Prodrugs for improved ocular drug delivery

Advancements in Transdermal Drug Delivery Systems: Harnessing the Potential of Macromolecular Assisted Permeation Enhancement and Novel Techniques

Transdermal drug delivery (TDD) represents a transformative paradigm in drug administration, offering advantages such as controlled drug release, enhanced patient adherence, and circumvention of hepatic first-pass metabolism. Despite these benefits, the inherent barrier function of the skin, primarily attributed to the stratum corneum, remains a significant impediment to the efficient permeation of therapeutic agents. Recent advancements have focused on macromolecular-assisted permeation enhancers, including carbohydrates, lipids, amino acids, nucleic acids, and cell-penetrating peptides, which modulate skin permeability by transiently altering its structural integrity. Concurrently, innovative methodologies such as iontophoresis, electroporation, microneedles, ultrasound, and sonophoresis have emerged as potent tools to enhance drug transport by creating transient microchannels or altering the skin's microenvironment. Among the novel approaches, the development of nanocarriers such as Liposome, niosomes, and transethosomes etc. has garnered substantial attention. These elastic vesicular systems, comprising lipids and edge activators, exhibit superior skin penetration owing to their deformability and enhanced payload delivery capabilities. Furthermore, the integration of nanocarriers with physical enhancement techniques demonstrates a synergistic potential, effectively addressing the limitations of conventional TDD systems. This comprehensive convergence of macromolecular-assisted enhancers, advanced physical techniques, and next-generation nanocarriers underscores the evolution of TDD, paving the way for optimized therapeutic outcomes.

Ocular drug-metabolizing enzymes: focus on esterases

This review describes current knowledge on the expression of ocular phase I and II drug-metabolizing enzymes in the main animal species used in ocular drug development and in humans, with a focus on ocular esterases and their prodrug substrates. The eye possesses a unique metabolic profile, exhibiting a lower and restricted expression of major cytochrome P450s (CYPs) and most transferases apart from glutathione S-transferases (GST) when compared to the liver. In contrast, hydrolytic enzymes are abundant in many ocular tissues. These enzymes have attracted interest because of their role in prodrug activation and drug elimination. A literature survey suggests profound variations in tissue expression levels and activities between different species but also points out significant gaps in knowledge. These uncertainties highlight a need for more detailed characterization of enzymes in individual ocular tissues and across species to aid future translational studies in ophthalmic drug research. Thus, an in-depth analysis of ocular drug metabolism and species differences is crucial for ocular drug development.

Topical drug delivery to the retina: obstacles and routes to success

INTRODUCTION

Retinal diseases are one of the main reasons for vision loss where all available drug treatments are based on invasive drug administration such as intravitreal injections. Despite huge efforts and some promising results in animal models, almost all delivery technologies tested have failed in human trials. There are however examples of clinically effective topical delivery systems such as fast dissolving aqueous eye drop suspensions.

AREAS COVERED

Six obstacles to topical drug delivery to the eye have been identified and discussed in some details. These obstacles consist of static membrane barriers to drug permeation into the eye, dynamic barriers such as the lacrimal drainage and physiochemical barriers such as low thermodynamic activity. It is explained how and why these obstacles hamper drug permeation and how different technologies, both those that are applied in marketed drug products and those that are under investigation, have addressed these obstacles.

EXPERT OPINION

The reason that most topical drug delivery systems have failed to deliver therapeutic drug concentrations to the retina is that they do not address physiochemical barriers such as the thermodynamic activity of the permeating drug molecules. Topical drug delivery to the retina has only been successful when the static, dynamic, and physiochemical barriers are addressed simultaneously.

Enhancing Permeation of Drug Molecules Across the Skin via Delivery in Nanocarriers: Novel Strategies for Effective Transdermal Applications

The transdermal route of administration provides numerous advantages over conventional routes i.e., oral or injectable for the treatment of different diseases and cosmetics applications. The skin also works as a reservoir, thus deliver the penetrated drug for more extended periods in a sustained manner. It reduces toxicity and local irritation due to multiple sites for absorption and owes the option of avoiding systemic side effects. However, the transdermal route of delivery for many drugs is limited since very few drugs can be delivered at a viable rate using this route. The stratum corneum of skin works as an effective barrier, limiting most drugs' penetration posing difficulty to cross through the skin. Fortunately, some non-invasive methods can significantly enhance the penetration of drugs through this barrier. The use of nanocarriers for increasing the range of available drugs for the transdermal delivery has emerged as a valuable and exciting alternative. Both the lipophilic and hydrophilic drugs can be delivered via a range of nanocarriers through the stratum corneum with the possibility of having local or systemic effects to treat various diseases. In this review, the skin structure and major obstacle for transdermal drug delivery, different nanocarriers used for transdermal delivery, i.e., nanoparticles, ethosomes, dendrimers, liposomes, etc., have been discussed. Some recent examples of the combination of nanocarrier and physical methods, including iontophoresis, ultrasound, laser, and microneedles, have also been discussed for improving the therapeutic efficacy of transdermal drugs. Limitations and future perspectives of nanocarriers for transdermal drug delivery have been summarized at the end of this manuscript.

Review of Approaches for Increasing Ophthalmic Bioavailability for Eye Drop Formulations

Ophthalmic diseases represent a significant problem as over 2 billion people worldwide suffer from vison impairment and blindness. Eye drops account for around 90% of ophthalmic medications but are limited in success due to poor patient compliance and low bioavailability. Low bioavailability can be attributed to short retention times in the eye caused by rapid tear turnover and the difficulty of drug diffusion through the multi-layered structure of the eye that includes lipid-rich endothelial and epithelial layers as well as the stroma which is high in water content. In addition, there are barriers such as tight junctional complexes in the corneal epithelium, lacrimal turnover, nasolacrimal drainage, blinking reflexes, efflux transporters, drug metabolism by ocular enzymes, and drug binding to or repulsion from conjunctival mucins, tear proteins, and melanin. In order to maximize transport through the cornea while minimizing drug loss through other pathways, researchers have developed numerous methods to improve eye drop formulations including the addition of viscosity enhancers, permeability enhancers, mucoadhesives, and vasoconstrictors, or using formulations that include puncta occlusion, nanocarriers, or prodrugs. This review explains the mechanism behind each of these methods, examines their history, analyzes previous and current research, evaluates future applications, and discusses the pros and cons of each technique.

Thermoresponsive sol-gel improves ocular bioavailability of Dipivefrin hydrochloride and potentially reduces the elevated intraocular pressure in vivo

The present study involves the development of Dipivefrin hydrochloride (DV) containing Poloxamers (P407 and P188)-Carbopol-934 (CP) based thermoresponsive-gels for the management of elevated intraocular pressure (IOP). Optimal formulation was evaluated for gelation temperature (T_gel), physicochemical and viscoelastic properties, in-vitro gel dissolution and drug release studies. The in-vivo safety, precorneal retention, ocular pharmacokinetics and efficacy in reducing IOP were also evaluated. T_gel of DV-containing thermoresponsive-gels were between 35.1 and 38.9 °C and it was Poloxamers and CP concentrations dependent. The optimal formulation (F8), composed of 20% P407, 5% P188 and 0.15% CP (w/v), had a T_gel of 35 °C. Its viscosity indicated good flow at room temperature and ability to convert to gel at ocular temperature and the rheology studies revealed favorable characteristics for its ocular use. In precorneal retention experiment, F8 indicated significantly higher area under concentrations curves as compared to DV-aqueous suspension (DV-AqS). In-vivo ocular pharmacokinetics indicated a significant improvement in ophthalmic bioavailability of epinephrine (active form of DV). F8 was non-irritant to the eyes and showed a successful, continuous and superior ability to reduce IOP compared to DV-AqS in rabbits. In conclusion, our developed system could be an appropriate substitute to the conventional DV eye preparations in the management of elevated IOP.

Ketoconazole-conjugated ZnO nanoparticles based semi-solid formulation and study their impacts on skin disease

In this study, the ketoconazole-conjugated zinc oxide (ZnO) nanoparticles were prepared in a single-step approach using dextrose as an intermediate compound. The physical parameters confirmed the drug conjugation with ZnO and their size was around 70-75 nm. The drug loading and in vivo drug release studies indicated that the -CHO group from the dextrose increase the drug loading up to 65% and their release kinetics were also studied. The anti-fungal studies indicated that the prepared nanoparticles exhibit strong anti-fungal activity and the minimum concentration needed is 10 mg/ml. The nanoparticles loaded semi-solid gel was prepared using carbopol, methylparaben, propyl paraben and propylene glycol. The in vitro penetration of the ketoconazole-conjugated nanoparticles was studied using the skin. The results indicated that the semi-solid gel preparations influenced the penetration and also favoured the accumulation into the skin membrane. The veterinary clinical studies indicated that the prepared gel is highly suitable for treatment of Malassezia.

The penetration and distribution of topical atropine in animal ocular tissues

PURPOSE

To conduct a multi-tissue investigation on the penetration and distribution of topical atropine in myopia treatment, and determine if atropine is detectable in the untreated contralateral eye after uniocular instillation.

METHODS

Nine mature New Zealand white rabbits were evenly divided into three groups. Each group was killed at 5, 24 and 72 hr, respectively, following uniocular instillation of 0.05 ml of 1% atropine. Tissues were sampled after enucleation: conjunctiva, sclera, cornea, iris, ciliary body, lens, retina, aqueous, and vitreous humors. The assay for atropine was performed using liquid chromatography-mass spectrometry (LC-MS), and molecular tissue distribution was illustrated using matrix-assisted laser desorption ionization-imaging mass spectrometry (MALDI-IMS) via an independent experiment on murine eyes.

RESULTS

At 5 hr, the highest (mean ± SEM) concentration of atropine was detected in the conjunctiva (19.05 ± 5.57 ng/mg, p < 0.05) with a concentration gradient established anteriorly to posteriorly, as supported by MALDI-IMS. At 24 hr, preferential binding of atropine to posterior ocular tissues occurred, demonstrating a reversal of the initial concentration gradient. Atropine has good ocular bioavailability with concentrations of two magnitudes higher than its binding affinity in most tissues at 3 days. Crossing-over of atropine to the untreated eye occurred within 5 hr post-administration.

CONCLUSION

Both transcorneal and transconjunctival-scleral routes are key in atropine absorption. Posterior ocular tissues could be important sites of action by atropine in myopic reduction. In uniocular atropine trials, cross-over effects on the placebo eye should be adjusted to enhance results reliability. Combining the use of LC-MS and MALDI-IMS can be a viable approach in the study of the ocular pharmacokinetics of atropine.

Targeted intracorneal delivery-Biodistribution of triamcinolone acetonide following topical iontophoresis of cationic amino acid ester prodrugs

The aim was to investigate intracorneal iontophoresis of biolabile triamcinolone acetonide (TA) amino acid ester prodrugs (TA-AA). Arginine and lysine esters of TA (TA-Arg and TA-Lys, respectively) were synthesized and characterized; quantification was performed by HPLC-UV and UHPLC-MS/MS. The aqueous solubility of the prodrugs (at pH 5.5) was ∼1000-fold greater than TA. Anodal iontophoresis (10min at 3mA/cm²) of TA-AA was investigated using isolated porcine cornea. Although no statistically significant difference was observed in total intracorneal delivery of TA (468.25±59.70 and 540.85±79.16nmol_TA/cm², for TA-Arg and TA-Lys, respectively), the different susceptibilities of the prodrugs to hydrolysis influenced intracorneal biodistribution. Quantification of TA in twenty-five 40μm thick corneal lamellae revealed significantly deeper penetration of TA following TA-Lys iontophoresis. Its superior resistance to hydrolysis enabled sustained electromigration into the deeper cornea suggesting judicious prodrug selection might enable targeted regioselective drug delivery. The intracorneal biodistribution following anodal iontophoresis of TA-Arg (2.3mM; 10min, 3mA/cm²) was visualized by full field optical coherence tomography providing qualitative confirmation of the extensive intracorneal penetration of TA. Short duration iontophoresis of TA-AA prodrugs may improve deep corneal bioavailability and efficacy in vivo, constituting a "single-shot" treatment option for corneal allograft rejection.

Advances in the use of prodrugs for drug delivery to the eye

INTRODUCTION

Ocular drug delivery is presented with many challenges, taking into account the distinctive structure of the eye. The prodrug approach has been, and is being, employed to overcome such barriers for some drug molecules, utilizing a chemical modification approach rather than a formulation-based approach. A prodrug strategy involves modification of the active moiety into various derivatives in a fashion that imparts some advantage, such as membrane permeability, site specificity, transporter targeting and improved aqueous solubility, over the parent compound. Areas covered: The following review is a comprehensive summary of various novel methodologies and strategies reported over the past few years in the area of ocular drug delivery. Some of the strategies discussed involve polymer and lipid conjugation with the drug moiety to impart hydrophilicity or lipophilicity, or to target nutrient transporters by conjugation with transporter-specific moieties and retrometabolic drug design. Expert opinion: The application of prodrug strategies provides an option for enhancing drug penetration into the ocular tissues, and overall ocular bioavailability, with minimum disruption of the ocular diffusion barriers. Although success of the prodrug strategy is contingent on various factors, such as the chemical structure of the parent molecule, aqueous solubility and solution stability, capacity of targeted transporters and bioreversion characteristics, this approach has been successfully utilized, commercially and therapeutically, in several cases.

Protective effect of Coenzyme Q(10) against oxidative damage in human lens epithelial cells by novel ocular drug carriers

The evaluation of N-trimethyl chitosan (TMC)-coated liposomes containing Coenzyme Q(10) as potential ophthalmic drug delivery system was carried out. Firstly, transcorneal permeation studies were conducted at 34°C using a side-by-side diffusion apparatus. The transport process of the fluorescent marker, rhodamine B, across the corneal epithelium was visualized with confocal laser scanning microscopy. Secondly, the human lens epithelial cells (HLECs) were cultured without or with Coenzyme Q(10) followed by addition of H(2)O(2). The cell viability and apoptosis were evaluated. The permeability coefficient for rhodamine B with TMC-coated liposomes increased more than two times in comparison with the value obtained for solution as control, from (0.42±0.018)×10(5)cms(-1) to (1.31±0.030)×10(5)cms(-1). Confocal laser scanning microscopy revealed that a TMC coating enhanced the transepithelial transport, dependent on the TMC concentration and contacting time. Coenzyme Q(10) elevated the cell viability and reduced the oxidative damage with the decreased percentage of apoptotic cells in a positive concentration-dependent manner. The ATP content of liposome-treated cells was increased about 2-fold compared with that of H(2)O(2)-treated cells. Together, our findings demonstrate that with the enhanced permeation effect of the TMC coating, Coenzyme Q(10)-loaded TMC-coated liposomes appear to be a promising ophthalmic drug delivery carrier with an efficacy in protecting HLECs against H(2)O(2)-induced oxidative damage.

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.

Roles of the conjunctiva in ocular drug delivery: a review of conjunctival transport mechanisms and their regulation

Conjunctiva plays many roles including protection of ocular surface, production of tear film, and a conduit for drug clearance (depending on drug properties) into the systemic circulation or for drug transport to the deep tissues of the eye. The conjunctiva, which is a moderately tight epithelium, endowed with various transport processes for the homeostasis of ions, solutes, and water in the conjunctival surface and tear film. Modulation of ion transport in the conjunctiva leads to alterations in transconjunctival fluid flow that may become useful for treatment of dry-eye state in the eye. As a possible drug delivery route to the posterior portion of the eye, conjunctiva is an attractive route due to both larger surface area than that of cornea and expression of several key transport processes. Tear contains D-glucose and many amino acids, in addition to the usual ions in the body fluids. Several ion-coupled solute transport processes for absorption of amino acids, D-glucose, monocarboxylate, nucleosides, and dipeptides are expressed in the conjunctiva. Thanks to the rich endowment of these transport processes, drug transport across the conjunctiva into the intraocular tissues may become quite feasible. Subconjunctival injection of microparticles and matrix materials (which allows sustained release of drugs) is shown to maintain reasonable levels of various drugs in the vitreous, perhaps attesting to the fact that conjunctiva per se may contribute as a part of multiple transport barrier(s) in ocular drug delivery. In addition, several conjunctival approaches have been investigated to optimize treatment of dry-eye syndrome and intraocular diseases, and more can be accomplished in the coming years.

In vitro corneal permeation of unoprostone isopropyl (UI) and its metabolism in the isolated pig eye

The corneal permeability, hydrolysis, and metabolism of unoprostone isopropyl (UI), a docosanoid, were examined in isolated porcine ocular tissues. The apparent permeability coefficient (Papp) of the esterified prodrug and of the acid metabolite were determined in a modified Valia-Chien permeation chamber and quantified by high performance liquid chromatography. Enzymatic hydrolysis and subsequent metabolism were examined in isolated tissue homogenates. The prodrug (ester form) was found to permeate the isolated intact porcine cornea with a Papp of 9.47 x 10(-7) cm/sec. Only the acid metabolite could be detected in the receiver chamber, indicating the requirement of hydrolysis for permeation. The acid metabolite could not permeate the intact cornea but was able to cross an epithelium-denuded cornea with a Papp of 1.22 x 10(-6) cm/sec. Enzymatic hydrolysis of UI was confined to the isolated intact cornea and epithelium, indicating that the esterase activity was localized in the corneal epithelium. Incubations with different porcine ocular tissues, conjunctiva, iris-ciliary body, trabeculum, as well as aqueous humor, did not reveal other metabolites. These findings demonstrate that the ocular penetration of UI is dependent on its uptake into the epithelium and subsequent hydrolysis prior to its penetration into the anterior chamber, a very common pathway for ophthalmic drugs. In the pig eye, unoprostone does not appear to be further metabolized.

Ocular drug delivery in veterinary medicine

This paper provides a comprehensive overview of the various approaches currently used in the development of ocular drug delivery systems for the treatment of ocular diseases in animals. It is obvious from the literature that most of the products that are currently available are derived from human medicine without consideration given to the differences which exist between the anatomy and physiology of the eye of various animal species which ultimately affect product design and performance. As a result, many of the products for animal use seem in many circumstances inappropriate for animal care. The article deals with some aspects of eye anatomy and physiology of different animals, and then provides an overview of the most commonly encountered pathologies. The paper then discusses the currently available drug products and finally reviews new delivery concepts. Several hundred references are included in the paper and provide access to further information on the subject.

Comparison of enzymatic hydrolysis of pilocarpine prodrugs in human plasma, rabbit cornea, and butyrylcholinesterase solutions

Various bispilocarpic acid diesters (double prodrugs of pilocarpine) were synthesized, and their in vitro esterase catalyzed hydrolysis was evaluated in diluted human plasma, rabbit cornea homogenate, and specific butyrylcholinesterase solution. The structural changes greatly affected the rate of enzymatic hydrolysis of the prodrugs. Bispilocarpic acid with 2 cyclopropane substituents was the most stable derivative, whereas bispilocarpic acid with 2 cyclobutane substituents was the most labile derivative. The charged bispilocarpic acid diester hydrolyzed more slowly than the unchanged form. Comparison of the results obtained from different plasma and cornea homogenate batches is difficult because of the variety of the enzyme systems involved. This variety also makes comparing the results between different laboratories difficult.

Delivery of antiglaucoma drugs: ocular vs systemic absorption

In order to reduce the intraocular pressure antiglaucoma drugs must penetrate into the inner eye. Ocular bioavailability is determined by the ability of drug to penetrate through the cornea and conjunctiva/sclera, and on the other hand, by its elimination from the conjunctival sac. Major part of this elimination is by systemic drug absorption via conjunctiva. Typically conjunctival systemic absorption of drugs is an order of magnitude greater than their ocular absorption. In addition substantial systemic absorption of ophthalmic drugs takes place via nasal mucosa. Systemic absorption of antiglaucoma drugs like beta blocking agents may cause systemic side-effects. The risk of systemic side-effects might be decreased by increasing the ocular/systemic ratio of drug absorption. Several approaches can be used to improve ocular/systemic drug absorption ratio. Firstly, corneal drug permeability is improved. This can be done using different formulations or prodrug derivatives. Secondly, systemic absorption can be decreased e.g. with kinetic drug interactions or drug formulations. Thirdly, the rate of drug delivery can be changed thereby affecting especially the peak concentrations of drug in systemic circulation. Different methods for improvement of ocular delivery relative to the systemic absorption of antiglaucoma drugs are summarized and the impact of systemic pharmacokinetics on the viability of each approach is discussed.

Minimizing systemic absorption of topically administered ophthalmic drugs

Due to absorption several ocularly applied medications give rise to systemic side-effects. The problem of systemic drug absorption should be taken into account in designing ocular drug and dosage forms so that oculospecificity of the medications is optimized. In this review we summarize the current knowledge about the systemic absorption of ocularly applied topical drugs. Special emphasis is directed to the methods that can be used to minimize systemic absorption and increase the oculospecificity of drugs, e.g., reducing volume and increasing viscosity of eyedrops, controlling drug release from depot preparations, prodrug-derivatization, and addition of vasoconstrictive agents.

Preliminary evaluation of a series of amphiphilic timolol prodrugs: possible evidence for transscleral absorption

A series of amphiphilic esters of timolol malonate (octanoyl, decanoyl, dodecanoyl, myristoyl and palmitoyl timolol) were tested in rabbits for their capacity to antagonise the isoproterenol-induced ocular hypotension, using timolol maleate as reference standard. The most active prodrug, palmitoyl timolol malonate (PTM) was also evaluated for its capacity: (a) to decrease IOP in a model of bethamethasone-induced ocular hypertension, and (b) to permeate "in vitro" through rabbit corneal tissues. PTM, the prodrug with the longest aliphatic chain and therefore the greatest amphiphilic/lipophilic character, showed "in vivo" significant activity differences with respect to timolol maleate: the beta-antagonism was more important at earlier and later experimental times, and the IOP decrease was more marked at longer times. The prodrug, however, showed "in vitro" an inferior corneal permeability when compared with timolol maleate. The significant differences observed for the beta-antagonism of PTM at earlier times of the test might be attributed to transscleral absorption, due to the physicochemical characteristics of the prodrug, while the prolonged action (also observed in the IOP-depression test) might be due to sustained release, resulting from accumulation of the prodrug in the corneal epithelium. The present preliminary results are indicative of the potentiality of amphiphilic properties in a prodrug molecule.

Formulation influence on ocular and systemic absorption of topically applied atenolol in the pigmented rabbit

The objective of this study was to determine the relative efficacy of various formulations in maximizing the ratio of ocular to systemic absorption of topically applied atenolol following solution instillation in the pigmented rabbit. Formulations of various pH's and tonicities and containing various preservatives and polymers were tested. Ocular absorption was determined by monitoring atenolol concentrations in various anterior segment tissues at 30 min following solution instillation, while systemic absorption was determined by monitoring the time course of atenolol concentration over 480 min. Reversed phase HPLC was the analytical methodology. All formulations except those containing 0.025% benzalkonium chloride or 0.5% EDTA showed similar drug concentration vs. time profiles in plasma, attaining a peak concentration of 30-50 ng/ml at about 100 min. For benzalkonium chloride and EDTA, there was an undesirable increase in systemic absorption, although ocular absorption was also increased. By contrast, lowering the solution tonicity to 80 mOsm/kg increased the ratio of aqueous humor to plasma peak concentrations 2 times and the ratio of iris-ciliary body to plasma peak concentrations 3 times. Incorporation of 3.75% poly(vinyl alcohol) into the formulation afforded yet a larger increase in the iris-ciliary body to plasma drug concentration ratio (52 times.) It may therefore be concluded that, for a hydrophilic drug like atenolol, formulation changes that increase membrane permeability and/or enhance noncorneal drug access may be more promising than those that increase drug residence in the conjunctival sac with respect to maximizing the ratio of ocular to systemic drug absorption.

Determination of physicochemical properties, stability in aqueous solutions and serum hydrolysis of pilocarpic acid diesters

New alkyl and aralkyl pilocarpic acid diesters, prodrugs of pilocarpine, were synthesized with the aim of improving the bioavailability of pilocarpine by increasing its corneal permeability. These esters were several orders of magnitude more lipophilic than pilocarpine as determined by their apparent partition coefficients between 1-octanol and phosphate buffer (pH 7.40) (log P). Good correlation between log P and HPLC capacity factors of the compounds was observed. All the compounds are stable in acidic aqueous solution; in serum, however, pilocarpic acid diesters are hydrolysed enzymatically to pilocarpic acid monoester, which undergoes spontaneous cyclization to active pilocarpine and inactive isopilocarpine. The half-lives of the diesters in serum varied from 6-232 min. In addition to the direct effects of the R2, R1 moiety had a remarkable effect on the rate of enzyme-catalysed hydrolysis taking place in moiety R2. The formed pilocarpine was analysed with a new HPLC method which allowed good resolution of pilocarpine, isopilocarpine, pilocarpic acid and isopilocarpic acid. Rates for pilocarpine formation were both determined by experiment and calculated using the STELLA simulation programme with known degradation rate constants of pilocarpic acid diesters and monoesters. Since the simulations were in good agreement with the experimental results, it is concluded that STELLA simulation programme is useful in predicting pilocarpine formation.