Medicines from Plants: Digitalis and Taxol
Medicinal Plant Chemicals
Scientists have identified many medicinal chemicals in plants. There are probably many more still to be discovered. Digitalis and taxol are two important medicines from the plant kingdom. Digitalis, which strengthens and slows the heartbeat, is found in foxgloves. Taxol, which is used as an anti-cancer drug, is found in yew trees.
Foxgloves are tall plants with rows of beautiful, tubular flowers ranging in color from purple to white. The flowers are frequently spotted. The plants contain poisonous as well as medicinal chemicals. They grow in the wild and as cultivated plants in gardens and landscaped areas.
Yew trees are coniferous and have needle-shaped leaves. The female or seed cone looks like a red berry instead of a typical conifer cone. Taxol was discovered in the Pacific yew and has since been found in other plants.
Digitalis and Foxgloves
The word "digitalis" is often used as the name of the medicine in foxgloves. Digitalis actually consists of a mixture of chemicals called cardiac glycosides. These chemicals are used as drugs to treat atrial fibrillation and congestive heart failure. Two cardiac glycosides in digitalis are digitoxin and digoxin. In North America, digoxin is the chemical that is generally used as a heart medicine today. One brand name of digoxin is Lanoxin. The medicines are generally obtained from foxglove leaves.
Digitalis or one of its components can be very helpful, but it must be used in the correct dose as prescribed by a doctor. It’s dangerous if too much is ingested. In addition to digitalis chemicals, foxglove contains other substances which are biologically active and are toxic to humans and animals. The entire foxglove plant is poisonous.
Although all foxgloves contain digitalis, today heart medicines are usually obtained from Digitalis lanata, or the woolly foxglove. The plant gets its common name from the hairs on the underside of its leaves.
How Does Digitalis Work?
In atrial fibrillation, the heartbeat is rapid and irregular. The inefficient pumping of the heart increases the risk of a stroke. Digitalis helps to treat atrial fibrillation by increasing the action of the parasympathetic nervous system on the heart. One job done by this system is to slow the heartbeat.
In congestive heart failure, the heart is unable to pump enough blood around the body. As a result, blood may back up in the blood vessels, causing fluid to leave the blood and enter the tissues. Fluid may build up in the lungs, the arms and legs, the digestive tract and the liver. This fluid buildup is called edema. Digitalis increases the amount of calcium in the heart cells. Increased calcium leads to a stronger heartbeat. Since digitalis strengthens the contraction of the heart, the heart can pump more blood and edema is reduced.
William Withering was a physician and botanist. He discovered the benefit of foxgloves for dropsy in 1775. Dropsy was the old name for edema due to heart problems. Withering's discovery was based on a remedy prescribed by a local herbalist.
The Common Foxglove in British Columbia
A patient taking any type of digitalis medicine has to be monitored carefully. A toxic dose is not much larger than a therapeutic dose, so it's very important that a patient follows instructions.
Someone suffering from digitalis toxicity may experience loss of appetite, nausea, vomiting and diarrhea. There may be confusion, an irregular heartbeat and vision problems. Vision may be blurred and objects being viewed may have a yellow tinge, a condition known as xanthopsia. The person may also see halos of light around objects. In addition, he or she may experience depression and hallucinations.
Taxol from Yew Trees
In the 1960s, researchers from the USDA (United States Department of Agriculture) collected yew bark and other plant samples in an attempt to find natural substances that could fight cancer. The survey was done at the request of the National Cancer Institute. Taxol was discovered in the bark of the Pacific yew tree in 1967.
Taxol's name is derived from Taxus brevifolia, the scientific name for the Pacific yew. The chemical is most abundant in the bark of the tree but is present in the needles as well. It has also been found in other species of yew and has recently been discovered in a number of fungi.
At first the use of taxol was controversial, since removing the bark from yew trees to extract the medicine kills the trees. This is a serious problem because Pacific yew grows very slowly. Trees that have been killed for taxol extraction can't be quickly replaced.
Nowadays taxol is often obtained by a Taxus cell culture method that doesn't involve killing trees. The cells are cultured with a fungus that normally lives in the yew's bark. The fungus also makes taxol. Another method used to obtain the medicine is to extract a precursor chemical from needles of yew trees and then convert this chemical to taxol in the lab, making the process semisynthetic.
Uses of Taxol or Paclitaxel
Taxol is used to treat several different cancers, including breast cancer, ovarian cancer and one type of lung cancer (non-small cell lung cancer). It’s also used to treat AIDS-related Kaposi’s sarcoma.
Taxol is also known as paclitaxel. It's used on its own or in combination with other chemotherapy drugs. It has proved to be a very helpful medication and is very popular, although like other chemotherapy drugs its success at treating cancer depends on a variety of factors.
How Does Taxol Fight Cancer?
The predominant theory to explain the anti-cancer action of taxol says that the drug works by interfering with the cytoskeleton of a cell. The cytoskeleton is made of protein filaments and tubules (microtubules) that form a network in the cell. The cytoskeleton gives the cell strength and supports and moves cell structures. It's said to be "dynamic", since it's continually breaking down and assembling, as shown in the animation below.
Just before a cell divides, its nucleus divides. Nuclear division is known as mitosis. Microtubules are assembled and disassembled at different stages in mitosis. Taxol stops microtubule breakdown, thereby interfering with the process of mitosis and inhibiting cell division.
Cancer cells multiply rapidly compared to most body cells and so have a high rate of mitosis. By preventing microtubule breakdown taxol can act as an anti-cancer drug.
Microtubules and Activity inside a Cell
Side Effects of Taxol Treatment
Unfortunately, in addition to hindering cancer cell replication, taxol can also interfere with the division of normal cells in the body that have a high rate of mitosis. Stem cells in the red bone marrow divide frequently to produce the blood cells. One of the side effects of taxol may therefore be a low red blood cell count (resulting in anemia), a low white blood cell count (which can lead to increased infections) or a low platelet count (which can lead to an increased risk of bruising and bleeding). Cells lining the gastrointestinal tract also have a high rate of division and may be affected by taxol.
Some of the side effects of taxol treatment may be due to the solvent carrying the medicine or to another chemical in the mixture instead of the medicine itself. More than one formulation of taxol is available.
Additional Side Effects
There may be additional side effects of taxol treatment, but not everyone will experience them. If the side effects do occur, they may be minor. It’s impossible to predict ahead of time how severe the effects will be for a particular patient, but quite often people find taxol treatment less unpleasant than other chemotherapy treatments. Additional medications can often relieve the effects that do appear.
The most common side effects of taxol treatment include low blood counts, nausea, vomiting, diarrhea, temporary hair loss (until the taxol treatment is stopped), mouth sores, muscle or joint pain, numbness and tingling. Other possible side effects are fluid retention in the feet, ankles or abdomen and nail darkening.
Some people have an allergic reaction to taxol treatment, but this is generally due to the substance used to dissolve the taxol so that it can enter the bloodstream rather than to the taxol itself. Taxol is not water soluble. Doctors usually prescribe corticosteroids to reduce the chance of an allergic reaction when taxol is given to a patient.
Discovery of Digitalis and Taxol
Foxglove leaf (or foxglove leaf extract) has long been used as a herbal remedy to treat heart problems. Taxol was discovered when scientists representing the National Cancer Institute in the United States commissioned a plant survey to find new chemotherapy drugs. By destroying so many plant habitats around the world humans are almost certainly denying themselves the opportunity to find many new medicines. This is one reason why the conservation of wild areas is so important.
References and Further Reading
© 2011 Linda Crampton
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