Molecular targeting of HER2 for diagnosis and therapy of breast cancer.

Abstract #6004 Background: Expression of HER2 receptors in breast cancer is correlated with poor prognosis and may be different in distant metastases as compared to the primary tumor. We are developing methods to assess global expression of HER2 in vivo and to deliver therapeutic agents specifically to HER2-positve cells.
 Materials and Methods: As the targeting agent we use an Affibody molecule (http://www.affibody.com). These very stable and highly soluble proteins are relatively small (8.3 kDa) and bind to HER2 receptors with high affinity (22 pM). For imaging with PET, SPECT, or optical methods, an appropriate group containing the imaging beacon can be attached by a selective chemical reaction to a unique C-terminal cysteine residue of Affibody. For therapeutic purposes, they can be conjugated to multifunctional nanoparticles containing both imaging and therapeutic agents. We have conjugated affibody molecules with thermo-sensitive liposomes or gold nanoparticles (subsequently activated with neutrons) and characterized their biodistribution using optical imaging and SPECT, respectively. We used PET imaging with 18 F-Z HER2 -Affibody to monitor the down-regulation of HER2 following four doses (50 mg/kg) of 17-dimethylaminoethylamino-17-demethoxy-geldanamycin, 17-DMAG, an inhibitor of Hsp90 known to decrease HER2 expression. Animals were scanned before and after treatment. Immediately after the last scan, the mice were euthanized and tumors were frozen for ex-vivo analysis of receptor expression. For optical imaging, we used AlexaFluor dyes conjugated with affibody molecules containing an albumin binding domain that extended their circulation time. We have attached nanoparticles (liposomes and gold) to Affibody molecules using the same type of maleimide chemistry.
 Results: Our results showed that Affibody molecules do not affect the targeted cells and that their binding does not interfere with either the binding or the effectiveness of trastuzumab. 18 F-Z HER2 -Affibody was eliminated quickly from blood and normal tissues, providing high tumor/blood and tumor/muscle ratios by 1h post injection. The signal obtained from PET and optical imaging correlated well with the number of receptors expressed in the studied tumors as assessed by western blot, ELISA, and IHC. Following 17-DMAG treatment, the level of HER2 expression, estimated by PET imaging, in BT474 and Mcf-7/clone18 tumors decreased 70% and 30%. This change was confirmed by the biodistribution studies, ELISA and western blot.
 Discussion: This strategy, involving assessment of target presence and distribution in an individual patient followed by optimized, target-specific drug delivery, may significantly improve efficacy of breast cancer treatment while reducing side effects. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6004.