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Objectives. This study aimed to evaluate the efficacy and safety of once-per-cycle balugrastim versus pegfilgrastim for neutro-phil support in breast cancer patients receiving myelosuppressive chemotherapy. Methods. Breast cancer patients (n =256) were randomized to 40 or 50 mg of subcutaneous balugrastim or 6 mg of pegfilgrastim ≈24 hours after chemotherapy (60 mg/m² doxorubicin and 75 mg/m² docetaxel, every 21 days for up to 4 cycles).The primary efficacy parameter was the duration of severe neutropenia (DSN) in cycle 1. Secondary parameters included DSN (cycles 2-4), absolute neutrophil count (ANC) nadir, febrile neutropenia rates, and time to ANC recovery (cycles 1-4). Safety, pharmacokinetics, and immunogenicity were assessed. Results. Mean cycle 1 DSN was 1.0 day with 40 mg of balugrastim, 1.3 with 50 mg of balugrastim, and 1.2 with pegfilgrastim (upper limit of 95% confidence intervals for between-group DSN differences was <1.0 day for both balugrastim doses versus pegfilgrastim). Between-group efficacy parameters were comparable except for time to ANC recovery in cycle 1 (40 mg of balugrastim, 2.0 days; 50 mg of balugrastim, 2.1; pegfilgrastim, 2.6). Median terminal elimination half-life was ≈37 hours for 40 mg of balugrastim, ≈36 for 50 mg of balugrastim, and ≈45 for pegfilgrastim. Antibody response to balugrastim was low and transient, with no neutralizing effect. Conclusion. Once-per-cycle balugrastim is not inferior to pegfilgrastim in reducing cycle 1 DSN in breast cancer patients receiving chemotherapy; both drugs have comparable safety profiles. [ABSTRACT FROM AUTHOR]