Drug Delivery to the Oral Cavity or Mouth

ANATOMY AND PHYSIOLOGY The oral cavity The palate The tongue The teeth

Organisation of the oral mucosa Functions of the oral mucosa Salivary secretion

Salivary glands Saliva

MIGRATION AND CLEARANCE OF SUBSTANCES FROM THE ORAL CAVITY ABSORPTION OF DRUGS ACROSS THE ORAL MUCOSA Disadvantages of oral mucosal delivery Effect of position on drug delivery Gingival penetration

Improving penetration through the mucosa MEASUREMENT OF ORAL MUCOSAL DRUG ABSORPTION DOSAGE FORMS FOR THE ORAL CAVITY Chewable formulations Fast-dissolving dosage forms Bioadhesive dosage forms Dental systems DRUGS ADMINISTERED VIA THE ORAL MUCOSA Nitrates Steroids Analgesics Antibiotics Antifungals Others CONCLUSIONS REFERENCES

ANATOMY AND PHYSIOLOGY The oral cavity

The oral cavity or mouth is the point of entry of food and air into the body and the mouth and lips are essential to humans to allow speech by modifying the passage of air. This structure is also referred to as the buccal cavity, but strictly speaking this should be confined to the inner cheek area. The mouth extends from the lips to the oropharynx at the rear and is divided into two regions: (a) the outer oral vestibule, which is bounded by the cheeks and lips, and (b) the interior oral vestibule, which is bounded by the dental maxillary and mandibular arches. The oral cavity proper is located between the dental arches on which the teeth are situated. It is partly filled by the tongue, a large muscle anchored to the floor of the mouth by the frenulum linguae (Figure 3.1). At the back of the oral cavity are large collections of lymphoid tissue forming the tonsils; small lymphoid nodules may occur in the mucosa of the soft palate. This tissue plays an important role in combating infection.

The palate

The palate is located in the roof of the mouth. It separates the nasal and oral cavities. It consists of an anterior hard palate of bone and, in mammals, a posterior soft palate that has no skeletal support and terminates in a fleshy, elongated projection called the uvula. The hard palate, which composes two-thirds of the total palate area, is a plate of bone covered by a moist, durable layer of mucous-membrane tissue, which secretes small amounts of mucus. This layer forms several ridges that help grip food while the tongue agitates it during chewing. The hard palate provides space for the tongue to move freely and supplies a rigid floor to the nasal cavity so that pressures within the mouth do not close off the nasal passage. The soft palate is composed of muscle and connective tissue, which give it both mobility and support. This palate is very flexible; when elevated for swallowing and sucking, it completely blocks and separates the nasal cavity and nasal portion of the pharynx from the mouth and the oral part of the pharynx. While elevated, the soft palate creates a vacuum in the oral cavity, which keeps food out of the respiratory tract.

Gingival Palatal

Membrane Membrane

Gingival Palatal

Membrane Membrane

Sublingual Membrane

Figure 3.1 Cross section through the oral cavity

Oesophagus

Sublingual Membrane

Figure 3.1 Cross section through the oral cavity

The tongue

In humans, the tongue aids in creating negative pressure within the oral cavity that enables sucking, and it is an important accessory organ in chewing, swallowing and speech. The tongue consists of a mass of interwoven, striated muscles interspersed with glands and fat and covered with mucous membrane. The ability of the tongue to touch the lips and teeth aids swallowing and speech. The top surface, or dorsum, contains numerous projections of the mucous membrane called papillae. They contain taste buds sensitive to food flavours and serous glands that secrete some of the fluid in saliva. The base, or upper rear portion, of the tongue has no papillae, but aggregated lymphatic tissue (lingual tonsils) and serous and mucus-secreting glands are present. The inferior, or under surface leads from the tip of the tongue to the floor of the mouth; its mucous membrane is smooth and purple in colour from the many blood vessels present. The root, the remainder of the underside that lies on the floor of the mouth contains bundles of nerves, arteries, and muscles that branch to the other tongue regions. Nerves from the tongue receive chemical stimulation from food in solution which gives the sensation of taste. There are four fundamental taste sensations: salt and sweet, the receptors for which are primarily located at the tip of the tongue, bitter at the base, and acid or sour along the borders. The flavour of a food comes from the combination of taste, smell, touch, texture or consistency, and temperature sensations.

The teeth

Teeth cut and grind food to facilitate digestion. A tooth consists of a crown and one or more roots. In humans, they are attached to the to the tooth-bearing bone of the jaws by a fibrous ligament called the periodontal ligament or membrane. The neck of the root is embedded in the fleshy gum tissue. Cementum is a thin covering to the root and serves as a medium for attachment of the fibres that hold the tooth to the surrounding tissue (periodontal membrane). Gum is attached to the alveolar bone and to the cementum by fibre bundles.

Caries, or tooth decay, is the most common disease of the teeth among humans. Tooth decay originates in the build-up of a yellowish film called plaque on teeth, which tends to harbour bacteria. The bacteria that live on plaque ferment the sugar and starchy-food debris found there into acids that destroy the tooth's enamel and dentine by removing the calcium and other minerals from them. Alkali production from urea by bacterial ureases in the oral cavity is thought to have a major impact on oral health and on the physiology and ecology of oral bacteria1. Another common dental disorder is inflammation of the gum, or gingivitis. It usually commences at or close to the gum margin, often between adjacent teeth. Pockets form between the gum and the adjacent teeth, sometimes penetrating deeply into the tissues. This leads to further infection, with inflammation and bleeding from the infected gums. A principal cause of gingivitis is the build-up of plaque on teeth, which causes irritation of the gums and thus leads to their inflammation and infection.

Organisation of the oral mucosa

The oral cavity and vestibule are entirely lined by relatively smooth mucous membranes containing numerous small glands (Figure 3.2). It is divided into a) the oral epithelium, b) the basement membrane, which connects the epithelium to the connective tissue, c) the lamina propria, which is underlying connective tissue and d) an area which contains loose fatty or glandular connective tissue and major blood vessels and nerves. It is often referred to as the muco-periostium. These tissues are laid over a layer of muscle or bone. To a certain extent, the structure of the oral mucosa resembles that of the skin.

Figure 3.2 Cross section through the oral mucosa

The epithelial lining of the oral mucosa is composed of squamous cells with a characteristic layered structure formed by the process of cell maturation. The composition and thickness of this layer varies according to the tissue functions; the hard palate and tongue, for example, is composed of keratinized epithelium whilst the lining of the cheek is distensible and non-keratinized (Table 3.1). Keratinised tissue is dehydrated, tough and resistant to chemical damage and it covers approximately 50% of the surface. Non-keratinised tissue is more flexible and occupies about 30% of the surface area. The oral mucosa has a turnover time of 3-8 days.

Sebaceous glands are found in the mucosa of 60 to 75% of adults and are seen as pale yellow spots in the upper lip and buccal mucosa. The openings of minor salivary glands are evident in many areas. In general, the oral mucosa has a more concentrated network of vessels than is present in the skin. Almost all venous return from the oral mucosa enters the internal jugular vein. Lymphatic capillaries are also present in the lamina propria and arise as "blind" beginnings in the papillae.

Table 3.1 Characteristics of the mucosae found in the oral cavity

Tissue Location Thickness mm Type

Table 3.1 Characteristics of the mucosae found in the oral cavity

Tissue Location Thickness mm Type

Gingival

Gums

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