ANÁLISIS DE LA CAPACIDAD SATELITAL SOBRE LA PRUEBA

Generally bioadhesive drug delivery systems have been formulated as solid compacts, flexible films, gels and visco-elastic semi-solids (Ahuja et al, 1997), Therefore many of the tests involved in the characterisation of such materials are not suited to the assessment of liquid formulations. Force of detachment tests are simple to perform and are a popular means of assessing the strength of the interaction between a bioadhesive formulation and a biological substrate. Rheological techniques are also a common tool used to probe the interaction between a bioadhesive formulation and a mucus substrate.

One of the first methods used to assess bioadhesion was developed by Park & Robinson (1984). This method used cultured epithelial cells that had been fluorescently labelled. A polymeric suspension was added to the cells and the change in fluorescence was indicative of the extent of interaction between the polymeric formulation and the epithelial cells. This technique is applicable to liquid formulations, however, disadvantages include the process involving a great deal of preparation plus the labelling of epithelial cells may greatly affect their inherent surface properties, thus the relevance of this model is limited.

Tensile strength measurements are a widely used method to measure the strength of the adhesive interaction. Such tests, developed by Smart et al (1984), measure the forces involved in pulling apart a biological substrate and an adhesive formulation. Both materials involved in the adhesive process must be gripped in some manner for an effective measurement to be obtained. Liquid formulations cannot be gripped, thus this test is unsuitable here. A second disadvantage of such a test includes the point of detachment. To effectively measure the strength of the adhesive bond it is this bond that should be broken in the test. However, generally it is the weakest interface that separates. This interfacial associative strength may be assessed but the relevance of such data is dubious.

There are only limited numbers of papers that investigate the bioadhesive properties of liquid-like formulations; these include papers that discuss the in situ gelling of suppositories (Kim et al, 1998) and those that discuss prolonging drug delivery to the

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eye (Cohen et al, 1997; Davies et al, 1991). An ideal method to assess the retention of an alginate dose on œsophageal tissue requires:

• A suitable substrate

• An accurate method to assess the amount of the dose that adheres to the substrate

• Physiological similarity to the real situation

• A means of measuring the duration of the adhesion

• Minimalisation of the errors involved in measuring the adhesion of a dose

Many models were reviewed and investigated in order to reproduce an optimum method for measuring the adhesion of an alginate solution to œsophageal tissue. Teng & Ho (1987) used a method that investigated the retention of polymer coated latex beads on intestinal mucus; this method is alternatively termed the “falling film” method. Within this method coated beads are dispensed onto the intestine substrate then washed with an appropriate medium. The number of particles dispensed was known and those washed off were calculated using a Coulter counter. A similar method was used by Rao & Buri (1989), although they used glass beads and analysed the percentage of beads recovered by gravimetric analysis of the collected material. The falling film method, as described, has proved to be a useful means of assessing the strength and duration of adhesion of polymer coated particles and/or microparticles (Ascentiis et al, 1995).

Other methods that specifically target the œsophagus were also explored to assess their potential as a valid method for this study. These methods included an everted rat œsophagus model used to determine the retention of a sucralfate suspension (Dobrozsi et al, 1999). This is a novel method that uses everted rat œsophageal tissue that is immersed into a solution of the relevant polymer. This method is an adaptation of the tensile strength methods whereby the method has been adapted for use with liquid formulations. The initial mass of polymer retained on the tissue was assessed gravimetrically. The tissue was then washed in an appropriate medium and the mass retained at set time points was measured. This model has been shown to assess the in vitro potential of œsophageal mucoadhesive viscous liquids. One

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disadvantage of this technique is that the method would be difficult to replicate using porcine œsophageal tissue due to the difference in size between the two tissue substrates. The advantages of porcine œsophageal tissue include its histological similarity to human œsophageal tissue and also the low expense and easy availability of this material.

A second method considered was a continuous-flow adhesion cell developed by LeRay et al (1999). A schematic representation of the apparatus used is shown in Figure 4.1. The method consisted of a glass tube with an opening at either end. The substrate was placed inside the tube and the bioadhesive formulation dispensed into the upper end of the tube. The substrate was washed with an appropriate medium and the effluent collected. The formulation was associated with a dye; this enabled colourimetric analysis of the effluent to determine the amount of the formulation removed from the biological substrate. Two substrates were compared; inert polymer tubing and ex vivo intestinal tissue.

Water “ jacket at 37 T

Washing medium input

Substrate

► Collected effluent

Figure 4.1. The continuous-flow adhesion cell apparatus used by LeRay et al (1999)

Another study that investigated adhesion onto the œsophagus used magnetic granules coated with a bioadhesive polymer to localise the formulation within the œsophagus (Nagano et al, 1997). These particles were administered then localised using a magnet. During the period for which the particles were held in contact with the mucosal surface the bioadhesive polymeric material hydrated and adhered to the tissue substrate. This type of formulation has applications in the treatment of

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œsophageal cancer whereby side effects may be minimised due to localisation of the drug therapy.

The method used to assess alginate retention on œsophageal tissue was based on a combination of the falling film method, the everted rat œsophagus and the continuous flow adhesion model. In developing a method for use within this study the first consideration was in producing a method that was similar to the real physiological situation. The substrate used was porcine œsophageal tissue; this was prepared as described in Chapter 2. The method required an alginate dose to be dispensed onto œsophageal tissue then washed with a suitable medium that mimicked saliva flow. The percentage of the dose remaining on and washed off the tissue was calculated as a means of assessing the retention of an alginate dose. The final method used was developed by Banning (1999). Further details about the technique and the apparatus used are provided in the next section.

The aim of this project was to characterise the adhesion of an alginate solution on œsophageal tissue; in this case the role of the bioadhesive agent is in protection of the œsophagus and limiting the damage caused to this organ by the reflux of the stomach contents. The experimental work was divided into three sections; method optimisation, characterisation of the adhesion of an alginate solution on œsophageal tissue and using the adhesive layer as a means of supporting model drug particles.