Solubilization Behavior of Polyene Antibiotics in Nanomicellar System: Insights from Molecular Dynamics Simulation of the Amphotericin B and Nystatin Interactions with Polysorbate 80

Abstract

Amphotericin B (AmB) and Nystatin (Nys) are the drugs of choice for treatment of systemic and superficial mycotic infections, respectively, with their full clinical potential unrealized due to the lack of high therapeutic index formulations for their solubilized delivery. In the present study, using a coarse-grained (CG) molecular dynamics (MD) simulation approach, we investigated the interaction of AmB and Nys with Polysorbate 80 (P80) to gain insight into the behavior of these polyene antibiotics (PAs) in nanomicellar solution and derive potential implications for their formulation development. While the encapsulation process was predominantly governed by hydrophobic forces, the dynamics, hydration, localization, orientation, and solvation of PAs in the micelle were largely controlled by hydrophilic interactions. Simulation results rationalized the experimentally observed capability of P80 in solubilizing PAs by indicating (i) the dominant kinetics of drugs encapsulation over self-association; (ii) significantly lower hydration of the drugs at encapsulated state compared with aggregated state; (iii) monomeric solubilization of the drugs; (iv) contribution of drug-micelle interactions to the solubilization; (v) suppressed diffusivity of the encapsulated drugs; (vi) high loading capacity of the micelle; and (vii) the structural robustness of the micelle against drug loading. Supported from the experimental data, our simulations determined the preferred location of PAs to be the core-shell interface at the relatively shallow depth of 75% of micelle radius. Deeper penetration of PAs was impeded by the synergistic effects of (i) limited diffusion of water; and (ii) perpendicular orientation of these drug molecules with respect to the micelle radius. PAs were solvated almost exclusively in the aqueous poly-oxyethylene (POE) medium due to the distance-related lack of interaction with the core, explaining the documented insensitivity of Nys solubilization to drug-core compatibility in detergent micelles. Based on the obtained results, the dearth of water at interior sites of micelle and the large lateral occupation space of PAs lead to shallow insertion, broad radial distribution, and lack of core interactions of the amphiphilic drugs. Hence, controlled promotion of micelle permeability and optimization of chain crowding in palisade layer may help to achieve more efficient solubilization of the PAs.

Keywords: Amphotericin B; Nystatin; Polysorbate 80; drug delivery systems; drug formulation; molecular dynamics simulation; nanomedicine; polyene antibiotics; solubilization.

Figure 1

Trajectory snapshots of micellation of Polysorbate 80 (P80) in the aqueous medium at different simulation times (water particles have been removed for clarity).

Figure 2

Density distribution of water and head and tail of P80 from micelle center of mass.

Figure 3

Average radial pair distribution functions for various moieties of (a) a single Amphotericin B (AmB) molecule; and (b) a single Nystatin (Nys) molecule against water particles.

Figure 4

Trajectory snapshots of aggregation of AmB in the aqueous medium at different simulation times (water particles have been removed for clarity).

Figure 5

Comparison of the time-evolution of (a) solvent accessible surface area (SASA) of AmB and Nys; and (b) average number of water particles around AmB and Nys molecules within 7 Å, during self-association process.

Figure 6

Average radial pair distribution functions for various moieties of (a) AmB; and (b) Nys against single P80 molecule in aqueous medium.

Figure 7

Average radial pair distribution functions for head and tail of P80 against (a) AmB; and (b) Nys in aqueous medium.

Figure 8

Average radial pair distribution functions for different molecular groups of (a) P80-AmB mixed micelle; and (b) P80-Nys mixed micelle against water particles.

Figure 9

The trajectory snapshots of encapsulation of AmB molecules into P80 micelle at different simulation times (water particles have been removed for clarity).

Figure 10

(a) Solvent accessible surface area of AmB and Nys molecules; and (b) average number of water particles around AmB and Nys molecules within 7 Å, during and after encapsulation into the P80 micelle.

Figure 11

Time-evolution of average energy of interactions (a) between AmB and AmB, P80, and water; and (b) between Nys and Nys, P80, and water.

Figure 12

Radial density distributions for various molecular groups of (a) P80-AmB mixed micelle; and (b) P80-Nys mixed micelle.

Figure 13

Bead4-Bead7-COM angle distribution for the encapsulated AmB and Nys.

Figure 14

Radial distribution functions for head and tail of P80 against (a) AmB; and (b) Nys in P80₆₀-AmB₉ and P80₆₀-Nys₉ mixed micelles, respectively.

Figure 15

(a) Course-grain mapping of P80; (b) coarse-grain model of P80, comprising 30 interaction sites: 21 sites for the poly-oxyethylene (POE) chain, 5 for alkyl chain, and 3 for terminal polar groups and 1 for the ester group; (c,e) coarse-grain mapping of AmB and Nys, respectively; (d,f) coarse-grain model of AmB and Nys, respectively, with 17 interaction sites: 4 sites for the polyene chain, 1 for carboxyl group, 1 for amine group, and 11 for the hydroxyl and other groups.