(324f) Self-Assembly and Nanoparticle Formation of a Novel Bioresorbable and Crosslinkable Terpolymer

INTRODUCTION: Cancer is the leading cause of death in the US and more than 0.5 M Americans die of cancer each year. One approach to targeted tumor delivery is to trap antitumor drugs in nanoparticles to increase selectivity of drugs towards cancer cells while reducing their systemic toxicity. Bioresorbable nanoparticles are especially attractive as a delivery system because they can be degraded by hydrolysis or enzymatic pathways to low molecular weight compounds, thereby reducing their accumulation in the interstitium. We have developed in our laboratory a novel bioresorbable macromer that self-assemble into nanoparticles in the aqueous solution. The macromer, hereafter abbreviated by PLEOF, is a terpolymer of lactide (LA), ethylene oxide (EG), and fumaric acid. The LA and EG units are FDA approved and fumaric acid occurs naturally in the Kreb's cycle. The objective of this study was to determine the effects of LA to EG ratio on nanoparticle size and distribution.

EXPERIMENTAL: PLEOF was synthesized by condensation polymerization of low molecular weight PLA and poly(ethylene oxide) (PEG) with fumaryl chloride (FC). PLEOF terpolymer was synthesized using PEG and PLA with Mn of 4300 and 1200 Da and polydispersity of 1.1 and 1.4, respectively. The weight ratio of the PLA to PEG in the polymerization mixture was varied from 90/10 to 60/40. After completion of the reaction, triethylamine hydrochloride salt was removed by filtration and the macromer was purified by precipitation twice in ethyl ether. The structure of the PLEOF macromer was characterized by 1H-NMR and FTIR. A 25% w/w solution of PLEOF in distilled deionized (DI) water was vortexed to obtain a stable colloidal suspension of nanoparticles. To crosslink the nanoparticles, 0.3 M ammonium persulfate solution and 0.3 M tetramethyethylenediamine were added to the suspension and allowed to crosslink at ambient conditions for 1 h. The morphology of the nanoparticles was examined using a JEOL 100 CXII transmission electron microscope at an accelerating voltage of 100 kV. A 1% osmium tetroxide aqueous solution was used to stain the nanoparticles. The size distribution of the nanoparticles was measured by dynamic light scattering with a NICOMP Submicron Particle Sizer.

RESULTS: The molecular weight of the PLEOF macromers was measured by GPC. The PLEOF macromers with 90/10, 80/20, 70/30, and 60/40 PLA/PEG ratios had Mn of 16, 14, 13, and 14 KDa and polydispersity of 1.3, 1.4, 1.5, and 1.8, respectively. The nanoparticles have a spherical morphology with a relatively narrow size distribution. The size of the particles decreased after crosslinking. The effect of PLA:PEG ratio on the number and volume average size was investigated. The sample with 100% PLA did not disperse in aqueous solution and the sample with 60:40 PLA:PEG completely dissolved after heating to 50?aC. The 90:10, 80:20, and 70:30 ratios had number average size of 123, 52.1, and 17.9 nm and volume average size of 131, 57.1, and 18.2 nm, respectively. All three ratios produced nanoparticles with very narrow size distribution, as supported by the values of the polydispersity index. The number average size of the 80:20 nanoparticles decreased from 28 to 24 nm after crosslinking while the polydispersity index remained at 1.9.

CONCLUSION: Unsaturated terpolymers of lactide, ethylene oxide, and fumaric acid (PLEOF) with lactide to ethylene oxide ratios in the 90:10 to 70:30 spontaneously self-assembled into stable nanoparticles without the need for addition of emulsifying agent or sonication. The lowest particles size was produced with lactide to ethylene oxide ratio of 70 to 30. After self-assembly, the unsaturated fumaric acid groups in PLEOF can be used to crosslink or graft cell-responsive biomolecules to the nanoparticles for targeted delivery of antitumor drugs. These nanoparticles are potentially useful as a cell-responsive targeted delivery system for anitumor drug delivery.