Introduction
MMPs are proteases produced by a wide variety of cells in the human body. They are responsible for breaking down the extracellular matrix around cells – the network of protein and carbohydrate found between cells to aid in cell adhesion1 . There are over 20 different types of these proteases, each of which break down particular compounds in the extracellular matrix. In a healthy human body, MMPs are involved in stimulating apoptosis (programmed cell death), wound healing, and embryonic development, among other physiological processes. However, they are also involved in a range of diseases, and they have been studied extensively in their relation to cancer2
MMPs are highly expressed in practically all types of cancerous tumours. One method of detecting this expression is by using immunohistochemistry techniques. This uses antibodies that bind to specific antigens present on the MMP protein. The antibodies themselves are tagged with a visible label, such as fluorescent marker, so that when they bind to the antigens they can be seen under a microscope3 .
MMPs play various roles in the development of cancer; they are linked to tumour growth and are also involved in metastasis. Metastasis is the process in which cancer cells from the original tumour break away and invade other parts of the body. In order for this to occur, MMPs are produced by the cancerous cells to break down the basement membranes of blood vessels and lymphatic vessels so that they can enter and leave the circulatory system4 . Once the cancer cells have migrated to another location, they multiply to form small tumours. This presents the danger of forming blockages, and means that the cancer is harder to treat as tumours may be found in numerous places around the body.
An important process required for these metastatic tumours to be sustained is angiogenesis – the formation of new blood vessels. This is needed so that the cancerous cells are able to exchange substances with blood: they can obtain oxygen and glucose needed to continue multiplying, and waste products can be removed. During angiogenesis, the endothelial cells which make up part of the blood vessel secrete MMPs. This allows the endothelial cells to migrate into the surrounding tumour, and here they can divide to form a network of blood vessels5 . Specific MMPs have been linked to cancer progression in some studies. For example, mice without the gene for producing MMP-2 (created by a technique known as gene knockout) were found to have reduced tumour angiogenesis6 , whilst MMP-11 is thought to inhibit cancer-cell apoptosis, allowing tumour growth2 .
Given that metastasis is the primary cause of death among people with cancer7 , one method of treatment being researched is to develop inhibitors of MMPs to interrupt metastasis. There are many ways to target MMPs, such as inhibiting transcription of the gene that produces MMPs, as well as their translation, secretion, activation and mechanism of action.
One example is marimastat, a competitive inhibitor which binds to the zinc atom on the active site of the MMP enzymes. As with any new drug it was first tested on humans in Phase I clinical trials, where it was taken by healthy volunteers to test for toxicity, dosage and pharmacokinetics (the way in which the drug moves through the body)8 . Results from these tests showed that marimastat appeared to have low toxicity and were well absorbed into the blood from the gastrointestinal tract.
However, in subsequent Phase II trials (using patients with various forms of cancer) the drug was found to have an inflammatory effect on joints, which still persisted for over 8 weeks when the treatment was stopped. Another problem encountered was that the concentration of marimastat in the plasma was well below the range thought to have a biologically relevant effect. Even so, these plasma concentrations only show how much of the drug is circulating in the blood, and does not take into account the amount of activated drug bound to MMPs9 .
Whilst the majority of these trials were tested on patients with advanced forms of cancer, it has been suggested that MMP inhibitors would have a more significant effect on those with early stages of cancer – as preventing metastasis early on would lead to an improved prognosis. More randomised trials are needed to define the optimum stage at which to use marimastat and other MMP inhibitors, and to observe their effects in combination with other drugs usually administered in chemotherapy10 .
An alternative method of utilising these proteases is to exploit the fact that there are elevated levels of MMPs in tumour tissue compared to normal tissue. Using this, an improved drug targeting system could be achieved, in which the drugs administered have a minimised effect on healthy cells and tissues and only affect the tumour11 . One way of doing this is to determine a specific sequence of amino acids where MMPs bind. Once this has been done, this amino acid sequence can be added to a drug to create a ‘prodrug’. This ‘prodrug’ would circulate around the body as a stable compound until it reaches the tumour. Here the MMPs would break down the sequence of amino acids and release the drug with its anticancer properties. This would mean the drug is not activated until at the site of the tumour, and so side effects would be reduced.
Research into such a system has been successful to some extent. Patterns of amino acids have been identified as substrates of many MMPs, such as ‘PXXXHy’. The ‘P’ represents the amino acid proline, ‘X’ represents any amino acid, and ‘XHy’ represents a hydrophobic amino acid, usually isoleucine or leucine. However, as this general sequence is shared by many MMPs, some MMPs that are produced by healthy cells could metabolize the prodrug and lead to the drug affecting normal tissues. The challenge is to find substrates that are particular to only one MMP known to have a specific role in cancer, with the hope that this can be used to create a prodrug that is highly selective6 .