A hamster pulmonary embolism model for the evaluation of the thrombolytic and pharmacokinetic properties of thrombolytic agents

Abstract

A pulmonary embolus model in adult hamsters (body weight less than 100 g) was developed for the quantitative evaluation of the thrombolytic and pharmacokinetic properties of plasminogen activators. A clot was made from a mixture of 125I-labeled fibrinogen, citrated human plasma, thrombin and CaCl 2, and was injected into the jugular vein of heparinized hamsters. The time course of in vivo clot lysis was continuously measured with an external gamma counter, placed over the thorax. The extent of thrombolysis within 90 min was determined both as the difference between the initial and final external gamma count and as the difference between the radioactivity injected in the jugular vein and that recovered in the heart and lungs. In control animals infused with solvent for 60 min, spontaneous progressive clot lysis occurred resulting in 25 ± 1 % lysis (mean ± SEM; n = 87) by ex vivo isotope recovery and in 15 ± 2% lysis (n = 31) by external gamma counting. Recombinant tissue-type plasminogen activator (Activase®), recombinant single chain urokinase-type plasminogen activator (rscu-PA) and urokinase (Winkinase®), infused intravenously over 60 min, caused dose-dependent progressive thrombolysis. With 1.0 mgAkg Activase®, lysis was 91 ± 3% (n = 4) by ex vivo isotope recovery and 61 ± 6% (n = 4) by external gamma counting. With streptokinase (Streptase®) infusion or with bolus injection of anisoylated plasminogen streptokinase activator complex (Eminase®), dose-response curves were not linear. Fibrinogen and α 2-antiplasmin levels at the end of the experiments were unchanged with Activase® and rscu-PA, and moderately decreased with Winkinase®, Streptase® and Eminase®. Linear regression analysis of all results of the percent lysis at 90 min, determined by ex vivo isotope recovery (IR) and by external gamma counting (EC) (n = 101) yielded the equation (IR) = 1.08 (EC) + 21 with r = 0.87. The failure rate of the thrombolysis experiments, due to technical reasons or to death of the animals, was less than 5 %. After i.v. bolus injection of plasminogen activators in hamsters, the pharmacokinetic properties were determined from the disappearance rate of antigen from plasma, which were for Activase®, i.e., an initial half-life of 1.5 min and a plasma clearance of 2.8 ml/min. The hamster pulmonary embolism model is simple, reproducible, quantitative, and allows for one investigator to perform up to 10 experiments per day. This model may be useful for the evaluation of the potency, fibrin-specificity and pharmacokinetics of thrombolytic agents.