Sarosh Khan

Functional sugar-based polymers and nanostructures comprised of degradable poly(D-glucose carbonate)s

Fundamental synthetic methodology was advanced to allow for the preparation of a reactive glucose-based block copolycarbonate, which was conveniently transformed into a series of amphiphilic block copolymers that underwent aqueous assembly into functional nanoparticle morphologies having practical utility in biomedical and other applications. Two degradable D-glucose carbonate monomers, with one carrying alkyne functionality, were designed and synthesized to access well-defined block polycarbonates (Đ < 1.1) via sequential organocatalytic ring opening polymerizations (ROPs). Kinetic studies of the organocatalyzed sequential ROPs showed a linear relationship between the monomer conversion and the polymer molecular weight, which indicated the controlled fashion during each polymerization. The pendant alkyne groups underwent two classic click reactions, copper-catalyzed azide–alkyne dipolar cycloaddition (CuAAC) and thiol–yne addition reactions, which were employed to render hydrophilicity for the alkyne-containing block and to provide a variety of amphiphilic diblock poly(D-glucose carbonate)s (PGCs). The resulting amphiphilic PGCs were further assembled into a family of nanostructures with different sizes, morphologies, surface charges and functionalities. These non-ionic and anionic nanoparticles showed low cytotoxicity in RAW 264.7 mouse macrophage cells and MC3T3 healthy mouse osteoblast precursor cells, while the cationic nanoparticles exhibited significantly higher IC50 (162 μg mL−1 in RAW 264.7; 199 μg mL−1 in MC3T3) compared to the commercially available cationic lipid-based formulation, Lipofectamine (IC50 = 31 μg mL−1), making these nanomaterials of interest for biomedical applications.

Rapidly-cured isosorbide-based cross-linked polycarbonate elastomers

The rapid synthesis of an optically-transparent, flexible elastomer was performed utilizing the naturally-derived source, isosorbide. A novel monomer based on isosorbide (isosorbide dialloc, IDA) was prepared by installing carbonate functionalities along with external olefins for use in thiol–ene click chemistry. Cross-linked networks were created using the commercially-available cross-linker, trimethylolpropane tris(3-mercaptopropionate) (TMPTMP) and resulted in IDA-co-TMPTMP, an optically-transparent elastomer. Systematically, IDA-co-TMPTMP networks were synthesized using a photoinitiator, a UV cure time of one minute and varied post cure times (0–24 h, 125 mm Hg) at 100 °C to observe effects on mechanical, thermal and surface alterations. The mechanical properties also had limited changes with post cure time, including a modulus at 25 °C of 1.9–2.8 MPa and an elongation of 220–344%. The thermal decomposition temperatures of the networks were consistent, ca. 320 °C, while the glass transition temperature remained below room temperature for all samples. A cell viability assay and fluorescence imaging with adherent cells are also reported in this study to show the potential of the material as a biomedical substrate. A degradation study for 60 days resulted in 8.3 ± 3.5% and 97.7 ± 0.3% mass remaining under accelerated (1 M NaOH, 60 °C) and biological conditions (pH 7.4 PBS at 37 °C), respectively. This quickly-synthesized material has the potential to hydrolytically degrade into biologically-benign and environmentally-friendly by-products and may be utilized in renewable plastics and/or bioelastomer applications.

Chemical Design of Both a Glutathione-Sensitive Dimeric Drug Guest and a Glucose-Derived Nanocarrier Host to Achieve Enhanced Osteosarcoma Lung Metastatic Anticancer Selectivity

Although nanomedicines have been pursued for nearly 20 years, fundamental chemical strategies that seek to optimize both the drug and drug carrier together in a concerted effort remain uncommon yet may be powerful. In this work, two block polymers and one dimeric prodrug molecule were designed to be coassembled into degradable, functional nanocarriers, where the chemistry of each component was defined to accomplish important tasks. The result is a poly(ethylene glycol) (PEG)-protected redox-responsive dimeric paclitaxel (diPTX)-loaded cationic poly(d-glucose carbonate) micelle (diPTX@CPGC). These nanostructures showed tunable sizes and surface charges and displayed controlled PTX drug release profiles in the presence of reducing agents, such as glutathione (GSH) and dithiothreitol (DTT), thereby resulting in significant selectivity for killing cancer cells over healthy cells. Compared to free PTX and diPTX, diPTX@CPGC exhibited improved tumor penetration and significant inhibition of tumor cell growth toward osteosarcoma (OS) lung metastases with minimal side effects both in vitro and in vivo, indicating the promise of diPTX@CPGC as optimized anticancer therapeutic agents for treatment of OS lung metastases.

A Vinyl Ether-Functional Polycarbonate as a Template for Multiple Postpolymerization Modifications

A highly-reactive vinyl ether-functionalized aliphatic polycarbonate and its block copolymer were developed as templates for multiple post-polymerization conjugation chemistries. The vinyl ether-functional six-membered cyclic carbonate monomer was synthesized by a well-established two-step procedure starting from 2,2-bis(hydroxymethyl)propionic acid. An organobase-catalyzed ring-opening polymerization of the synthesized monomer afforded polycarbonates with pendant vinyl ether functionalities (PMVEC). The vinyl ether moieties on the resulting polymers were readily conjugated with hydroxyl- or thiol-containing compounds via three different post-polymerization modification chemistries - acetalization, thio-acetalization, and thiol-ene reaction. Acetal-functionalized polycarbonates were studied in depth to exploit their acid-labile acetal functionalities. Acetalization of the amphiphilic diblock copolymer of poly(ethylene glycol) methyl ether (mPEG) and PMVEC, mPEG113-b-PMVEC13, with the model hydroxyl compound 4- methylbenzyl alcohol resulted in a maximum of 42% acetal and 58% hydroxyl side chain groups. Nonetheless, the amphiphilicity of the block polymer allowed for its self-assembly in water to afford nanostructures, as characterized via dynamic light scattering and transmission electron microscopy. The kinetics of acetal cleavage within the block polymer micelles were examined in acidic buffered solutions (pH 4 and 5). In addition, mPEG-b-PMVEC and its hydrolyzed polymer mPEG-b-PMHEC (i.e., after full cleavage of acetals) exhibited minimal cytotoxicity to RAW 264.7 mouse macrophages, indicating that this polymer system represents a biologically non-hazardous material with pH-responsive activity.

Functional, Degradable Zwitterionic Polyphosphoesters as Biocompatible Coating Materials for Metal Nanostructures

A zwitterionic polyphosphoester (zPPE), specifically l-cysteine-functionalized poly(but-3-yn-1-yloxy)-2-oxo-1,3,2-dioxaphospholane (zPBYP), has been developed as a poly(ethylene glycol) (PEG) alternative coating material for gold nanoparticles (AuNPs), the most extensively investigated metal nanoparticulate platform toward molecular imaging, photothermal therapy, and drug delivery applications. Thiol-yne conjugation of cysteine transformed an initial azido-terminated and alkynyl-functionalized PBYP homopolymer into zPBYP, offering hydrolytic degradability, biocompatibility, and versatile reactive moieties for installation of a range of functional groups. Despite minor degradation during purification, zPPEs were able to stabilize AuNPs presumably through multivalent interactions between combinations of the side chain zwitterions (thioether and phosphoester groups of the zPPEs with the AuNPs). 31P NMR studies in D2O revealed ca. 20% hydrolysis of the phosphoester moieties of the repeat units had occurred during the workup and purification by aqueous dialysis at pH 3 over ca. 1 d, as observed by the 31P signal of the phosphotriesters resonating at ca. -0.5 to -1.7 shifting downfield to ca. 1.1 to -0.4 ppm, attributed to transformation to phosphates. Further hydrolysis of side chain and backbone units proceeded to an extent of ca. 75% over the next 2 d in nanopure water (pH 5-6). The NMR degradation results were consistent with the broadening and red-shift of the surface plasmon resonance (SPR) observed by UV-vis spectroscopy of the zPPE-coated AuNPs in water over time. All AuNP formulations in this study, including those with citrate, PEG, and zPPE coatings, exhibited negligible immunotoxicity, as determined by cytokine overexpression in the presence of the nanostructures relative to those in cell culture medium. Notably, the zPPE-coated AuNPs displayed superior antifouling properties, as assessed by the extent of cytokine adsorption relative to both the PEGylated and citrate-coated AuNPs. Taken together, the physicochemical and biological evaluations of zPPE-coated AuNPs in conjunction with PEGylated and citrate-coated analogues indicate the promise of zPPEs as favorable alternatives to PEG coatings, with negligible immunotoxicity, good antifouling performance, and versatile reactive groups that enable the preparation of highly tailored nanomaterials for diverse applications.

Development of Fully Degradable Phosphonium-Functionalized Amphiphilic Diblock Copolymers for Nucleic Acids Delivery

To expand the range of functional polymer materials to include fully hydrolytically degradable systems that bear bioinspired phosphorus-containing linkages both along the backbone and as cationic side chain moieties for packaging and delivery of nucleic acids, phosphonium-functionalized polyphosphoester- block-poly(l-lactide) copolymers of various compositions were synthesized, fully characterized, and their self-assembly into nanoparticles were studied. First, an alkyne-functionalized polyphosphoester- block-poly(l-lactide) copolymer was synthesized via a one pot sequential ring opening polymerization of an alkyne-functionalized phospholane monomer, followed by the addition of l-lactide to grow the second block. Second, the alkynyl side groups of the polyphosphoester block were functionalized via photoinitiated thiol-yne radical addition of a phosphonium-functionalized free thiol. The polymers of varying phosphonium substitution degrees were self-assembled in aqueous buffers to afford formation of well-defined core-shell assemblies with an average size ranging between 30 and 50 nm, as determined by dynamic light scattering. Intracellular delivery of the nanoparticles and their effects on cell viability and capability at enhancing transfection efficiency of nucleic acids (e.g., siRNA) were investigated. Cell viability assays demonstrated limited toxicity of the assembly to RAW 264.7 mouse macrophages, except at high polymer concentrations, where the polymer of high degree of phosphonium functionalization induced relatively higher cytotoxicity. Transfection efficiency was strongly affected by the phosphonium-to-phosphate (P+/P-) ratios of the polymers and siRNA, respectively. The AllStars Hs Cell Death siRNA complexed to the various copolymers at a P+/P- ratio of 10:1 induced comparable cell death to Lipofectamine. These fully degradable nanoparticles might provide biocompatible nanocarriers for therapeutic nucleic acid delivery.

Harnessing the Chemical Diversity of the Natural Product Magnolol for the Synthesis of Renewable, Degradable Neolignan Thermosets with Tunable Thermomechanical Characteristics and Antioxidant Activity

Magnolol, a neolignan natural product with antioxidant properties, contains inherent, orthogonal, phenolic, and alkenyl reactive groups that were used in both direct thermoset synthesis, as well as the stepwise synthesis of a small library of monomers, followed by transformation into thermoset materials. Each monomer from the small library was prepared via a single step functionalization reaction of the phenolic groups of magnolol. Thermoset materials were realized through solvent-free, thiol-ene reactions, and the resulting cross-linked materials were each comprised of thioether and ester linkages, with one retaining the hydrophilic phenols from magnolol, another having the phenols protected as an acetonide, and two others incorporating the phenols into additional cross-linking sites via hydrolytically labile carbonates or stable ether linkages. With this diversity of chemical compositions and structures, the thermosets displayed a range of thermomechanical properties including glass transition temperatures, Tg, 29-52 °C, onset of thermal degradation, Td, from about 290-360 °C, and ultimate strength up to 50 MPa. These tunable materials were studied in their degradation and biological properties with the aim of exploiting the antioxidant properties of the natural product. Hydrolytic degradation occurred under basic conditions (pH = 11) in all thermosets, but with kinetics that were dependent upon their chemical structures and mechanical properties: 20% mass loss was observed at 5, 7, 27, and 40 weeks for the thermosets produced from magnolol directly, acetonide-protected magnolol, bis(allyl carbonate)-functionalized magnolol, and bis(allyl ether)-functionalized magnolol, respectively. Isolated degradation products and model compounds displayed antioxidant properties similar to magnolol, as determined by both UV-vis and in vitro reactive oxygen species (ROS) assays. As these magnolol-based thermosets were found to also allow for extended cell culture, these materials may serve as promising degradable biomaterials.