Ronald L. Fournier

A transient mathematical model of oxygen depletion during photodynamic therapy.

A transient one-dimensional mathematical model is presented to help visualize the qualitative and quantitative effects on inter-capillary tissue undergoing photodynamic therapy (PDT). The model is solved by a Crank-Nicholson finite difference formulation to provide time-dependent concentrations of the Type II mechanisms photo-oxidation species in the tissue surrounding a capillary. The time-dependent solution allows educated decisions to be made as to the optimum timing of light fractionation (on/off) cycles. Qualitative and quantitative optimization of the PDT process is considered along with a case study of data in the literature, the main goal being to provide optimized light therapy regimens for eventual clinical use.

Predictions of Mathematical Models of Tissue Oxygenation and Generation of Singlet Oxygen during Photodynamic Therapy

Photodynamic therapy (PDT) is a relatively new protocol for cancer treatment which has recently been approved for limited clinical use. Traditionally, the success of treatment with PDT has been compared on the basis of total light delivery. Using the mathematical model of Henning et al. (Radiat. Res. 142, 221-226, 1995), we have determined that when oxygen is not depleted from the tissue, the concentration of singlet oxygen that is generated is directly proportional to the product of the light fluence rate (phi) and the concentration of the photosensitizer (Cs). Therefore, phiCs is an appropriate parameter for comparing the potential success of PDT protocols under these conditions. For a treatment of time t, the observed photodynamic effect resulting from singlet oxygen exposure should be directly related to phiCst. For high phiCs, the model predicts that oxygen depletion occurs within the tumor tissue. As a result, the photodynamic effect is no longer proportional to phiCst. We have expanded the model of Henning et al. to include the changes in oxygen concentration which occur within the capillary as blood flows through the tissue. Our new predictions with the mathematical model for optimal PDT treatment conditions are significantly different from those predicted by the previous models. Predictions of the model are given using parameters relevant for treatment of solid tumors with Photofrin.

Helicobacter pylori survival in gastric mucosa by generation of a pH gradient.

Helicobacter pylori has been established as the major causative agent of human active gastritis and is an essential factor in peptic ulcer disease and gastric cancer. The mechanism that has been proposed for H. pylori to control its inhospitable microenvironment happens to coincide with the pH control technique developed by us. This technique was developed to separate an acidic environment from a basic environment for a sequential enzymatic reaction by the hydrolysis of urea within a thin layer of immobilized urease. In this paper, a mathematical model is presented to consider how H. pylori survives the gastric acidity. The computed results explain well the experimental data available involving H. pylori.

Application of a convective-dispersion model to predict in vivo hepatic clearance from in vitro measurements utilizing cryopreserved human hepatocytes.

Growing interest in the prediction of in vivo pharmacokinetic data from purely in vitro data has grown into a process known as the in vitro-in vivo correlation (IVIC). IVIC can be used to determine the viability of new chemical entities in the early drug development phases, leading to a reduction of resource spending by many large pharmaceutical companies. Here, a convective-dispersion model was developed to predict the total hepatic clearance of six drugs using pharmacokinetic data obtained from in vitro metabolism studies in which the drug disappearance from suspensions of human cryopreserved hepatocytes was measured. Predicted in vivo hepatic clearances estimated by the convective-dispersion model were ultimately compared to the actual clearance values and to in vivo hepatic clearances that were scaled based on the well-stirred model. Finally, sensitivity studies were performed to determine the dependence of hepatic clearance on a number of physiological model parameters. Results reaffirmed that low clearance drugs exhibit rate-limited metabolism, and their hepatic clearances are thus independent of blood flow characteristics, whereas drugs with relatively higher clearance values show a more pronounced dependence on the flood flow properties of dispersion and convection. Absent a priori knowledge about the flow-dependent properties of a drugs clearance, the convective dispersion model applied to disappearance data acquired from cryopreserved human hepatocytes is likely to provide satisfactory estimates of hepatic drug clearance.

Demonstration of pH control in a commercial immobilized glucose isomerase

The synthesis of a variety of important biochemicals involves multistep enzyme-catalyzed reactions. In many cases, the optimal operating pH is much different for the individual enzymatic steps of such synthesis reactions. Yet, it may be beneficial if such reaction steps are combined or paired, allowing them to occur simultaneously, in proximity to one another, and at their respective optimal pH. This can be achieved by separating the micro-environments of the two steps of a reaction pathway using a thin urease layer that catalyzes an ammonia-forming reaction. In this article, the pH control system in a commercial immobilized glucose (xylose) isomerase pellet, which has an optimal pH of 7.5, is demonstrated. This system allows the glucose isomerase to have near its optimal pH activity when immersed in a bulk solution of pH 4.6. A theoretical analysis is also given for the effective fraction of the immobilized glucose isomerase, which remains active when the bulk pH is at 4.6 in the presence of 20 mM urea versus when the bulk pH is at its optimal pH of 7.5. Both theoretical and experimental results show that this pH control system works well in this case. (c) 1996 John Wiley & Sons, Inc.

Evaluation of an immunoisolation membrane formed by incorporating a polyvinyl alcohol hydrogel within a microporous filter support.

An immunoisolation membrane formed by incorporating a high water content polyvinyl alcohol (PVA) hydrogel into a microporous polyether sulfone (PES) filter has been investigated in this study. The PVA hydrogel is formed in situ within the filter pores via glutaraldehyde (GA) crosslinking under acidic conditions. The tortuous nature of the microporous filter pores securely anchors the embedded hydrogel to provide excellent structural integrity. The high void fraction of the PES filter support (>80%) and high water content of the PVA hydrogel (>85% water by weight) allow excellent solute transport rates, while an appropriate level of glutaraldehyde crosslinking supplies the required molecular size selectivity. In vitro permeability measurements made with solutes covering a wide range of molecular sizes demonstrate high transport rates for small nutrient molecules with rapidly diminishing permeabilities above a molecular weight of approximately 1,000 Dalton. Implantation experiments show that the membrane properties are not deleteriously affected by prolonged in vivo exposure or common sterilization techniques. Thus, this hybrid hydrogel/filter membrane system offers a promising approach to the immunoisolation of implanted cells.

A FEASIBILITY ANALYSIS OF A NOVEL APPROACH FOR THE CONVERSION OF XYLOSE TO ETHANOL

Economic production of ethanol from plant biomass could be significantly increased if the feedstock for the fermentation is more completely utilized. Currently, simple sugars (mostly D-glucose and D-xylose) can be recovered from lignocellulose by enzymatic or acid hydrolysis. However, while glucose can be readily converted to ethanol by yeasts, the xylose is not fermentable by many of the same species of yeasts that are able to convert glucose into ethanol. Nevertheless, xylose can be converted to its ketose isomer, xylulose, by the enzyme xylose isomerase and this isomer can be converted to ethanol. A major obstacle, however, in converting the xylose to xylulose and then simultaneously converting the xylulose to ethanol is that the pH at which xylose isomerase displays its optimal activity (pH of 7.0-8.0) is much different from the pH at which the fermentation of the xylulose and glucose is best carried out (pH of 4.0-5.0). Herein we propose a novel scheme to provide a means by which the isomerization an...

Numerical investigation of a novel spiral wound membrane sandwich design for an implantable bioartificial pancreas

A novel spiral wound membrane sandwich (SWMS) design for an implantable bioartificial pancreas is presented and is numerically evaluated using a comprehensive model of the glucose-insulin kinetics within the device. The spiral blood flow pattern in this design induces a convective flow of blood ultrafiltrate directly through the islet chamber. Use of ultrafiltration in addition to diffusion allows rapid transmission of blood glucose changes to the islet chamber and efficient transport of insulin from the islet chamber back to the blood stream. Simulation results suggest that the implantable bioartificial pancreas design presented in this paper may offer a means of improving the control of blood glucose levels in type I diabetics.

The inhibition of Escherichia coli lac operon Gene expression by antigene oligonucleotides-mathematical modeling

Gene transcription is regulated by transcription factors that can bind to specific regions on DNA. Antigene oligonucleotides (oligos) can bind to specific regions on DNA and form a triplex with the double-stranded DNA. The triplex can competitively inhibit the binding of transcription factors and, as a result, transcription can be inhibited. A genetically structured model has been developed to quantitatively describe the inhibition of the Escherichia coli lac operon gene expression by triplex-forming oligos. The model predicts that the effect of triplex-forming oligos on the lac operon gene expression depends on their target sites. Oligonucleotides targeted to the operator are much more effective than those targeted to other regulatory sites on the lac operon. In some cases, the effect of oligo binding is similar to that of a mutation in the lac operon. The model provides insight as to the specific binding site to be targeted to achieve the most effective inhibition of gene expression. The model is also capable of predicting the oligo concentration needed to inhibit gene expression, which is in general agreement with results reported by other investigators.

Development and characterization of erythrosine nanoparticles with potential for treating sinusitis using photodynamic therapy.

BACKGROUND Antimicrobial therapy for sinusitis has been shown to reduce or eliminate pathologic bacteria associated with rhinosinusitis and improve the symptoms associated with the disease. However, the continuing rise in antibiotic resistance, the ongoing problem with patient compliance, and the intrinsic difficulty in eradication of biofilms complicates antibiotic therapy. The introduction of photodynamic antimicrobial therapy (PAT) using erythrosine, a photosensitizer, could eliminate the bacteria without inducing antibiotic resistance or even requiring daily dosing. In the present study, erythrosine nanoparticles were prepared using poly-lactic-co-glycolic acid (PLGA) and evaluated for their potential in PAT against Staphylococcus aureus cells. METHODS PLGA nanoparticles of erythrosine were prepared by nanoprecipitation technique. Erythrosine nanoparticles were characterized for size, zeta potential, morphology and in vitro release. Qualitative and quantitative uptake studies of erythrosine nanoparticles were carried out in S. aureus cells. Photodynamic inactivation of S. aureus cells in the presence of erythrosine nanoparticles was investigated by colony forming unit assay. RESULTS Nanoprecipitation technique resulted in nanoparticles with a mean diameter of 385nm and zeta potential of -9.36mV. Erythrosine was slowly released from nanoparticles over a period of 120h. The qualitative study using flow cytometry showed the ability of S. aureus cells to internalize erythrosine nanoparticles. Moreover, erythrosine nanoparticles exhibited a significantly higher uptake and antimicrobial efficacy compared to pure drug in S. aureus cells. CONCLUSION In conclusion, erythrosine-loaded PLGA nanoparticles can be a potential long term drug delivery system for PAT and are useful for the eradication of S. aureus cells.