Amphiphilic polymers: crystallization-assisted Self-assembly and applications in pharmaceutical formulation

University of Minnesota Ph.D. dissertation. April 2013. Major: Chemistry. Advisor: Marc A. Hillmyer. 1 computer file (PDF); xx, 251 pages.Amphiphilic polymers are macromolecules that simultaneously contain hydrophobic and hydrophilic components. These molecules not only attract much attention in academic research but also are important materials in industry. Application areas include detergency, oil field, paints, agriculture, food, cosmetics, and pharmaceutics. This dissertation highlights my efforts since the November of 2007 on three separate systems of amphiphilic polymers, which addresses both the fundamental self-assembly behavior in solution and applications in pharmaceutical formulation. Chapter 2 describes the self-assembled micelles in water that contain semicrystalline polyethylene (PE) as the core-forming material. Poly(N,N-dimethylacrylamide)-polyethylene (AE) diblock copolymers were chosen as the model system. An AE diblock copolymer with relatively low PE composition resulted in micelles with oblate ellipsoidal cores in water, in which crystalline PE existed as flat disks at the center and rubbery PE resided on both sides. In contrast, a control sample with a rubbery polyolefin as the hydrophobic component resulted in micelles with spherical cores in water. The morphology transition was ascribed to the crystallization of PE. The heat-assisted direct dissolution for sample preparation was identified as a stepwise "micellization-crystallization" procedure. In addition, the morphology of the aggregates exhibited much dependence on the composition of AE copolymers, and wormlike micelles and bilayered vesicles were obtained from samples with relatively high PE compositions. Chapter 3 demonstrates the precise synthesis of glucose-containing diblock terpolymers from a combination of anionic and reversible addition-fragmentation chain-transfer (RAFT) polymerizations. The resulting micelles exhibited excellent stability in several biologically-relevant media under in vitro conditions, including 100% fetal bovine serum. These particles may find applications as serum-stable nanocarriers of hydrophobic drugs for intravenous administration. Chapter 4 presents the development of novel cellulose derivatives as matrices in amorphous solid dispersions for improving the bioavailability of poorly water-soluble drugs in oral administration. Hydroxypropyl methylcellulose (HPMC) was modified with monosubstituted succinic anhydrides using facile anhydride chemistry, and the resulting materials simultaneously contained hydrophobic, hydrophilic, and pH-responsive moieties. Several HPMC esters of substituted succinates exhibited more effective crystallization inhibition of phenytoin under in vitro conditions than a commercial hydroxypropyl methylcellulose acetate succinate (HPMCAS). (341 words)Yin, Ligeng. (2013). Amphiphilic polymers: crystallization-assisted Self-assembly and applications in pharmaceutical formulation. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/163033

Preparation and Performance of Hydroxypropyl Methylcellulose Esters of Substituted Succinates for <i>in Vitro</i> Supersaturation of a Crystalline Hydrophobic Drug

We prepared hydroxypropyl methylcellulose (HPMC) esters of substituted succinates and examined their performance for improving the aqueous solubility of crystalline hydrophobic drugs in spray-dried dispersions (SDDs). From one HPMC, we synthesized five HPMC esters using various monosubstituted succinic anhydrides. These HPMC esters along with a commercial HPMC acetate succinate (HPMCAS) were spray-dried from solutions with phenytoin. The SDDs with different matrices at 10 wt % loading had very similar bulk properties with a minimal amount of detectable crystalline phenytoin as revealed by scanning electron microscopy (SEM), powder X-ray diffraction (powder XRD), and differential scanning calorimetry (DSC). In solution, while the SDD with HPMCAS was very effective at achieving high levels of phenytoin supersaturation initially, it was not competent at maintaining such supersaturation due to the rapid crystallization of the dissolved phenytoin. Alternatively, SDDs with several synthesized HPMC esters of substituted succinates not only achieved rather high initial supersaturation but also maintained high concentrations for extended time (i.e., 1.5 h and longer). Such maintenance was largely ascribed to the inhibition of phenytoin nucleation. Structure–property relationships were established, and the most successful systems contained a high degree of substitution and a combination of a thioether with neighboring weak electron-withdrawing groups in the substituted succinic anhydrides. The effective maintenance of supersaturated solutions was only found in SDDs with rather low drug loadings, which indicates the significance of sufficiently high concentrations of polymer additives in the dissolution media

A Stepwise “Micellization–Crystallization” Route to Oblate Ellipsoidal, Cylindrical, and Bilayer Micelles with Polyethylene Cores in Water

Micellar polymorphism from block copolymers has been well documented, but most attention has focused on noncrystalline hydrophobic systems. We have investigated the micellization in water of model diblock copolymers with semicrystalline polyethylene (PE) as the core-forming component. Poly­(<i>N</i>,<i>N</i>-dimethylacrylamide)–polyethylene (AE) diblock copolymers were synthesized by a combination of anionic and RAFT polymerizations. The bulk nanostructures were probed by small-angle X-ray scattering (SAXS) and AE diblock copolymers were found to be moderately segregated at 140 °C. Dispersions of AE amphiphiles in water were prepared by direct dissolution at 120 °C (i.e., above the melting transition of PE) followed by cooling to 25 °C. By manipulating the composition of AE diblock copolymers, discrete structures with oblate ellipsoidal, cylindrical, and bilayer morphologies were produced, as evidenced in cryogenic transmission electron microscopy (cryo-TEM). The self-assembled aggregates were also studied by small-angle neutron scattering (SANS) and dilute solution rheology. The semicrystalline nature of the nanostructures was further revealed by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). A stepwise “micellization–crystallization” process was proposed as the micelle formation mechanism, as supported by the existence of similar nanostructures at 120 °C using SANS. This strategy holds promise for a general protocol toward the production of giant wormlike micelles and vesicles with semicrystalline polymeric cores

Glucose-Functionalized, Serum-Stable Polymeric Micelles from the Combination of Anionic and RAFT Polymerizations

Poly­(ethylene-<i>alt</i>-propylene)–poly­[(<i>N</i>,<i>N</i>-dimethylacrylamide)-<i>grad</i>-(2-methacrylamido glucopyranose)] (PEP–poly­(DMA-<i>grad</i>-MAG), or PG) diblock terpolymers were synthesized by combining anionic and reversible addition–fragmentation chain transfer (RAFT) polymerizations. An ω-trithiocarbonate-functionalized PEP homopolymer served as the macromolecular chain transfer agent (macroCTA), and RAFT copolymerizations of DMA and a trimethylsilyl-protected MAG (TMS-MAG) monomer gave a family of PG diblock terpolymers after hydrolysis. The terpolymers had similar degrees of polymerization, and the MAG content ranged from 3.5 to 39 mol % in the hydrophilic block. At 70 °C, the reactivity ratios of DMA (1) and TMS-MAG (2) were determined to be <i>r</i>₁ = 1.86 ± 0.07 and <i>r</i>₂ = 0.16 ± 0.01, and thus the poly­(meth)­acrylamide blocks in the PG diblock terpolymers were likely to be gradient copolymers. Micellar dispersions from PG diblock polymers in water were examined by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS). Spherical micelles with core radii of ca. 7 nm and overall hydrodynamic radii of ca. 15 nm were the predominant morphologies observed in all samples prepared by sequential nanoprecipitation and dialysis. The electron-dense MAG moieties greatly increased the native contrast of the micellar coronae, which were clearly viewed as gray halos around the micellar cores in samples with relatively large MAG content. The stability of the glucose-installed micelles was tested in four biologically relevant media, from simple phosphate-buffered saline (PBS) to fetal bovine serum (FBS), using a combination of DLS and cryo-TEM measurements. Micellar dispersions from a PG diblock terpolymer with 16 mol % of MAG of the hydrophilic block were stable in 100% FBS over at least 14 h, suggesting their minimal interactions with serum proteins. Control experiments suggested that micelles composed of PDMA alone in the corona had similar serum stabilities. These sugar-functionalized micelles hold promise as <i>in vivo</i> drug delivery vehicles to possibly prolong circulation time after intravenous administration