Mechanochemical Solid-State Polymerization (XI): Effect of Water-Insoluble Pharmaceutical Aids on Drug Release from Mechanically Synthesized Polymeric Prodrugs

We discuss here the effect of water-insoluble pharmaceutical aids on the nature of drug release from composite polymeric prodrugs synthesized by mechanochemical solid-state polymerization. Magnesium stearate (Mgst) and hydrogen castor oil (HCO) were used as water-insoluble pharmaceutical aids. Composite polymeric prodrugs were synthesized by the mechanochemical solid-state polymerization of a vinyl monomer of 5-fluorouracil (I) in the presence of Mgst or HCO. The molecular weight of the resulting polymeric prodrugs increased with increasing the content of Mgst or HCO. Prodrug hydrolysis was carried out in a heterogeneous system in phosphate buffer at pH 6.8 and 37 degrees C. The rate of drug release from the composite polymeric prodrug containing Mgst (Poly-Mgst) was faster than that from polymeric prodrug containing no pharmaceutical aids (Poly-Non), while hydrolysis of the composite polymeric prodrug containing HCO (Poly-HCO) was slower than Poly-Non. Scanning electron microscope (SEM) photos showed the surface of Poly-HCO was smoother than that of Poly-Non and Poly-Mgst. It was suggested that the slower drug release from Poly-HCO may be responsible for the smaller specific surface area than that of Poly-Non. It was also shown that the rate of drug release from the composite polymeric prodrugs decreases with increasing the content of Mgst or HCO. Hence, novel composite polymeric prodrugs with a variety of drug release rates can be prepared by mechanochemical solid-state polymerization in a totally dry process.

Solubility of guaifenesin in the presence of common pharmaceutical additives

The aqueous solubility of guaifenesin, a highly water-soluble drug, in the presence of salts, sugars, and cosolvents was determined at 25 degrees C and 40 degrees C. The solubility of drug at both temperatures was reduced with increasing concentrations of salts and sugars. The extent of reduction in drug solubility was dependent on the type of salts and sugars used. The salting-out coefficient of additives was calculated by plotting log-linear solubility profiles of the drug against the concentrations of the additives. The solubility of guaifenesin, a neutral compound, was found to be higher at lower pH values, which could be due to hydrogen-bonding effects. At 25 degrees C, glycerin, PEG 300, and propylene glycol increased the solubility of drug at low solvent concentrations while the solubility was reduced at high concentrations. At 40 degrees C, the solubility of drug was reduced at all concentrations of cosolvents. The thermodynamic events accompanying the solubility process were discussed to explain the solubility phenomena observed in the presence of additives. The reduced aqueous solubility of guaifenesin in the presence of additives greatly improved the entrapment of drug into controlled-release wax microspheres.

Investigations on the drug releasing mechanism from an asymmetric membrane-coated capsule with an in situ formed delivery orifice

Asymmetric membrane-coated capsules with in situ formation of a delivery orifice were examined for their improved osmotic effects. The release mechanisms were investigated for drugs with both moderate to high water solubility and those with poor water solubility. The capsule wall membrane was produced by a phase-inversion process, in which an asymmetric membrane was formed on stainless steel mold pins by dipping the mold pins into a coating solution containing a polymeric material followed by dipping into a quenching solution. In situ formation of a delivery orifice in the thin membrane was proven by visualization of a jet stream of chlorophyll being released from the capsule. The release mechanism for drugs with moderate to high water solubility was mainly controlled by the osmotic effect, which is a function of the drug's solubility. Permeability across the asymmetric membrane of the capsule was determined to be 4.28 x 10(-6) cm(2)/h-atm at 37 degrees C for drugs with water solubilities in a moderate to high range. Accordingly, the poorly water-soluble drug, nifedipine, was unable to create enough of an osmotic effect to activate drug release. Solubilization either by the addition of the solubility enhancer, SLS, or by a solid dispersion with HPMC could increase the solubility of nifedipine to a sufficient extent to activate drug release. It was found that the suspending ability induced by the viscous nature of HPMC further interacted with SLS to synergistically increase the maximal percent release and the release rate of nifedipine. The osmotic effect of this suspension ability was proposed as the underlying mechanism responsible for the release of poorly water-soluble drugs, i.e. nifedipine, from this system.

Effect of excipients on the stability and transport of recombinant human epidermal growth factor (rhEGF) across Caco-2 cell monolayers

The effect of sixteen excipients on the transport of recombinant human epidermal growth factor (rhEGF) across Caco-2 cell monolayers was examined at 37 degrees C. The apparent apical to basolateral (A-B) permeability (Papp) of 30 microM rhEGF was 8.15 x 10(-7) cm/sec, indicative of a poor level of absorption in the GI tract. The Papp was 1.7- and 6.3-fold greater than the Papp in the basolateral to apical (B-A) direction and the A-B permeability of mannitol, respectively, and decreased dramatically to a negligible level at 4 degrees C, consistent with a receptor mediated transcytosis of rhEGF. The stability of rhEGF was very poor, undergoing more than 85% degradation in 2 h in the transport medium at 37 degrees C. A significant increase in the Papp could be achieved by the addition of certain excipients, as exemplified by 23, 21, 20 and 16-fold increases, in the presence of sodium taurochenodeoxycholate (NaTCDC), sodium taurodeoxycholate (NaTDC), sodium glycodeoxycholate (NaGDC) and sodium laurylsulfate (SLS) (all at a concentration of 1% w/v), respectively. A significant increase in stability could also be achieved by the addition of some of the excipients, as represented by 1% SLS, which nearly completely stabilized the rhEGF. Unfortunately, however, an increase in the Papp of rhEGF could not be achieved without a simultaneous and extensive decrease in the integrity of the cell membranes. Thus, more efficient excipients, that specifically enhance the permeation of rhEGF and do not alter the membrane integrity, should be pursued in order to safely enhance the permeation of rhEGF.

Effect of additives on the crystallization and the permeation of ketoprofen from adhesive matrix

The crystallization of drug in a matrix may significantly affect the efficacy and quality of the transdermal drug delivery system. Therefore, the control of drug crystallization is of particular interest in the development of efficient transdermal delivery systems. In this study, we investigated the effects of various additives on the crystallization of ketoprofen in polyisobutylene (PIB) adhesive matrix. The effects of various additives on the permeation of ketoprofen from PIB matrix across hairless mouse skin were also examined. Poly(vinyl pyrrolidone) (PVP) K-30 was found to be the most effective crystallization inhibitor. Also, Poloxamer, Tween 80 and Labrasol significantly inhibited the crystallization of ketoprofen in a PIB matrix. In case of Tween 80, Labrasol, and PVP K-30, the flux of ketoprofen decreased as the loading content of the additives increased. However, the addition of Tween 80, Labrasol, or PVP K-30 significantly reduced the decrease in the flux of ketoprofen within the PIB matrix during a storage time of 3 weeks.

An investigation of pulsatile release tablets with ethylcellulose and Eudragit L as film coating materials and cross-linked polyvinylpyrrolidone in the core tablets

To develop new pulsatile release tablets, which can suppress drug release in stomach and release the drug rapidly after a predetermined lag time of about 3 h in intestine, the use of tablets with ethylcellulose/Eudragit L as a coating film and cross-linked polyvinylpyrrolidone in the core tablets was investigated. The release of diltiazem hydrochloride (DIL) as a model drug in the core tablets was investigated in vitro. The lag time (t10) was prolonged with an increase of the coating level, whereas the drug release rate was almost constant, irrespective of the coating level. The water-uptake study and electron microscope photographs suggested the mechanism of pulsatile release of drug. Pulsatile release tablets containing 60 mg DIL with 4.4 h of lag time (t10) in vitro were administrated to eight volunteers. The mean plasma concentration curves showed 4.9 h of lag time (tlag), 8.0 h of time to maximum concentration (tmax) and 3.1 h of time between tmax and tlag (t(psi)) in vivo. Relative bioavailability was 1.05 for pulsatile release tablets compared to conventional tablets.

Evaluation of the functional equivalence of crospovidone NF from different sources. II. Standard performance test

Current NF monographs do not provide tests that reflect on the functionality of Crospovidone NF from multiple sources. Physical characterization studies such as particle size and distribution, surface area, porosity, and surface morphology revealed major differences among the crospovidones from different sources (Shah, U.; Augsburger, L.L. J. Pharm. Dev. Technol. 2001, 6 (1), 39-51). Differences in disintegration and dissolution were also observed for a model drug in an insoluble filler system (see Shah and Augsburger, 2001). The objective of this study was to determine the relationship between physical differences observed and disintegrant functionality and to develop standard performance test. Tests performed included settling volume studies, measurement of initial rate as well as extent of liquid uptake of the loose disintegrant powder, and simultaneous measurement of the axial and radial disintegrating forces along with the rate and extent of liquid uptake of the pure disintegrant compacts. Significant differences among the crospovidones were observed for all tests performed. Settling volume, liquid uptake, and disintegration force are recommended as standard performance tests to determine differences among crospovidones from different sources.

Statistical optimization of gastric floating system for oral controlled delivery of calcium

The development of an optimized gastric floating drug delivery system is described. Statistical experimental design and data analysis using response surface methodology is also illustrated. A central, composite Box-Wilson design for the controlled release of calcium was used with 3 formulation variables: X1 (hydroxypropyl methylcellulose [HPMC] loading), X2 (citric acid loading), and X3 (magnesium stearate loading). Twenty formulations were prepared, and dissolution studies and floating kinetics were performed on these formulations. The dissolution data obtained were then fitted to the Power Law, and floating profiles were analyzed. Diffusion exponents obtained by Power Law were used as targeted response variables, and the constraints were placed on other response variables. All 3 formulation variables were found to be significant for the release properties (P <.05), while only HPMC loading was found to be significant for floating properties. Optimization of the formulations was achieved by applying the constrained optimization. The optimized formulation delivered calcium at the release rate of 40 mg/hr, with predicted n and T50% values at 0.93 and 3.29 hours, respectively. Experimentally, calcium was observed to release from the optimized formulation with n and T50% values of 0.89 (+/- 0.10) and 3.20 (+/- 0.21) hours, which showed an excellent agreement. The quadratic mathematical model developed could be used to further predict formulations with desirable release and floating properties.

Antitumor activity of tumor necrosis factor-alpha conjugated with polyvinylpyrrolidone on solid tumors in mice

We attempted the development of a novel polymer conjugation to further improve the therapeutic potency of antitumor cytokines compared with PEGylation for clinical application. Compared with native tumor necrosis factor (TNF)-alpha in vitro, specific bioactivities of polyvinyl-pyrrolidone (PVP)-modified TNF-alphas (PVP-TNF-alphas) were decreased by increasing the degree of PVP attachment. PVP-TNF-alpha fraction 3, Mr 101,000, had the most effective antitumor activity of the various PVP-TNF-alphas in vivo. PVP-TNF-alpha fraction 3 had >200-fold higher antitumor effect than native TNF-alpha, and the antitumor activity of PVP-TNF-alpha fraction 3 was >2-fold higher than that of MPEG-TNF-alpha (Mr 108,000), which had the highest antitumor activity among the polyethylene glycol (PEG)-conjugated TNF-alphas. Additionally, a high dose of native TNF-alpha induced toxic side effects such as body weight reduction, piloerection. and tissue inflammation, whereas no side effects were observed after i.v. administration of PVP-TNF-alpha fraction 3. The plasma half-life of PVP-TNF-alpha fraction 3 (360 min) was about 80- and 3-fold longer than those of native TNF-alpha (4.6 mm) and MPEG-TNF-alpha (122 min), respectively. The mechanism of increased antitumor effect in vivo caused the prolongation of plasma half-life and increase in stability. These results suggested that PVP is a useful polymeric modifier for bioconjugation of TNF-alpha to increase its antitumor potency, and multifunctionally bioconjugated TNF-alpha may be a potentiated antitumor agent for clinical use.

Iodine absorption after intraoperative bowel irrigation with povidone-iodine

PURPOSE

Povidone-iodine is a commonly used intrarectal tumoricidal agent in patients undergoing colorectal surgery. The aim of this study was to assess systemic absorption of total iodine and its effect on thyroid function after intrarectal application.

METHODS

Twenty patients with carcinoma of the rectum received intraoperative irrigation with either povidone-iodine (Group A; n = 10) or physiologic saline (Group B; n = 10). Ten patients with carcinoma of the sigmoid colon (group C) were treated the same as Group A. Electrolyte, total iodine, triiodothyronine, thyroxine, and thyroid-stimulating hormone values were measured in serum preoperatively and before intraoperative irrigation and immediately, ten minutes, 1 hour, 6 hours, 24 hours, and two weeks after irrigation.

RESULTS

No significant changes occurred in serum electrolytes. A significant uptake of the total iodine was demonstrated in each group. Total iodine levels examined immediately, ten minutes, and one hour after irrigation in Group C were significantly higher than those examined in Group B. Maximum values were obtained one hour after irrigation in Groups A and B and six hours after irrigation in Group C. No significant changes occurred in triiodothyronine, thyroxine, and thyroid-stimulating hormone levels among the three groups. The decrease in triiodothyronine levels after surgery was demonstrated in each group. We noted a decrease after surgery in thyroxine levels for Groups A and B and in thyroid-stimulating hormone levels for Group B. Those hormones were not affected by the administration of povidone-iodine.

CONCLUSION

High serum levels of iodine did not cause organ toxicity, suggesting that a single use of intraoperative bowel irrigation with povidone-iodine may be performed with practically negligible risk.

Drug release from an ensemble of swellable crosslinked polymer particles

This paper presents a new model suitable to describe the drug release from drug delivery systems constituted by an ensemble of drug loaded crosslinked polymer particles. The model accounts for the main factors affecting the drug release such as the particle size distribution, the physical state and the concentration profile of the drug inside the polymeric particles, the viscoelastic properties of the polymer-penetrant system and the dissolution-diffusion properties of the loaded drug. In order to check the validity of the model, release experiments were performed by using crosslinked polyvinyl-pyrrolidone (PVP) particles and two different model drugs, MAP (medroxyprogesterone acetate) and TEM (Temazepam). MAP and TEM were chosen because of their completely different dissolution behaviours in water. In particular, TEM undergoes a phase transition to the crystalline state upon dissolution when it is loaded in the polymeric network in the amorphous state. The comparison with the experimental results confirms that the most important factors determining the drug release kinetics can be properly accounted for.

Biodistribution of fluoresceinated dextran using novel nanoparticles evading reticuloendothelial system

The rapid clearance of circulating nanoparticles from the blood stream coupled with their high uptake by liver and spleen has thus far been overcome by reducing the particle size, and by making the particle surface hydrophilic with poloxamers and poloxamines. We have prepared hydrogel nanoparticles of polyvinylpyrrolidone of a size less than 100 nm diameter with precise size distribution. Since the inner cores of these particles are also hydrophilic, these particles are capable of encapsulating water-soluble compounds. Biodistribution of these particles shows practically negligible (<1%) uptake by the macrophages in liver and spleen, and approximately 5-10% of these particles remain in circulation even 8 h after i.v. injection. Increasing the surface hydrophobicity as well as particle size can increase the RES uptake of these particles. Because of longer residence in blood, the hydrogel nanoparticles have potential therapeutic applications particularly in cancer: the water-soluble cytotoxic agents encapsulated in these particles can be targeted to tumors while minimizing the likelihood of toxicity to reticuloendothelial system (RES).

Drug/cyclodextrin/hydroxy acid multicomponent systems. Properties and pharmaceutical applications

The objective of this mini-review is to summarize the findings concerning the properties and the pharmaceutical applications of multicomponent complexes made of a sparingly water-soluble amino-type drug, a cyclodextrin, and a hydroxy carboxylic acid. Simultaneous complexation and salt formation with these acids significantly increase the solubilizing power, allowing us to reduce the amount of cyclodextrin necessary for making the targeted formulation. In many cases, the aqueous solubility of the hydrophobic drug can be enhanced by several orders of magnitude, while that of CD can be enhanced more than 10-fold. The mechanism through which these complexes elicit their synergetic effects on the drug solubility is also discussed. Finally, some general observations are made concerning the structural requirements of the drug necessary for exploiting the aforementioned effect.

Bioconjugation of laminin-related peptide YIGSR with polyvinyl pyrrolidone increases its antimetastatic effect due to a longer plasma half-life

Polyvinyl pyrrolidone (PVP) which can be radically synthesized and have a long blood residency was used to modify the laminin-related peptide YIGSR, and its inhibitory effect on experimental lung metastasis of B16-BL6 melanoma cells was examined. The antimetastatic effect of PVP-conjugated YIGSR (PVP-YIGSR) was more than 100-fold greater than that of native YIGSR. When injected intravenously, PVP-YIGSR showed more than a 15-fold longer plasma half-life relative to native YIGSR. In addition, the stability of YIGSR in plasma was increased by conjugation with PVP. These findings suggest that PVP is a useful polymeric modifier for increasing the antimetastatic activity of YIGSR.

A genetically modified recombinant tumor necrosis factor-alpha conjugated to the distal terminals of liposomal surface grafted polyethyleneglycol chains

A genetically modified recombinant tumor necrosis factor (TNF)-alpha (rKRKTNF) was conjugated to the terminal carboxyl groups of liposome grafted polyethyleneglycol (PEG) chains. The long-circulating liposomes were composed of egg phosphatidylcholine, cholesterol (chol) and 7% carboxyl PEG-phosphatidylethanolamine. The conjugation efficiency of the genetically modified rKRKTNF under the conditions described in the text was approximately 55%. The biological activity of liposomal rKRKTNF, as tested with an in vitro cytotoxicity assay was reduced compared to the free, unconjugated rKRKTNF. In vivo biodistribution studies showed that conjugation of as little as 0. 13% of the grafted PEG chains resulted in a rapid elimination of the formulation from the blood stream. It is speculated that both non-selective conjugate chemistry and inherent recognition of the TNF by the components of the reticuloendothelial system (RES) are responsible for the short blood half life of the rKRKTNF-PEG-liposomes. The result suggest that conjugating a rapidly clearing recombinant cytokine to long-circulating liposomes provides little advantage in modifying the pharmacokinetic parameters of the cytokine.

Evaluation of transdermal iontophoresis of enoxacin from polymer formulations: in vitro skin permeation and in vivo microdialysis using Wistar rat as an animal model

Polymers were used in vehicles to form hydrogel matrices in this study to evaluate the in vitro permeation and in vivo microdialysis of enoxacin. The highest transdermal delivery determined by area under flux-time curve (AUC) and intracutaneous enoxacin concentration were observed in methylcellulose (MC) and polyvinylpyrrolidone (PVP) hydrogels, respectively. To avoid the pH shift in vehicles during iontophoresis, buffer species were added to formulations to increase the buffer capacity. As expected, the permeability of enoxacin of anodal iontophoresis was larger than that of cathodal iontophoresis. Combination of benzalkonium chloride, a cationic surfactant as an enhancer, and iontophoresis exerted an enhancing effect for anionic enoxacin at pH 10.0. However, no effect or a negative effect was detected for cationic enoxacin in deionized water or pH 5.0 buffer, due to the shielding of the negative charge in the skin. The skin residue of enoxacin was slightly increased after the incorporation of Azone in PVP hydrogel. The result of in vivo microdialysis was in accordance with that of in vitro study. The effect of Azone on the intracutaneous enoxacin was more significant for in vivo microdialysis than in the in vitro study indicating the clinical feasibility of Azone for iontophoretic delivery. Microdialysis can be considered as a useful technique to investigate the pharmacokinetics of transdermal iontophoresis in vivo.

Biotransformation of coumarin by rodent and human cytochromes P-450: metabolic basis of tissue-selective toxicity in olfactory mucosa of rats and mice

Coumarin was previously found to cause tissue-selective toxicity in the olfactory mucosa (OM) of rats and mice, with rats being the more sensitive species. The aim of this study was to explore the role of target tissue biotransformation in OM-selective toxicity and the metabolic basis of the species differences in coumarin toxicity. At least six coumarin metabolites were detected in OM microsomal reactions, with o-hydroxyphenylacetaldehyde (o-HPA) being the most abundant. Formation of o-HPA was inhibited by reduced glutathione, confirming its origin from a reactive intermediate. There were significant differences in the rates and metabolite profiles of coumarin metabolism in the livers of Wistar rats and C57BL/6 mice. The rates of metabolic activation of coumarin, as indicated by the formation of o-HPA, were comparable in OM microsomes of the two species but about 25- and 3-fold higher in OM than in liver microsomes of rats and mice, respectively. Thus, target tissue activation seems to play an important role in the tissue-selective toxicity, whereas differences in the rates of hepatic metabolism may be responsible for the species difference in olfactory toxicity. Purified, heterologously expressed mouse CYP2A5 and CYP2G1 produced 7-hydroxycoumarin and o-HPA as the predominant products, respectively. Kinetic analysis and immunoinhibition studies indicated that the OM-specific CYP2G1 plays the major role in metabolic activation of coumarin. Furthermore, of 13 human cytochrome P-450s (P-450s) examined, five (CYP1A1, CYP1A2, CYP2B6, CYP2E1, and CYP3A4) were active in the metabolic activation of coumarin, suggesting a potential risk of coumarin toxicity in humans.

Polyethyleneglycol-stabilized manganese-substituted hydroxylapatite as a potential contrast agent for magnetic resonance imaging: particle stability in biologic fluids

RATIONALE AND OBJECTIVES

Polymer-stabilized manganese(II)-substituted hydroxylapatite (MnHA) has been investigated as a particulate contrast agent for magnetic resonance imaging. The MnHA core requires a polymer coating to retard opsonization, thereby prolonging its systemic persistence. Therefore, the aim of this study was to assess the stability of various formulations in biologic media in vitro.

METHODS

Polyethyleneglycol-coated manganese(II)-substituted hydroxylapatite particles were studied in bovine plasma as a function of the concentration of polymer in the formulation. Particle sizing techniques and nuclear magnetic resonance proton relaxometry were used to evaluate both in vitro and in vivo stability.

RESULTS

A small-sized particle (approximately 10 nm diameter) that is stable in bovine plasma and rabbit whole blood was formed in formulations with high amounts of polymer concentration. In formulations with low amounts of polymer concentration, larger-sized particles (approximately 100 nm diameter) were present along with the small-sized population. The larger particles de-aggregated into the small-size particle distribution on dispersion in bovine plasma and rabbit whole blood.

CONCLUSIONS

Ultrasmall particles with high surface coat were stable in plasma, whereas larger aggregates de-aggregated. Unlike Mn2+, the interaction of polyethyleneglycol-stabilized manganese(II)-substituted hydroxylapatite with plasma proteins was weak.

Codiffusion of propylene glycol and dimethyl isosorbide in hairless mouse skin

The in vitro percutaneous fluxes of propylene glycol (PG), cis-oleic acid (OA) and dimethyl isosorbide (DI) were determined and their effect on nifedipine (N) flux and lag time evaluated. PG, OA and DI flux through hairless mouse (HM) skin was measured in vitro by beta-scintigraphy and N permeation was measured by HPLC under finite and infinite dose conditions. Evaluation of each of the solvents separately showed that pure DI possessed the inherent ability to traverse the skin (12% in 24 h). For the tested formulation after 24 h, 57% of the PG and 40% of the DI had permeated across the skin with nearly linear permeation between 4 and 18 h and the relative order of permeation was PG > DI > N. DI permeation was further aided in the presence of PG and OA. N flux was dependent on concomitant solvent permeation. Over a 24-h test period a dose dependent response was observed for N, with 4.9-15.6 mg of N delivered from the lowest and highest doses, respectively, and the highest dose yielding zero-order flux of 146 (g/h per cm2).

In vitro percutaneous penetration through hairless rat skin: influence of temperature, vehicle and penetration enhancers

Theoretically, a positive relation is expected between skin temperature and the percutaneous penetration of topically applied substances. The aim of the present study was to evaluate the influence of the temperature on the in vitro percutaneous penetration of dihydrotestosterone. Hairless rat skin was mounted in static diffusion cells placed in a water bath at different temperatures (28.6, 35.1 and 38.2 degrees C, respectively). Different vehicles were tested as well as the addition of penetration promoting molecules such as oleic acid and limonene. A saline buffer was used as the receptor phase. Penetration through the skin was evaluated by means of scintillation counting of the radiolabelled dihydrotestosterone. Experiments were followed for a period of 29 h. The total amount of penetrant, dihydrotestosterone, as well as the flux, were calculated from these kinetics. Our results demonstrate a temperature effect with the highest penetration at 38.2 degrees C. The vehicle was also found to influence the penetration of dihydrotestosterone in a significant way. Furthermore, limonene presented better penetration promoting properties compared with oleic acid.

Characterization of paracellular and aqueous penetration routes in cornea, conjunctiva, and sclera

PURPOSE

To characterize quantitatively the paracellular permeation routes in rabbit cornea, conjunctiva, and sclera using polyethylene glycol (PEG) oligomers.

METHODS

Corneal, conjunctival, and scleral tissues from New Zealand white rabbits were tested individually in a modified two-chamber Ussing apparatus with the mixture of PEGs with mean molecular weights 200, 400, 600, and 1000 in glutathione bicarbonated Ringer's solution buffer on the donor side of the chamber. The samples and standards were analyzed with high-performance liquid chromatography-thermospray mass spectrometry method. The pore sizes and the pore densities of the corneal and conjunctival epithelia were calculated using an effusion-like approach.

RESULTS

The conjunctival and scleral tissues were 15 to 25 times more permeable than the cornea and the molecular size affected the conjunctival permeability less than that of the cornea. The palpebral and bulbar conjunctivas had equal permeabilities. The scleral permeability was approximately half of that in the conjunctiva and approximately 10 times more than in the cornea. The conjunctival epithelia had 2 times larger pores and 16 times higher pore density than the cornea. The total paracellular space in the conjunctiva was estimated to be 230 times greater than that in the cornea.

CONCLUSIONS

The conjunctival epithelium, due to its higher membrane permeability and larger absorptive and intercellular space surface areas, is the most viable route for ocular delivery of peptides and oligonucleotides.

Corneal epithelial permeability after instillation of ophthalmic solutions containing local anaesthetics and preservatives

The effect of the local anaesthetics oxybuprocaine (OBu) and tetracaine (Tetra) and the preservatives chlorhexidine (CH) and benzalkonium chloride (BAK) on corneal epithelial permeability was studied by fluorophotometry in normal human eyes. Five instillations of one drop ophthalmic solution of the compounds were administered to one eye at 2-minute intervals; a control solution was instilled into the fellow eye. The increase in corneal epithelial permeability, expressed as the permeability ratio between the treated and control eye, was not significant after instillation of the anaesthetics. The preservatives and the combination OBu + CH increased corneal epithelial permeability significantly (P less than 0.05). OBu + BAK and Tetra + BAK increased permeability to a far greater extent (P less than 0.005).

Absorption of topical disodium cromoglycate and its preservatives by soft contact lenses

Disodium cromoglycate (2%) (DSCG) was administered four times daily for one month to 10 patients using extended wear contact lenses. The same regimen was also followed for five days by 25 patients using daily wear soft contact lenses. The contact lenses and their soaking solutions were collected at the end of the wearing period and analyzed for DSCG, benzalkonium chloride (BAK), 2-phenylethanol, and EDTA. The lens soaking solutions and eluates prepared from the lenses were tested. In the extended wear group, small amounts of DSCG were detected in both the eluates and the soaking solutions. In the daily wear group DSCG was detected in small amounts in the soaking solutions but not in the eluates. BAK, EDTA, and 2-phenylethanol were not detected in any of the eluates or soaking solutions. During the study, no side effects of DSCG were observed in any of the patients. In animal experiments, radioactive DSCG was applied once to a series of rabbit eyes. Four hours after administration of the labelled DSCG, the animals' tears, cornea, and aqueous humor were examined for DSCG. Of the instilled dose, approximately 0.2% was found in the cornea, and less than 0.04% was found in the aqueous humor. We conclude that commercial DSCG applied topically to contact lenses does not result in the accumulation of either the drug or its preservatives in lenses and that DSCG can be safely applied directly onto a worn contact lens.