The Transport of Organic Anions by the Rabbit Eye: II. In Vivo Transport of Iodopyracet (Diodrast)^*

Probenecid is a highly lipid soluble benzoic acid derivative originally used to increase serum antibiotic concentrations. It was later discovered to have uricosuric effects and was FDA approved for gout therapy. It has recently been found to be a potent agonist of transient receptor potential vanilloid 2 (TRPV2). We have shown that this receptor is in the cardiomyocyte and report a positive inotropic effect of the drug. Using echocardiography, Langendorff and isolated myocytes, we measured the change in contractility and, using TRPV2^−/− mice, proved that the effect was mediated by TRPV2 channels in the cardiomyocytes. Analysis of the expression of Ca^2 + handling and β-adrenergic signaling pathway proteins showed that the contractility was not increased through activation of the β-ADR. We propose that the response to probenecid is due to activation of TRPV2 channels secondary to SR release of Ca^2 +.

We evaluated biodegradable scleral implants as a controlled intraocular delivery system of ganciclovir (GCV) for the treatment of cytomegalovirus retinitis in rabbits. The scleral implants (weight, 8.5 mg; length, 5 mm) were made of poly(dl-lactide) (PLA) or poly(dl-lactide-co-glycolide) (PLGA) and contained various amounts of GCV. The in vitro release studies demonstrated a triphasic release pattern. The 10% GCV-loaded scleral implant made from PLA with a molecular weight of 130 000 released GCV for 6 months. The in vivo release and biodegradation were studied using the 25% GCV-loaded implant made from PLGA($\text{75}\text{25}$) with a molecular weight of 121 000 in pigmented rabbits. The GCV concentration in the range of ED₉₀ for human CMV was maintained in the vitreous for over 3 months and in the retina/choroid for over 5 months. The GCV concentration was greater in the retina/choroid than in the vitreous throughout the study. The scleral implants showed two phases of biodegradation: lagtime and erosion. In the erosion phase, the weight of PLGA dropped remarkably. All the scleral implants were separated into two pieces at the site of scleral penetration and displaced into the vitreous 10 weeks after implantation. The fragments disappeared from the vitreous and the subconjunctival space 5 months after implantation. Our findings suggest that the biodegradable scleral implant may be a promising device for the intraocular drug delivery of GCV.

A patient with acquired immune deficiency syndrome with bilateral cytomegalovirus retinitis was treated with intravitreal 200-ug/ 0.1-ml doses of ganciclovir (9-[2-hydroxy-1-(hydroxymethyl) ethoxymethyl]guanine). The ganciclovir serum and intravitreal concentrations were measured with an enzyme-linked immunosorbent assay and pharmacokinetic factors were determined. There was no evidence of systemic absorption of ganciclovir from the eye. The elimination half-life of ganciclovir from the vitreous was estimated to be 13.3 hours. The intravitreal concentration remained above the ID50 of cytomegalovirus for approximately 62 hours after a single injection. Clinically, the patient retained useful vision in his right eye for three months. A total of 28 intravitreal injections were given on an outpatient basis under topical anesthesia and were well tolerated. There was no evidence of retinal toxicity from the drug.

A patient with secondary syphilis had positive serum and cerebroespinal fluid findings. Fluorescent-antibody darkfield testing demonstrated spirochetes (Treponema pallidum) in the aqueous. There were clinical signs of secondary syphilis, including palmar skin lesions and frontal balding, and bilateral nasal altitudinal visual field loss. The discrete, waxy, yellow-white retinal lesions became increasingly pigmented and later migrated anteriorly into the vitreous chamber. The patient was treated with large amounts of penicillin (total dosage, 440 million units) and probenecid. Visual acuity improved to R.E.: 6/9 (20/30) and L.E.: 6/12 (20/40) and the inflammatory reaction disappeared. A second aqueous tap eight weeks after therapy ended showed no spirochetes by fluorescent-antibody testing.

Capillaries were isolated from bovine retina by homogenization and glass bead filtration in order to study their ability to transport certain solutes. Viability of the microvessels was demonstrated by their ability to maintain linear rates of substrate oxidation for more than two hours. Hexose uptake (measured using 3-o-methyl-d-glucose) could be inhibited by cytochalasin B, phloretin and phlorizin, but not by 2,4-dinitrophenol or ouabain. d-Glucose, 2-deoxy-d-glucose, d-mannose, d-galactose and d-xylose inhibited 3-o-methyl-d-glucose uptake, while l-glucose, d-ribose and d-fructose did not. When incubated at 37°C with 5 mm-d-glucose, the microvessels contained much more free d-glucose than d-glucose metabolites. Thus, transport was not rate limiting for metabolism. l-Glucose entered capillaries more slowly than other hexoses and served as a marker for simple diffusion of sugars into the cells. ⁸⁶Rb⁺ was used as an indicator of K⁺ transport activity. Uptake of ⁸⁶Rb⁺ was temperature sensitive and markedly inhibited by I mm ouabain, thus indicating the presence of an active K⁺ transport system. Na⁺-dependent amino acid transport was demonstrated using α-(methylamino)isobutyric acid as a model substrate. Capillary uptake of this neutral amino acid analogue was inhibited after abolishing the Na⁺ gradient with 1 mm-ouabain. Uptake of the organic anion p-aminohippuric acid (PAH) was slightly greater than uptake of the extracellular marker sucrose. There was a small inhibition of PAH uptake by fluorescein and penicillin but not by probenicid. Our results indicate the presence of several transport processes in retinal capillary endothelial cells which may be important for the maintenance of homeostasis within the retina.