HEPARIN-INDUCED PLATELET AGGREGATION (H-IPA): DOSE/RESPONSE RELATIONSHIPS FOR TWO LOW MOLECULAR WEIGHT (LMW)HEPARIN PREPARATIONS (CY 216 and CY 222)

We have previously demonstrated that unfractionated heparin causes platelet aggregation (>50%) in about 40% of normal healthy donors tested. H-IPA occurs in a dose-dependent manner and can be inhibited by antogonists of the thromboxane pathway. Using a LMW heparin preparation (PK 10169) and fractions of this agent separated on the basis of molecular weight (MW) by gel permeation chromatography, we showed that H-IPA was dependent upon the MW of the agents tested. In order to further examine this MW dependence, we tested two other LMW heparin preparations, CY 216 (Mol. wt: 5600) and CY 222 (mol. wt: 3800), and 9 subfractions of each of these agents separated on the basis of MW. Blood was drawn from the same donors whose platelets aggregated when heparin was added to their platelet-rich plasma (PRP), and placed into citrate anticoagulant. PRP was prepared, various concentrations of the agents or their fractions were added and aggregation was monitored for 40 minutes at 37°C. Dose/response curves were constructed from the data obtained with each agent. Compared to unmodified heparin with Mr = 15,000 daltons (D), the dose/response curves for CY 216 (Mr = 5000 D) and CY 222 (Mr = 3,500 D) were shifted progressively down and to the right. Dosq' response curves for each of the fractions of CY 216 and CY 222 demonstrated that as the molecular weight of the fractions decreased, the dose/response curves were also shifted progressively down and to the right. These results indicate that as MW decreases, higher concentrations of the fractions are required to cause aggregation, and the maximum aggregation obtained decreases. Fractions with MW less than 2,500 daltons caused aggregation only at concentrations exceeding supra therapeutic range. Since heparin and LMW fractions have inhibitory activity to the activated clotting factors Ila and Xa and LMW fractions have higher anti-Xa than anti-IIa activity, we measured these activities and attempted to correlate them with the ability to cause H-IPA. No correlation between AXa and H-IPA was found. We conclude that the ability to cause H-IPA is an inherent property of heparin and is molecular weight dependent.