Photocontrolled Reversible Binding between the Protein A-Derived Z Domain and Immunoglobulin G.

Photoisomerization of the trans and cis isomers of azobenzene derivatives has been used to control the function of biomolecules in a reversible and nondestructive manner. In this study, affibody molecules, representing a class of small, helical proteins that can be engineered for binding to a wide range of target proteins, have been investigated by the incorporation of a photoswitchable azobenzene derivative in the molecule. Three different Z domain variants were produced by solid phase peptide synthesis and conjugated by thiol-directed chemistry to an azobenzene-based photoswitch. The proteins were screened for binding to and light elution from an IgG-sepharose affinity column. One of the tested Z variants, ZC3, showed efficient binding to the column and could be eluted by irradiation with light at 400 nm. In a reverse affinity chromatography assay, where the ZC3 variant was coupled to sepharose, human IgG1 could be captured to the column and partially eluted by light. Further studies of the azobenzene-conjugated ZC3 domain by surface plasmon resonance (SPR) confirmed the high affinity binding to IgG, and circular dichroism (CD) spectroscopy showed that the protein has a high α-helical secondary structure content.

Incorporation of a triglutamyl spacer improves the biodistribution of synthetic affibody molecules radiofluorinated at the N-terminus via oxime formation with (18)F-4-fluorobenzaldehyde.

Affibody molecules are a class of affinity agents for molecular imaging based on a non-immunoglobulin protein scaffold. Previous studies have demonstrated high contrast for in vivo imaging of cancer-associated molecular abnormalities using Affibody molecules. Using the radionuclide (18)F for labeling and PET as the imaging modality, the sensitivity of molecular imaging using Affibody molecules can be further increased. The use of oxime formation between an aminooxy-functionalized peptide and (18)F-fluorobenzaldehyde ((18)F-FBA) is a promising way of radiolabeling of targeting peptides. However, previous studies demonstrated that application of this method to Affibody molecules is associated with high liver uptake. We hypothesized that incorporation of a triglutamyl spacer between the aminooxy moiety and the N-terminus of a synthetic Affibody molecule would decrease the hepatic uptake of the (18)F-N-(4-fluorobenzylidine)oxime) ((18)F-FBO)-labeled tracer. To verify this, we have produced two variants of the HER2-targeting ZHER2:342 Affibody molecule by peptide synthesis: OA-PEP4313, where aminooxyacetic acid was conjugated directly to the N-terminal alanine, and OA-E3-PEP4313, where a triglutamyl spacer was introduced between the aminooxy moiety and the N-terminus. We have found that the use of the spacer is associated with a minor decrease of affinity, from KD = 49 pM to KD = 180 pM. Radiolabeled (18)F-FBO-E3-PEP4313 demonstrated specific binding to HER2-expressing ovarian carcinoma SKOV-3 cells and slow internalization. Biodistribution studies in mice demonstrated that the use of a triglutamyl linker decreased uptake of radioactivity in liver 2.7-fold at 2 h after injection. Interestingly, radioactivity uptake in kidneys was also reduced (2.4-fold). Experiments in BALB/C nu/nu mice bearing SKOV-3 xenografts demonstrated HER2-specific uptake of (18)F-FBO-E3-PEP4313 in tumors. At 2 h pi, the tumor uptake (20 ± 2% ID/g) exceeded uptake in liver 5-fold and uptake in kidneys 3.6-fold. The tumor-to-blood ratio was 21 ± 3. The microPET/CT imaging experiment confirmed the biodistribution data. In conclusion, the use of a triglutamyl spacer is a convenient way to improve the biodistribution profile of Affibody molecules labeled at the N-terminus using (18)F-FBA. It provides a tracer capable of producing high-contrast images of HER2-expressing tumors.

Influence of nuclides and chelators on imaging using affibody molecules: comparative evaluation of recombinant affibody molecules site-specifically labeled with 68Ga and ¹¹¹In via maleimido derivatives of DOTA and NODAGA.

Accurate detection of cancer-associated molecular abnormalities in tumors could make cancer treatment more personalized. Affibody molecules enable high contrast imaging of tumor-associated protein expression shortly after injection. The use of the generator-produced positron-emitting radionuclide (68)Ga should increase sensitivity of HER2 imaging. The chemical nature of radionuclides and chelators influences the biodistribution of Affibody molecules, providing an opportunity to further increase the imaging contrast. The aim of the study was to compare maleimido derivatives of DOTA and NODAGA for site-specific labeling of a recombinant ZHER2:2395 HER2-binding Affibody molecule with (68)Ga. DOTA and NODAGA were site-specifically conjugated to the ZHER2:2395 Affibody molecule having a C-terminal cysteine and labeled with (68)Ga and (111)In. All labeled conjugates retained specificity to HER2 in vitro. Most of the cell-associated activity was membrane-bound with a minor difference in internalization rate. All variants demonstrated specific targeting of xenografts and a high tumor uptake. The xenografts were clearly visualized using all conjugates. The influence of chelator on the biodistribution and targeting properties was much less pronounced for (68)Ga than for (111)In. The tumor uptake of (68)Ga-NODAGA-ZHER2:2395 and (68)Ga-DOTA-ZHER2:2395 and tumor-to-blood ratios at 2 h p.i. did not differ significantly. However, the tumor-to-liver ratio was significantly higher for (68)Ga-NODAGA- ZHER2:2395 (8 ± 2 vs 5.0 ± 0.3) offering the advantage of better liver metastases visualization. In conclusion, influence of chelators on biodistribution of Affibody molecules depends on the radionuclides and reoptimization of labeling chemistry is required when a radionuclide label is changed.

Direct comparison of 111In-labelled two-helix and three-helix Affibody molecules for in vivo molecular imaging.

PURPOSE: Radiolabelled Affibody molecules have demonstrated a potential for visualization of tumour-associated molecular targets. Affibody molecules (7 kDa) are composed of three alpha-helices. Recently, a smaller two-helix variant of Affibody molecules (5.1 kDa) was developed. The aim of this study was to compare two- and three-helix HER2-targeting Affibody molecules directly in vivo. METHODS: The three-helix Affibody molecule ABY-002 and the two-helix Affibody molecule PEP09239 were labelled with (111)In at the N-termini via DOTA chelator. Tumour-targeting properties were directly compared at 1 and 4 h after injection in mice bearing SKOV-3 xenografts with high HER2 expression and LS174T xenografts with low HER2 expression. RESULTS: The dissociation constants (K(D)) for HER2 binding were 78 pM for the three-helix Affibody molecule and 2.1 nM for the two-helix Affibody molecule. (111)In-PEP09239 cleared more rapidly from the blood. In xenografts with high HER2 expression, the uptake of (111)In-ABY-002 was significantly higher than that of (111)In-PEP09239. The tumour-to-blood ratio was higher for (111)In-PEP09239 at 4 h after injection, while there was no significant difference in other tumour-to-organ ratios. The tumour uptake of (111)In-ABY-002 was eightfold higher than that of (111)In-PEP09239 in xenografts with low expression. Tumour-to-blood ratios were equal in this case, but other tumour-to-organ ratios were appreciably higher for the three-helix variant. CONCLUSION: For tumours with high HER2 expression, two-helix HER2-targeting Affibody molecules can provide higher tumour-to-blood ratio at the cost of lower tumour uptake. In the case of low expression, both tumour uptake and tumour-to-organ ratios are appreciably higher for three-helix than for two-helix HER2-targeting Affibody molecules.

Imaging of EGFR expression in murine xenografts using site-specifically labelled anti-EGFR 111In-DOTA-Z EGFR:2377 Affibody molecule: aspect of the injected tracer amount.

INTRODUCTION: Overexpression of epidermal growth factor receptor (EGFR) is a prognostic and predictive biomarker in a number of malignant tumours. Radionuclide molecular imaging of EGFR expression in cancer could influence patient management. However, EGFR expression in normal tissues might complicate in vivo imaging. The aim of this study was to evaluate if optimization of the injected protein dose might improve imaging of EGFR expression in tumours using a novel EGFR-targeting protein, the DOTA-Z(EGFR:2377) Affibody molecule. METHODS: An anti-EGFR Affibody molecule, Z(EGFR:2377), was labelled with (111)In via the DOTA chelator site-specifically conjugated to a C-terminal cysteine. The affinity of DOTA-Z(EGFR:2377) for murine and human EGFR was measured by surface plasmon resonance. The cellular processing of (111)In-DOTA-Z(EGFR:2377) was evaluated in vitro. The biodistribution of radiolabelled Affibody molecules injected in a broad range of injected Affibody protein doses was evaluated in mice bearing EGFR-expressing A431 xenografts. RESULTS: Site-specific coupling of DOTA provided a uniform conjugate possessing equal affinity for human and murine EGFR. The internalization of (111)In-DOTA-Z(EGFR:2377) by A431 cells was slow. In vivo, the conjugate accumulated specifically in xenografts and in EGFR-expressing tissues. The curve representing the dependence of tumour uptake on the injected Affibody protein dose was bell-shaped. The highest specific radioactivity (lowest injected protein dose) provided a suboptimal tumour-to-blood ratio. The results of the biodistribution study were confirmed by gamma-camera imaging. CONCLUSION: The (111)In-DOTA-Z(EGFR:2377) Affibody molecule is a promising tracer for radionuclide molecular imaging of EGFR expression in malignant tumours. Careful optimization of protein dose is required for high-contrast imaging of EGFR expression in vivo.

Affibody molecules for epidermal growth factor receptor targeting in vivo: aspects of dimerization and labeling chemistry.

UNLABELLED: Noninvasive detection of epidermal growth factor receptor (EGFR) expression in malignant tumors by radionuclide molecular imaging may provide diagnostic information influencing patient management. The aim of this study was to evaluate a novel EGFR-targeting protein, the ZEGFR:1907 Affibody molecule, for radionuclide imaging of EGFR expression, to determine a suitable tracer format (dimer or monomer) and optimal label. METHODS: An EGFR-specific Affibody molecule, ZEGFR:1907, and its dimeric form, (ZEGFR:1907)2, were labeled with 111In using benzyl-diethylenetriaminepentaacetic acid and with 125I using p-iodobenzoate. Affinity and cellular retention of conjugates were evaluated in vitro. Biodistribution of radiolabeled Affibody molecules was compared in mice bearing EGFR-expressing A431 xenografts. Specificity of EGFR targeting was confirmed by comparison with biodistribution of non-EGFR-specific counterparts. RESULTS: Head-to-tail dimerization of the Affibody molecule improved the dissociation rate. In vitro, dimeric forms demonstrated superior cellular retention of radioactivity. For both molecular set-ups, retention was better for the 111In-labeled tracer than for the radioiodinated counterpart. In vivo, all conjugates accumulated specifically in xenografts and in EGFR-expressing tissues. The retention of radioactivity in tumors was better in vivo for dimeric forms; however, the absolute uptake values were higher for monomeric tracers. The best tracer, 111In-labeled ZEGFR:1907, provided a tumor-to-blood ratio of 100 (24 h after injection). CONCLUSION: The radiometal-labeled monomeric Affibody molecule ZEGFR:1907 has a potential for radionuclide molecular imaging of EGFR expression in malignant tumors.

Design, synthesis and biological evaluation of a multifunctional HER2-specific Affibody molecule for molecular imaging.

PURPOSE: The purpose of this study was to design and evaluate a novel platform for labelling of Affibody molecules, enabling both recombinant and synthetic production and site-specific labelling with (99m)Tc or trivalent radiometals. METHODS: The HER2-specific Affibody molecule PEP05352 was made by peptide synthesis. The chelator sequence SECG (serine-glutamic acid-cysteine-glycine) was anchored on the C-terminal to allow (99m)Tc labelling. The cysteine can alternatively serve as a conjugation site of the chelator DOTA for indium labelling. The resulting (99m)Tc- and (111)In-labelled Affibody molecules were evaluated both in vitro and in vivo. RESULTS: Both conjugates retained their capacity to bind to HER2 receptors in vitro and in vivo. The tumour to blood ratio in LS174T xenografts was 30 at 4 h post-injection for both conjugates. Biodistribution data showed that the (99m)Tc-labelled Affibody molecule had a fourfold lower kidney accumulation compared with the (111)In-labelled Affibody molecule while the accumulation in other organs was similar. Gamma camera imaging of the conjugates could clearly visualise the tumours 4 h after injection. CONCLUSION: Incorporation of the C-terminal SECG sequence in Affibody molecules provides a general multifunctional platform for site-specific labelling with different nuclides (technetium, indium, gallium, cobalt or yttrium) and for a flexible production (chemical synthesis or recombinant).

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

A radiolabeled anti-HER2 Affibody molecule (Z(HER2:342)) targets HER2-expressing xenografts with high selectivity and gives good imaging contrast. However, the small size (approximately 7 kDa) results in rapid glomerular filtration and high renal accumulation of radiometals, thus excluding targeted therapy. Here, we report that reversible binding to albumin efficiently reduces the renal excretion and uptake, enabling radiometal-based nuclide therapy. The dimeric Affibody molecule (Z(HER2:342))(2) was fused with an albumin-binding domain (ABD) conjugated with the isothiocyanate derivative of CHX-A''-DTPA and labeled with the low-energy beta-emitter (177)Lu. The obtained conjugate [CHX-A''-DTPA-ABD-(Z(HER2:342))(2)] had a dissociation constant of 18 pmol/L to HER2 and 8.2 and 31 nmol/L for human and murine albumin, respectively. The radiolabeled conjugate displayed specific binding to HER2-expressing cells and good cellular retention in vitro. In vivo, fusion with ABD enabled a 25-fold reduction of renal uptake in comparison with the nonfused dimer molecule (Z(HER2:342))(2). Furthermore, the biodistribution showed high and specific uptake of the conjugate in HER2-expressing tumors. Treatment of SKOV-3 microxenografts (high HER2 expression) with 17 or 22 MBq (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) completely prevented formation of tumors, in contrast to mice given PBS or 22 MBq of a radiolabeled non-HER2-binding Affibody molecule. In LS174T xenografts (low HER2 expression), this treatment resulted in a small but significant increase of the survival time. Thus, fusion with ABD improved the in vivo biodistribution, and the results highlight (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) as a candidate for treatment of disseminated tumors with a high level of HER2 expression.

111In-benzyl-DTPA-ZHER2:342, an affibody-based conjugate for in vivo imaging of HER2 expression in malignant tumors.

UNLABELLED: Data on expression of the HER2 (erbB-2) receptor in breast carcinoma make it possible to select the most efficient treatment. There are strong indications that HER2 expression possesses prognostic and predictive values in ovarian, prostate, and lung carcinomas as well. Visualization of HER2 expression using radionuclide targeting can provide important diagnostic information. The Affibody Z(HER2:342) is a short (approximately 7 kDa) phage-display-selected protein that binds HER2 with an affinity of 22 pmol/L. The goal of this study was to evaluate whether (111)In-labeled HER2:342 can be used for imaging of HER2 overexpression in vivo. METHODS: Z(HER2:342) was labeled with (111)In via isothiocyanate-benzyl-DTPA (DTPA is diethylenetriaminepentaacetic acid) and the conjugate was characterized in vitro and in vivo. RESULTS: (111)In-Benzyl-DTPA-Z(HER2:342) preserved the capacity to bind living HER2-expressing cells specifically. The affinity of In-benzyl-DTPA-Z(HER2:342) to HER2 was 21 pmol/L according to surface plasmon resonance measurements. In nude mice bearing HER2-expressing SKOV-3 xenografts, a tumor uptake of 12% +/- 3% injected activity per gram and a tumor-to-blood ratio of about 100 were obtained 4 h after injection. Tumor uptake in vivo was receptor specific, as it could be blocked with an excess of nonlabeled Z(HER2:342). HER2-expressing xenografts were clearly imaged 4 h after injection using a gamma-camera. CONCLUSION: (111)In-Benzyl-DTPA-Z(HER2:342) is a promising candidate for visualization of HER2 expression in carcinomas, using the single-photon detection technique.

Site-specific radiometal labeling and improved biodistribution using ABY-027, a novel HER2-targeting affibody molecule-albumin-binding domain fusion protein.

UNLABELLED: Because of their better penetration, smaller targeting proteins may be superior to antibodies for radioimmunotherapy of solid tumors. Therefore, Affibody molecules (6.5 kDa) have a potential for being suitable as targeted moiety for radiolabeled therapeutic proteins. Previous studies have demonstrated that a fusion of an Affibody molecule with an albumin-binding domain (ABD) provides a strong noncovalent binding to albumin in vivo. This strong noncovalent binding can be used for reduction of the renal uptake of the Affibody molecule while maintaining a size smaller than that of an antibody, which is important when using residualizing radionuclide labels conjugated to Affibody molecules. The goal of this study was to design and evaluate a new targeting Affibody-ABD fusion protein with improved biodistribution properties for radionuclide therapy. METHODS: A novel Affibody-based construct, ZHER2:2891-ABD035-DOTA (ABY-027), was created by fusion of the reengineered HER2-binding Affibody molecule ZHER2:2891 to the N terminus of the high-affinity ABD035, and a maleimido-derivative of DOTA was conjugated at the C terminus of the construct. Binding and processing of (177)Lu-ABY-027 by HER2-expressing cells were evaluated in vitro. Targeting of HER2-expressing SKOV-3 xenografts was evaluated in BALB/C nu/nu mice and compared with targeting of previously reported ABD-(ZHER2:342)2. RESULTS: The binding affinity (dissociation constant) of ABY-027 to HER2 (74 pM) was the same as for the parental ZHER2:2891 (76 pM). ABY-027 was stably labeled with (177)Lu and (111)In with preserved specific binding to HER2-expressing cells in vitro. In vivo receptor saturation experiments demonstrated that targeting of SKOV-3 xenografts in BALB/C nu/nu mice was HER2-specific. (177)Lu-ABY-027 demonstrated substantially (2- to 3-fold) lower renal and hepatic uptake than previously assessed HER2-specific Affibody-based albumin-binding agents. Tumor uptake of radiolabeled ABY-027 at 48 h after injection was 2-fold higher than that for previously reported ABD-(ZHER2:342)2. CONCLUSION: An optimized molecular design of an ABD fusion protein resulted in an Affibody molecule construct with better properties for therapy. Fully preserved in vivo targeting of the fusion protein was shown in xenografted mice. Site-specific coupling of DOTA provides a uniform conjugate and creates the potential for labeling with a broad range of therapeutic radionuclides. The biodistribution of (177)Lu-ABY-027 in a murine model suggests it is more suitable for therapy than alternative approaches.

Evaluation of backbone-cyclized HER2-binding 2-helix affibody molecule for in vivo molecular imaging.

INTRODUCTION: Affibody molecules, small scaffold proteins, have demonstrated an appreciable potential as imaging probes. Affibody molecules are composed of three alpha-helices. Helices 1 and 2 are involved in molecular recognition, while helix 3 provides stability. The size of Affibody molecules can be reduced by omitting the third alpha-helix and cross-linking the two remaining, providing a smaller molecule with better extravasation and quicker clearance of unbound tracer. The goal of this study was to develop a novel 2-helix Affibody molecule based on backbone cyclization by native chemical ligation (NCL). METHODS: The HER2-targeting NCL-cyclized Affibody molecule ZHER2:342min has been designed, synthesized and site-specifically conjugated with a DOTA chelator. DOTA-ZHER2:342min was labeled with (111)In and (68)Ga. The binding affinity of DOTA-ZHER2:342min was evaluated in vitro. The targeting properties of (111)In- and (68)Ga-DOTA-ZHER2:342min were evaluated in mice bearing SKOV-3 xenografts and compared with the properties of (111)In- and (68)Ga-labeled PEP09239, a DOTA-conjugated 2-helix Affibody analogue cyclized by a homocysteine disulfide bridge. RESULTS: The dissociation constant (KD) for DOTA-ZHER2:342min binding to HER2 was 18nM according to SPR measurements. DOTA-ZHER2:342min was labeled with (111)In and (68)Ga. Both conjugates demonstrated bi-phasic binding kinetics to HER2-expressing cells, with KD1 in low nanomolar range. Both variants demonstrated specific uptake in HER2-expressing xenografts. Tumor-to-blood ratios at 2h p.i. were 6.1±1.3 for (111)In- DOTA-ZHER2:342min and 4.6±0.7 for (68)Ga-DOTA-ZHER2:342min. However, the uptake of DOTA-ZHER2:342min in lung, liver and spleen was appreciably higher than the uptake of PEP09239-based counterparts. CONCLUSIONS: Native chemical ligation enables production of a backbone-cyclized HER2-binding 2-helix Affibody molecule (ZHER2:342min) with low nanomolar target affinity and specific tumor uptake.

Tumor targeting using affibody molecules: interplay of affinity, target expression level, and binding site composition.

UNLABELLED: Radionuclide imaging of cancer-associated molecular alterations may contribute to patient stratification for targeting therapy. Scaffold high-affinity proteins, such as Affibody molecules, are a new, promising class of probes for in vivo imaging. METHODS: The effects of human epidermal growth factor receptor 2 (HER2) affinity and binding site composition of HER2-binding Affibody molecules, and of the HER2 density on the tumor targeting, were studied in vivo. The tumor uptake and tumor-to-organ ratios of Affibody molecules with moderate (dissociation constant [K(D)] = 10(-9) M) or high (K(D) = 10(-10) M) affinity were compared between tumor xenografts with a high (SKOV-3) and low (LS174T) HER2 expression level in BALB/C nu/nu mice. Two Affibody molecules with similar affinity (K(D) = 10(-10) M) but having alternative amino acids in the binding site were compared. RESULTS: In SKOV-3 xenografts, uptake was independent of affinity at 4 h after injection, but high-affinity binders provided 2-fold-higher tumor radioactivity retention at 24 h. In LS174T xenografts, uptake of high-affinity probes was already severalfold higher at 4 h after injection, and the difference was increased at 24 h. The clearance rate and tumor-to-organ ratios were influenced by the amino acid composition of the binding surface of the tracer protein. CONCLUSION: The optimal affinity of HER2-binding Affibody molecules depends on the expression of a molecular target. At a high expression level (>10(6) receptors per cell), an affinity in the low-nanomolar range is sufficient. At moderate expression, subnanomolar affinity is desirable. The binding site composition can influence the imaging contrast. This information may be useful for development of imaging agents based on scaffold affinity proteins.

Imaging of human epidermal growth factor receptor type 2 expression with 18F-labeled affibody molecule ZHER2:2395 in a mouse model for ovarian cancer.

UNLABELLED: Affibody molecules are small (7 kDa) proteins with subnanomolar targeting affinity. Previous SPECT studies in xenografts have shown that the Affibody molecule (111)In-DOTA-Z(HER2)(:2395) can discriminate between high and low human epidermal growth factor receptor type 2 (HER2)-expressing tumors, indicating that radiolabeled Affibody molecules have potential for patient selection for HER2-targeted therapy. Compared with SPECT, PET with positron-emitting radionuclides, such as (18)F, may improve imaging of HER2 expression because of higher sensitivity and improved quantification of PET. The aim of the present study was to determine whether the (18)F-labeled NOTA-conjugated Affibody molecule Z(HER2)(:2395) is a suitable agent for imaging of HER2 expression. The tumor-targeting properties of (18)F-labeled Z(HER2)(:2395) were compared with (111)In- and (68)Ga-labeled Z(HER2)(:2395) in mice with HER2-expressing SK-OV-3 xenografts. METHODS: Z(HER2)(:2395) was conjugated with NOTA and radiolabeled with (18)F, (68)Ga, and (111)In. Radiolabeling with (18)F was based on the complexation of Al(18)F by NOTA. The 50% inhibitory concentration values for NOTA-Z(HER2)(:2395) labeled with (19)F, (69)Ga, and (115)In were determined in a competitive cell-binding assay using SK-OV-3 cells. Mice bearing subcutaneous SK-OV-3 xenografts were injected intravenously with radiolabeled NOTA-Z(HER2)(:2395). One and 4 h after injection, PET/CT or SPECT/CT images were acquired, and the biodistribution was determined by ex vivo measurement. RESULTS: The 50% inhibitory concentration values for (19)F-, (69)Ga-, and (115)In-NOTA-Z(HER2)(:2395) were 5.0, 6.3, and 5.3 nM, respectively. One hour after injection, tumor uptake was 4.4 ± 0.8 percentage injected dose per gram (%ID/g), 5.6 ± 1.6 %ID/g, and 7.1 ± 1.4 %ID/g for (18)F-, (68)Ga-, and (111)In-NOTA-Z(HER2)(:2395), respectively, and the respective tumor-to-blood ratios were 7.4 ± 1.8, 8.0 ± 1.3, and 4.8 ± 1.3. Tumor uptake was specific, because uptake could be blocked efficiently by coinjection of an excess of unlabeled Z(HER2)(:2395). PET/CT and SPECT/CT images clearly visualized HER2-expressing SK-OV-3 xenografts. CONCLUSION: This study showed that (18)F-NOTA-Z(HER2)(:2395) is a promising new imaging agent for HER2 expression in tumors. Affibody molecules were successfully labeled with (18)F within 30 min, based on the complexation of Al(18)F by NOTA. Further research is needed to determine whether this technique can be used for patient selection for HER2-targeted therapy.

Design of an optimized scaffold for affibody molecules.

Affibody molecules are non-immunoglobulin-derived affinity proteins based on a three-helical bundle protein domain. Here, we describe the design process of an optimized Affibody molecule scaffold with improved properties and a surface distinctly different from that of the parental scaffold. The improvement was achieved by applying an iterative process of amino acid substitutions in the context of the human epidermal growth factor receptor 2 (HER2)-specific Affibody molecule Z(HER2:342). Replacements in the N-terminal region, loop 1, helix 2 and helix 3 were guided by extensive structural modeling using the available structures of the parent Z domain and Affibody molecules. The effect of several single substitutions was analyzed followed by combination of up to 11 different substitutions. The two amino acid substitutions N23T and S33K accounted for the most dramatic improvements, including increased thermal stability with elevated melting temperatures of up to +12 degrees C. The optimized scaffold contains 11 amino acid substitutions in the nonbinding surface and is characterized by improved thermal and chemical stability, as well as increased hydrophilicity, and enables generation of identical Affibody molecules both by chemical peptide synthesis and by recombinant bacterial expression. A HER2-specific Affibody tracer, [MMA-DOTA-Cys61]-Z(HER2:2891)-Cys (ABY-025), was produced by conjugating MMA-DOTA (maleimide-monoamide-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to the peptide produced either chemically or in Escherichia coli. ABY-025 showed high affinity and specificity for HER2 (equilibrium dissociation constant, K(D), of 76 pM) and detected HER2 in tissue sections of SKOV-3 xenograft and human breast tumors. The HER2-binding capacity was fully retained after three cycles of heating to 90 degrees C followed by cooling to room temperature. Furthermore, the binding surfaces of five Affibody molecules targeting other proteins (tumor necrosis factor alpha, insulin, Taq polymerase, epidermal growth factor receptor or platelet-derived growth factor receptor beta) were grafted onto the optimized scaffold, resulting in molecules with improved thermal stability and a more hydrophilic nonbinding surface.

Evaluation of maleimide derivative of DOTA for site-specific labeling of recombinant affibody molecules.

Affibody molecules are a new class of small (7 kDa) scaffold affinity proteins, which demonstrate promising properties as agents for in vivo radionuclide targeting. The Affibody scaffold is cysteine-free and therefore independent of disulfide bonds. Thus, a single thiol group can be engineered into the protein by introduction of one cysteine. Coupling of thiol-reactive bifunctional chelators can enable site-specific labeling of recombinantly produced Affibody molecules. In this study, the use of 1,4,7,10-tetraazacyclododecane-1,4,7-tris-acetic acid-10-maleimidoethylacetamide (MMA-DOTA) for 111 In-labeling of anti-HER2 Affibody molecules His 6-Z HER2:342-Cys and Z HER2:2395-Cys has been evaluated. The introduction of a cysteine residue did not affect the affinity of the proteins, which was 29 pM for His 6-Z HER2:342-Cys and 27 pM for Z HER2:2395-Cys, comparable with 22 pM for the parental Z HER2:342. MMA-DOTA was conjugated to DTT-reduced Affibody molecules with a coupling efficiency of 93% using a 1:1 molar ratio of chelator to protein. The conjugates were labeled with 111 In to a specific radioactivity of up to 7 GBq/mmol, with preserved binding for the target HER2. In vivo, the non-His-tagged variant 111 In-[MMA-DOTA-Cys61]-Z HER2:2395-Cys demonstrated appreciably lower liver uptake than its His-tag-containing counterpart. In mice bearing HER2-expressing LS174T xenografts, 111 In-[MMA-DOTA-Cys61]-Z HER2:2395-Cys showed specific and rapid tumor localization, and rapid clearance from blood and nonspecific compartments, leading to a tumor-to-blood-ratio of 18 +/- 8 already 1 h p.i. Four hours p.i., the tumor-to-blood ratio was 138 +/- 8. Xenografts were clearly visualized already 1 h p.i.