Summary
Poloxamer 188, N.F. (RheothRx®), a nonionic block copolymer composed of 2 hydrophilic polyoxyethylene chains connected
by a hydrophobic polyoxypropylene chain, normalizes the viscosity of whole blood high
in soluble fibrin(ogen) complexes. Normalization may be via a poloxamer 188-induced
decrease in fibrin(ogen)-red cell interaction. Our study examined the influences of
poloxamer 188 on fibrin assembly and structure. Studies were performed in both purified
and plasma systems with a poloxamer 188 concentration range of 0.1-20 mg/ml and specific
clotting conditions (fibrinogen 1 mg/ml, thrombin 1 NIH u/ml, pH 7.4 [Tris 0.05 M],
ionic strength 0.15 and calcium 5 mM). Fibrin assembly was accelerated in the presence
of poloxamer 188. As poloxamer 188 concentration was increased from 0 to 8 mg/ml in
plasma: a) the lag phase prior to initial turbidity rise decreased from 25 to <5 s;
b) the final gel optical density (OD) increased from 0.65 to 1.28 and c) fiber size
(mass/length ratio [ε]) increased from 4.3 to 12.6 × 1013 daltons/cm. Similar results were seen in the purified system with a poloxamer 188
concentration range of 0-8 mg/ml. OD increased from 0.26 to 0.51, and ε increased
from 2.3 to 5.3 × 1013 daltons/cm. Above 8 mg/ml, precipitation of fibrinogen was noted in this system.
Since large fibrin fibers tend to be degraded more rapidly, possible poloxamer mediated
enhancement of r-tPA-mediated clot lysis was investigated. With r-tPA (70 lu/ml) present
at the time of clotting, clot lysis in the presence of ploxamer 188 (8 mg/ml) was
50% complete at 1,600 s compared to 2,540 s for the control. Thus, poloxamer 188-induced
alterations in fibrin structure and fibrin-cell interactions may explain some of this
agent's interesting hemorrheologic and antithrombotic properties.
