How is cyclosporin A able to traverse through membranes and achieve high oral bioavailability despite having a molecular weight that would usually predispose others to a less remarkable fate? Its cyclic backbone, lipophilicity and pattern of N-methylation are partly responsible. These structural features have been more generally exploited to enhance membrane permeability of other small cyclic peptides. For example, I have showed that N-methylation at specific backbone sites, as guided by NMR, can result in peptides that have small-molecule-like oral bioavailability. Here, I will present recent results on the role of dynamics, a feature that is less well understood and difficult to study using chemical methods because the factors underpinning membrane permeability are strongly interdependent. I have used extended time frame molecular dynamics simulations to observe and manipulate the conformational behavior of cyclosporin A (as well as other cyclic peptides) in explicit water, chloroform, water/chloroform mixture, cyclohexane, and in the presence of a POPC lipid bilayer. The simulations are validated against NMR experiments, measuring conformational exchange, nuclear spin relaxation, and structures in matched solutions, such as DPC micelles, to mimic the lipid environment. I have found that conformational dynamics is a key determinant of the membrane permeability of cyclic peptides, and affects all stages of passage across membranes. Not only does dynamics enable cyclic peptides to transition into conformations that have physicochemical properties that favor membrane insertion but also enable them to wiggle their way through the dynamic polymer-like network of the membrane core.
References
Wang CK, et al. (2014) Rational design and synthesis of an orally bioavailable peptide guided by NMR amide temperature coefficients. Proc Natl Acad Sci USA 111:17504-17509; Wang CK, et al. (2015) Exploring experimental and computational markers of cyclic peptides: Charting islands of permeability. Eur J Med Chem 97:202-213. Wang CK, et al. (2016) Cyclic peptide oral bioavailability: Lessons from the past. Biopolymers 106:901-909.