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9.2 Lifestyle chemistry: 5. The solubility of drugs
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Syllabus reference (October 2002 version) | ||
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5. The solubility material used in drugs has an effect on the way in which the body responds to them |
Students learn to: |
Students: |
Extract from Senior Science Stage 6 Syllabus (Amended October 2002). Board of Studies, NSW
[Edit: 11 Jun 11]
Prior learning: Stages 4-5, Outcomes 4.8 (content 4.8.5a), Outcome 4.7 (content 4.7.5 b and c)
Recall statements in Preliminary course: Module 8.2, subsection 1.
Background:
Most of the powerful drugs used for the relief of pain, such as morphine and pethidine, are administered by injection, while some other drugs, like aspirin, codeine, pseudoephedrine and penicillin, are administered by ingestion, using a variety of carriers, such as hard white tablets or capsules. Although some oral medications act faster than others, they are by no means as fast-acting as intravenous injections.
A drug, having reached the intended part of the digestive system, is absorbed
through the lining and is then delivered to many different parts of the body by
the bloodstream.
identify the parts of the digestive system
- The following diagram identifies the main parts of the digestive system.
outline the role of the stomach and the small intestine in breaking down food
- The stomach has the role of beginning the breakdown of protein into amino acids. It does this by churning the food with gastric juice. The gastric juice, which is excreted from the walls of the stomach, contains enzymes, to break down the proteins, and acid, to kill harmful bacteria.
- The small intestine produces enzymes that complete the digestion process. It is also the organ from which digested foods are absorbed into the bloodstream. In the first section of the small intestine, called the duodenum, the partly digested food is mixed with bile and pancreatic juice. The bile helps to break down fat and oil drops into smaller droplets. The pancreatic juice helps to break down carbohydrates to simple sugars, helps to complete the digestion of proteins and to break down fats into fatty acids. The pancreatic juice also helps neutralise the acid from the stomach.
discuss the difference in pH of the stomach and the small intestine
- The stomach has a pH of around 3, owing to the presence of hydrochloric acid. The small intestine has a pH of around 8.
- The low pH of the stomach allows it to kill harmful bacteria. Also, this pH provides the conditions necessary for the enzyme pepsin to begin the breakdown of the proteins present in the food that has been ingested.
- In the small intestine, on the other hand, bile from the gall bladder emulsifies the fat present in the food. This bile is alkaline and helps to neutralise the stomach acid in the food. The alkaline environment allows other enzymes like lipase, amylase and protease to continue the digestion of the protein, starch and fat molecules present in the food.
gather, process and analyse information from first-hand or secondary sources to relate the significance of solubility of the medication to its action on/in the body
- You could gather information from first-hand sources by conducting your own experiments to see how well tablets containing these drugs dissolve, remembering that a tablet will most often contain other substances.
- To gather information from secondary sources, the solubility of the pure form of each drug will be needed in order for you to make valid judgements. You will also need to know where and how well each drug is absorbed in the digestive tract. Find out how each drug is taken into the body. (Is it administered as a solid because it will dissolve when it reaches its destination? Is it dissolved in alcohol because its solubility in water is very low?). Collate information from a number of sources and from a first-hand investigation in order to analyse this problem. You would need to gather information about the solubility and absorption of some common drugs such as aspirin, paracetamol, codeine, pseudoephedrine and penicillin. To quantitatively analyse the data, record all solubilities in grams per litre (g L-1). Likely sources include textbooks, pharmaceutical releases and drug databases on the Internet.
- You should process this information by relating the solubility of each drug to its action in the digestive system.
- Analyse this information by explaining the relationship between how a drug is administered and its solubility in water, alcohol and solutions of varying pH.
Simulated Stomach Lab Data Sheet 
Science NetLinks, AAAS, USA
account for the absorption of a drug and its action on the body in terms of its solubility
- The absorption of a drug can be affected by its solubility. Generally, the more soluble a drug, the quicker will be its absorption and the quicker the action on the body. A drug must be stable enough to survive the body's digestive system. Absorption can also be affected by the presence of other substances and the nature of membranes through which the drug must be absorbed.
- Drugs taken orally may be soluble in water, soluble in alcohol or not soluble in either.
- When an acidic drug is given in the form of a salt, it may precipitate in the stomach initially but can be readily redissolved in the intestine and be absorbed.
- Some drugs are prepared as sprays to act directly on the surface of the throat or nose, or to be absorbed through the lining. Some sprays are designed to be inhaled into the lungs. This type of administration can be very fast-acting. Nasal sprays are usually aqueous solutions. Some sprays are very fine particles of powder; others are a fine mist of dissolved particles.
- Skin applications are usually intended to reach the skin surface or the lining of the sebaceous glands under the skin. They include creams (water-based) and ointments (oil-based), as well as gels, lotions (which may be water- or alcohol-based) and liniments (usually alcohol-based). Many ingredients of skin applications are soluble in fats and oils, such as the type found in the layers of the skin.
- Highly soluble drugs can be easily carried in the bloodstream and so can act more quickly on the body.
identify that the manner of administration of a drug may be related to its solubility
- If a drug is soluble in water, it may be administered conveniently in a number of ways, including orally (as solutions, or in capsules or tablets), by inhalation, or as eye-drops, nose-drops or ear-drops.
- Some drugs will dissolve better in water-alcohol or glycerol solvents and may then be administered as solutions, suspensions or emulsions.
- In some circumstances, a skin application may be intended to be absorbed into the blood stream by passing through the lining of the glands. To help penetrate the lining they need to be fat-soluble. In this case, transdermal patches are used which have been impregnated with drugs such as oestrogen. or nicotine.
- Subdermal implants, on the other hand, are implanted into the body and the drugs are released into the lymphatic system, not the bloodstream. They may be placed inside the thigh or arm or in the stomach. This system ensures a continuous slow release of the drug required.
explain why the knowledge of the solubility of materials can be used to design drugs for specific purposes
- When drugs are being designed, scientists consider carefully where in the digestive tract the drugs should be dissolved, ready to be absorbed into the bloodstream. Scientists may need to ensure that a drug:
- dissolves when it reaches a location with a specific pH
- reaches its destination without being broken down. It may have been dissolved but it must still be chemically effective
- is released within an appropriate timeframe.
- The solubility of a drug can be affected by the nature of the solvent, the amount of agitation and the temperature.
- Soluble tablets will dissolve rapidly in water producing a liquid solution. Liquids entering the digestive system are absorbed more rapidly than solids. This is useful when fast-acting medication is desirable.
- Some substances that are not very soluble in water can be dissolved by the addition of other substances to produce a solution without changing the therapeutic effect of the drug itself.
- Drugs that dissolve in acidic solutions will be absorbed through the lining of the stomach and are then taken to other parts of the body by the bloodstream. Those that dissolve in alkaline conditions will be absorbed through the walls of the small intestine and are similarly carried by the bloodstream.
- Medication may be designed especially to dissolve in the intestine rather than the stomach.
- It is important for scientists to test medicines thoroughly because the digestive tract can affect the drug and because the drugs may cause damaging side effects. Some drugs are broken down in the presence of acids and this may alter their effectiveness.
- Some drugs may cause damage to the lining of the stomach. For example, some common drugs used in the treatment of arthritis are enteric-coated to prevent their solution in the stomach, where they may cause irritation or gastric upset. The coating is not dissolved in the acidic environment of the stomach.
- Medication may be designed to dissolve over a period of time in order to provide access to the drug without further ingestion.
Pharmacokinetics ForCon Forensic Consulting, Canada, 2004. This website has a lot of information about different ways drugs can be administered and absorbed.
More information can be found at NSW Higher School Certificate, Senior Science, 9.2 Lifestyle Chemistry, Section 5 from the Catholic Education System. Scroll down to pages 7 and 8.
identify vitamins that are water-soluble and those that are fat-soluble
- Some vitamins are water-soluble (including the eight B-group vitamins and Vitamin C) and some are fat-soluble (including Vitamins A, D, E and K).
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