Delivery Of Peptide Drugs Research Articles

Thiolated Chitosans: Design and In Vivo Evaluation of a Mucoadhesive Buccal Peptide Drug Delivery System

Intravenous application of pituitary adenylate cyclase-activating polypeptide (PACAP) has been identified as a promising strategy for the treatment of type 2 diabetes. To generate a more applicable formulation, it was the aim of this study to develop a sustained buccal delivery system for this promising therapeutic peptide. 2-Iminothiolane was covalently bound to chitosan to improve the mucoadhesive and permeation-enhancing properties of chitosan used as drug carrier matrix. The resulting chitosan-4-thiobutylamidine conjugate was homogenized with the enzyme inhibitor and permeation mediator glutathione (gamma-Glu-Cys-Gly), Brij 35, and PACAP (formulation A). The mixture was lyophilized and compressed into flat-faced discs (18 mm in diameter). One formulation was additionally coated on one side with palm wax (formulation B). Tablets consisting of unmodified chitosan and PACAP (formulation C) or of unmodified chitosan, Brij 35, and PACAP (formulation D) served as controls. Bioavailability studies were performed in pigs by buccal administration of these test formulations. Blood samples were analyzed via an ELISA method. Formulations A and B led to an absolute bioavailability of 1%, whereas PACAP did not reach the systemic circulation when administered via formulations C and D. Moreover, in the case of formulations A and B, a continuously raised plasma level of the peptide drug being in the therapeutic range could be maintained over the whole period of application (6 h). Formulations A and B were removed by moderate force from the buccal mucosa after 6 h, whereas formulations C and D detached from the mucosa 4 h after application. The study reveals this novel mucoadhesive delivery system to be a promising approach for buccal delivery of PACAP.

The potential of cystine-knot microproteins as novel pharmacophoric scaffolds in oral peptide drug delivery

Within this study, the potential of three clinically relevant microproteins (SE–AG–AZ, SE–EM and SE–EP) with cystine-knot architecture as pharmacophoric scaffolds for oral peptide delivery was investigated. Cystine-knot microproteins (CKM) were analysed regarding their stability towards the most important gastrointestinal secreted and membrane bound proteases in physiological concentrations. In addition, their permeation behaviour through freshly excised rat intestinal mucosa as well as important parameters such as aggregation behaviour, stability in rat plasma and isoelectric point were evaluated and compared to the properties of the model peptide drugs bacitracin and insulin. Aggregation studies indicate that under physiological conditions between 25 and 70% of the CKMs occur as monomers, whereas the rest forms di- and trimers. Pepsin and elastase cause no or only minor degradation to CKMs, whereas trypsin and chymotrypsin degrade CKMs extensively. Removing the theoretical chymotrypsin cleavage site from a CKM, however, led to stabilization towards this protease. Two of the three evaluated CKMs are stable against membrane bound proteases. Papp values were determined to be 5.96 ± 0.98 × 10− 6 and 6.63 ± 0.47 × 10− 6 cm/s. In conclusion, this study indicates that CKM are promising novel pharmacophoric scaffolds for oral peptide delivery.

New absorption promoter for the buccal delivery: Preparation and characterization of lysalbinic acid

Drug delivery across the buccal mucosa is convenient and safe transport method. The efficiency of the buccal system of peptide delivery is, however, not yet satisfactory. To improve the buccal transport new absorption promoters should be developed to be sufficiently active and at the same time causing no side effects like irritation or unpleasant taste. We have found that lysalbinic acid, a product of the alkaline hydrolysis of egg albumin and a mild detergent, meets those requirements. The preparation and some physicochemical properties of lysalbinic acid are described. Hamster cheek pouch was used as a model for the penetration process studies lysalbinic acid was shown to increase significantly an oral mucosa permeability for α-interferon and insulin. So this substance of the natural origin can be applied as an absorption enhancer for the buccal delivery of peptide drugs.

Thiolated chitosan microparticles: A vehicle for nasal peptide drug delivery

The goal of this study was to develop a microparticulate delivery system based on a thiolated chitosan conjugate for the nasal application of peptides. Insulin was used as model peptide. For thiolation of chitosan 2-iminothiolane was covalently linked to chitosan. The resulting chitosan–TBA (chitosan-4-thiobutylamidine) conjugate featured 304.89 ± 63.45 μmol thiol groups per gram polymer. 6.5% of these thiol groups were oxidised. A mixture of the chitosan–TBA conjugate, insulin and the permeation mediator reduced glutathione were formulated to microparticles. Control microparticles comprised unmodified chitosan and insulin. As second control served mannitol–insulin microparticles. All microparticulate systems were prepared via the emulsification solvent evaporation technique. In 100 mM phosphate buffer pH 6.8 chitosan–TBA–insulin microparticles swelled 4.39 ± 0.52-fold in size, whereas chitosan based microparticles did not swell at all. Chitosan–TBA microparticles showed a controlled release of fluorescein isothiocyanate (FITC)-labelled insulin over 6 h. Nasal administered chitosan–TBA–insulin microparticles led to an absolute bioavailability of 7.24 ± 0.76% (means ± S.D.; n = 3) in conscious rats. In contrast, chitosan–insulin microparticles and mannitol–insulin microparticles exhibited an absolute bioavailability of 2.04 ± 1.33% and 1.04 ± 0.27%, respectively (means ± S.D.; n = 4). Because of these results microparticles comprising chitosan–TBA and reduced glutathione seem to represent a useful formulation for the nasal administration of peptides.

Sustained insulin release with biodegradation of chitosan gel beads prepared by copper ions

Chitosan (CS) gel beads were prepared with chelated copper (II) ions as a vehicle for the delivery of peptide and protein drugs. Insulin, which is a model of peptide and protein drugs, was scarcely released from the CS gel beads in vitro, presumably due to the nature of interactions occurring between insulin, CS and the copper (II) ions. The efficacy of insulin released from the CS gel beads was confirmed by implantation into diabetic mice. A consistent reduction in blood glucose level was observed in vivo due to insulin release as the CS gel beads were degraded. Control over insulin release was achieved by altering the properties of the CS. Thus, CS gel beads are promising as a biocompatible and biodegradable vehicle by which peptide and protein drugs can be delivered.

An enteric-coated dry emulsion formulation for oral insulin delivery

A novel oral dosage formulation of insulin consisting of a surfactant, a vegetable oil, and a pH-responsive polymer has been developed. First, a solid-in-oil (S/O) suspension containing a surfactant–insulin complex was prepared. Solid-in-oil-in-water (S/O/W) emulsions were obtained by homogenizing the S/O suspension and the aqueous solution of hydroxypropylmethylcellulose phthalate (HPMCP). A microparticulate solid emulsion formulation was successfully prepared from the S/O/W emulsions by extruding them to an acidic aqueous solution, followed by lyophilization. The insulin release from the resultant dry emulsion responded to the change in external environment simulated by gastrointestinal conditions, suggesting that the new enteric-coated dry emulsion formulation is potentially applicable for the oral delivery of peptide and protein drugs.

Comparison of monomeric and oligomeric transferrin as potential carrier in oral delivery of protein drugs

Oligomeric transferrin (Tf) was used to investigate the effect that cross-linking of transferrin receptors (TfR) has on intracellular trafficking of the Tf–TfR complex and to determine whether or not the Tf-oligomer would be a better carrier than monomeric Tf for the oral delivery of protein drugs. The intracellular retention and transcytosis of the Tf-oligomer was determined by performing pulse chase studies on enterocyte-like Caco-2 cells. The intracellular retention of the Tf-oligomer was 2-fold higher than that of monomeric Tf while there was no significant difference in transcytosis. However, in vivo studies in CF-1 mice showed that the plasma concentrations of Tf from the orally administered Tf-oligomer were approximately 2-, 3- and 60-fold higher than that of orally administered monomeric Tf at 24, 48 and 72 h post-administration, respectively. In addition, the retention of the Tf-oligomer in the intestine was higher than that of monomeric Tf, which was consistent with in vitro studies. Insulin (In), when conjugated to the Tf-oligomer (Agg-Tf–S–S–In), was more effective than monomeric Tf–In conjugate (Mono-Tf–S–S–In) in reducing blood glucose levels when orally administered to streptozotocin (STZ)-induced diabetic rats. Post-oral administration of Agg-Tf–In, a delayed onset and prolonged hypoglycemic effect was observed. These results demonstrate that the cross-linking of TfR induced by the binding of the oligomeric Tf alters the intracellular trafficking and increases the intracellular retention of Tf–TfR complexes in polarized Caco-2 cells. The alteration of TfR trafficking could conceivably have caused the increase of insulin transport across the intestinal barrier when Agg-Tf–S–S–In was administered orally to STZ-induced diabetic rats. The delayed onset and prolonged effect of Agg-Tf–S–S–In in hypoglycemia strongly suggests that the Tf-oligomer can act as a sustained release carrier in the oral delivery of protein and peptide drugs.

Cyclodextrins in drug delivery: An updated review

The purpose of this review is to discuss and summarize some of the interesting findings and applications of cyclodextrins (CDs) and their derivatives in different areas of drug delivery, particularly in protein and peptide drug delivery and gene delivery. The article highlights important CD applications in the design of various novel delivery systems like liposomes, microspheres, microcapsules, and nanoparticles. In addition to their well-known effects on drug solubility and dissolution, bioavailability, safety, and stability, their use as excipients in drug formulation are also discussed in this article. The article also focuses on various factors influencing inclusion complex formation because an understanding of the same is necessary for proper handling of these versatile materials. Some important considerations in selecting CDs in drug formulation such as their commercial availability, regulatory status, and patent status are also summarized. CDs, because of their continuing ability to find several novel applications in drug delivery, are expected to solve many problems associated with the delivery of different novel drugs through different delivery routes.

Emerging strategies for the transdermal delivery of peptide and protein drugs

Transdermal delivery has been at the forefront of research addressing the development of non-invasive methods for the systemic administration of peptide and protein therapeutics generated by the biotechnology revolution. Numerous approaches have been suggested for overcoming the skin’s formidable barrier function; whereas certain strategies simply act on the drug formulation or transiently increase the skin permeability, others are designed to bypass or even remove the outermost skin layer. This article reviews the technologies currently under investigation, ranging from those in their early-stage of development, such as laser-assisted delivery to others, where feasibility has already been demonstrated, such as microneedle systems, and finally more mature techniques that have already led to commercialisation (e.g., velocity-based technologies). The principles, mechanisms involved, potential applications, limitations and safety considerations are discussed for each approach, and the most advanced devices in each field are described.

Development of Neuropeptide Drugs that Cross the Blood-Brain Barrier

In recent years, there have been several important advancements in the development of neuropeptide therapeutics. Nevertheless, the targeting of peptide drugs to the CNS remains a formidable obstacle. Delivery of peptide drugs is limited by their poor bioavailability to the brain due to low metabolic stability, high clearance by the liver, and the presence of the blood brain barrier (BBB). Multiple strategies have been devised in an attempt to improve peptide drug delivery to the brain, with variable results. In this review, we discuss several of the strategies that have been used to improve both bioavailability and BBB transport, with an emphasis on antibody based vector delivery, useful for large peptides/small proteins, and glycosylation, useful for small peptides. Further development of these delivery methods may finally enable peptide drugs to be useful for the treatment of neurological disease states.

Oral Delivery of Peptide Drugs

A wide variety of peptide drugs are now produced on a commercial scale as a result of advances in the biotechnology field. Most of these therapeutic peptides are still administered by the parenteral route because of insufficient absorption from the gastrointestinal tract. Peptide drugs are usually indicated for chronic conditions, and the use of injections on a daily basis during long-term treatment has obvious drawbacks. In contrast to this inconvenient and potentially problematic method of drug administration, the oral route offers the advantages of self-administration with a high degree of patient acceptability and compliance. The main reasons for the low oral bioavailability of peptide drugs are pre-systemic enzymatic degradation and poor penetration of the intestinal mucosa. A considerable amount of research has focused on overcoming the challenges presented by these intestinal absorption barriers to provide effective oral delivery of peptide and protein drugs. Attempts to improve the oral bioavailability of peptide drugs have ranged from changing the physicochemical properties of peptide molecules to the inclusion of functional excipients in specially adapted drug delivery systems. However, the progress in developing an effective peptide delivery system has been hampered by factors such as the inherent toxicities of absorption-enhancing excipients, variation in absorption between individuals, and potentially high manufacturing costs. This review focuses on the intestinal barriers that compromise the systemic absorption of intact peptide and protein molecules and on the advanced technologies that have been developed to overcome the barriers to peptide drug absorption.

Buccal Delivery Systems for Peptides

This review aims to illustrate the potential of the buccal route for delivery of peptide drugs. Compared with the oral, nasal, and rectal routes for drug delivery, the buccal route presents advantages such as an efficient blood supply and relatively low enzymatic activity. Moreover, the buccal mucosa is easily accessible and acceptable to patients; it allows the patient to interrupt drug administration by simply removing the drug delivery system. On the other hand, the buccal route is characterized by some intrinsic limitations (barrier properties of the mucosa, small area available for drug absorption, short residence time of the formulation caused by physiologic-removal mechanisms), which have to be considered in the design of buccal drug delivery systems. After a description of the relevant anatomic and physiologic features of the buccal mucosa and of the factors affecting mucosal absorption, strategies adopted to promote the buccal absorption of peptides are discussed. In particular, recent developments in formulation design are presented. Attention is focused on the employment of penetration enhancers, enzyme inhibitors, and mucoadhesive agents as functional excipients in buccal formulations. Currently, research is oriented towards the design of new materials combining all of these latter properties. In the last few years some chitosan derivatives have been proven to fulfil such requirements. Different formulations have been studied: gels, tablets, and films. Recently a novel liquid aerosol formulation of insulin has been developed and it is now in clinical phase II trials.

Feasibility study on the retention of superporous hydrogel composite polymer in the intestinal tract of man using scintigraphy

In recent years, many complex oral drug delivery systems have been developed using various polymers in order to achieve better drug targeting and drug absorption in the intestinal tract. Superporous hydrogel (SPH) and SPH composite (SPHC)-based drug delivery systems were also developed for the targeted delivery of peptide drugs into the intestinal tract. In the present study, the retention time of SPHC polymer is studied in man using the scintigraphy technique. To that purpose, SPHC polymers were radiolabelled with Tc-99m and administered orally in an enteric-coated gelatin capsule. The location of the radiolabelled polymer was monitored in five healthy volunteers while the subjects were sitting in front of a large field of view gamma camera. The results showed that enteric-coated gelatin capsules remained in the stomach for 75 to 150 min after oral administration to fasted volunteers and that the SPHC polymers thereafter attached to the upper part of the small intestine for at least 45 to 60 min due to their mechanical fixation properties. No discomfort was observed in any of the volunteers after oral administration of these polymers, which indicates that they are safe to be applied for oral drug delivery systems in man.

Correlation of tetradecylmaltoside induced increases in nasal peptide drug delivery with morphological changes in nasal epithelial cells

The effect of tetradecylmaltoside (TDM) on nasal peptide drug absorption was assessed with four peptides of distinctmolecular size: insulin (5.7kDa), leptin (16kDa), somatropin (22.1kDa), and epoetin alfa (30.4kDa). The nasal uptake of the smallest peptides, insulin and leptin, was significantly increased at a TDM concentration of only 0.06%. The uptake of somatropin was significantly increased when concentrations of 0.125% or more were used. The uptake of the largest peptide, epoetin alfa, was not significantly increased, in the presence of 0.125−0.5% TDM. Light microscopy revealed that formulations containing 0.125% TDM caused moderate alterations in nasal epithelial cell morphology, while higher concentrations of TDM (0.5%), caused more extensive morphological changes. Following treatment with 0.125% TDM, the distribution of cilia was altered and the number of pinocytotic vesicles was increased, at a time that correlated with increased nasal absorption of insulin. Consistent with these findings, FITC-insulin applied nasally in the absence of TDM did not enter nasal epithelial cells, whereas FITC-insulin co-administered with 0.125% TDM was internalized into the cells, with a uniform distribution, consistent with transcellular movement of the peptide through the cells.

Chemistry of polymer biodegradation and implications on parenteral drug delivery

Most polymeric implants are biodegraded by one of two common chemical degradation mechanisms: (i) hydrolysis and (ii) oxidation. The chemical structure is among the most important factors which affect the biodegradation of polymeric implants. Hydrolytic biodegradations are often accompanied by substantial decrease of pH, whilst oxidative biodegradation processes are usually very slow due to consumption of stoichiometric amounts of oxidising agents. A dramatic acceleration of the biodegradation can be expected, if the biodegradation can be initiated by catalytic amounts of oxidation agents. Poly(ethylene carbonate) (PEC) and poly(trimethylene carbonate) (PTMC) are presumably biodegraded by such catalytic oxidation processes. Their biodegradation shows all the characteristics of surface erosion. Poly(ethylene carbonate) is utilised as a surface eroding biocompatible polymer for controlled delivery of peptide and protein drugs.

Trimethylated chitosan as polymeric absorption enhancer for improved peroral delivery of peptide drugs

The absorption enhancing effects of chitosan and its derivatives have been intensively studied in recent years. It has been shown that these compounds are potent absorption enhancers. Chitosan is only soluble in acidic environments and is therefore incapable of enhancing absorption in the small intestine, the main absorption area in the gastrointestinal tract. Special emphasis has been placed on the absorption enhancing properties of N -trimethyl chitosan chloride (TMC), a partially quaternised derivative of chitosan, due to its solubility in neutral and basic environments. TMC is prepared by the reductive methylation of chitosan. The degree of quaternisation can be altered by increasing the number of reaction steps or by increasing the reaction time. Although the molecular weight of the polymer increases with addition of the methyl groups, a net decrease in the molecular weight is observed due to a decrease in the chain length of the polymer. TMC, like chitosan, possesses mucoadhesive properties. In vitro studies performed on Caco-2 cell monolayers showed a pronounced reduction in the transepithelial electrical resistance (TEER). TMC is also able to increase the permeation of hydrophilic compounds such as [ 14 C]-mannitol and [ 14 C] polyethylene glycol 4000 ([ 14 C] PEG 4000, MW4000) across the cell monolayers. It was also shown that the degree of quaternisation of the polymer plays an important role on its absorption enhancing properties, especially in neutral environments where chitosan is ineffective as an absorption enhancer. The reduction in TEER is an indication of the opening of the tight junctions located between epithelial cells. Opening of the tight junctions will result in enhancement of absorption via the paracellular route. Confocal laser scanning microscopy confirmed transport of large hydrophilic compounds via the paracellular route as well as the mechanism of action of the polymer in which redistribution of the cytoskeletal F-actin is provoked, which leads to the opening of the tight junctions. Various in vivo studies in different animal models confirmed the ability of TMC to increase the absorption of the peptide drugs buserelin and octreotide after intraduodenal or -jejunal administration. However, TMC has always been administered as a solution in these studies. The impracticality of administering a solution, as well as the fact that most peptides are unstable in the presence of water, have led to the need for a solid oral dosage form with which TMC can be administered together with peptide drugs. Recent studies have focused on the development and in vivo evaluation of solid oral dosage forms.

Chemistry of polymer biodegradation and implications on parenteral drug delivery

Most polymeric implants are biodegraded by one of two common chemical degradation mechanisms: (i) hydrolysis and (ii) oxidation. The chemical structure is among the most important factors which affect the biodegradation of polymeric implants. Hydrolytic biodegradations are often accompanied by substantial decrease of pH, whilst oxidative biodegradation processes are usually very slow due to consumption of stoichiometric amounts of oxidising agents. A dramatic acceleration of the biodegradation can be expected, if the biodegradation can be initiated by catalytic amounts of oxidation agents. Poly(ethylene carbonate) (PEC) and poly(trimethylene carbonate) (PTMC) are presumably biodegraded by such catalytic oxidation processes. Their biodegradation shows all the characteristics of surface erosion. Poly(ethylene carbonate) is utilised as a surface eroding biocompatible polymer for controlled delivery of peptide and protein drugs.

Microencapsulation of water-soluble macromolecules with acrylic terpolymers by the Wurster coating process for colon-specific drug delivery

The aim of this work is to explore a microcapsule formulation applicable for a peroral colon-specific delivery of water-soluble macromolecules such as peptide drugs, based on our previous delayed-release formulations using self-made colloidal terpolymers of ethyl acrylate, methyl methacrylate and 2-hydroxyethyl methacrylate (poly(EA/MMA/HEMA)). Peptides are thermally unstable and less permeable through a polymeric membrane in general. Thus, the present work was intended to establish the acrylic terpolymers having the following characteristics: an excellent film-formability even at a low process-temperature of 40 °C to preserve the stability of heat-sensitive drugs and a desired permeability allowing a delayed-release profile of macromolecular drugs. Using various types of poly(EA/MMA/HEMA) synthesized with different monomer molar ratios, microencapsulation of the particles consisting of a lactose core and a surface-layer of water-soluble fluorescence-labeled dextran (FITC-Dex, Mw 9500), a model of macromolecular solute, was carried out by the Wurster process. The release profiles of FITC-Dex were highly dependent on the terpolymer compositions. The increased EA content made the poly(EA/MMA/HEMA) films flexible and less permeable, leading to significant decrease in release rate of FITC-Dex. Contrary, it was found that the increase in HEMA content enhanced the FITC-Dex release due to the enhanced hydration of the films, as expected from its higher hydrophilicity than the other monomers. Among a series of poly(EA/MMA/HEMA) prepared here, the poly(EA/MMA/HEMA) with the molar ratio of 95:85:40 exhibited a good film-formability in the post-thermal curing process at 40 °C. The established coating films allowed delayed-release of the microencapsulated FITC-Dex; the profile was characterized by a lag time and subsequent rapid release. This delayed-release was probably due to time-dependent change of the semi-permeable nature of the hydrated coating films as a consequence of anomalous film-deformation arising from the osmotic pressure inside the microcapsules. Moreover, the release profiles of FITC-Dex were found to be pH-independent. Based on our findings, the use of poly(EA/MMA/HEMA) with molar ratio of 95:85:40 could be proposed as a feasible way to prepare a delayed-release microcapsule containing water-soluble macromolecules by the Wurster coating process, toward the future application to colon-specific delivery of peptide drugs.

Transport of Octreotide and Evaluation of Mechanism of Opening the Paracellular Tight Junctions Using Superporous Hydrogel Polymers In Caco-2 Cell Monolayers

The purpose of this study was to investigate the mechanism of opening of tight junctions in Caco-2 cell monolayers using superporous hydrogel (SPH) and SPH composite (SPHC) polymers as permeation enhancers for peptide drug delivery. Moreover, the transport of octreotide across Caco-2 cell monolayers was assessed by application of SPH and SPHC polymers on Caco-2 cell monolayers. In these experiments, N,N,N-trimethyl chitosan chloride with 60% quaternization (TMC60) was used as a positive control for opening of tight junctions. Transepithelial electrical resistance (TEER) studies showed that all three polymers (TMC60, SPH, and SPHC) were able to decrease TEER values to ∼30% of the initial values, indicating the ability of these polymers to open the tight junctions. Recovery TEER studies showed that the effects of the polymers on Caco-2 cell monolayers were reversible, indicating viability of the cells after incubation with polymers. Both SPH and SPHC (compared with TMC60) were able to increase the paracellular transport of octreotide by their mechanical pressures on tight junctions. The mechanistic studies showed that junctional proteins, including actin, occludin, and claudin-1, were influenced by application of SPH and SPHC polymers to the Caco-2 cell monolayers. SPH and SPHC induced clear changes in the staining pattern of all three proteins compared with the control, indicating that the expression of these proteins in the tight junctions was increased, most likely due to the mechanical pressure of the polymers on the junctional proteins. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association

PLGA microspheres for the ocular delivery of a peptide drug, vancomycin using emulsification/spray-drying as the preparation method: in vitro/in vivo studies

The aim of this study was an in vitro/in vivo investigation on poly(lactide-co-glycolide) (PLGA) microspheres as carriers for the topical ocular delivery of a peptide drug vancomycin (VA). The microspheres were prepared by an emulsification/spray-drying technique that can be proposed as an alternative to the double emulsion method for preparation of peptide-loaded microparticles. The drug encapsulation efficiencies were close to the theoretical values (84.2–99.5%); the average particle size, expressed as d vs, was about 11 μm. The microspheres were able to modulate the in vitro drug release of VA with a behavior dependent on their composition: the highest drug content corresponded to the highest release rate. In vivo studies were carried out by assessing the pharmacokinetic profile of VA in the aqueous humor of rabbits after topical administration of aqueous suspensions of microspheres. High and prolonged VA concentrations and increased AUC values (2-fold) with respect to an aqueous solution of the drug were observed. Increasing the viscosity of the microsphere suspension by addition of a suspending-viscosizing agent (hydroxypropylcellulose) did not produce an increase of the ocular bioavailability. PLGA microspheres can be proposed as a system for ocular delivery of peptide drugs.