Development of Experimental Models of Antithrombin-independent Heparin Resistance Using Platelet Factor 4 and the Effect of Antithrombin in These Models.

Objectives: Antithrombin (AT) deficiency is considered the primary cause of heparin resistance (HR); however, some patients with HR have normal AT activity (AT-independent HR). Supplementation with concentrated human AT is recommended for patients with AT-deficient HR, whereas treatment for AT-independent HR has not been established. Interestingly, the efficacy of concentrated human AT for AT-independent HR has recently been reported. Therefore, this study was designed to experimentally investigate the effects of AT on AT-independent HR using experimentally developed models with platelet factor 4 (PF4), a potent heparin inhibitor and a potential risk factor for AT-independent HR.

Design: In vitro and in vivo experimental studies.

Setting: Experimental laboratory.

Participants: Normal human plasma, whole blood, and 8- or 9-week-old male Institute of Cancer Research mice.

Interventions: Recombinant PF4 (rPF4), heparin, and AT were added or administered.

Measurements and main results: Coagulation parameters, including activated partial thromboplastin time and clotting time using the INTEM assay of rotational thromboelastometry, were assessed. In addition, the AT activity of the rPF4-containing plasma was measured, and the interaction of rPF4 or AT with heparin was evaluated. Recombinant PF4 shortened activated partial thromboplastin time and clotting time prolonged by heparin, without affecting AT activity. AT ameliorated this shortening in a dose-dependent manner. The binding affinity of AT for heparin was weaker than that of rPF4.

Conclusions: This work experimentally demonstrated that AT has the potential to ameliorate AT-independent HR. The potential mechanism was considered to involve an increased absolute number of AT-heparin complexes. This report provides insights into therapeutic strategies for AT-independent HR.