Overview, Advantage And Disadvantage Of Pharmaceutical Sublingual Tablet

Sublingual tablets generally do not have a coating and are designed so that they will dissolve when placed under the tongue. Substances diffuse into the blood through tissues under the tongue which is predominantly a mucous gland that produces a thick mucinous fluid and lubricates the oral cavity which allows for swallowing, initiating digestion, buffering pH, and dental hygiene.

Sublingual tablet

It involves placing the drug beneath the tongue, to be absorbed and dissolved into the bloodstream through the mucous membranes. These drugs are in the form of tablets that dissolve, sprays, or films.

Anatomical Structure Of The Oral Mucosa

Oral mucosal drug absorption is governed by (a) the permeability of the oral mucous membrane and the anatomy of the underlying tissues, (b) the physicochemical properties of the drugs, and (c) the formulation design.

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The oral cavity has four distinct regions that can absorb drugs-the sublingual, buccal, gingival, and palatal regions. These regions differ from one another in histological structure and biochemical composition of the mucosal membrane, and their ability to retain the dosage form long enough to allow complete drug absorption.

The mucosal lining consists of three distinct layers. The outermost layer is the epithelial membrane, which consists of stratified squamous epithelial cells and has a protective barrier function. The innermost layer of the epithelial membrane is called the basement membrane that replenishes the epithelium. Below the epithelium lies the lamina propria followed by the submucosa. The lamina propria is a hydrated and less dense layer of connective tissue containing collagen and elastic fibers. The oral submucosa is also richly supplied with blood vessels.

Following absorption through the mucous membrane in the sublingual region, the drug instantly diffuses into venous blood. The venous blood from the sublingual region of the oral cavity drains into a common trunk, which then drains via the internal jugular vein, the subclavian vein, and the brachiocephalic vein directly into the superior vena cava. Thus, venous return from these regions enters the systemic circulation, bypassing the pre-systemic drug elimination. Direct drainage into systemic circulation results in immediate systemic availability of the drug and rapid onset of action. It should be noted that smoking, which causes vasoconstriction, may affect drug absorption.

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Permeability Of The Oral Mucosa And Drug Absorption

The salivary glands present in the oral cavity secrete saliva that has a pH of 5.5-7.0. Saliva consists of proteins and carbohydrate complexes called mucus and enzymes such as amylase and carboxylesterase. Mucus is negatively charged at the physiological pH, forming a cohesive gelatinous film on all oral cavity surfaces. This cohesiveness permits mucoadhesion of the drug to the epithelial tissue leading to drug absorption.

The epithelial membrane is 100–200 µm thick in the sublingual region and 500–600 µm thick in the buccal region (6). The epithelial membrane in both regions is non-keratinized. The permeability of the mucosa varies from region to region in the oral cavity depending on thickness and degree of keratinization of the epithelial membrane.

The profuse blood supply, combined with the relative thinness and higher permeability of the sublingual mucosa, permits rapid absorption and desirable bioavailability of certain drugs following sublingual administration.

Mechanism Of Mucosal Drug Absorption

Following sublingual administration, the drugs are absorbed across the mucous membrane by one of the following mechanisms:

i. Passive diffusion

ii. Active or carrier-mediated transport

iii. Endocytosis

Although the process of passive diffusion is spontaneous, the rate of diffusion is dependent on the molecular weight and solubility of the drug, concentration gradient, temperature, the surface area of the membrane, and the proximity of the molecule to the membrane. Lipids present in the oral mucous membrane offer the main barrier to the permeability of hydrophilic drugs. On the other hand, well-hydrated connective tissues provide resistance to lipophilic drugs. For efficient absorption through the oral mucosa, the drug must be hydrophobic enough to partition into the lipid bilayer, but not so hydrophobic such that once it is in the bilayer, it will not partition out again.

Physical Characteristics Of Sublingual Tablet

The physical and mechanical characteristics of a tablet, such as size, hardness, porosity, and wettability, affect its disintegration time. A smaller tablet size, with low hardness and high porosity, disintegrates more rapidly than a larger or harder tablet. However, a tablet with a high porosity and low hardness is more friable, and this presents problems in tablet packaging and handling. During development, all approaches to increase the mechanical strength of tablets should be studied, without compromising disintegration and dissolution.

The amount and type of disintegrants also play a significant role in achieving rapid disintegration. Effervescent agents have been used to facilitate disintegration. The inclusion of water-soluble excipients, such as saccharides, helps in achieving rapid dissolution by enhancing the wettability of the tablet matrix.

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Although several technologies are available to manufacture sublingual tablets, usually compression molding, direct compression, and freeze drying have been commonly used for commercial manufacture of sublingual tablets. Presently, the direct compression and freeze-drying methods are commonly exploited for commercial manufacture of sublingual tablets.

Advantages Of Sublingual Tablet

1. Medicine is absorbed quickly into the bloodstream

2. These drugs are easy to take for patients who experience trouble in digesting or swallowing medication

3. These drugs are not metabolized in the liver since they are not passing in the digestive system. Therefore, you may take a lower dosage and still get equivalent results

4. Suitable for drugs that are destroyed by gastric enzyme

5. Self administration is possible

6. Dosage can be determinated anytime by spitting it out

7. Reduced interaction with other medications and foods

Disadvantages Of Sublingual Tablet

1. Smoking, drinking or eating while using sublingual medication can affect the drug absorption and how efficient the results would be

2. Drugs that need to be absorbed slowly into the system could not work well in this form of administration

3. Medicine can irritate the mouth if you have open sores

4. Very small dose can be administered hence the drug must be potent

5. Tendency to swallow some of the dissolve medication and swallow the whole tablet

6. Bitter tasting drugs cannot be design this way