Development of lead structures that are cytotoxic and are derived from macrocycle natural products

This thesis describes the development of lead structures from two macrocyclic peptide-based natural products: Sansalvamide A (San A) and Sanguinamide B (San B). San A, a macrocyclic depsipeptide, exhibits a low micromolar cytotoxicity against 60 NCI-cancer cell lines. Exchanging the ester with an amide improves the potency of San A by 10-fold. The San A’s biological target is heat shock protein 90 (hsp90). Hsp90, a major molecular chaperone responsible for the folding and stabilization of unfolded proteins, plays a pivotal role in cancer cell survival. My first project involved synthesizing peptidomimetic analogues of San A peptide in order to improve its cytotoxicity. I incorporated thiazoles and oxazoles into the San A’s backbone, rigidifying the macrocycle. These macrocycles were anticipated to have enhanced binding affinity to hsp90 compared to the peptide-based macrocycles. However, the peptidomimetics were very poor hsp90 inhibitors, thereby indicating that the peptide inhibitors have a unique 3D conformation that is vital for activity. Thus our focus turned to other routes to enhance binding affinity to hsp90. Since hsp90 functions as a dimer, and has two binding sites for each inhibitor, my second project developed San A-based dimeric inhibitors of hsp90. Tethering two San A units with a PEG linker, created a dimeric inhibitor. The appropriate linker length was based on the published crystal structures of hsp90. The results showed that dimerization of the San A inhibitors enhanced the inhibitory effect by two-fold compared to monomer, and the best linker length was 9 PEG units. Sanguinamide B (San B), a heterocycle-containing macrocyclic peptide, has two thiazoles, one oxazole, and five hydrophobic amino acids along its macrocyclic backbone. San B possesses some unqiue structural features that made the molecule synthetically interesting: a bis-azole moiety and dual prolines. My final project involved establishing the structure-activity relationship (SAR) of San B against colon cancer cells and bacteria. An amino acid scan was performed on three peptide positions, including an N-Me amino acid, a glycine, an L- and D-phenylalanine. The results showed that the aromatic residues (L- or D-Phe) locked the macrocycle into a single conformer, and showed promising biological activity