PHYSIOLOGY OF MUCOSAE AND MECHANISM OF

TRANSMUCOSAL PERMEATION

Prepared by :Ms Arpita Patel

Guided by:Mr Ravish Patel

Ramanbhai Patel of college of pharmacy

INTRODUCTION TO MUCOSA

Mucosa is moist tissue that lines some organs and body cavities throughout the body, including your nose, mouth, lungs, and digestive tract.

Glands along the mucosa release mucus (a thick fluid). Mucus is a translucent and viscid secretion which forms a thin, continuous gel blanket adherent to the mucosal epithelial surface.

It has the following general composition: Water 95% Glycoprotiens and lipids 0.5-5% Mineral salts 1% Free proteins 0.5-1%

TRANSMUCOSAL ROUTES OF DRUG DELIVERY

The various routes of transmucosal delivery includes the mucosal linings of the

Nasal,

Rectal,

Vaginal,

Ocular, and

Oral cavity

Advantages of Transmucosal RoutesPossible bypass of first pass metabolism

Avoidance of presystemic elimination within the GI tract,

Depending on the particular drug, a better enzymatic flora for drug absorption.

Because of dual biophysical and biochemical nature of these mucosal membranes drugs with hydrophilic and /or lipophilic characteristics can be readily absorbed.

Disadvantages of transmucosal routes

For buccal route:

The tablet must be kept in place and not chewed or swallowed.

Excessive salivary flow may cause a very rapid dissolution and absorption of the tablet and wash it away.

A bad tasting tablet will have a low patient acceptability.

For nasal route: Disease conditions of the nose may result in impaired absorption.

Dose is limited because of relatively small area available for absorption.

Time available for absorption is limited.

Little is known of the effect of the common cold on transmucosal drug delivery and it is likely that instilling a drug into a blocked nose or a nose with surplus of watery rhinorrhea may expel the medication from the nose.

Not applicable to all the drugs. Polar drugs and some macromolecules are absorbed in sufficient concentration because of poor membrane permeability, rapid clearance and enzymatic degradation into nasal cavity.

For rectal route:

Some hydrophilic drugs such as antibiotics and peptide drugs are not easily absorbed from the rectum and absorption enhancers are needed.

Drugs may cause rectal irritation and sometimes proctitis with ulceration and bleeding.

Human mucosaNASAL MUCOSA :

The surface of human nasal mucosa is lined with both ciliated columnar epithelium, which covers the nasal septum and turbinates, and squamous cutaneous epithelium.

An individual cilium on the columnar epithelial surface is approximately 5 microns in length and 0.2 microns in width. Within the mucosa there exist goblet cells, which produce nasal secretions with a pH of 7.4.

On the surface of each goblet cell there are hundreds of club like microvilli. Beneath the ciliated epithelium and goblet cells several layers of flat polygonal basal cells, which have microvilli-like processes, exist.

In the intercellular spacings there exists a composition of homogenous mucoprotein-like substances. The basement membrane is composed of parallel and transverse reticulum, fibres and connective tissues.

ORAL MUCOSA:

It consists of two parts: the underlying epithelium and the connective tissues. The human oral mucosal epithelium shows several distinct patterns of maturation that may be related to the different functions of the mucosa at the various regions of the oral cavity.

The epithelium is of the stratified squamous type and varies in its thickness and the extent of keratinized epithelium may coexist with, or be replaced by, Para keratinized epithelium.

The major function of the oral epithelium is to provide a protective surface layer between the oral environment and the deeper tissues.

The epithelium of the buccal mucosa is about 40-50 cell layers thick, while that of the sublingual epithelium contains somewhat fewer. The epithelial cells increase in size and become flatter as they travel from the basal layers to the superficial layers.

The oral cavity also consists of specialized epithelial tissues that surround the teeth and serve as a lining. These tissues are: masticatory mucosa, lining mucosa, and specialized mucosa.

A) Masticatory Mucosa: Masticatory mucosa is comprised of the tissue that covers the hard palate and the gingiva.

Masticatory mucosa is usually light pink in color and is keratinized. Keratinized tissue has a horny, tough, protective outer layer of tissue. Characteristics of masticatory mucosa are: no submucosa lies under the masticatory mucosa, held in place firmly to bone and does not move, has a dense, hard covering, and functions to withstand the active process of chewing and swallowing food.

B) Lining Mucosa: Lining mucosa is found on the inside of the lips, cheeks,

vestibule, soft palate, and under the tongue. It consists of a thin, fragile tissue that is very vascular. Lining mucosa is brighter red in color than masticatory mucosa. Also included in the lining mucosa is alveolar mucosa. It lies apical to the mucogingival junction and is loosely attached.

C) Specialized Mucosa: GINGIVA. The gingiva is specialized masticatory mucosa covering the alveolar process. In a healthy mouth, gingiva is firmly in place encircling the necks of the teeth. It aids in the support of the teeth, and protects the alveolar process and periodontal ligament from bacterial invasion. Healthy gingiva is firm and resilient. Healthy gingiva under normal flossing and brushing activities does not bleed. The color of healthy gingiva can range from pale pink to darker shades (purple to black) depending on each individual's pigmentation.

Within the oral mucosal cavity, delivery of drugs is classified into three categories:

Sublingual delivery, which is systemic delivery of drugs through the mucosal membranes lining the floor of the mouth.

Buccal delivery, which is drug administration through the mucosal membranes lining the cheeks (buccal mucosa)

Local delivery, which is drug delivery into the oral cavity.

Some advantages of oral route:

Highly acceptable by patients The mucosa is relatively permeable with a rich blood supply Shows short recovery times after stress or damage The virtual lack of Langerhans cells makes the oral mucosa

tolerant to potential allergens.

RECTAL MUCOSA: The human rectal mucosa is composed of the epithelium, the lamina propria,

and the double –layer muscularis mucosae.

The epithelial surface consists of closely packed columnar cells with some areas interrupted by crypt regions. Within the crypt regions there are mucus producing goblet cells.

The lamina propria consists of two layers: a dense acellular collagen layer and a loose connective tissue layer. Within the lamina propria there exist superficial blood vessels and inflammatory cells, including macrophages, eosinophils, and lymphocytes

The muscularis mucosa contains smooth muscle cells and larger blood vessels. The total volume of mucous is estimated as approximately 3 ml, spread over a total surface area of approximately 300cm2. The pH of the mucous layer is reported approximately 7.5.

VAGINAL MUCOSA

The vagina is the lower part of the female reproductive tract. It is a muscular tube lined with mucous membrane which is covered with a layer of stratified squamous epithelium with an underlying layer of connective tissue (lamina propria).

The human vaginal epithelium is composed of noncornified, stratified squamous cells, similar to those of the buccal mucosa and somewhat similar to the skin epithelium.

The vaginal epithelium is composed of five different cell layers:- Superficial (about 10 rows of cells): large polygonal cells with a high

degree of proliferation.

Transitional (about 10 rows of cells)

Intermediate (about 10 rows of cells)

Parabasal (2 rows of cells)

Basal (single row of cells)

The lamina propria is composed of dense connective tissue, consisting of collagen fibres, ground substance, and cells such as fibroblasts, macrophages, mast cells, lymphocytes, langerhans cells, plasma cells, neutrophils, and eosinophils .

It contains a blood supply, a lymphatic drainage system, and a network of nerve fibres.

It is through the blood vessels in the lamina propria that drugs can gain entry to the systemic ciruculation.

Lymph drainage from the vagina takes place to the iliac sacral, gluteal, rectal, and inguinal lymphatic nodes.

OCULAR MUCOSA: Conjunctiva (named because it conjoins the eyeball to the lids) is a thin, transparent mucous membrane that lines the posterior surface of the lids, and is then reflected forwards on the eye.

The conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. It is made of epithelial tissue.

The palpebral portion lines the eyelids; that portion joining the eyeball is the bulbar conjunctiva and that forming the conjunctival sac and reflecting on the eye is called the fornix. The palpebral conjunctiva is subdivided into marginal, tarsal, and orbital zones.

The marginal zone transitions between skin and conjunctiva and shows minimal keratinization. The tarsal conjunctiva is a fairly flat layer. The orbital zone shows more numerous Goblet cells

The limbus (1) is the junction of the conjunctiva and cornea.

The bulbar conjunctiva (2) covers the eyeball and extends into the recess created by forniceal conjunctiva (3).

The tarsal conjunctiva (4) covers the tarsus.

The marginal conjunctiva (6) is at the eyelid margin where the epithelium will begin to be keratinized.

The punctum (5) is also shown.

The histology of the conjunctiva varies according to its topographic location. The bulbar conjunctiva is relatively less undulating and contains fewer Goblet cells.

In this electron micrograph the microvilli cover the superficial layer of the epithelium (arrow 1) and the mucin granules (2) of the Goblet cell are captured in a plane lacking contiguity with the surface. The Goblet cells produce gel forming mucins called MUC5AC that may be critical to provide lubrication to the ocular surface.

The tarsal conjunctiva shows a stratified squamous epithelium (1) that has few Goblet cells (none seen here) overlying a very dense fibrous stroma, tarsus (2). Meibomian glands are embedded in the tarsus

Biochemical and Metabolic characteristics of Mucosa

The surface environment of the nasal, rectal and vaginal mucosae is influenced, to a great extent, by the presence of mucus secreted by the goblet cells. These secretions, which contain proteolytic enzymes and immunoglobulins, may add both enzymatic and diffusional barriers to the mucosal absorption of drugs.

Drugs may be subject to metabolism during the course of transmucosal permeation, either in the mucosal surface microenvironment or in the mucosal membrane.

For e.g. Peptidases have been noted to be present in the nonoral mucosae, including nasal, rectal, and vaginal mucosal homogenates.

Vaginal secretions also contain both glucose and glycogen; both are converted by enzymes and bacteria to lactic acid, resulting in mucus with pH of 4-5.

Intestinal mucosa:

First-pass intestinal metabolism. This includes brush border metabolism and intracellular metabolism. The former occurs at the surface of the enterocytes by the enzymes present within the brush border membrane.

Furthermore, the brush border activity is generally greater in the proximal small intestine (duodenum _ jejunum > ileum _ colon) and involves enzymes such as alkaline phosphatase, sucrase, isomaltase, and a considerable number of peptidases.

The intracellular metabolism occurs in the cytoplasm of enterocytes and involves the major class of phase I metabolizing enzymes (i.e., cytochromeP450s, in particular CYP3A4); several phase II conjugating enzymes, and others such as esterases.

It is obvious that intestinal epithelium as a site of preabsorptive metabolism may significantly contribute to the low bioavailability of therapeutic peptides and ester type drugs like aspirin, although it could serve as a key site for targeted delivery of ester or amide prodrugs.

Nasal mucosa:

The nasal route of drug administration avoids the first pass metabolism in liver, but nasal mucosa does possess enzymatic activity as a protective mechanism against a protective mechanism against exogenous chemicals.

Nasal first pass metabolism may be a significant factor in the absorption of some drugs. For example there is a high content of cytochrome P450 enzymes; P450 monooxygenases can oxidize many nasally administered drugs, such as nasal decongestants and anaesthetics

There are many other types of enzymes in the nasal mucosa which can act on conventional drugs. Examples include dehydrogenases, hyroxylases, carboxylesterases, carbonic anhydrase and various phase II conjugative enzymes.

Transmucosal systemic delivery of drugs:

Trans mucosal drug delivery has the potential to achieve greater systemic bioavailability for orally metabolized drugs, including organic and peptide based pharmaceuticals.

For e.g. for organic based pharmaceuticals, such as progesterone, it was demonstrated that Trans mucosal delivery high systemic bioavailability, especially the nasal route, which has bioavailability 5-10 times greater than that by oral administration.

Peptide/protein drugs are increasingly becoming a very important class of therapeutic agents. These drugs are easily degraded by proteolytic enzymes in the gastrointestinal tract and thus are generally not suitable for oral administration.

Currently, they are mostly delivered by parenteral administration. Because they are extremely short-acting, repeated injections are often required. To minimize the health hazard by constant injection, there was an urgent need to search for non-parenteral routes of administration as well as to develop formulations with controlled delivery features

Routes of administration that have been widely investigated for this purpose include nasal, ocular, rectal, buccal and transdermal.

However, the transmucosal delivery of peptide-based pharmaceuticals, such as insulin, has achieved a much lower systemic bioavailability than parenteral administration. The lower extent of transmucosal absorption of insulin and many other peptide-based pharmaceuticals is probably due to a combined effect of poor mucosal permeability and extensive metabolism at the absorption site.

Mechanisms of Transmucosal Permeation:

There are two permeation pathways for passive drug transport across the oral mucosa:

PARA CELLULAR ROUTE.

TRANS CELLULAR ROUTE.

The Para cellular route:

Low molecular weight, water soluble compounds may transverse the mucosa via the Para cellular route, moving between the junctions of the epithelial cells.

The major junctional attachment between the epithelial cells is the desmosome, which displays minimal impedance to intercellular diffusion.

Thus in the majority of the cases, drug absorption for small hydrophilic moieties is thought to occur via Para cellular penetration, moving between the cells, as claimed for drug transport through the epidermis of the skin.

The Transcellular route:

Transcellular passive diffusion:

Low molecular weight, lipophilic drugs may be absorbed transcellularly, by passive diffusion across the cells of the epithelium.

Again, movement occurs down a concentration gradient, according to Fick’s Law. The stratified nature of epithelium means that lipophilic moieties must permeate across several layers of cells to reach the underlying blood capillaries.

Carrier mediated processes

The oral mucosa contains active or carrier mediated systems for small molecules such as monosaccharides and amino acids. However, these processes have not been fully characterized in terms of location, transport capacity or specificity

Endocytic processes:

Endocytosis means a process of cellular ingestion by which the plasma membrane folds inward to bring substances into the cell.

These are presently poorly characterized in the oral mucosa. However, as the oral cavity becomes increasingly important as a potential site for systemic absorption, particularly for high molecular weight drugs which are generally thought to cross epithelial cells endocytically

Factors affecting Transmucosal Permeation

MOLECULAR SIZE AND WEIGHT: The absorption of small compounds is high at around 80%, but it decreases as molecular weight increases.

EFFECT OF pH AND PARTITION CO-EFFICIENT: More lipophilic drugs are likely to travel probably by partitioning across the mucosal cell membranes and diffusing through the cells (Transcellular route) at a slower rate than through the paracellular route. Most drugs can be ionized and their partition co-efficients are dependent upon environmental pH. The effect of pH on peptide drugs is more complex than on conventional drugs, as peptides have a large number of ionizable groups of either charge. Peptides are characterized by their isoelectric point, the pH at which they have no net charge and where their solubility is often lowest.

SOLUBILITY OF DRUG AND DISSOLUTION RATE: This is quite important when drug is give as solid dosage form (e.g. powder), as it must be able to cross the mucous layer before it can be absorbed by the epithelial cells. In addition, powder morphology and particle size influence the deposition of the drug inside the cavities.

Ability of compound to form hydrogen bond with the component of the membrane.

SPECIFICALLY FOR NASAL ROUTE: The rate of nasal secretion: the greater the rate of secretion, the

lesser the bioavailability of drug. Ciliary movement: the greater the ciliary movement, the lesser

the bioavailability of drug. Vascularity of the nose: increase of blood flow increases the

absorption of drugs.

DISEASE CONDITION: example common cold, conjunctivitis, keratitis, etc.

SURFACE AREA OF MEMBRANE: the greater the surface area, more will be the absorption.

OTHER ADDITIVES: The inclusion of variety of additives to the formulation also affects the transmucosal permeation. Example- inclusion of viscosity increasing additives like colloidal silicon oxide or aluminium monostearate will create gel like system with a slower rate of the drug.

Permeation Enhancers: For increasing the transmucosal permeability we can do the

following:

1) Improve the residence time of drug in the respective cavities:

e.g. Mucociliary clearance acts to remove foreign bodies and substances from the nasal mucosa as quickly as possible. The preparation can be formulated with polymers like HPMC, MC, or polyacrylic acid (carbopol), which increase the viscosity of the formulation and act as bioadhesives with the mucus.

2) Modify the structure of the drug to change the physicochemical properties. E.g. increasing the aqueous solubility or improving the partitioning characteristics. However this task requires the regulatory approval, therefore its too costly and time consuming.

3) Enhancement of Absorption: absorption enhancers work by increasing the rate at which drugs pass through the mucosa. Many act by altering the structure of the epithelial cells in some way, but they should accomplish this while causing no damage or permanent change.

General requirements of an ideal absorption enhancer are:

It should be pharmacologically inert

Non irritating, Non toxic, Non allergic

Its effect on the mucosa should be reversible

Compatible with the drug

Should remain in contact with the mucosa long enough to achieve maximal

effects

Should not have any offensive odor or taste

Readily available and inexpensive

The major reason for developing enhancers is to increase the absorption of peptides and proteins, because their size leads to a relatively poor bioavailability.

Various permeation enhancers:

BILE SALTS: Bile salts have greater potential as they appear to possess much of enhancing activity but less of damaging potential of surfactants. Commonly studied bile salts include sodium cholate, sodium deoxycholate, sodium glycocholate, and glycodeoxycholate, sodium taurocholate and taurodeoxycholate.

Mechanisms by which the increase in permeation occurs by bile salts are:

Increasing cell membrane permeability by forming temporary channels through the lipid structure.

Forming intercellular aqueous pores by opening the tight junctions between cells.

Increasing the lipophilicity of charged drugs by forming ion pairs.

Inhibition of proteolytic enzymes.

Opening of intercellular channels is most likely than increasing cell permeability, as the latter would cause massive disruption of cell.

SURFACTANTS: Sodium tauro-24, 25-dihydrofusidate (STDHF) is an enhancer with similar structure to bile salts. It has good aqueous solubility and stability and is surface active, forming micelles at a critical micelle concentration of 2.5Mm.

Another group of surface active materials is the phosphatidylcholines example lysophosphatidylcholine. They disrupt the cell membranes and increase its permeability. They may also inhibit the proteolytic enzymes and lysophosphatidylcholine is also mucolytic.

Azone (laurocapram) is used for transdermal as well as for buccal drug delivery. It is a lipophilic surfactant. Other surfactants include sodium dodecyl sulfate, polysorbates, benzalkonium chloride. These are predominantly water soluble and can form associations in aqueous solution.

FATTY ACIDS AND THEIR SALTS AND ESTERS:

Oleic acid, lauric acid, and cod liver extract and their salts like sodium laurate, sodium caprate and esters like gylceryl monostearate, diethylene glycol monoethyl ether and various sucrose fatty acids esters are widely used.

The unsaturated fatty acids such as oleic acid acts by decreasing lipid order and increasing fluidity due to their kinded molecular conformation arising from the double bond in the hydrocarbon chain.

CONCLUSION:

Interest in TRANSMUCOSAL DRUG DELIVERY as a non-invasive route for drug delivery continues to grow rapidly.

The mucosa offers numerous benefits as a target issue for drug delivery, such as a large surface area for delivery, rapid drug onset, potential for central nervous system delivery, and no first-pass metabolism.

A wide variety of therapeutic compounds can be delivered, including relatively large molecules such as peptides and proteins, particularly in the presence of permeation enhancers

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