Ocular parameters

To remain safe and efficacious on the eye, contact lenses must maintain clear and wetted surfaces, provide an adequate supply of atmospheric oxygen to and adequate expulsion of carbon dioxide from the cornea, allow adequate flow of the eye s tear fluid, and avoid excessive abrasion of the ocular surface or eyeflds, all under a variety of environmental conditions. The clinical performance of a contact lens is controlled by the nature of the lens material the lens design the method and quaUty of manufacture the lens parameters or specifications prescribed by the practitioner and the cleaning, disinfection, and wearing procedures used by the patient. 

Although these studies were designed to look for ocular effects, no adverse effects of these xanthophylls were reported where clinical or biochemical parameters were also examined. 

The correlation (or lack of correlation) of other physiochemical characteristics has not yet been established. For instance, are all surfactants irritants Can one classify severity by the size of the molecule Can octanol water partition coefficients predict irritation potential does a propensity to partition out of the ocular fluid mean that a compound presents more of an irritation hazard than one which is more water soluble Theoretically, these data should reflect the ability of a compound to penetrate the eye and cause an irreversible lesion. However, until definitive data are available, physical and chemical parameters will probably have limited utility in an overall assessment of irritation. 

Scholz M, Lin JE, Lee VE1, Keipert S. Pilocarpine permeability across ocular tissues and cell cultures Influence of formulation parameters. J Ocul Pharmacol Ther 18 455-468 (2002). 

Table 1 shows the products used and tested as ocular endotamponades and their characteristics. In addition to the specific density, the interfacial tension against water is also an important parameter for the use. The drainage of subret-inal fluids is achieved, but the interfacial tension prevents the passage of the PFCL into the subretinal space. 

All these parameters are very close to the requirements which a long-term ocular endotamponade has to fulfil. Also the in vivo tests in a rabbit eye model were extremely promising no emulsification, no changes in the vascular structure of the retina and no increase of the intra-ocular pressures. All negative side effects, seen with the monomeric FCLs, seemed to be eliminated. In addition, some additional advantages could be claimed reduced tissue penetration and the potential to dissolve drugs [44,45]. 

In any type of ocular bum and later on rinsing therapy, we have found that the speed of the penetration was roughly correlated to the concentration of the corrosive and the type of corrosive. This question is still scientifically open but estimations of penetration of sodium hydroxide are from about 5-8 pm/s depth propagation into the tissues, derived from measurements of Rihawi et al. on rabbit corneas [43]. Theoretical work on penetration characteristics of different chemicals have been published by Pospisil and Holzhuetter [44]. They have proved that, in first order estimation, the chemical properties like molecular size and shape, partition coefficients, and the type of interaction with the intrinsic membrane parameters determine the penetration characteristics. In very good estimations, they have shown that, for a various set of test substances, the penetration is almost exactly predicted by their modelization. 

Current density was calculated from the current applied and the surface contact area of the iontophoretic probe, if mentioned in this chapter. It is clear from this summary that the two major parameters influencing drug penetration are the current density and iontophoretic duration. Ocular iontophoresis can be delivered by two approaches transcorneal and trans-scleral iontophoresis. 

The dose of chemicals applied to filter paper may be modified to view concentration-dependent parameters of test substances. By establishing the dose response to ocular irritancy, comparisons can be made to the mild, moderate and severe grading systems previously validated (Stokes 2003). An alternative exposure of test chemicals may be performed by direct application of test solution dropwise onto the center surface of the comeal (Xu et al. 2000). The test solution then dissolves into the culture media. 

Various lecithin-based MEs were also characterized by Hasse and Keipert [131]. The formulations were tested in terms of their physicochemical parameters (pH, refractive index, osmolality, viscosity, and surface tension) and physiological compatibility (HET-CAM and Draize test). In addition, in vitro and in vivo evaluations were performed. The tested MEs showed favorable physicochemical parameters and no ocular irritation as well as a prolonged pilocarpine release in vitro and in vivo. 

Baeyens, V., and Gurny, R. (1997), Chemical and physical parameters of tears relevant for the design of ocular drug delivery formulations, Pharm. Acta Ilelv., 72(4), 191-202. 

It has to be clear that, once diluted and injected (or administered in ocular and other routes), the emulsion stability and fate are determined by three measurable parameters. The first is the partition coefficient of each emulsion component (including added drugs and agents) between the emulsion assembly and the medium. To some extent this partition coefficient is related to oil-water and/or octanol-water partition coefficients. For example, it was well demonstrated that per component of which logP is lower than 8, the stability upon intravenous (IV) injection is questionable [42,138], The other two parameters are kQff, a kinetic parameter which describes the desorption rate of an emulsion component from the assembly, and kc, the rate of clearance of the emulsion from the site of administration. This approach is useful to decide if and what application a drug delivery system will have a chance to perform well [89],... 

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