Transport of acyclovir ester prodrugs through rabbit cornea and SIRC-rabbit corneal epithelial cell line

In Vitro and Ex Vivo Models for Assessing Drug Permeation across the Cornea

Drug permeation across the cornea remains a major challenge due to its unique and complex anatomy and physiology. Static barriers such as the different layers of the cornea, as well as dynamic aspects such as the constant renewal of the tear film and the presence of the mucin layer together with efflux pumps, all present unique challenges for effective ophthalmic drug delivery. To overcome some of the current ophthalmic drug limitations, the identification and testing of novel drug formulations such as liposomes, nanoemulsions, and nanoparticles began to be considered and widely explored. In the early stages of corneal drug development reliable in vitro and ex vivo alternatives, are required, to be in line with the principles of the 3Rs (Replacement, Reduction, and Refinement), with such methods being in addition faster and more ethical alternatives to in vivo studies. The ocular field remains limited to a handful of predictive models for ophthalmic drug permeation. In vitro cell culture models are increasingly used when it comes to transcorneal permeation studies. Ex vivo models using excised animal tissue such as porcine eyes are the model of choice to study corneal permeation and promising advancements have been reported over the years. Interspecies characteristics must be considered in detail when using such models. This review updates the current knowledge about in vitro and ex vivo corneal permeability models and evaluates their advantages and limitations.

Optically Clear Silk Fibroin Films with Tunable Properties for Potential Corneal Tissue Engineering Applications: A Process-Property-Function Relationship Study

Owing to the shortage of donor corneas and issues associated with conventional corneal transplantation, corneal tissue engineering has emerged as a promising therapeutic alternative. Biocompatibility and other attractive features make silk fibroin a biomaterial of choice for corneal tissue engineering applications. The current study presents three modes of silk fibroin film fabrication by solvent casting with popular solvents, viz. aqueous (aq), formic acid (FA), and hexafluoroisopropanol (HFIP), followed by three standard modes of postfabrication annealing with water vapor, methanol vapor, and steam, and systematic characterization studies including corneal cell culture in vitro. The results indicated that silk fibroin films made from aq, FA, and HFIP solvents had surface roughness (Rq) of 1.39, 0.32, and 0.13, contact angles of 73°, 85°, and 89°, water uptake% of 58, 29, and 27%, swelling ratios of 1.58, 1.3, and 1.28, and water vapor transmission% of 39, 26, and 22%, respectively. The degradation rate was in the order of aq > HF > FA, whereas the tensile strength was in the order of aq < HF < FA. Further, the results of the annealing process indicated notable changes in morpho-topographical, physical, degradation, and tensile properties. However, the films showed no detectable changes in chemical composition and remained optically clear with >90% transmission in the visible range, irrespective of fabrication and postfabrication processing conditions. The films were noncytotoxic against L929 cells and were cytocompatible with rabbit cornea-derived SIRC cells in vitro. The study demonstrated the potential of fine-tuning various properties of silk fibroin films by varying the fabrication and postfabrication processing conditions.

Tricyclic Derivative of Acyclovir and Its Esters in Relation to the Esters of Acyclovir Enzymatic Stability: Enzymatic Stability Study

The 3,9-dihydro-3-[(2-hydroxyethoxy)methyl]-6-(4-methoxyphenyl)-9-oxo-5H-imidazo[1,2-a]-purine (6-(4-MeOPh)-TACV) was selected to assess the enzymatic stability of the tricyclic acyclovir derivatives from the imidazo[1,2-a]-purine group. The parent compound and its esters (acetyl, isobutyryl, pivaloyl, nicotinic, ethoxycarbonyl) were subjected to kinetic studies and compared with the stability of analogous acyclovir (ACV) esters. The enzymatic hydrolysis was observed in vitro in a medium of 80% human plasma in the absence and presence of porcine liver esterase (PLE). The tests were carried out at 37 °C. To determine the kinetic parameters (k_obs., t_0.5) of the observed reaction, the validated HPLC-UV method in the reversed phase was used. The HPLC-MS/MS method was used to identify the degradation products under the tested conditions. In summary, it was found that 6-(4-MeOPh)-TACV esters are more susceptible to esterase metabolism than ACV esters. It was confirmed by HPLC-MS/MS that in the plasma, the main product of their hydrolysis is 6-(4-MeOPh)-TACV and not ACV, which confirms that their antiviral activity observed in vitro does not result from ring degradation.

Topical ophthalmic lipid nanoparticle formulations (SLN, NLC) of indomethacin for delivery to the posterior segment ocular tissues

PURPOSE

The objective of the present study was to formulate indomethacin (IN)-loaded solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) and to investigate their potential use in topical ocular delivery.

METHODS

IN SLNs (0.1% w/v) and NLCs (0.8% w/v) were prepared, characterized and evaluated. Their in vitro release and flux profiles across the cornea and sclera-choroid-RPE (trans-SCR) tissues and in vivo ocular tissue distribution were assessed. Furthermore, chitosan chloride (CS) (mol. wt.<200kDa), a cationic and water-soluble penetration enhancer, was used to modify the surface of the SLNs, and its effect was investigated through in vitro transmembrane penetration and in vivo distribution tissue studies.

RESULTS

For the IN-SLNs, IN-CS-SLNs and IN-NLCs, the particle size was 226±5, 265±8, and 227±11nm, respectively; the zeta potential was -22±0.8, 27±1.2, and -12.2±2.3mV, respectively; the polydispersity index (PDI) was 0.17, 0.30, and 0.23, respectively; and the entrapment efficiency (EE) was 81±0.9, 91.5±3.2 and 99.8±0.2%, respectively. The surface modification of the SLNs with CS increased the ocular penetration of IN. The NLCs maintained significantly higher IN concentrations in all ocular tissues tested compared to the other formulations evaluated in vivo.

CONCLUSION

The results suggest that lipid-based particulate systems can serve as viable vehicles for ocular delivery. The NLC formulations demonstrated increased drug loading capability, entrapment and delivery to anterior and posterior segment ocular tissues.

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.

Prevalidation of a human cornea construct as an alternative to animal corneas for in vitro drug absorption studies

The use of ophthalmic drugs has increased consistently over the past few decades. Currently, most research is conducted using in vivo and ex vivo animal experiments; however, they have many disadvantages, including ethical concerns, high costs, the questionable extension of animal results to humans, and poor standardization. Although several cell culture-based cornea models have been developed, none have been validated and accepted for general use. In this study, a standardized, three-dimensional model of the human cornea (Hemicornea, HC) based on immortalized human corneal cells and cultivated in serum-free conditions was developed for drug absorption studies and prevalidated using compounds with a wide range of molecular characteristics (sodium fluorescein, rhodamine B, fluorescein isothiocyanate-labeled dextran, aciclovir, bimatoprost, dexamethasone, and timolol maleate). The HC model was independently cultured in three different laboratories, and the intralaboratory and interlaboratory reproducibility was analyzed and compared with the rabbit cornea. This analysis showed that the HC has a barrier in the same range as excised animal corneas, although with a higher reproducibility and lower variability. Because of the demonstrated transferability, the HC represents a promising in vitro alternative to the use of ex vivo tissue and offers a well-defined and standardized system for drug absorption studies.

Effects of poly(amidoamine) dendrimers on ocular absorption of puerarin using microdialysis

PURPOSE

The purpose of this study was to investigate the effects of poly(amidoamine) (PAMAM) dendrimers on ocular absorption of puerarin.

METHODS

The samples of rabbits' aqueous humor were collected by in vivo microdialysis. The ocular pharmacokinetic properties of puerarin were measured to evaluate the effects of PAMAM dendrimers on ocular absorption of puerarin.

RESULTS

In the pharmacokinetic studies, the AUC(0∼∞) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers (G3.0, G4.0, G5.0) were greater than those in the control group by 2.3-, 3.5-, and 2.1-folds, respectively. The C(max) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers were 1.5, 2.5, and 1.3 times the values of the control group and the t(1/2) values were significantly longer than that of control group.

CONCLUSIONS

Using PAMAM dendrimers might be a promising strategy to enhance the absorption of puerarin. The generation of PAMAM dendrimers may play a key role in increasing the ocular absorption of puerarin.

Ocular sustained release nanoparticles containing stereoisomeric dipeptide prodrugs of acyclovir

PURPOSE

The objective of this study was to develop and characterize polymeric nanoparticles of appropriate stereoisomeric dipeptide prodrugs of acyclovir (L-valine-L-valine-ACV, L-valine-D-valine-ACV, D-valine-L-valine-ACV, and D-valine-D-valine-ACV) for the treatment of ocular herpes keratitis.

METHODS

Stereoisomeric dipeptide prodrugs of acyclovir (ACV) were screened for bioreversion in various ocular tissues, cell proliferation, and uptake across the rabbit primary corneal epithelial cell line. Docking studies were carried out to examine the affinity of prodrugs to the peptide transporter protein. Prodrugs with optimum characteristics were selected for the preparation of nanoparticles using various grades of poly (lactic-co-glycolic acid) (PLGA). Nanoparticles were characterized for the entrapment efficiency, surface morphology, size distribution, and in vitro release. Further, the effect of thermosensitive gels on the release of prodrugs from nanoparticles was also studied.

RESULTS

L-valine-L-valine-ACV and L-valine-D-valine-ACV were considered to be optimum in terms of enzymatic stability, uptake, and cytotoxicity. Docking results indicated that L-valine in the terminal position increases the affinity of the prodrugs to the peptide transporter protein. Entrapment efficiency values of L-valine-L-valine-ACV and L-valine-D-valine-ACV were found to be optimal with PLGA 75:25 and PLGA 65:35 polymers, respectively. In vitro release of prodrugs from nanoparticles exhibited a biphasic release behavior with initial burst phase followed by sustained release. Dispersion of nanoparticles in thermosensitive gels completely eliminated the burst release phase.

CONCLUSION

Novel nanoparticulate systems of dipeptide prodrugs of ACV suspended in thermosensitive gels may provide sustained delivery after topical administration.

Development of a serum-free human cornea construct for in vitro drug absorption studies: the influence of varying cultivation parameters on barrier characteristics

The increased use of ophthalmic products in recent years has led to an increased demand for in vitro and in vivo transcorneal drug absorption studies. Cell-culture models of the human cornea can avoid several of the disadvantages of widely used animal experimental models, including ethical concerns and poor standardisation. This study describes the development of a serum-free cultivated, three-dimensional human cornea model (Hemicornea, HC) for drug absorption experiments. The impact of varying cultivation conditions on the corneal barrier function was analysed and compared with excised rabbit and porcine corneas. The HC was cultivated on permeable polycarbonate filters using immortalised human keratocytes and a corneal epithelial cell line. The equivalence to native tissue was investigated through absorption studies using model substances with a wide range of molecular characteristics, including hydrophilic sodium fluorescein, lipophilic rhodamine B and fluorescein isothiocyanate (FITC)-labelled macromolecule dextran. To study the intra-laboratory repeatability and construct cultivation, the permeation studies were performed independently by different researchers. The HC exhibited a permeation barrier in the same range as excised animal corneas, high reproducibility and a lower standard deviation. Therefore, the HC could be a promising in vitro alternative to ex vivo corneal tissues in preclinical permeation studies.

In vitro cell culture models to study the corneal drug absorption

INTRODUCTION

Many diseases of the anterior eye segment are treated using topically applied ophthalmic drugs. For these drugs, the cornea is the main barrier to reaching the interior of the eye. In vitro studies regarding transcorneal drug absorption are commonly performed using excised corneas from experimental animals. Due to several disadvantages and limitations of these animal experiments, establishing corneal cell culture models has been attempted as an alternative.

AREAS COVERED

This review summarizes the development of in vitro models based on corneal cell cultures for permeation studies during the last 20 years, starting with simple epithelial models and moving toward complex organotypical 3D corneal equivalents.

EXPERT OPINION

Current human 3D corneal cell culture models have the potential to replace excised animal corneas in drug absorption studies. However, for widespread use, the contemporary validation of existent systems is required.

Novel vehicle based on cubosomes for ophthalmic delivery of flurbiprofen with low irritancy and high bioavailability

AIM

To develop a novel vehicle based on cubosomes as an ophthalmic drug delivery system for flurbiprofen (FB) to reduce ocular irritancy and improve bioavailability.

METHODS

FB-loaded cubosomes were prepared using hot and high-pressure homogenization. Cubosomes were then characterized by particle size, zeta potential, encapsulation efficiency, particle morphology, inner cubic structure and in vitro release. Corneal permeation was evaluated using modified Franz-type cells. Ocular irritation was then evaluated using both the Draize method and histological examination. The ocular pharmacokinetics of FB was determined using microdialysis.

RESULTS

The particle size of each cubosome formulation was about 150 nm. A bicontinuous cubic phase of cubic P-type was determined using cryo-transmission electron microscopy (cryo-TEM) observation and small angle X-ray scattering (SAXS) analysis. In vitro corneal permeation study revealed that FB formulated in cubosomes exhibited 2.5-fold (F1) and 2.0-fold (F2) increase in P(app) compared with FB PBS. In the ocular irritation test, irritation scores for each group were less than 2, indicating that all formulations exhibited excellent ocular tolerance. Histological examination revealed that neither the structure nor the integrity of the cornea was visibly affected after incubation with FB cubosomes. The AUC of FB administered as FB cubosome F2 was 486.36+/-38.93 ng.mL(-1).min.microg(-1), which was significantly higher than that of FB Na eye drops (P<0.01). Compared with FB Na eye drops, the T(max) of FB cubosome F2 was about 1.6-fold higher and the MRT was also significantly longer (P<0.001).

CONCLUSION

This novel low-irritant vehicle based on cubosomes might be a promising system for effective ocular drug delivery.

Small Neutral Amino Acid Ester Prodrugs of Acyclovir Targeting Amino Acid Transporters on the Cornea: Possible Antiviral Agents against Ocular HSV-1 Infections

The aim of this study was to characterize the affinity and permeability patterns of the amino acid ester prodrugs of acyclovir (ACV), L-alanine-ACV (AACV), L-serine-ACV (SACV), L-serine-succinate-ACV (SSACV) and L-cysteine-ACV (CACV) on rabbit primary corneal epithelial cell culture (rPCEC) and on rabbit cornea. Amino acid prodrugs of acyclovir, AACV, SACV, SSACV and CACV were synthesized in our laboratory. Chemical hydrolysis in aqueous buffer, enzymatic hydrolysis in corneal homogenates and transport across freshly excised rabbit cornea of these prodrugs were studied. SSACV inhibited the uptake of [3H] L-alanine on rPCEC and across the intact rabbit cornea. Lineweaver-Burk plot transformation revealed competitive inhibition between L-alanine and SSACV. In corneal tissue homogenate, the half lives of SSACV, SACV and CACV (t1/2) were observed to be 3.5 ± 0.4, 9.2 ± 0.6 and 1.8 ± 0.1 hr respectively, whereas AACV was readily converted to the active parent drug acyclovir exhibiting complete degradation before 5 min. Interestingly translocation of SACV across cornea was inhibited in the presence of 5 mM arginine (~51%), a specific substrate for cationic transport system and in presence of BCH (~38%), a substrate specific for large neutral amino acid transport system (LAT) or cationic and neutral amino acid transport system (B0,+). SACV exhibited higher permeability across cornea along with excellent antiviral activity against herpes simplex virus (HSV-1) and varicella-zoster virus (VZV) in comparison to ACV. Recognition by multiple transporters, stability in corneal homogenate and changes in physico-chemical properties contributed to the increased permeability of SACV across cornea.

Expression of multidrug resistance associated protein 5 (MRP5) on cornea and its role in drug efflux

PURPOSE

The purpose of this manuscript is to investigate the presence of nucleoside/nucleotide efflux transporter in cornea and to evaluate the role in ocular drug efflux.

METHODS

RT-PCR, immunoprecipitation followed by Western blot analysis and immunostaining were employed to establish molecular presence of multidrug resistance associated protein 5 (MRP5) on cornea. Corneal efflux by MRP5 was studied with bis(POM)-PMEA and acyclovir using rabbit and human corneal epithelial cells along with MRP5 over expressing cells (MDCKII-MRP5). Ex vivo studies using excised rabbit cornea and in vivo ocular microdialysis in male New Zealand white rabbits were used to further evaluate the role of MRP5 in conferring ocular drug resistance.

RESULTS

RT-PCR confirms the expression of MRP5 in both rabbit and human corneal epithelial cells along with MDCKII-MRP5 cells. Immunoprecipitation followed by Western blot analysis using a rat (M511-54) monoclonal antibody that reacts with human epitope confirms the expression of MRP5 protein in human corneal epithelial cells and MDCKII-MRP5 cells. Immunostaining performed on human cornea indicates the localization of this efflux pump on both epithelium and endothelium. Efflux studies reveal that depletion of ATP decreased PMEA efflux significantly. MRP5 inhibitors also diminished PMEA and acyclovir efflux. However, depletion of glutathione did not alter efflux. MDR1 and MRP2 did not contribute to PMEA efflux. However, MRP2 is involved in acyclovir efflux while MDR1 do not participate in this process. TLC/autoradiography suggested the conversion of bis(POM)-PMEA to PMEA in rabbit and human corneal epithelial cells. Two well known antiglaucoma drugs, bimatoprost and latanoprost were rapidly effluxed by MRP5. Ex vivo study on intact rabbit corneas demonstrated accumulation of PMEA in cornea in the presence of ATP-depleting medium. In vivo ocular pharmacokinetics also revealed a significant increase in maximum aqueous humor concentration (C(max)) and area under the aqueous humor time curve (AUC) of acyclovir in the presence of MK-571, a specific MRP inhibitor.

CONCLUSIONS

Taken together immunolocalization on human cornea, in vitro efflux in human, rabbit corneal and MRP5 over expressing cells, ex vivo and in vivo studies in intact rabbit cornea suggest that MRP5 on cornea can significantly lower the permeability of antiviral and glaucoma drugs. These findings may be valuable in developing formulation strategies to optimize ocular bioavailability of topically administered ocular agents.

Identification and Characterization of a Na+-Dependent Neutral Amino Acid Transporter, ASCT1, in Rabbit Corneal Epithelial Cell Culture and Rabbit Cornea

PURPOSE

The aim of this study was to investigate the presence of a Na+-dependent neutral amino acid transporter, ASCT1, in rabbit primary corneal epithelial cell culture and rabbit cornea.

METHODS

Uptake studies were carried out on rabbit primary corneal epithelial culture (rPCEC) cells using 12-well plates. Transport studies were conducted with isolated rabbit corneas at 34 degrees C. Uptake and transport of L-alanine was determined at various concentrations. Inhibition studies were conducted in presence of various L- and D-amino acids, metabolic inhibitors like ouabain and sodium azide, and in the absence of sodium to delineate the functional characteristics of L-alanine uptake and transport. Reverse transcription-polymerase chain reaction (RT-PCR) was performed on total RNA harvested from rabbit cornea and rPCEC cells for identification of ASCT1.

RESULTS

Uptake of L-Ala was found to be saturable with a Km of 0.71 mM and a Vmax value of 0.84 micromoles min(-1) mg(-1) protein. Uptake was independent of pH and energy but depends on sodium. It was inhibited by serine, threonine, cysteine, and glutamine but did not respond to BCH (2-aminobicyclo [2,2,1] heptane-2-carboxylic acid) and MeAIB (alpha -methylaminoisobutyric acid). Transport of L-Ala across rabbit cornea was also saturable (Km 6.52 mM and Vmax 1.09 x 10(-2) micromoles min(-1) cm(-2)), energy independent, and subject to similar competitive inhibition. Presence of ASCT1 on rPCEC and on rabbit cornea was identified by RT-PCR.

CONCLUSIONS

L-Alanine, the chosen model substrate, was actively transported by Na+-dependent, neutral amino acid exchanger ASCT1, which was identified and functionally characterized on rPCEC cells and rabbit cornea.

Characterization of human corneal epithelial cell model as a surrogate for corneal permeability assessment: metabolism and transport

The recently introduced Clonetics human corneal epithelium (cHCE) cell line is considered a promising in vitro permeability model, replacing excised animal cornea to predict corneal permeability of topically administered compounds. The purpose of this study was to further characterize cHCE as a corneal permeability model from both drug metabolism and transport aspects. First, good correlation was found in the permeability values (P(app)) obtained from cHCE and rabbit corneas for various ophthalmic drugs and permeability markers. Second, a previously established real-time quantitative polymerase chain reaction method was used to profile mRNA expression of drug-metabolizing enzymes (major cytochromes P450 and UDP glucuronosyltransferase 1A1) and transporters in cHCE in comparison with human cornea. Findings indicated that 1) the mRNA expression of most metabolizing enzymes tested was lower in cHCE than in excised human cornea, 2) the mRNA expression of efflux transporters [multidrug resistant-associated protein (MRP) 1, MRP2, MRP3, and breast cancer resistance protein], peptide transporters (PEPT1 and PEPT2), and organic cation transporters (OCTN1, OCTN2, OCT1, and OCT3) could be detected in cHCE as in human cornea. However, multidrug resistance (MDR) 1 and organic anion transporting polypeptide 2B1 was not detected in cHCE; 3) cHCE was demonstrated to possess both esterase and ketone reductase activities known to be present in human cornea; and 4) transport studies using probe substrates suggested that both active efflux and uptake transport may be limited in cHCE. As the first detailed report to delineate drug metabolism and transport characteristics of cHCE, this work shed light on the usefulness and potential limitations of cHCE in predicting the corneal permeability of ophthalmic drugs, including ester prodrugs, and transporter substrates.

Vitreal pharmacokinetics of biotinylated ganciclovir: role of sodium-dependent multivitamin transporter expressed on retina

PURPOSE

The objective of this study was to investigate the role of sodium-dependent multiple vitamin transporter (SMVT) on Biotin-Ganciclovir (biotin-GCV) uptake on both human retinal pigmented epithelium cell line (ARPE-19) and rabbit retina. Study also aims to delineate the vitreal pharmacokinetics of biotin-GCV.

METHOD

ARPE-19 was employed to study the in vitro uptake experiments. New Zealand white albino rabbits were used to study in vivo retinal uptake and vitreal pharmacokinetics following intravitreal administration of biotin-GCV. In vitro uptake kinetics of [3H] biotin was determined at various initial concentrations. Competitive inhibition studies were conducted in the presence of unlabelled biotin, desthiobiotin, pantothenic acid, and lipoic acid. Various other uptake studies were performed to functionally characterize the transporter. To provide the molecular evidence of this transporter, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) studies were also conducted. In vivo retinal/choroidal uptake studies were carried out with New Zealand albino rabbits. Unconscious animal ocular microdialysis studies were performed in order to evaluate intravitreal pharmacokinetics of GCV and Biotin-GCV.

RESULTS

Uptake of [3H] biotin into ARPE-19 was linear over 7 min, and found to be saturable with K(m) of 138.25 muM and Vmax of 38.85 pmol/min/mg protein. Both pantothenic acid and lipoic acid decreased significantly in uptake of biotin in the concentration-dependent manner. Uptake of biotin into ARPE-19 was found to be temperature, energy, and Na+ dependent but Cl(-)independent. Further, RT-PCR studies identified a band exhibiting presence of hSMVT on ARPE-19. Biotin-GCV is recognized by SMVT system present on the ARPE-19 and rabbit retina. Vitreal Pharmacokinetics profile reveals that most of the parameters were not significantly different for GCV and Biotin-GCV. However, use of Biotin-GCV may result in sustain levels of regenerated GCV in vitreous.

CONCLUSIONS

SMVT was identified and functionally characterized on ARPE-19 cells. Further, Biotin-GCV shares this transport system. Vitreal pharmacokinetics of the conjugate was determined in unconscious rabbit model.

Molecular design for enhancement of ocular penetration

Over the past two decades, many oral drugs have been designed in consideration of physicochemical properties to attain optimal pharmacokinetic properties. This strategy significantly reduced attrition in drug development owing to inadequate pharmacokinetics during the last decade. On the other hand, most ophthalmic drugs are generated from reformulation of other therapeutic dosage forms. Therefore, the modification of formulations has been used mainly as the approach to improve ocular pharmacokinetics. However, to maximize ocular pharmacokinetic properties, a specific molecular design for ocular drug is preferable. Passive diffusion of drugs across the cornea membranes requires appropriate lipophilicity and aqueous solubility. Improvement of such physicochemical properties has been achieved by structure optimization or prodrug approaches. This review discusses the current knowledge about ophthalmic drugs adapted from systemic drugs and molecular design for ocular drugs. I propose the approaches for molecular design to obtain the optimal ocular penetration into anterior segment based on published studies to date.

Cell culture models of the human cornea - a comparative evaluation of their usefulness to determine ocular drug absorption in-vitro

Cell culture models of the cornea are continually developed to replace the isolated animal cornea for transcorneal drug absorption studies. The aim of this study was to determine and compare epithelial tightness and permeability of currently available corneal cell culture models to avoid interlaboratory variability and to assess their usefulness for in-vitro permeation studies. Pure epithelial cell culture models (CEPI, SIRC and HCE-T cell lines), primary cultures of human corneal epithelium (HCEpiC) and the two commercially available models (RHC and Epiocular), as well as organotypic human cornea constructs (HCC, HCC-HCE-T), were investigated and data were compared with those obtained from the excised bovine cornea. Barrier properties were assessed by measurements of transepithelial electrical resistance (TEER) and permeability of three passively absorbed substances (mannitol, testosterone and timolol maleate) with different physico-chemical properties. TEER experiments revealed weak barrier functions for all of the investigated epithelial models (<or=100-200 Omega cm2), except the HCE-T cell line. Transport studies confirmed TEER results insofar that models showing low TEER values also had higher permeation rates in comparison with the excised bovine cornea. However, models based on HCE-T cells demonstrated similar barrier properties to isolated corneal tissue. The corneal models investigated in our laboratory show clear differences in epithelial barrier function. In-vitro systems comprising the HCE-T cell line seem to be most appropriate to replace excised animal cornea for assessing corneal permeability.

Corneal epithelial cell cultures as a tool for research, drug screening and testing

Understanding of visual system function and the development of new therapies for corneal diseases and damages depend upon comprehension of the biological roles of the tissue. The in vitro cultivation of corneal epithelial cells and cell lines derived from them has become a powerful tool to analyze and understand such issues. Currently, researchers have developed well-defined and precisely described culture protocols and a collection of corneal epithelial cell lines. These cell lines have been obtained through different experimental approaches: (1) the ectopic expression of oncogenes, (2) the inactivation of p16 and p53 pathways and hTERT expression, and (3) the spontaneous establishment after serial cultivation of cells. The advantages or disadvantages for these approaches are discussed. In conclusion, the availability of several culture protocols and immortalized cell lines that express corneal epithelial phenotype will be useful for investigating issues such as gene regulation and tissue development, or for validating alternative methods in toxicology.

Synthesis and antiviral properties of aza-analogues of acyclovir

Aza-analogues of Acyclovir were obtained from N-(2-pivaloyloxyethyl)-N-(pivaloyloxymethyl)-p-toluenesulfonamide via a one-pot base silylation/nucleoside coupling procedure. The antiviral activities of all aza-nucleosides in vitro against a variety of viruses were evaluated. None of these compounds displayed any specific antiviral effects.

Study of the extent of ocular absorption of acetazolamide from a developed niosomal formulation, by microdialysis sampling of aqueous humor

Acetazolamide, a carbonic anhydrase inhibitor is used orally (no topical formulation being available in the market) for the reduction of intraocular pressure (IOP) in patients suffering from glaucoma. Two major reasons responsible for the failure to develop a topically effective formulation of acetazolamide are its low solubility (0.7mg/ml) and its low permeability coefficient. It is generally recognized that topical acetazolamide formulation possessing efficacy similar to that achieved upon oral administration would be a significant advancement in the treatment of glaucoma. In order to enhance the bioavailability of acetazolamide by topical route and to improve the corneal permeability of the drug, the niosomes of acetazolamide were prepared (by reverse phase evaporation method) and coated with Carbopol for the latter's bioadhesive effect. The pharmacodynamic studies showed 33% fall in IOP with the developed formulation, and the effect was sustained for 6h after instillation. The effect compared well with a four times higher concentration of dorzolamide (Dorzox, a topical CAI available in the market. In the present study, the aqueous humor disposition of the drug from the developed bioadhesive coated niosomal formulation (ACZREVbio) is compared with the aqueous suspension of the drug (containing 1% (w/v) Tween 80 as a dispersing agent) at similar concentrations. The concentration of acetazolamide absorbed in the aqueous humor at various times from the control suspension and from ACZREVbio was determined by microdialysis in male albino rabbits. Microdialysis provides a complete concentration versus time profile and hence is an important advance to the regional sampling of tissues. The peak concentration of drug absorbed in the aqueous humor from the ACZREVbio formulation (14.94microg/ml) was almost two times of that obtained with the equivalent amount of acetazolamide control suspension (6.93microg). The results show a significant broadening of peak from 80 to 120min with the concentration of more than 13microg being maintained at these times, for the bioadhesive coated niosomal formulation (ACZREVbio). An important observation was the fact that a high drug concentration of 12.02microg reached immediately, i.e., 20min after instillation of ACZREVbio indicating a high penetration being achieved, while a meagre concentration of only 3.53microg is obtained at 60min after instillation of the control suspension. The aqueous humor disposition indicates peaks and troughs in drug concentration which may be related to the decrease in aqueous humor formation, such that the drug concentration/volume increased at these points.

Identification and functional expression of a carrier-mediated riboflavin transport system on rabbit corneal epithelium

PURPOSE

To investigate the functional expression of a carrier-mediated transport mechanism for riboflavin (vitamin B2) across cultured rabbit primary corneal epithelial cells (rPCECs) and intact rabbit cornea. The secondary objective was to understand the physiological significance behind the presence of such a transport system for riboflavin on the apical side of the corneal epithelium.

METHODS

rPCECs and freshly excised rabbit corneas were selected as in vitro and ex vivo models, respectively. Transport and uptake characteristics of [3H]riboflavin were determined at various time points, concentrations, temperatures, and pH. Substrate specificity, energy, and ion dependence studies were carried out to characterize the translocation mechanism. Rabbit tear analysis was done with liquid chromatography/tandem mass spectrometry (LC-MS/MS) to understand the physiological relevance of this transporter.

RESULTS

The uptake process in rPCECs was found to be concentration dependent and saturable at higher concentrations. The process was also independent of pH, Na+, and Cl- but dependent on energy and temperature. Unlabeled riboflavin and its structural analogues caused significant inhibition, whereas unrelated vitamins did not interfere with the process. Transport of [3H]riboflavin across rabbit cornea was also saturable at higher concentration and energy dependent but Na+ independent. Substrate specificity studies across intact rabbit cornea produced results similar to the uptake studies in cultured rPCECs. LC-MS/MS analysis of rabbit tears showed the presence of riboflavin.

CONCLUSIONS

Results suggest the presence of a specialized, high-affinity transport mechanism for riboflavin that is expressed on the apical side of rabbit corneal epithelium and may in turn be responsible for influx of riboflavin from tears to cornea.

Biotin uptake by rabbit corneal epithelial cells: role of sodium-dependent multivitamin transporter (SMVT)

PURPOSE

The objective of this research was to investigate the presence of sodium-dependent multivitamin transporter (SMVT) on rabbit corneal epithelial cells.

METHODS

Primary cultured rabbit corneal epithelial cells (rPCECs)and freshly excised rabbit corneas were used for characterization of biotin uptake and transport, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of SMVT. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis was performed to examine the presence of biotin in rabbit tears.

RESULTS

Uptake of biotin by rPCECs was found to be time and concentration dependent with Km of 32.52 microM and Vmax of 10.43 pmol min- 1 mg protein- 1. Biotin was significantly inhibited in the presence of pantothenic acid and lipoic acid. Biotin uptake was found to be energy and Na+ dependent but H+ and Cl- independent. The uptake was inhibited by valeric acid in a concentration-dependent manner but not much affected in the presence of biotin methyl ester and biocytin with no free carboxyl group. Modulators of both PKC- and PKA-mediated pathways had no effect on biotin uptake, but calcium-calmodulin inhibitor significantly inhibited its uptake. Sodium-dependent multivitamin transporter was identified by RT-PCR in rPCECs. Transport experiments across the rabbit corneas revealed the functional localization of SMVT on the apical side of the cornea, and thereby corroborating with in vitro results with cultured corneal cells. Finally, LC-MS/MS analysis showed the presence of biotin in rabbit tears.

CONCLUSIONS

Results obtained from both in vitro and exvivo studies suggest the possible role of SMVT expressed on corneal epithelial cells for the uptake of biotin, which co-transports pantothenic acid and lipoic acid. Further, the presence of biotin in tears suggests the physiological significance of this transporter in rabbit corneal epithelium.

Modulation of P-glycoprotein-mediated efflux by prodrug derivatization: an approach involving peptide transporter-mediated influx across rabbit cornea

The aim of this study was to investigate the modulation of efflux mechanisms using transporter- targeted prodrug derivatization of a model P-gp substrate, quinidine. The L-valine, L-valine-valine esters of quinidine, val-quinidine (VQ), and val-val-quinidine (VVQ) were synthesized in our laboratory, respectively. [(14)C] erythromycin was chosen to delineate the affinity of quinidine (Q) toward P-gp. [(3)H] glycylsarcosine (GS, or glysar) was chosen as a model peptide transporter (PEPT) substrate. Uptake studies were performed on rPCEC (rabbit primary corneal epithelial culture) using 12-well plates. Transport studies were conducted with isolated rabbit corneas at 34 degrees C. Efflux of [(14)C] erythromycin was significantly increased in the presence of quinidine, whereas it was unaltered in the presence of VQ and VVQ. VVQ was more stable, both in buffers and tissue homogenate. Transport of VQ and VVQ was inhibited with GS, and their permeability values were 1.5 and 3 times higher than the permeability of quinidine, respectively. Results from this study clearly indicate that prodrug derivatization of quinidine can modulate P-gp-mediated efflux. These prodrugs have a reduced or diminished affinity toward P-gp and were further recognized by the peptide transporter- mediated process. Enhanced permeabilities of the prodrugs indicate that drug derivatization can be a viable strategy for overcoming P-gp-mediated efflux.

Mechanism of L-ascorbic acid uptake by rabbit corneal epithelial cells: evidence for the involvement of sodium-dependent vitamin C transporter 2

PURPOSE

To investigate the mechanism of L-ascorbic acid uptake by rabbit corneal epithelial cells and to functionally characterize the specific transporter involved in this translocation process.

METHODS

Uptake studies were carried out with SIRC (Statens Seruminstitut Rabbit Cornea) and rPCEC (rabbit Primary corneal epithelial cell culture) in 12-well plates using [14C] Ascorbic acid (AA). Uptake was done in the presence of L-ascorbic acid and D-isoascorbic acid to delineate stereospecificity. Inhibition studies were performed in the presence of D-glucose a substrate for GLUT and also para amino hippuric acid (PAHA) a substrate for organic anion transporter. Effects of pH and sodium on the uptake of AA were characterized. Concentration dependency studies were performed. Energy dependence of AA uptake was investigated with ouabain and sodium azide in rPCEC. Reverse Transcription-polymerase chain reaction (RT-PCR) was also performed.

RESULTS

Uptake of AA was inhibited by about 90% and 50% respectively in the presence of L-ascorbic acid and D-isoascorbic acid in both SIRC and rPCEC. Uptake was unaltered by D-glucose and PAHA. The process was sodium dependent and saturable at higher concentrations. Ouabain and sodium azide significantly diminished the uptake process. It also decreased with a reduction in pH. The RT-PCR results showed the presence of SVCT2 but not SVCT1.

CONCLUSIONS

Uptake of AA across rabbit corneal epithelial cells appears to be a carrier mediated active process. A saturable, sodium dependent, and pH sensitive transporter with high specificity for L-ascorbic acid was functionally characterized and was identified as SVCT2.

Enzymes involved in the bioconversion of ester-based prodrugs

Enzymes are essential for the activation of many prodrugs. In this review, the most important enzymes (e.g., paraoxonase, carboxylesterase, acetylcholinesterase, cholinesterase) involved in the bioconversion of ester-based prodrugs will be discussed in terms of their biology and biochemistry. Most of these enzymes fall into the category of hydrolytic enzymes. However, nonhydrolytic enzymes, including cytochrome P450s, can also catalyze the bioconversion of ester prodrugs and thus will be discussed here. Other factors influencing the ability of these enzymes to catalyze the bioconversion of ester-based prodrugs, particularly species and interindividual differences and stereochemical and structural features of the prodrugs, will be discussed.

Esteraseaktivität eines organotypischen humanen Kornea-Konstrukts (HCC) als In-vitro-Modell für Permeationsuntersuchungen

Organotypic cornea equivalents are used as in vitro models for permeation studies. Many ophthalmic drugs are applied as ester prodrugs to achieve a higher bioavailability. The esterase activity of three corneal human cell lines (epithelial, stromal, endothelial cells) as well as of excised porcine cornea, human donor cornea and human cornea construct (HCC) was investigated and compared. Esterase activity was determined using p-nitrophenyl acetate and hydrocortisone acetate (HCA) as esterase substrates. Hydrocortisone acetate permeation across porcine cornea, human donor cornea and HCC was studied in vitro using Franz-diffusion cells. Corneal epithelial cells showed the highest esterase activity and only small differences to keratocytes and endothelial cells were detectable. The permeation barrier properties of the different corneal tissues were very similar in the case of HCA permeation whereas HCA metabolism rates were in the ranking order of porcine cornea > HCC > human donor cornea. Permeation and metabolism studies indicate that the in vitro permeation model HCC is able to adequately convert hydrocortisone acetate to hydrocortisone.

Human cornea construct HCC-an alternative for in vitro permeation studies? A comparison with human donor corneas

Transcorneal in vitro permeation studies of ophthalmic drugs are normally performed with either excised animal corneas or latterly corneal cell culture models. A good correlation between these models and excised animal corneas regarding permeation behaviour of drugs has already been shown. However, comparisons between corneal in vitro models containing human cells and excised human corneas do not exist yet. Therefore in the present study the transcorneal permeation of six different model drugs (pilocarpine hydrochloride, befunolol hydrochloride, hydrocortisone, diclofenac sodium, clindamycin hydrochloride and timolol maleate) across our previously described three-dimensional organotypic human cornea construct (HCC) was tested using Franz diffusion cells and compared with permeation data obtained from human donor corneas. The HCC showed a similar permeation behaviour compared with human donor cornea for all substances. The permeabilities (permeation coefficients P) of the human cornea equivalent versus the human donor cornea were the same in the case of diclofenac, clindamycin, timolol, but marginally decreased for hydrocortisone and slightly increased for pilocarpine and befunolol. These small differences of permeation coefficients were expressed as factors and only varied from 0.8 to 1.4. The results indicate that the HCC may be an alternative for in vitro permeation studies and appropriate for predicting drug absorption into the human eye.

Development of Cultured Rabbit Corneal Epithelium for Drug Permeation Studies: A Comparison with Excised Rabbit Cornea

The aim of this study was to prepare and test an artificial corneal epithelium (reconstituted rabbit corneal epithelium, RRCE) exhibiting barrier characteristics and paracellular permeability similar to those of native rabbit cornea. The RRCE was obtained from a rabbit corneal epithelium (RCE) cell line grown for 8 days in submerged culture, then for 7 days in air-interface conditions on Snapwell polyester membranes. Permeation studies on the RRCE were carried out in comparison with rabbit excised corneas in vitro, using timolol maleate (TM) as the test drug, alone and in association with the following ocular permeation enhancers: benzalkonium chloride, ethylene-diaminetetraacetic acid sodium salt, polyethoxylated castor oil, polyoxyethylene stearyl ether, sodium deoxycholate, and escin. The integrity of the RRCE was assessed by measuring the transepithelial electrical resistance (TEER) during culture time and after every permeation experiment. When TM was tested alone, the permeation parameters (apparent permeability coefficient, lag time) obtained with the RRCE were similar to those of excised rabbit corneas. The artificial epithelium, however, was less sensitive than native cornea to the effect of permeation enhancers.

Ultrasound-Enhanced Transcorneal Drug Delivery

PURPOSE

Ultrasound has been shown to enhance, by up to 10 times, the corneal permeability to different compounds such as beta-blockers and fluorescein. Here, we report on our investigation of the mechanisms of ultrasound-enhanced drug delivery through the cornea using light and electron microscopy.

METHODS

Enhancement of permeability for a hydrophilic compound, sodium fluorescein, in rabbit cornea in vitro was achieved using ultrasound at a frequency of 880 kHz and intensities of 0.19-0.56 W/cm2 with an exposure duration of 5 minutes. Light and electron microscopy (transmission and scanning) were used to observe ultrasound-induced structural changes in the cornea.

RESULTS

The permeability increased by 2.1, 2.5, and 4.2 times when ultrasound was applied at 0.19, 0.34, and 0.56 W/cm2, respectively (P<0.05). The surface cells of corneal epithelium exposed to ultrasound appeared swollen and lighter in color (indications of membrane rupture) as compared with the control cells. Some of the surface epithelial cells were absent. The cells in the inner layers of the epithelium were occasionally lighter in color. Also, holes 3-10 microm in diameter were observed on the epithelial surface. No structural changes were observed in the stroma.

CONCLUSION

Ultrasound enhancement of drug delivery through the cornea appears to result from minor structural alterations in the epithelium. Careful investigation of the recovery of cornea structure and barrier function after the ultrasound application, in vivo, is needed.

Membrane transporter/receptor-targeted prodrug design: strategies for human and veterinary drug development

The bioavailability of drugs is often severely limited due to the presence of biological barriers in the form of epithelial tight junctions, efflux proteins and enzymatic degradation. Physicochemical properties, such as lipophilicity, molecular weight, charge, etc., also play key roles in determining the permeation properties of drug candidates. As a result, many potential drug candidates may be dropped from the initial screening portfolio. Prodrug derivatization targeting transporters and receptors expressed on mammalian cells holds tremendous potential. Enhanced cellular delivery can significantly improve drug absorption. Such approaches of drug targeting and delivery have been the subject of intense research. Various prodrugs have been designed that demonstrate enhanced bioavailability and tissue specificity. This approach is equally applicable to human and veterinary pharmaceuticals since most of the transporters and receptors expressed by human tissues are also expressed in animals. This review highlights studies conducted on the use of transporters and receptors in an effort to improve drug bioavailability and to develop targeted drug delivery systems.

Effect of hydroxypropyl beta cyclodextrin complexation on aqueous solubility, stability, and corneal permeation of acyl ester prodrugs of ganciclovir

The purpose of the study was to investigate the effect of hydroxypropyl beta cyclodextrin (HPbetaCD) on aqueous solubility, stability, and in vitro corneal permeation of acyl ester prodrugs of ganciclovir (GCV). Aqueous solubility and stability of acyl ester prodrugs of Ganciclovir (GCV) were evaluated in pH 7.4 isotonic phosphate buffer solution (IPBS) in the presence and absence of HPbetaCD. Butyryl cholinesterase-mediated enzymatic hydrolysis of the GCV prodrugs was studied using various percentage w/v HPbetaCD. In vitro corneal permeation of GCV and its prodrugs (with and without 5% HPbetaCD) across isolated rabbit cornea was studied using side-by-side diffusion cells. HPbetaCD-prodrug complexation was of the A(L) type with values for complexation constants ranging between 12 and 108 M(-1). Considerable improvement in chemical and enzymatic stability of the GCV prodrugs was observed in the presence of HPbetaCD. The stabilizing effect of HPbetaCD was found to depend on the degree of complexation and the degradation rate of prodrug within the complex. Five percent w/v HPbetaCD was found to enhance the corneal permeation of only the most lipophilic prodrug GCV dibutyrate (2.5-fold compared with 0% HPbetaCD). All other prodrugs showed little or no difference in transport in the presence of 5% w/v HPbetaCD. Agitation in the donor chamber largely influenced the transport kinetics of GCV dibutyrate across cornea. Results indicate the presence of an unstirred aqueous diffusion layer at the corneal surface that restricts the transport of the highly lipophilic GCV dibutyrate prodrug. HPbetaCD improves corneal permeation by solubilizing the hydrophobic prodrug and delivering it across the mucin layer at the corneal surface.

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 degrees C. Thymidine uptake by SIRC displayed saturable kinetics (K(m) = 595.9 +/- 80.4 microM, and V(max) = 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 (K(m) = 14.4 +/- 2.3 microM, V(max) = 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.

Mechanism of corneal permeation of L-valyl ester of acyclovir: targeting the oligopeptide transporter on the rabbit cornea

PURPOSE

To delineate mechanisms associated with the corneal transport of a L-valine prodrug of an antiviral agent, acyclovir.

METHOD

The permeability and enzymatic hydrolysis of L-Val-ACV were evaluated using freshly excised rabbit cornea. Transport mechanism across rabbit cornea was investigated through a competitive inhibition study of L-Val-ACV with other substrates of human peptide transporter (hPepT1).

RESULTS

L-Valyl ester of Acyclovir (L-Val-ACV) was approximately threefold more permeable across the intact rabbit cornea than acyclovir (ACV). Dipeptides, beta-lactam antibiotics, and angiotensin converting enzyme (ACE) inhibitors, strongly inhibited the transport of L-Val-ACV indicating that a carrier mediated transport system specific for peptides is primarily responsible for the corneal permeation of L-Val-ACV. L-Val-ACV transport was found to be saturable (Km = 2.26 +/- 0.34 mM, Jmax = 1.087 +/- 0.05 nmoles cm(-2) min(-1)), energy and pH dependent. CONCLUSIONS Functional evidence of an oligopeptide transport system present on the rabbit cornea has been established. The peptide transporter on the corneal epithelium may be targeted to improve the ocular bioavailability of poorly absorbed drugs.

Corneal permeation of ganciclovir: mechanism of ganciclovir permeation enhancement by acyl ester prodrug design

Ganciclovir (GCV), a promising antiviral compound, has poor ocular bioavailability as a result of its relatively low partition coefficient. In this study, lipophilic ester prodrugs of GCV were synthesized in an effort to improve its uptake into ocular tissues. In vitro permeability studies were conducted on isolated rabbit corneal membranes using aliphatic mono-acyl ester prodrugs of GCV to determine the effect of lipophilicity and corneal hydrolysis rate on transcorneal diffusion. The GCV prodrugs showed a progressive decrease in solubility and a corresponding increase in Log P values as the chain length was ascended. Permeation studies using freshly isolated rabbit corneas showed that all prodrugs permeated as intact prodrug as well as hydrolyzed GCV. Corneal permeability coefficients increased with increasing lipophilicity for mono-ester prodrugs having more than three carbon atoms in the side chain. The permeability of GCV increased about 6-fold in ascending from the parent drug-ganciclovir (3.82 +/- 0.19 x 10(-6) cm sec(-1)) to its valerate ester prodrug (23.70 +/- 1.36 x 10(-6) cm sec(-1)). Among the prodrugs studied, the valerate ester showed the highest permeability and holds the most potential for development. Overall prodrug permeability correlated linearly with increased susceptibility of the GCV esters to undergo hydrolysis in the cornea. The present work indicates that the ideal prodrug is one that not only possesses enhanced partitioning characteristics, but also high enzyme susceptibility. Concentration of active GCV penetrating the corneal epithelium was substantially increased through the bio-reversible ester prodrug strategy.

Carrier mediated uptake of L-tyrosine and its competitive inhibition by model tyrosine linked compounds in a rabbit corneal cell line (SIRC)--strategy for the design of transporter/receptor targeted prodrugs

The objective of this study was to investigate the presence of amino acid transporters on the corneal epithelium and to enhance corneal drug absorption through prodrug modification targeted to the amino acid transporters. SIRC was used as a model cell line representing the corneal epithelium. Uptake studies were carried out using [3H] L-tyrosine at 37 degrees C. Temperature, energy and pH dependence studies were carried out. The uptake seems to be composed of a major saturable and minor non-saturable component (V(max) =2.9+/-0.62 nmoles/min/mg protein, K(m) =71+/-21 microM, K(d) =2.6+/-0.6 nl/min/mg protein). No significant inhibition of uptake was observed in the presence of metabolic inhibitors or in the absence of sodium. Competitive inhibition studies were performed in the presence of various amino acids and model tyrosine conjugates (p-nitro and p-chloro benzyl ether conjugate of L-tyrosine). Uptake was inhibited by neutral aromatic and large neutral aliphatic amino acids. L-Tyrosine uptake was inhibited by its ether conjugates in a concentration dependent manner suggesting that these compounds may be sharing the same transport mechanism. This study provides biochemical evidence of the presence of a large neutral amino acid transport system on the corneal epithelium, which may be utilized to enhance the corneal drug transport.