Ocular membrane permeability of hydrophilic drugs for ocular peptide delivery

Therapeutic contact lenses for the treatment of corneal and ocular surface diseases: advances in extended and targeted drug delivery

The eye is one of the most important organs in the human body providing critical information on the environment. Many corneal diseases can lead to vision loss affecting the lives of people around the world. Ophthalmic drug delivery has always been a major challenge in the medical sciences. Since traditional methods are less efficient (∼ 5%) at delivering drugs to ocular tissues, contact lenses have generated growing interest in ocular drug delivery due to their potential to enhance drug bioavailability in ocular tissues. The main techniques used to achieve sustained release are discussed in this review, including soaking in drug solutions, incorporating drug into multilayered contact lenses, use of vitamin E barriers, molecular imprinting, nanoparticles, micelles and liposomes. The most clinically relevant results on different eye pathologies are presented. In addition, this review summarizes the benefits of contact lenses over eye drops, strategies for incorporating drugs into lenses to achieve sustained release, results of in vitro and in vivo studies, and the recent advances in the commercialization of therapeutic contact lenses for allergic conjunctivitis.

Peptide Inhibitors of MARCKS Suppress Endotoxin Induced Uveitis in Rats

Purpose: To determine if inhibition of Myristoylated Alanine Rich C Kinase Substrate (MARCKS) protein, using novel MARCKS inhibitor peptides, will reduce the severity of endotoxin-induced uveitis (EIU) in rats. Methods: EIU was induced in Lewis rats using subcutaneous administration of lipopolysaccharide. In the first phase of the study, 3 different novel MARCKS inhibitor peptides that mimic the N-terminal region of MARCKS (BIO-11006, or lower molecular weight analogs BIO-91201 or BIO-91202; Biomarck Pharmaceuticals, Ltd., Newtown, PA) were administered intravitreally (IVT) at 50 and 100 μM. In the second phase, BIO-91201 was administered IVT at 10, 50, and 100 μM and topically at the 100 μM concentration. The efficacy of MARCKS inhibitor peptides was assessed by clinical examination using slit lamp biomicroscopy, optical coherence tomography (OCT) anterior chamber cell counts, histopathology, and aqueous humor cytokine analysis. Results: Clinical scores were significantly reduced 24 h following uveitis induction in the first phase of the study in the following treatment groups: BIO-11006 50 μM IVT and 100 μM IVT, BIO-91201 50 μM IVT, and BIO-91202 100 μM IVT (P < 0.05). OCT anterior chamber cell counts were significantly reduced in the first phase of the study in all treatment groups (P < 0.001). OCT anterior chamber cell counts and histopathology scores were significantly reduced in the second phase of the study in the BIO-91201 50 μM IVT group (P < 0.05). No effect was seen with topical administration. Conclusion: MARCKS inhibitor peptides were effective in reducing the severity of ocular inflammation and cellular influx in EIU.

Ex Vivo Conjunctival Retention and Transconjunctival Transport of Poorly Soluble Drugs Using Polymeric Micelles

This paper addresses the problem of ocular delivery of lipophilic drugs. The aim of the paper is the evaluation of polymeric micelles, prepared using TPGS (d-α-Tocopheryl polyethylene glycol 1000 succinate), a water-soluble derivative of Vitamin E and/or poloxamer 407, as a vehicle for the ocular delivery of dexamethasone, cyclosporine, and econazole nitrate. The research steps were: (1) characterize polymeric micelles by dynamic light scattering (DLS) and X-ray scattering; (2) evaluate the solubility increase of the three drugs; (3) measure the in vitro transport and conjunctiva retention, in comparison to conventional vehicles; (4) investigate the mechanisms of enhancement, by studying drug release from the micelles and transconjunctival permeation of TPGS; and (5) study the effect of micelles application on the histology of conjunctiva. The data obtained demonstrate the application potential of polymeric micelles in ocular delivery, due to their ability to increase the solubility of lipophilic drugs and enhance transport in and across the conjunctival epithelium. The best-performing formulation was the one made of TPGS alone (micelles size ≈ 12 nm), probably because of the higher mobility of these micelles, an enhanced interaction with the conjunctival epithelium, and, possibly, the penetration of intact micelles.

Impact of physiochemical properties on pharmacokinetics of protein therapeutics

Physicochemical properties, such as molecular weight, size, partition coefficient, acid dissociation constant and solubility have a great impact on pharmacokinetics of traditional small molecule drugs and substantially used in development of small drugs. However, predicting pharmacokinetic fate (absorption, distribution, metabolism and elimination) of protein therapeutics from their physicochemical parameters is extremely difficult due to the macromolecular nature of therapeutic proteins and peptides. Their structural complexity and immunogenicity are other contributing factors that determine their biological fate. Therefore, to develop generalized strategies concerning development of therapeutic proteins and peptides are highly challenging. However, reviewing the literature, authors found that physiochemical properties, such as molecular weight, charge and structural modification are having great impact on pharmacokinetics of protein therapeutics and an attempt is made to provide the major findings in this manuscript. This manuscript will serve to provide some bases for developing protein therapeutics with desired pharmacokinetic profile.

Paracellular and passive transcellular permeability in immortalized human corneal epithelial cell culture model

A cell culture model of human corneal epithelium (HCE-model) was recently introduced [Invest. Ophthalmol. Vis. Sci. 42 (2001) 2942] as a tool for ocular drug permeation studies. In this study, passive permeability and esterase activity of the HCE-model were characterised. Immortalised human corneal epithelial cells were grown on collagen coated filters under air-lift. The sensitivity of transcellular permeability to lipophilicity was tested in studies using nine beta-blockers. The size selectivity of the paracellular route was investigated using 16 polyethylene glycol oligomers (PEG). An effusion-like approach was used to estimate porosity and pore sizes of the paracellular space in HCE membrane. Permeability and degradation of fluorescein diacetate to fluorescein in HCE-cells was used to probe the esterase activity of the HCE-model. Drug concentrations were analyzed using HPLC (beta-blockers), LC-MS (PEGs), and fluorometry (fluorescein). Permeabilities were compared to those in the excised rabbit cornea. Penetration of beta-blockers increased with lipophilicity according to a sigmoidal relationship. This was almost similar to the profile in excised cornea. No apical to basolateral directionality was seen in the permeation of beta-blockers. Paracellular permeability of the HCE-model was generally slightly higher than that of the excised rabbit cornea. The HCE-model has larger paracellular pores, but lower pore density than the excised cornea, but the overall paracellular space was fairly similar in both models. The HCE-model shows significant esterase activity (i.e. fluorescein diacetate was converted to free fluorescein). These data on permeability of 27 compounds demonstrate that the barrier of the HCE-model closely resembles that of the excised rabbit cornea. Therefore, the HCE-model is a promising alternative corneal substitute for ocular drug delivery studies.

Ocular drug permeation following experimental excimer laser treatment on the isolated pig eye

Excimer laser photorefractive keratectomy (PRK) is a well-established procedure which is frequently applied to correct myopia. Since structural alterations of the corneal epithelium occur after the treatment, a different drug permeation can be assumed. To investigate the effects of PRK on drug permeation, excimer laser ablations with varying depths were performed on isolated pig eyes. The permeation of lipophilic (diclofenac-sodium; D-Na) and hydrophilic (pilocarpine-hydrochloride; P-HCl model drugs were studied in vitro. Under these experimental conditions, P-HCl demonstrated a significant (p < 0.05) enhancement of permeation in relation to the ablation depth. In contrast, corneal epithelial thickness scarcely influenced the permeation rate of D-Na. Not until removing the entire epithelium did a significantly increased permeability occur, when compared to untreated cornea. These results suggest that PRK may significantly reduce the corneal barrier function and alter pharmacokinetics of topical medication.

Comparison of vaginal aminopeptidase enzymatic activities in various animals and in humans

The specific enzymatic activity of four different aminopeptidases (aminopeptidase N, leucine aminopeptidase, aminopeptidase A and aminopeptidase B) in vaginal homogenates from rabbit, rat, guinea-pig, sheep and humans was compared. The purpose of the study was to find an appropriate animal model that can be used in degradation studies of protein and peptide drugs. Different substrates were used as the relative specific substrates for the determination of aminopeptidase enzymatic activity: 4-methoxy-2-naphthylamide of L-alanine for aminopeptidase N, 4-methoxy-2-naphthylamide of L-leucine for leucine aminopeptidase, 4-methoxy-2-naphthylamide of L-glutamic acid for aminopeptidase A and 4-methoxy-2-naphthylamide of L-arginine for aminopeptidase B. The vaginal aminopeptidase enzymatic activity of different species was determined spectrofluorometrically. The inhibition of aminopeptidase activity in the presence of bestatin and puromycin inhibitors was also investigated. The results showed the presence of aminopeptidase enzymatic activity in all vaginal homogenates in the order: sheep > guinea-pig > rabbit > or = human > or = rat. Based on the results of the hydrolysis and inhibition of the 4-methoxy-2-naphthylamide substrates, it was difficult to have an exact decision on the aminopeptidase type in the vaginal homogenates from the species studied. It was found that the aminopeptidase activity in rat, rabbit and humans was not statistically different. Therefore, we suggest that rats and rabbits could be used as model animals for vaginal enzymatic activity studies and for determination of the degradation of protein and peptide drugs in the vagina.

A compartment model to calculate time-dependent concentration profiles of topically applied chemical compounds in the anterior compartments of the rabbit eye

Hitherto, none of the existing in vitro methods has been convincingly demonstrated to be suitable as a replacement for the Draize rabbit eye irritation test. We examine the hypothesis that one reason for this is that insufficient consideration has been given to the differences in the effective concentrations at which chemicals operate in vitro and in vivo. When a chemical is applied topically to the eye, the strength of the observed irritation that it elicits depends both on its toxic potential toward cells or tissues, and its effective concentration in the tissues of the eye. Most of the existing in vitro methods are based on isolated cells or tissues, and thus may be useful in assessing the cytotoxic potentials of chemicals. However, a reliable approach to assessing the effective concentrations of chemicals within the various tissues of the eye is lacking. A simplified compartment model is presented for calculating the time-dependent, intra-ocular concentration profiles of topically applied chemicals. The model encompasses the outer surface of the eye, three distinct segments of the cornea (subdivided into the epithelium, stroma and endothelium) and the conjunctiva. Transport through the membranes of these compartments is described as passive diffusion. For the transport coefficients, rate equations are established that contain, as free parameters, the molecular size and the partition coefficient of the chemical, as well as some intrinsic membrane parameters, such as thickness, viscosity and pore density. Numerical values for the unknown membrane parameters were estimated by fitting the theoretical rate equations to measured permeability coefficients. The compartment model was applied to an independent set of 52 test chemicals compiled from the European Commission/UK Home Office validation study. The calculated passage times (required to let 95% of the chemical reach the posterior eye tissues) varied between 0.33 minutes and 50.6 minutes, and are generally much shorter than the typical duration of observed impairments in the cornea or conjunctiva. This finding suggests that short-term contacts of the eye tissues with a chemical are sufficient to elicit long-term eye irritation. An example is given, showing how the conventional approach of using in vitro endpoints as predictors of eye irritation can be improved significantly by incorporating into the prediction the calculated intra-ocular concentration of a chemical.

Vitreal elimination kinetics of large molecular weight FITC-labeled dextrans in albino rabbits using a novel microsampling technique

A novel sampling technique that allowed for continuous vitreal sampling of high molecular weight compounds was developed. This technique generated consistent and reproducible results. Fluorescein isothiocyanate-linked dextrans (FITC-dextrans) with average molecular weights of 4.4, 9.3, and 38.9 kD were selected for the study. A 100 microgram dose was administered into the vitreous by a short-term infusion (100 microL) over a period of 45 s, and sampling was carried out for 10 h. The vitreal elimination of these dextrans was found to follow apparent first-order elimination kinetics, having half-lives of 246 min, 275 min, and 484 min, respectively. Aqueous levels were also determined at the end of 10 h and were correlated with vitreal dextran concentrations. The FITC-dextrans displayed an initial equilibration phase of about 200 min followed by linear first-order elimination. Apparent diffusion coefficients in the vitreous have been calculated to be 7.56 x 10(-6) and 6.18 x 10(-6) cm(2)/s for 4.4 and 9.3 kD dextrans, respectively. Furthermore, it became evident that with progressively higher molecular weight FITC-dextrans the vitreal elimination rate constant gradually decreased. The elimination rate constant was found to be inversely related to the logarithm of molecular weight with a correlation coefficient of 0.983. Results obtained suggest an elimination mechanism primarily involving the transretinal route possibly with some involvement of the aqueous pathway.