Identification of Covalent Cyclic Peptide Inhibitors in mRNA Display.

Peptides have historically been underutilized for covalent inhibitor discovery, despite their unique abilities to interact with protein surfaces and interfaces. This is in part due to a lack of methods for screening and identifying covalent peptide ligands. Here, we report a method to identify covalent cyclic peptide inhibitors in mRNA display. We combine co- and post-translational library diversification strategies to create cyclic libraries with reactive dehydroalanines (Dhas), which we employ in selections against two model targets. The most potent hits exhibit low nanomolar inhibitory activities and disrupt known protein-protein interactions with their selected targets. Overall, we establish Dhas as electrophiles for covalent inhibition and showcase how separate library diversification methods can work synergistically to dispose mRNA display to novel applications like covalent inhibitor discovery.

Enabling Genetic Code Expansion and Peptide Macrocyclization in mRNA Display via a Promiscuous Orthogonal Aminoacyl-tRNA Synthetase.

mRNA display is revolutionizing peptide drug discovery through its ability to quickly identify potent peptide binders of therapeutic protein targets. Methods to expand the chemical diversity of display libraries are continually needed to increase the likelihood of identifying clinically relevant peptide ligands. Orthogonal aminoacyl-tRNA synthetases (ORSs) have proven utility in cellular genetic code expansion, but are relatively underexplored for in vitro translation (IVT) and mRNA display. Herein, we demonstrate that the promiscuous ORS p-CNF-RS can incorporate noncanonical amino acids at amber codons in IVT, including the novel substrate p-cyanopyridylalanine (p-CNpyrA), to enable a pyridine-thiazoline (pyr-thn) macrocyclization in mRNA display. Pyr-thn-based selections against the deubiquitinase USP15 yielded a potent macrocyclic binder that exhibits good selectivity for USP15 and close homologues over other ubiquitin-specific proteases (USPs). Overall, this work exemplifies how promiscuous ORSs can both expand side chain diversity and provide structural novelty in mRNA display libraries through a heterocycle forming macrocyclization.

mRNA Display Reaches for the Clinic with New PCSK9 Inhibitor.

Merck & Co. recently reported one of the first mRNA display-derived clinical candidates in a bioavailable inhibitor of proprotein convertase subtilisin/kexin type 9 (PCSK9). Herein, we discuss the chemical and pharmacological challenges surmounted in bringing this compound to trials and the current outlook for mRNA display-based therapeutic development.

Expanding the Chemical Diversity of Genetically Encoded Libraries.

The power of ribosomes has increasingly been harnessed for the synthesis and selection of molecular libraries. Technologies, such as phage display, yeast display, and mRNA display, effectively couple genotype to phenotype for the molecular evolution of high affinity epitopes for many therapeutic targets. Genetic code expansion is central to the success of these technologies, allowing researchers to surpass the intrinsic capabilities of the ribosome and access new, genetically encoded materials for these selections. Here, we review techniques for the chemical expansion of genetically encoded libraries, their abilities and limits, and opportunities for further development. Importantly, we also discuss methods and metrics used to assess the efficiency of modification and library diversity with these new techniques.