Exploring Passive Permeability Profiles of Cyclic Heptapeptide Chemical Space Uncovers Bioactivity of Mortiamide Scaffold Driven by Colloidal Aggregation

We investigated the passive permeability and cytotoxicity of cyclic heptapeptides based on the mortiamide family of natural products. Of all possible stereoisomeric backbones, the natural product's scaffold was among the two most lipophilic as measured by hydrocarbon-water partition coefficients. Using one-bead-one-compound synthesis, we generated a ∼66,000-member library based on the mortiamide scaffold and identified numerous compounds that showed low micromolar cytotoxicity against synovial sarcoma and breast cancer cell lines. Physicochemical characterization revealed that these compounds, including the known mortiamides, have very low aqueous solubilities and form amyloid-like fibril aggregates. Multiple lines of evidence-including detergent-reversible enzyme inhibition, thioflavin T fluorescence, transmission electron microscopy, and equipotent enantiomeric pairs-demonstrate that the observed bioactivity arises from colloidal aggregation rather than specific target engagement. Our findings highlight the critical importance of early physicochemical evaluation in natural product-inspired drug discovery and underscore how aggregation-prone scaffolds can generate misleading structure-activity relationships.