Pharmacokinetics and pharmacodynamics of a therapeutic dose of unfractionated heparin (200 U/kg) administered subcutaneously or intravenously to healthy dogs

Objectives: To evaluate the effects of 200 U/kg of sodium unfractionated heparin (UFH) on coagulation times in dogs after IV and SC administration and to compare these effects with plasma heparin concentrations assessed by its anti Xa activity. Methods: 200 U/kg of UFH were administered Intravenously (IV) and Subcutaneously (SC) to five healthy adult Beagle dogs with a wash out period of at least 3 days. Activated Partial Thromboplastin Time (APTT), Prothrombin Time (PT) and plasma anti-factor Xa (aXa) activity were determined in serial blood samples. Results: After IV injection, PT remained unchanged except for a slight increase in one dog; APTT was not measurable (> 60 s) for 45 to 90 min, then decreased regularly and returned to baseline values between 150 and 240 min. High plasma heparin concentrations were observed (C max = 4.64±1.4 aXa U/mL) and decreased according to a slightly concave-convex pattern on a semi-logarithmic curve but returned to baseline slightly more slowly (t240 to t300 min). After SC administration, APTT was moderately prolonged (mean±SD prolongation: 1.55±0.28 x APTT t0, range [1.35-2.01]) between 1 and 4 hours after administration. Plasma anti-factor Xa activity reached a maximum of 0.56±0.20 aXa U/mL, range: [0.42 - 0.9] after 132±26.8 min and this lasted for 102±26.8 min. Heparin concentrations were grossly correlated to APTT; prolongation of APTT of 120 to 160% corresponded to plasma heparin concentrations range of 0.3-0.7 aXa U/mL, considered as the therapeutic range in human medicine. Conclusions: As in human, pharmacokinetic of UFH in dogs is non linear. Administration of 200 U/kg of UFH SC in healthy dogs results in sustained plasma heparin concentrations in accordance with human recommendations for thrombosis treatment or prevention, without excessively increased bleeding risks. In these conditions, APTT can be used as a surrogate to assess plasma heparin concentrations. This has to be confirmed in diseased animals