What do the six main types of chemotherapy actually do?

Chemotherapy is a general term for any medicine that is used to kill cancer cells or stop them from growing and replicating. These medicines, known as chemotherapy agents or drugs, might be given on their own, in combination with other chemotherapy drugs, or with other treatments like radiotherapy.  

We often talk about chemotherapy as if it’s just one treatment, but there are actually over 100 different chemotherapy medicines.  These all work in different ways and can roughly be grouped into six main types.

So, lets take a look at some of the different types of chemotherapy and how they help fight cancer in children and young people. 

The DNA-binders

‘Alkylating agents’ are a type of chemotherapy drug that bind to DNA, the genetic code contained within your cells that tells the cells how to behave, keeps your body functioning and makes you unique. DNA is made up of two strands, which wind together into a spiral shape, with the tiny proteins that make up the cell’s instructions safely in the middle.

When making new cells, whether cancerous or healthy, a cell undergoes a number of changes that lead to it splitting into two cells. One of the first steps is copying the genetic code in the cell so that there are two copies of the instructions – one for each new cell. To copy the DNA, the two strands need to be split apart.

Alkylating agents create strong bonds between the two strands of DNA, so that they cannot be split apart.  This means that the cancer cell can’t make copies of its DNA, and therefore can’t make copies of itself. It also means that as the affected cancer cells age and die, they won’t be replaced by new cancer cells. And fewer cancer cells mean less cancer – whether it’s fewer leukaemia cells in the blood stream, or a smaller tumour.

The fake building blocks

‘Antimetabolites’ look a lot like the building blocks that cells need to make DNA. As you know, cancer cells (and healthy cells) need to be able to make copies of their DNA to function and make new cells. This means that the ingredients for DNA, like tiny proteins and nitrogen molecules, are very important to cells.

Some of these medicines work by stopping the cell from putting the right ingredients together. They behave like a molecule or protein that is involved in making the ingredient, tricking the cell into using them. However, they actually prevent the ingredient being made instead because they are not quite right.

Sometimes, the antimetabolites pretend to be the final ingredient, rather than stopping the ingredient from being made. Cells use the antimetabolite molecules to make their DNA copies instead of the right ingredients, which causes serious errors in the genetic code. 

Antimetabolite chemotherapy drugs result in problems making new copies of DNA, meaning that the cell cannot create copies of itself. This can help kill tumours or make them grow more slowly.

The DNA-cutters

In cells, most processes are controlled by enzymes, such as DNA replication and repair. ‘Topoisomerase inhibitors’ are a chemotherapy medicine which work against one of these enzymes, called topoisomerase. This is a really important enzyme in the DNA-copying process.

Because of DNA’s spiral shape, as the two strands are separated for copying, the bit that’s still joined becomes more and more twisted. This stops the other enzymes from separating the two strands and making the DNA copies that are needed for cell replication and function. The topoisomerase enzyme makes tiny cuts in the twisted bits of DNA, allowing the DNA to untwist, then joins the cut ends back together.

Topoisomerase inhibitors let the enzyme continue making cuts in DNA – however, they stop it from joining the cut ends back together. This causes serious problems for DNA replication, meaning that the cell can’t make new copies and will eventually die.

The super strong antibiotics

‘Anti-tumour antibiotics’ are similar to the antibiotics that you might take if you had a chest infection – but a lot stronger. They’re actually considered too strong to use for bacterial infections, as they can damage organs like the liver and kidneys, but are very good at causing problems for cancer cells. There are four different types, which all work in different ways:

• Bleomycin binds to DNA within a cell and cuts the strands, a little like the topoisomerase inhibitors, which eventually leads to the cell dying.
• Dactinomycin inserts itself between the two strands of DNA and holds them together, stopping the strands from being separated and copied.
• Mitomycin creates bonds between DNA strands, like the alkylating agents, but is more often used for adult cancers.
• Finally, anthracyclines are a group of antibiotics that are used for a range of cancers. They do a number of things that harm cancer cells, including inserting itself into DNA to stop the strands being copied and causing damage to DNA. They also stop DNA errors from being repaired and change how the cells can absorb nutrients.

The ones which cause a mess

‘Mitotic inhibitors’ fight cancer in a different way. When cells are copying themselves, the DNA is wound up into two sets of identical chromosomes after it has been copied. These sets of chromosomes then line up along the centre of the cell so that, when the cell splits in half, each half has a full copy of the DNA instructions.

It’s amazing to think that all over your body right now, thousands of cells have chromosomes neatly lined up in their centres, ready to divide – and that it happens without any input from you.

Mitotic inhibitors mess up this nice order – they break the tiny filaments which organise the chromosomes and hold them in place. This stops the cell from being able to divide properly, meaning that the cancer can’t grow and spread.

The support team

‘Corticosteroids’ are similar to chemicals that our bodies naturally make, which affect things like blood pressure and the immune system. As a medicine, they’re a lot stronger than the ones in your body and they are used for a wide range of diseases.

If they’re used in cancer, they are a form of chemotherapy. Whilst some types of steroids can kill cancer cells, they can also be given alongside other chemotherapy medicines to help with the effects of cancer and its treatments.

Corticosteroids can help with allergic reactions to cancer treatments, to improve appetite and reduce sickness and swelling. In short, they can make a patient’s quality of life a lot better during treatment, which is a vital part of cancer treatment. Patients have to be properly supported or the side effects might become so difficult that they are in more immediate danger from the treatment than from the cancer. They also need to be healthy enough to be able to endure these toxic treatments.

So, that’s some of the main types of chemotherapy! It’s important to remember that they don’t all work for all cancers, and doctors have to carefully choose the right medicine for each cancer.

You’ll also notice that these medicines don’t make any distinctions between cancer cells and healthy cells - this is the cause of many of the side effects for cancer treatments. Whilst targeted treatments that just attack cancer cells are being developed, most treatments are still not specific to cancer cells.

The reason that these medicines have a bigger effect on cancer cells, and don’t just kill all of your healthy cells, is because of how cancer cells are different. Due to normal wear and tear, our cells are constantly dividing and replacing themselves. However, cells have strict limits on how many copies they can make of themselves, and how fast they can make them. This makes sure your cells don’t copy themselves too much and grow out of control.

Cancer cells have found ways around those limits, meaning that they can replicate themselves endlessly, picking up more and more errors as they go. You’ll have noticed that many chemotherapy drugs have an impact on how a cell replicates itself – from preventing DNA being copied to messing up the way the cell divides. Whilst these do still affect healthy cells, the impact on cancer is thought to be much higher due to how often the cancer cells are replicating themselves.

Despite new treatments like immunotherapy and proton beam therapy, chemotherapy is still a vital tool in any cancer doctor’s arsenal. Whilst researchers are hard at work on new innovative treatments, there is still a lot of work needed to find new chemotherapy medicines and improve existing ones. Together, this will help build a brighter future with better, safer and kinder treatments for all children and young people with cancer.