Different Effects of Absorption Promoters on Corneal and Conjunctival Penetration of Ophthalmic Beta-Blockers

Purpose. The purpose of this study was to investigate the improvement in corneal penetration of ophthalmic beta-blockers of various lipophilicities afforded by absorption promoters and to compare the corneal against conjunctival penetration in response to absorption promoters. Methods. The penetration of the beta-blockers, atenolol, carteolol, tilisolol, timolol, and befunolol, in the presence of absorption promoters, across the isolated corneal and conjunctival membranes of albino rabbits was measured using a two-chamber glass diffusion cell. EDTA, taurocholic acid, capric acid, and saponin were used as the absorption promoters. Results. The absorption promoters significantly increased the corneal permeability of most beta-blockers, especially the hydrophilic agents. The absorption promoters also enhanced the conjunctival permeability of beta-blockers, although their effect in promoting conjunctival penetration was less than that on corneal penetration. There was a differing penetration of instilled beta-blockers in the cornea and conjunctiva in response to absorption promoters. Capric acid and saponin showed significant promoting action on corneal penetration, but not on conjunctival penetration. Taurocholic acid had a significant effect on conjunctival penetration but not on corneal penetration. Saponin caused slight irritation. Conclusions. Absorption promoters can improve the ocular delivery of beta-blockers and a selective use of absorption promoter can improve the extent and pathway of drug ocular absorption.     

A Compartmental Model for the Ocular Pharmacokinetics of Cyclosporine in Rabbits

Studies were conducted in rabbits to determine the ocular distribution and elimination of cyclosporine, with the objective of developing a comprehensive pharmacokinetic model. Following a bolus dose into the anterior chamber, drug levels were measured in the aqueous humor, cornea, iris/ciliary body, lens, sclera, and conjunctiva. Cyclosporine was rapidly eliminated from the aqueous, but drug levels in ocular tissues persisted for in excess of 48 hours, particularly in the cornea and iris/ciliary body. The terminal elimination half life from these tissues was 45 hr and 30 hr, respectively, providing evidence that these tissues could act as a reservoir for the drug. It was found that a compartmental model accurately described the experimental data. A single compartment was used for each of the tissues and fluids sampled, except for the cornea, which was subdivided into two compartments, representing its tissue and aqueous regions.     

Ocular drug delivery

Drug delivery to the eye is hampered by anatomical factors, including the corneal epithelium, the blood–aqueous barrier and the blood–retinal barrier. This review aims to outline the major routes of ocular drug delivery, including systemic, topical, periocular and intravitreal. The pharmacokinetics, the disadvantages and the clinical relevance of these drug delivery routes have been emphasised. Recent advances in surgical techniques, therapeutic approaches and material sciences have produced exciting new therapies for ocular diseases. The role of ophthalmic drug formulation in targeting the desired ocular tissue and enhancing drug delivery by the chosen route whilst minimising side effects is also discussed.     

Pharmacokinetics and efficacy of a ketorolac-loaded ocular coil in New Zealand white rabbits

Eye drops are considered standard practice for the delivery of ocular drugs. However, low patient compliance and low drug levels compromise its effectiveness. Our group developed a ketorolac-loaded ocular coil for sustained drug delivery up to 28 days. The aim of this study was to gain insight into the pharmacokinetics and efficacy of the ocular coil. The pharmacokinetics of the ketorolac-loaded ocular coil versus eye drops were tested in New Zealand White rabbits by repetitive sampling for 28 days. Efficacy of the ocular coil was also tested in New Zealand White rabbits. Ocular inflammation was induced where after the ocular coil was inserted, or eye drops, or no treatment was provided. The total protein concentration and cytokine levels were measured in tears, aqueous humor, and plasma at 4 h, 8 h, 24 h, 4 d, 7 d, 14 d, 21 d, and 28 d. Four h after inserting the ocular coil in the eye, ketorolac levels in aqueous humor and plasma were higher in the ocular coil group than in the eye drop group. Ketorolac released from the ocular coil could be detected up to 28 d in tears, up to 4 d in aqueous humor and up to 24 h in plasma. After inducing inflammation, both the ocular coil and eye drops were able to suppress prostaglandin E₂, TNFα and IL-6 levels in aqueous humor and plasma as compared to the group that received no treatment. To conclude, the ocular coil facilitated a sustained release of the drug and showed similar therapeutic benefit in suppressing post-operative inflammation as eye drops.     

The Kinetics of Timolol in the Rabbit Lens: Implications for Ocular Drug Delivery

This study examines the uptake and distribution of timolol in the rabbit lens following topical instillation using a heuristic approach. The implications of anisotropic drug diffusion through the lens are presented here and discussed in the context of actual in vivo data. The dynamics of timolol in the lens involve an initial, rapid uptake of the drug by the capsule and epithelium followed by slower, anisotropic diffusion through the cortex body. Kinetically, the capsule and epithelium can be treated as a separate compartment which is distinct from the cortex and which serves to provide a concentration gradient for subsequent diffusion of timolol into the dense interior structures of the lens. Model simulations support the hypothesis that the preferred route of penetration of timolol into the vitreous humor via the lens is the diffusion of drug around the capsule/epithelium and peripheral cortical layers. It is also shown that due to the high and increasing diffusional resistance toward the center of the lens, as well as the diminishing drug concentrations in the capsule and epithelium, steady-state levels in the lens may be extremely difficult to achieve in some therapeutic situations. This phenomenon could have a significant impact on the success or failure of a drug treatment involving the lens and ocular tissues.     

Efficiency of Drug Targeting: Steady-State Considerations Using a Three-Compartment Model

Physiological models have often been used to investigate the processes involved in drug targeting. Such a model is used to investigate some aspects of drug targeting, including the pharmacodynamics of therapeutic and toxic effects. A simple pharmacodynamic model is incorporated in a three-compartment pharmacokinetic model. Conventional administration and drug targeting are compared at steady state for the same degree of therapeutic effect. The efficiency of drug targeting is quantified as the ratio (TA) of the rates of administration of free drug or of a drug–carrier complex required to achieve this effect. Also, the ratios of drug concentrations in the toxicity compartment (DTI) or of the consequent degree of toxic effects (TI) are used to compare conventional administration with drug targeting. The kinetic characteristics of the drug–carrier complex, rate of elimination, and rate of free drug release, influence TA but not DTI or TI. The importance of these characteristics depends on the cost and toxicity of the drug–carrier complex or of the carrier alone. The pharmacodynamics of the free drug in both the target and the toxicity compartments have an important influence on TI but not on TA or DTI. As the pharmacological selectivity of the drug increases, so does TI. However, a drug with good pharmacological selectivity may not be suitable for drug targeting. TI is also very dependent on the shape of the effect–concentration curves, particularly that for toxicity. While TA increases as the rate of elimination of free drug from either central or target compartments increases, TI may actually be reduced if release of free drug is not confined to the target compartment.     

Pharmacokinetics and Tissue Distribution of Pilocarpine After Application to Eyelid Skin of Rats

Extending the delivery of drugs into the eyes while reducing systemic bioavailability is of utmost importance in the management of chronic ocular diseases. Topical application onto the lower eyelid skin, as an alternative to eye drops, is seen to be a valuable strategy in the treatment of chronic eye diseases. To elucidate the critical value of delivering drugs in solution onto the eyeball through the eyelid skin, pharmacokinetic studies of pilocarpine were conducted, and the results were verified using a direct pharmacodynamic study in rats. The mean residence time of pilocarpine after topical eyelid application to the eyelid skin, conjunctiva, eyeball, and plasma were 14.9, 8.50, 6.29, and 8.11 h, respectively. Conjunctiva and eyeball concentrations of pilocarpine at 8 h were 80-fold and 8-fold higher after topical eyelid application, respectively, than those for eye drops. Pupillary constriction was sustained over 8 h after topical eyelid application. Topical eyelid skin application exhibited a localized drug absorption and specific drug accumulation in the ocular tissues. Hence, it is rational to prepare topical formulations directed onto the eyelid skin, which is suitable for drugs required for long-term treatment.     

Ocular drug delivery system: a reference to natural polymers

Introduction: Ocular drug delivery is a very challenging endeavor due to the unique anatomical and physiological barriers. The low ocular bioavailability (<10%) obtained from conventional formulations has forced the scientists to develop new formulations to deliver drugs to ocular tissues at a controlled rate to reduce frequent instillations. The natural polymers have represented the potential to deliver drugs topically through the limited precorneal area and release over a prolonged time period.

Areas covered: The important points to be considered during the fabrication of ophthalmic formulations for example, properties of drug molecule and polymer which affect the release rate are discussed. Novel polymers, like arabinogalactan, xyloglucan, gum cordia, locust bean gum, carrageenan and Bletilla striata polysaccharide, besides the conventional polymers like chitosan, starch, sodium alginate, sodium hyaluronate, xanthan gum, gelatin, gellan gum, guar gum, collagen and albumin, have demonstrated the potential to safely deliver drugs at a controlled rate in different ophthalmic formulations.

Expert opinion: The limitations of topical delivery of genes and chemotherapeutic drugs can be overcome by using natural polymers with characteristic properties. Despite the wide applicability, tremendous efforts are required to establish natural polymers in novel formulations on a commercial scale.     

纳米粒在目艮部给药系统中的研究近况

介绍了近年来纳米粒在眼部给药系统中的研究情况。传统眼用制剂的主要缺陷为生物利用度低,药物在眼部保留时间短,刺激性较强,且药物一般只能到达眼前段组织;而新型纳米粒制剂不但能克服传统眼用统制剂存在的问题,还可实现靶向给药,故极具开发价值。     

Ocular Availability of Gentamicin in Small Animals After Topical Administration of a Conventional Eye Drop Solution and a Novel Long Acting Bioadhesive Ophthalmic Drug Insert

Purpose. Gentamicin eye drop solutions have a short precorneal residence time. The present study investigates the effect of gentamicin using a new long acting delivery Bioadhesive Ophthalmic Insert (BODI) in healthy dogs and rabbits and compares the results with a conventional regimen using an eye drop solution. Methods. In vivo assays were performed on animals after deposition of one BODI and instillations of an eye drop solution. Tear samples were collected over 72 hours and 60 minutes, in the case of inserts and eye drop solution respectively. The gentamicin concentration profiles in tear fluid (determined by a fluorescent polarization immunoassay technique) was individually analyzed, in each animal, in relation with the minimum inhibitory concentration observed in vitro against some bacteria. A non classical pharmacokinetic approach was used for the analysis of the topically applied drug substance, involving two parameters: the efficacy area under the curve (AUC_eff) and the efficacy time (t_eff). Results. In the case of the eye drop solution, the AUC_eff were higher in dogs (2.80 10³ – 3.64 10³ [µg ml^–1 h]) than in rabbits (0.64 · 10³ – 0.95 · 10³ [µg ml^–1h]); the t_eff had a similar behavior: 6-15 [h] in dogs and 2-6 [h] in rabbits. In the case of BODIs, the AUC_eff and the t_eff were quite similar between dogs and rabbits: 190 10³ – 205 10³ [µg ml^–1 h] and 70-76 [h], respectively. The AUC_eff and the t_effwere always much higher in the case of BODIs than for the eye drop solution both in dogs and rabbits. Conclusions. This study shows that topical administration of gentamicin using BODIs can improve treatment due to the decreasing number of applications while ensuring an effective level of antibiotic in tears controlled by the device.     

Ocular Pharmacokinetics and Pharmacodynamics of Phenylephrine and Phenylephrine Oxazolidine in Rabbit Eyes

The aqueous humor concentration of phenylephrine and its corresponding mydriatic response were measured over time in New Zealand albino rabbit eyes following a 10-µl topical instillation of a phenylephrine HC1 viscous solution (10%) or a phenylephrine oxazolidine (prodrug) suspension in sesame oil (1 and 10%). The bioavailability of a 1% prodrug suspension in the rabbit eye (AUC of aqueous humor concentration vs time) was 30% lower than that of a 10% phenylephrine solution (P < 0.1) with the exception that the peak time occurred 34 min earlier with the prodrug. A 10% prodrug suspension increased the aqueous humor bioavailability approximately eightfold but improved the mydriatic activity (AUC of mydriasis vs time) only fourfold. The pharmacokinetic parameters, apparent absorption, and elimination rate constants, of phenylephrine and the prodrug were determined from aqueous humor concentration–time and mydriasis–time profiles. The study showed that the kinetic parameters of phenylephrine estimated from its mydriasis profile do not accurately reflect the kinetics of drug distribution in the iris. These parameters also varied with the instillation of phenylephrine solution or prodrug suspensions. A mydriatic tolerance of the pupil response was apparent after the topical instillation of phenylephrine solution. The mydriatic tolerance may be due to the decrease in receptor number in the iris dilator muscle.     

辛伐他汀自微乳化释药系统在Beagle犬体内的药动学特征

目的 研究辛伐他汀自微乳化释药系统(SV-SMEDDS)在Beagle犬体内的药动学特征.方法 采用Waters OASIS[R]HLB固相萃取小柱提取样品,RP-HPLC法测定Beagle犬血浆药物浓度.双周期交叉实验设计,与辛伐他汀混悬液(SV-Sus)比较,考察单剂量灌胃给予含辛伐他汀40 mg后SV-SMEDDS的体内药动学.结果 SV-SMEDDS与SV-Sus在犬体内的药动学均符合二室模型;tmax分别为(0.84±0.26)和(0.99±0.32)h,ρmax分别为(39.73±9.11)和(28.54±7.76)μg·L-1;SV-SMEDDS相对生物利用度为184.84%(以AUC0→∞计).结论 自微乳化释药系统可以提高辛伐他汀的生物利用度.     

纳米给药系统的药动学及毒理学研究进展

纳米给药是药剂学领域研究颇多的1种新型药物递送体系,具有超微体积及特殊结构,在控释、缓释、靶向给药以及黏膜、局部给药中,可提高难溶性药物与多肽药物的生物利用度,降低不良反应,在基因工程等领域中也显示出独特的优势。笔者综述了近年来报道的纳米给药系统在药动学、毒理学方面的研究,并简要介绍了纳米给药系统在药剂学领域中的研究进展。     

Ocular application of chitosan

Introduction: A major problem in ocular therapeutics with classical formulations is the maintenance of an effective drug concentration at the site of action for a long period of time. Enhancement of ocular bioavailability with increased dose penetration and longer retention time at desired sites can be achieved by recent formulations. Chitosan stands out with its unique structural advantageous characteristics for different types of formulations like in situ gelling systems, micro- and nanoparticles, inserts, etc.

Areas covered: In this review, the authors focus on ocular therapeutics and the characteristics that make chitosan more acceptable in ocular applications.

Expert opinion: Chitosan seems to be one of the most promising polymeric carriers for both hydrophilic and lipophilic drugs for ocular application.     

Pharmacokinetics of Substrate Uptake and Distribution in Murine Brain After Nasal Instillation

No HeadingPurpose. This study was conducted to develop a physiologically relevant mathematical model for describing brain uptake and disposition of nasally administered substrates.Methods. [¹⁴C]-antipyrine, [¹⁴C]-diazepam, [³H]-sucrose, or [³H]-verapamil was administered nasally to CF-1 mice. P-glycoprotein (P-gp)-deficient mice also received [³H]-verapamil to probe the influence of P-gp on uptake/distribution. Mice were sacrificed at selected intervals, and 20 serial 300-m coronal brain sections were obtained to determine radioactivity. A series of compartmental pharmacokinetic models was developed and fit to concentration vs. time/distance data.Results. After nasal instillation, substrate concentration was highest in the olfactory bulb and decreased with distance. In the absence of transport-mediated flux, peak brain exposure occurred at 6 h. A catenary pharmacokinetic model with slice-specific brain-to-blood efflux rate constants and slice-to-slice diffusivity factors was capable of fitting the data. P-gp limited fractional absorption of [³H]-verapamil via efflux from the nasal cavity and olfactory epithelium. P-gp also increased the rate constants associated with [³H]-verapamil efflux 1.5- to 190-fold, depending on brain region. P-gp limited [³H]-verapamil uptake from the nasal cavity into brain and facilitated removal of [³H]-verapamil from brain during rostral-to-caudal distribution.Conclusions. Taken together, the data and associated modeling provide a comprehensive assessment of the influence of P-gp on brain uptake and disposition of nasally administered substrates.     

眼表局部给药的屏障及克服屏障的方法研究进展

眼部结构的复杂性使药物经眼表局部给药的生物利用度极低。基于纳米制剂的新型眼部药物递送系统,因其更易克服眼部给药屏障,使得药物经眼表局部给药后有效递送至眼组织内成为可能。介绍了眼表局部给药后药物递送至眼内需克服的屏障,总结了克服给药屏障的重要方法及眼用新剂型的临床进展,并对眼表局部给药系统的前景进行了展望,以期为眼部疾病的治疗提供新思路。     

环孢素A眼用微乳的制备及稳定性初步研究

以环孢素A为模型药物，根据伪三元相图筛选适宜的眼用微乳处方，采用自微乳化方法制备载药眼用微乳。测得3批制品中药物平均浓度为（49．8±2．5）mg·（100ml）^-1，平均粒径为（20．5±1．0）nm。稳定性初步试验表明，制品在60℃、4℃、（4500±500）1x条件放置10d，其含量、pH和平均粒径均无明显变化。     

Ocular and systemic bioavailability of ophthalmic flurbiprofen

Flurbiprofen, a nonsteroidal antiinflammatory agent which is not ocularly metabolized, was employed as a probe compound to investigate the drug kinetic relationship between systemic and ocular humoral circulation. The ocular and systemic bioavailabilities of topically applied flurbiprofen were also quantitated. Anesthetized albino female rabbits received flurbiprofen doses intracamerally, topically, and intravenously at 2 to 4 week intervals. Aqueous humor and plasma were used as the sampling compartments. Plasma clearance values of flurbiprofen were 6.77 and 7.87 ml/min, after 6-mg and 208-g intravenous doses, respectively. These values were not significantly different and indicated no dose-dependent disposition kinetics over a 30-fold dose range. Both ocular and systemic flurbiprofen dispositions followed a biexponential pattern with a rapid distribution phase. The systemic and ocular distribution half-lives of flurbiprofen were 12 min and 15 min, respectively. The plasma elimination half-life was 74 min and the aqueous humor elimination half-life was 93 min. The latter approximated the turnover rate of aqueous humor and suggested that aqueous humor drainage was the major process of flurbiprofen elimination from the globe. About 99% of flurbiprofen is bound to plasma protein. At distribution equilibrium, the plasma and aqueous humor concentrations of fluobiprofen differed by a hundredfold, suggesting that only free drug entered the aqueous humor after the administration of a systemic dose. In the ophthalmic studies, right eyes were instilled with 50 l of 0.3% flurbiprofen in saline (dose = 150 g), and left eyes were instilled with 50 l of 0.15% flurbiprofen in saline (dose=75 g). When the area of the aqueous humor concentration-versus-time curve values was normalized by the administered dose, the 75-g dose was 30% more available to ocular tissues than was the 150-g dose. This demonstrated a disproportionate relationship between the administered dose and the fraction absorbed. The intracameral dose was considered to be completely bioavailable for intraocular effects. The ocular bioavailability of the ophthalmic dose was defined by using intracameral administration as a standard measurement. The ocular bioavailabilities of the 75-g and 150-g topical flurbiprofen doses were 10% and 7%, respectively. Systemic bioavailability after topical administration of 225 g of flurbiprofen was 74%.     

Pharmacokinetics and Pharmacodynamics of Escalating Doses of SHetA2 After Vaginal Administration to Mice

SHetA2 is a novel compound with strong potential to treat cervical dysplasia, but its low aqueous solubility limits its oral bioavailability. A vaginal suppository achieved SHetA2 cervix concentrations that were severalfold above the predicted therapeutic levels. Thus, we aimed at determining the minimum dose that would achieve SHetA2 therapeutic levels while reducing cyclin D1 levels, the pharmacodynamic end point. The disposition of SHetA2 after vaginal administration of escalating SHetA2 doses and the corresponding reduction in cyclin D1 levels was compared to that after the conventional oral treatment. Vaginal administration of a 15-mg/kg dose achieved an area under the cervix concentration versus time curve (AUC_cervix) that was ～120 times larger than that after a 60 mg/kg administered orally. AUC_cervix and C_max-cervix did not increase proportionally with respect to the dose, with the 30-mg/kg dose resulting in higher AUC_cervix and C_max-cervix (1368.53 μg.mL/h and 155.38 μg/g, respectively) compared to the 15 mg/kg (334.98 μg.mL/h and 121.78 μg/g, respectively) or 60 mg/kg (1178.55 μg.mL/h and 410.38 μg/g, respectively). Likewise, the 30-mg/kg dose caused a larger reduction in cyclin D1 levels than the other doses. Thus, the 30-mg/kg dose was selected for future efficacy studies in a mouse model of cervical neoplasia.     

Pharmacokinetics of the Aldose Reductase Inhibitor Imirestat Following Topical Ocular Administration

The pharmacokinetics of imirestat following topical ocular administration were evaluated in a series of studies in rabbits and dogs. Following single topical doses to both albino and pigmented rabbits, imirestat was subject to rapid uptake into the cornea followed by an initial rapid decline and then very slow elimination, with a t _1/2 of approximately 130 hr. Drug was rapidly absorbed into aqueous humor, with concentrations declining to nondetectable levels by 12 hr. Imirestat was retained in the lens following topical dosing similar to that in cornea, with an apparent elimination t _1/2 of 140 hr. Vitreous humor concentrations of drug were detectable for up to 72 hr after dosing. There was no apparent difference in the disposition of the drug between albino and pigmented rabbits. Bioavailability following topical dosing increased with dose, although not in a linear fashion. Formulation pH did not have an appreciable effect on ocular bioavailability. There was detectable systemic absorption following topical dosing, with plasma concentrations in rabbit being 50 to 75% of that observed following an equivalent intravenous dose. However, drug levels in the dosed eyes were significantly higher than in contralateral undosed eyes. Multiple dosing of imirestat for 6 weeks resulted in accumulation of drug in rabbit lens. Concentrations were higher in lens cortex than lens nucleus, with the time course for accumulation being different for the two. Our data suggest that imirestat penetrates into ocular tissue following topical dosing and is retained in lens and cornea, potential sites of action for the drug.