The structures of ecteinascidin anticancer agents bound to DNA shed light on their mechanism of action.

Ecteinascidins constitute a family of alkaloid compounds, originally isolated from marine tunicates, that exhibit strong antitumor activity. They act through binding to the DNA minor groove and forming covalent adducts with guanine residues. However, the limited availability of structural data restricts mechanistic insights into their mode of action and hampers the discovery of novel compounds. We report crystal structures of duplex DNA adducts with first-, second-, and third-generation ecteinascidins. The structures show that trabectedin, lurbinectedin, and PM54 bind through their shared A- and B-subunits, forming a covalent bond with the N2 atom of guanine and an extensive network of noncovalent interactions, leading to significant minor groove widening. In contrast, their C-subunit, which differs across the compounds, establishes distinct contacts with the modified strand that affect binding strength and sequence specificity. These structural findings, supported by Förster resonance energy transfer and biochemical assays, reveal the molecular determinants underlying differential sequence selectivity and reactivity. Our results provide a mechanistic framework for the anticancer activity of ecteinascidins and a structural basis to guide the design of next-generation analogues with improved therapeutic potential.