Erythropoietin pharmacokinetics in premature infants: developmental, nonlinearity, and treatment effects.

Erythropoietin (EPO) pharmacokinetic studies were performed in premature infants (birth weight < 1.25 kg) and normal adults. Infants were divided into two subgroups on the basis of whether they received chronic treatment with recombinant human EPO (rhEPO; 500 IU.kg-1.wk-1 for 6 wk) beginning at 2-4 wk of life. Ten adults and seven rhEPO-treated infants underwent intravenous pharmacokinetic studies at escalating rhEPO doses: 10, 100, and 500 IU/kg. To test for pharmacokinetic developmental and treatment effects, an equal number of non-EPO- and EPO-treated infants were studied with 100 IU/kg on the last day of treatment. Compared with adults, very low birth weight infants demonstrated significantly greater plasma clearance and distribution volume and significantly shorter fractional elimination times (FET) and mean residence time (MRT) at all three rhEPO doses. Both infants and adults demonstrated nonlinear EPO elimination, i.e., increasing rhEPO dosing was associated with decreasing plasma clearance and increasing FET and MRT. In the absence of rhEPO treatment there were no pharmacokinetic differences between the two subgroups of infants studied 6 wk apart. In contrast, the rhEPO-treated infant subgroup demonstrated a significant increase in clearance and a decrease in FET and MRT following 6 wk of treatment. Enhancement of rhEPO efficacy in the prevention and treatment of anemia in premature infants may require higher doses administered in a progressively increasing fashion.