Tips and tricks for mucoadhesion testing

Mucoadhesion is the process by which a drug delivery device is designed to stick to a part of the gut or other mucosae, thus delivering drug to a precise site in the body for an extended period.  

This gives more effective treatment of some diseases and can also protect drugs from some of the harsh conditions in the body. Mucoadhesive drug delivery systems are used to treat several conditions in the mouth and have been investigated as treatments for stomach ulcers and cancer. 

The majority of infections affecting man and animals take place or start in mucous membranes. The ability to retain pharmacologically-active agents for extended periods of time on any mucosal epithelium, including those of the nose, mouth, rectum or vagina confers several potential therapeutic advantages.  


MEASURING MUCOADHESION

The Mucoadhesion Rig fitted to the TA.XTplus
Texture Analyser; the Gel Mucoadhesion Probe

The TA.XTplus Texture Analyser has emerged as a useful tool for measuring bioadhesion bond strength. The determination of the adhesive properties of pharmaceutical dosage forms is important in their development and several methods have been developed for these measurements.

Tests of solid dosage forms, semi-solids such as ointments and gels and even systems which solidify on contact with the target organ can be performed using alternative measuring probes.

The measurement of adhesive properties has already been reviewed for transdermal adhesive products and the same adhesive test guidelines and curve analysis techniques apply for the measurement of mucoadhesion. Sample preparation and testing method techniques, however, vary depending upon the nature of the sample. The following points aim to provide an understanding of the alternative sample preparation methods, probe options and necessary method adjustments to assist in the design of custom tests, particularly for mucoadhesion testing.

Developed at the University of Strathclyde, and since adopted by a number of groups in Britain and Europe, the Mucoadhesion Rig offers a number of advantages over systems previously used for the assessment of mucoadhesion. Where conditions close to those found in vivo are required, the Mucoadhesion Rig provides the ability to set-up the tissue samples in a vessel of temperature regulated gastric fluid and lower a probe with the attached solid or semi-solid dosage form onto the tissue.
Sample Preparation Alternatives

Porcine mucosa is the membrane typically used for bioadhesive measurements. As the tissue itself is often inconsistent, the sample preparation does demand consistent harvesting, trimming, storing and environmental conditioning of the sample so as not to pose a barrier to the experimentation required to optimise the test methods. Artificial membranes have also been used as these simplify the sample preparation difficulties when using biological membrane.

Several published papers examine the bond of bioadhesives against glass or metal cylinders. Even though bioadhesives can adhere differently to different materials than to tissue, often the relative changes in the adhesiveness correlate well against expectations (due to changes in bioadhesive ingredients). For this reason solid materials (stainless steel, aluminium, acrylic, glass, etc) may be acceptable to examine relative adhesive behaviours, with the added benefit that sample handling protocols are much simpler. 

Where the mucosa is held in ambient conditions without suspension in, for example, gastric fluid, a fixed volume of buffer is generally pipetted onto the mucosa to standardise the hydration prior to testing.

The most common probe sizes and dimensions for bioadhesive testing are acrylic or similar cylinders with a diameter of 7-10 mm. Solid materials such as tablets and films (of standard section size) are usually attached to the underside of the upper testing probe using cyanoacrylate adhesive or double-sided tape (see Donnelly et al).  

For the assessment of powders, the application of the powder can be performed by immersing the probe (with adhesive tape attached to the underside) into a powder bed and thereafter gently shaking the probe to remove any excess, to achieve a monolayer of particles (as investigated by Bredenberg et al). When selecting a double-sided tape the thinnest and stiffest tape possible should be chosen since the material must not be allowed to flex or loosen during debonding.

Samples of gastrospheres have been successfully tested (see Murphy) by previously immersing in simulated gastric fluid for predetermined time intervals, covering both the probe and the test platform with simulated gastric membrane and measuring the bioadhesion (the force of detachment) of samples after applying a force of 2N.

Researchers Skulason, Kristmundsdottir and Holbrook in the Faculty of Pharmaceutical Sciences at the University of Iceland recently presented a technique for evaluation of the adhesion of hydrogel compositions. The use of bioadhesive polymers in the formulation of dosage forms for mucosal drug delivery is receiving increasing attention and whilst the mucoadhesion of solid dosage forms using the Mucoadhesion Rig has been frequently employed by researchers in this field, the measurement of hydrogel mucoadhesion had not previously been thoroughly investigated. 

Mucin discs have been reported to have been prepared and attached to the underside of a cylinder probe using double sided adhesive tape and then lowered onto the surface of the gel formulation applying a downward force for a predefined time and measuring the force required to detach the mucin disc from the surface of each formulation. However, a particular advantage of the Mucoadhesion Rig is the ability to test tissue samples in the conditions in which they are normally found. 

An alternative method is now available which enables attachment of gels to a probe. The Gel Mucoadhesion Probe consists of an inverted cone shape at its end which has machined concentric grooves. These grooves encourage the attachment of a controlled volume of hydrogel sample to the probe surface area. For constant gel volume application the use of a syringe is recommended. A PTFE collar is supplied to support larger volumes of hydrogel loading which is removed when the gel is set. 

Analysis of Adhesion Test Results

As previously explained, during an adhesive test the probe (usually a cylinder probe for mucoadhesion applications) descends to begin the bonding process and maintains the pre-determined compression force for the dwell time. After this time, the probe withdraws from the mucosal tissue and the adhesive properties are measured. Shown here is an annotated adhesive curve. The shape of these graphs have been described by Chuang et al and can be applied to almost every bioadhesive.  

A plot of the measurement from
time zero, when force is being applied
until detachment begins

Once an adhesive test is generated, analysing the debonding behaviour can be much more complex than simply tracking peak force which has commonly been collected in the past. Whilst peak forces for different products can be similar, the debonding behaviour shown by the calculation of parameters from other regions of the curve, such as work of adhesion and debonding distance, can be very different. For the measurement of tensile stress (N/cm2) the detachment force is divided by the surface area of the tablet/gel sample or probe surface area (for films and powders).

A mucoadhesive which must hold a drug against throat linings, where food, liquid and muscles are capable of dislodging it, must hold under very aggressive conditions. Area of work is a much better measure of its adhesive capability than peak force, since the product must withstand the dislodging  forces. Few real forces in many mucoadhesive situations are sharp enough to generate  momentarily high peak forces.

For most adhesive plots, if the distance after the peak force is great (relative to the distance to the peak - and without regard to the areas of work) then the product has likely failed internally, indicating poor cohesion. The same behaviour tends to be true for the area of work after the peak. Thus low ratios of either (post distance)/(pre distance) or (post area)/(pre area) indicate strong relative cohesion; and high ratios would indicate poor cohesion. 

The stringiness or ‘legs’ of bioadhesives are quantifiable by the displacement distances to the peak force and to the final debonding. The adhesive bond of a highly-cohesive bioadhesive (e.g. a transdermal patch) is typically weaker than its adhesiveness to the probe, so separation from the probe can be immediate. Very cohesive products typically leave no residue on the probe since the failure is at the adhesive surface and the force drop-off is crisp.

A few complex polymers will experience early adhesive and cohesive failures, followed by increasing resistance due to the changing and strengthening adhesive filaments or to strain hardening behaviours. It is desirable that a mucoadhesive be extremely cohesive so that it can stay intact as the body attempts to void it.

Poorly cohesive bioadhesives (for example, medicinal eye-drops & mucoadhesives) must nevertheless be sufficiently strong to stay in place during the initial application. When poorly-cohesive bioadhesive debond, they tend to deform in an ‘hourglass’ shape before failing cohesively. These products typically leave residue on the probe surface since the failure is entirely within the adhesive, and the force drop-off tends to be gradual. For such poorly cohesive products higher areas of work indicates that the product has remained intact, perhaps sufficiently for a smaller drug dose to be adequate for the medical challenge.
 
References:

BREDENBERG, S. & NYSTROM, C. (2003).  In-vitro evaluation of bioadhesion in particulate systems and possible improvement using interactive mixtures.  Journal of Pharmacy & Pharmacology, 55, 169-177.

CHUANG, H. K., CHUI, C. & PANIAGUA, R. (1997). Avery Adhesive Test Yields More Performance Data Than Traditional Probe. Adhesives Age, September, 18-23.

DONNELLY, R. F., McCARRON, P. A., ZAWISLAK, A. A. & WOOLFSON, A. D. (2006).  Design and physicochemical characterisation of a bioadhesive patch for dose-controlled topical delivery of imiquimod.  International Journal of Pharmaceutics, 307 (2), 318-25.

MURPHY, C. S., PILLAY, V. & CHOONARA, Y. E. (2008). Design and development of retentive gastrospheres for the delivery of narrow absorption window and low bioavailable drugs.  Paper presented at 35th CRS Meeting, New York, 2008.

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