Effect of acylation on the ocular disposition of acyclovir. II: Corneal permeability and anti-HSV 1 activity of 2'-esters in rabbit epithelial keratitis

Drug delivery to the anterior segment of the eye: A review of current and future treatment strategies

Research in the development of ophthalmic drug formulations and innovative technologies over the past few decades has been directed at improving the penetration of medications delivered to the eye. Currently, approximately 90% of all ophthalmic drug formulations (e.g. liposomes, micelles) are applied as eye drops. The major challenge of topical eye drops is low bioavailability, need for frequent instillation due to the short half-life, poor drug solubility, and potential side effects. Recent research has been focused on improving topical drug delivery devices by increasing ocular residence time, overcoming physiological and anatomical barriers, and developing medical devices and drug formulations to increase the duration of action of the active drugs. Researchers have developed innovative technologies and formulations ranging from sub-micron to macroscopic size such as prodrugs, enhancers, mucus-penetrating particles (MPPs), therapeutic contact lenses, and collagen corneal shields. Another approach towards the development of effective topical drug delivery is embedding therapeutic formulations in microdevices designed for sustained release of the active drugs. The goal is to optimize the delivery of ophthalmic medications by achieving high drug concentration with prolonged duration of action that is convenient for patients to administer.

Enzyme Models-From Catalysis to Prodrugs

Enzymes are highly specific biological catalysts that accelerate the rate of chemical reactions within the cell. Our knowledge of how enzymes work remains incomplete. Computational methodologies such as molecular mechanics (MM) and quantum mechanical (QM) methods play an important role in elucidating the detailed mechanisms of enzymatic reactions where experimental research measurements are not possible. Theories invoked by a variety of scientists indicate that enzymes work as structural scaffolds that serve to bring together and orient the reactants so that the reaction can proceed with minimum energy. Enzyme models can be utilized for mimicking enzyme catalysis and the development of novel prodrugs. Prodrugs are used to enhance the pharmacokinetics of drugs; classical prodrug approaches focus on alternating the physicochemical properties, while chemical modern approaches are based on the knowledge gained from the chemistry of enzyme models and correlations between experimental and calculated rate values of intramolecular processes (enzyme models). A large number of prodrugs have been designed and developed to improve the effectiveness and pharmacokinetics of commonly used drugs, such as anti-Parkinson (dopamine), antiviral (acyclovir), antimalarial (atovaquone), anticancer (azanucleosides), antifibrinolytic (tranexamic acid), antihyperlipidemia (statins), vasoconstrictors (phenylephrine), antihypertension (atenolol), antibacterial agents (amoxicillin, cephalexin, and cefuroxime axetil), paracetamol, and guaifenesin. This article describes the works done on enzyme models and the computational methods used to understand enzyme catalysis and to help in the development of efficient prodrugs.

Ocular prodrugs: Attributes and challenges

Ocular drug delivery is one of the most attention-grabbing and challenging endeavors among the numerous existing drug delivery systems. From a drug delivery point of view, eye is an intricate organ to investigate and explore. In spite of many limitations, advancements have been made with the intention of improving the residence time or permeation of the drug in the ocular region. Poor bioavailability of topically administered drugs is the major issue pertaining to ocular drug delivery. Several efforts have been made towards improving precorneal residence time and corneal penetration, e.g. iontophoresis, prodrugs and ion-pairing, etc. Prodrug approach (chemical approach) has been explored by the formulation scientists to optimize the physicochemical and biochemical properties of drug molecules for improving ocular bioavailability. Formulation of ocular prodrugs is a challenging task as they should exhibit optimum chemical stability as well as enzymatic liability so that they are converted into parent drug after administration at the desired pace. This review will encompass the concept of derivatization and recent academic and industrial advancements in the field of ocular prodrugs. The progression in prodrug designing holds a potential future for ophthalmic drug delivery.

Ocular Drug Delivery

Although the eye is an accessible organ for direct drug application, ocular drug delivery remains a major challenge due to multiple barriers within the eye. Key barriers include static barriers imposed by the cornea, conjunctiva, and retinal pigment epithelium and dynamic barriers including tear turnover and blood and lymphatic clearance mechanisms. Systemic administration by oral and parenteral routes is limited by static blood-tissue barriers that include epithelial and endothelial layers, in addition to rapid vascular clearance mechanisms. Together, the static and dynamic barriers limit the rate and extent of drug delivery to the eye. Thus, there is an ongoing need to identify novel delivery systems and approaches to enhance and sustain ocular drug delivery. This chapter summarizes current and recent experimental approaches for drug delivery to the anterior and posterior segments of the eye.

MODERN ETHIOTROPIC CHEMOTHERAPY OF HERPESVIRUS INFECTIONS: ADVANCES, NEW TRENDS AND PERSPECTIVES. ALPHAHERPESVIRINAE (part I)

Modern therapy of infections caused by alpha-herpesviruses is based on drugs belonging to the class of modified nucleosides (acyclovir) and their metabolic progenitors - valine ester of acyclovir and famciclovir (prodrug of penciclovir). The biological activity of these compounds is determined by the similarity of their structure to natural nucleosides: modified nucleosides compete with natural nucleosides for binding to DNA-polymerase and, due to their structural features, inhibit its activity. However, the emergence of variants of viruses resistant to the antiviral drugs available in the arsenal of modern medicine necessitates the search for new compounds able of effectively inhibiting the reproduction of viruses. These compounds should be harmless to the macroorganisms, convenient to use, and overcoming the drug resistance barrier in viruses. The search for literature in international databases (PubMed, MedLine, RINC, etc.) in order to obtain information on promising developments that open new possibilities for treating herpesvirus infection and subsequent analysis of the collected data made it possible to determine not only the main trends in the search for new antiviral agents, but also to provide information on the compounds most promising for the development of anti-herpesvirus drugs.

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.

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.

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.

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.

Ocular pharmacokinetics of acyclovir amino acid ester prodrugs in the anterior chamber: evaluation of their utility in treating ocular HSV infections

PURPOSE

To evaluate in vivo corneal absorption of the amino acid prodrugs of acyclovir (ACV) using a topical well model and microdialysis in rabbits.

METHODS

Stability of L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) prodrugs was evaluated in various ocular tissues. Dose-dependent toxicity of these prodrugs was also examined in rabbit primary corneal epithelial cell culture (rPCEC) using 96-well based cell proliferation assay. In vivo ocular bioavailability of these compounds was also evaluated with a combination of topical well infusion and aqueous humor microdialysis techniques.

RESULTS

Among the amino acid ester prodrugs, SACV was most stable in aqueous humor. Enzymatic degradation of EACV was the least compared to all other prodrugs. Cellular toxicity of all the prodrugs was significantly less compared to trifluorothymidine (TFT) at 5mM. Absorption rate constants of all the compounds were found to be lower than the elimination rate constants. All the prodrugs showed similar terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately two-fold increase in area under the curve (AUC) relative to ACV (p<0.05). Clast (concentration at the last time point) of SACV was observed to be 8+/-2.6microM in aqueous humor which is two and three times higher than VACV and ACV, respectively.

CONCLUSIONS

Amino acid ester prodrugs of ACV were absorbed through the cornea at varying rates (ka) thereby leading to varying extents (AUC). The amino acid ester prodrug, SACV owing to its enhanced stability, comparable AUC and high concentration at last time point (Clast) seems to be a promising candidate for the treatment of ocular HSV infections.

Preparation and ocular pharmacokinetics of ganciclovir liposomes

Ophthalmic liposomes of ganciclovir (GCV) were prepared by the reverse phase evaporation method, and their ocular pharmacokinetics in albino rabbits were compared with those obtained after dosing with GCV solution. The in vitro transcorneal permeability of GCV liposomes was found to be 3.9-fold higher than that of the solution. After in vivo instillation in albino rabbits, no difference was found in the precorneal elimination rate of GCV from liposome vs solution dosing. The aqueous humor concentration-time profiles of both liposomes and solution were well described by 2-compartmental pharmacokinetics with first-order absorption. The area under the curve of the aqueous humor concentration-time profiles of GCV liposomes was found to be 1.7-fold higher than that of GCV solution. Ocular tissue distribution of GCV from liposomes was 2 to 10 times higher in the sclera, cornea, iris, lens, and vitreous humor when compared with those observed after solution dosing. These results suggested that liposomes may hold some promise in ocular GCV delivery.

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.

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.

Interactions of the dipeptide ester prodrugs of acyclovir with the intestinal oligopeptide transporter: competitive inhibition of glycylsarcosine transport in human intestinal cell line-Caco-2

The oligopeptide transporter may be exploited to enhance the absorption of drugs by synthesizing their dipeptide ester prodrugs, which may be recognized as its substrates. Various dipeptide esters of acyclovir (ACV), an antiviral nucleoside analog, were synthesized. Enzymatic hydrolysis and affinity of the prodrugs toward the human intestinal peptide transporter hPEPT1 were studied using the human intestinal Caco-2 cell line. Affinity studies were performed by inhibiting the uptake of [(3)H]glycylsarcosine by the prodrugs. The uptake of glycylsarcosine was found to be saturable at higher concentrations and was competitively inhibited by the prodrugs of ACV. All prodrugs except Tyr-Gly-ACV demonstrated a higher affinity (1.41-4.96 mM) toward hPEPT1 than cephalexin (8.19 +/- 2.12 mM), which was used as a positive control. Two prodrugs, Gly-Val-ACV and Val-Val-ACV, showed comparable affinity to Val-ACV, an amino acid prodrug of ACV recognized by PEPT1/PEPT2. The permeability of Gly-Val-ACV (2.99 +/- 0.59 x 10(-6) cm/s) across Caco-2 was comparable with that of Val-ACV (3.01 +/- 0.21 x 10(-6) cm/s) and was significantly inhibited (63%) in presence of glycylsarcosine. The transport of GVACV across Caco-2 was saturable at higher concentrations, and the parameters were calculated as K(m) 3.16 +/- 0.31 mM and V(max) 0.014 +/- 0.00058 nmol cm(-2) min(-1). Overall, the results suggest that the dipeptide prodrugs of ACV have a high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of ocular and oral herpesvirus infections, because cornea and intestinal epithelia seem to express the oligopeptide transporters.

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.

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

The purpose of this study is to assess the permeability of acyclovir (ACV) prodrugs through the rabbit corneal cell line (SIRC) as well as the cornea, and characterize the SIRC cell line for transport and metabolism studies of ester prodrugs. Prodrug derivatization of an acycloguanosine antiviral agent, acyclovir, was employed to improve its permeability across the cornea. New Zealand albino rabbits were used as an animal model for corneal studies. The SIRC cell line grown on polyester membranes was used for transport of these prodrugs. SIRC cells grown on the membrane support for 10 days developed four to six layers of epithelial cells, and this is comparable to the normal rabbit corneal epithelial layer. Transport experiments were conducted across the rabbit cornea and confluent SIRC cells using side-by-side diffusion-cell apparatus. Enzymatic hydrolysis of these compounds was evaluated in SIRC cell lysates. Appropriate reversed phase HPLC method(s) were employed for quantitation of both the prodrug and ACV simultaneously. Corneal permeabilities of some of these prodrugs (Malonyl ACV and Acetyl ACV) were higher relative to ACV. The SIRC cell line permeability values of all the prodrugs were higher compared to that of the intact cornea. The total amount of ACV-prodrugs transported, i.e., unhydrolyzed prodrugs and regenerated ACV, across the SIRC cell line was more relative to ACV. Hydrolytic studies in the SIRC cell line homogenate demonstrated the bioreversion potential of the prodrugs and the presence of enzymes, particularly the cholinesterase in the SIRC cell line. It may be concluded that the SIRC cell line is leakier compared to the cornea. Keeping in mind the limitations, the SIRC cell line after further characterization may be used for transport and metabolism studies of ester prodrugs.

In-vitro hydrolysis, permeability, and ocular uptake of prodrugs of N-[4-(benzoylamino)phenylsulfonyl]glycine, a novel aldose reductase inhibitor

To enhance the ocular uptake of N-[4-(benzoylamino)phenylsulfonyl]glycine (BAPSG), two ester (methyl and isopropyl) prodrugs were synthesized and evaluated for their stability in various buffers (pH 1-9), hydrolysis in rabbit ocular tissues (cornea, conjunctiva, iris-ciliary body, lens, aqueous humor, and vitreous humor), transport across cornea and conjunctiva, and in-vivo uptake following topical administration. Over the pH range of 1-9, the rate constants for degradation ranged from 5.67 to 218.9 x 10(-3) h(-1) for the methyl ester and from 3.14 to 4.45 x 10(-3) h(-1) for the isopropyl ester. At all pH conditions, the isopropyl ester was more stable when compared with the methyl ester. A change in buffer concentration at pH 7.4 did not influence the stability of the prodrugs. The prodrugs were rapidly hydrolysed in the tissue homogenates, with the rate constants for hydrolysis ranging from 1.98 to 7.2x 10(-3) min(-1) for the methyl ester and 3.32 to 6.53 x 10(-3) min(-1) for the isopropyl ester. The in-vitro permeability of the methyl ester was less than the parent drug across cornea and conjunctiva. Isopropyl ester levels were not detectable in the receiver chamber even at the end of the 4-h transport study. Following topical administration of BAPSG and the two prodrugs at a dose of 60 microg/eye, the lowest levels were seen in vitreous humor for parent compound and its methyl ester. In general, the tissue uptake of methyl ester was less than BAPSG. Isopropyl ester levels were below detection limits in all the ocular tissues. Lipophilic ester prodrugs of BAPSG showed good aqueous solution stability in tissue homogenates. However, these prodrugs lacking the free carboxylate anion exhibited reduced in-vitro permeability and in-vivo uptake, suggesting the importance of free carboxylate anion in the delivery of BAPSG.

Acyclovir-containing liposomes for potential ocular delivery. Corneal penetration and absorption

The in vitro corneal penetration and in vivo corneal absorption of acyclovir from an acyclovir-containing liposome system were investigated. Results of in vitro corneal penetration demonstrated that positively charged liposomes resulted in a penetration rate lower than those of negatively charged liposomes and free acyclovir in solution. An in vivo study indicated that the extent of acyclovir absorption from positively charged liposomes was higher that those from negatively charged liposomes and free acyclovir. The acyclovir concentration in the cornea after administration of positively charged liposomes showed that an acyclovir deposition in the cornea was greater than those of negatively charged liposomes and free acyclovir. From morphological observation of the cornea surface treated with liposomes, it was suggested that positively charged liposomes formed a completely coated layer on the cornea surface. These liposomes would bind intimately on the cornea surface, leading to an increase of residence time. Therefore, positively charged liposomes resulted in an increase of acyclovir (ACV) absorption.

Characterization and in-vivo ocular absorption of liposome-encapsulated acyclovir

The potential of liposomes as an in-vivo ophthalmic drug delivery system for acyclovir was investigated. The drug-membrane interaction was evaluated by means of differential scanning calorimetry analysis. These experiments showed that acyclovir is able to interact with both positively and negatively charged membranes via electrostatic or hydrogen bonds. No interaction was observed with neutral membranes made up of dipalmitoylphosphatidylcholine. Different liposome preparation procedures were carried out to encapsulate acyclovir. The drug encapsulation mainly depends on the amount of water which the liposome system is able to entrap. In the case of multilamellar vesicles, charged systems showed the highest encapsulation efficiency. No particular difference in the encapsulation efficiency was observed for oligolamellar vesicles prepared with the reverse-phase evaporation technique. Oligolamellar liposomes showed the highest acyclovir encapsulation parameters and had release profiles similar to those of multilamellar liposomes. In-vivo experiments using male New Zealand albino rabbits were carried out to evaluate the aqueous humour concentration of acyclovir bioavailability. The most suitable ophthalmic drug delivery system was oligolamellar systems made up of dipalmitoylphosphatidylcholine-cholesterol-dimethyldioctadecyl glycerole bromide (7:4:1 molar ratio), which presented the highest encapsulation capacity and were able to deliver greater amounts of the drug into the aqueous humour than a saline acyclovir solution or a physical liposome/drug blend.

Bioadhesive microspheres for ophthalmic administration of acyclovir

The bioavailability of acyclovir to the ophthalmic epithelium is low and when the drug is administered in ophthalmic ointment it must be applied every four hours. An emulsification technique has been used to prepare acyclovir-loaded chitosan microspheres with the aim of promoting the prolonged release of drug and increasing its ocular bioavailability. The microparticulate drug-delivery systems obtained have been characterized for their morphology and physicochemical characteristics by in-vitro dissolution tests and in-vivo ocular administration to rabbits. The results show that the microspheres obtained are always quite small--the diameters of 90% of the particles are < or = 25 microns (i.e. d 90% never exceeds 25 microns) and physicochemical characterization shows that the drug is homogeneously dispersed in an amorphous state inside the microspheres. The in-vitro dissolution profile of acyclovir from chitosan microspheres is slower than that for the raw drug. Results from in-vivo ocular administration of acyclovir-loaded microspheres to the rabbit eye show prolonged high concentrations of acyclovir and increased AUC values. The microparticulate drug-carrier seems a promising means of topical administration of acyclovir to the eye.