A Primate Model (Macaca Mulatta) to Study the Pharmacokinetics of Heparin and Its Fractions

Abstract

We have extensively studied the hemostatic parameters and the responses to the anticoagulant action of heparin and its fractions in the primate model (M. mulatta) and found these to be identical to those obtained in humans. The functional properties of antithrombin III, alpha 2-antiplasmin, and platelet factor 4 were also identical to humans in amidolytic and coagulant assays. Human antibodies against FPA, B beta 15-42 peptide, platelet factor 4, and thromboxane B2 reacted with the primate antigen, and assays were developed to measure these parameters in primates. Infusion of activated prothrombin complex concentrates (more than 100 U/kg/day) on a continual basis up to 3 days resulted in a hypercoagulable state manifested by an elevation of FPA, thromboxane B2, and changes in the thrombelastographic patterns. Similarly, infusion of homologous primate serum also resulted in a hypercoagulable state, as was evident by a sharp increase in the FPA levels. The antithrombotic effects of intravenous and subcutaneous administration of heparin, its low molecular fraction, and their constituents were studied after intravenous and subcutaneous injections. The low molecular weight fractions showed the most effective antithrombotic effects, whereas somewhat milder protection was observed with the starting material and highly anionic fraction. The prolongation of global tests, such as the APTT, TT, and changes in the thromboelastogram did not correlate with the reduction in the blood markers of hypercoagulable state. A modified simplate bleeding time method was used to study the effect of heparin and its fractions on the bleeding profile of heparin fractions. The components of fibrinolytic systems were also measurable in both the clot-based and amidolytic methods to predict the profibrinolytic actions of heparin fractions in its mode. These studies suggest that plasma markers, such as the platelet release proteins, products of thrombin activation, and prostaglandin metabolites, may provide better indices in the monitoring of the antithrombotic actions of newer heparins and antithrombotic drugs. Studies suggest that the pathophysiologic responses after a thrombogenic trigger in the primate model are close to humans, and drug modulation of these may provide relevant clinical information. This model provides the most similar preclinical model to study the actions of heparin fractions.

Validity of the Newly Established Low-Molecular-Weight Heparin Standard in Cross-Referencing Low-Molecular-Weight Heparins

“…These studies were conducted in an estab lished primate model [9], In the intravenous studies, the relative bioavailability of the anti-Xa effects were similar in all of the LMWHs except for the Novo LMWH and heparin. Wider variations in the anti-IIa ac tions were noted after a 50 IU/kg of the var ious LMWHs.…”

“…The cumulative bioavailabil ity spectrum of each of the potency-activated lieparins was evaluated in the partial thromboplastine time, anti-Xa, Alla and Heptest. assays in a primate model in both the intravenous and subcutaneous route [9]. Bleeding studies were performed in a mod ified rabbit ear bleeding time model as reported pre viously [4], Protamine titration studies were con ducted in vitro by supplementing 10 pg/ml of prot amine to 1.0 IU/ml of each of the LMWHs and the anti-Xa activity was measured prior to and after the addition of protamine using the aca® (Du Pont; Wil mington, Del.)…”

Comparative Study on the in vitro and in vivo Activities of Seven Low-Molecular-Weight Heparins

“…The bioavailability of each agent was studied in a primate model of pharmacokinetics/pharmacodynamics as previously developed in our laboratory [3]. Bioavailability was measured in terms of Heptest®, antifactor Xa and antifactor Ila actions.…”

Pharmacokinetics of Heparin and Low Molecular Weight Heparins

“…The ratio between this different dynamic effects changes with time after a single administration and is de pendent on the route of drug delivery (intra venous, subcutaneous, intraperitoneal) [15]; moreover, this ratio changes when different doses are employed [16], This suggests that, even though the kinetics of LMWHs is li near, as previously demonstrated, their var ious components and/or the related dynamic effects might depend on saturable pro cesses.…”

“…The presence of this active process of heparin biodegradation might account for the systematic differenes that may be demonstrated when compar ing the antithrombotic activities of equal amounts of heparin in vitro and in vivo [9]; the higher in vivo activity of heparin in fact might be explained by the transformation to which Heparin is subjected. The higher bio availability of LMWHs after subcutaneous adminstration with respect to intravenous administration might also be explained by a similar process [16], All experimental evidences suggest that no simple model may be used to describe the pharmacokinetics of heparin and LMWHs; only a complex model including a central pooling compartment where the heparin is stored and transformed might explain the various peculiarities of heparin kinetics [23] that are still uncompletely understood be cause of serious methodological shortcom ings, such as those resulting from the fre quent use of indirect measures of heparin concentrations.…”