Stephanie T. Lopina

Penicillin V-conjugated PEG-PAMAM star polymers

Starburst® PAMAM dendrimers are potential carriers for drug delivery due to their unique structure. Drug-delivery scaffolds were designed and built up based on the polyethylene glycolpolyamidoamine (PEG-PAMAM) star polymer. Penicillin V was used as a model carboxylic group containing drug to conjugate with full- and half-generation PAMAM dendrimers. G2.5 PAMAM (with 32 carboxylic groups on the surface) dendrimers and G3.0 (with 32 primary amine groups on the surface) were typically chosen. There are two strategies given in the paper where a drug carrying a carboxylic group (e.g. penicillin V) was coupled to star polymer via amide and ester bonds, respectively. FT-IR, UV-Vis and 1H-NMR were used to characterize the intermediates and drug-star polymer conjugates. A single-strain bacterium, Staphylococcus aureus, was grown up for penicillin-conjugated PEG-PAMAM (G3.0) star polymer activity test. The result verified the bioavailability of modified penicillin after the ester bond was cleaved.

Synthesis and characterization of L-tyrosine based polyurethanes for biomaterial applications

The use of amino acid based polymers for biomaterial applications enhance biocompatibility and ensure biodegradability. Two polyurethanes based on L-tyrosine based diphenolic dipeptide, desaminotyrosyl tyrosine hexyl ester as chain extender are synthesized with polyethylene glycol (PEG) and polycaprolactone diol (PCL) as soft segment and hexamethylene diisocyanate as diisocyanate. The chemical structure and molecular characteristics of the polymers were studied by 1H NMR, FTIR, and gel permeation chromatography. Results of DSC and TGA analysis were used for examining the thermal behavior of the polyurethanes. In addition, DSC results were used to analyze the morphology of the polymers, which shows characteristic microphase behavior of the polyurethanes. The tensile properties of the polyurethanes are primarily controlled by the soft segment and are higher in PCL based polymers. Contact angle, water vapor permeation, release of model drug, and water absorption characteristics of the polymers were studied and analyzed in terms of structure of the polyurethanes. In vitro degradation studies show that PEG based polyurethane is more degradable than PCL based polyurethane. The difference in the soft segment structure offers significant variation in the properties of the polyurethanes. These polyurethanes show the potential for use in a variety of biomaterial applications including tissue engineering.

Effects of sterilization on poly(ethylene glycol) hydrogels

The past few decades have witnessed a dramatic increase in the development of polymeric biomaterials. These biomaterials have to undergo a sterilization procedure before implantation. However, many sterilization procedures have been shown to profoundly affect polymer properties. Poly(ethylene glycol) hydrogels have gained increasing importance in the controlled delivery of therapeutics and in tissue engineering. We evaluated the effect of ethylene oxide (EtO), hydrogen peroxide (H(2)O(2)), and gamma sterilization of poly(ethylene glycol) hydrogels on properties relevant to controlled drug delivery and tissue engineering. We observed that the release of cyclosporine (CyA) (an immunosuppressive drug that is effective in combating tissue rejection following organ transplantation) was significantly affected by the type of sterilization. However, that was not the case with rhodamine B, a dye. Hence, the drug release characteristics were observed to be dependent not only on the sterilization procedure but also on the type of agent that needs to be delivered. In addition, differences in the swelling ratios for the sterilized and unsterilized hydrogels were statistically significant for 1:1 crosslinked hydrogels derived from the 8000 MW polymer. Significant differences were also observed for gamma sterilization for 1:1 crosslinked hydrogels derived from the 3350 MW polymer and also the 2:1 crosslinked hydrogels derived from the 8000 MW polymer. Atomic force microscopy (AFM) studies revealed that the roughness parameter for the unsterilized and EtO-sterilized PEG hydrogels remained similar. However, a statistically significant reduction of the roughness parameter was observed for the H(2)O(2) and gamma-sterilized samples. Electron spin resonance (ESR) studies on the unsterilized and the sterilized samples revealed the presence of the peroxy and the triphenyl methyl carbon radical in the samples. The gamma and the H(2)O(2)-sterilized samples were observed to have a much higher concentration of the radical pecies when compared with the EtO and the unsterilized samples.

Nanospheres formulated from L-tyrosine polyphosphate as a potential intracellular delivery device.

Current delivery devices for drugs and genes such as films and microspheres are usually formulated from polymers that degrade over a period of months. In general, these delivery systems are designed to achieve an extracellular release of their encapsulated drugs. For drugs that require interaction with cellular machinery, the efficacies of both macroscopic and microscopic delivery systems are normally low. In contrast, nano-sized drug delivery vehicles could achieve high delivery efficiencies, but they must degrade quickly, and the delivery system itself should be nontoxic to cells. In this aspect, biodegradable nanospheres formulated from l-tyrosine polyphosphate (LTP) have been produced from an emulsion of oil and water for the potential use as an intracellular delivery device. Scanning electron microscopy (SEM) and dynamic laser light scattering (DLS) show that LTP nanospheres possess a diameter range between 100 and 600 nm. SEM reveals nanospheres formulated from LTP are spherical and smooth. Additionally, DLS studies demonstrate that nanospheres degrade hydrolytically in 7 days. Confocal microscopy reveals LTP nanospheres are internalized within human fibroblasts. Finally, the cell viability after exposure to LTP nanospheres and determined with a LIVE/DEAD Cell Viability Assay is comparable to a buffer control. In conclusion, our nanospheres have been shown to be nontoxic to human cells, possess the appropriate size for endocytosis by human cells, and degrade within 7 days. Therefore LTP nanospheres can be used for a sustained intracellular delivery device.

L-Tyrosine-based backbone-modified poly(amino acids)

Tyrosine-based pseudo-peptide polymers, first introduced in 1987 by Kohn and Langer, have been identified for potential biomaterial applications. These materials combine the desired polypeptide properties of biocompatibility, biodegradability, non-toxicity, and non-immunogenicity with good processing properties including solubility, thermal stability, and moldability which arise from alternating non-amide bonds along the polymer backbone. This paper focuses on the analysis of two such polymers based on the natural amino acid L-tyrosine. Starting from L-tyrosine and its deaminated analogue, 3-(4-para-hydroxy)-phenylpropionic acid, a diphenolic structure containing an amide linkage, was synthesized following standard procedures of peptide synthesis. This diphenolic structure was then used as a monomer to synthesize a polyiminocarbonate using a cyanogen bromide-initiated reaction and a polycarbonate using a triphosgene-initiated reaction. The polyiminocarbonate has iminocarbonate linkages and the polycarbonate has carbonate linkages alternating with amide linkages in the respective polymer backbone. Analytical studies were performed regarding the feasibility of the reaction procedures, the physical properties of the polymers, and their degradation processes, to gain insight into the potential biomaterial applications of these polymers. These results independently reaffirm the studies published by Kohn et al. working on similar polymeric systems.

Sustained local drug delivery from a novel polymeric ring to inhibit intimal hyperplasia

The long-term clinical success of autologous vein and synthetic vascular grafts are limited because of the development of anastomotic intimal hyperplasia (IH). We have previously published data suggesting that cyclosporine (CyA) may reduce the development of IH in a canine model (Hirko et al., J Vasc Surg 1993;17:877-887). However, systemic administration of CyA could create serious adverse effects. Therefore, it is our long-term goal to test the hypothesis that the controlled local release of CyA from a polymeric vascular wrap would prevent the development of IH. To test this hypothesis, we developed a controlled release polymeric ring that could be placed around anastomotic sites to deliver therapeutic drugs locally. The ring is a composite polymeric device consisting of poly(DL-lactide-co-glycolide) (PLGA) microspheres embedded in a poly(ethylene glycol) hydrogel. Several in vitro studies were conducted to evaluate the effects of different sterilization procedures on the properties of the device. It was determined that gamma sterilization was the preferred sterilization method of choice for this device. In vivo studies were conducted on a swine model to evaluate the biocompatibility of the ring. The histological findings of the ring implants at 2 and 4 weeks demonstrate the biocompatibility of this device.

Electron spin resonance studies of the effects of sterilization on poly(ethylene glycol) hydrogels

The effects of several sterilization procedures on a poly(ethylene glycol) (PEG) hydrogel have been examined by electron spin resonance (ESR) spectroscopy. The crosslinked polyurethanes were synthesized by reacting PEG with a tri-functional isocyanate. The free radical concentration of unsterilized, ethylene oxide (EtO), hydrogen peroxide (H(2)O(2)), and gamma sterilized hydrogels were monitored over time. Free radical presence was observed for all the treatments, unsterilized and sterilized PEG hydrogels. The unsterilized and the EtO sterilized samples elicited similar levels of free radical intensity whereas, the H(2)O(2) and gamma sterilized samples had a significantly higher free radical concentration. The spectra reveal overlapping resonances of a peroxy and a triphenylmethyl radical. The concentration of the free radicals increase for all the treatments over time except for the gamma sterilized sample. The increase is significantly higher in the H(2)O(2) sterilized sample. A tentative model is proposed to explain the reaction pathway leading to the production of the free radicals. The observed increases in the free radical concentrations of the EtO and hydrogen peroxide sterilized hydrogels over a five-month-period make it difficult to predict properties that are affected by free radical concentrations. In that light, gamma sterilization, that does not induce a change in free radical concentrations over a five month period, could be the sterilization method of choice for PEG hydrogels that could potentially be stored for undetermined periods of time prior to application.