N-Alkylated cyclic peptides represent an attractive modality for targeting intracellular PPI, but the lack of sufficient synthetic method has prevented progress in the development of this modality. We established a versatile and durable method for synthesizing highly N-alkylated drug-like cyclic peptides. This is the first reported method for synthesizing such peptides in parallel with a high success rate and acceptable purity that does not require optimizations for a particular sequence.

Using this newly established method, we successfully synthesized thousands of cyclic peptides to gain a detailed understanding of the drug-likeness of cyclic peptides, and to innovate the whole drug discovery process from hit generation to lead optimization. As a result, we discovered the clinical cyclic peptide LUNA18 (RAS inhibitor) and showed the value of cyclic peptides as a medium molecule modality.

In the course of this study, we set up each reaction condition by overcoming the three main issues reported in previous literatures: (1) diketopiperazine (DKP) formation as a side reaction during Fmoc removal, (2) insufficient amide bond formation due to the steric hindrance of the N-Me amino acid, and (3) instability of N-Me rich peptides under acidic conditions. In this presentation, we will discuss how to overcome those issues by the established conditions.