Haemophilia is a congenital bleeding disorder resulting from genetic deficiencies in Factor VIII or Factor IX that together comprises the intrinsic tenase complex, the keystone of the thrombin amplification loop in normal coagulation. Current treatment modalities rely on prophylactic or on demand replacement therapy, however, these treatments can lead to the development of inhibitory antibodies against the replacement coagulation factors and the necessity of so-called “bypassing” agents. Several strategies have been developed to bypass the lack of a functional tenase complex; however, generation of thrombin-activatable Factor X variants for the first time provides the opportunity to re-establish the feed-back amplification loop in the coagulation cascade.
We have approached the design of thrombin-activatable Factor X compounds using a two-tiered strategy. Firstly, to identify the most effective thrombin sensitive cleavage sites to be engineered into Factor X, we designed a positional scanning quenched fluorescence substrate library with a core cleavage sequence of XXPR-IVGG having the P4 and P3 residues fully randomized while maintaining a preferred P2 proline and the prime side –IVGG motif of Factor X. Direct screening and counter screening of the quenched-fluorescence library with alpha-thrombin and Factor Xa, respectively, returned both expected and unique preferred sequence motifs of which more than 20 were selected for further evaluation. Secondly, thrombin sensitive cleavage sequences were introduced into the activation peptide of Factor X in combination with strategies anticipated to augment cleavage rates and minimise the insertion size and number of substitutions necessary to create a productive cleavage site. The generated FX variants were evaluated with respect to in vitro thrombin activation rates, thrombin generation and efficacy in thromboelastography. In conclusion, this work proved the feasibility of generating FX variants which can be effectively activated by thrombin as well as demonstrated that overall efficacy was highly dependent on a combination of the preferred cleavage sequence and the insertion strategy.