Further studies on the mechanism for the antithrombotic effects of naroparcil, an orally active thioxyloside compound

The antithrombotic beta-D-xyloside, naroparcil, has previously been shown t o induce a dose-related increase of circulating glycosamino-glycans (GAGs) together with an antithrombin activity (anti-IIa) via heparin cofactor II ( HCII) in the rabbit. In order to go further in the mechanisms, the relation ship between the antithrombotic activity, the HCII-mediated anti-IIa activi ty and the plasma GAG content was investigated. We showed that the in vitro specific activity on the inhibition of thrombin by HCII of the plasma GAG extract from naroparcil-treated rabbits was increased by a factor of 60 whe n compared to controls. In addition, the fractionation of the plasma GAG ex tract by affinity chromatography on immobilized HCII led to a more potent m aterial whereas the low-affinity fraction was shown to be inactive in throm bin inhibition by HCII. The qualitative analysis of GAGs showed the presence of the Delta Di-4S DS disaccharide, undetectable in control, which accounted for 22% in the unfra ctionated GAG extract and for 60% in the high affinity fraction. lit vitro experiments using immune-depleted plasma in antithrombin III (ATIII), HCII or both, indicated that the anti-IIa activity of the plasma GAG extract fro m naroparcil-treated rabbits was mainly due to HCII potentialisation. The u nfractionated GAG extract and the high affinity fraction were shown to be a ntithrombotic in a Wessler-based model in the rat, giving ED80 Values of 61 0 UA/kg and 56 UA/kg respectively whereas the low-affinity fraction was dev oid of any antithrombotic activity. These results show that the antitbrombo tic activity of naroparcil is dependent on modification in the plasma GAG p rofile which inactivates thrombin via the HCII.