Turn the clock back 60,000 years and, according to evidence from a burial site in Iraq, people in the Middle East were using herbal remedies to combat illness.
“The beginning of pharmacy was medicinal plants,” says Dr Taghread Hudaib, a pharmacologist from Jordan.
“In the old days, people used to [use] plants or leaves for different sicknesses; they understood,” she says.
In medieval times, Islamic physicians in Arabia used plant material for medicinal purposes, such as when they prescribed the consumption of poppy seeds to relieve pain.
“There was something called Islamic medicine, it’s built on plant extracts, people have this connection with medicinal plants from this kind of history,” says Dr Hudaib.
Fast-forward to the present day and the technology has moved on regarding how plants are used − particular chemicals can be extracted and purified, offering improved consistency over using the plants themselves − but the principles are the same: plant material is used to combat disease.
Scientists led by Dr Rabah Iratni of UAE University have shown that extracts of the shrub and small tree Rhus coriaria can stop certain breast cancer cells from spreading, causing them to die and making it harder for tumours to form.
Rhus coriaria contains more than 200 chemicals, and many other plants also produce dozens of such “secondary metabolites” − substances not essential for the organism’s growth and reproduction.
In thousands of cases, plant extracts have been shown to be of value in tackling human disease.
Dr Iratni’s laboratory demonstrated that the herb marjoram helped to combat cancer.
“Each plant has a different set of compounds,” says Dr Hudaib, a lecturer at the University of Lincoln, in England.
“Sometimes you will find hundreds of compounds in a leaf extract, sometimes you don’t find so many.”
Many of these chemicals are produced by the plant to help it defend itself.
“Because [plants] are rooted in the ground, if they are targeted by pests or parasites, they can’t run away and they can’t fight back physically, they have to use chemicals,” says Dr Paula Rowe, a senior lecturer in biochemistry at the UK’s University of Swansea who researches the medicinal properties of plant chemicals.
“Plants also produce chemicals to target bacterial and fungal infections,” she says.
Those antibacterial and antifungal compounds have properties that help to combat infections in people, especially given the similarities at the cellular level between widely divergent organisms.
However, she says, “It’s not so easy to understand why plant compounds should have anticancer activity”.
“My feeling is that, if plants get infected with multicellular organisms, they might produce compounds that block cell division, to stop the multicellular organism growing or reproducing,” she says.
Research has shown plant compounds that target, for example, the cytoskeleton of cells − a network of structural elements in cells that provide shape − often have anticancer properties and are sometimes effective in combating parasites.
“So that’s what’s in it for the plant [in producing these anticancer substances] − it may be stopping the growth and proliferation of very small worms, or amoebic organisms perhaps, to protect itself,” says Dr Rowe.
Many secondary metabolites provide defence by acting as enzyme inhibitors, blocking the active site of an enzyme, says Dr Declan Naughton, a professor of biomolecular science at Kingston University London. “Thus, a plant may contain hundreds of potential inhibitors – and inhibitors are a major class of medicines,” he says.
Among them are monoamine oxidase (MAO) inhibitors, which prevent the function of enzymes that cause the oxidation of monoamines, a type of neurotransmitter. In doing so, MOA inhibitors have potential for being more widely used to treat Parkinson’s, Alzheimer’s and other neurodegenerative disorders, as well as depression.
“Many plant isolates showed remarkable MAO inhibition in recent years,” said a group of scientists from the Grace College of Pharmacy in Kerala, India, in a 2014 paper, Central Nervous System Agents in Medicinal Chemistry.
Given the wealth of chemicals they contain, many plants can be “considered as ‘libraries’ of natural products that have the potential to act on biological systems”, says Dr Naughton.
Sometimes plant extracts are effective on their own, while on other occasions their activity depends upon the presence of other substances.
One example of this was highlighted in a paper in the journal BMC Complementary and Alternative Medicine.
It showed that the antimicrobial effect of pomegranate rind extract (PRE) was much greater when a chemical called cupric sulphate was present.
After the addition of this substance, PRE was better able to combat a bacterium, Staphylococcus aureus, including strains of the bacterium resistant to the antibiotic methicillin that causes the condition MRSA.
“I see the trend moving to combinations as opposed to a single chemical entity as a magic bullet,” says Dr Naughton.
So, much remains to be discovered about how plant extracts can combat disease, despite many hundreds of years of traditional use of plant material and the wealth of modern-day research.
“There are hundreds of thousands of research papers published on different phytochemicals and active compounds but the area is still open because there are lots of plants that haven’t been analysed, there’s great potential,” says Dr Hudaib.
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