Radionuclide therapy of HER2-positive microxenografts using a 177Lu-labeled HER2-specific Affibody molecule

Dimeric HER2-specific affibody molecules inhibit proliferation of the SKBR-3 breast cancer cell line

HER2-specific affibody molecules in different formats have previously been shown to be useful tumor targeting agents for radionuclide-based imaging and therapy applications, but their biological effect on tumor cells is not well known. In this study, two dimeric ((Z(HER2:4))(2) and (Z(HER2:342))(2)) and one monomeric (Z(HER2:342)) HER2-specific affibody molecules are investigated with respect to biological activity. Both (Z(HER2:4))(2) and (Z(HER2:342))(2) were found to decrease the growth rate of SKBR-3 cells to the same extent as the antibody trastuzumab. When the substances were removed, the cells treated with the dimeric affibody molecules continued to be growth suppressed while the cells treated with trastuzumab immediately resumed normal proliferation. The effects of Z(HER2:342) were minor on both proliferation and cell signaling. The dimeric (Z(HER2:4))(2) and (Z(HER2:342))(2) both reduced growth of SKBR-3 cells and may prove therapeutically useful either by themselves or as carriers of radionuclides or other cytotoxic agents.

Evaluation of a maleimido derivative of CHX-A'' DTPA for site-specific labeling of affibody molecules

Affibody molecules are a new class of small targeting proteins based on a common three-helix bundle structure. Affibody molecules binding a desired target may be selected using phage-display technology. An Affibody molecule Z HER2:342 binding with subnanomolar affinity to the tumor antigen HER2 has recently been developed for radionuclide imaging in vivo. Introduction of a single cysteine into the cysteine-free Affibody scaffold provides a unique thiol group for site-specific labeling of recombinant Affibody molecules. The recently developed maleimido-CHX-A'' DTPA was site-specifically conjugated at the C-terminal cysteine of Z HER2:2395-C, a variant of Z HER2:342, providing a homogeneous conjugate with a dissociation constant of 56 pM. The yield of labeling with (111)In was >99% after 10 min at room temperature. In vitro cell tests demonstrated specific binding of (111)In-CHX-A'' DTPA-Z 2395-C to HER2-expressing cell-line SKOV-3 and good cellular retention of radioactivity. In normal mice, the conjugate demonstrated rapid clearance from all nonspecific organs except kidney. In mice bearing SKOV-3 xenografts, the tumor uptake of (111)In-CHX-A'' DTPA-Z 2395-C was 17.3 +/- 4.8% IA/g and the tumor-to-blood ratio 86 +/- 46 (4 h postinjection). HER2-expressing xenografts were clearly visualized 1 h postinjection. In conclusion, coupling of maleimido-CHX-A'' DTPA to cysteine-containing Affibody molecules provides a well-defined uniform conjugate, which can be rapidly labeled at room temperature and provides high-contrast imaging of molecular targets in vivo.

DARPins: a new generation of protein therapeutics

DARPins (designed ankyrin repeat proteins) are a novel class of binding molecules with the potential to overcome limitations of monoclonal antibodies, hence allowing novel therapeutic approaches. DARPins are small, single domain proteins (14 kDa) which can be selected to bind any given target protein with high affinity and specificity. These characteristics make them ideal agonistic, antagonistic or inhibitory drug candidates. Furthermore, DARPins can be engineered to carry various effector functions or combine multiple binding specificities, enabling completely new drug formats. Taken together, DARPins are a prominent member of the next generation of protein therapeutics with the potential to surpass existing antibody drugs.

Affibody molecules for in vivo characterization of HER2-positive tumors by near-infrared imaging

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 ZHER2:342, dimer (ZHER2:477)2, and albumin-binding domain-fused-(ZHER2:342)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-(ZHER2:342)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.

Small-animal PET imaging of human epidermal growth factor receptor type 2 expression with site-specific 18F-labeled protein scaffold molecules

Human epidermal growth factor receptor type 2 (HER2) is a well-established tumor biomarker that is overexpressed in a wide variety of cancers and that serves as a molecular target for therapeutic intervention. HER2 also serves as a prognostic indicator of patient survival and as a predictive marker of the response to antineoplastic therapy. The development of (18)F-labeled biomolecules for PET imaging of HER2 (HER2 PET) is very important because it may provide a powerful tool for the early detection of HER2-positive tumor recurrence and for the monitoring of HER2-based tumor treatment.

METHODS

In this study, anti-HER2 monomeric and dimeric protein scaffold molecules [Z(HER2:477) and (Z(HER2:477))(2), respectively] were radiofluorinated at a reasonable radiochemical yield (13%-18%) by use of site-specific oxime chemistry. The resulting radiofluorinated protein scaffold molecules were then evaluated as potential molecular probes for small-animal HER2 PET by use of a SKOV3 tumor-bearing mouse model.

RESULTS

The 4-(18)F-fluorobenzaldehyde conjugated aminooxy-protein scaffolds [(18)F-N-(4-fluorobenzylidene)oxime (FBO)-Z(HER2:477) and (18)F-FBO-(Z(HER2:477))(2)] both displayed specific HER2-binding ability in vitro. Biodistribution and small-animal PET imaging studies further revealed that (18)F-FBO-Z(HER2:477) showed rapid and high SKOV3 tumor accumulation and quick clearance from normal tissues, whereas (18)F-FBO-(Z(HER2:477))(2) showed poor in vivo performance (low tumor uptake and tumor-to-normal tissue ratios). The specificity of (18)F-FBO-Z(HER2:477) for SKOV3 tumors was confirmed by its lower uptake on pretreatment of tumor-bearing mice with the HER2-targeting agents Z(HER2) and trastuzumab. Moreover, small-animal PET imaging studies revealed that (18)F-FBO-Z(HER2:477) produced higher-quality tumor imaging than (18)F-FBO-(Z(HER2:477))(2). (18)F-FBO-Z(HER2:477) could clearly identify HER2-positive tumors with good contrast.

CONCLUSION

Overall, these data demonstrate that (18)F-FBO-Z(HER2:477) is a promising PET probe for imaging HER2 expression in living mice. It has a high potential for translation to clinical applications. The radiofluorination method developed can also be used as a general strategy for the site-specific labeling of other proteins with (18)F. The protein scaffold molecules used here are attractive for the further development of PET probes for other molecular targets.

Stabilization of a beta-hairpin in monomeric Alzheimer's amyloid-beta peptide inhibits amyloid formation

According to the amyloid hypothesis, the pathogenesis of Alzheimer's disease is triggered by the oligomerization and aggregation of the amyloid-beta (Abeta) peptide into protein plaques. Formation of the potentially toxic oligomeric and fibrillar Abeta assemblies is accompanied by a conformational change toward a high content of beta-structure. Here, we report the solution structure of Abeta(1-40) in complex with the phage-display selected affibody protein Z(Abeta3), a binding protein of nanomolar affinity. Bound Abeta(1-40) features a beta-hairpin comprising residues 17-36, providing the first high-resolution structure of Abeta in beta conformation. The positions of the secondary structure elements strongly resemble those observed for fibrillar Abeta. Z(Abeta3) stabilizes the beta-sheet by extending it intermolecularly and by burying both of the mostly nonpolar faces of the Abeta hairpin within a large hydrophobic tunnel-like cavity. Consequently, Z(Abeta3) acts as a stoichiometric inhibitor of Abeta fibrillation. The selected Abeta conformation allows us to suggest a structural mechanism for amyloid formation based on soluble oligomeric hairpin intermediates.

Differences in radiosensitivity between three HER2 overexpressing cell lines

PURPOSE

HER2 is a potential target for radionuclide therapy, especially when HER2 overexpressing breast cancer cells are resistant to Herceptin(R) treatment. Therefore, it is of interest to analyse whether HER2 overexpressing tumour cells have different inherent radiosensitivity.

METHODS

The radiosensitivity of three often used HER2 overexpressing cell lines, SKOV-3, SKBR-3 and BT-474, was analysed. The cells were exposed to conventional photon irradiation, low linear energy transfer (LET), to characterise their inherent radiosensitivity. The analysis was made with clonogenic survival and growth extrapolation assays. The cells were also exposed to alpha particles, high LET, from (211)At decays using the HER2-binding affibody molecule (211)At-(Z(HER2:4))(2) as targeting agent. Assays for studies of internalisation of the affibody molecule were applied.

RESULTS

SKOV-3 cells were most radioresistant, SKBR-3 cells were intermediate and BT-474 cells were most sensitive as measured with the clonogenic and growth extrapolation assays after photon irradiation. The HER2 dependent cellular uptake of (211)At was qualitatively similar for all three cell lines. However, the sensitivity to the alpha particles from (211)At differed; SKOV-3 was most resistant, SKBR-3 intermediate and BT-474 most sensitive. These differences were unexpected because it is assumed that all types of cells should have similar sensitivity to high-LET radiation. The sensitivity to alpha particle exposure correlated with internalisation of the affibody molecule and with size of the cell nucleus.

CONCLUSION

There can be differences in radiosensitivity, which, if they also exist between patient breast cancer cells, are important to consider for both conventional radiotherapy and for HER2-targeted radionuclide therapy.

Multimodality imaging of the HER-kinase axis in cancer

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases controls critical pathways involved in epithelial cell differentiation, growth, division, and motility. Alterations and disruptions in the function of the HER-kinase axis can lead to malignancy. Many therapeutic agents targeting the HER-kinase axis are approved for clinical use or are in preclinical/clinical development. The ability to quantitatively image the HER-kinase axis in a noninvasive manner can aid in lesion detection, patient stratification, new drug development/validation, dose optimization, and treatment monitoring. This review summarizes the current status in multimodality imaging of the HER-kinase axis using PET, SPECT, optical, and MR imaging. The targeting ligands used include small-molecule tyrosine kinase inhibitors, peptides, proteins, antibodies, and engineered antibody fragments. EGFR and HER2 imaging have been well documented in the past, and imaging of HER3, HER4, HER heterodimers, and HER-kinase mutants deserves significant research effort in the future. Successful development of new HER-kinase-targeted imaging agents with optimal in vivo stability, targeting efficacy, and desirable pharmacokinetics for clinical translation will enable maximum benefit in cancer patient management.

Alternative non-antibody scaffolds for molecular recognition

Originally proposed one decade ago, the idea of engineering proteins outside the immunoglobulin family for novel binding functions has evolved as a powerful technology. Several classes of protein scaffolds proved to yield reagents with specificities and affinities in a range that was previously considered unique to antibodies. Such engineered protein scaffolds are usually obtained by designing a random library with mutagenesis focused at a loop region or at an otherwise permissible surface area and by selection of variants against a given target via phage display or related techniques. Whereas a plethora of protein scaffolds has meanwhile been proposed, only few of them were actually demonstrated to yield specificities towards different kinds of targets and to offer practical benefits such as robustness, smaller size, and ease of expression that justify their use as a true alternative to conventional antibodies or their recombinant fragments. Currently, the most promising scaffolds with broader applicability are protein A, the lipocalins, a fibronectin domain, an ankyrin consensus repeat domain, and thioredoxin. Corresponding binding proteins are not only of interest as research reagents or for separation in biotechnology but also as potential biopharmaceuticals, especially in the areas of cancer, autoimmune and infectious diseases as well as for in vivo diagnostics. The medical prospects have boosted high commercial expectations, and many of the promising scaffolds are under development by biotech start-up companies. Although some issues still have to be addressed, for example immunogenicity, effector functions, and plasma half-life in the context of therapeutic use or low-cost high-throughput selection for applications in proteomics research, it has become clear that scaffold-derived binding proteins will play an increasing role in biotechnology and medicine.

Phage display selection of Affibody molecules with specific binding to the extracellular domain of the epidermal growth factor receptor

Affibody molecules specific for the epidermal growth factor receptor (EGFR) have been selected by phage display technology from a combinatorial protein library based on the 58-residue, protein A-derived Z domain. EGFR is overexpressed in various malignancies and is frequently associated with poor patient prognosis, and the information provided by targeting this receptor could facilitate both patient diagnostics and treatment. Three selected Affibody variants were shown to selectively bind to the extracellular domain of EGFR (EGFR-ECD). Kinetic biosensor analysis revealed that the three monomeric Affibody molecules bound with similar affinity, ranging from 130 to 185 nM. Head-to-tail dimers of the Affibody molecules were compared for their binding to recombinant EGFR-ECD in biosensor analysis and in human epithelial cancer A431 cells. Although the dimeric Affibody variants were found to bind in a range of 25-50 nM affinities in biosensor analysis, they were found to be low nanomolar binders in the cellular assays. Competition assays using radiolabeled Affibody dimers confirmed specific EGFR-binding and demonstrated that the three Affibody molecules competed for the same epitope. Immunofluorescence microscopy demonstrated that the selected Affibody dimers were initially binding to EGFR at the cell surface of A431, and confocal microscopy analysis showed that the Affibody dimers could thereafter be internalized. The potential use of the described Affibody molecules as targeting agents for radionuclide based imaging applications in various carcinomas is discussed.