The Chemistry of Foxgloves

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The vibrancy of foxgloves belies their poisonous nature – ingesting even a small amount of the plant can cause unpleasant effects, and in some cases death. However, the same compounds that make it poisonous can also have medicinal uses. This graphic takes a look at them in detail.

The mantra ‘the dose makes the poison’ is oft-repeated in the field of toxicology; the foxglove perhaps provides one of the best examples of how true this is. The compounds in foxglove that lend it both its toxicity and medicinal use are called cardiac glycosides. A glycoside is a molecule which contains a steroid portion bonded to a sugar portion. The glycosides in foxgloves are found in higher concentrations in the leaves, but they’re still found in all other parts of the plant as well.

Ingestion of a small amount of parts of a foxglove can cause symptoms including nausea, vomiting, and diarrhoea. Though it may seem like an unlikely turn of events, the leaves of foxgloves can easily be confused with other edible plants – there’s a case of a man mistaking the leaves for that of another plant, and brewing a herbal tea from them. Larger amounts can result in death; although cases of this are rare, they have been documented.

Despite its toxicity, foxglove has actually been used in medicine for a number of centuries. Back then, it was used as a treatment for ‘dropsy’, what we now recognise as edema, an excess of fluid collecting in the tissues of various parts of the body. We now also know that this condition is often a side-effect of heart problems.

The use of foxglove for treating dropsy was trialled by an English doctor, William Withering. In the course of recording and eventually publishing his findings he found that foxglove extract was an effective method for treating dropsy and heart failure – though he also discovered it could have unpleasant effects if given in too high a dose. One of the more curious of these is a yellowing of the vision.

The foxglove extract, the key constituents of which are the cardiac glycosides digoxin and digitoxin, is known as digitalis after the Latin name for the plant. After Withering’s work, it became a common treatment for heart issues, including heart failure. Unusually for a drug that has persisted from antiquity to the present day, digoxin is still extracted from foxgloves, as it’s difficult for chemists to synthesise it in a cost-effective and efficient manner.

So how does digoxin exert its beneficial effects, and why is the line between its ability to heal and harm so fine? To answer this, we need more insight into how it affects the body. Though its exact mechanism of action is still unclear, it’s thought that it affects the sodium-potassium ion pumps in heart cells. These usually remove sodium ions from the cells. Digoxin stops sodium being removed from the cells, which has a knock on effect of causing the concentration of calcium ions in the cells to rise. This, in turn, interferes with the electrical signals that keep the heart beating, causing its pumping to become more forceful but slower.

This ability to interfere with and slow the heart rate makes digoxin useful for treating heart arrhythmias. It can still be used to treat heart failure, though its use for this has declined with the advent of other drugs. Its therapeutic range (the range in which it exerts beneficial medicinal effects) is very close to its toxic level – the point at which unpleasant effects start to be seen. In excess, it slows the heart rate so much that the brain becomes starved of oxygen; the body’s reflex response is to try and increase the heart rate, and this eventually results in a heart attack.

Finally, there are, of course, plenty of cases of people using foxglove and digitalis for more nefarious means. ‘Nature’s Poisons’ cites the case of a German doctor who murdered his girlfriend by administering the poison to his girlfriend rectally (!). Charles Cullen, New Jersey’s most prolific serial killer, also used digoxin to kill a number of his victims.

(It goes without saying, but do be wary of foxgloves, particularly if you’re into foraging. You need to ingest very little to experience toxic effects; if you notice symptoms, or think you or someone you know might have accidentally ingested some part of a foxglove, seek medical attention immediately.)

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