Human epidermal growth factor receptor 2 (HER2) is a well-studied therapeutic target as well as a biomarker of breast cancer. HER2-targeting affibody (Z _HER2:342) is a novel small scaffold protein with an extreme high affinity against HER2 screened by phage display. However, the small molecular weight of Z_HER2:342 has limited its pharmaceutical application. Human serum albumin (HSA), as the main protein in plasma, has been commonly used to extend the small peptides serum half-life. Its high solubility, stability and excellent ability to carry multiple ligands in blood stream make it a good candidate for drug delivery.

Two HSA and Z_HER2:342 fusion proteins, one with a single Z _HER2:342 domain fused to the C terminus of HSA (rHSA-ZHER2) and the other with two tandem copies of Z_HER2:342 (rHSA-(ZHER2)₂), have been constructed, expressed, and purified. Both fusion proteins possessed the HER2 and fatty acid (FA) binding abilities demonstrated by in vitro assays. Interestingly, rHSA-(ZHER2)₂, not rHSA-ZHER2, was able to inhibit the proliferation of SK-BR-3 cells at a relatively low concentration, and the increase of HER2 and ERK1/2 phosphorylation followed by rHSA-(ZHER2) ₂ treatment has been observed. However, the inhibition effect on HER2-overexpressing cells is cell linedependent. Fusion protein rHSA-(ZHER2)₂ showed preferred accumulation in tumor tissues in xenograft model.

HSA fusion proteins are easy and economical to express, purify, and formulate. Two formulation strategies have been explored, one is to complex the fusion protein with FA modified chemo drugs, and the other is to make them into nanoparticles. As expected, HSA fusion proteins and fusion protein-bound fatty acid-modified fluorescein isothiocyanate (FITC) could be efficiently taken up by cells. FA-Taxol/albumin formulation showed its advantages over Taxol/albumin treatments on in vitro cell growth inhibition. Nanoparticles containing rHSA-ZHER2 produced by desolvation method displayed optimal size distribution, satisfactory stability and preferred binding/uptake on HER2-overexpressing cells. These results proved the feasibility of using HSA fusion proteins as therapeutic agents as well as carriers for targeted drug delivery.