Initial identification of fXa inhibitors relied on high throughput screening assays based on inhibition of fXa induced chromogenic or clotting activity the first leads contained isoxazole derivatives such as benzamidine guanidine or napthylamine which are thought to mimic Glu-Gly-Arg the sequence in prothrombin that is recognized by fXa the first such compound was Dx-9065a which contained highly basic amidine groups as non peptidic mimetic of the Arg residue in the prothrombin recognition sequence because it exploited interactions with the S4 and S1 subsites of the active site ( figure 2) DX-9065a was specific for fXa and inhibited the enzyme with a K value of 46 nmol/l. After establishing its antithrombotic activity in a variety of animal models of venous and arterial thrombosis DX-9065a was compared with heparin in a small phase II dose-finding study in ACS patients. Although the results of the study were promising development of DX-9065a was halted because the oral bioavailability of the drug in humans was only 2 % to 3% because of its highly basic amidine content . 42 Nonetheless DX-9065a provided the groundwork future orally active fXa inhibitors
once lead compounds were identified through high throughput screening a structure -based approach was used to characterize their interaction with fXa. Using molecular modeling based on the crystallographic structure of fXa in complex with the first generation of fXa inhibitors such as Dx- 9065a quantitative structure - activity relationship analyses were used to determine how modifications affected the potency of the leads. 48-51 these techniques revealed that the highly basic amidine moiety in the P1 position of isoxazoline derivatives participates in a two-component interaction with the carboxylate group of the Asp residue at position 189 in the floor of the S1 pocket that flanks the catalytic triad of fXa. the first component involved a coulombic interraction between positive and negative charges( so called ion pairing) whereas the second reflected a bidentate hydrogen bond.52 Elimination of either of these interactions increased the k by one to two orders of magnitude. 52 Building on this information the amidine group in the P1 position was replaced with nonbasic moieties in an attempt to increase oral bioavailability .53 various linker or P4 elements were then added to overcome the subsequent loss in potency by maximizing interactions at the S4 siteonce lead compounds were identified through high throughput screening a structure -based approach was used to characterize their interaction with fXa. Using molecular modeling based on the crystallographic structure of fXa in complex with the first generation of fXa inhibitors such as Dx- 9065a quantitative structure - activity relationship analyses were used to determine how modifications affected the potency of the leads. 48-51 these techniques revealed that the highly basic amidine moiety in the P1 position of isoxazoline derivatives participates in a two-component interaction with the carboxylate group of the Asp residue at position 189 in the floor of the S1 pocket that flanks the catalytic triad of fXa. the first component involved a coulombic interraction between positive and negative charges( so called ion pairing) whereas the second reflected a bidentate hydrogen bond.52 Elimination of either of these interactions increased the k by one to two orders of magnitude. 52 Building on this information the amidine group in the P1 position was replaced with nonbasic moieties in an attempt to increase oral bioavailability .53 various linker or P4 elements were then added to overcome the subsequent loss in potency by maximizing interactions at the S4 site. A decade of development led to the synthesis of several orally active fXa inhibitors.54 ofฺ these rivaroxaban (Bayer Healthcare) 13 apixaban (ฺBristol-Myers Squibb) 14 and edoxaban (Daiichi Sankyo) 15 are the drugs in the most advanced stages of development. All of these agents contain nonbasic moieties in the P1 position. chlorothiophene in rivaroxaban methoxyaryl in apixaban and chloro-substituted pyridine rings in edoxaban 54 (Figure 3) with the drugs developed and their antithrombotic activities established in animal models testing in humans was