Background: Eflapegrastim (E) is a novel long-acting GCSF comprised of recombinant human GCSF covalently linked to human IgG4 Fc fragment via a PEG linker (MW, 72 kDa). E showed increased potency vs pegfilgrastim (P) in preclinical and Phase I and II trials. Two identically designed Phase III pivotal trials (NCT02643420, NCT02953340) were conducted globally with a fixed dose of 13.2 mg E containing 3.6 mg GCSF to evaluate E vs P (6 mg) in pts receiving chemotherapy for early-stage breast cancer. Methods: Each open-label trial randomized pts 1:1 to a single subcutaneous dose of E 13.2 mg/0.6 mL or P 6 mg/0.6 mL on Day 2 of each of four 21-day cycles following Day 1 adj/neoadj docetaxel 75 mg/m² + cyclophosphamide 600mg/m² (TC). The primary endpoint was to demonstrate E non-inferiority (NI) to P as measured by mean duration of severe neutropenia (DSN) in Cycle 1. Results: A total 643 intent-to-treat pts (314 E/329 P) with median age 60 yrs (24–88) were enrolled. Cycle 1 mean (SD) DSN was 0.24 (0.581) vs 0.36 (0.789) days for E and P, confirming NI (p < .0001) and suggesting statistical superiority (p < .029). DSN NI was also shown across cycles 2–4. Among subgroups, including elderly (≥65 yrs) and overweight ( > 75kg) pts, DSN was reduced for E vs P. In Cycle 1, E showed an absolute risk reduction for severe neutropenia of 6.5% vs P (27.1% relative risk reduction, p < .043). Neutropenic complications (hospitalization and/or anti-infective use) were 2.9% and 4.0% for E and P (p = ns). Incidence of FN was low for both E and P, 1.6% vs 1.8% in Cycle 1 and 3.2% vs 3.0% overall. ANC profiles showed sustained increased levels for E vs P in the recovery phase across all cycles. Safety profiles were similar for E and P, including primarily for expected hematologic AEs and for bone pain and other musculoskeletal pain. Conclusions: These integrated pivotal trial results confirm a similar safety profile and non-inferiority in reducing neutropenic risk for E at a lower GCSF dose vs P. The data also suggests the potential for increased potency of E to deliver improved clinical benefit, a possibility that warrants further clinical trials. Clinical trial information: NCT02643420, NCT02953340