Mechanistic Insights from Differences in the Aggregate Morphologies of Amphotericin B and Its Less Toxic Variant Am-2-19 (Turletricin)

Amphotericin B (AmB) is a potent, life-saving antifungal, but its clinical use is limited by toxicity due to poor selectivity for ergosterol over cholesterol. The AmB derivative Am-2-19 (turletricin) exhibits improved ergosterol selectivity and reduced toxicity, which is attributed to enhanced ergosterol extraction kinetics via C16-amidation rather than altered sterol binding affinity. Using all-atom molecular dynamics simulations, we reveal key differences in the aggregate morphologies of AmB and Am-2-19 that provide mechanistic insights into the increased ergosterol extraction kinetics. In both the absence and presence of sterols, Am-2-19 aggregates slower than AmB, forming less dense and more solvated aggregates, driven by the hydrogen-bonding capacity of the C16-amidation. <i>In vivo</i>, these looser aggregates likely enable a greater exchange of sterols, increasing the likelihood of cholesterol-ergosterol exchange. Our findings provide the first atomistic-level insight into Am-2-19's mechanism, emphasizing the critical role of aggregation in structure-activity relationships in a physiologically relevant solution environment.