How Does Releuko Work? Mechanism of Action Explained in Plain English

Understanding how Releuko works can help you feel more confident about your treatment. This article explains the science in simple terms — no medical degree required.

The Problem Releuko Solves: Neutropenia

To understand how Releuko works, you first need to understand what it's solving. Chemotherapy is designed to kill rapidly dividing cancer cells — but it also damages other rapidly dividing healthy cells, including the stem cells in your bone marrow that produce white blood cells. The result is neutropenia: a dangerous drop in neutrophils, a type of white blood cell that is your body's primary defense against bacterial and fungal infections.

When neutrophils are too low, even a minor cut or common bacteria can cause a life-threatening infection. This is why G-CSF therapy — drugs like Releuko — is such a critical part of cancer care.

What Is G-CSF (Granulocyte Colony-Stimulating Factor)?

Your body naturally produces a protein called granulocyte colony-stimulating factor (G-CSF). Think of G-CSF as a signal sent by your body to the bone marrow that says: "Make more white blood cells — and fast." When you have an infection, your body ramps up G-CSF production to flood your system with neutrophils to fight it.

Filgrastim — the active ingredient in Releuko — is a laboratory-made (recombinant) version of this exact protein. It is produced using E. coli bacteria that have been genetically programmed to make the human G-CSF protein. The result is a molecule that behaves identically to your body's own G-CSF.

How Releuko Works Step by Step

Injection: Releuko is injected under the skin (subcutaneous) or into a vein (intravenous). It enters your bloodstream and travels to the bone marrow.

Receptor binding: Releuko binds to specific G-CSF receptors on the surface of neutrophil precursor cells (myeloid progenitors) in the bone marrow. Think of the receptor as a lock and Releuko as the key.

Signal activation: When the key (Releuko) fits the lock (receptor), it triggers a cascade of signals inside the cell through the JAK/STAT signaling pathway. These signals tell the cell to: proliferate (multiply), differentiate (mature into functioning neutrophils), and survive longer than they normally would.

Neutrophil release: The mature neutrophils are released from the bone marrow into the bloodstream. This typically begins within 24–48 hours of the first dose. Your ANC (absolute neutrophil count) — measured by a CBC blood test — rises as the neutrophils accumulate.

Infection protection: Once your ANC rises to a safe level (typically above 1,000–2,000 cells/mm³), your immune system is back online and can effectively fight infection.

Why Does Releuko Cause Bone Pain?

The mechanism of action also explains why bone pain is the most common side effect. When Releuko stimulates the bone marrow to work overtime producing neutrophils, the marrow physically expands within the rigid walls of bones. This pressure causes the characteristic deep, aching pain — most commonly felt in the lower back, hips, and sternum (breastbone). The pain is a sign the drug is working.

What Is the Difference Between Filgrastim and Pegfilgrastim?

Releuko (filgrastim-ayow) and Neulasta (pegfilgrastim) both work through the G-CSF mechanism — binding to G-CSF receptors and stimulating neutrophil production. The difference is duration:

Filgrastim (Releuko): Short-acting. Leaves the body relatively quickly. Must be given daily (or for several consecutive days) for sustained effect.

Pegfilgrastim (Neulasta): Long-acting. Pegylation — attaching polyethylene glycol molecules to filgrastim — slows its clearance from the body. A single dose after each chemotherapy cycle can sustain neutrophil stimulation for 2 weeks.

Your oncologist will determine which formulation is best based on your treatment schedule, insurance coverage, and clinical needs.

What Makes Releuko a Biosimilar Rather Than a Generic?

This is a common question. Regular generics (like generic aspirin or generic metformin) are simple chemical compounds that can be made identically to the original. Biologics like filgrastim are made from living cells — in this case, genetically engineered bacteria. Because the manufacturing process involves living organisms, no two batches are ever perfectly identical at the molecular level. That's why these copies are called biosimilars rather than generics. The FDA requires biosimilars to demonstrate "no clinically meaningful differences" in safety, purity, and potency — which Releuko has done.

For a full overview of what Releuko is and its approved uses, see our guide What Is Releuko? And if you need to find Releuko near you, visit medfinder.com.