The Carmaphycins: New Proteasome Inhibitors Exhibiting an α,β‐Epoxyketone Warhead from a Marine Cyanobacterium

Abstract

Two new peptidic proteasome inhibitors were isolated as trace components from a Curaçao collection of Symploca sp. marine cyanobacteria. Carmaphycin A (1) and carmaphycin B (2) feature a leucine-derived α, β -epoxyketone warhead directly connected to either methionine sulfoxide or methionine sulfone. Their structures were elucidated on the basis of extensive NMR/MS analyses and confirmed by total synthesis, which in turn provided more material for further biological evaluations. Pure carmaphycins A and B were found to inhibit the β5 subunit (chymotrypsin-like activity) of the S. cerevisiae 20S proteasome in the low nanomolar range. Additionally, they exhibited strong cytotoxicity to lung and colon cancer cell lines, as well as exquisite antiproliferative effects in the NCI60 cell line panel. These assay results as well as initial structural biology studies suggest a distinctive binding mode for these new inhibitors.

Molecular Targets of Anticancer Agents from Filamentous Marine Cyanobacteria

“…27.7) having potent anti-proteasome properties were isolated in small amounts from Symploca sp. obtained from CARMABI beach, Curacao [88]. These compounds are structurally related to the known proteasome inhibitor, epoxomicin, in having a terminal α,β-epoxyketone moiety and a methionine sulfoxide in 40 or methionine sulfone in 41.…”

“…These compounds are structurally related to the known proteasome inhibitor, epoxomicin, in having a terminal α,β-epoxyketone moiety and a methionine sulfoxide in 40 or methionine sulfone in 41. As part of the drug development process, their total synthesis was also accomplished using an efficient and scalable convergent method [88]. The inhibitory properties of carmaphycins A and B were evaluated against Saccharomyces cerevisiae 20S proteasome and found to possess comparable IC 50 values of 2.5 nM and 2.6 nM, respectively.…”

Biological targets and mechanisms of action of natural products from marine cyanobacteria

“…6). 123 These compounds are characterized by a tripeptide moiety fused to a hexanoic acid and α,β-epoxyketone on the N- and C-terminal ends, respectively. Insights into the MOA of the modified peptides carmaphycins A and B were obtained from the structural similarity of these compounds with the epoxomicin class of compounds.…”

“…Insights into the MOA of the modified peptides carmaphycins A and B were obtained from the structural similarity of these compounds with the epoxomicin class of compounds. 123 Carmaphycins and epoxomicins are characterized by an α,β-epoxyketone that has been demonstrated to be one of the key features in the latter for inhibiting the proteasome, forming a covalent bond with the catalytic Thr residue of the β5 subunit. 121 Carmaphycins inhibited the chymotrypsin-like (β5 subunit) activity of the 20S proteasome, with comparable potency to epoxomicin, culminating in potent inhibition of the growth of cancer cells, particularly those which harbor KRAS/tp53 mutations.…”

“…121 Carmaphycins inhibited the chymotrypsin-like (β5 subunit) activity of the 20S proteasome, with comparable potency to epoxomicin, culminating in potent inhibition of the growth of cancer cells, particularly those which harbor KRAS/tp53 mutations. 123 Using the X-ray crystal structure of the 20S proteasome derived from the complex with epoxomicins, the molecular interactions of carmaphycins were modeled. 123 Extensive hydrogen bonding, van der Waals and solvent interactions were observed.…”