The spliceostatin class of natural products was reported to be potent cytotoxic agents via inhibition of the spliceosome, a key protein complex in the biosynthesis of mature mRNA. As part of an effort to discover novel leads for cancer chemotherapy, we re-examined this class of compounds from several angles, including fermentation of the producing strains, isolation and structure determination of new analogues, and semisynthetic modification. Accordingly, a group of spliceostatins were isolated from a culture broth of Burkholderia sp. FERM BP-3421, and their structures identified by analysis of spectroscopic data. Semisynthesis was performed on the major components 4 and 5 to generate ester and amide derivatives with improved in vitro potency. With their potent activity against tumor cells and unique mode of action, spliceostatins can be considered potential leads for development of cancer drugs.

Significance Spliceostatins are bacterial natural products that show promising anticancer activity. Understanding how the bacterium makes spliceostatins will aid efforts toward a sustainable route for their production. Moreover, altering the chemical structure of a natural product is usually necessary to improve its pharmaceutical properties. For example, the parent spliceostatin molecule contains an unstable hemiketal chemical group. Contrary to previous hypotheses, we report on the identification of a dioxygenase enzyme responsible for hemiketal biosynthesis. Deletion of the corresponding dioxygenase gene led to a strain that produces exclusively spliceostatin congeners that are more stable than, and as active as, the parent compound, when derivatized to increase cell permeability. The strain generated in this study will be the basis for future development.

As part of our efforts to develop new classes of tubulin inhibitor payloads for antibody-drug conjugate (ADC) programs, we developed a tubulysin ADC that demonstrated excellent in vitro activity but suffered from rapid metabolism of a critical acetate ester. A two-pronged strategy was employed to address this metabolism. First, the hydrolytically labile ester was replaced by a carbamate functional group resulting in a more stable ADC that retained potency in cellular assays. Second, site-specific conjugation was employed in order to design ADCs with reduced metabolic liabilities. Using the later approach, we were able to identify a conjugate at the 334C position of the heavy chain that resulted in an ADC with considerably reduced metabolism and improved efficacy. The examples discussed herein provide one of the clearest demonstrations to-date that site of conjugation can play a critical role in addressing metabolic and PK liabilities of an ADC. Moreover, a clear correlation was identified between the hydrophobicity of an ADC and its susceptibility to metabolic enzymes. Importantly, this study demonstrates that traditional medicinal chemistry strategies can be effectively applied to ADC programs.

Theopapuamide (1), a new cytotoxic peptide has been isolated from the lithistid sponge Theonella swinhoei from Papua New Guinea. The structure was established by analysis of NMR, mass spectrometry and chemical methods. The undecapeptide (1) contains several unusual amino acid residues, of which the occurrence of β-methoxyasparagine and 4-amino-5-methyl-2,3,5-trihydroxyhexanoic acid (Amtha) is unprecedented in natural peptides. Compound 1 also contains a amide linked fatty acid moiety, 3-hydroxy-2,4,6-trimethyl-octanoic acid (Htoa). Theopapuamide (1) was cytotoxic against CEM-TART and HCT-116 cell lines with EC 50 values of 0.5 μM and 0.9 μM respectively.Marine sponges have proven to be a significant source of biologically active cyclic peptides and depsipeptides. 1 Among these sponge depsipeptides, callipeltin A (from New Caledonian sponge Callipelta sp.), 2 neamphamide A (from a Papua New Guinea sponge Neamphius huxleyi) 3 and Papuamides A-B (from Papua New Guinea sponges Theonella mirabilis and Theonella swinhoei) 4 are well known for their potent HIV-inhibitory activity and their structurally unique features incorporating several modified amino acid residues. For instance, the atypical amino acid residues 3,4-dimethyl-L-glutamine and β-methoxytyrosine are common to all of the above mentioned marine depsipeptides but to date have not been described elsewhere. The rarity of these atypical amino acid residues has inspired their chemical synthesis. 5 The structural diversity found among lithistid sponge metabolites (genus Theonella and Callipelta) has been attributed to symbiotic microorganisms. 1 As part of our continuing studies on Theonella swinhoei from Papua New Guinea, 6 the aqueous CH 3 CN extract of the sponge was analyzed and proved active in an in vitro anti-HIV assay. Fractionation of the active extract resulted in the isolation of a new cyclic depsipeptide, theopapuamide (1). This paper describes the isolation, structure elucidation, and stereochemical analysis of theopapuamide (1).The crude aqueous CH 3 CN extract of T. swinhoei (family Theonellidae) was concentrated under vacuum and fractionated by C18 flash column chromatography. Further purification on Diaion HP-20 resin followed by CN-HPLC afforded the new cyclic undecapeptide, *To whom correspondence should be addressed. Tel: (801) In conjunction with NMR analysis, the gross structure elucidation of 1 was guided by standard amino acid analysis, 9 which revealed molar concentrations of ~1:2:1 for Asx, Thr and Leu respectively. The presence of two N-methylated amino acid residues were suggested based on the characteristic 1 H and 13 C chemical shifts of the N-methyl groups at δ H 2.81 (δ C 30.8) and δ H 2.88 (δ C 31.8). HMBC correlations were used to identify these N-methylated amino acids as NMeLeu and NMeGln, respectively. Additionally, the presence of a methoxy-bearing amino acid residue was suggested by the characteristic 1 H and 13 C chemical shifts of the O-methyl group at δ H 3.34 (δ C 60.3). Based on HMBC correlations, the methoxy...

[structure: see text]. A new alkyl-substituted nonenolide, microcarpalide 1, has been isolated from fermentation broths of an unidentified endophytic fungus. Microcarpalide is weakly cytotoxic to mammalian cells and acts as a microfilament disrupting agent. The structure of 1 was elucidated by application of spectroscopic methods. The absolute configuration was determined by the exciton chirality method.