Genomics-Guided Discovery of Thailanstatins A, B, and C As Pre-mRNA Splicing Inhibitors and Antiproliferative Agents from <i>Burkholderia thailandensis</i> MSMB43

Mining the genome sequence of <i>Burkholderia thailandensis</i> MSMB43 revealed a cryptic biosynthetic gene cluster resembling that of FR901464 (<b>4</b>), a prototype spliceosome inhibitor produced by <i>Pseudomonas</i> sp. No. 2663. Transcriptional analysis revealed a cultivation condition in which a regulatory gene of the cryptic gene cluster is adequately expressed. Consequently, three new compounds, named thailanstatins A (<b>1</b>), B (<b>2</b>), and C (<b>3</b>), were isolated from the fermentation broth of <i>B. thailandensis</i> MSMB43. Thailanstatins are proposed to be biosynthesized by a hybrid polyketide synthase–nonribosomal peptide synthetase pathway. They differ from <b>4</b> by lacking an unstable hydroxyl group and by having an extra carboxyl moiety; those differences endow thailanstatins with a significantly greater stability than <b>4</b> as tested in phosphate buffer at pH 7.4. <i>In vitro</i> assays showed that thailanstatins inhibit pre-mRNA splicing as potently as <b>4</b>, with half-maximal inhibitory concentrations in the single to sub-μM range. Cell culture assays indicated that thailanstatins also possess potent antiproliferative activities in representative human cancer cell lines, with half-maximal growth inhibitory concentrations in the single nM range. This work provides new chemical entities for research and development and new structure–activity information for chemical optimization of related spliceosome inhibitors