Chemotherapy
Oncologist Matthias Dobbelstein – about the stages of development of chemotherapy, DNA damage and anti-oncogene p53.
There are many interpretations of the term "chemotherapy". What we call chemotherapeutic drugs are substances that destroy cancer cells, but to be more precise, these compounds destroy cell proliferation. This means interference with the integrity and replication of DNA or disruption of the division spindle.
Development of chemotherapy
The term "chemotherapy" appeared about 100 years ago when the German doctor Paul Ehrlich, who discovered that the chemically synthesized compound salvarsan is active against pale treponema, a microorganism responsible for the previously incurable disease syphilis. After learning about this discovery, the German emperor proposed to expand the concept of chemotherapy for the treatment of not only infectious diseases, but also cancer.
So what was the first anti-cancer drug? During the world Wars, various types of chemical weapons were used on the battlefields. Among them, nitrogenous mustard gas, which are alkylating agents of DNA, are of particular interest. An incident occurred on the Italian island of Bari: a ship loaded with nitrogenous mustard gas exploded, as a result of which a large amount of gas was released on the residents of the city. This tragic event is interesting because the survivors subsequently suffered from anemia. The bone marrow's ability to generate new blood cells has been compromised. This class of alkylating agents, as it later became clear, prevents the proliferation of leukemic cells. Alkylating agents are one of the first chemotherapeutic agents used in treatment.
Another approach to chemotherapy was invented by Boston scientist Sidney Farber. He discovered aminopterin, an antimetabolite of folic acid, which interfered with DNA synthesis. In an experimental trial, he injected aminopterin into a young patient with leukemia, as a result, the child went into remission. Unfortunately, the remission did not last long, the disease returned, but there was an understanding that chemicals can suppress the growth of cancer cells.
Targeted therapies
So the hunt for anti-cancer drugs began. But it soon became clear that this search turned out to be a complex and rather unscientific enterprise. The drugs were developed on the basis of data from tests of their anticancer potency, which were carried out by trial and error, and not by trying to explain the molecular component of therapy.
In the late 1990s, a new molecular approach was used to find new anticancer agents. Targeted therapy has become an alternative to classical chemotherapy. This molecular approach uses compounds that are specifically targeted at a specific molecule in a cancer cell – usually a signal-transmitting protein that serves as a link for cellular reproduction. The search for these therapies is rarely successful, but some compounds lead to impressive remission of certain subtypes of cancer. For example, the small molecule inhibitor Glivec (imatinib), which interferes with the activity of the Bcr-Abl protein, can lead to permanent remission in a significant number of cases of chronic myeloid leukemia (CML) in patients.
However, such powerful compounds have not been found for most other cancers. On the one hand, cancer cells often manage to avoid targeted drugs and develop independently of the targeted factor, using, for example, an alternative method of signal transmission through kinases. Resistance can also be developed in relation to classical chemotherapy, but since it targets whole molecular apparatuses, resistance is much rarer. Consequently, the main anti-cancer therapy still consists of classical chemotherapy and has not yet been replaced by targeted.
The role of the p53 gene
In more than 50% of cancers, a mutation of the anti-oncogene p53 has been detected. P53 is often called the "guardian of the genome", and it serves as the main regulator, according to which the cell makes a choice: to correct the damage and survive or undergo apoptosis, that is, programmed cell death. As a result, it was predicted that tumor cells without p53 would be more sensitive to classical chemotherapeutic agents, since they would not have an important regulatory mechanism.
In the late 1990s, one of Bert Vogelstein's colleagues named Binz removed p53 from the HCT116 collateral cell line. Then it was a great scientific achievement. After that, they compared cell lines with and without the presence of p53, checking how they would react to various classes of classical chemotherapeutic drugs. They expected that cell lines without p53 would be much more sensitive to chemotherapeutic agents, since they do not have a master regulator. However, the results were somewhat disappointing: different classes of chemotherapeutic drugs had different effects on cells with and without p53.
What is the reason for this? The answer to this question lies in the variety of activation effects of p53. P53 is able to stop the cell cycle by activating the transcription factor p21, which regulates the subsequent links of signaling cascades. And a stopped cell cycle is a pretty favorable location for a cell if it has been hit with a large dose of a chemotherapeutic agent. Then the process can develop in two directions: either the cell decides to correct the lesion and restarts the cell cycle, or it undergoes apoptosis. Thus, whether p53 is beneficial or toxic to the cell cycle depends on the stage of the cell cycle and the class of chemotherapy. Therefore, now p53 does not play a big role in predicting clinical methods of cancer treatment.
Treatment through p53 activation
The status of p53 cancer cells is the most common defining characteristic: only half of all cancers have p53 mutations, which makes these cells different from healthy patient cells. How could we use this difference? One idea is to pharmacologically activate p53 and try to protect healthy cells from chemotherapeutic drugs acting in sensitive cell phases, such as DNA replication and cell division. This is an interesting approach, since patients receiving classical chemotherapeutic treatment often have side effects: anemia, diseases of the gastrointestinal tract, hair loss. All this is due to the damage of stem cells in the tissues. The advantages of protective methods are obvious: the side effects of chemotherapy not only limit the dosage of drugs, but also create physiological and psychological problems and even become life-threatening.
So is there a way to activate p53 pharmacologically? It turned out that a small nutlin inhibitor stabilizes p53 in a non-genotoxic way. It makes it resistant to its antagonist and negative regulator MDM2. In vitro tests of cell lines show encouraging results, and treatment with nutlin stops the cell cycle and protects cells from replicative and mitotic toxic drugs. In animal experiments, however, nutlin stimulates anemia, from which it is intended to protect.
Somehow the inhibitor causes apoptosis rather than stopping the cell cycle in the blood stem cell population. One way to improve the situation may be the inhibition of apoptosis with the help of certain compounds of small molecules. This approach is relevant and of scientific interest, since various auto-inflammatory, neurodegenerative diseases, as well as liver diseases cause apoptosis during their pathogenesis. With a better understanding of the molecular biology of nutlin's action, p53 can become a protective agent in the combination therapy of classical chemotherapeutic agents and targeted therapy.
Chemotherapy and DNA
An extremely important macromolecule, which is the target of many classes of classical chemotherapy, contains our genetic code – DNA. Thus, the maintenance and repair of DNA are important components of the cell's ability to survive. They also represent an interesting area for the development of anti-cancer therapies. Biochemically, DNA damage manifests itself in the form of chemical modification of bases, such as alkylation and single and double DNA breaks. These DNA changes lead to the activation of various proteins that respond to DNA damage.
Initially, only a few factors were identified: ATM, ATR, Chk1 and Chk2 kinases. Over the years, the factors regulating the subsequent links of the signal cascades have been identified. There are quite a lot of them, each of them consists of different types of proteins. They include mainly kinases, but also other proteins such as ubiquitin ligases, one of which is the well–known BRCA1 (breast cancer 1) - a factor associated with breast and ovarian cancer. BRCA1 has become an important prognostic marker for both classical and targeted chemotherapy. This means that its presence or absence has a strong influence on the doctor's decision regarding therapy. Nevertheless, BRCA1 is an exceptional case among the factors of DNA damage: only it has a prognostic influence on clinical decisions.
Initially, cancer researchers dreamed of measuring levels of DNA damage factors and designing targeted therapies according to the results, but this approach proved largely unsuccessful. On the other hand, a new approach is being tested, in particular, inhibiting the DNA damage reaction by inhibiting DNA damage of ATM, Chk1 and Wee1 kinases using specific small molecule inhibitors against them. In in vitro cell lines, as well as in animal models, it was found that a weak response to DNA damage can enhance the damage caused by classical chemotherapeutic drugs. In 2016, this approach entered the initial stage of clinical trials.
The Future of Chemotherapy
Classical chemotherapy is not able to cure all types of cancer, but in some cases, especially in the case of cancers in children, it is able to prolong life or even cure the disease. The search for effective anticancer compounds has been slow but steady. In such a complex and multifaceted field as cancer treatment, even just steady improvements are desirable, even if the process is too slow to meet high expectations. These sustained improvements should encourage us to continue to invest more and more effort in research to get improvements in the treatment of cancer patients.
About the author:
Matthias Dobbelstein – Professor of Molecular Oncology, Head of Department of Molecular Oncology, Georg-August-Universität Göttingen
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16.01.2017