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Overexpression of human epidermal growth factor receptor 2 (HER2) tyrosine kinase cell surface receptor occurs in 25 to 30% of all breast cancer and is linked to aggressive phenotype and high-mortality disease. HER2 is a clinically important target in diagnosis and treatment of breast cancer but despite its pivotal role there are no established tools for quantitative clinical imaging of the extent and location of HER2-positive (HER2+) tumours in patients. Such a tool could provide important clinical diagnostic information by early detection of subclinical HER2+ disease, optimal management of current anti-HER2 therapies and response assessment of novel therapeutics. We aimed to generate recombinant proteins that would achieve sensitive and specific detection of HER2+ tumours in the clinic using radioimmunoimaging.
Two different HER2-binding molecules were investigated: C6.5 a small dimeric antibody fragment (diabody), which is approximately 1/3 of the size of an antibody; and G3, a small monomeric designed ankyrin repeat protein (DARPin) that is 1/10 the size of an antibody. The agents were generated in the yeast Pichia pastoris system using processes compliant with good manufacturing practice (GMP). C6.5 and G3 production strains were constructed to allow methanol-inducible, soluble expression. The expressed proteins were purified using expanded-bed adsorption-immobilized metal affinity chromatography.
For C6.5 the final product was homogeneous, stable and free of host cell and other relevant contaminants. The protein was stable during storage, with no evidence of aggregation. In addition, affinity for HER2, as measured by Biacore analysis, was not compromised by storage at either 4 or -80°C. Preliminary results with the G3 DARPin indicate that this protein is also amenable to GMP production in P. pastoris. The relative efficacy of these agents for specific radioimmunoimaging of HER2+ tumours in vivo is currently under investigation.