INHIBITION OF RED BLOOD CELL-INDUCED PLATELET-AGGREGATION IN WHOLE-BLOOD BY A NONIONIC SURFACTANT, POLOXAMER 188 (RHEOTHRX(R) INJECTION)

RheothRx Injection, an aqueous solution of a nonionic block copolymer (poloxamer 188) formulated for intravenous administration, was investigated as an inhibitor of red blood cell (RBC)-induced platelet aggregation at plasma concentrations of 0.05-5mgmL(-1). Platelet aggregation was determined by measuring the fall in single platelet counts after mechanical agitation of 2mL aliquots of citrated whole blood in a 37 degrees C shaking waterbath. Inhibition of RBC-induced platelet aggregation of >95% was observed for poloxamer 188 at a concentration of 1mgmL(-1), and 41% inhibition was observed at 0.05mgmL(-1). Poloxamer 188 was observed to be a more effective inhibitor of RBC-induced platelet aggregation than 2-chloradenosine (2-ClAd) or phosphoenolpyruvate/pyruvate kinase (PEP/PK). Studies using platelet rich plasma (PRP) showed that platelet aggregation could not be induced by shaking in the absence of RBC, though aggregation was induced by the addition of exogenous adenosine diphosphate(ADP). Poloxamer 188 did not inhibit ADP-induced platelet aggregation. We propose that poloxamer 188 protects RBC from mechanical trauma by non-specific adsorption of copolymer to the RBC surface (via the hydrophobic polyoxypropylene moiety), and that this effect prevents mechanical damage and hence leakage of ADP from RBC. RheothRx Injection has been shown to have value in the treatment of acute ischemic disorders such as myocardial infarction. The observation of significant inhibition of RBC-induced platelet aggregation at clinically relevant concentrations suggests that RheothRx Injection may have antithrombotic properties in vivo, and may therefore have potential not only in acute ischemia but also to prevent thrombosis within vascular prostheses or to prevent rethrombosis after angioplasty or endarterectomy. Poloxamers are nonionic ABA block copolymers of polyoxyethylene (A) and polyoxypropylene (B) (Fig. 1) and are commercially available over a wide range of molecular weights (1000 to 14000 gmol(-1)) and polyoxyethylene/polyoxypropylene ratios. These copolymers are produced commercially under several tradenames [e.g. Pluronics (BASF, Wyandotte, MI.), Synperonic PE nonionic surfactants (ICI, Middlesbrough, U.K.)] and have numerous industrial uses (1-3) for example as emulsion stabilizers, foaming, defoaming, antistatic and wetting agents. The desired hydrophobic/hydrophilic balance is achieved by varying the total molecular weight and polyoxypropylene/polyoxyethylene ratio.