The epidermal growth factor receptor (EGFR) is often overexpressed during prostate cancer (PCa) progression towards androgen-independence after hormone therapy, but the overexpression is lower than in other types of cancers. Despite the low expression, EGFR has emerged as a promising therapeutic target for patients with castration-resistant PCa. Non-invasive methods for determination of EGFR expression in PCa can serve for patient stratification and therapy response monitoring. Radionuclide imaging probes based on affibody molecules (7 kDa) provide high contrast imaging of cancer-associated molecular targets. We hypothesized that the anti-EGFR affibody molecule DOTA-Z(EGFR:2377) labeled with Co-55 (positron-emitter, T1/2=17.5 h) would enable imaging of EGFR expression in PCa xenografts. The human PCa cell line DU-145 was used for in vitro and in vivo experiments and Co-57 was used as a surrogate for Co-55 in the present study. Binding of Co-57-DOTA-Z(EGFR:2377) to EGFR-expressing xenografts was saturable with anti-EGFR monoclonal antibody cetuximab, which would motivate the use of this tracer for monitoring the receptor occupancy during treatment. A significant dose-dependent difference in radioactivity accumulation in tumors and normal organs was observed when the biodistribution was studied 3 h after the injection of 10 and 35 mu g of Co-57-DOTA-Z(EGFR:2377): At lower doses the tumor uptake was 2-fold higher although tumor-to-organ ratios were not altered. For clinically relevant organs for PCa, tumor-to-organ ratios increased with time, and at 24 h pi were 2.2 +/- 0.5 for colon, 7 +/- 2 for muscle, and 4.0 +/- 0.7 for bones. Small animal SPECT/CT images confirmed the capacity of radiocobalt labeled DOTA-Z(EGFR:2377) to visualize EGFR expression in PCa. In conclusion, the present study demonstrated the feasibility of using the radiocobalt labeled anti-EGFR affibody conjugate Z(EGFR:2377) as an imaging agent for in vivo visualization of low EGFR-expressing tumors, like PCa, and for monitoring of receptor occupancy during cetuximab therapy as well as the importance of optimal dosing in order to achieve higher sensitivity molecular imaging.

Human epidermal growth factor receptor type 3 (HER3) plays a crucial role in the progression of many cancer types. In vivo radionuclide imaging could be a reliable method for repetitive detection of HER3-expression in tumors. The main challenge of HER3-imaging is the low expression in tumors together with endogenous receptor expression in normal tissues, particularly the liver. A HER3-targeting affibody molecule labeled with radiocobalt via a NOTA chelator [Co-57]Co-NOTA-Z(08699) has demonstrated the most favorable biodistribution profile with the lowest unspecific hepatic uptake and high activity uptake in tumors. We hypothesized that specific uptake of labeled affibody monomer might be selectively blocked in the liver but not in tumors by a co-injection of non-labeled corresponding trivalent affibody (Z(08699))(3). Biodistribution of [Co-57]Co-NOTA-Z(08699) and [In-111]ln-DOTA-(Z(08699))(3) was studied in BxPC-3 xenografted mice. [Co-57]Co-NOTA-Z(08699) was co-injected with unlabeled trivalent affibody DOTA-(Z(08699))(3) at different monomer:trimer molar ratios. HER3-expression in xenografts was imaged using [Co-57]Co-NOTA-Z(08699) and [Co-57]Co-NOTA-Z(08699): DOTA-(Z(08699))(3). Hepatic activity uptake of [Co-57] Co-NOTA-Z(08699): DOTA-(Z(08699))(3) decreased with increasing monomer:trimer molar ratio. The tumor activity uptake and tumor-to-liver ratios were the highest for the 1:3 ratio. SPECT/CT images confirmed the biodistribution data. Imaging of HER3 expression can be improved by co-injection of a radiolabeled monomeric affi body-based imaging probe together with a trivalent affibody.

The HER3 receptor plays a strong role in disease progression and resistance to therapies in several cancer types. Due to its endogenous expression and low overexpression in malignant tumours, it is a particularly challenging target. The primary aim of this thesis project was to develop, evaluate and characterize affibody molecules for theranostic applications in HER3-expressing malignant tumours.

Paper I investigated the in vivo targeting properties and therapeutic efficacy of a bivalent affibody construct fused with an albumin binding domain, Z_HER3-ABD-Z_HER3. This construct could slow down the growth of HER3-expressing tumour xenografts without causing health problems or side effects in mice.

Paper II compared the in vitro and in vivo properties of two HER3-targeting affibody molecules (Z₀₈₆₉₈ and Z₀₈₆₉₉) to select an imaging probe for HER3 diagnostics. While the two constructs had similar properties, Z₀₈₆₉₈ demonstrated better blood clearance and better radioactivity retention in tumours.

Paper III and IV present the development of a HER3 imaging probe for PET using gallium and cobalt isotopes. We demonstrated that imaging of HER3 expression could be obtained as soon as 3 h pi using gallium-68. Additionally, we demonstrated that affibody molecules labelled with a neutral cobalt-NOTA complex had a lower radioactivity uptake in the liver than molecules radiolabelled with a positive gallium-NOTA complex. Imaging contrast increased over time. As the dose of the injected protein increased, the activity uptake in normal organs decreased, whereas the tumour uptake remained the same, which improved the imaging contrast and allowed discrimination between xenografts with high and low HER3 expression. This modification did not influence tumour activity uptake.

Paper V presents the HER3-targeting affibody molecule trimer as a tool to block hepatic uptake in order to increase the imaging contrast in the liver. The trimer demonstrated its ability to bind to endogenous receptors in the liver, which decreased the hepatic uptake of the radiolabelled monomer. This phenomenon enabled the monomer to pass the liver barrier, which increased tumour radioactivity uptake and improved imaging contrast.

Human epidermal growth factor receptor type 3 (HER3) is recognized to be involved in resistance to HER targeting therapies. A number of HER3-targeting monoclonal antibodies are under clinical investigation as potential cancer therapeutics. Smaller high-affinity scaffold proteins are attractive non-Fc containing alternatives to antibodies. A previous study indicated that anti-HER3 affibody molecules could delay the growth of xenografted HER3-positive tumors. Here, we designed a second-generation HER3-targeting construct (TAM-HER3), containing two HER3-specific affibody molecules bridged by an albumin-binding domain (ABD) for extension of blood circulation. Receptor blocking activity was demonstrated in vitro. In mice bearing BxPC-3 xenografts, the therapeutic efficacy of TAM-HER3 was compared to the HER3-specific monoclonal antibody seribantumab (MM-121). TAM-HER3 inhibited heregulin-induced phosphorylation in a panel of HER3-expressing cancer cells and was found to be equally as potent as seribantumab in terms of therapeutic efficacy in vivo and with a similar safety profile. Median survival times were 60 days for TAM-HER3, 54 days for seribantumab, and 41 days for the control group. No pathological changes were observed in cytopathological examination. The multimeric HER3-binding affibody molecule in fusion to ABD seems promising for further evaluation as candidate therapeutics for treatment of HER3-overexpressing tumors.

The human epidermal growth factor receptor 3 (HER3) is involved in the development of cancer resistance towards tyrosine kinase-targeted therapies. Several HER3‑targeting therapeutics are currently under clinical evaluation. Non-invasive imaging of HER3 expression could improve patient management. Affibody molecules are small engineered scaffold proteins demonstrating superior properties as targeting probes for molecular imaging compared with monoclonal antibodies. Feasibility of in vivo HER3 imaging using affibody molecules has been previously demonstrated. Preclinical studies have shown that the contrast when imaging using anti-HER3 affibody molecules can be improved over time. We aim to develop an agent for PET imaging of HER3 expression using the long-lived positron-emitting radionuclide cobalt-55 (55Co) (T1/2=17.5 h). A long-lived cobalt isotope 57Co was used as a surrogate for 55Co in this study. The anti-HER3 affibody molecule HEHEHE-ZHER3-NOTA was labelled with radiocobalt with high yield, purity and stability. Biodistribution of 57Co-HEHEHE-ZHER3-NOTA was measured in mice bearing DU145 (prostate carcinoma) and LS174T (colorectal carcinoma) xenografts at 3 and 24 h post injection (p.i.). Tumour-to-blood ratios significantly increased between 3 and 24 h p.i. (p<0.05). At 24 h p.i., tumour-to-blood ratios were 6 for DU145 and 8 for LS174T xenografts, respectively. HER3‑expressing xenografts were clearly visualized in a preclinical imaging setting already 3 h p.i., and contrast further improved at 24 h p.i. In conclusion, the radiocobalt-labelled anti-HER3 affibody molecule, HEHEHE-ZHER3-NOTA, is a promising tracer for imaging of HER3 expression in tumours.

Overexpression of human epidermal growth factor receptor 3 (HER3) is involved in resistance to several therapies for malignant tumours. Currently, several anti-HER3 monoclonal antibodies are under clinical development. We introduce an alternative approach to HER3-targeted therapy based on engineered scaffold proteins, i.e. affibody molecules. We designed a small construct (22.5 kDa, denoted 3A3), consisting of two high-affinity anti-HER3 affibody molecules flanking an albumin-binding domain ABD, which was introduced for prolonged residence in circulation. In vitro, 3A3 efficiently inhibited growth of HER3-expressing BxPC-3 cells. Biodistribution in mice was measured using 3A3 that was site-specifically labelled with In-111 via a DOTA chelator. The residence time of In-111-DOTA-3A3 in blood was extended when compared with the monomeric affibody molecule. In-111-DOTA-3A3 accumulated specifically in HER3-expressing BxPC-3 xenografts in mice. However, In-111-DOTA-3A3 cleared more rapidly from blood than a size-matched control construct In-111-DOTA-TAT, most likely due to sequestering of 3A3 by mErbB3, the murine counterpart of HER3. Repeated dosing and increase of injected protein dose decreased uptake of In-111-DOTA-3A3 in mErbB3-expressing tissues. Encouragingly, growth of BxPC-3 xenografts in mice was delayed in an experimental (pilot-scale) therapy study using 3A3. We conclude that the 3A3 affibody format seems promising for treatment of HER3-overexpressing tumours.