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4386 Introduction: A neutralization of anticoagulant activity occurs when heparin binds to a variety of positively charged substances such as protamine and platelet factor 4 (PF4). PMX60056 (PolyMedix, Radnor, PA) is a novel compound that is being developed as a heparin antagonist. Since heparin-PF4 complexes are antigenic, with antibodies against this complex activating platelets to trigger thrombin generation, thrombus formation and associated morbidities, it is of interest to determine whether heparin:PMX60056 complexes affect platelet function. This study compares the effect of PMX60056 and protamine, alone and complexed to heparin, on platelet function as assessed by platelet aggregometry. Materials and Method: Whole blood, collected from 10 healthy individuals, was anticoagulated with 3.2% sodium citrate and centrifuged to make platelet rich plasma (PRP). PRP was supplemented with 10 μ g/ml heparin (~1.5 IU/ml), 10 μ g/ml heparin antagonist (protamine or PMX 60056) or a complex of 10 μ g/ml heparin and 10 μ g/ml heparin antagonist. Platelet aggregation was stimulated by the addition of ADP (5 or 10 μ M final concentration) or serum from a patient with heparin-induced thrombocytopenia. Result: ADP-induced platelet aggregation was not affected by the addition of heparin, protamine, PMX 60056, or complexes of heparin with heparin antagonist. In the HIT system, heparin + HIT serum led to a significant increase in platelet aggregation vs. saline (46.7 ± 3.2 % vs. 8.2 ± 2.9%). HIT serum + heparin antagonist did not induce platelet aggregation (PMX60056: 10.2 ± 4.4%; protamine: 11.6 ± 3.5%). The aggregation responses to HIT serum + heparin (46.7 ± 3.2%), HIT serum + heparin:PMX60056 (43.6 ± 5.8%) and HIT serum + heparin:protamine (47.8 ± 3.8%) were not significantly different. Conclusion: When mixed at equigravimetric amounts, protamine and PMX60056 do not prevent formation of immune complexes consisting of HIT antibody and heparin which lead to platelet activation. Previous data from human trials has suggested that smaller amounts of PMX60056 (less than equigravimetric) may effectively neutralize heparin. Thus, it is speculated that smaller heparin:PMX60056 complexes may induce less antibody formation than larger heparin:protamine complexes. Validation of this hypothesis in animal models or clinical studies is warranted. Disclosures: McAllister: PolyMedix, Inc.: Employment.

3329 PMX-60056 is a small-molecule first-in-class new chemical entity designed to bind to the pentasaccharide group in unfractionated heparin (UFH) and low-molecular-weight heparins (LMWH), and reverse their anticoagulation effects. It has been shown to reverse UFH and the LMWH tinzaparin to date, and without rebound anticoagulation. When no UFH or LMWH is present, PMX-60056 dosing has been limited by transient hypotension at doses over 0.4 mg/kg in volunteers, but its use for heparin reversal should eliminate this effect as PMX-60056 preferentially binds to heparin. A study with UFH at 70 U/kg and reversal with 0.3 mg/kg PMX-60056 showed full efficacy, and mild blood-pressure reductions in half the subjects that suggested excess PMX-60056. Therefore, we examined the dose-response, efficacy, and safety at higher doses of UFH as used in cardiac surgery. Hemodynamic effects were recorded with lithium-dilution cardiac output, and blood pressures were continuous intra-arterial measurements. PMX-60056 dose was titrated using activated clotting times (ACTs), and protamine requirements were also estimated by heparin-protamine titrations to determine heparin levels. 350 U/kg of heparin was reversed in 6 normal volunteers, after an initial 10-minute infusion of 0.7 mg/kg PMX-60056 and subsequent smaller doses as needed to normalize ACT. Hemodynamics were unaffected until total PMX-60056 dose exceeded 1 mg/kg – the initial dose of 0.7 mg/kg never produced a change. When a hemodynamic effect did occur (in 3 of the 6), it was initiated by a fall in systemic vascular resistance. Only 1 of these 6 subjects had a clinically significant hypotension, which lasted 15 minutes with pressor agents and limb elevation; this subject was subsequently found to have a past history suggestive of vaso-vagal instability. The other 5 subjects had no appreciable change in blood pressure. Per protocol the antagonist doses were in discrete amounts, so some overshoot was inevitable. Therefore, the total dose of PMX-60056 to the point immediately preceding full reversal of ACT was correlated with ACT, with initial protamine requirement according to heparin levels, and with estimated residual heparin (calculated from dose given and time elapsed). These data were highly correlated with a straight line from zero: R-squared was 0.90 with initial ACT, 0.96 with initial protamine requirement according to heparin levels, and 0.97 with estimated residual heparin (calculated). These data suggest that measurements routinely available during cardiac surgery are sufficient for predicting a single reversing dose of PMX-60056 that will safely and effectively neutralize UFH-induced anticoagulation. After reversal, the ability to re-anticoagulate is sometimes important. The same study also established that reversal of anticoagulation with PMX-60056 did not inhibit subsequent repeat anticoagulation with heparin a few minutes later, which was then also reversed with PMX-60056. This study was conducted according to cGCP at Duke University's Anesthesiology Research Unit. More results will be forthcoming as the analysis proceeds. Disclosures: McAllister: PolyMedix, Inc.: Employment.