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that slower GI motility may not significantly increase the absorption of drugs, Chinese herbs that promote peristalsis may significantly decrease the absorption of many drugs. These herbs include all herbs that purge (e.g., Da Huang, Fan Xie Ye, and etc.), and some herbs that promote digestion (e.g., Shan Zha, Lai Fu Zi, Ji Nei Jin, and etc).
2.1.3 Change of drug absorption due to the binding effect of herbs
Herbs that may interfere drug absorption due to their binding effects include:
- Some gelatin products, e.g., E Jiao, Lu Jiao Jiao, Bie Jia and Gui Ban Jiao; -Herbsthatcontainhighinflavoneswhichmaybe bound with drugs such as calcium carbonate, iron preparations, aluminums hydroxynate.
- Herbs that contain metal ions may cause binding problems: e.g., Hua Shi , Yang Qi Shi, and Hu Po (Magnesium), Ming Fan, Chi Shi Zhi (Aluminums), Dai Zhe Shi, Yang Qi Shi, Yu Yu Lian, Zhi Ran Tong, and Chi Shi (Iron)
- Herbs that contain tannins including: Di Yu, Wu Bei Zi, Hu Zhang, He Zi, Bian Xu, Da Huang, Shi Liu Pi, Huang Yao Zi, Jin Qian Cao, and Mu Gua. These tannin-containing herbs should not be used orally together with:
- Enzymes such as, trypsin and pepsin
- Drugs with metal ions such as Zn, Fe, or Calcium - Cardiac glycoside such as digoxin, digitalis to form salt products
- Drugs with amidopyrine;
- Vitamins B1 and B6
- Any product with NaHCO3
- Some antibiotics such as tetracycline,
sufanilamide, erythromycin, chloramphenicol and rifampicin.
2.1.4 Change of drug absorption due to the effect of herb or drug’s effect on intestinal flora
Alternation of bowel flora (e.g., by concomitant use of antibiotics) by Chinese herbs can interrupt enterophepatic recycle and result in decrease of activity of some drugs (e.g., the oestrogen contraceptive pill), or increase of activity of some drugs (e.g., digoxin). Chinese herbs that have antibiotic effect are usually in the category of heat-clearing and detoxifying. Herbs such as Huang Lian, Huang Bai, Zhi Zi, and Ku Shen are high in berberine. They should be monitored carefully when used together with drugs whose intestinal absorption is related to the intestinal flora.
2.2 Distribution
Distribution refers to the process in which herbs or drugs are carried and released to different parts of the body. Protein binding is by far the biggest factor when determining interactions. Theoretically drugs or herbs with active ingredients that are highly protein bound are very susceptible to interactions. Serious interactions occur during the distribution phase if the drug has a narrow range of safety index and is highly protein-bound. For example, Coumadin (warfarin) is an anticoagulant medication that is very highly bound to protein and has a very narrow range of safety
index. Other drugs that are highly protein bound include phenytoin, oral contraceptive pills and some NSAIDs. There is not much information about the protein binding of chemical components from herbs. The best precautionary measure is to monitor of medications on clinical observations of therapeutic effects and tolerability.
2.3 Metabolism
Drug metabolism is biochemical modification or degradation of the drugs, usually through specialized enzymatic systems. Drug metabolism often converts lipophilic chemical compounds into more readily excreted polar products. Its rate is an importantdeterminantofthedurationand intensity of the pharmacological action of drugs and herbs.
Most herbs and drugs are metabolized by the liver. Metabolism, or biotransformation of substances, has a large potential for interactions. One of the biggest targets of interactions within the realm of metabolism is the cytochrome P450 (CYP) system. The most common reaction catalysed by CYP450 is a monooxygenase reaction.
Human CYPs are primarily membrane-associated proteins, located either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells. CYPs metabolize thousands of endogenous and exogenous compounds. Most CYPs can metabolize multiple substrates, and many can catalyze multiple reactions, which accounts for their central importance in metabolizing the extremely large number of endogenous and exogenous molecules. Hepatic cytochromes P450 are the most widely studied.
Major human CYP isoforms in drug metabolism include CYP450 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5 and 3A7 (6, 7). The followings are some terms related to the discussion of CYP450 system and drug metabolism:
1. Isoforms of CYP450: Different subtypes of enzymes.
2. Substrates: In biochemistry, a substrate (a drug) is a molecule upon which an enzyme acts. The herb metabolism is not fully understood. Therefore the information on the herb as a substrate is very limited.
3. Inhibitors: A substance (a drug or herb) that decreases the enzyme's activity. Inhibitors affect the
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