Proline, a privileged fragment in drug design: advances and future perspectives

Proline, a unique cyclic α-amino acid with a pyrrolidine ring structure, plays a pivotal role in modulating the conformational properties and biological activity of peptides and small-molecule drugs. Its distinctive steric and electronic characteristics have been widely exploited in pharmaceutical design, contributing to enhanced binding affinity, receptor selectivity and metabolic stability. With six potential sites for chemical modification (N1, C2, C3, C4, C5, and C6), proline-based structures support a broad range of mono-, di-, tri-, poly-substituted, and spirocyclic or fused-ring architectures. In this review, we provide a systematic overview of 216 proline-containing bioactive molecules published in the last five years (2020 to 2025) to elucidate research advances and recent trends, and categorize them according to their substitution patterns and ring structures. A comprehensive summary of 58 marketed small-molecule drugs (including both chemical drugs and small peptides) incorporating proline fragments is also provided. This review highlights the versatility of proline as a privileged fragment in drug discovery and aims to provide insights that may inspire the rational design of novel drugs based on proline-containing frameworks.