Targeting the Eph System with Peptides and Peptide Conjugates

Abstract

Eph receptor tyrosine kinases and ephrin ligands constitute an important cell communication system that controls development, tissue homeostasis and many pathological processes. Various Eph receptors/ephrins are present in essentially all cell types and their expression is often dysregulated by injury and disease. Thus, the 14 Eph receptors are attracting increasing attention as a major class of potential drug targets. In particular, agents that bind to the extracellular ephrin-binding pocket of these receptors show promise for medical applications. This pocket comprises a broad and shallow groove surrounded by several flexible loops, which makes peptides particularly suitable to target it with high affinity and selectivity. Accordingly, a number of peptides that bind to Eph receptors with micromolar affinity have been identified using phage display and other approaches. These peptides are generally antagonists that inhibit ephrin binding and Eph receptor/ephrin signaling, but some are agonists mimicking ephrin-induced Eph receptor activation. Importantly, some of the peptides are exquisitely selective for single Eph receptors. Most identified peptides are linear, but recently the considerable advantages of cyclic scaffolds have been recognized, particularly in light of potential optimization towards drug leads. To date, peptide improvements have yielded derivatives with low nanomolar Eph receptor binding affinity, high resistance to plasma proteases and/or long in vivo half-life, exemplifying the merits of peptides for Eph receptor targeting. Besides their modulation of Eph receptor/ephrin function, peptides can also serve to deliver conjugated imaging and therapeutic agents or various types of nanoparticles to tumors and other diseased tissues presenting target Eph receptors.

A small peptide promotes EphA2 kinase-dependent signaling by stabilizing EphA2 dimers

“…The YSA peptide (YSAYPDSVPMMS) binds specifically to EphA2(41). Prior work with this peptide utilized a biotinylated variant containing a C-terminal GSGSK linker through which biotin was attached(40).…”

“…Prior work with this peptide utilized a biotinylated variant containing a C-terminal GSGSK linker through which biotin was attached(40). The biotinylated YSA peptide has a higher antagonistic potency than the unmodified YSA, with a dissociation constant ( K D ) of ~200 nM measured by surface plasmon resonance(40),(41). Thus, we have used the YSA peptide with the GSGSK linker (YSAYPDSVPMMSGSGSK).…”

“…Using phage display, we previously identified two dodecameric peptides that bind selectively to EphA2 with low micromolar affinity (40, 41). These peptides interact with the ephrin-binding pocket in the ligand-binding domain of EphA2 and compete with ephrin ligands for binding (40, 42).…”

Evaluation of EphA2 and EphB4 as Targets for Image-Guided Colorectal Cancer Surgery

“…Targeting of Eph receptors for tumor imaging purposes could be achieved using various agents, such as peptides or antibodies [23,24]. Antibodies offer the advantage of high specificity and affinity and, thanks to molecular engineering, recently even bi-and tri-specific antibodies have been developed, which could enhance tumor specificity [13,25,26].…”

Modifications of a Nanomolar Cyclic Peptide Antagonist for the EphA4 Receptor To Achieve High Plasma Stability

“…2,6,8 Among the EphA4-targeting peptides identified using phage display screens and other strategies, the linear dodecameric KYL (KYLPYWPVLSSL) was initially considered the most promising. 2,9 Although the ephrin ligands promiscuously bind multiple Eph receptors, KYL binds only to EphA4. Despite its modest (∼1 μM) binding affinity, KYL has been instrumental not only as a research tool but also in demonstrating the potential therapeutic value of pharmacologically inhibiting EphA4 in in vitro and in vivo models of ALS, Alzheimer's disease, spinal cord injury, and breast cancer.…”