Chemical & Biomolecular Engineering

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Formerly known as the Department of Chemical Engineering.

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2004 - 20102

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Subject: search.filters.subject.Health Sciences, Ophthalmology

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    Metabolic Acid Transport in Human Retinal Pigment Epithelium
    (2010) Adijanto, Jeffrey; Wang, Nam S; Miller, Sheldon S; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    At the back of our eyes, photoreceptors capture light and convert it into electrical signals that we perceive in our brain as vision. Photoreceptor function is energy expensive, even more so than many other processes in the body. Furthermore, photoreceptor metabolism increases in the dark and releases more metabolic by-products (CO2, lactic acid, and water) into the photoreceptor extracellular space (SRS). The retinal pigment epithelium (RPE) maintains photoreceptor health by transporting these metabolic acids from the SRS to the choroidal blood supply. By using native and cultured fetal human RPE, we show that the apical membrane is significantly more permeable to CO2 than the basolateral membrane. This feature traps CO2 in the cell and drives carbonic anhydrase (CA)-mediated hydration of CO2 into HCO3, which is subsequently transported out of the basolateral membrane by a Na-linked HCO3 co-transporter (NBC). This process increases net steady-state fluid absorption, thus maintaining retinal adhesion to the RPE. Oxidative metabolism generates significantly more ATP than glycolysis, but photoreceptors derive 50% of their total ATP consumed from glycolysis due to the low oxygen level at the photoreceptor inner segment. Furthermore, lactic acid production and release into the SRS almost doubles in the dark. We show that the RPE transports lactic acid from the SRS via a proton-linked monocarboxylate transporter (MCT1), and this process activates pHi-regulatory mechanisms at the RPE apical membrane: Na/H exchanger (NHE) and Na-linked HCO3 transporters (NBC1 & NBC3). These mechanisms also facilitate MCT1-mediated lactic acid transport by preventing buildup of a proton-gradient across the RPE apical membrane. We show that an increase in SRS CO2 or lactic acid level causes RPE cell swelling. The RPE alleviates swell-induced osmotic stress by activating apical membrane K-channel (Kir 7.1) and basolateral membrane Cl -channel (ClC-2), which drives KCl (and fluid) out of the cell to decrease cell volume. In this study, we identified the cellular mechanisms in RPE that prevent acidosis and fluid accumulation in the SRS caused by increased photoreceptor metabolism in the dark. These homeostatic processes maintain the close anatomical relationship between photoreceptors and RPE, thus protecting photoreceptor health and preserving visual function.
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    Study of ocular transport of drugs released from a sustained release device
    (2004-05-03) Kim, Hyuncheol; Wang, Nam Sun; Chemical Engineering
    Delivering ocular therapeutics to a target site with minimal side effects requires detailed information about the distribution and elimination pathways. This knowledge can guide the development of new drug delivery devices. In this study, we investigated the movement of two drug surrogates, H-110, which is lipophilic, and Gd-DTPA, which is hydrophilic, released from polymer-based implants using a fluorescein technique and magnetic resonance imaging (MRI). We also studied the pharmacokinetics of intravitreally injected triamcinolone acetonide, a low water soluble drug used for treating sight-threatening diseases such as diabetic retinopathy and choroidal neovascularization associated with age-related macular degeneration (AMD). At 24 hour post implantation, H-110 released from an intravitreal implant was detected in the subretinal space. However, following a subconjunctival implant, very little H-110 fluorescence was detected in the subretinal region. H-110 most likely reached the subretinal space from an intravitreal implant by diffusion through the vitreous and retina. However, most of the H-110 released from a subconjunctival implant is thought to dissipate through the choroidal blood flow. Concentration profiles of Gd-DTPA, which was released from an intravitreal implant in a New Zealand white rabbit, approached pseudo-steady state within 7 to 8 hours and showed gradients at the rabbit's vitreous-retina border suggesting that diffusion was occurring into the retinal-choroidal-scleral membrane. Parametric analysis with a finite element mathematical model of the rabbit eye yielded for Gd-DTPA a diffusion coefficient of 2.8 × 10-6 cm2/sec in the vitreous and a permeability of 1.0 × 10-5 cm/sec in the composite retina-choroid-sclera membrane. Gd-DTPA concentration decreased away from the implant. Such regional concentration variations throughout the vitreous may have clinical significance when the ubiquitous eye diseases are treated using a single positional implant. Subconjunctival implants in vivo delivered a mean total of 2.7 µg of Gd-DTPA over 8 hours into the vitreous representing only 0.12% of the total amount of compound released from the implant in vitro. No Gd-DTPA was detected in the posterior segment of the eye. Ex vivo, the Gd-DTPA concentration in the vitreous was 30 fold higher suggesting the elimination of significant in vivo barriers to the movement of drugs from the subconjunctival space into the vitreous. We developed a new preservative-free formulation for intravitreal injections of triamcinolone acetonide for the treatment of diabetic macular edema, and choroidal neovascularization associated with AMD in human clinical trials at the National Institutes of Health. A pharmacokinetic study in rabbits was done to estimate elimination rate of two injection amounts of triamcinolone acetonide, 4 mg and 16 mg, from the vitreous. From our pharmacokinetic model, we found the half-lives for 4 mg and 16 mg injection in the vitreous to be 18.6 days and 37.6 days, respectively. We subsequently estimated the half-lives of 1 mg and 8 mg triamcinolone acetonide injection in order to predict therapeutic exposure in human. There are three components in this thesis: the study of lipophilic H-110 transport with fluorescence, the study of hydrophilic transport of Gd-DTPA with MRI, and the pharmacokinetic analysis of triamcinolone acetonide. They have each contributed to further insights into our fundamental understanding of drug movement in the eye and the implication on optimal therapeutic delivery.