APB-F1, a long-acting feline granulocyte colony-stimulating factor fusion protein, created by exploiting FL335, a chimeric Fab specific for feline serum albumin

Vet Immunol Immunopathol. 2021 Oct;240:110322. doi: 10.1016/j.vetimm.2021.110322. Epub 2021 Aug 28.


Off-label use of a human granulocyte colony stimulating factor (hG-CSF) has been allowed to treat dogs and cats with neutropenia. However, repeated administration of hG-CSF induces undesirable anti-drug antibody (ADA) responses, implying the necessity of animal-derived G-CSF as a therapeutic reagent, preferably with a long-acting capability. Herein, we generated a recombinant fusion protein by genetically combining FL335, a chimeric Fab specific for feline serum albumin (FSA), and feline G-CSF (fG-CSF), with the ultimate goal of developing a long-acting therapeutic fG-CSF for cats. The resulting FL335-fG-CSF fusion protein, referred to as APB-F1, was produced well as a functional form in a Chinese hamster ovary (CHO) expression system. In in vitro analyses, APB-F1 bound to FSA at high affinity (KD = 400 pM) and possessed 0.78 × 107 U/mg G-CSF biological activity, clearly proving its biological functionality. Pharmacokinetic (PK) and pharmacodynamic (PD) studies using healthy cats revealed that the serum half-life (t1/2) of APB-F1 was increased five times compared with that of fG-CSF (t1/2 = 13.3 h vs. 2.7 h) in subcutaneous (SC) injections. Additionally, APB-F1 induced a profound and sustained increase in white blood cell (WBC) and actual neutrophil count (ANC) up to 10 days, which was far superior to other G-CSF preparations, including filgrastim (Neupogen™) and even pegfilgrastim (Neulasta™). Conclusively, a long-acting fG-CSF with potent in vivo bioactivity was successfully created by using FL335; thus, we provided evidence that our "anti-serum albumin Fab-associated" (SAFA) technology can be applied reliably in developing valuable long-acting biologics in veterinary medicine.

Keywords: Antigen-binding fragment (fab); Cat; Granulocyte-colony stimulating factor (G-CSF); Neutropenia; Pegfilgrastim; Serum albumin.

Publication types

  • Review

MeSH terms

  • Animals
  • Antibodies
  • CHO Cells
  • Cat Diseases / therapy
  • Cats
  • Cricetinae
  • Cricetulus
  • Dogs
  • Granulocyte Colony-Stimulating Factor / pharmacology*
  • Immunoglobulin Fab Fragments / pharmacology
  • Neutropenia / therapy
  • Recombinant Proteins / pharmacology
  • Serum Albumin / immunology*


  • Antibodies
  • Immunoglobulin Fab Fragments
  • Recombinant Proteins
  • Serum Albumin
  • Granulocyte Colony-Stimulating Factor