Plasma levels of anti-Xa activity increase in the first 2 to 3 hours following SC injection of tinzaparin sodium and reach a maximum within 4 to 5 hours. Maximum concentrations (Cmax) of 0.25 and 0.87 IU/mL are achieved following a single SC fixed dose of 4,500 IU (approximately 64.3 IU/kg) and weight-adjusted dose of 175 IU/kg of tinzaparin sodium, respectively. Based on the extent of absorption (AUC0-), a comparison of 4,500 IU and 12,250 IU single doses indicates that increases in anti-Xa activity are greater than dose proportional relative to the increase in dose. Following a single SC injection of tinzaparin sodium, the mean anti-Xa to anti- IIa activity ratio, based on the area under the anti-Xa and anti-IIa time profiles, is 2.8 and is higher than that of unfractionated heparin (approximately 1.2). The absolute bioavailability (following 4,500 IU SC and intravenous [IV] administrations) is 86.7% based on anti-Xa activity.

Distribution

The volume of distribution of tinzaparin sodium ranges from 3.1 L to 5.0 L. These values are similar in magnitude to blood volume, suggesting that the distribution of anti-Xa activity is limited to the central compartment.

Metabolism

Low molecular weight heparins are partially metabolized by desulphation and depolymerization.

Elimination

In healthy volunteers, the elimination half-life following SC administration of 4,500 IU or 175 IU/kg tinzaparin sodium is approximately 3-4 hours based on anti-Xa activity. Clearance following IV administration of 4,500 IU tinzaparin sodium is approximately 1.7 L/hr. The primary route of elimination is renal. Anti-Xa activity did not accumulate with once daily dosing of 175 IU/kg for five days in healthy volunteers. (See Figure 1 and Table 2.)

Special Populations

Population Pharmacokinetics

Anti-Xa concentrations from approximately 180 patients receiving SC tinzaparin sodium once daily (175 IU/kg body weight) as the treatment of proximal DVT and approximately 240 patients undergoing elective hip replacement surgery receiving SC tinzaparin sodium once daily (~65 IU/kg body weight) were analyzed by population pharmacokinetic methods. The results indicate that neither age nor gender significantly alter tinzaparin sodium clearance based on anti-Xa activity (see PRECAUTIONS , General ). However, a reduction in tinzaparin sodium clearance was observed in patients with impaired renal function (reduced calculated creatinine clearance). Weight is also an important factor for the prediction of tinzaparin sodium clearance, consistent with the recommendation that INNOHEP therapy be based on weight-adjusted dosing (see DOSAGE AND ADMINISTRATION ).

Renal Impairment

Clinical Studies

In patients being treated with tinzaparin sodium (175 IU/kg) for DVT, a population pharmacokinetic (PK) analysis determined that tinzaparin sodium clearance based on anti-Xa activity was related to creatinine clearance calculated by the Cockroft Gault equation. In this PK analysis, a reduction in tinzaparin sodium clearance in moderate (30-50 mL/min) and severe ( < 30 mL/min) renal impairment was observed. Patients with severe renal impairment exhibited a 24% reduction in tinzaparin sodium clearance relative to patients with normal renal function (>80 mL/min).

Hemodialysis Studies

In a study of 12 chronic renal failure patients undergoing hemodialysis, anti-Xa clearance was reduced 28%, consistent with estimates from the population PK analyses. In another study of 6 patients undergoing hemodialysis, the half-life of anti-Xa activity following a single IV dose of 75 IU/kg of tinzaparin sodium on an off-dialysis day was prolonged relative to that for healthy volunteers (5.2 versus 1.6 hours).

Patients with severe renal impairment should be dosed with caution (see PRECAUTIONS).

Hepatic Impairment

No prospective studies have assessed tinzaparin sodium pharmacokinetics or pharmacodynamics in hepatically-impaired patients. However, the hepatic route is not a major route of elimination of low molecular weight heparins (see WARNINGS , Hemorrhage ).

Elderly

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