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Filgrastim

Written by The Peptide Record editorialClinical review not yet completedLast reviewed 19 September 2026

Filgrastim (Neupogen) is a real, FDA-approved recombinant G-CSF, used to reduce infection risk during chemotherapy by stimulating neutrophil production — approved in 1991, and the drug whose first biosimilar (Zarxio, 2015) was the very first biosimilar of any kind approved in the US.

On this page
  1. A neutrophil-stimulating growth factor
  2. The approval and pivotal trial
  3. Safety
  4. Current status: the first US biosimilar

In brief

Should you care? Relevant if you or someone you know is undergoing chemotherapy and managing neutropenia risk.

The short version

  • Recombinant human G-CSF, stimulating the bone marrow to produce more neutrophils, the white blood cells most critical for fighting infection.
  • Approved February 1991, based on a real placebo-controlled trial in chemotherapy patients.
  • Its 2015 biosimilar, Zarxio, was the first biosimilar of any kind approved in the US — a genuinely significant regulatory first.

A neutrophil-stimulating growth factor

Filgrastim is recombinant human granulocyte colony-stimulating factor (G-CSF), produced in E. coli. It binds the G-CSF receptor on myeloid progenitor cells in bone marrow, stimulating proliferation and functional activation of neutrophils — the white blood cells most responsible for fighting bacterial infection, and the ones chemotherapy most reliably depletes.

The approval and pivotal trial

The FDA approved filgrastim (as Neupogen) on 20 February 1991 to reduce the incidence of infection, as measured by febrile neutropenia, in patients with non-myeloid cancers receiving chemotherapy that suppresses bone marrow function. The pivotal trial (Crawford et al., N Engl J Med 1991, PMID 1711156) randomized 210 small-cell lung cancer patients receiving chemotherapy to filgrastim or placebo, finding significant reductions in febrile neutropenia, culture-confirmed infections, the duration and severity of severe neutropenia, and days spent on IV antibiotics or in the hospital. Indications later expanded to bone marrow transplant settings and other neutropenia conditions.

Safety

The most notable labeled risk is splenic rupture, including rare fatal cases — a real, if uncommon, Warnings and Precautions item (not a boxed warning) across the whole filgrastim-class label; patients are told to report left upper abdominal or shoulder-tip pain for evaluation. Other labeled risks shared across this drug class: acute respiratory distress syndrome, serious allergic reactions, sickle cell crises in patients with sickle cell disease, and capillary leak syndrome.

Current status: the first US biosimilar

Still marketed by Amgen as Neupogen. Filgrastim-sndz (Zarxio, Sandoz/Novartis), approved 6 March 2015, was the first biosimilar of any kind approved in the US under the Biologics Price Competition and Innovation Act pathway — a genuinely notable regulatory milestone beyond just this drug's own story. Additional filgrastim biosimilars have followed since.

References

  1. Crawford J, Ozer H, Stoller R, et al., "Reduction by Granulocyte Colony-Stimulating Factor of Fever and Neutropenia Induced by Chemotherapy in Patients with Small-Cell Lung Cancer," N Engl J Med 1991;325(3):164-170, PMID 1711156.
  2. FDA approval history for Neupogen (filgrastim), 20 February 1991. Zarxio (filgrastim-sndz), first US biosimilar of any kind, approved 6 March 2015.