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Pegylation of a protein can improve not only its formulation properties, but also both its pharmacokinetic and pharmacodynamic performance. Pegfilgrastim was made by linking a 20-kDa polyethylene glycol molecule to filgrastim, producing a long-acting cytokine requiring less frequent dosing than its parent drug. This review describes the clinical development of pegfilgrastim, and discusses its potential benefits to patients and caregivers in the prophylaxis of chemotherapy-induced neutropenia. Pegfilgrastim has a longer half-life and slower elimination rate than filgrastim, resulting in an increased serum concentration over time. Serum levels of pegfilgrastim are maintained until after a chemotherapy-induced neutrophil nadir and then decline rapidly as the neutrophil count recovers, consistent with a neutrophil-mediated clearance mechanism. In two pivotal phase III studies of women receiving chemotherapy for breast cancer, a single injection of pegfilgrastim per chemotherapy cycle, dosed either by body weight (100 μg/kg) or as a fixed dose (6 mg), was comparable to daily filgrastim (5 μg/kg) for all efficacy parameters, including duration of severe neutropenia and depth of neutrophil nadir. Analysis of pooled data from these studies showed a significantly lower incidence of febrile neutropenia in patients receiving pegfilgrastim compared with filgrastim (11 versus 19%, p<0.05), and a trend towards a lower risk of hospitalization and use of i.v. anti-infectives. The safety profiles of pegfilgrastim and filgrastim were similar. Pegfilgrastim given once per chemotherapy cycle is as effective and well tolerated as daily injections of filgrastim. With its more convenient dosing regimen, pegfilgrastim has the potential to improve quality of life and compliance in patients, and to be more cost effective.