Study Objective. To examine the serum concentrations of pegfilgrastim during recovery of absolute neutrophil count (ANC) in patients with cancer who received pegfilgrastim after chemotherapy.

Design. Retrospective analysis.

Data Source. Data were pooled from seven pegfilgrastim registrational clinical trials: four open‐label phase I or II studies and three randomized phase II or III studies.

Patients. A total of 370 patients with non–small cell lung cancer, Hodgkin's lymphoma, non–Hodgkin's lymphoma, or breast cancer.

Measurements and Main Results. Chemotherapy was given every 3 weeks, and pegfilgrastim was given once/chemotherapy cycle, 24 hours after chemotherapy completion. Data were available from 187 patients for the serum pegfilgrastim concentration analysis and from 319 patients for the ANC data analysis. Recovery of ANC to normal levels (≥ 1 × 10³/mm³) correlated well with the decline of pegfilgrastim concentrations to subtherapeutic levels; this inverse correlation was observed across different tumor types. By day 12 after pegfilgrastim administration, all patients experienced ANC recovery to normal levels, and none had a serum pegfilgrastim concentration above 2 ng/ml, considered the lowest concentration to elicit clinically meaningful granulopoiesis. After administration of pegfilgrastim, a steady postnadir recovery of the ANC to normal levels was noted, and postnadir peaks of 30 × 10³/mm³ or higher were observed in only three patients.

Conclusion. Serum concentrations of pegfilgrastim were consistently cleared to subtherapeutic levels by day 12 after pegfilgrastim administration, and subtherapeutic pegfilgrastim levels were predicted by ANC recovery to 1 × 10³/mm³ or greater.