Chemical Epitope Targeting: Review of a Novel Screening Technology

Chemical Epitope Targeting is a novel technology developed for designing peptide ligands with high affinity and specificity against specific regions of a protein that may be inaccessible to small molecules or antibodies. In this review, we summarize the key steps and significant applications of this technology. Operating on the same principles as antibody-antigen interactions, this technique involves chemically synthesizing the region of interest on the protein, called the epitope, as a polypeptide with a biotin detection tag and a strategically placed alkyne or azide presenting amino acid. The constructed epitope is screened against a comprehensive linear or cyclic One Bead One Compound library of the corresponding azide or alkyne presenting peptides with approximately 2 million unique members. Binders in the correct orientation undergo proximity catalyzed azide-alkyne cycloaddition reaction and are detected using this copper free in situ click chemistry. Subsequent binding assays against the full protein identify high affinity peptide binders with dissociation constants in the nanomolar range. These monoligand peptides can be further developed into biligands and triligands, larger macromolecules with two or three peptide ligands connected by linkers, which have improved binding affinity for continuous or discontinuous epitopes. Application of this technology has yielded protease-resistant and cell-permeable compounds for potential therapeutic and diagnostic purposes. In this review, we highlight the different Chemical Epitope Targeted ligands developed for a range of applications, from differential detection of biomarkers and receptor isoforms to the inhibition of enzyme functions and stabilization of protein folding states

Epitope Targeting of Tertiary Protein Structure Enables Target-Guided Synthesis of a Potent In-Cell Inhibitor of Botulinum Neurotoxin

Botulinum neurotoxin (BoNT) serotype A is the most lethal known toxin and has an occluded structure, which prevents direct inhibition of its active site before it enters the cytosol. Target-guided synthesis by in situ click chemistry is combined with synthetic epitope targeting to exploit the tertiary structure of the BoNT protein as a landscape for assembling a competitive inhibitor. A substrate-mimicking peptide macrocycle is used as a direct inhibitor of BoNT. An epitope-targeting in situ click screen is utilized to identify a second peptide macrocycle ligand that binds to an epitope that, in the folded BoNT structure, is active-site-adjacent. A second in situ click screen identifies a molecular bridge between the two macrocycles. The resulting divalent inhibitor exhibits an in vitro inhibition constant of 165 pM against the BoNT/A catalytic chain. The inhibitor is carried into cells by the intact holotoxin, and demonstrates protection and rescue of BoNT intoxication in a human neuron model

Iterative in Situ Click Chemistry Assembles a Branched Capture Agent and Allosteric Inhibitor for Akt1

We describe the use of iterative in situ click chemistry to design an Akt-specific branched peptide triligand that is a drop-in replacement for monoclonal antibodies in multiple biochemical assays. Each peptide module in the branched structure makes unique contributions to affinity and/or specificity resulting in a 200 nM affinity ligand that efficiently immunoprecipitates Akt from cancer cell lysates and labels Akt in fixed cells. Our use of a small molecule to preinhibit Akt prior to screening resulted in low micromolar inhibitory potency and an allosteric mode of inhibition, which is evidenced through a series of competitive enzyme kinetic assays. To demonstrate the efficiency and selectivity of the protein-templated in situ click reaction, we developed a novel QPCR-based methodology that enabled a quantitative assessment of its yield. These results point to the potential for iterative in situ click chemistry to generate potent, synthetically accessible antibody replacements with novel inhibitory properties

In situ click chemistry: from small molecule discovery to synthetic antibodies

Advances in the fields of proteomics, molecular imaging, and therapeutics are closely linked to the availability of affinity reagents that selectively recognize their biological targets. Here we present a review of Iterative Peptide In Situ Click Chemistry (IPISC), a novel screening technology for designing peptide multiligands with high affinity and specificity. This technology builds upon in situ click chemistry, a kinetic target-guided synthesis approach where the protein target catalyzes the conjugation of two small molecules, typically through the azide–alkyne Huisgen cycloaddition. Integrating this methodology with solid phase peptide libraries enables the assembly of linear and branched peptide multiligands we refer to as Protein Catalyzed Capture Agents (PCC Agents). The resulting structures can be thought of as analogous to the antigen recognition site of antibodies and serve as antibody replacements in biochemical and cell-based applications. In this review, we discuss the recent progress in ligand design through IPISC and related approaches, focusing on the improvements in affinity and specificity as multiligands are assembled by target-catalyzed peptide conjugation. We compare the IPISC process to small molecule in situ click chemistry with particular emphasis on the advantages and technical challenges of constructing antibody-like PCC Agents

A protein-targeting strategy used to develop a selective inhibitor of the E17K point mutation in the PH domain of Akt1

Ligands that can bind selectively to proteins with single amino-acid point mutations offer the potential to detect or treat an abnormal protein in the presence of the wild type (WT). However, it is difficult to develop a selective ligand if the point mutation is not associated with an addressable location, such as a binding pocket. Here we report an all-chemical synthetic epitope-targeting strategy that we used to discover a 5-mer peptide with selectivity for the E17K-transforming point mutation in the pleckstrin homology domain of the Akt1 oncoprotein. A fragment of Akt1 that contained the E17K mutation and an I19[propargylglycine] substitution was synthesized to form an addressable synthetic epitope. Azide-presenting peptides that clicked covalently onto this alkyne-presenting epitope were selected from a library using in situ screening. One peptide exhibits a 10:1 in vitro selectivity for the oncoprotein relative to the WT, with a similar selectivity in cells. This 5-mer peptide was expanded into a larger ligand that selectively blocks the E17K Akt1 interaction with its PIP3 (phosphatidylinositol (3,4,5)-trisphosphate) substrate

A Chemical Epitope-Targeting Strategy for Protein Capture Agents: The Serine 474 Epitope of the Kinase Akt2

Target and click: Peptide ligands targeted to the C-terminal motif of the kinase Akt2 were obtained by combining phosphate recognition of a dinuclear zinc(II) complex with in situ click chemistry to target this epitope. The peptide ligands (shown as XXXXX) selectively bind the C-terminal polypeptide of Akt2, and are selective for Akt2 relative to the Akt1 and Akt3 isoforms. The ligands differentially modulate Akt2 activity

Iterative in situ click chemistry creates antibody-like protein-capture agents

Iterative in situ click chemistry (see scheme for the tertiary ligand screen) and the one-bead-one-compound method for the creation of a peptide library enable the fragment-based assembly of selective high-affinity protein-capture agents. The resulting ligands are water-soluble and stable chemically, biochemically, and thermally. They can be produced in gram quantities through copper (I)-catalyzed cycloaddition

A General Synthetic Approach for Designing Epitope Targeted Macrocyclic Peptide Ligands

We describe a general synthetic strategy for developing high-affinity peptide binders against specific epitopes of challenging protein biomarkers. The epitope of interest is synthesized as a polypeptide, with a detection biotin tag and a strategically placed azide (or alkyne) presenting amino acid. This synthetic epitope (SynEp) is incubated with a library of complementary alkyne or azide presenting peptides. Library elements that bind the SynEp in the correct orientation undergo the Huisgen cycloaddition, and are covalently linked to the SynEp. Hit peptides are tested against the full-length protein to identify the best binder. We describe development of epitope-targeted linear or macrocycle peptide ligands against 12 different diagnostic or therapeutic analytes. The general epitope targeting capability for these low molecular weight synthetic ligands enables a range of therapeutic and diagnostic applications, similar to those of monoclonal antibodies