Novel biotinylated lipid prodrugs of acyclovir for the treatment of herpetic keratitis (HK): transporter recognition, tissue stability and antiviral activity.

PURPOSE: Biotinylated lipid prodrugs of acyclovir (ACV) were designed to target the sodium dependent multivitamin transporter (SMVT) on the cornea to facilitate enhanced cellular absorption of ACV. METHODS: All the prodrugs were screened for in vitro cellular uptake, interaction with SMVT, docking analysis, cytotoxicity, enzymatic stability and antiviral activity. RESULTS: Uptake of biotinylated lipid prodrugs of ACV (B-R-ACV and B-12HS-ACV) was significantly higher than biotinylated prodrug (B-ACV), lipid prodrugs (R-ACV and 12HS-ACV) and ACV in corneal cells. Transepithelial transport across rabbit corneas indicated the recognition of the prodrugs by SMVT. Average Vina scores obtained from docking studies further confirmed that biotinylated lipid prodrugs possess enhanced affinity towards SMVT. All the prodrugs studied did not cause any cytotoxicity and were found to be safe and non-toxic. B-R-ACV and B-12HS-ACV were found to be relatively more stable in ocular tissue homogenates and exhibited excellent antiviral activity. CONCLUSIONS: Biotinylated lipid prodrugs demonstrated synergistic improvement in cellular uptake due to recognition of the prodrugs by SMVT on the cornea and lipid mediated transcellular diffusion. These biotinylated lipid prodrugs appear to be promising drug candidates for the treatment of herpetic keratitis (HK) and may lower ACV resistance in patients with poor clinical response.

Transfected MDCK cell line with enhanced expression of CYP3A4 and P-glycoprotein as a model to study their role in drug transport and metabolism.

The aim of this study was to characterize and utilize MDCK cell line expressing CYP3A4 and P-glycoprotein as an in vitro model for evaluating drug-herb and drug-drug of abuse interactions. MDCK cell line simultaneously expressing P-gp and CYP3A4 (MMC) was developed and characterized by using expression and activity studies. Cellular transport study of 200 µM cortisol was performed to determine their combined activity. The study was carried across MDCK-WT, MDCK-MDR1 and MMC cell lines. Similar studies were also carried out in the presence of 50 µM naringin and 3 µM morphine. Samples were analyzed by HPLC for drug and its CYP3A4 metabolite. PCR, qPCR and Western blot studies confirmed the enhanced expression of the proteins in the transfected cells. The Vivid CYP3A4 assay and ketoconazole inhibition studies further confirmed the presence of active protein. Apical to basal transport of cortisol was found to be 10- and 3-fold lower in MMC as compared to MDCK-WT and MDCK-MDR1 respectively. Higher amount of metabolite was formed in MMC than in MDCK-WT, indicating enhanced expression of CYP3A4. Highest cortisol metabolite formation was observed in MMC cell line due to the combined activities of CYP3A4 and P-gp. Transport of cortisol increased 5-fold in the presence of naringin in MMC and doubled in MDCK-MDR1. Cortisol transport in MMC was significantly lower than that in MDCK-WT in the presence of naringin. The permeability increased 3-fold in the presence of morphine, which is a weaker inhibitor of CYP3A4. Formation of 6ß-hydroxy cortisol was found to decrease in the presence of morphine and naringin. This new model cell line with its enhanced CYP3A4 and P-gp levels in addition to short culture time can serve as an invaluable model to study drug-drug interactions. This cell line can also be used to study the combined contribution of efflux transporter and metabolizing enzymes toward drug-drug interactions.

Functional characterization and molecular identification of vitamin C transporter (SVCT2) in human corneal epithelial (HCEC) and retinal pigment epithelial (D407) cells.

PURPOSE: The main goal of this study is to investigate the existence of sodium-dependent vitamin C transport system (SVCT2) and to define time-dependent uptake mechanism and intracellular regulation of ascorbic acid (AA) in human corneal epithelial (HCEC) and human retinal pigment epithelial (D407) cells. METHODS: Uptake of [(14)C] AA was studied in HCEC and D407 cells. Functional aspects of [(14)C] AA uptake were studied in the presence of different concentrations of unlabeled AA, pH, temperature, metabolic inhibitors, substrates and structural analogs. Molecular identification of SVCT2 was examined with reverse transcription-polymerase chain reaction (RT-PCR). RESULTS: Uptake of [(14)C] AA was observed to be sodium, chloride, temperature, pH and energy-dependent in both cell lines. [(14)C] AA uptake was found to be saturable, with Km values of 46.14 ± 6.03 and 47.26 ± 3.24 µM and Vmax values of 17.34 ± 0.58 and 31.86 ± 0.56 pmol/min/mg protein, across HCEC and D407 cells, respectively. The process is inhibited by structural analogs (L-AA and D-Iso AA) but not by structurally unrelated substrates (glucose and PAHA). Ca(++)/calmodulin and protein kinase pathways play an important role in modulating uptake of AA. A 626 bp band corresponding to a vitamin C transporter (SVCT2) has been identified by RT-PCR analysis in both the cell lines. CONCLUSION: This research article reports regarding the ascorbic acid uptake mechanism, kinetics and regulation by sodium dependent vitamin C transporter (SVCT2) in HCEC and D407 cells. Also, SVCT2 can be utilized for targeted delivery in enhancing ocular permeation and bioavailability of highly potent ophthalmic drugs.

Interaction Studies of Resolvin E1 Analog (RX-10045) with Efflux Transporters.

PURPOSE: Screening interactions of a resolvin E1 analog (RX-10045) with efflux transporters (P-glycoprotein [P-gp], multidrug resistance-associated protein [MRP2], and breast cancer-resistant protein [BCRP]). METHODS: Madin-Darby canine kidney cells transfected with P-gp, MRP2, and BCRP genes were selected for this study. [3H]-Digoxin, [3H]-vinblastine and [3H]-abacavir were selected as model substrates for P-gp, MRP2, and BCRP. Uptake and permeability studies across cell monolayer in both apical to basal (AP-BL) and BL-AP of these substrates were conducted in the presence of specific efflux pump inhibitors and RX-10045. Cell viability studies were conducted with increasing concentrations of RX-10045. RESULTS: Uptake studies showed a higher accumulation in the presence of inhibitors (GF120918 and ketoconazole for P-gp; MK571 for MRP2; and ß-estradiol for BCRP) as well as RX-10045. Similarly, dose-dependent inhibition studies demonstrated higher accumulation of various substrates ([3H]-digoxin, [3H]-vinblastine, and [3H]-abacavir) in the presence of RX-10045. IC50 values of dose-dependent inhibition of RX-10045 for P-gp, MRP2, and BCRP were 239±11.2, 291±79.2, and 300±42 µM, respectively. Cell viability assay indicated no apparent toxicity up to 350 µM concentration. Enhanced permeability for model substrates was observed in the presence of RX-10045. Uptake studies in human corneal epithelial cells suggest that RX-10045 is a strong inhibitor of organic cation transporter-1 (OCT-1). CONCLUSIONS: In summary, the resolvin analog (RX-10045) was identified as a substrate/inhibitor for efflux transporters (MRP2 and BCRP). Also, RX-10045 appears to be a strong inhibitor/substrate of OCT-1. Novel formulation strategies such as nanoparticles, nanomicelles, and liposomes for circumventing efflux barriers and delivering higher drug concentrations leading to a higher therapeutic efficacy may be employed.

Role of membrane transporters and metabolizing enzymes in ocular drug delivery.

Ocular disorders can significantly lower a patient's quality of life. Centers for Disease Control and Prevention's Vision Health Initiative have estimated that the number of people affected by age-related ocular diseases may be doubled in the United States by 2030. Although availability of newer therapeutics has improved the prognosis of ocular diseases, poor ocular bioavailability still remains a major concern. Combinations of pharmacodynamic and pharmacokinetic barriers have been known to determine the amount of drug delivered to the target tissue. However, presence of membrane transporters and metabolizing enzymes pose a significant challenge to ocular drug disposition. Scientific literature confirms the expression of efflux/ATP-binding cassette transporters, influx/solute carrier transporters and several metabolic enzymes including oxidoreductases, hydrolases and transferases in different ocular tissues. Therefore, this review article describes the anatomical features of the eye and various barriers regulating ocular drug disposition. Differential expression of membrane transporters and metabolizing enzymes in normal and diseased states are briefly discussed. Further, the significance of transporter- metabolism interplay in ophthalmic drug design and various ocular drug delivery strategies are also outlined.

Synthesis and Characterization of Ganciclovir Long Chain Lipid Prodrugs.

Ganciclovir (GCV) is indicated for the treatment of human cytomegalo virus (HCMV) retinitis in immunocompromised patients. Sub-optimal physicochemical properties prevent GCV from reaching therapeutic concentrations in back of the eye (retina) tissue after oral and intravenous administration. Chronic high dose administration results in systemic toxicity. Local intravitreal injections suffer from poor ocular bioavailability and require repeated administration which can cause retinal detachment, retinal/vitreal hemorrhage and endophthalmitis. In the current study, we synthesized long chain acyl ester derivatives of GCV to improve lipophilicity and bioavailability. Ester conjugates (C5, C10 and C13 mono- and di-(O-acyl)) of GCV were synthesized in one step reaction following conventional esterification reaction. Purity of the novel prodrugs was determined with reversed phase high performance liquid chromatography. Conjugation of long lipid chain to GCV was confirmed with proton (1H) and carbon (13C) nuclear magnetic resonance and mass spectroscopy. Also, melting point and lipophilicity for the prodrugs and GCV were determined. MTS assay was used to assess in vitro toxicity of GCV and its long chain lipid prodrugs on human retinal pigment epithelial cell line (ARPE-19) cells. Results indicated that long chain lipid GCV prodrugs are nontoxic, safe and well-tolerated by ARPE-19 cells. These results suggest that novel long chain lipid GCV prodrugs may be further evaluated for ocular delivery and treatment of HCMV retinitis.

Aqueous nanomicellar formulation for topical delivery of biotinylated lipid prodrug of acyclovir: formulation development and ocular biocompatibility.

PURPOSE: The objective of this study was to develop a clear, aqueous nanomicellar formulation and evaluate its in vitro ocular biocompatibility as a novel carrier for topical ocular delivery of biotinylated lipid prodrug for the treatment of herpetic keratitis. METHODS: Micellar formulation of Biotin-12Hydroxystearic acid-acyclovir (B-12HS-ACV) was prepared by solvent evaporation/film hydration method with two nonionic surfactants, vitamin E TPGS and octoxynol-40. The optimized formulation was characterized for various parameters including micelle size, polydispersity index (PDI), and zeta-potential and in vitro prodrug release. Human corneal epithelial cells (HCECs) were employed for studying the cytotoxicity of the formulation. Further, mRNA expression levels of various cytokines were also studied with quantitative real-time PCR (qPCR). RESULTS: Average size was 10.46±0.05 nm with a PDI of 0.086 for blank nanomicelles, and 10.78±0.09 nm with a PDI of 0.075 for prodrug-loaded nanomicelles. Both unloaded and prodrug-loaded nanomicelles had low negative zeta potential. Prodrug encapsulation efficiency of mixed nanomicelles was calculated to be ∼90%. Transmission electron microscopy analysis revealed that nanomicelles were spherical, homogenous, and devoid of aggregates. B-12HS-ACV release from nanomicelles was slow with no significant burst effect. Results show a sustained release of the prodrug from nanomicelles over a period of 4 days. Neither the blank formulation nor the prodrug-loaded micellar formulation demonstrated any cytotoxic effects. Further, incubation of HCECs with blank and prodrug-loaded nanomicellar groups did not significantly alter the expression levels of IL-1ß, IL-6, IL-8, IL-17, TNF-α, and IFN-γ. CONCLUSIONS: In summary, a topical clear, aqueous nanomicellar formulation comprised of vitamin E TPGS and octoxynol-40 loaded with 0.1% B-12HS-ACV was successfully developed. B-12HS-ACV-loaded nanomicelles are small in size, spherical, and homogenous, without any aggregates. The micellar formulations were perfectly transparent similar to pure water. Ocular biocompatibility studies indicated that mixed nanomicelles were nontoxic and noninflammatory to corneal epithelial cells. Therefore, nanomicellar technology represents a promising strategy for the delivery of biotinylated lipid prodrugs of ACV.

Binary and ternary combinations of anti-HIV protease inhibitors: effect on gene expression and functional activity of CYP3A4 and efflux transporters.

BACKGROUND: The purpose of this study is to identify the effect of binary and ternary combinations of anti-HIV protease inhibitors (PIs) on the expression of metabolizing enzyme (CYP3A4) and efflux transporters [multidrug resistance-associated protein 2 (MRP2), P-glycoprotein (P-gp) and breast cancer resistant protein (BCRP)] in a model intestinal cell line (LS-180). METHODS: LS-180 cells were treated with various combinations of PIs (amprenavir, indinavir, saquinavir and lopinavir), and the mRNA expression levels of metabolizing enzyme and efflux transporters were measured using quantitative reverse transcription polymerase chain reaction. The alteration of gene expression was further correlated to the expression of nuclear hormone receptor PXR. Uptake of fluorescent and radioactive substrates was carried out to study the functional activity of these proteins. Cytotoxicity and adenosine triphosphate (ATP) assays were carried out to measure stress responses. RESULTS: Binary and ternary combinations of PIs appeared to modulate the expression of CYP3A4, MRP2, P-gp and BCRP in a considerable manner. Unlike the individual PIs, their binary combinations showed much greater induction of metabolizing enzyme and efflux proteins. However, such pronounced induction was not observed in the presence of ternary combinations. The observed trend of altered mRNA expression was found to correlate well with the change in expression levels of PXR. The gene expression was found to correlate with activity assays. Lack of cytotoxicity and ATP activity was observed in the treatment samples, suggesting that these alterations in expression levels were probably not stress responses. CONCLUSIONS: In the present study, we demonstrated that combinations of drugs can have serious consequences toward the treatment of HIV infection by altering their bioavailability and disposition.

Hypoxia-inducible factor-1 (HIF-1): a potential target for intervention in ocular neovascular diseases.

Constant oxygen supply is essential for proper tissue development, homeostasis and function of all eukaryotic organisms. Cellular response to reduced oxygen levels is mediated by the transcriptional regulator hypoxia-inducible factor-1 (HIF-1). It is a heterodimeric complex protein consisting of an oxygen dependent subunit (HIF-1α) and a constitutively expressed nuclear subunit (HIF-1ß). In normoxic conditions, de novo synthesized cytoplasmic HIF-1α is degraded by 26S proteasome. Under hypoxic conditions, HIF-1α is stabilized, binds with HIF-1ß and activates transcription of various target genes. These genes play a key role in regulating angiogenesis, cell survival, proliferation, chemotherapy, radiation resistance, invasion, metastasis, genetic instability, immortalization, immune evasion, metabolism and stem cell maintenance. This review highlights the importance of hypoxia signaling in development and progression of various vision threatening pathologies such as diabetic retinopathy, retinopathy of prematurity, age-related macular degeneration and glaucoma. Further, various inhibitors of HIF-1 pathway that may have a viable potential in the treatment of oxygen-dependent ocular diseases are also discussed.

Novel strategies for anterior segment ocular drug delivery.

Research advancements in pharmaceutical sciences have led to the development of new strategies in drug delivery to anterior segment. Designing a new delivery system that can efficiently target the diseased anterior ocular tissue, generate high drug levels, and maintain prolonged and effective concentrations with no or minimal side effects is the major focus of current research. Drug delivery by traditional method of administration via topical dosing is impeded by ocular static and dynamic barriers. Various products have been introduced into the market that prolong drug retention in the precorneal pocket and to improve bioavailability. However, there is a need of a delivery system that can provide controlled release to treat chronic ocular diseases with a reduced dosing frequency without causing any visual disturbances. This review provides an overview of anterior ocular barriers along with strategies to overcome these ocular barriers and deliver therapeutic agents to the affected anterior ocular tissue with a special emphasis on nanotechnology-based drug delivery approaches.

Approaches for enhancing oral bioavailability of peptides and proteins.

Oral delivery of peptide and protein drugs faces immense challenge partially due to the gastrointestinal (GI) environment. In spite of considerable efforts by industrial and academic laboratories, no major breakthrough in the effective oral delivery of polypeptides and proteins has been accomplished. Upon oral administration, gastrointestinal epithelium acts as a physical and biochemical barrier for absorption of proteins resulting in low bioavailability (typically less than 1-2%). An ideal oral drug delivery system should be capable of (a) maintaining the integrity of protein molecules until it reaches the site of absorption, (b) releasing the drug at the target absorption site, where the delivery system appends to that site by virtue of specific interaction, and (c) retaining inside the gastrointestinal tract irrespective of its transitory constraints. Various technologies have been explored to overcome the problems associated with the oral delivery of macromolecules such as insulin, gonadotropin-releasing hormones, calcitonin, human growth factor, vaccines, enkephalins, and interferons, all of which met with limited success. This review article intends to summarize the physiological barriers to oral delivery of peptides and proteins and novel pharmaceutical approaches to circumvent these barriers and enhance oral bioavailability of these macromolecules.

Mechanisms of drug resistance in cancer chemotherapy: coordinated role and regulation of efflux transporters and metabolizing enzymes.

Cancer remains one of the major leading causes of death worldwide. Acquisition of multidrug resistance (MDR) remains a major impediment to successful chemotherapy. As the name implies, MDR is not limited only to one drug but often associated to structurally and functionally unrelated chemotherapeutics. Extensive research and investigations have identified several mechanisms underlying the development of MDR. This process of drug resistance is considered to be multifactorial including decreased drug accumulation, increased efflux, increased biotransformation, drug compartmentalization, modification of drug targets and defects in cellular pathways. In the first part of the review, these pharmacokinetic and pharmacodynamic mechanisms have been described in brief. Although the pathways can act independently, they are more often intertwined. Of the various mechanisms involved, up-regulation of efflux transporters and metabolizing enzymes constitute a major resistance phenotype. This review also provides a general biological overview of important efflux transporters and metabolizing enzymes involved in MDR. Further, synergistic action between efflux transporters and metabolizing enzymes leading to MDR could possibly arise due to two different factors; overlapping substrate specificity and coordinated regulation of their expression. The expression of efflux transporters and metabolizing enzymes is governed by nuclear receptors, mainly pregnane X receptor (PXR). The pharmacological role of PXR and advances in the development of PXR antagonists to overcome MDR are outlined.

Biotin uptake by T47D breast cancer cells: functional and molecular evidence of sodium-dependent multivitamin transporter (SMVT).

The objective of this study was to investigate functional and molecular evidence of carrier mediated system responsible for biotin uptake in breast cancer (T47D) cells and to delineate mechanism of intracellular regulation of this transporter. Cellular accumulation of [3H] biotin was studied in T47D and normal mammary epithelial (MCF-12A) cells. Reverse transcription polymerase chain reaction (RT-PCR) was carried out to confirm the molecular expression of sodium dependent multivitamin transporter (SMVT) in T47D cells. Quantitative real time PCR analysis was also performed to compare the relative expression of SMVT in T47D and MCF-12A cells. [3H] biotin uptake by T47D cells was found to be concentration dependent with K(m) of 9.24 µM and V(max) of 27.34 pmol/mg protein/min. Uptake of [3H] biotin on MCF-12A cells was also found to be concentration dependent and saturable, but with a relatively higher K(m) (53.10 µM) indicating a decrease in affinity of biotin uptake in normal breast cells compared to breast cancer cells. [3H] biotin uptake appears to be time-, temperature-, pH- and sodium ion-dependent but independent of energy and chloride ions. [3H] biotin uptake was significantly inhibited in the presence of biotin, its structural analog desthiobiotin, pantothenic acid and lipoic acid. Concentration dependent inhibition of biotin uptake was evident in the presence of valeric acid which possesses free carboxyl group and biocytin and NHS biotin which are devoid of free carboxyl group. No significant inhibition was observed in the presence of structurally unrelated vitamins (ascorbic acid, folic acid, nicotinic acid, thiamine, pyridoxine and riboflavin). Modulators of PTK, PKC and PKA mediated pathways had no effect, but uptake in presence of calmidazolium (calcium-calmodulin inhibitor) was significantly inhibited. [3H] biotin uptake in the presence of calmidazolium was found to be saturable with a K(m) and V(max) values of 13.49 µM and 11.20 pmol/mg protein/min, respectively. A band of SMVT mRNA at 774 bp was identified by RT-PCR analysis. Quantitative real time PCR confirmed higher expression of SMVT in T47D cells relative to MCF-12A cells. All these studies demonstrated for the first time the functional and molecular expression of sodium dependent multivitamin transporter (SMVT), a specific carrier-mediated system for biotin uptake, in human derived breast cancer (T47D) cells. The present study also indicated that cancer cells could import more vitamin compared to normal breast cells possibly for maintaining high proliferative status. We investigated the likelihood of selecting this cell line (T47D) as an in vitro cell culture model to study biotin-conjugated anti-cancer drugs/drug delivery systems.

Molecular expression and functional activity of efflux and influx transporters in hypoxia induced retinal pigment epithelial cells.

A decrease in tissue oxygen levels (aka hypoxia) mediates a number of vascular retinal diseases. Despite introduction of novel therapeutics, treatment of retinal disorders remains challenging, possibly due to complex nature of hypoxia signaling. To date, the differential effect of hypoxia on expression of efflux and influx transporters in retinal cells has not been studied. Therefore, the objective of this study was to delineate molecular and functional expression of membrane transporters in human retinal pigment epithelial (RPE) cells cultured under normoxic and hypoxic conditions. Quantitative real time polymerase chain reaction (qPCR), ELISA and immunoblot analysis were performed to examine the RNA and protein expression levels of transporters. Further, functional activity was evaluated by performing the uptake of various substrates in both normoxic and hypoxic conditions. qPCR analysis showed elevated expression of efflux transporters (P-glycoprotein, multidrug resistant protein 2, breast cancer resistant protein) and influx transporters (folate receptor-α, cationic and neutral amino acid transporter, sodium dependent multivitamin transporter) in a time dependent manner. Immunoblot analysis further confirmed elevated expression of breast cancer resistant protein and sodium dependent multivitamin transporter. A decrease in the uptake of efflux transporter substrates (digoxin, lopinavir and abacavir) and enhanced uptake of influx transporter substrates (arginine, folic acid and biotin) in hypoxia relative to normoxia further confirmed elevated expression of transporters, respectively. This study demonstrates for the first time that hypoxic conditions may alter expression of efflux and influx transporters in RPE cells. These findings suggest that hypoxia may further alter disposition of ophthalmic drugs.

Novel Nanomicellar Formulation Approaches for Anterior and Posterior Segment Ocular Drug Delivery.

One of the most challenging areas of pharmaceutical research is ocular drug delivery. The unique anatomy and physiology of the eye impedes drug permeation to deeper ocular tissues. Nanosized carrier systems such as nanoparticles, liposomes, suspensions, dendrimers, and nanomicelles are being explored for ocular drug delivery. In this review, we have focused on application of emerging nanomicellar carrier systems in ocular drug delivery. Nanomicelles are nanosized vesicular carriers formed from amphiphilic monomer units. Surfactant and polymeric micellar nanocarriers provide an amenable means to improve drug solubilization, develop clear aqueous formulations and deliver drugs to anterior and posterior ocular tissues. Nanomicelles due to their amphiphilic nature encapsulate hydrophobic drugs and aid in drug delivery. Various methods are employed to develop nanosized micellar formulations depending upon the physicochemical properties of the drug. Nanomicellar carriers appear to be promising vehicles with potential applications in ocular drug delivery. In this review, we attempted to discuss about the progress in ocular drug delivery research using nanomicelles as carriers from the published literature and issued patents. Also, with regards to ocular static and dynamic barriers which prevent drug permeation, a brief discussion about nanomicelles, types of nanomicelles, their methods of preparation and micellar strategy to overcome ocular barriers, delivering therapeutic levels of drugs to anterior and posterior ocular tissues are discussed.

Recent Patents on Emerging Therapeutics for the Treatment of Glaucoma, Age Related Macular Degeneration and Uveitis.

Advancements in the field and rising interest among pharmaceutical researchers have led to the development of new molecules with enhanced therapeutic activity. Design of new drugs which can target a particular pathway and/or explore novel targets is of immense interest to ocular pharmacologists worldwide. Delivery of suitable pharmacologically active agents at proper dose (within the therapeutic window) to the target tissues without any toxicity to the healthy ocular tissues still remain an elusive task. Moreover, the presence of static and dynamic barriers to drug absorption including the corneal epithelium (lipophilic), corneal and scleral stroma (hydrophilic), conjunctival lymphatics, choroidal vasculature and the blood-ocular barriers also pose a significant challenge for achieving therapeutic drug concentrations at the target site. Although many agents are currently available, new compounds are being introduced for treating various ocular diseases. Deeper understanding of the etiology and complex mechanisms associated with the disease condition would aid in the development of potential therapeutic candidates. Novel small molecules as well as complex biotechnology derived macromolecules with superior efficacy, safety and tolerability are being developed. Therefore, this review article provides an overview of existing drugs, treatment options, advances in emerging therapeutics and related recent patents for the treatment of ocular disorders such as glaucoma, age related macular degeneration (AMD) and uveitis.

Ocular toxicity from systemically administered xenobiotics.

INTRODUCTION: The eye is considered as the most privileged organ because of the blood-ocular barrier that acts as a barrier to systemically administered xenobiotics. However, there has been a significant increase in the number of reports on systemic drug-induced ocular complications. If such complications are left untreated, then it may cause permanent damage to vision. Hence, knowledge of most recent updates on ever-increasing reports of such toxicities has become imperative to develop better therapy while minimizing toxicities. AREAS COVERED: The article is mainly divided into anterior and posterior segment manifestations caused by systemically administered drugs. The anterior segment is further elaborated on corneal complications where as the posterior segment is focused on optic nerve, retinal and vitreous complications. Furthermore, this article includes recent updates on acute and chronic ocular predicaments, in addition to discussing various associated symptoms caused by drugs. EXPERT OPINION: Direct correlation of ocular toxicities due to systemic drug therapy is evident from current literature. Therefore, it is necessary to have detailed documentation of these complications to improve understanding and predict toxicities. We made an attempt to ensure that the reader is aware of the characteristic ocular complications, the potential for irreversible drug toxicity and indications for cessation.

Sodium dependent multivitamin transporter (SMVT): a potential target for drug delivery.

Sodium dependent multivitamin transporter (SMVT; product of the SLC5A6 gene) is an important transmembrane protein responsible for translocation of vitamins and other essential cofactors such as biotin, pantothenic acid and lipoic acid. Hydropathy plot (Kyte-Dolittle algorithm) revealed that human SMVT protein consists of 635 amino acids and 12 transmembrane domains with both amino and carboxyl termini oriented towards the cytoplasm. SMVT is expressed in various tissues such as placenta, intestine, brain, liver, lung, kidney, cornea, retina and heart. This transporter displays broad substrate specificity and excellent capacity for utilization in drug delivery. Drug absorption is often limited by the presence of physiological (epithelial tight junctions), biochemical (efflux transporters and enzymatic degradation) and chemical (size, lipophilicity, molecular weight, charge etc.) barriers. These barriers may cause many potential therapeutics to be dropped from the preliminary screening portfolio and subsequent entry into the market. Transporter targeted delivery has become a powerful approach to deliver drugs to target tissues because of the ability of the transporter to translocate the drug to intracellular organelles at a higher rate. This review highlights studies employing SMVT transporter as a target for drug delivery to improve bioavailability and investigate the feasibility of developing SMVT targeted drug delivery systems.

Functional and molecular aspects of biotin uptake via SMVT in human corneal epithelial (HCEC) and retinal pigment epithelial (D407) cells.

Sodium-dependent multivitamin transporter (SMVT) is a vital transmembrane protein responsible for translocating biotin and other essential cofactors such as pantothenate and lipoate. Unlike primary cultures of corneal and retinal pigment epithelial (RPE) cells, immortalized cells can be subcultured many times, yet maintain their physiological properties. Hence, the purpose of this study was to delineate the functional and molecular aspects of biotin uptake via SMVT on immortalized human corneal epithelial (HCEC) and RPE (D407) cells. Functional aspects of [(3)H] biotin uptake were studied in the presence of different concentrations of unlabeled biotin, pH, temperature, metabolic inhibitors, ions, substrates, structural analogs and biotinylated prodrug (Biotin-Acyclovir (B-ACV)). Molecular identity of SMVT was examined with reverse transcription-polymerase chain reaction. Biotin uptake was found to be saturable in HCEC and D407 cells with K (m) of 296.2 ± 25.9 and 863.8 ± 66.9 µM and V (max) of 77.2 ± 2.2 and 308.3 ± 10.7 pmol/mg protein/min, respectively. Uptake was found to be pH, temperature, energy, and sodium-dependent. Inhibition of biotin uptake was observed in the presence of structural analogs and specific substrates. Further, uptake was lowered in the presence of B-ACV indicating the translocation of biotinylated prodrug by SMVT. A distinct band at 774 bp confirmed the molecular existence of SMVT in both the cells. This study shows for the first time the functional and molecular presence of SMVT in HCEC and D407 cells. Therefore, these cell lines may be utilized as in vitro models to study the cellular translocation of biotin-conjugated prodrugs.

Differential expression of folate receptor-alpha, sodium-dependent multivitamin transporter, and amino acid transporter (B (0, +)) in human retinoblastoma (Y-79) and retinal pigment epithelial (ARPE-19) cell lines.

PURPOSE: The overall objective of this study was to investigate the differential expression of folate receptor-alpha (FR-α), sodium-dependent multivitamin transporter (SMVT), and amino acid transporter [B ((0, +))] in retinoblastoma (Y-79) and retinal pigment epithelial (ARPE-19) cells. METHODS: Polymerase chain reaction (PCR) analysis was performed to confirm the existence of FR-α, SMVT, and B ((0, +)) in Y-79 and ARPE-19 cell lines. Quantitative real-time PCR was also performed to determine the relative expression of FR-α, SMVT, and B ((0, +)) at mRNA level in these cell lines. Quantitative uptake of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was studied in Y-79 and ARPE-19 cells. Further, saturation kinetics of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was performed in the presence of various concentrations of respective cold substrates to determine the kinetic parameters (K(m) and V(max)) in Y-79 and ARPE-19 cells. RESULTS: PCR analysis had confirmed the existence of FR-α, SMVT, and B ((0, +)) in Y-79 and ARPE-19 cells. Quantitative real-time PCR analysis had shown significantly higher expression of FR-α, SMVT, and B ((0, +)) mRNA levels in Y-79 cells compared with ARPE-19 cells. Quantitative uptake of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was found to be significantly higher in Y-79 cells relative to ARPE-19 cells. [(3)H] Folic acid uptake process followed saturation kinetics with apparent K(m) of 8.29 nM and V(max) of 393.47 fmol/min/mg protein in Y-79 cells and K(m) of 80.55 nM and V(max) of 491.86 fmol/min/mg protein in ARPE-19 cells. [(3)H] Biotin uptake process also displayed saturation kinetics with K(m) of 8.53 µM and V(max) of 14.12 pmol/min/mg protein in Y-79 cells and K(m) of 138.25 µM and V(max) of 38.85 pmol/min/mg protein in ARPE-19 cells. [(14)C] Arginine uptake process followed saturation kinetics with K(m) of 16.77 µM and V(max) of 348.27 pmol/min/mg protein in Y-79 cells and K(m) of 52.03 µM and V(max) of 379.21 pmol/min/mg protein in ARPE-19 cells. CONCLUSIONS: This work demonstrated for the first time the higher expression and affinity of FR-α, SMVT, and B ((0, +)) mRNA levels in retinoblastoma (Y-79) cells compared with retinal pigment epithelial (ARPE-19) cells.