Chapter 15 Methods of administration
• Enteral: administered through the gastrointestinal tract, including the mucosa of the mouth, stomach, small intestine, colon and rectum.
Enteral Administration
• Advantages
• Ease of administration: patients are used to ingesting food, so this form of administration is generally acceptable.
• Ease of absorption: in many places, the lining of the digestive tract is only one cell thick and moist, which is ideal for absorption.
Administration via the Rectum
Different Types of Oral Delivery System
General Gastrointestinal Tract Health
The pH can affect the degree of disassociation of a substance, resulting in increased absorption if the substance remains fully associated and non-polar (see Figures 8.2, 8.3 and 8.4, pp. 55–57). Drugs are designed for certain pHs, if these are not correct (due to poor gut health) then the drugs will not be as effective.
A healthy gastrointestinal tract ensures the correct content of commensals, as these are an important part of the gut chemistry, particularly the enterohepatic cycle (see Chapter 17 ‘Metabolism’, p. 131). If there is an imbalance then drugs might not be utilized properly or might remain in the body for an extended period of time, resulting in toxicity.
Factors Affecting Absorption
• Disease affecting the lining of the gastrointestinal tract: e.g. Crohn’s disease, in which flattening of the villi leads to a decrease of the surface area available for absorption.
• Short bowel syndrome: surgical removal of the small intestine due to diseases such as advanced Crohn’s disease and carcinoma of the small intestine.
• Alcohol abuse: damages not only the liver but also the lining of the gut. Vitamin B12 deficiency is a common side effect of this problem (see Chapter 13 ‘Vitamins and minerals’, p. 105).
• Parasitic infections: parasites can absorb nutrients (this is particularly relevant in vitamin and mineral supplementation).
• Disease affecting the gall bladder: substantially affects fat absorption. The bile salts are responsible for creating micelles – small globules – that make it much easier for the gut wall to absorb the fats and provide a larger surface area for enzymes to work on (see Figure 4.5, p. 25).
• Other diseases such as myasthenia gravis, anorexia, chronic gastritis and pancreatitis also affect absorption.
• Poor blood flow: the small intestine has a very good blood supply. Combined with the increased surface area formed by the villi, this explains why the most absorption occurs here.
• Presence of food: certain foods increase or decrease the absorption of drugs in the gastrointestinal tract.
• Certain chemicals found in foods can interfere with absorption by creating an insoluble complex. Phytates found in cereals are an example of this as they can bind minerals such as zinc, iron and calcium (see Chapter 13 ‘Vitamins and minerals’, pp. 95, 96, 98) and interfere with remedy absorption.
Parenteral Administration
Mucous Membranes
• The mouth, rectum, vagina and nose all contain mucous membranes. As these are thin and moist and absorption is usually fairly efficient.
• Drugs given orally and absorbed from the gastrointestinal tract are carried via the hepatic portal vein to the liver, where they are at least partly, if not completely, metabolized. In some cases, the drug is removed from the system before it has time to work. This factor is appreciated by pharmaceutical chemists, and is the reason that glyceryl trinitrate, given for angina, is taken sublingually.
Lungs
This is a possible method of delivering herbal remedy, e.g. steam bath for respiratory conditions.
Vagina
Possible method of delivering herbal remedy for the local treatment of conditions such as thrush.
Topical/Transdermal
Qualities required for absorption by this method:
• Areas that facilitate absorption contain plenty of hair follicles and sweat glands (neck, shoulders, extremities and back of hands).
• If substance is to be carried further than the local area, the rate of blood supply to the area of application needs to be reasonable.
• Carriers are usually used to get a remedy through the skin. Glycerol is easily absorbed but is hydroscopic (attracts water from the environment) and therefore has a drying effect. Lanolin mixes the best of both worlds; while being easily absorbed by the lipid layer of the skin it is able to take on board a large amount of water, by forming an emulsion. This is because part of it is ionic and therefore hydrophilic and part is non-polar and hydrophobic and therefore fat soluble (see Chapter 4 ‘Bonds continued’, p. 24).
• Disadvantages
• Generally, the chemicals in remedies need to be fat soluble if they are going to be effective. The skin is generally water resistant but soaking the area for some time will improve absorption, which is why herbal baths are used to assist faster absorption. Occlusive dressings or ointments can do this on a small scale. This is important to note in terms of Chinese medicine; where a skin condition is deemed to be damp this type of dressing will exacerbate the condition.
