Therapeutic Peptides Are Preferentially Solubilized in Specific Microenvironments within PEG-PLGA Polymer Nanoparticles

Abstract

Polymeric nanoparticles are a highly promising drug delivery formulation. However, a lack of understanding of the molecular mechanisms that underlie their drug solubilization and controlled release capabilities has hindered the efficient clinical translation of such technologies. Polyethylene glycol-poly(lactic-co-glycolic) acid (PEG-PLGA) nanoparticles have been widely studied as cancer drug delivery vehicles. In this letter, we use unbiased coarse-grained molecular dynamics simulations to model the self-assembly of a PEG-PLGA nanoparticle and its solubulization of the anticancer peptide, EEK, with good agreement with previously reported experimental structural data. We applied unsupervised machine learning techniques to quantify the conformations that polymers adopt at various locations within the nanoparticle. We find that the local microenvironments formed by the various polymer conformations promote preferential EEK solubilization within specific regions of the NP. This demonstrates that these microenvironments are key in controlling drug storage locations within nanoparticles, supporting the rational design of nanoparticles for therapeutic applications.

Keywords: Molecular dynamics simulations; PEG; PLGA; drug delivery vehicles; polymer nanoparticles.

Figure 1

Internal composition of the PLGA–PEG nanoformulation. (a) Spherical density of various components of the polymeric NP and its aqueous environment, where glycolic acid (GA) is shown in light pink, lactic acid (LA) in fucsia, EO in light blue, EEK in orange, and water in navy blue. (b) Percentage of the NP core made up by each polymer block, EEK (“peptide”), and water. EEK peptides account for 0.5% of the beads in the core. (c) Snapshot of the cross section of the polymeric NP loaded with EEK peptides, where PLGA is shown in pink, PEG in light blue, peptide in orange, and water in navy blue. The two peptides located in the inner core can be clearly seen in the middle of the NP core. This representation is not to scale.

Figure 2

Time-averaged cargo contacts and the hydration difference between peptide storage locations. (a) Difference between the time-averaged water–EEK contacts (hydration) per amino acid between the peptides within the core and at the core–corona interface. (b) Difference between the time-averaged polymer–EEK contacts per amino acid between the peptides within the core and at the core–corona interface. A positive value corresponds to more contacts between a specific EEK residue and either a polymer bead or water in the center of the core than at the core–corona interface and vice versa for a negative value.

Figure 3

Unsupervised learning reveals location-specific polymer conformations. (a) Bar chart with the percentage of each cluster of polymer conformations within the NP with corresponding snapshots of a random polymer within each cluster. In the snapshots, PLGA is shown in pink and PEG in cyan. (b) Normalized intrinsic density profile of the various clusters within the NP. The normalization takes into account the cluster population. Normalized per polymer cluster. (c) Snapshot of NP with the polymers colored in correspondence to their cluster. The colors applied in the snapshot are the same as those used for the different clusters in (a) and (b). Polymer and NP snapshots are not to scale.

Figure 4

Cargo interactions with polymer conformational clusters. Average enrichment fraction of contacts between polymers in each of the different polymer clusters within the NP and (a) peptides at the core–shell interface and (b) peptides inside the core. Note that all error bars show the 90% CI. The enrichment takes into account the relative cluster population, such that an enrichment value greater than 1 means the peptides have a tendency to interact with that polymer conformation, an enrichment value of less than 1 suggests that there is a depletion of the polymers in that cluster around the peptide, and a value of 1 means that the peptide and the polymers from that cluster interact randomly. In (b), there is a snapshot showing a peptide interacting with the most preferential polymer cluster, cluster 4. In this snapshot, PLGA is colored pink, PEG is light blue, the peptide is orange, and water is navy blue. The error bar of cluster 3 in (b) is colored in gray due to poor statistics for this cluster, as there are not many interactions between the peptides captured inside the core and this cluster. The theoretical background for this calculation can be found in the SI.