The apparent volume of distribution of etanercept is about 121 after single-dose administration. This is larger than the plasma volume (about 3.75 1 in a 75-kg person) but less than that of extracellular water (about 161) (Zhou 2005). Based on its macromolecular size, extra-vascular distribution would likely be limited for etanercept (a 150-kDa macromolecule). Despite its molecular size, however, data suggest that etanercept penetrates into synovial fluid at levels similar to those seen in serum (Zhou 2005).
Although no formal studies have been done in lac-tating women, the estimated amount of etanercept excreted in breast milk was reported in a case study (Ostensen and Eigenmann 2004; Zhou 2005). The patient was a 30-year-old woman with rheumatoid arthritis who received 25 mg etanercept twice weekly for 4 weeks. The concentration of etanercept found in the breast milk ranged from 0.025 to 0.075 mg/l. Considering the small amount of drug excreted into breast milk, and the lack of oral bioavailability of etanercept, the risk of attaining pharmacologic concentrations of etanercept in a nursing infant remains speculative but is not likely (Zhou 2005). Nonetheless, in the absence of formal data, the drug label recommends that a decision should be made to discontinue nursing or discontinue etanercept while nursing.
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