Thrombin activatable fibrinolysis inhibitor (TAFI) forms a molecular link between coagulation and fibrinolysis and is a putative target to develop profibrinolytic drugs. Out of a panel of monoclonal antibodies (MA) raised against TAFI-ACIIYQ, we selected MA-TCK11A9, MA-TCK22G2 and MA-TCK27A4, which revealed high affinity towards human TAFI-TI-wt. MA-TCK11A9 was able to inhibit mainly plasmin-mediated TAFI activation, MA-TCK22G2 inhibited plasmin- and thrombin-mediated TAFI activation and MA-TCK27A4 inhibited TAFI activation by plasmin, thrombin and thrombin/thrombomodulin (T/TM) in a dose-dependent manner. These MA did not interfere with TAFIa activity. Using an eight-fold molar excess of MA over TAFI, all three MA were able to reduce clot lysis time significantly, i.e. in the presence of exogenous TM, MA-TCK11A9, MA-TCK22G2 and MA-TCK27A4 reduced clot lysis time by 47 ± 9.1%, 80 ± 8.6% and 92 ± 14%, respectively, compared to PTCI. This effect was even more pronounced in the absence of TM i.e. MA-TCK11A9, MA-TCK22G2 and MA-TCK27A4 reduced clot lysis time by 90 ± 14%, 140 ± 12% and 147 ± 29%, respectively, compared to PTCI. Mutagenesis analysis revealed that residues at position 268, 272 and 276 are involved in the binding of MA-TCK11A9, residues 147 and 148 in the binding of MA-TCK22G2 and residue 113 in the binding of MA-TCK27A4. The present study identified three MA, with distinct epitopes, that impair the activation of human TAFI and demonstrated that MA-TCK11A9 which mainly impairs plasmin-mediated TAFI activation can also reduce significantly clot lysis time in vitro.

To cite this article: Develter J, Booth NA, Declerck PJ, Gils A. Bispecific targeting of thrombin activatable fibrinolysis inhibitor and plasminogen activator inhibitor-1 by a heterodimer diabody. J Thromb Haemost 2008; 6: 1884-91.Summary. Background and objectives: Thrombin activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor-1 (PAI-1) play important roles in fibrinolysis. Both reduce plasmin generation, but they exert their antifibrinolytic effects via different mechanisms. This study reports the cloning and characterization of a heterodimer diabody that inhibits TAFI and PAI-1 simultaneously. Methods and results: The diabody was derived from two inhibiting monoclonal antibodies, i.e. MA-33H1F7, an anti-PAI-1 antibody that induces noninhibitory substrate behavior of PAI-1, and MA-T12D11, an anti-TAFI antibody that inhibits activation of TAFI by the thrombin-thrombomodulin complex. A single-chain variable fragment (scFv) was derived from MA-T12D11 that displayed slightly reduced binding and inhibitory properties as compared to MA-T12D11. Characterization of the diabody revealed a similar affinity for TAFI and PAI-1 as that of the parental antibodies. Furthermore, the inhibitory properties of MA-33H1F7 and MA-T12D11 were fully preserved in the diabody format. In platelet-free plasma (PFP) clots, addition of the diabody had a stronger effect in shortening lysis times than either MA-T12D11 or MA-33H1F7. A similar reduction in clot lysis time was observed in platelet-rich plasma (PRP) clots. The same effect on clot lysis times in PFP and PRP was also achieved by the combined addition of MA-T12D11 and MA-33H1F7. The lysis rate of human model thrombi, made from whole blood, was approximately doubled after addition of the diabody. Moreover, this effect was significantly better than after the combined addition of the individual antibodies.Conclusions: These observations demonstrate that simultaneous inhibition of TAFI and PAI-1 results in faster lysis of the formed thrombus.

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Thrombin activatable fibrinolysis inhibitor (TAFI) attenuates fibrinolysis and is considered as an attractive drug target. We generated two different antibody fragments, an antigen-binding fragment (Fab) and a single-chain variable fragment (scFv), derived from three distinct monoclonal antibodies (MAs) that inhibit the activation of TAFI by the thrombin/thrombomodulin complex (T/TM) and plasmin (MA-T1C10 and MA-T94H3) or by T/TM alone (MA-T12D11). The Fabs were obtained by papain digestion of the purified MAs, whereas the scFvs were cloned and subsequently expressed in bacteria. All antibody fragments revealed similar or slightly decreased affinities compared to those of the respective MAs, except scFv-T94H3. In the presence of a 16-fold molar excess of all antibody fragments, activation of TAFI by T/TM was completely blocked. Furthermore, Fab and scFv-derivatives from MA-T1C10 and MA-T94H3 were capable of interfering with the plasmin-mediated activation of TAFI. Addition of 850 nM of MA, Fab or scFv to an in-vitro clot lysis assay caused a significant reduction of clot lysis time (except for scFv-T94H3) and this effect was comparable to that of potato tuber carboxypeptidase inhibitor, a well-known TAFIa inhibitor. Dose-response experiments with the antibody (derivatives) in clot lysis and chromogenic assay revealed that the inhibitory capacity of the Fabs was comparable to that of the MAs, whereas the scFvs had a more reduced potency. In conclusion, these highly specific TAFI inhibitors are interesting tools to further evaluate the concept of TAFI inhibition in various in-vitro and in-vivo models.

Introduction:TAFIa exerts an antifibrinolytic effect by removing C-terminal lysines from partially degraded fibrin degradation products thereby abolishing their cofactor function in the activation of plasminogen by t-PA. A previous study identified 14 monoclonal antibodies, raised against human TAFI, that exert a TAFI inhibitory effect by impairment of the activation of TAFI into TAFIa by thrombin/thrombomodulin. Surprisingly, only three of these antibodies could also hamper substantially the activation by plasmin even though activation by plasmin and thrombin involves the same cleavage site, i.e. Arg92. Epitope analysis identified Gly66 as the major residue for the neutralizing antibodies that inhibit exclusively the activation of TAFI by thrombin/thrombomodulin, and this group is represented by MA-T12D11. Objective: To study the biochemical properties of MA-T12D11 derivatives in order to gain more insights in the working mechanism of MA-T12D11. Methods and results: Two antibody derivatives of MA-T12D11 were generated. Firstly, a Fab-T12D11 fragment was produced by papain digestion of MA-T12D11, followed by protein A purification. Secondly, a single-chain variable fragment (scFv) of MA-T12D11 was cloned. Therefore, cDNA was isolated from MA-T12D11 producing hybridomas, and VH and VL regions were amplified with the appropriate primers and assembled using a DNA linker segment (encoding (Gly4Ser)3). As expected, sequencing of this scFv revealed the typical presence of CDR regions. Surprisingly, affinity constants of Fab-T12D11 and scFv-T12D11 for TAFI-AT (KA = 2.8 ± 1.0 x 10e9 1/M and KA = 1.7 ± 0.7 x 10e9 1/M, respectively) were only 2-to 3-fold lower compared to that of MA-T12D11 (KA = 5.4 ± 1.5 x 10e9 1/M). Evaluation of the dose-response curves of the Fab and scFv fragment on the inhibition of TAFI activation revealed a shift towards slightly higher concentrations. This is in line with the small decrease in affinity and eventually resulted in a similar maximum effect as the corresponding MA-T12D11 (i.e. 98 ± 1%, 99 ± 1% and 91 ± 3% TAFIa inhibition using a 8-fold molar excess of MA-T12D11, Fab-T12D11 and scFv-T12D11, respectively). Evaluation of the profibrinolytic effect of MA-T12D11 derivatives was tested in a clot lysis assay and compared to potato tuber carboxypeptidase inhibitor (PTCI), a known inhibitor of TAFIa. In the absence of an inhibitor, the clot lysis time was 120 min. Preincubation of plasma with PTCI (45-fold molar excess) shortened the clot lysis time to 32 min. Preincubation with MA-T12D11(8-fold molar excess) and Fab-T12D11(8-fold molar excess) shortened the clot lysis time to 25 min and 26 min, respectively. The effect of scFv-T12D11 in the clot lysis assay will be tested. Conclusion: Characterization of MA-T12D11 derivatives reveals that these fragments have similar affinity constants as their parental MA and have a similar mode of action on TAFI activation by thrombin/thrombomodulin. The availability of the cloned scFv and its sequence also allows to further explore the paratope characteri...