Akira Ohtori

Pharmacokinetic model of intravitreal drug injection

A dynamic mathematical model is developed to describe the distribution and elimination behavior of a drug in the vitreous body following intravitreal injection. The effects of three elimination pathways--the annular gap between the lens and the ciliary body (the posterior chamber), the lens, and the retina-choroid-sclera membrane--upon the concentration distribution in the vitreous body and the time course of the rate of elimination have been quantitatively demonstrated. The effects of metabolism in the vitreous body and the site of injection are also simulated. The annular gap between the lens and the ciliary body (the posterior chamber) is found to be a main route of elimination for large molecules injected into the vitreous body. For small or highly lipophilic molecules, however, both the posterior chamber and the retina-choroid-sclera membrane act as major routes of elimination. The lens pathway may contribute negligibly to the escape of drugs from the vitreous body. The concentration on the surface of the retina is appreciably affected by the site of injection or the initial distribution profiles, while the concentration gradient on the lens surface remains almost independent of the site of injection. To maintain the therapeutic concentration in the vitreous body or in the retina for a prolonged period of time, the drug must be injected into the posterior area of the vitreous body. When the drug is injected into the anterior segment of the vitreous body, the drug molecules quickly escape into the posterior chamber from the annular gap between the lens and the ciliary body. The present mathematical model describes well in vivo elimination profile of lomefloxacin following intravitreal injection.

Analysis of an anti-inflammatory steroidal drug, difluprednate, in aqueous humor by combination of semi-micro HPLC and column switching method

A specific and sensitive method for the determination for difluprednate (DFBA) and its metabolite (deacetylated DFBA, DFB) in aqueous humor was developed. DFBA and DFB were initially absorbed on a Pinkerton-type column, then analyzed by high-performance liquid chromatography using a semi-micro column after column switching. Under the optimized conditions, calibration curves for DFBA and DFB showed good linearity over the range of 1.0-50 and 0.5-50 ng/ml, respectively. We applied the method to the analysis of DFBA and DFB in rabbit aqueous humor, and found that DFBA in rabbit aqueous humor 1 h after instillation of 0.002% DFBA ophthalmic emulsion was not detected, but DFB was present at the concentration of 4.3+/-3.1 ng/ml.

Evaluation of ophthalmic suspensions using surface tension

Uniformity and precision of single dose are required for ophthalmic suspensions including water-insoluble ingredients. Solid sediments formed after standing still must be immediately re-dispersible and distributed homogeneously before use. However, selection of an appropriate water-soluble polymer as suspending agent is a challenging problem. In this report, the relationship between the surface tension and the re-dispersibility of suspensions was investigated. The surface tension of 0.1 w/v% fluorometholone suspensions began to decline from 74 mN/m at 0.0001 w/v% of hydroxypropylmethylcellulose (HPMC) and became almost constant at 52 mN/m at 0.01 w/v% of HPMC. Re-dispersion time was less than 4 s when HPMC was present at concentrations between 0.0001 w/v% and 0.01 w/v%. At these concentrations, aggregation of suspended particles was not observed. When indomethacin suspensions at 1.0 w/v% concentration were used, the surface tension began to decline from 73 mN/m at 0.0005 w/v% HPMC and became constant at 50 mN/m at 0.005 w/v% HPMC. The suspension also showed good re-dispersibility, and a uniform suspension was obtained between 0.0005 w/v% and 0.005 w/v% of HPMC. The time required for re-dispersion was less than 17 s. The change of surface tension showed a good correlation with the concentration of HPMC in ophthalmic suspensions having good re-dispersibility. Measurement of the surface tension of suspensions provided the optimal concentration of the water-soluble polymers for the suspensions of well re-dispersible characteristics. Evaluation of ophthalmic suspension using surface tension is a good strategy for formulation of suspending pharmaceutical products in the ophthalmic area.

A pharmacokinetic model of intravitreal delivery of ganciclovir.

A pharmacokinetic model of intravitreal drug delivery was developed for describing the elimination and distribution of ganciclovir in the eye following intravitreous polymeric delivery. The model was based on Ficks second law of diffusion and assumed a cylindrical vitreous body. The model parameters such as the diffusion coefficient and the partition coefficient of the drug in the vitreous body and its surrounding tissues were determined from in vitro experiments using rabbit tissues. The time course of in vivo mean concentration of ganciclovir in the rabbit vitreous body agreed well with the profile calculated from the present pharmacokinetic model for both membrane-controlled polymeric devices and biodegradable rod-matrix systems. The clinical vitreous concentration following implantation of the membrane-controlled delivery system was the same order of magnitude but approximately four times lower than that predicted from the present model. This may indicate the metabolism of ganciclovir and/or the facilitated transport across the retina/choroid membrane in the human eye.

Corneal Critical Barrier against the Penetration of Dexamethasone and Lomefloxacin Hydrochloride: Evaluation by the Activation Energy for Drug Partition and Diffusion in Cornea

The cornea is a solid barrier against drug permeation. We searched the critical barrier of corneal drug permeation using a hydrophobic drug, dexamethasone (DM), and a hydrophilic drug, lomefloxacin hydrochloride (LFLX). The activation energies for permeability of DM and LFLX across the intact cornea were 88.0 and 42.1 kJ/mol, respectively. Their activation energies for permeability across the cornea without epithelium decreased to 33.1 and 16.6 kJ/mol, respectively. The results show that epithelium is the critical barrier on the cornea against the permeation of a hydrophobic drug of DM as well as a hydrophilic drug of LFLX. The activation energy of partition for DM (66.8 kJ/mol) was approximately 3-fold larger than that of diffusion (21.2 kJ/mol). The results indicate that the partition for the hydrophobic drug of DM to the corneal epithelium is the primary barrier. Thermodynamic evaluation of activation energy for the drug permeation parameters is a good approch to investigate the mechanism of drug permeability.

Hydrogel Ring for Topical Drug Delivery to the Ocular Posterior Segment

Abstract Purpose: To investigate the efficacy of a topical hydrogel ring for drug delivery to the posterior segment of the rabbit eye. Materials and methods: Novel hydrogel corneal lenses (CL), scleral/corneal lenses (S/CL), and rings were prepared using poly(hydroxyethyl methacrylate). The devices were immersed in 0.3% ofloxacin ophthalmic solution (OOS) to homogeneously distribute the drug throughout the hydrogel. The medicated CL, S/CL, Ring 1 (standard ring), or Ring 2 (shape-optimized ring) was applied to the surface of the cornea, cornea/bulbar conjunctiva, or bulbar conjunctiva of albino rabbits, respectively. Medicated rings did not touch the corneal surface. In another group, one OOS drop was administered to the eye. After 0.25–8 hours, the hydrogel devices were removed and ocular tissues were harvested. High-performance liquid chromatography (HPLC) was used to measure the ofloxacin concentration in the devices and tissues. The drug concentrations in the posterior segment tissues were compared among ofloxacin delivery methods. Results: One hour after placement, eyes treated with Ring 1 or S/CL had markedly higher ofloxacin levels in the posterior segment tissues (conjunctiva, sclera, and retina/choroid) than eyes treated with topical OOS or a CL. Lower levels of ofloxacin were found in anterior segment tissues (cornea and aqueous humor) in eyes treated with Ring 1 compared to those treated with S/CL. Ring 2 most effectively delivered ofloxacin to the retina/choroid. The tissue ofloxacin concentration in the fellow eye was markedly lower than the eye treated with Ring 2. Conclusions: Our results suggest that hydrogel rings are effective in delivering topical ophthalmic drugs to the posterior segment. The drugs are most likely delivered via the transconjunctival/scleral route by lateral diffusion across the bulbar conjunctiva and through the sclera. Systemic drug delivery to the posterior segment is minimal.

Topical FK962 Facilitates Axonal Regeneration and Recovery of Corneal Sensitivity After Flap Surgery in Rabbits

PURPOSE To test if the drug FK962 (N-(1-acetylpiperidin-4-yl)-4-fluorobenzamide) facilitates axonal elongation and recovery of corneal sensitivity after creation of a corneal flap in rabbits. DESIGN Animal study. METHODS Primary cultures of rabbit trigeminal ganglion cells were used to test if FK962 promoted nerve elongation in vitro. A 130 μm-thick×8.6 mm-diameter flap was created in rabbit corneas where topical 10(-6) M FK962 was administered 4 times daily. After treatment of 7 days, corneal mechanical sensitivity was measured using a Cochet-Bonnet esthesiometer. Whole-mount corneal sections were prepared, sensory nerve axons were stained with antibody for neurofilament, and axonal elongation from transected nerve termini were scored using standardized criteria. Ocular pharmacokinetics modeling was used to predict permeation of topical FK962 into cornea. RESULTS FK962 accelerated sprouting and elongation of neurites in cultured neuronal cells from rabbit trigeminal ganglia. In the in vivo rabbit model, distal axons from transected nerve termini in corneas disappeared soon after flap surgery; but with time, axons regenerated and elongated. Topical application of 10(-6) M FK962 for 7 days significantly enhanced axonal elongation and increased corneal sensitivity. Increased corneal sensitivity was directly and significantly correlated with axonal elongation, suggesting functional enhancement of re-innervation by FK962. CONCLUSIONS Results from a rabbit model of laser in situ keratomileusis (LASIK) surgery showed that topical FK962 facilitated corneal re-innervation leading to recovery of sensitivity. Results suggested that topical application of FK962 might decrease complications in patients after LASIK surgery.

Effect of dosing interval on the efficacy of topical ophthalmic gatifloxacin against Enterococcus faecalis in an in vitro pharmacokinetic model simulating the local eye compartment

Improved dosing regimens have been proposed for various antimicrobial agents by application of pharmacokinetic/pharmacodynamic (PK/PD) principles. However, for topical ophthalmic use there are several limitations to changing the dosing regimen, such as drug formulation and bioavailability. In this study, we investigated the relationship between dosing interval and antibacterial efficacy in an in vitro PK model mimicking post-operative endophthalmitis. The in vitro PK model simulated the aqueous humour concentration following topical application of 0.3% gatifloxacin ophthalmic solution to rabbit eyes. A clinical isolate of Enterococcus faecalis was exposed to gatifloxacin three times repeatedly at various intervals from 0 h to 8 h. The area between the control growth curve and the bacterial killing and re-growth curve for 24 h (ABBC) was used to evaluate efficacy. The ABBC showed bell-shaped dependence on the dosing interval with a peak at 3h. Under limited condition of total exposure amount, i.e. area under the concentration-time curve, the antimicrobial efficacy appears to be associated with the cumulative time of a 24-h period such that the concentration exceeds the minimum inhibitory concentration (T>MIC) rather than the peak concentration:MIC ratio. The length of intermission of T>MIC during repeated dosing appears to be proportional to the decrease in efficacy of gatifloxacin against E. faecalis. A longer dosing interval, as long as T>MIC is continuous, would likely be more efficient at preventing post-operative enterococcal endophthalmitis. However, further investigation is necessary to explore whether this model is applicable to a variety of pathogens and drugs.

In vitro study of aldose reductase inhibitor concentrations in the lens and inhibitory effect on sugar alcohol accumulation

Rat lenses were cultured in a medium containing either 30mM xylose or 30mM glucose. Inhibitory effects of two aldose reductase inhibitors, AD-5467 and CT-112, on the accumulation of sugar alcohols and the concentrations of the drugs in the lens were measured. Inhibitory effect of lens opacification by the drugs were also observed. Lens opacification was apparently inhibited in a medium containing each of the drugs. Inhibition rates for sugar alcohol accumulation by AD-5467 and CT-112 were unrelated to the kinds of added sugars. 50% inhibition-concentration in the medium was 0.82-1.02 x 10(-7) M for AD-5467 and 3.78-5.92 x 10(-7) M for CT-112, and the resulting concentration of drug in the lens was 2.33-3.08 nmole/g lens for AD-5467 vs. 21.19-24.63 nmole/g lens for CT-112. The results above indicated that the concentration of CT-112 should be 4.6-5.8 times of that of AD-5467 in the medium to attain the same inhibitory effect on sugar alcohol accumulation, and 8.0-9.1 times more CT-112 is needed in the lens. A linear correlation was found on a bilogarithmic graph of concentrations in the medium and in the lens for both drugs. This correlation is an empirical adsorption isotherm shown by Freundrich. So this suggested that both drugs were adsorbed into the lens. The rate of adsorption varied with the drugs. Even when the drug concentrations in the medium were the same, the concentration of CT-112 in the lens was 2-4 times higher than that of AD-5467.(ABSTRACT TRUNCATED AT 250 WORDS)

In Silico Ocular Pharmacokinetic Modeling: Delivery of Topical FK962 to Retina

PURPOSES To establish the in silico ocular pharmacokinetic modeling for eye drops, and to simulate the dose regimen for FK962 in human choroid/retinal diseases. METHODS Pharmacokinetics for FK962 in vivo was performed by a single instillation of drops containing 0.1% 14C-FK962 in rabbit eyes. Permeation of FK962 across the cornea, sclera, and choroid/retina was measured in vitro. Neurite elongation by FK962 was measured in cultured rat retinal ganglion cells. Parameters from the experimental data were used in an improved in silico model of ocular pharmacokinetics of FK962 in man. RESULTS The mean concentration of FK962 in ocular tissues predicted by in silico modeling was consistent with in vivo results, validating the in silico model. FK962 rapidly penetrated into the anterior and posterior segments of the eye and then diffused into the vitreous body. The in silico pharmacokinetic modeling also predicted that a dose regimen of 0.0054% FK962 twice per day would produce biologically effective concentrations of FK962 in the choroid/retina, where FK962 facilitates rat neurite elongation. CONCLUSIONS Our in silico model for ocular pharmacokinetics is useful (1) for predicting drug concentrations in specific ocular tissues after topical instillation, and (2) for suggesting the optimal dose regimens for eye drops. The pharmacodynamics for FK962 produced by this model may be useful for clinical trials against retinal neuropathy.