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Discovery and Basic Research

(T1230-04-20) Mechanism of Curcumin Encapsulation by GCPQ Polymers: Unexpected Polymer Mesh (Matrix) Configuration Revealed by Atomistic Molecular Dynamics Simulations

Tuesday, October 22, 2024
12:30 PM – 1:30 PM MT
Purpose: GCPQ (quaternary ammonium palmitoyl glycol chitosan) is an amphiphilic polymer that self-assembles into nano-scale particles at a very low critical micellar concentration (CMC~2µM) and acts as a penetration enhancer for hydrophobic drugs across various epithelia barriers, resulting in new nanomedicines in clinical development [1-3]. However, at the molecular level the mechanisms behind the ability of the polymer to carry high payloads and mode of encapsulation remain uncertain. Also, defining the correct parameters for encapsulation remains by and large an iterative and empirical process. We have previously reported the use of coarse-grained MD simulations to elucidate self-assembly and drug encapsulation [4]. Here, we propose that atomistic molecular dynamics simulations may be required to capture essential details of the drug encapsulation process.

Methods: The GCPQ polymer was defined with MW (~12.7 kDa, 55 monomers) and a random distribution of pendant hydrophobic palmitoyl GC residues (9%) and quaternary ammonium GC residues (9%). The initial conformation of the polymer was generated by PSFGEN Tcl script. The 3D structure of model drug – curcumin was obtained from PubChem. The protonation of amine groups was used to reflect pH values from pH 5.5 to pH 8, which affected the overall charge of the polymer at different pHs. GCPQ systems containing 10 polymer chains with curcumin molecules (mass ratio from 5:1 to 68:1) initially packed by PackMol were used to build molecular systems for all-atom MD simulations using Maestro Desmond as a graphical user interface. System size was controlled at around 315k atoms. All systems were firstly compacted in implicit solvation system and then re-solvated by adding water molecules  explicitly  using   SPC  model, 0.15M NaCl and additional chloride counterions to neutralise the system. The models were then energy-minimised, followed by a 500 ps NVT simulation heating from 10 K to 300 K and a 1.2 ns NVT simulation for equilibration. Production runs were conducted at 300 K for 200 ns, with timestep at 1.2 ps. The resulting trajectories were analysed using Maestro and VMD software.

Results: Self-assembly into curcumin-GCPQ aggregates and the formation of curcumin-GCPQ pockets were observed via MD trajectories (showed in Figure 1). In general, the loading of curcumin molecules occurred through conformation changes of polymer chains to form dynamic intra- and inter-molecular pockets to accommodate curcumin molecules. These pockets show an isotropic distribution throughout the loose self-assembled matrix of the particle. Encapsulation led to an increase in solvent-accessible surface area of GCPQ polymer in comparison to the pure polymer. As the loading concentrations of curcumin increases, the role of curcumin-curcumin interaction becomes more important, resulting in the formation of curcumin clusters, corresponding to the decrease of solvent-accessible surface area per drug shown in Table 1.  These curcumin clusters are significantly smaller compared to aggregates formed in absence of polymers. We also looked to understand the effect of pH on the self-assembled matrix, choosing typical formulation conditions with mass ratio GCPQ: curcumin=5:1. The results suggest that a lower pH leads to a stronger tendency of curcumin-curcumin aggregation, but also a stronger polymer-polymer repulsion interactions and looser matrix being formed. The most curcumin-GCPQ polymer interactions occurred at pH 7. As shown in Figure 2, the average radius of gyration of GCPQ polymer at pH 5.5 was the largest, reflecting the least compact conformation.

Conclusion: Our results show that drug encapsulation occurs through the accommodation of curcumin into binding pockets formed spontaneously with the process of GCPQ polymers self-assembling into an isotropic nano-sized matrix with various entanglements. The encapsulation process was mainly driven by the hydrophobic contacts with pendant hydrophobic palmitoyl chains and hydrogen bonding with GC backbones.

References: (1) Godfrey, L.; Iannitelli, A.; Garrett, N. L.; Moger, J.; Imbert, I.; King, T.; Porreca, F.; Soundararajan, R.; Lalatsa, A.; Schätzlein, A. G.; et al. Nanoparticulate peptide delivery exclusively to the brain produces tolerance free analgesia. Journal of Controlled Release 2018, 270, 135-144. DOI: https://doi.org/10.1016/j.jconrel.2017.11.041.
(2) Badr, M. Y.; Abdulrahman, N. S.; Schatzlein, A. G.; Uchegbu, I. F. A polymeric aqueous tacrolimus formulation for topical ocular delivery. International Journal of Pharmaceutics 2021, 599, 120364. DOI: https://doi.org/10.1016/j.ijpharm.2021.120364.
(3) Badr, M. Y.; Halwani, A. A.; Odunze, U.; Eskandarpour, M.; Calder, V. L.; Schätzlein, A. G.; Uchegbu, I. F. The topical ocular delivery of rapamycin to posterior eye tissues and the suppression of retinal inflammatory disease. International Journal of Pharmaceutics 2022, 621, 121755. DOI: https://doi.org/10.1016/j.ijpharm.2022.121755.
(4) Ahmad, S.; Johnston, B. F.; Mackay, S. P.; Schatzlein, A. G.; Gellert, P.; Sengupta, D.; Uchegbu, I. F. In silico modelling of drug-polymer interactions for pharmaceutical formulations. J R Soc Interface 2010, 7 Suppl 4 (Suppl 4), S423-433. DOI: 10.1098/rsif.2010.0190.focus From NLM Medline.


Figure 1: (a)Snapshots at t=0 ns and t=200 ns of production run, showing the self-assembly of curcumin-GCPQ at 4 pHs. Colour codes: GCN in silver, GCP in green, GCQ in yellow, GCC in pink, curcumin in cyan. (b) Formation of curcumin-GCPQ pockets during self-assembly process at pH7. Colour codes: GCN in silver, GCP in green, GCQ in yellow, GCC in pink, curcumin in cyan.


Figure 2: (a) Average radius of gyration of GCPQ at four pHs in last 50 ns of production run. (b) Average solvent-accessible surface area of curcumin, GCPQ and GCP monomers at four pHs, respectively, in last 50 ns of production run. (c) Number of curcumin-GCPQ interactions in last 50ns of production run.


Table 1: Summary table for curcumin-GCPQ systems in different drug concentrations and in different pHs, respectively. Eccentricity was calculated by measuring moments of inertia of GCPQ polymers to interpret the shape of the polymer structure. Simulated loading efficiency was calculated by the equation: (Number of curcumin loaded)⁄(Total number of curcumin molecules), while the criteria for loaded curcumin followed as the distance between centre of mass of curcumin and polymer aggregates is smaller than the radius of gyration of GCPQ at t=200 ns.