Amphotericin B (AmB) is among the most effective antifungal drugs for treating invasive fungal infections but causes severe host toxicity. This is due to poor selectivity for the fungal-specific target ergosterol over the mammalian off-target cholesterol, and the drug’s tendency to aggregate. After decades of unsuccessful efforts, Maji et al recently developed AmB variant, Am2-19 with reduceds AmB toxicity (Maji et al., Nature, 2023). While the detailed mechanism of Am2-19 remains unclear, data suggests that Am2-19 improves the kinetics of sterol binding, resulting in selective increase of ergosterol sequestration from fungal cells. Differences in the aggregation behaviour of Am2-19 relative to AmB may play a critical role in elucidating the mechanism, as well as aiding the development of stable formulations.
We use Molecular Dynamics simulations to show that Am2-19 aggregates significantly slower than AmB, and that the resulting aggregates are less dense than those formed by AmB. This is due to improved solvation of Am2-19 resulting in aggregates with higher water content. We also show that the looser aggregates formed by Am2-19 enables sterol contacts to form much faster than with AmB. Specifically, amidation in Am2-19, reported to play a key role in ergosterol sequestration kinetics, drives this faster contact formation and higher water content. Together, our findings suggest a mechanism whereby Am2-19 exhibits improved sterol binding kinetics due to the formation of looser aggregates, resulting in a reduction in the kinetic trapping for cholesterol extracted from host cells. Our findings add to our understanding of Am2-19’s reduced toxicity and may have implications for developing suitable formulations.