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Affibody Molecules for In vivo Characterization of HER2-Positive Tumors by Near-Infrared Imaging

Authors' Affiliations: ¹ Radiation Oncology Branch, Center for Cancer Research, National Cancer Institute and ² Section on Biomedical Stochastic Physics, Laboratory of Integrative and Medical Biophysics, National Institute of Child Health and Human Development, NIH, Bethesda, Maryland; and ³ Protein Chemistry Laboratory, SAIC-Frederick, Inc., NCI-Frederick, Frederick, Maryland

Requests for reprints: Jacek Capala, 10 Center Drive, Building 10, Room 1B-37A, Bethesda, MD 20892. Phone: 301-435-5882; Fax: 301-480-5532; E-mail: capalaj{at}mail.nih.gov.

Purpose: HER2 overexpression has been associated with a poor prognosis and resistance to therapy in breast cancer patients. We are developing molecular probes for in vivo quantitative imaging of HER2 receptors using near-infrared (NIR) optical imaging. The goal is to provide probes that will minimally interfere with the studied system, that is, whose binding does not interfere with the binding of the therapeutic agents and whose effect on the target cells is minimal.

Experimental Design: We used three different types of HER2-specific Affibody molecules [monomer Z_HER2:342, dimer (Z_HER2:477)₂, and albumin-binding domain-fused-(Z_HER2:342)₂] as targeting agents and labeled them with Alexa Fluor dyes. Trastuzumab was also conjugated, using commercially available kits, as a standard control. The resulting conjugates were characterized in vitro by toxicity assays, Biacore affinity measurements, flow cytometry, and confocal microscopy. Semiquantitative in vivo NIR optical imaging studies were carried out using mice with s.c. xenografts of HER2-positive tumors.

Results: The HER2-specific Affibody molecules were not toxic to HER2-overexpressing cells and their binding to HER2 did interfere with neither binding nor effectives of trastuzumab. The binding affinities and specificities of the Affibody-Alexa Fluor fluorescent conjugates to HER2 were unchanged or minimally affected by the modifications. Pharmacokinetics and biodistribution studies showed the albumin-binding domain-fused-(Z_HER2:342)₂-Alexa Fluor 750 conjugate to be an optimal probe for optical imaging of HER2 in vivo.

Conclusion: Our results suggest that Affibody-Alexa Fluor conjugates may be used as a specific NIR probe for the noninvasive semiquantitative imaging of HER2 expression in vivo.