Selecting optimal dosage volumes for eye irritation tests in the rabbit

Development and Characterization of Nanoemulsions for Ophthalmic Applications: Role of Cationic Surfactants

The eye is a very complex organ comprising several physiological and physical barriers that compromise drug absorption into deeper layers. Nanoemulsions are promising delivery systems to be used in ocular drug delivery due to their innumerous advantages, such as high retention time onto the site of application and the modified release profile of loaded drugs, thereby contributing to increasing the bioavailability of drugs for the treatment of eye diseases, in particular those affecting the posterior segment. In this review, we address the main factors that govern the development of a suitable nanoemulsion formulation for eye administration to increase the patient’s compliance to the treatment. Appropriate lipid composition and type of surfactants (with a special emphasis on cationic compounds) are discussed, together with manufacturing techniques and characterization methods that are instrumental for the development of appropriate ophthalmic nanoemulsions.

Therapeutic nanoemulsions in ophthalmic drug administration: Concept in formulations and characterization techniques for ocular drug delivery

The eye is the specialized part of the body and is comprised of numerous physiological ocular barriers that limit the drug absorption at the action site. Regardless of various efforts, efficient topical ophthalmic drug delivery remains unsolved, and thus, it is extremely necessary to advance the contemporary treatments of ocular disorders affecting the anterior and posterior cavities. Nowadays, the advent of nanotechnology-based multicomponent nanoemulsions for ophthalmic drug delivery has gained popularity due to the enhancement of ocular penetrability, improve bioavailability, increase solubility, and stability of lipophilic drugs. Nanoemulsions offer the sustained/controlled drug release and increase residence time which depend on viscosity, compositions, and stabilization process, etc.; hence, decrease the instillation frequency and improve patient compliance. Further, due to the nanosized of nanoemulsions, the sterilization process is easy as conventional solutions and cause no blur vision. The review aims to summarizes the various ocular barriers, manufacturing techniques, possible mechanisms to the retention and deep penetration into the eye, and appropriate excipients with their under-lying selection principles to prevent destabilization of nanoemulsions. This review also discusses the characterization parameters of ocular drug delivery to spike the interest of those contemplating a foray in this field. Here, in short, nanoemulsions are abridged with concepts to design clinically advantageous ocular drug delivery.

The use of low-volume dosing in the eye irritation test

The Draize rabbit eye test was developed to provide a method for assessing the irritation potential of materials that might come in contact with human eyes. The method involves the instillation of 0.1 ml of a test liquid (100 mg solid) into the conjunctival sac of an animal's eye. A refinement of the Draize test is the low-volume eye test in which 0.01 ml of a substance is placed directly on the cornea of the eye. Studies indicate that the low-volume method provides a better correlation to human eye irritation experience for some substances. The Interagency Regulatory Alternatives Group (IRAG) proposes that the low-volume eye test can be used to substantiate the irritancy of suspect severe ocular irritants that have not been eliminated by various pre-eye test 'screens'. A substance testing positive by the low-volume method can be classified as an irritant; one that tests negative will require further testing by the use of the 0.1-ml volume procedure. For all other definitive testing, the Draize test (0.1 ml) should be used. Results from a questionnaire distributed at the IRAG workshop showed that many workshop participants thought that the low-volume test should be used as an eye irritation screening procedure.

Quantification of Eye Irritation Based upon In Vitro Changes of Corneal Thickness

Measurement of corneal swelling is an objective assessment of irritation in in vivo tests. The same measurement may be made in in vitro tests using the isolated eye test (IET), which has been recommended by the European Community as an alternative for in vivo eye irritation tests. In order to compare the corneal swelling in vivo and in vitro, tests were performed with 34 substances. The in vitro test results were assessed for their ability to predict eye-irritant potential.

The corneal swelling data in vivo after 4, 24, 48 and 72 hours were compared with the in vitro data obtained after 2 and 4 hours. Slight linear correlation was found between the corneal swelling in vivo after 4 hours and the corneal swelling in vitro after 4 hours (r=0.77).

The substances tested can be divided into two groups, according to their ability to create opacity in either the epithelial layer or in the stroma. When the substances causing epithelial opacity were omitted from the comparison, a much better linear correlation was obtained between the mean corneal swelling 121 vitro over 2 and 4 hours and the mean corneal swelling calculated in vivo for all animals and over three observation times (24, 48 and 72 hours; r=0.91), the latter being the observation times prescribed by EC legislation (1). A comparison of the mean corneal opacity scores observed in vivo and the mean percentage corneal swelling in vitro gave a satisfactory linear correlation (r=0.89). From this study it can be deduced that a mean corneal swelling of 55%, obtained in isolated eyes over 2 and 4 hours, corresponds with the limit for classification as irritant, to the eye. When this criterion was applied to all substances causing no epithelial opacity (n=28), only one false positive and no false negatives were found.