A single subcutaneous administration of 25 mg etanercept in patients with rheumatoid arthritis resulted in a mean half-life of 102 h (about 4 days) and a clearance rate of 160 ml/h, with no significant change in these pharma-cokinetic parameters observed with repeated dosing (Enbrel: US Full Prescribing Information 2005; Zhou 2005). Although not confirmed, it is thought that the elimination of etanercept occurs after it binds to TNF, and metabolism of the complex could occur through either peptide or amino acid degradation pathways. Amino acids could then be recycled or eliminated through the bile or urine (Culy and Keating 2002; Zhou 2005). The metabolites of etanercept are not bioactive.
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