In Vivo Evaluation and Dosimetry Estimate for a High Affinity Affibody PET Tracer Targeting PD-L1.

PURPOSE: In vivo imaging of programmed death ligand 1 (PD-L1) during immunotherapy could potentially monitor changing PD-L1 expression and PD-L1 expression heterogeneity within and across tumors. Some protein constructs can be used for same-day positron emission tomography (PET) imaging. Previously, we evaluated the PD-L1-targeting Affibody molecule [18F]AlF-NOTA-ZPD-L1_1 as a PET tracer in a mouse tumor model of human PD-L1 expression. In this study, we evaluated the affinity-matured Affibody molecule ZPD-L1_4, to determine if improved affinity for PD-L1 resulted in increased in vivo targeting of PD-L1. PROCEDURES: ZPD-L1_4 was conjugated with NOTA and radiolabeled with either [18F]AlF or 68Ga. [18F]AlF-NOTA-ZPD-L1_4 and [68Ga]NOTA-ZPD-L1_4 were evaluated in immunocompromised mice with LOX (PD-L1+) and SUDHL6 (PD-L1-) tumors with PET and ex vivo biodistribution measurements. In addition, whole-body PET studies were performed in rhesus monkeys to predict human biodistribution in a model with tracer binding to endogenous PD-L1, and to calculate absorbed radiation doses. RESULTS: Ex vivo biodistribution measurements showed that both tracers had > 25 fold higher accumulation in LOX tumors than SUDHL6 ([18F]AlF-NOTA-ZPD-L1_4: LOX: 8.7 ± 0.7 %ID/g (N = 4) SUDHL6: 0.2 ± 0.01 %ID/g (N = 6), [68Ga]NOTA-ZPD-L1_4: LOX: 15.8 ± 1.0 %ID/g (N = 6) SUDHL6: 0.6 ± 0.1 %ID/g (N = 6)), considerably higher than ZPD-L1_1. In rhesus monkeys, both PET tracers showed fast clearance through kidneys and low background signal in the liver ([18F]AlF-NOTA-ZPD-L1_4: 1.26 ± 0.13 SUV, [68Ga]NOTA-ZPD-L1_4: 1.11 ± 0.06 SUV). PD-L1-expressing lymph nodes were visible in PET images, indicating in vivo PD-L1 targeting. Dosimetry estimates suggest that both PET tracers can be used for repeated clinical studies, although high kidney accumulation may limit allowable radioactive doses. CONCLUSIONS: [18F]AlF-NOTA-ZPD-L1_4 and [68Ga]NOTA-ZPD-L1_4 are promising candidates for same-day clinical PD-L1 PET imaging, warranting clinical evaluation. The ability to use either [18F] or [68Ga] may expand access to clinical sites.

Intramolecular Thioether Crosslinking to Increase the Proteolytic Stability of Affibody Molecules.

Protein therapeutics suffer from low oral bioavailability, mainly due to poor membrane permeability and digestion by gastrointestinal proteases. To improve proteolytic stability, intramolecular thioether crosslinks were introduced into a three-helix affibody molecule binding the human epidermal growth factor receptor (EGFR). Solid-phase peptide synthesis was used to produce an unmodified control protein domain and three different crosslinked protein domain variants: one with a thioether crosslink between the N-terminal lysine residue and a cysteine residue in the second loop region (denoted K4), a second with a crosslink between the C-terminal lysine residue and a cysteine residue in the first loop region (denoted K58), and a third with crosslinks in both positions (denoted K4K58). Circular dichroism (CD) and surface-plasmon-resonance-based (SPR-based) biosensor studies of the protein domains showed that the three-helix structure and high-affinity binding to EGFR were preserved in the crosslinked protein domains. In vitro digestion by gastrointestinal proteases demonstrated that the crosslinked protein domains showed increased stability towards pepsin and towards a combination of trypsin and chymotrypsin.

Specific Binding of Cu(II) Ions to Amyloid-Beta Peptides Bound to Aggregation-Inhibiting Molecules or SDS Micelles Creates Complexes that Generate Radical Oxygen Species.

Aggregation of the amyloid-beta (Aß) peptide into insoluble plaques is a major factor in Alzheimer's disease (AD) pathology. Another major factor in AD is arguably metal ions, as metal dyshomeostasis is observed in AD patients, metal ions modulate Aß aggregation, and AD plaques contain numerous metals including redox-active Cu and Fe ions. In vivo, Aß is found in various cellular locations including membranes. So far, Cu(II)/Aß interactions and ROS generation have not been investigated in a membrane environment. Here, we study Cu(II) and Zn(II) interactions with Aß bound to SDS micelles or to engineered aggregation-inhibiting molecules (the cyclic peptide CP-2 and the ZAß3(12-58)Y18L Affibody molecule). In all studied systems the Aß N-terminal segment was found to be unbound, unstructured, and free to bind metal ions. In SDS micelles, Aß was found to bind Cu(II) and Zn(II) with the same ligands and the same KD as in aqueous solution. ROS was generated in all Cu(II)/Aß complexes. These results indicate that binding of Aß to membranes, drugs, and other entities that do not interact with the Aß N-terminal part, appears not to compromise the N-terminal segment's ability to bind metal ions, nor impede the capacity of N-terminally bound Cu(II) to generate ROS.

Intramolecular thioether crosslinking of therapeutic proteins to increase proteolytic stability.

Protein-based pharmaceuticals typically display high selectivity and low toxicity, but are also characterized by low oral availability, mainly because of enzymatic degradation in the gastrointestinal tract and poor permeability across the intestinal wall. One way to increase the proteolytic stability of peptides and proteins is by intramolecular crosslinking, such as the introduction of disulfide bridges. However, disulfide bridges are at risk of thiol-disulfide exchange or reduction during production, purification, and/or therapeutic use, whereas thioether bridges are expected to be stable under the same conditions. In this study, thioether crosslinking was investigated for a 46 aa albumin-binding domain (ABD) derived from streptococcal protein G. ABD binds with high affinity to human serum albumin (HSA) and has been proposed as a fusion partner to increase the in vivo half-lives of therapeutic proteins. In the study, five ABD variants with single or double intramolecular thioether bridges were designed and synthesized. The binding affinity, secondary structure, and thermal stability of each protein was investigated by SPR-based biosensor analysis and CD spectroscopy. The proteolytic stability in the presence of the major intestinal proteases pepsin (found in the stomach) and trypsin in combination with chymotrypsin (found in pancreatin secreted to the duodenum by the pancreas) was also investigated. The most promising crosslinked variant, ABD_CL1, showed high thermal stability, retained high affinity in binding to HSA, and showed dramatically increased stability in the presence of pepsin and trypsin/chymotrypsin, compared to the ABD reference protein. This suggests that the intramolecular thioether crosslinking strategy can be used to increase the stability towards gastrointestinal enzymes.

A GLP-1 receptor agonist conjugated to an albumin-binding domain for extended half-life.

Glucagon-like peptide 1 (GLP-1) and related peptide agonists have been extensively investigated for glycaemic control in Type 2 diabetes, and may also have therapeutic applications for other diseases. Due to the short half-life (t1/2 < 2 min) of the endogenous peptide, caused by proteolytic degradation and renal clearance, different strategies for half-life extension and sustained release have been explored. In the present study, conjugates between a GLP-1 analogue and a 5 kDa albumin-binding domain (ABD) derived from streptococcal protein G have been chemically synthesized and evaluated. ABD binds with high affinity to human serum albumin, which is highly abundant in plasma and functions as a drug carrier in the circulation. Three different GLP-1-ABD conjugates, with the two peptides connected by linkers of two, four, and six PEG units, respectively, were synthesized and tested in mouse pancreatic islets at high (11 mM) and low (3 mM) glucose concentration. Insulin release upon stimulation was shown to be glucose-dependent, showing no significant difference between the three different GLP-1-ABD conjugates and unconjugated GLP-1 analogue. The biological activity, in combination with the high affinity binding to albumin, make the GLP-1-ABD conjugates promising GLP-1 receptor agonists expected to show extended in vivo half-life.

Engineered non-fluorescent Affibody molecules facilitate studies of the amyloid-beta (Aß) peptide in monomeric form: low pH was found to reduce Aß/Cu(II) binding affinity.

Aggregation of amyloid-beta (Aß) peptides into oligomers and amyloid plaques in the human brain is considered a causative factor in Alzheimer's disease (AD). As metal ions are over-represented in AD patient brains, and as distinct Aß aggregation pathways in presence of Cu(II) have been demonstrated, metal binding to Aß likely affects AD progression. Aß aggregation is moreover pH-dependent, and AD appears to involve inflammatory conditions leading to physiological acidosis. Although metal binding specificity to Aß varies at different pH's, metal binding affinity to Aß has so far not been quantitatively investigated at sub-neutral pH levels. This may be explained by the difficulties involved in studying monomeric peptide properties under aggregation-promoting conditions. We have recently devised a modified Affibody molecule, Z(Aß3)(12-58), that binds Aß with sub-nanomolar affinity, thereby locking the peptide in monomeric form without affecting the N-terminal region where metal ions bind. Here, we introduce non-fluorescent Aß-binding Affibody variants that keep Aß monomeric while only slightly affecting the Aß peptide's metal binding properties. Using fluorescence spectroscopy, we demonstrate that Cu(II)/Aß(1-40) binding is almost two orders of magnitude weaker at pH 5.0 (apparent K(D)=51 µM) than at pH 7.3 (apparent K(D)=0.86 µM). This effect is arguably caused by protonation of the histidines involved in the metal ligandation. Our results indicate that engineered variants of Affibody molecules are useful for studying metal-binding and other properties of monomeric Aß under various physiological conditions, which will improve our understanding of the molecular mechanisms involved in AD.

A native chemical ligation approach for combinatorial assembly of affibody molecules.

Affinity molecules labeled with different reporter groups, such as fluorophores or radionuclides, are valuable research tools used in a variety of applications. One class of engineered affinity proteins is Affibody molecules, which are small (6.5 kDa) proteins that can be produced by solid phase peptide synthesis (SPPS), thereby allowing site-specific incorporation of reporter groups during synthesis. The Affibody molecules are triple-helix proteins composed of a variable part, which gives the protein its binding specificity, and a constant part, which is identical for all Affibody molecules. In the present study, native chemical ligation (NCL) has been applied for combinatorial assembly of Affibody molecules from peptide fragments produced by Fmoc SPPS. The concept is demonstrated for the synthesis of three different Affibody molecules. The cysteine residue introduced at the site of ligation can be used for directed immobilization and does not interfere with the function of the investigated proteins. This strategy combines a high-yield production method with facilitated preparation of proteins with different C-terminal modifications.

N-terminal engineering of amyloid-ß-binding Affibody molecules yields improved chemical synthesis and higher binding affinity.

The aggregation of amyloid-ß (Aß) peptides is believed to be a major factor in the onset and progression of Alzheimer's disease. Molecules binding with high affinity and selectivity to Aß-peptides are important tools for investigating the aggregation process. An Aß-binding Affibody molecule, ZAß3 , has earlier been selected by phage display and shown to bind Aß(1-40) with nanomolar affinity and to inhibit Aß-peptide aggregation. In this study, we create truncated functional versions of the ZAß3 Affibody molecule better suited for chemical synthesis production. Engineered Affibody molecules of different length were produced by solid phase peptide synthesis and allowed to form covalently linked homodimers by S-S-bridges. The N-terminally truncated Affibody molecules ZAß3 (12-58), ZAß3 (15-58), and ZAß3 (18-58) were produced in considerably higher synthetic yield than the corresponding full-length molecule ZAß3 (1-58). Circular dichroism spectroscopy and surface plasmon resonance-based biosensor analysis showed that the shortest Affibody molecule, ZAß3 (18-58), exhibited complete loss of binding to the Aß(1-40)-peptide, while the ZAß3 (12-58) and ZAß3 (15-58) Affibody molecules both displayed approximately one order of magnitude higher binding affinity to the Aß(1-40)-peptide compared to the full-length Affibody molecule. Nuclear magnetic resonance spectroscopy showed that the structure of Aß(1-40) in complex with the truncated Affibody dimers is very similar to the previously published solution structure of the Aß(1-40)-peptide in complex with the full-length ZAß3 Affibody molecule. This indicates that the N-terminally truncated Affibody molecules ZAß3 (12-58) and ZAß3 (15-58) are highly promising for further engineering and future use as binding agents to monomeric Aß(1-40).