Determination of the anti-factor Xa activity of the synthetic pentasaccharide SR 90107A/ORG 31540 and of two structural analogues.

The anti-factor Xa activity of the synthetic pentasaccharide SR 90107A/ORG 31540 was assayed by a chromogenic method at pH 8.4 and pH 7.35, comparatively to the 4th International Heparin Standard (IHS) or to the Ist International Low Molecular Weight Heparin Standard (LMWHS). At pH 8.4, SR 90107A/ORG 31540 was found to have a specific anti-factor Xa activity of 639 +/- 14 and 659 +/- 19 IU/mg (mean +/- sem, n = 6) when assayed in comparison with the 4th ISH and the Ist LMWHS respectively. At pH 7.35, the corresponding figures were 864 +/- 6 and 1160 +/- 51 IU/mg (mean +/- sem, n = 6) respectively. The dissociation constants of the ATIII-pentasaccharide complex formed by SR 90107A/ORG 31540 and by two close analogues: SR 80327A and SR 80027A in the presence of purified human ATIII were found to be 41 +/- 8, 96 +/- 1 and 3 +/- 1.4 nM (mean +/- sem, n = 3) respectively. For the three compounds, the pseudo-first order molar catalytic constants for factor Xa inactivation by the ATIII-pentasaccharide complex were shown to be statistically comparable, in the range of 7-8 x 10(7) min-1 per mole. It is concluded that the differences in specific anti-factor Xa activities between SR 90107A/ORG 31540 and its synthetic chemical analogues can be attributed to variations of the dissociation constants whereas the catalytic constants for factor Xa inactivation remain unchanged.