Soumyajit Majumdar

Drug delivery to the retina: challenges and opportunities.

Retinal drug delivery is a challenging area in the field of ophthalmic drug delivery. An ideal drug delivery system for the retina and vitreous humor has not yet been found, despite extensive research. Drug delivery to retinal tissue and vitreous via systemic administration is constrained due to the presence of a blood–retinal barrier (BRB) which regulates permeation of substances from blood to the retina. Although intravitreal administration overcomes this barrier, it is associated with several other problems. In recent years, transporter targeted drug delivery has become a clinically significant drug delivery approach for enhancing the bioavailabilities of drug molecules with poor membrane permeability characteristics. Various nutrient transporters, which include peptide, amino acid, folate, monocarboxylic acid transporters and so on, have been reported to be expressed on the retina and BRB. Prodrug derivatisation of drug molecules which target these transporters could result in enhanced ocular bioavailability. Highlighted in this review are various strategies currently employed for drug delivery to the posterior chamber, and novel opportunities that can be exploited to enhance ocular bioavailability of drugs.

Role of Metabolism in Ocular Drug Delivery

Metabolism is one of the primary routes of drug elimination from the body. This process comprises of mechanisms, such as oxidation and conjugation, which lead to inactivation and/or elimination from hepatic, biliary, pulmonary, renal and ocular tissues. Enzymes involved in metabolism are expressed in various tissues of the body, liver being the primary site. Studies involving ocular tissues have demonstrated the expression of several metabolic enzymes such as esterases, peptidases, ketone reductases, and CYP-450s in these tissues. These enzymes play an important role in ocular homeostasis by preventing entry and/or eliminating xenobiotics from the ocular tissues. Scientists have targeted these enzymes in drug design and delivery through prodrug derivatization. The prodrugs undergo biotransformation to the parent drug by ocular enzymatic degradation. This review examines the distribution pattern of various metabolic enzymes in the ocular tissues, their physiological role and utility in targeted prodrug delivery.

Indomethacin-Loaded Solid Lipid Nanoparticles for Ocular Delivery: Development, Characterization, and In Vitro Evaluation

PURPOSE The goal of this study was to develop and characterize indomethacin-loaded solid lipid nanoparticles (IN-SLNs; 0.1% w/v) for ocular delivery. METHODS Various lipids, homogenization pressures/cycles, Tween 80 fraction in the mixture of surfactants (Poloxamer 188 and Tween 80; total surfactant concentration at 1% w/v), and pH were investigated in the preparation of the IN-SLNs. Compritol(®) 888 ATO was selected as the lipid phase for the IN-SLNs, as indomethacin exhibited a highest distribution coefficient and solubility in this phase. RESULTS Homogenization at 15,000 psi for 6 cycles resulted in the smallest particle size. Increase in the Poloxamer 188 fraction resulted in decrease in the entrapment efficiency (EE). The mean particle size, polydispersity index, zeta-potential, and EE of the optimized formulation were 140 nm, 0.16, -21 mV, and 72.0%, respectively. IN-SLNs were physically stable post-sterilization and on storage for a period of 1 month (last timepoint tested). A dramatic increase in the chemical stability and in vitro corneal permeability of indomethacin was observed with the IN-SLN formulation in comparison to the indomethacin solution- (0.1% w/v) and indomethacin hydroxypropyl-beta-cyclodextrin-based formulations (0.1% w/v). CONCLUSION Results from this study suggest that topical IN-SLNs could significantly improve ocular bioavailability of indomethacin.

Functional differences in nucleoside and nucleobase transporters expressed on the rabbit corneal epithelial cell line (SIRC) and isolated rabbit cornea

The purpose of this study was to investigate the expression of nucleoside/nucleobase transporters on the Statens Seruminstitut rabbit corneal (SIRC) epithelial cell line and to evaluate SIRC as an in vitro screening tool for delineating the mechanism of corneal permeation of nucleoside analogs. SIRC cells (passages 410–425) were used to study uptake of [3H]thymidine, [3H]adenine, and [3H]ganciclovir. Transport of [3H]adenine and [3H]ganciclovir was studied across isolated rabbit cornea. Uptake and transport studies were performed for 2 minutes and 120 minutes, respectively, at 34°C. Thymidine uptake by SIRC displayed saturable kinetics (Km=595.9±80.4μM, and Vmax=289.5±17.2 pmol/min/mg protein). Uptake was inhibited by both purine and pyrimidine nucleosides but not by nucleobases. [3H]thymidine uptake was sodium and energy independent but was inhibited by nitrobenzylthioinosine at nanomolar concentrations. Adenine uptake by SIRC consisted of a saturable component (Km=14.4±2.3μM, Vmax=0.4±0.04 nmol/min/mg protein) and a nonsaturable component. Uptake of adenine was inhibited by purine nucleobases but not by the nucleosides or pyrimidine nucleobases and was independent of sodium, energy, and nitrobenzylthioinosine. [3H]ganciclovir uptake involved a carrier-mediated component and was inhibited by the purine nucleobases but not by the nucleosides or pyrimidine nucleobases. However, transport of [3H]adenine across the isolated rabbit cornea was not inhibited by unlabeled adenine. Further, corneal permeability of ganciclovir across a 100-fold concentration range remained constant, indicating that ganciclovir permeates the cornea primarily by passive diffusion. Nucleoside and nucleobase transporters on rabbit cornea and corneal epithelial cell line, SIRC, are functionally different, undermining the utility of the SIRC cell line as an in vitro screening tool for elucidating the corneal permeation mechanism of nucleoside analogs.

Functional expression of a sodium dependent nucleoside transporter on rabbit cornea: Role in corneal permeation of acyclovir and idoxuridine

Purpose. The major objectives were to investigate functional expression of nucleoside transporters on the rabbit cornea and to delineate mechanism of corneal permeation of acyclovir (ACV) and idoxuridine (IDU). Methods. Transport studies were conducted with isolated rabbit corneas at 34°C using [ 3 H]thymidine, [ 3 H]ACV and [ 3 H]IDU. Results. Thymidine transport across rabbit cornea comprised of saturable (K m = 14.9 ± 9.7µM and V max = 0.045 ± 0.0087 nmol/min) and non saturable (k d = 0.00015 ± 0.000013µl/min) components. Both purine and pyrimidine nucleosides including inosine inhibited transport of [ 3 H]thymidine. However, nucleobases adenine and thymine did not have any inhibitory effect on thymidine transport which was sodium dependent with a Na + : thymidine coupling ratio of greater than 1 : 1 indicating that the nucleoside transporter is of the N3 type. Although IDU inhibited transport of [ 3 H]thymidine, unlabeled IDU and thymidine did not inhibit [ 3 H]IDU transport suggesting that IDU was binding to the transporter but was not translocated by it. ACV did not affect transport of [ 3 H]thymidine. Moreover, thymidine, adenine or unlabeled ACV did not inhibit [ 3 H]ACV transport. Permeability coefficients of ACV and IDU over a 4 fold concentration range did not show any significant difference confirming that these antiviral agents permeate the cornea by passive diffusional mechanism. Conclusion. Functional expression of a N3 type sodium dependent nucleoside transporter has been demonstrated on the rabbit cornea. Antiviral nucleoside analogs ACV and IDU are not substrates for this transporter and appear to permeate the cornea by simple passive diffusion.

Mechanism of ganciclovir uptake by rabbit retina and human retinal pigmented epithelium cell line ARPE-19

Purpose. The objective of this study was to elucidate the mechanism of ganciclovir uptake by the rabbit retina and the human retinal pigmented epithelium cell line ARPE-19. Materials and methods. [3H]Adenine, [3H]adenosine, [3H]thymidine, and [3H]ganciclovir were used to elucidate the mechanism of ganciclovir uptake by the ARPE-19 cell line and the isolated rabbit retinal tissue. Uptake studies using ARPE-19 cell line and isolated rabbit retina were carried out at 37°C and 25°C, respectively, for 5 min. Results. Uptake of [3H]adenine by ARPE-19 cells decreased by 95% in the presence of unlabeled adenine. Other nucleobases such as guanine, thymine, and uracil and the nucleosides adenosine, guanosine, thymidine, uridine, and inosine also reduced uptake of [3H]adenine by the ARPE-19 cells. Although [3H]adenosine and [3H]thymidine uptake was inhibited by nucleosides, nucleobases did not demonstrate any inhibitory effect, indicating that nucleosides can only bind to the nucleobase transporter but are not translocated by it. Uptake of the nucleosides and nucleobases by the ARPE-19 cells was sodium and pH independent. [3H]adenosine and [3H]thymidine uptake by the ARPE-19 cells was inhibited by nanomolar quantities of nitrobenzylthioinosine. Uptake of [3H]adenine by the isolated rabbit retina was drastically reduced in the presence of unlabeled adenine. Unlabeled thymidine and guanosine, and removal of sodium from the uptake medium, inhibited uptake of [3H]thymidine by the rabbit retina. Nucleosides, nucleobases, and unlabeled ganciclovir did not exhibit any inhibitory effect on [3H]ganciclovir uptake by the isolated rabbit retina or ARPE-19 cells. Conclusions. Our results indicate that although the rabbit retina and the ARPE-19 cell line express nucleoside and nucleobase transporters, translocation of ganciclovir does not involve any carrier-mediated transport process. Rather, ganciclovir uptake by the rabbit retina and ARPE-19 cells is governed primarily by passive diffusion.

Expression of Peptide Transporters on the Rabbit Retina: A Strategy to Improve Retinal Delivery of Ganciclovir

The objective of this study was to investigate functional expression of peptide transporters on the rabbit retina. In vivo and ex vivo retina/choroidal uptake studies were carried out with New Zealand albino rabbits. [3H]Gly-Sar was used as the model peptide transporter substrate. [3H]Gly-Sar solutions, in the presence and absence of specific inhibitors, were added to the vitreal side of the retina. Results indicate that retinal uptake of [3H]Gly-Sar was significantly inhibited in the presence of other known peptide transporter substrates, such as Gly-Pro and cephalexin. Importantly, valganciclovir, a peptidomimetic prodrug of ganciclovir, also inhibited uptake of [3H]Gly-Sar. These studies therefore, indicate that peptide transporters are functionally expressed on the rabbit retina and retinal concentrations of GCV may be enhanced by targeting these transporters through prodrug derivatization.

Evaluation of the Intravenous and Topical Routes for Ocular Delivery of Hesperidin and Hesperetin

PURPOSE The objective of this study was to determine the ocular bioavailability of hesperidin and hesperetin, especially with respect to their distribution into the posterior segment of the eye, following systemic and topical administration in rabbits. METHODS Hesperidin and hesperetin were administered either intravenously or topically to male New Zealand white (NZW) rabbits. Vitreous humor and plasma samples were collected after intravenous administration and analyzed to estimate the concentrations of the parent compounds and their metabolites. Ocular tissue concentrations, obtained on topical administration of hesperidin and hesperetin, were also determined. RESULTS In the systemic circulation, hesperidin and hesperetin were rapidly metabolized into their glucuronides, which are extremely hydrophilic in nature. Vitreal samples did not demonstrate any detectable levels of hesperidin/hesperetin following intravenous administration. Topical administration produced significant concentrations of hesperidin/hesperetin in all the ocular tissues tested at the 1 and 3 hours time points postdosing, with hesperetin showing higher levels compared to hesperidin. However, only low levels were generated in the vitreous humor. Inclusion of a penetration enhancer, benzalkonium chloride (BAK), improved the back-of-the-eye hesperetin levels. CONCLUSIONS Ocular delivery of hesperidin/hesperetin via the systemic route does not seem to be feasible considering the rapid generation of the hydrophilic metabolites. Topical application appears to be more promising and needs to be further developed/refined.

Bioreversion and Oral Bioavailability of the l‐Valine Dipeptide Ester Prodrug of Acyclovir, Val‐Valacyclovir, in Sprague‐Dawley Rats

The overall objective of this study was to compare bioavailability of acyclovir (ACV) in Sprague‐Dawley (SD) rats following oral administration of the ACV prodrugs valacyclovir and val‐valacyclovir (10 mg/kg body weight ACV equivalent). ACV, valacyclovir and val‐valacyclovir were estimated using a high pressure liquid chromatography system equipped with a fluorescence detector. Intact prodrugs, valacyclovir and val‐valacyclovir could not be detected in the plasma. Differences in Cmax (4.47 ± 1.68 µg/ml and 2.92 ± 0.90 µg/ml), Tmax (51.66 ± 27.14 and 27.14 and 25.83 ± 11.14 min) and AUC (366.02 ± 146.78 and 231.3 ± 90.88 min. µg/ml) achieved for ACV derived from valacyclovir and val‐valacyclovir, respectively, were not statistically significant. Val‐valacyclovir was rapidly hydrolyzed in rat enterocyte and whole intestinal homogenates (half‐lives of 6.5 and 15 min respectively) and plasma (half‐life of 8.4 min). Valacyclovir demonstrated comparatively longer half‐lives in plasma (half‐life 466.9 min) and whole intestinal homogenates (half‐life 271.7 min) but was rapidly hydrolyzed to ACV in the enterocyte homogenates (half‐life 24.2 min). Both valacyclovir and val‐valacyclovir were rapidly hydrolyzed in liver homogenates. The short half‐lives of val‐valacyclovir in the biological matrices suggest rapid presystematic bioreversion to valacyclovir. This study reports for the first time that oral val‐valacyclovir generates systematic ACV levels comparable to those achieved with oral valacyclovir.