(161ad) Synthetically Tunable PLA-PEG Analogues for Fabricating Drug-Loaded Nanoparticles

Background: Linear polylactic acid-polyethylene glycol (PLA-PEG) block copolymers have been widely used to fabricate nanoparticle (NP) drug delivery vehicles due to their ability to self-assemble^1-5. PEG moieties are commonly employed in drug delivery vehicles to impart ‘stealth’ properties through the presentation of a hydrophilic face^4-6. PLA due to its relatively hydrophobic nature is commonly used as a hydrophobic block in NP formulations as it is both degradable cytocompatible^4-5. However, the step growth chemistry of polymerization makes these materials challenging to chemically functionalize, as is often desirable for targeting and/or tuning drug release kinetics.

The use of methacrylate-based monomers in which the PEG and PLA functionality is incorporated into the side chain rather than the polymer backbone offers a potential solution to this challenge, enabling facile functionalization of the polymers by standard or controlled radical polymerization processes while maintaining the key physical properties of PEG-PLA block copolymers that have motivated their interest in drug delivery applications. Herein, we assessed the drug delivery potential of brush-brush and linear-brush copolymer analogues of PLA-PEG, specifically incorporating poly(oligo(lactic acid) methacrylate) (POLAMA, PLA-mimetic) and/or poly(oligo(ethylene glycol) methacrylate) (POEGMA, PEG-mimetic) in one or both blocks, compared to linear PLA-PEG.

Methodology: Atom transfer radical polymerization (ATRP) and activator regenerator by electron transfer (ARGET) ATRP were used to synthesize the brush-brush and linear-brush copolymer analogues POEGMA-POLAMA and PLA-POEGMA. The composition and molecular weight of these polymers were characterized by nuclear magnetic resonance spectroscopy and gel permeation chromatography respectively. Blank and drug-loaded NPs were fabricated through flash nanoprecipitation via a confined impinging jet device (CIJD) using tetrahydrofuran as the solvent phase and phosphate-buffered saline as antisolvent. Multiple solvents were tested to determine the optimal solvent for NP size and drug loading. NP size and stability was measured using dynamic light scattering. Cytocompatibility was assessed via metabolic resazurin and live/dead assays using NIH 3T3 mouse fibroblast cells. In vivo stability and biodistribution was assessed using C57BL/6 mice implanted with CT26 tumors. 10 days after implanting the tumor, mice were intravenously injected with Cy5-labelled NPs. The biodistribution of the nanoparticles as a function of block type was monitored at 2, 8, and 24 timepoints using an IVIS imaging system by harvesting the lungs, heart, spleen, liver, tumor, and kidneys at each time point.

Results: NPs fabricated from synthesized brush-brush and linear-brush copolymers ranged from 60-500 nm in size, with the size tunable based on the fabrication technique used (i.e. solvent type, mixer shear) and the block morphology and block length used for fabrication. The comonomer tert-butyl methacrylate (tBMA) was incorporated in varying ratios to either block, demonstrating the versatility of the technique and the potential for facile polymer functionalization following tBMA hydrolysis (to generate derivatizable -COOH groups) that represents a clear advantage of this approach relative to conventional PEG-PLA materials. POEGMA-POLAMA NPs yielded encapsulation efficiencies >96% and 41% for paclitaxel and doxorubicin hydrochloride, respectively. Biodistribution studies showed significant concentrations of NPs in the tumor, spleen, liver, and kidneys, with accumulation occurring within the tumor as early as the 2-hour timepoint for both POEGMA-POLAMA and PLA-POEGMA polymers.

Conclusions: PLA-PEG brush copolymer analogues prepared through ATRP or ARGET ATRP are viable alternatives to their linear counterparts while providing superior tunability. NP sizes can be changed through varying organic solvents and changing shear conditions of the CIJD. NPs fabricated from PLA-PEG analogues have been shown to be cytocompatible and can bioaccumulate in tumors without any modification.

References:

1. Jain, R. A., The Manufacturing techniques of various drug loaded biodegradable poly(lactide-co-glycolide) (PLGA) Devices. Biomaterials 2000, 21, 2475-2490.
2. Zizelmann, C.; Schoen, R.; Metzger, M. C.; Schmelzeisen, R.; Schramm, A.; Dott, B.; Bormann, K. H.; Gellrich, N. C., Bone formation after sinus augmentation with engineered bone. Clin Oral Implants Res 2007, 18 (1), 69-73.
3. Danhier, F.; Ansorena, E.; Silva, J. M.; Coco, R.; Le Breton, A.; Preat, V., PLGA-based nanoparticles: an overview of biomedical applications. J Control Release 2012, 161 (2), 505-22.
4. Hoare, T. R.; Kohane, D. S., Hydrogels in drug delivery: Progress and challenges. Polymer 2008, 49 (8), 1993-2007.
5. Wang, J.; Li, S.; Han, Y.; Guan, J.; Chung, S.; Wang, C.; Li, D., Poly(Ethylene Glycol)-Polylactide Micelles for Cancer Therapy. Front Pharmacol 2018, 9, 202.
6. Li, S. D.; Huang, L., Stealth nanoparticles: high density but sheddable PEG is a key for tumor targeting. J Control Release 2010, 145 (3), 178-81.