Drugs: A Primer

A drug, broadly defined, is any chemical that affects living processes. Historically, most drugs were derived from natural sources such as plants. VP-16 (Etoposide), a drug used in some BMT preparative regimens, is one such drug.

More recently, drugs have been manufactured synthetically, based on our increased knowledge of the function of chemicals in the human body. Antihistamines, for example, are drugs developed based on our understanding of how a body chemical called "histamine" stimulates allergic reactions.

The newest development in pharmacology is the use of biotechnology to produce drugs. With biotechnology, genes that produce human proteins can be put into another living cell such as bacteria or yeast, and stimulated to produce more of the human protein. "Colony-stimulating factors" (G-CSF and GM-CSF) are examples of drugs produced by biotechnology processes. (See the November 1992 issue of BMT Newsletter for more on colony stimulating factors.)

In order for drugs be effective, they must reach their intended target site. In some cases, the target site is easily accessible by "topical" medications_drugs that can be administered directly to the affected area. Eye drops and ointments are examples of topical drugs. Another example is "intrathecal" medications_drugs that are injected directly into the fluid that bathes the brain.

In most cases, however, drugs must be carried to the target site by the bloodstream. For these drugs, two methods of administration are possible: oral and parenteral.

Oral administration is sometimes referred to by the abbreviation "PO" which stands for the Latin words "per os" or "by mouth." (When ingestion of food or medications is to be avoided, the abbreviation "NPO" for "nil per os" or "nothing by mouth" is commonly used.) Oral medications are absorbed by the bloodstream from the stomach and intestines and carried to the target site.

Parenteral medications include those administered by injection into a vein (intravenous), under the skin (subcutaneous) or into muscles (intramuscular). More recently, certain drugs have been incorporated into "patches" that are applied directly to the skin (intradermal administration). Drugs administered by these methods eventually reach the bloodstream which carries them to the target site.

Although oral administration of drugs is generally preferred, many drugs cannot be given orally. Some are not made in oral form. Others are broken down by stomach acids or intestinal enzymes before reaching the bloodstream, and therefore never reach the target site.

Many chemotherapy drugs are given intravenously rather than orally because they tend to cause nausea and vomiting. If the patient vomits a medication, it can't reach the bloodstream and have its intended effect on tumor cells and/or the immune system.

Even when it's possible to administer a drug orally, only a fraction of the dose may actually reach the bloodstream. This fraction is called the drug's "bioavailability," and is expressed as a percentage ranging from 0 to 100%. For some drugs, fluctuations in bioavailability can affect both the good (therapeutic) and bad (toxic) effects of the drug.

Many factors influence a drug's bioavailability such as the presence of food in the stomach, and sometimes even the type of food. To ensure you are getting a consistent dosage of a drug, it's important to be consistent. Don't take a pill or capsule with a meal on one day, and on an empty stomach the next.

Every drug, if given in sufficient quantity, can have toxic side effects. Physicians carefully consider the risks and benefits of drugs before choosing the appropriate dosage. In some cases, however, the toxic side effects of a drug are unavoidable.

The high dose chemotherapy included in BMT preparative regimens is a good example. In order for the drugs to produce the desired good effects (ie. destruction of cancerous cells and/or suppression of the immune system) they must be given in very high doses_doses that also produce toxic side effects. Fortunately, these toxic side effects are usually mild or moderate and completely reversible.

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