Total synthesis and absolute stereochemistry of the proteasome inhibitors cystargolides A and B

Cryptic enzymatic assembly of peptides armed with β-lactone warheads

Nature has evolved biosynthetic pathways to molecules possessing reactive warheads that inspired the development of many therapeutic agents, including penicillin antibiotics. Peptides armed with electrophilic warheads have proven to be particularly effective covalent inhibitors, providing essential antimicrobial, antiviral and anticancer agents. Here we provide a full characterization of the pathways that nature deploys to assemble peptides with β-lactone warheads, which are potent proteasome inhibitors with promising anticancer activity. Warhead assembly involves a three-step cryptic methylation sequence, which is likely required to reduce unfavorable electrostatic interactions during the sterically demanding β-lactonization. Amide-bond synthetase and adenosine triphosphate (ATP)-grasp enzymes couple amino acids to the β-lactone warhead, generating the bioactive peptide products. After reconstituting the entire pathway to β-lactone peptides in vitro, we go on to deliver a diverse range of analogs through enzymatic cascade reactions. Our approach is more efficient and cleaner than the synthetic methods currently used to produce clinically important warhead-containing peptides.

Cystargolide-based amide and ester Pz analogues as proteasome inhibitors and anti-cancer agents

A series of cystargolide-based β-lactone analogues containing nitrogen atoms at the Pz portion of the scaffold were prepared and evaluated as proteasome inhibitors, and for their cytotoxicity profile toward several cancer cell lines. Inclusion of one, two or even three nitrogen atoms at the Pz portion of the cystargolide scaffold is well tolerated, producing analogues with low nanomolar proteasome inhibition activity, in many cases superior to carfilzomib. Additionally, analogue 8g, containing an ester and pyrazine group at Pz, was shown to possess significant activity toward RPMI 8226 cells (IC₅₀ = 21 nM) and to be less cytotoxic toward the normal tissue model MCF10A cells than carfilzomib.

Global analysis of biosynthetic gene clusters reveals conserved and unique natural products in entomopathogenic nematode-symbiotic bacteria

Microorganisms contribute to the biology and physiology of eukaryotic hosts and affect other organisms through natural products. Xenorhabdus and Photorhabdus (XP) living in mutualistic symbiosis with entomopathogenic nematodes generate natural products to mediate bacteria–nematode–insect interactions. However, a lack of systematic analysis of the XP biosynthetic gene clusters (BGCs) has limited the understanding of how natural products affect interactions between the organisms. Here we combine pangenome and sequence similarity networks to analyse BGCs from 45 XP strains that cover all sequenced strains in our collection and represent almost all XP taxonomy. The identified 1,000 BGCs belong to 176 families. The most conserved families are denoted by 11 BGC classes. We homologously (over)express the ubiquitous and unique BGCs and identify compounds featuring unusual architectures. The bioactivity evaluation demonstrates that the prevalent compounds are eukaryotic proteasome inhibitors, virulence factors against insects, metallophores and insect immunosuppressants. These findings explain the functional basis of bacterial natural products in this tripartite relationship.

Entomopathogenic nematodes carrying Xenorhabdus and Photorhabdus bacteria prey on insect larvae in the soil. Now, a comprehensive analysis of the bacterial genome has revealed ubiquitous and unique families of biosynthetic gene clusters. Evaluation of the bioactivity of the natural products expressed by the most prevalent cluster families explains the functional basis of bacterial natural products involved in bacteria–nematode–insect interactions.

Natural product scaffolds as inspiration for the design and synthesis of 20S human proteasome inhibitors

The 20S proteasome is a valuable target for the treatment of a number of diseases including cancer, neurodegenerative disease, and parasitic infection. In an effort to discover novel inhibitors of the 20S proteasome, many reseaarchers have looked to natural products as potential leads for drug discovery. The following review discusses the efforts made in the field to isolate and identify natural products as inhibitors of the proteasome. In addition, we describe some of the modifications made to natural products in order to discover more potent and selective inhibitors for potential disease treatment.

Chemically synthesized LYRM03 could inhibit the metastasis of human breast cancer MDA-MB-231 cells in vitro and in vivo

Aminopeptidase N (APN) belongs to the aminopeptidase family, which is widely distributed throughout the animal and plant kingdoms. APN is thought to be a very important target for cancer therapy as it is linked to cancer progression and metastasis. However, bestatin (Ubenimex) is the only approved drug that targets various aminopeptidases for the treatment of acute myelocytic leukemia and lymphedema. A compound 3-amino-2-hydroxy-4-phenylbutanoylvalylisoleucine (also known as LYRM03), isolated from a Streptomyces strain HCCB10043, exhibited more potent inhibitory activity than bestatin. In this work, we applied a chemical synthesis strategy to generate LYRM03 to overcome the low yields typically achieved from fermentation. Finally, we explored a suite of experiments to determine the bioactivity of LYRM03 and revealed that the metastasis of MDA-MB-231 cells was significantly restrained with LYRM03 treatment or injection both in vitro and in vivo. Because of its anti-metastasis capacity, further structure modifications of LYRM03 will be of interest for its use alone or in combination as a therapy in cancer.

Built to bind: biosynthetic strategies for the formation of small-molecule protease inhibitors

The discovery and characterization of natural product protease inhibitors has inspired the development of numerous pharmaceutical agents.

Design, synthesis, and evaluation of cystargolide-based β-lactones as potent proteasome inhibitors

The peptidic β-lactone proteasome inhibitors (PIs) cystargolides A and B were used to conduct structure-activity relationship (SAR) studies in order to assess their anticancer potential. A total of 24 different analogs were designed, synthesized and evaluated for proteasome inhibition, for cytotoxicity towards several cancer cell lines, and for their ability to enter intact cells. X-ray crystallographic analysis and subunit selectivity was used to determine the specific subunit binding associated with the structural modification of the β-lactone (P₁), peptidic core, (P_x and P_y), and end-cap (P_z) of our scaffold. The cystargolide derivative 5k, structurally unique at both P_y and P₁, exhibited the most promising inhibitory activity for the β5 subunit of human proteasomes (IC₅₀ = 3.1 nM) and significant cytotoxicity towards MCF-7 (IC₅₀ = 416 nM), MDA-MB-231 (IC₅₀ = 74 nM) and RPMI 8226 (IC₅₀ = 41 nM) cancer cell lines. Cellular infiltration assays revealed that minor structural modifications have significant effects on the ability of our PIs to inhibit intracellular proteasomes, and we identified 5k as a promising candidate for continued therapeutic studies. Our novel drug lead 5k is a more potent proteasome inhibitor than carfilzomib with mid-to-low nanomolar IC₅₀ measurements and it is cytotoxic against multiple cancer cell lines at levels approaching those of carfilzomib.

A total synthesis of (-)-Hortonone C

A total synthesis of the cytotoxic terpenoid hortonone C was accomplished and its absolute stereochemistry confirmed. Intermediate (+)-4 was synthesized using either an asymmetric conjugate addition strategy, or by elaboration of the Hajos-Parrish ketone. Reduction of (+)-4 under dissolving-metal conditions and trapping the enolate intermediate served to control the cis-stereochemistry at the ring fusion and provide a silyl enol ether necessary for ring expansion. Comparison of optical rotation data confirmed that the absolute configuration of natural hortonone C is (6S,7S,10S).