Polymers as Drugs

Bioactive Synthetic Polymers

Synthetic polymers are omnipresent in society as textiles and packaging materials, in construction and medicine, among many other important applications. Alternatively, natural polymers play a crucial role in sustaining life and allowing organisms to adapt to their environments by performing key biological functions such as molecular recognition and transmission of genetic information. In general, the synthetic and natural polymer worlds are completely separated due to the inability for synthetic polymers to perform specific biological functions; in some cases, synthetic polymers cause uncontrolled and unwanted biological responses. However, owing to the advancement of synthetic polymerization techniques in recent years, new synthetic polymers have emerged that provide specific biological functions such as targeted molecular recognition of peptides, or present antiviral, anticancer, and antimicrobial activities. In this review, the emergence of this generation of bioactive synthetic polymers and their bioapplications are summarized. Finally, the future opportunities in this area are discussed.

Polymers from S-vinyl monomers: reactivities and properties

This review summarises the work of several decades on the polymerisation of S-vinyl monomers, ranging from the early reports of suitable polymerisation techniques for these monomers to their recent renaissance in various applications.

Poly(ethylene glycol) induces cell toxicity in melanoma cells by producing a hyperosmotic extracellular medium

Poly(ethylene glycol) is a polymer that is widely used as a biomaterial and has been approved in a host of applications. While generally viewed as inert, recent studies with poly(ethylene glycol) suggest that it may have some effects on cells and tissues, making it potentially attractive as a therapeutic agent. In this study, the effect of poly(ethylene glycol) on the cell viability, membrane transport and apoptotic markers of metastatic melanoma cells was examined. The data were combined with observed effects of the polymer on the cell media, including osmolality and viscosity, in order to elucidate any structure-function relationship between the polymer and cells. It was observed that poly(ethylene glycol) reduced the cellular viability of A375 cells, and that the effect was dependent on poly(ethylene glycol) molecular weight and concentration. The mechanism was highly correlated with changes in the osmolality of the cell medium, which is determined by the inherent structure of poly(ethylene glycol), and in particular the ethylene oxide units. This mechanism was specific to poly(ethylene glycol) and was not observed with the similar linear, hydrophilic polymer poly(vinyl pyrrolidone). Overall, the data suggest that poly(ethylene glycol) and poly(ethylene glycol)-like compounds have a distinct effect on cellular activity, presumably mediated in part by their osmotic effects, supporting the further investigation of these polymers as pharmaceutically active compounds.

Synthesis of tailor-made bile acid sequestrants by supplemental activator and reducing agent atom transfer radical polymerization

This work reports the synthesis of tailor-made polymeric bile acid sequestrants (BAS) by supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) using ecofriendly conditions.

Synthesis of polymeric phosphonates for selective delivery of radionuclides to osteosarcoma

Discussed in detail is the synthesis and primary structure characterization of two polymers aimed at advancing the treatment of pediatric osteosarcoma. These polymers are designed to systemically deliver radiometals specifically to osteosarcomas using the passive targeting mechanism of enhanced permeability and retention (the EPR effect). The approach begins with the synthesis of a polymer capable of binding radiometals, for which prior data show improved site-specific targeting of solid tumors. Building on this success, a second polymer has been designed for improving the efficacy of currently available radionuclide therapies by incorporating the FDA-approved small-molecule ligand Quadramet directly onto the polymer structure. Time-activity curves of the phosphonate-functionalized polymers show rapid clearance from the central compartment and nontargeted organs, with up to 65% of injected activity being excreted within 3 hours. Both polymer ligands demonstrate good osteosarcoma targeting capability with little to no uptake in organs associated with the dose-limiting bone marrow. Additionally, biodistribution studies in nonosseous tumor models demonstrate the tumor targeting mechanism of the polymer ligands, which appears to be influenced by the high affinity of the phosphonate functionality for the positively charged hydroxyapatite mineral found in bone tumors.

Synthesis, characterization, and assessment of cytotoxic, antiproliferative, and antiangiogenic effects of a novel procainamide hydrochloride-poly(maleic anhydride-co-styrene) conjugate

Poly(maleic anhydride-co-styrene) (MAST) was synthesized by a free-radical polymerization reaction. A bioactive molecule, procainamide hydrochloride (PH), was then conjugated to MAST. The conjugation product was named as MAST/PH. Structural characterization of MAST and MAST/PH was carried out by Fourier Transform Infrared and Nuclear Magnetic Resonance spectroscopy. Their molecular weights were determined by size-exclusion chromatography. A mechanism was then suggested for the conjugation reaction. The results of the cytotoxicity assay, employing a mouse fibroblast cell line (L929), indicated that MAST/PH had no cytotoxicity at concentrations [Formula: see text] 62 μg mL(-1) (p > 0.05). Antiproliferative activities of MAST/PH and PH were determined by the BrdU cell proliferation ELISA assay, using C6 and HeLa cell lines. In the experiment, two anticancer chemotherapy drugs, cisplatin and 5-fluorouracil, were included as positive control. Antiproliferative activity results demonstrated that MAST/PH yielded the highest suppression profile (approximately 42%) at 20 μg/ml, while free PH exerted the same activity at 100 μg/ml. Interestingly, both MAST/PH and PH suppressed the proliferation of only one of the cell lines, C6 cells. Both cisplatin and 5-fluorouracil yielded approximately 60% antiproliferative activity on C6 cells at 20 and 100 μg/ml concentrations. Antiangiogenic capacity of both MAST and MAST/PH was also investigated by using the chicken chorioallantoic membrane assay. Results obtained indicated that while MAST/PH could be included into the category of good antiangiogenic substances, the activity score of MAST was within the weak category.

Hyperbranched polydiselenide as a self assembling broad spectrum anticancer agent

This work presents a highly efficient, broad spectrum and self-delivery anticancer agent, which is the hyperbranched polydiselenide (HPSe) consisting of alternative hydrophobic diselenide groups and hydrophilic phosphate segments in the backbone framework. The data of systematic evaluations demonstrate that HPSe is very potent to inhibit the proliferation of many forms of cancer cell. The dose of HPSe required for growth inhibition of 50% (IC(50)) in all of the tested cancer cell lines is within the concentration range between 1 and 2.5 μg mL(-1) with the incubation time of 72 h. Furthermore, the amphiphilic HPSe can self-assembly into nanomicelles with an average diameter of 50 nm and spontaneously enter into tumor cells by the enhanced permeability and retention (EPR) effect. Besides, other hydrophobic anticancer drugs such as doxorubicin (DOX) can be encapsulated into HPSe micelles for combining therapy.

Targeting bacterial toxins

Protein toxins constitute the main virulence factors of several species of bacteria and have proven to be attractive targets for drug development. Lead candidates that target bacterial toxins range from small molecules to polymeric binders, and act at each of the multiple steps in the process of toxin-mediated pathogenicity. Despite recent and significant advances in the field, a rationally designed drug that targets toxins has yet to reach the market. This Review presents the state of the art in bacterial toxin targeted drug development with a critical consideration of achieved breakthroughs and withstanding challenges. The discussion focuses on A-B-type protein toxins secreted by four species of bacteria, namely Clostridium difficile (toxins A and B), Vibrio cholerae (cholera toxin), enterohemorrhagic Escherichia coli (Shiga toxin), and Bacillus anthracis (anthrax toxin), which are the causative agents of diseases for which treatments need to be improved.

Noninvasive detection of passively targeted poly(ethylene glycol) nanocarriers in tumors

The present studies noninvasively investigate the passive tumor distribution potential of a series of poly(ethylene glycol) (PEG) nanocarriers using a SkinSkan spectrofluorometer and an In Vivo Imaging System (IVIS) 100. Fluorescein conjugated PEG nanocarriers of varying molecular weights (10, 20, 30, 40, and 60 kDa) were prepared and characterized. The nanocarriers were administered intravenously to female balb/c mice bearing subcutaneous 4T1 tumors. Passive distribution was measured in vivo (λ(exc), 480 nm; λ(em), 515-520 nm) from the tumor and a contralateral skin site (i.e., control site). The signal intensity from the tumor was always significantly higher than that from the contralateral site. Trends in results between the two methods were consistent with tumor distribution increasing in a molecular weight-dependent manner (10 < 20 < 30 ≪ 40 ≪ 60 kDa). The 10 kDa nanocarrier was not detected in tumors at 24 h, whereas 40-60 kDa nanocarriers were detected in tumors for up to 96 h. The 30, 40, and 60 kDa nanocarriers showed 2.1, 5.3, and 4.1 times higher passive distribution in tumors at 24 h, respectively, as compared to the 20 kDa nanocarrier. The 60 kDa nanocarrier exhibited 1.5 times higher tumor distribution than 40 kDa nanocarrier at 96 h. Thus, PEG nanocarriers (40 and 60 kDa) with molecular weights close to or above the renal exclusion limit, which for globular proteins is ≥45 kDa, showed significantly higher tumor distribution than those below it. The hydrodynamic radii of PEG polymers, measured using dynamic light scattering (DLS), showed that nanocarriers obtained from polymers with hydrodynamic radii ≥8 nm exhibited higher tumor distribution. Ex vivo mass balance studies revealed that nanocarrier tissue distribution followed the rank order tumor > lung > spleen > liver > kidney > muscle > heart, thus validating the in vivo studies. The results of the current studies suggest that noninvasive dermal imaging of tumors provides a reliable and rapid method for the initial screening of nanocarrier tumor distribution pharmacokinetics.

Functional polymers as therapeutic agents: concept to market place

Biologically active synthetic polymers have received considerable scientific interest and attention in recent years for their potential as promising novel therapeutic agents to treat human diseases. Although a significant amount of research has been carried out involving polymer-linked drugs as targeted and sustained release drug delivery systems and prodrugs, examples on bioactive polymers that exhibit intrinsic therapeutic properties are relatively less. Several appealing characteristics of synthetic polymers including high molecular weight, molecular architecture, and controlled polydispersity can all be utilized to discover a new generation of therapies. For example, high molecular weight bioactive polymers can be restricted to gastrointestinal tract, where they can selectively recognize, bind, and remove target disease causing substances from the body. The appealing features of GI tract restriction and stability in biological environment render these polymeric drugs to be devoid of systemic toxicity that are generally associated with small molecule systemic drugs. The present article highlights recent developments in the rational design and synthesis of appropriate functional polymers that have resulted in a number of promising polymer based therapies and biomaterials, including some marketed products.

Polymer-Based Therapeutics

Polymeric materials have been applied in therapeutic applications, such as drug delivery and tissue regeneration, for decades owing to their biocompatibility and suitable mechanical properties. In addition, select polymer-drug conjugates have been used as bioactive pharmaceuticals owing to their increased drug efficacy, solubility, and target specificity compared with small-molecule drugs. Increased synthetic control of polymer properties has permitted the production of polymer assemblies for the targeted and controlled delivery of drugs, and polymeric sequestrants take advantage of their lack of solubility for the sequestration of target molecules in vivo. In more recent studies reviewed in greater detail here, the properties of polymers that distinguish them from small-molecule drugs, such as their high molecular weight and their ability to display multiple pendant moieties, have been specifically exploited for activating cellular targets or inhibiting the binding of pathogens. The elucidation of relevant structure-function relationships in investigations of this kind has relied on the combination of living polymerization methods with chemical conjugation methods, and protein engineering methods have shown increasing potential in the manipulation of architectural features of such polymer therapeutics. Garnering a detailed understanding of the various mechanisms by which multivalent polymers engage biological targets is certain to expand the role of polymers as therapeutics, by enabling highly specific activities of designed polymers in the biological environment.

Polymeric binders suppress gliadin-induced toxicity in the intestinal epithelium

BACKGROUND & AIMS

Celiac disease is a prevalent immune disorder caused by the ingestion of gliadin-containing grains. We investigated the ability of a polymeric binder to reverse the toxic effects induced by gliadin in human intestinal cells and gliadin-sensitive HCD4-DQ8 mice.

METHODS

Gliadin was neutralized by complexation to a linear copolymer of hydroxyethylmethacrylate (HEMA) and sodium 4-styrene sulfonate (SS). The ability of the polymeric binder to abrogate the damaging effect of gliadin on cell-cell contact was investigated in IEC-6, Caco-2/15, and primary cultured differentiated enterocytes. The efficacy of the polymeric binder in preventing gliadin-induced intestinal barrier dysfunction was assessed using gliadin-sensitive HLA-HCD4/DQ8 transgenic mice.

RESULTS

Poly(hydroxyethylmethacrylate-co-styrene sulfonate) [P(HEMA-co-SS)] complexed with gliadin in a relatively specific fashion. Intestinal cells exposed to gliadin underwent profound alterations in morphology and cell-cell contacts. These changes were averted by complexing the gliadin with P(HEMA-co-SS). More importantly, the P(HEMA-co-SS) hindered the digestion of gliadin by gastrointestinal enzymes, thus minimizing the formation of immunogenic peptides. Coadministration of P(HEMA-co-SS) with gliadin to HLA-HCD4/DQ8 mice attenuated gliadin-induced changes in the intestinal barrier and reduced intraepithelial lymphocyte and macrophage cell counts.

CONCLUSIONS

Polymeric binders can prevent in vitro gliadin-induced epithelial toxicity and intestinal barrier dysfunction in HCD4/DQ8 mice. They have a potential role in the treatment of patients with gluten-induced disorders.

Biologically active polymeric sequestrants: Design, synthesis, and therapeutic applications

In recent years, functional polymers exhibiting inherently biological activities have been receiving increasing attention as polymer-based human therapeutic agents. These polymeric drugs exhibit unique pharmaceutical properties that are fundamentally different from their traditional small-molecule counterparts. However, unlike polymeric drug delivery systems, examples of polymers possessing intrinsically therapeutic properties are relatively scarce. By virtue of their high-molecular-weight characteristics, these polymeric drugs can be confined to the gastrointestinal (GI) tract, where they can selectively recognize, bind, and remove target disease-causing substances from the body. Being confined to the GI tract and non-biodegradable, these polymeric drugs are free from toxic effects that are associated with traditional systemic drugs. This report highlights recent developments in the rational design and synthesis of appropriate functional polymers that have resulted in a number of promising polymer-based therapeutic agents, including some marketed products.