The human epidermal growth factor receptor type 3 (HER3) is overexpressed in different types of cancer and is a known contributor to disease progression and resistance to cancer therapy. This thesis is based on five original articles, which aimed to improve the diagnostic and therapeutic potential of affibody-based agents for management of HER3-expressing cancers. 

Papers I-III focused on the development and optimization of radiolabeled affibody molecules for radionuclide molecular imaging of HER3 expression. In particular, they investigated the influence of different radiometal/chelator complexes and hydrophilicity on the biodistribution and imaging properties of the HER3-targeting affibody molecule Z_HER3. Paper IV compared the optimized Z_HER3-based radiotracer with antibody and antibody-fragment based radiotracers for PET imaging of HER3 expression. In Paper V, a preclinical therapy study was conducted to investigate the efficacy of different monomeric and dimeric HER3-targeting affibody constructs for treatment of HER3-expressing cancer.

It was shown that by optimizing the radiometal/chelator complex and incorporation of a hydrophilic (HE)₃-tag the imaging properties of Z_HER3-based radiotracers could be improved (Papers I-III). Generally, replacing a positively charged radiometal/chelator complex with a neutral or negatively charged complex improved the image contrast by reducing the normal organ uptake, especially in the liver. Further, it was demonstrated that the optimized affibody-based tracer [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA could provide higher contrast PET images of HER3 expression than the ⁸⁹Zr-labeled antibody seribantumab and a seribantumab-derived F(ab’)₂ fragment (Paper IV). The therapy study showed that the arrangement of the molecular building blocks affected the therapeutic efficacy of Z_HER3-based affibody constructs. The monomeric and dimeric ABD-conjugated affibody constructs 3A and 3A3 showed the best therapeutic efficacy among the tested constructs and were able to delay tumor growth and prolong survival with the same efficacy as the therapeutic HER3-targeting antibody seribantumab (Paper V).

In conclusion, the results described in this thesis show that HER3-targeting affibody-based agents could be well-suited for molecular imaging of HER3 expression and HER3-targeted therapy in cancer. Careful optimization of the molecular design could improve the imaging properties and therapeutic efficacy of HER3-targeting affibody molecules. Most importantly, it was demonstrated that HER3-targeting affibody molecules could provide superior diagnostic images and similar therapeutic effect than more traditional approaches for management of HER3-expressing cancer.