
agent for prevention and treatment of prothrombotic events, and during initiation of
warfarin therapy, serum levels of FVII and protein C decrease more rapidly than that
of prothrombin, FIX, and X. Thus warfarin propagation of thrombin generation keeps
unaffected much longer, which lead to increased risk of inadequate anticoagulation
[71].
FIX is a vitamin K-dependent serine protease involved in the activation of intrinsic
coagulation pathway in the coagulation system [72]. In the coagulation cascade,
FIX is firstly activated into FIXa by FXIa-VIIa/TF complex after vascular injury
and then FVIII is activated. In the presence of Ca
2+ and membrane phospholipids,
the complex of FIXa-VIIIa-Ca
2+ -PF 3 activates FX into Xa, and FXa-Va-Ca
2+ -
PF 3 converts prothrombin into thrombin [73]. Finally, thrombin converts soluble
fibrinogen into soluble fibrin, and FXIIIa crosslinks soluble fibrin into insoluble
fibrin, and thus forms stable fibrin clots [21]. So FIX plays a vital role in an intermediate stage of the blood coagulation cascade and elevated FIX level is believed
to be correlated to venous thromboembolism [73]. Since FIXa activates FXa and
thrombin during clot propagation, FIXa is a more potent initiator of coagulation
compared with FXa or thrombin. FIX is a promising drug target for the development
of a novel anticoagulant.
Rusconi et al. selected a specific FIXa inhibiting aptamer (9.3t) from a pool of
more than 10
14 random nucleic acid sequences by using SELEX technology, and then
designed a complementary RNA antidote to reverse anti-FIXa activity of aptamer
[74, 75]. To increase the in vivo half-life and limit its volume of distribution, the initial
aptamer (9.3t) was linked to a 40 kDa PEG moiety. In vitro binding studies showed