Dr. Vijaya Khader Former Dean, Acharya N G Ranga Agricultural University Dr

Paper No. 1 : Novel Drug Delivery Systems I

Module No 7 : Factors affecting mucoadhesion and evaluation techniques

Development Team Prof. Farhan J Ahmad Principal Investigator Jamia Hamdard, New Delhi Dr. Vijaya Khader Former Dean, Acharya N G Ranga Agricultural University Dr. Zeenat Iqbal Paper Coordinator Jamia Hamdard, New Delhi

Content Writer Dr. Zeenat Iqbal Jamia Hamdard, New Delhi

ProfDr. KamlaA K Tiwarey Pathak ContentContent Reviewer PunjabiRajv academy University, of pharmacy, Patiala Mathura

Prof. Dharmendra.C.Saxena Dr. Vijaya Khader SLIET, Longowal

Dr. MC Varadaraj

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

Description of Module

Subject Name Pharmaceutical Sciences

Paper Name Novel Drug Delivery Systems I

Module Name/Title Factors affecting mucoadhesion and evaluation techniques

Module Id

Pre-requisites

Objectives

Keywords

Factors affecting mucoadhesion

Polymer related factors 1) Hydrophilicity: Bioadhesive polymers having hydrophilic functional groups, such as carboxyl and hydroxyl interact with the mucosal surface via hydrogen bonding. On contact with the 2

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

aqueous media, the polymer swells, resulting in exposure of potential anchor sites. Further, the swollen polymers have got the maximum distance between their chains which leads to greater level of flexibility and penetration of the polymer in the mucosa.

2) Molecular weight: The mucoadhesive ability of the polymer depends on its molecular weight. The bioadhesive force of the polymer increases up to 100,000 with the molecular weight. However, no further enhancement is observed beyond this value.

3) Cross linking and swelling: the density of cross linking density of the polymer is inversely related to its swelling. Low density leads to high degree of swelling and thus larger surface area and better mucoadhesion. It also leads to increased hydration rate and increased flexibility. However increased hydration and swelling may lead to slippery surface of the polymer which may result in its rapid removal from the biological substrate decreasing the retention time. The mucoadhesion can further be increased by including adhesion promoters such as polymers grafted onto the preformed network and free polymer chains.

4) Spatial Conformation: spatial conformation of the polymers determines its adhesive strength. Non-Linear polymers or polymers with helical structure may shield many active groups responsible for adhesion. E.g. dextran despite having a higher molecular weight of 19,500,500 have an adhesive strength similar to PEG with a much lower molecular weight of 200,000. This is because of the helical confirmation of dextran as compared to the linear PEG.

5) Concentration of active polymer: it has been found that the concentration of polymer influences bioadhesion. Beyond an optimum concentration of the polymer, a significant reduction in the bioadhesive capacity is observed. In concentrated solution, the coiled molecules become solvent deprived and the availability of chains to interpenetrate with the mucosal surface becomes less.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

This result is mainly applicable to liquid mucoadhesive formulations. However for solid dosage forms such as tablets, higher polymer concentration, results in stronger mucoadhesion.

6) Hydrogen bonding capacity: hydrogen bonding capacity of the polymer is an important factor that determines bioadhesion. The polymers must have functional groups that are able to form hydrogen bond for bioadhesion.

Environment related factors

1) pH: The pH at the interface of bioadhesive and the biological surface affects bioadhesion especially in case of bioadhesives with ionizable groups. Several bioadhesives are polyanions possessing carboxylic acid functional groups which are used in drug delivery. If the pH of the microenvironment is more than the pK of the polymer, it will be largely ionized and if the pH is below the pK of the polymer, it will be largely unionized. The maximum bioadhesive strength of poly (acrylic acid) polymers (pK=4-5) is observed around pH 4–5 and decreases gradually when it is above the pH of 6. There are numerous investigations which have shown that the carboxyl groups which are protonated rather than the ionized react with mucin molecules by forming hydrogen bonds with it.

2) Applied strength: For applying a mucoadhesive system, a definite strength is required. Higher shear force may lead to increased interaction and higher bioadhesive strength it affects the interaction of the adhesive with the mucous and hence determines the depth of penetration of the polymeric chains. . Also greater the contact time between the mucoadhesive and the mucosal surface greater the swelling and interpenetration of polymer chains.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

3) Moistening: Moistening or wetting is required to allow the mucoadhesive polymer to spread over the mucosal surface and create a macromolecular network of sufficient size for the interpenetration of the polymer and mucin molecules and to increase the mobility of the polymer chains.

4) Presence of metal ions: ionic strength of the surrounding environment affects mucoadhesive bond. Interaction with charged groups of polymers and or mucus can shields the number of interaction sites required for adhesion and tightness of mucoadhesive bonding. However some polymers like gellan are reliant on the presence of divalent cations for in situ gelation.

Physiological factors

Mucin turnover Physiological conditions such as rate of mucus turnover as in disease state or in the presence of food affects mucoadhesion. Mucosal turn over limits the residence time of the mucoadhesive system resulting in detachment and excretion of mucoadhesive. Thus the contact time of the mucoadhesive with the mucosal surface depends on the turnover rate. In some tissues like oral and vaginal the mucous turnover rate is low as compared to intestines which has a high mucous turnover rate limiting the contact time of the adhesive polymer to not more than a few hours. In addition mucous turnover results in increased amount of soluble mucin molecules which interacts with the mucoadhesives before interacting with the mucosal layer.

Disease state The properties of the mucus get changed in various disease conditions such as gastric ulcers, common cold, cystic fibrosis, ulcerative colitis, bacterial and fungal infections of reproductive tract and during inflammatory conditions of the eye. Thus mucoadhesives need to be evaluated used in such conditions.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

Rate of renewal of mucoadhesive cells

Rate of renewal of mucoadhesive cells varies for different types of mucosal surface and limits the residence time of the bioadhesive on the substrate.

Mucosal viscosity

Mucosal layer viscosity varies throughout the body, and also in disease states. Low mucus viscosity results in a weak interaction with the mucoadhesive polymer resulting in detachment and poor retention, whereas, a highly viscous mucus layer, such as those thickened due to white blood cell DNA, dead cells and inflammatory mediators restricts the interpenetration and increases the diffusion pathway through which the active agent must pass

Tissue movement

The movement of tissue in case of liquid and food uptake, speaking, peristalsis affects the retention of mucoadhesive system especially in case of gastroretentive systems.

Techniques for the determination of mucoadhesion:

The evaluation of bioadhesive properties is a fundamental tool in the development of novel bioadhesive drug delivery systems. The instrumental techniques used for the determination of mucoadhesion are in vitro methods for direct or indirect assessment of mucoadhesion of the system under study and involve the use of various techniques such as texture analysis, AFM (Atomic force microscopy), wetting measurements, rheological measurements and zeta potential measurements. The in vitro/ Ex vivo tests are important in the development of slow (controlled/ sustained) release mucoadhesive systems because they involve studies of mechanical and physical stability, interaction between formulation and mucous

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

membrane and strength of the bioadhesive bond. The majority of techniques for testing mucoadhesion are in vitro; however some in vivo methods have also been entailed.

Since there are no standard apparatus available for bioadhesive strength testing, however a few test methods have been employed:

Application of force: The commonly used technique of bioadhesive testing for the determination of force of separation is the application of force perpendicularly to the incised tissue/ adhesive interface, during which a state of tensile stress is established. But during the shear stress, the direction of the force is reoriented so that it acts along the joint interfaces. In both tensile strength and shear stress modes, an equal pressure is distributed across the contact area.

Peel test:

The aim of a peel test is to determine the adhesive strength of the material or the strength or force of the adhesive bond between two materials.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

The peel test is based on calculating the amount of energy required to detach the patch from the surface of the substrate. The peel test offers a limited use in most bioadhesive systems. Peel test is of value when the bioadhesive system is designed as a patch. In peel test stress is focused at the edge of the joint unlike in tensile and shear experiments in which stress is uniformly distributed over the adhesive joint.

The common types of peel tests for the measurement of adhesiveness are the T-peel test, 90 degree peel test, and the 180 degree peel test.

T-peel test: T-peel test is the type of tensile test that is performed upon two flexible substrates which have been bonded together and are placed into peel test grips such that one substrate sticks upside and the other sticks downwards while the bonded area sticks out horizontally so that the entire system forms a “T” shape.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

90 degree peel test: 90 degree peel test requires a fixture of 90 degree peel test to determine the adhesive strength between a rigid substrate (plate) and flexible (tape), where the plate lies in a horizontal position with the gripped end of the tape sticking up perpendicular while the rest of the system is bonded to the plate so that it forms the “L” shape.

180 degree peel test: 180 degree peel test is almost similar to the 90 degree peel test with an exception that the bonded area between the plate and tape is placed vertically between the peel test grips, although the free end of the of plate is held by the bottom and the free end of the tape is held by the top so that it forms a firmly tight “U” shape.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

b) Shear stress and Tensile Stress

It measures the force required to cause the bioadhesive to slide with respect to the mucus layer in a direction parallel to their plane of contact. Such force acts tangentially to a surface and can be calculated by dividing it by the area of the surface. It is the force per unit area required to sustain a constant rate of fluid movement. Smart and coworkers (Smart et al., 1984) had earlier described the Wilhelmy plate method primarily used for study of attachment of polymeric films to mucin. There are three typical definitions of tensile strength:

1) Yield Strength: The stress a material can withstand without permanent deformation.

2) Ultimate Strength: The maximum stress a material can withstand.

3) Breaking Strength: The stress coordinate on the stress– strain curve at the point of rupture

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

Rheological technique: The changes in the rheological properties of mucoadhesive polymer and the mucin may be due to physical entanglement (chain interlocking), conformational changes and chemical interactions including non-covalent bonds such as hydrogen, electrostatic and hydrophobic bonding and covalent bonds such as disulfide bridges, which may occur alone or in combination between the two macromolecular species. Hence, the rheological evaluation of mucus and polymer mixtures can give information on the extent and magnitude of interaction between mucus and polymer, since the increase in viscosity resulting from mixing of mucus and polymer has been claimed to correlate with mucoadhesion. The rheological method should not be used independently as a method for studying mucoadhesion properties of polymer gels. Hassan and Gallo proposed an empirical equation for the viscosity changes due to interaction between the polymer and mucin. In this method, rheological interactions between mucin and polymer gel solution was determined. The mucoadhesion ability of various polymers could be screened using their ηb values from the following equation:

ηt = ηm + ηp + ηb,

Where ηt is the viscosity of the solution containing both mucin and polymer, ηm and ηp are the individual viscosities of mucin and polymer, respectively, and ηb is the contribution that appears to be an outcome of molecular interaction between the polymer and mucin. Hence, viscosity synergism could be used as an in vitro parameter to measure and compare the mucoadhesion properties of various polymers.

The other parameters that were used to quantify the mucoadhesion interactions were the loss tangent and rheological synergism. The viscoelastic properties of the samples could be described using the loss tangent (tan δ), which is an indicator of the overall viscoelasticity of the sample, being a measure of the energy loss to the energy stored per cycle (G’’/G’). tan δ < 1 indicates a solid or gel-like component or elastic behavior, whereas tan δ > 1 indicates a liquid or viscous-like component. When tan δ becomes smaller, the elasticity of the material is increased with a reduction in viscous behavior. The rheological synergism or interaction between the polymer and mucin (∆G’ and ∆G’’), which is the difference between the actual viscoelastic values of the mixture of polymer and mucin (G’ mix and G’’ mix) and

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

the theoretical values defined as the sum of the viscoelastic components of polymer (G’p and G’’p) and mucin (G’ m and G’’ m), could be calculated from the following equations:

∆G’ = G’mix – (G’p + G’m) ,

∆G’’ = G’’mix – (G’ p + G’’ m) .

Sriamornsak and Wattanakorn studied the rheological synergism of mucin with various types of pectins and concluded that molecular interpenetration of the two macromolecules was involved in the mucoadhesion phenomenon as evidenced by an increase in dynamic modulus and a decrease in loss tangent. Moreover, the elastic modulus for a polymer alone could be compared with that of polymer/mucin mixture and an increase in the elastic modulus for the mixture compared to the polymer is interpreted as a positive interaction causing mucoadhesion. Madsen et al. investigated rheological synergism for a large range of putative mucoadhesive gels by dynamic oscillatory rheology and concluded that macromolecules possessing numerous hydrogen bond-forming groups and an open expanded network in the test environment gave pronounced rheological synergism.

In vivo testing: Recently in vivo aspects of mucoadhesive testing have been reported to observe the mucoadhesion on tissue surface such as the buccal cavity or GIT. However, there are only a limited number of in vivo studies reported in the literature which could be because of the cost, time and/ or ethical constrains. The commonly used in vivo techniques to observe mucoadhesion include GI transit times (residence time) of bioadhesive/ mucoadhesive-coated particles and drug release from in situ bioadhesive/ mucoadhesive devices.

The in vivo delivery time of a mucoadhesive-based dosage form can be measured by three techniques which include gamma scintigraphy, perfused intestinal loops and transit studies with radiolabelled dosage forms.

Ch’ng studied the in vivo transit time/ residence time for bioadhesive beads in the GIT of rat. He inserted 51Cr-labeled bioadhesive bead at specific time intervals. GITs of rat were removed and then cut into 20 equal segments. Finally the radioactivity was measured out

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques

The noninvasive in vivo technique was monitored by Davis to determine the transit time of mucoadhesive agent. In his study a formulation containing a gamma-emitting radionuclide was used. The release characteristics and subsequently the position polymer was examined by gamma scintigraphy.

Nowadays, magnetic resonance imaging (MRI), a noninvasive technique is widely used. Christian Kremser used MRI technique to detect the time and location of release of bioadhesive/ mucoadhesive formulation using dry Gd-DOTA powder.

Bioavailabilty Studies: A few research groups have investigated the bioavailability of bioadhesive dosage forms using human volunteers or animal models. The results are controversial because some results were contrary to the in vitro results and some did not show the same extent of improvement as shown in the in vitro test due to complications in the in vivo situations.

Pharmaceutical Novel Drug Delivery Systems I sciences Factors affecting mucoadhesion and evaluation techniques