Carbonic anhydrase IX (CAIX) is a cancer-associated molecular target for several classes of therapeutics. CAIX is overexpressed in a large fraction of renal cell carcinomas (RCC). Radionuclide molecular imaging of CAIX-expression might offer a non-invasive methodology for stratification of patients with disseminated RCC for CAIX-targeting therapeutics. Radiolabeled monoclonal antibodies and their fragments are actively investigated for imaging of CAIX expression. Promising alternatives are small non-immunoglobulin scaffold proteins, such as affibody molecules. A CAIX-targeting affibody ZCAIX:2 was re-designed with the aim to decrease off-target interactions and increase imaging contrast. The new tracer, DOTA-HE3-ZCAIX:2, was labeled with In-111 and characterized in vitro. Tumor-targeting properties of [In-111]In-DOTA-HE3-ZCAIX:2 were compared head-to-head with properties of the parental variant, [(99)mTc]Tc(CO)(3)-HE3-ZCAIX:2, and the most promising antibody fragment-based tracer, [In-111]In-DTPA-G250(Fab')(2), in the same batch of nude mice bearing CAIX-expressing RCC xenografts. Compared to the (99)mTc-labeled parental variant, [In-111]In-DOTA-HE3-ZCAIX:2 provides significantly higher tumor-to-lung, tumor-to-bone and tumor-to-liver ratios, which is essential for imaging of CAIX expression in the major metastatic sites of RCC. [In-111]In-DOTA-HE3-ZCAIX:2 offers significantly higher tumor-to-organ ratios compared with [In-111]In-G250(Fab']2. In conclusion, [In-111]In-DOTA-HE3-ZCAIX:2 can be considered as a highly promising tracer for imaging of CAIX expression in RCC metastases based on our results and literature data.

[Ga-68]Ga-ABY-025/PET-CT targeting human epidermal growth factor receptor type 2 (HER2) has demonstrated its potential clinical value for the detection and quantification of HER2 in a phase I clinical study with breast cancer patients. Previously, the radiopharmaceutical was prepared manually, however larger scale of multicenter clinical trials and routine healthcare requires automation of the production process to limit the operator radiation dose, improve tracer manufacturing robustness, and provide on-line documentation for good manufacturing practice (GMP) compliance. The production of [Ga-68]Ga-ABY-025 was implemented on the Modular-Lab PharmTrace synthesis platform (Eckert & Ziegler) and disposable cassettes were developed. Pharmaceutical grade Ge-68/Ga-68 generator (GalliaPharm (R)) was used in the study. The active pharmaceutical ingredient starting material ABY-025 (GMP grade) was provided by Affibody AB. The patient examinations were conducted using a Discovery MI PET/CT scanner (20 cm FOV, GE Healthcare). Reproducible and GMP compliant fully automated production of [Ga-68]Ga-ABY-025 was developed. The radiochemical purity of the product was 98.7 +/- 0.6% with total peptide content of 315 +/- 15 mu g (n = 3). Radionuclidic purity, sterility, endotoxin content, residual solvent content, and sterile filter integrity were controlled and met acceptance criteria. The product was stable at ambient temperature for at least 2 h. The primary tumor and metastasis were detected with SUVmax values of 8.3 and 16.0, respectively. Automated production of [Ga-68]Ga-ABY-025 was established and the process was validated enabling standardized multicenter phase II and III clinical trials and routine clinical use. Patient examinations conformed to the radiopharmaceutical biodistribution observed in the previous phase I study.

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.

Imaging of expression of therapeutic targets may enable patients' stratification for targeted treatments. The use of small radiolabeled probes based on the heavy-chain variable region of heavy-chain-only immunoglobulins or non-immunoglobulin scaffolds permits rapid localization of radiotracers in tumors and rapid clearance from normal tissues. This makes high-contrast imaging possible on the day of injection. This mini-review focuses on small proteins for radionuclide-based imaging that would allow same-day imaging, with the emphasis on clinical applications and promising preclinical developments within the field of oncology.

Affibody molecules can be used as tools for molecular recognition in diagnostic and therapeutic applications. There are several preclinical studies reported on diagnostic and therapeutic use of this molecular class of alternative scaffolds, and early clinical evidence is now beginning to accumulate that suggests the Affibody molecules to be efficacious and safe in man. The small size and ease of engineering make Affibody molecules suitable for use in multispecific constructs where AffiMabs is one such that offers the option to potentiate antibodies for use in complex disease.

Affibody molecules are small (6.5-kDa) affinity proteins based on a three-helix bundle domain framework. Since their introduction 20 years ago as an alternative to antibodies for biotechnological applications, the first therapeutic affibody molecules have now entered clinical development and more than 400 studies have been published in which affibody molecules have been developed and used in a variety of contexts. In this review, we focus primarily on efforts over the past 5 years to explore the potential of affibody molecules for medical applications in oncology, neurodegenerative, and inflammation disorders, including molecular imaging, receptor signal blocking, and delivery of toxic payloads. In addition, we describe recent examples of biotechnological applications, in which affibody molecules have been exploited as modular affinity fusion partners.

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.

Programmed death ligand 1 (PD-L1) is an immune regulatory ligand that binds to the T-cell immune check point programmed death 1. Tumor expression of PD-L1 is correlated with immune suppression and poor prognosis. It is also correlated with therapeutic efficacy of programmed death 1 and PD-L1 inhibitors. In vivo imaging may enable real-time follow-up of changing PD-L1 expression and heterogeneity evaluation of PD-L1 expression across tumors in the same subject. We have radiolabeled the PD-L1-binding Affibody molecule NOTA-Z(PD-L1_1) with F-18 and evaluated its in vitro and in vivo binding affinity, targeting, and specificity. Methods: The affinity of the PD-L1-binding Affibody ligand Z(PD-L1_1) was evaluated by surface plasmon resonance. Labeling was accomplished by maleimide coupling of NOTA to a unique cysteine residue and chelation of F-18-AlF. In vivo studies were performed in PD-L1-positive, PD-L1-negative, and mixed tumor-bearing severe combined immunodeficiency mice. Tracer was injected via the tail vein, and dynamic PET scans were acquired for 90 min, followed by gamma-counting biodistribution. Immunohistochemical staining with an antibody specific for anti-PD-L1 (22C3) was used to evaluate the tumor distribution of PD-L1. Immunohistochemistry results were then compared with ex vivo autoradiographic images obtained from adjacent tissue sections. Results: NOTA-Z(PD-L1_1) was labeled, with a radiochemical yield of 15.1% +/- 5.6%, radiochemical purity of 96.7% +/- 2.0%, and specific activity of 14.6 +/- 6.5 GBq/mu mol. Surface plasmon resonance showed a NOTA-conjugated ligand binding affinity of 1 nM. PET imaging demonstrated rapid uptake of tracer in the PD-L1-positive tumor, whereas the PD-L1-negative control tumor showed little tracer retention. Tracer clearance from most organs and blood was quick, with biodistribution showing prominent kidney retention, low liver uptake, and a significant difference between PD-L1-positive (percentage injected dose per gram [%ID/g] = 2.56 +/- 0.33) and -negative (% ID/g = 0.32 +/- 0.05) tumors (P = 0.0006). Ex vivo autoradiography showed excellent spatial correlation with immunohistochemistry in mixed tumors. Conclusion: Our results show that Affibody ligands can be effective at targeting tumor PD-L1 in vivo, with good specificity and rapid clearance. Future studies will explore methods to reduce kidney activity retention and further increase tumor uptake.

Overexpression of the enzyme carbonic anhydrase IX (CAIX) is documented for chronically hypoxic malignant tumors as well as for normoxic renal cell carcinoma. Radionuclide molecular imaging of CAIX would be useful for detection of hypoxic areas in malignant tumors, for patients' stratification of CAIX-targeted therapies and for discrimination of primary malignant and benign renal tumors. Earlier, we have reported feasibility of in vivo radionuclide based imaging of CAIX expressing tumors using Affibody molecules, small affinity proteins based on a non-immunoglobulin scaffold. In this study, we compared imaging properties of several anti-CAIX Affibody molecules having identical scaffold parts and competing for the same epitope on CAIX, but having different binding paratopes. Four variants were labeled using residualizing 99mTc and non-residualizing 125I labels. All radiolabeled variants demonstrated high-affinity detection of CAIX-expressing cell line SK-RC-52 in vitro and specific accumulation in SK-RC-52 xenografts in vivo. 125I-labeled conjugates demonstrated much lower radioactivity uptake in kidneys but higher radioactivity concentration in blood compared with 99mTc-labed counterparts. Although all variants cleared rapidly from blood and non-specific compartments, there was noticeably difference in their biodistribution. The best variant for imaging of expression of CAIX- in disseminated cancer was 99mTc-(HE)3-ZCAIX:2 providing tumor uptake of 16.3±0.9 %ID/g and tumor-to-blood ratio of 44±7 at 4 h after injection. For primary renal cell carcinoma, the most promising imaging candidate was 125I-ZCAIX:4 providing tumor-kidney ratio of 2.1±0.5. In conclusion, several clones of scaffold proteins should be evaluated to select the best variant for development of an imaging probe with optimal sensitivity for the intended application.