Syntheses of Anticancer Cyclic Peptides, Phakellistatin 12 & 13: A comparative study of various linkers and cyclization strategies employed in Solid-Phase Peptide Synthesis

Cyclic peptides are structurally diverse molecules exhibiting a broad spectrum of biological functions with inherent physiological advantages, ranging from insecticidal and antimicrobial to anti-inflammatory and immunosuppressing activities. The synthesis and study of cyclic peptide analogues thus presents a powerful advance in the design and discovery of novel peptide drugs. During the past 20 years, phakellistatins 1-18 have been isolated as prolin-rich cyclic peptides with potent activities against various cancer cell lines. The solid-phase total syntheses of two members of the series, phakellistatins 12 & 13, along with alanin-scanning analogues of phakellistatin 13 was carried out to verify the structure and bioactivities of the natural products. Interestingly, the synthetic peptides were found to be identical in structures to their natural counterparts, whereas the biological activity could not be reproduced. The present study is thus valuable for the readers interested in various methods of SPPS and structural characterization of peptides. Furthermore, the results of the present study highlight the potential uncertainty with the biologically active natural products.