Oxidation-induced micellization of a diblock copolymer containing stable nitroxyl radicals

Optimizing an Antioxidant TEMPO Copolymer for Reactive Oxygen Species Scavenging and Anti-Inflammatory Effects in Vivo

Oxidative stress is broadly implicated in chronic, inflammatory diseases because it causes protein and lipid damage, cell death, and stimulation of inflammatory signaling. Supplementation of innate antioxidant mechanisms with drugs such as the superoxide dismutase (SOD) mimetic compound 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) is a promising strategy for reducing oxidative stress-driven pathologies. TEMPO is inexpensive to produce and has strong antioxidant activity, but it is limited as a drug due to rapid clearance from the body. It is also challenging to encapsulate into micellar nanoparticles or polymer microparticles, because it is a small, water soluble molecule that does not efficiently load into hydrophobic carrier systems. In this work, we pursued a polymeric form of TEMPO [poly(TEMPO)] to increase its molecular weight with the goal of improving in vivo bioavailability. High density of TEMPO on the poly(TEMPO) backbone limited water solubility and bioactivity of the product, a challenge that was overcome by tuning the density of TEMPO in the polymer by copolymerization with the hydrophilic monomer dimethylacrylamide (DMA). Using this strategy, we formed a series of poly(DMA-co-TEMPO) random copolymers. An optimal composition of 40 mol % TEMPO/60 mol % DMA was identified for water solubility and O2•- scavenging in vitro. In an air pouch model of acute local inflammation, the optimized copolymer outperformed both the free drug and a 100% poly(TEMPO) formulation in O2•- scavenging, retention, and reduction of TNFα levels. Additionally, the optimized copolymer reduced ROS levels after systemic injection in a footpad model of inflammation. These results demonstrate the benefit of polymerizing TEMPO for in vivo efficacy and could lead to a useful antioxidant polymer formulation for next-generation anti-inflammatory treatments.

Synthesis of well-defined carboxyl poly(ε-caprolactone) by fine-tuning the protection group

Carboxyl functionalized polycaprolactone with a well-defined structure was synthesized via ring-opening polymerization (ROP) of substituted caprolactone monomer and acidic hydrolysis.

Core-shell hybrid upconversion nanoparticles carrying stable nitroxide radicals as potential multifunctional nanoprobes for upconversion luminescence and magnetic resonance dual-modality imaging

Nitroxide radicals, such as 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and its derivatives, have recently been used as contrast agents for magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). However, their rapid one-electron bioreduction to diamagnetic N-hydroxy species when administered intravenously has limited their use in in vivo applications. In this article, a new approach of silica coating for carrying stable radicals was proposed. A 4-carboxyl-TEMPO nitroxide radical was covalently linked with 3-aminopropyl-trimethoxysilane to produce a silanizing TEMPO radical. Utilizing a facile reaction based on the copolymerization of silanizing TEMPO radicals with tetraethyl orthosilicate in reverse microemulsion, a TEMPO radicals doped SiO2 nanostructure was synthesized and coated on the surface of NaYF4:Yb,Er/NaYF4 upconversion nanoparticles (UCNPs) to generate a novel multifunctional nanoprobe, PEGylated UCNP@TEMPO@SiO2 for upconversion luminescence (UCL) and magnetic resonance dual-modality imaging. The electron spin resonance (ESR) signals generated by the TEMPO@SiO2 show an enhanced reduction resistance property for a period of time of up to 1 h, even in the presence of 5 mM ascorbic acid. The longitudinal relaxivity of PEGylated UCNPs@TEMPO@SiO2 nanocomposites is about 10 times stronger than that for free TEMPO radicals. The core-shell NaYF4:Yb,Er/NaYF4 UCNPs synthesized by this modified user-friendly one-pot solvothermal strategy show a significant enhancement of UCL emission of up to 60 times more than the core NaYF4:Yb,Er. Furthermore, the PEGylated UCNP@TEMPO@SiO2 nanocomposites were further used as multifunctional nanoprobes to explore their performance in the UCL imaging of living cells and T1-weighted MRI in vitro and in vivo.

Organic-Inorganic Hybrid Nanoparticles via Photoinduced Micellation and Siloxane Core Cross-Linking of Stimuli-Responsive Copolymers

Photoacid-induced siloxane cross-linking of stimuli-responsive copolymer micelles allows the synthesis of well-defined organic-inorganic hybrid nanoparticles. Two conceptually different synthetic approaches are presented, both via photoinduced cross-linking of poly(4-hydroxystyrene-block-styrene) micelles and via one-pot photoacid-catalyzed micelle formation and siloxane cross-linking of poly(4-tert-butoxystyrene-block-styrene). The multistep synthetic route showed intermicellar cross-linking leading to agglomerates. In contrast to this, the formation of the nanoparticles via the one-pot synthesis yielded well-defined structures. The use of different siloxane cross-linking agents and their effects on the properties of the cross-linked micellar structures have been evaluated. Scanning electron microscopy and differential scanning calorimetry indicate rigid core cross-linked nanoparticles. Their size, molar mass, and swelling behavior were analyzed by dynamic and static light scattering. Cyclic siloxane cross-linking agents lead to residual C═C double bonds within the nanoparticle core that allow postsynthetic modification by, e.g., thiol-ene click reactions.

Creation of a blood-compatible surface: a novel strategy for suppressing blood activation and coagulation using a nitroxide radical-containing polymer with reactive oxygen species scavenging activity

Various polymeric materials have been used in medical devices, including blood-contacting artificial organs. Contact between blood and foreign materials causes blood cell activation and adhesion, followed by blood coagulation. Concurrently, the activated blood cells release inflammatory cytokines together with reactive oxygen species (ROS). We have hypothesized that the suppression of ROS generation plays a crucial role in blood activation and coagulation. To confirm this hypothesis, surface-coated polymers containing nitroxide radical compounds (nitroxide radical-containing polymers (NRP)) were designed and developed. The NRP was composed of a hydrophobic poly(chloromethylstyrene) (PCMS) chain to which 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) moieties were conjugated via condensation reaction of the chloromethyl groups in PCMS with the sodium alcoholate group of 4-hydroxy-TEMPO. Blood compatibility was investigated by placing NRP-coated beads in contact with rat whole blood. The amount of ROS generated on PCMS-coated beads used as a control increased significantly with time, while NRP-coated beads suppressed ROS generation. It is interesting to note that the suppression of inflammatory cytokine generation by NRP-coated beads was shown to be significantly higher than that by PCMS-coated beads. Both platelet and leukocyte adhesion to the beads were suppressed with increasing TEMPO incorporation in the polymer. These results confirm that the suppression of ROS by NRP prevents inflammatory cytokine generation, which in turn results in the suppression of blood activation and coagulation on the beads.

Nitroxide radicals and nanoparticles: a partnership for nanomedicine radical delivery

This article aims to provide a research update on nitroxide radical compounds for application of anti-oxidative stress therapy. Nitroxide compounds such as 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) can catalytically react with reactive oxygen species (ROS) and are anticipated as new anti-oxidant therapies for several diseases. However, low-molecular-weight nitroxide compounds pose several problems such as nonspecific dispersion in normal tissues, preferential renal clearance and rapid reduction of the nitroxide radical to the corresponding hydroxylamine. Nitroxide radical compounds are also known to show dose-related antihypertensive action accompanied by reflex tachycardia, increased skin temperature, and seizures. The author has recently designed novel nanoparticles, which possess nitroxide radicals in the core for novel bioimaging and nanotherapy. Nitroxide radical-containing nanoparticles (RNP) shows high safety, long blood circulation, magnetic resonance imaging and ESR imaging sensitive character and efficient therapeutic effects to several diseases such as cerebral and renal ischemia reperfusions, ulcerative colitis and Alzheimer’s disease models. RNPs are, thus, promising as new nanotherapeutic materials.

Micelle Formation Induced by Disproportionation of Stable Nitroxyl Radicals Supported on a Diblock Copolymer

Micelle formation induced by disproportionation was attained for a diblock copolymer containing 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed no self-assembly in 1,4-dioxane, a nonselective solvent. Light scattering studies demonstrated that the copolymer self-assembled into micelles in this solvent with the addition of hydrochloric acid (HCl). The hydrodynamic diameter of the copolymer was estimated to be ca. 55 nm based on the cumulant analysis of the complete micellization. A UV analysis confirmed that the micellizarion proceeded through the disproportionation of the TEMPO into the oxoaminium chloride and the hydroxylamine by the reaction with HCl, because the absorption based on the oxoaminium chloride increased with an increase in the amount of HCl. ESR verified that the radical concentration of the TEMPO decreased with an increase in the HCl. Before the addition of HCl, the PVTEMPO-b-PSt copolymer showed broad signals based on the random orientation. As the amount of HCl increased, the broad signals changed to the typical triplet of TEMPO, accompanied by a decrease in the signal intensity. The g values had a negligible change throughout the micellization. Finally, 40% of the TEMPO remained unreacted when the micellization was completed. The micellization prevented the dispropotionation of the TEMPO, because the PVTEMPO blocks formed the micellar cores which were covered with the micellar coronas of the PSt blocks. TEM observations demonstrated that PVTEMPO-b-PSt formed spherical micelles through the dispropotionation-induced micellization.