Cancer growth blockers

This page is about cancer growth blockers (also called cancer growth inhibitors). They are a type of biological therapy and include tyrosine kinase inhibitors, proteasome inhibitors, mTOR inhibitors, PI3K inhibitors, histone deacetylase inhibitors and hedgehog pathway blockers. You can find information about

Growth factors
What cancer growth blockers are
Types of cancer growth blockers
Possible side effects of cancer growth blockers

Growth factors are chemicals produced by the body that control cell growth. There are many different types of growth factors and they all work in different ways. Some tell cells what type of cells they should become (how they should specialise). Some make cells grow and divide into new cells. Some tell cells to stop growing or to die.

Growth factors work by plugging in to receptors on the cell surface. This sends a signal to the inside of the cell, which sets off a chain of complicated chemical reactions.

There are a number of different growth factors. Examples include

Epidermal growth factor (EGF) – controls cell growth
Vascular endothelial growth factor (VEGF) – controls blood vessel development
Platelet derived endothelial growth factor (PDGF) – controls blood vessel development and cell growth
Fibroblast growth factor(FGF) – controls cell growth

Each growth factor works by attaching to the corresponding receptor on the cell surface. For example, EGF binds to epidermal growth factor receptor (EGFR).

A cancer growth blocker blocks the growth factors that trigger the cancer cells to divide and grow. Scientists are looking at different ways of doing this such as

Lowering levels of the growth factor in the body
Blocking the growth factor receptor on the cancer cell
Blocking the signals inside the cell that start up when the growth factor triggers the receptor

Most of these treatments work by blocking the signalling processes that cancer cells use to divide.

Cancer cells are often very sensitive to growth factors. So if we can block them, we can stop some types of cancer from growing and dividing. Scientists are developing different inhibitors for the different types of growth factors.

It isn’t easy to group biological therapies into different types because the groups often overlap. This can get very confusing. Some cancer growth blockers block blood vessel growth to the growing cancer (anti angiogenic drugs). Others are also monoclonal antibodies. We have more explanation about the different types of biological therapies in this section.

There are different types of cancer growth blockers. They can be grouped according to the types of chemical that they block. Some of the main types are

Tyrosine kinase inhibitors
Proteasome inhibitors
mTOR inhibitors
PI3K inhibitors
Histone deacetylase inhibitors
Hedgehog pathway blockers

Tyrosine kinase inhibitors

Tyrosine kinase inhibitors are also called TKIs. They block chemical messengers (enzymes) called tyrosine kinases. Tyrosine kinases help to send growth signals in cells. So blocking them stops the cell growing and dividing. Cancer growth blockers can block one type of tyrosine kinase or more than one type. TKIs that block more than one type of tyrosine kinase are called multi-TKIs.

Single TKI

Multi TKI

TKIs in use or in clinical trials include

The links above take you to more information about these drugs. You take these TKIs as tablets or capsules, usually once or twice a day.

Proteasome inhibitors

Proteasomes are tiny, barrel shaped structures found in all cells. They help break down proteins that the cell doesn't need into smaller parts. The cell can then use them to make new proteins that it does need. Drug treatments that block proteasomes from working are called proteasome inhibitors. They cause a build up of unwanted proteins in the cell, which makes the cancer cells die.

Bortezomib (Velcade) is a proteasome inhibitor used to treat myeloma. You have it as an injection through a tube into a vein. There is information about bortezomib in the section about treating myeloma.

mTOR inhibitors

mTOR is a type of protein called a kinase protein. It can make cells produce chemicals such as cyclins that trigger cell growth. It may also make cells produce proteins that trigger the development of new blood vessels. Cancers need new blood vessels in order to grow.

In some types of cancer mTOR is switched on, which makes the cancer cells grow and produce new blood vessels. mTOR blockers (inhibitors) are a newer type of cancer growth blocker. They can stop the growth of some types of cancer. mTOR inhibitors include temsirolimus (Torisel), everolimus (Afinitor) and deforolimus.

PI3K inhibitors

PI3Ks are a group of closely related kinase proteins. Their full name is phospho inositide 3 kinases. They do a number of different things in cells. For example they act like switches in the cell – turning on other proteins such as mTOR (see above). Switching on PI3Ks may make cells grow and multiply, or trigger the development of blood vessels, or help cells to move around.

In some cancers PI3K is permanently switched on, which means that the cancer cells grow uncontrollably. Researchers are developing new treatments that block (inhibit) PI3K. They hope this will stop the cancer cells growing and make them die. This type of cancer growth blocker is only available within a clinical trial. It will be some time before we know how well it works in treating cancer.

Histone deacetylase inhibitors

Histone deacetylase inhibitors are also called HDAC inhibitors or HDIs. They block the action of a group of enzymes that remove chemicals called acetyl groups from particular proteins. This can stop cancer cells from growing and dividing and sometimes kills them completely.

HDACs are a newer type of cancer growth blocker. HDACs used in cancer treatment or in clinical trials include vorinostat (Zolinza), belinostat, panobinostat, entinostat and mocetinostat.

Hedgehog pathway blockers

Hedgehog pathway blockers are drugs that target a group of proteins known as the hedgehog pathway. In the developing embryo, these proteins send signals that help cells to grow in the right place and in the right way. The hedgehog pathway can also control the growth of blood vessels and nerves. In adults, hedgehog pathway proteins are not usually active. But in some people, changes in a gene switch them on. Hedgehog pathway blockers are designed to switch off the proteins and stop the growth of the cancer.

This type of biological therapy is quite new. Vismodegib (Erivedge) is an example of a hedgehog pathway blocker being looked at in clinical trials.

All treatments can cause side effects. While there are general side effects for a type of treatment, they vary for each individual drug. In general, cancer growth blockers can cause