Major Progress in the Liquid-Phase Synthesis of <i>N</i> -Alkyl-Rich Cyclic Peptides: Kilogram-Scale GMP Production of the KRAS Inhibitor LUNA18

This article describes the development of a liquid-phase peptide synthesis (LPPS) process for LUNA18 (paluratide), an N-alkyl-rich cyclic undecapeptide with KRAS inhibitory activity. Departing from conventional solid-phase peptide synthesis, we developed a high-yielding LPPS process by addressing key challenges: (1) low reactivity caused by steric hindrance of the N-alkylated amino acids, (2) incomplete hydrolysis of excess active esters during workup, (3) instability of N-alkyl-rich peptide backbone under acidic conditions, and (4) diketopiperazine formation in N-deprotected intermediates. Through a convergent synthetic route consisting of 24 telescoped chemical transformation steps followed by a final crystallization step, LUNA18 was synthesized with >98.5% purity (HPLC-UV) and >30% overall yield. This process enabled rapid supply of high-purity active pharmaceutical ingredients at the kilogram scale under Good Manufacturing Practice conditions, demonstrating the utility and scalability of our approach for N-alkyl-rich cyclic peptides.