**2. Cancer origins**

Cancer (proper medical name - malignant neoplasm) commonly considered to be a civilization disease, has in fact been traced to occur even before the ancestral species of man [5]. The oldest evidence of cancer dates back to several million years ago and has been found in fossilized remains (bones) of a dinosaur in Wyoming. The oldest specimens of cancer, a hominid malignant tumour (probably Burkitt's lymphoma) and bone cancer - were found in the remains of a body of either Homo erectus or an Australopithecus and in the remains of a female skull dating to the Bronze Age (1900-1600 B.C.), respectively. Bone cancers have been also discov‐ ered in mummies in the Great Pyramid of Giza and in mummified skeletal remains of Peruvian Incas. The earliest written records differentiating between benign and malignant cancers date back to ancient times (3000-1500 B.C., Mesopotamia and Egypt). Seven Egyptian Papyruses including the Edwin Smyth (2500 B.C.), Leyde (1500 B.C.), and George Ebers (1500 B.C.) described not only the symptoms but also the first primitive forms of treatment, i.e. the removal of a malignant tissue. The Hindu epic, the Ramayana (500 B.C.), mentioned not only cancer cases but also the first medicines in the form of arsenic pastes, for treatment of cancerous growth. Ancient Greek physician Hippocrates of Kos (ca. 460-370 B.C.) described many different types of cancer (breast, uterus, stomach, skin, and rectum) recognised the difference between benign and malignant tumours and formulated the humoral theory of cancer genesis. As the veins surrounding the tumour resembled the crab claws, he named the disease after the Greek word *carcinos*. Cornelus Celsus (ca. 25 B.C.-50 A.D.), who described the first surgeries on cancers, translated Greek carcinos into now commonly used Latin term cancer. Claudius Galen (129-216 A.D.), the most famous Roman Empire physician, who wrote about 500 medical treatises, left a comprehensive descriptions of many neoplasms. He introduced the Greek word *oncos* (swelling) to describe tumours. Nowadays the use of Hippocrates and Celsus term is limited to describe malignant tumours, while Galen's term is used as a part of the name of the branch of medicine that deals with cancer - that is oncology. Followers of his works in Constantinople, Alexandria, Athens explained the appearance of cancer as a result of an excess of black bile. This idea prevailed through up to the 16th century.

the overall survival time increased significantly, especially in high-developed countries, but in fact the metastases, not the cancer itself, are the major cause of death. In 1971, only 50% of people diagnosed with the cancer went on to live at least five years, while nowadays, the fiveyear survival rate is 63% [2]. However if a cancer has spread the chances of survival are only scarcely better than in the 1970s. These numbers indicate that although the knowledge about cancer in the last two decades raised, even to a larger degree than in all preceding centuries, but the problem of cancer diseases persists and our knowledge is still insufficient to solve it. Despite the remarkable progress in cancer prevention, early detection, and treatment, made during the last few decades, the methods of cancer diagnosis and treatment are still not

Not so long ago in the beginning of 20th century, neither carcinogens nor cellular targets were identified while the treatment was carried out exclusively by surgeries or natural products selected by trial and error. Modern cancer therapy based on the so-called holistic approach the combined use of surgical methods, radiotherapy, chemotherapy, hormonal therapy and immunotherapy - is applied in the treatment of cancer at most stages. In fact this approach originated from the ancient Sumerian, Akkadian, Babylonian, Assyrian and Egyptian medi‐ cine, and was largely influenced by the Roman and Greek ideas concerning anatomy, physi‐ ology as well as the achievements of practical medicine and natural science. The chemotherapy, hormonal therapy, immunotherapy and radiotherapy as the methods of cancer treatment joined to the oldest surgical one only in the 20th c. An important component of the combined therapy, but sometimes when cancer had already metastasised, the only available therapeutic method, is chemotherapy using natural or synthetic anticancer drugs and treated as curative, palliative, adjuvant or neoadjuvant. Over the centuries anticancer drugs evolved from natural products, discovered mainly from green plants and minerals to fully chemically synthesized chemotherapeutic agents. However, even today drugs of natural origin play an important role in the treatment of cancer as 14 of them were on the list of the top 35 drugs worldwide sales [4]. The process of anticancer drug discovery leading from natural products to chemothera‐ peutic agents, often illicitly limited only to cytostatic and antiproliferative, has evolved from serendipity to rational design based on advances in chemistry, physics and biology in a long and complicated process. Nowadays both cancer itself and anticancer drugs are investigated at the molecular level thus methods of drug discovery have changed diametrically. The dominant direction of contemporary aniticancer drug discovery is the search for the possibil‐ ities to influence the pathogenetic mechanisms specific of the tumour structures at the cellular

sufficiently specific and effective thus cancer still takes a heavy toll.

and molecular levels, which require the knowledge of cancer origins.

**2. Cancer origins**

36 Drug Discovery

This chapter will focus on the factors which influenced the direction of anticancer drug discovery methods from guessing to the targeted search i.e. from serendipity to rational design.

Cancer (proper medical name - malignant neoplasm) commonly considered to be a civilization disease, has in fact been traced to occur even before the ancestral species of man [5]. The oldest

The intensive studies in the field of anatomy and physiology during the Renaissance, resulted in advancement of surgery and development of rational therapies based on clinical observa‐ tions. Based on autopsies William Harvey (1578-1657) described the systemic circulation of blood through the heart and body. Although cancers were still incurable, their temporary inhibition was often observed thanks to complementary remedies including the most common arsenic-based creams and pastes. In the beginning of the 16th c. Zacutus Lusitani (1575-1642) and Nicholas Tulp (1593−1674) formulated the contagion theory and proposed isolation of patients in order to prevent the spread of cancer. Throughout the 17th and 18th centuries, this theory was so popular that the first cancer hospital founded in Reims, France, was forced to move outside the city. Nowadays, we know that their certain viruses, bacteria, and parasites can increase a risk of developing cancer. Gaspare Aselli (1581-1625), who discovered the lymphatic system, suggested a connection between the lymphatic system and cancer. Georg Ernst Stahl (1660-1734) and Friedrich Hoffman (1660-1742) proposed a concept that tumours grow from degenerating lymph constantly excreted by the blood. This idea was accepted by John Hunter (1728-1793), who described methods to identify surgically removable tumours. At that time the so-called humoral theory of cancer was replaced by the lymph theory. Claude-Deshasis Gendron (1663-1750) was convinced that cancer arises as a solid and growing mass untreatable with drugs, and must be completely removed. The discovery of a microscope by Antonie van Leeuwenhoek (1632-1723) in the late 17th century extended the knowledge about the cancer formation process and accelerated the search for the origin of cancer. It was realised that the progress in cancer treatment critically depends on the ability to distinguish between normal and malignant cells. Giovanni Battista Morgagni (1682-1771), father of pathomorphol‐ ogy, related the illness to pathological changes that laid the foundation for scientific oncology. This observation in connection with discovery of anaesthesia in 1844 by Horace Wells (1815-1848) enabled development of precise diagnosis of cancer and modern radical cancer surgery. In 1838, Johannes Peter Muller (1801-1858) indicated cells as basic units of tumours and proposed the blastema theory that cancer cells developed from budding elements (blastema) between normal tissues. In 1860, Karl Thiersch (1822-1895), showed that cancers metastasize through the spread of malignant cells and described establishment of secondary cancer as a result of their spread by lymph. Rudolf Virchow (1821-1902), the founder of cellular pathology, recognized leukaemia cells. He showed that cancer cells can be differentiated from surrounding normal cells from which they originated and the stage of cancer can be deter‐ mined using microscopic images. Virchow also properly recognized chronic irritation as one of the factors favouring cancer development. Nowadays, we are aware that cancers arise from sites of infection, chronic irritation and inflammation. The next key step in understanding the mechanism of cancer development was the discovery of chromosome and mitosis credited to German botanist Wilhelm Hofmeister (1824-1877). In 1902 Theodor Boveri (1862-1915) reasoned that a cancerous tumour begins with a single cell, which divided uncontrollably, while David Paul von Hansemann (1858–1920), included multipolar mitoses among the factors responsible for the arise of abnormal chromosome numbers in cells leading to tumour formation. In fact Hansemann formulated chromosomal theory of cancer, while Boveri proposed the existence of cell-cycle checkpoints, tumour suppressor genes, and oncogenes and speculated that uncontrolled growth might be caused by physical (radiation), chemical (some chemicals) or biological (microscopic pathogens) factors. Thomas Hunt Morgan (1866-1945) made a key observations of chromosomal changes and demonstrated in 1915 the correctness of this theory. But still the carcinogens like chemical agents or irradiation could not explain the fact that sometimes cancer seemed to run in families. Already in the 17th c. Lusitani and Tulp observed the appearance of breast cancer in whole families.

long arm of chromosome 13 at position 14.2 in humans. Its mutation results in retinoblastoma juvenile eye tumour. On the basis of the earlier (dated to 1953) findings of Carl Nordling, he has formulated the accepted till now "two-hit hypothesis" which assumes that both alleles coding a particular protein must be affected before an effect is manifested. Knudson provided an explanation of the relationship between the hereditary and non-hereditary origins of cancer and predicted the existence of tumor suppressor genes that can suppress cancer cell growth. It was later discovered that both classes of genes proto-oncogenes and tumor-suppressor genes encode many kinds of proteins controlling cell growth and proliferation and the mutations in

Anticancer Drug Discovery — From Serendipity to Rational Design

http://dx.doi.org/10.5772/52507

39

In 1976 John Michael Bishop and Harold Elliot Varmus discovered the presence of oncogenes in many organisms including humans. Nowadays, after human genome sequencing in 2004, we know that human DNA contains approximately 20,500 genes [6]. About 50 of them are known to be proto-oncogenes, while 30 tumour suppressor genes. The proto-oncogenes (*conc*) initiate the process of cell division and code enzymes which control grows and division of cells. Proto-oncogenes can be activated to oncogenes by many factors including chromosome rearrangements, gene duplication, mutation or overexpression. For example, a chromosome rearrangement results in formation of BCR-ABL gene which leads to chronic myeloid leukae‐ mia [7], acquired mutation activate the KIT gene which results in gastrointestinal stromal tumour [8], while inheritance of BRCA1 or BRCA2 increase the risk of breast, ovarian, fallopian tube, and prostate cancers [9]. To cause cancer most oncogenes require an additional step, for example mutations in another gene or introduction of foreign DNA (e.g. by viral infection). Infection and inflammation significantly contribute to about 25% of cancer cases. During the inflammatory response to viral infection the free radicals - reactive oxygen and nitrogen species - are generated as a physiological protective response. During chronic inflammation the mechanism is different - free radicals induce genetic and epigenetic changes including somatic mutations in cancer-related genes and posttranslational modifications in proteins involved in DNA repair or apoptosis. However, irrespective of the origins, the tumour microenvironment created by inflammatory cells, is an essential factor in the whole neoplastic process. It facilitates proliferation and survival of malignant cells, promotes angiogenesis and metastasis, subverts adaptive immune responses, and alters responses to chemotherapeutic agents. If a cell accumulates critical mutations in a few of these proto-oncogenes (five or six), it will survive instead of undergoing apoptosis, will proliferate and become capable of forming a tumour. The protecting mechanism involves the tumour suppressor genes, "anti-oncogenes", which protect from developing or growing cancer by repairing DNA damages (mutations), inhibiting cell division and cell proliferation or prevent reproduction by stimulating apoptosis. Mutation of these genes may lead to cessation of the inhibition of cell division. As a result the cell will divide uncontrollably, and produce daughter cells with the same defect. For example, mutation in the TP53 gene (initially after discovery in 1979 by Arnold Levine, David Lane and William Old incorrectly believed to be an oncogene), one of the most commonly mutated tumour suppressor genes which encoding tumour protein - so called p53 protein, a key element in stress-induced apoptosis, is involved in the pathophysiology of leukaemias, lymphomas, sarcomas, and neurogenic tumours [10,11]. Homozygous loss of p53 is found in 70% of colon cancers, 50% of lung cancers and 30–50% of breast cancers. Other important tumour suppressor

these genes can contribute to carcinogenesis.

A rapid progress in understanding the cancer origins was possible thanks to the scientific progress and appearance of instruments required to solve complex interdisciplinary problems of chemistry and biology. The turning points in the research on cancer were the mapping of locations of the fruit fly (Drosophila melanogaster) genes by Alfred Sturtevant (1891-1970), the discovery that DNA is the genetic material by Oswald Avery (1877-1955), Colin Munro MacLeod (1909-1972) and Maclyn McCarty (1911-2005) and the resolution of the exact chemical structure of DNA, the basic material in genes, by James Watson and Francis Crick (1916-2004). Their results indicated that DNA was the cellular target for carcinogens and that mutations were the key to understanding the mechanisms of cancer. In 1970 the first oncogene (SRC, from sarcoma) a defective proto-oncogene i.e. gene which after mutation, predispose the cell to become a cancerous (stimulate cell proliferation) was discovered by G. Steve Martin in a chicken retrovirus. One year later, but long before the human genome was sequenced, Alfred George Knudson identified first tumour suppressor gene, the Rb gene, located on a region of long arm of chromosome 13 at position 14.2 in humans. Its mutation results in retinoblastoma juvenile eye tumour. On the basis of the earlier (dated to 1953) findings of Carl Nordling, he has formulated the accepted till now "two-hit hypothesis" which assumes that both alleles coding a particular protein must be affected before an effect is manifested. Knudson provided an explanation of the relationship between the hereditary and non-hereditary origins of cancer and predicted the existence of tumor suppressor genes that can suppress cancer cell growth. It was later discovered that both classes of genes proto-oncogenes and tumor-suppressor genes encode many kinds of proteins controlling cell growth and proliferation and the mutations in these genes can contribute to carcinogenesis.

Antonie van Leeuwenhoek (1632-1723) in the late 17th century extended the knowledge about the cancer formation process and accelerated the search for the origin of cancer. It was realised that the progress in cancer treatment critically depends on the ability to distinguish between normal and malignant cells. Giovanni Battista Morgagni (1682-1771), father of pathomorphol‐ ogy, related the illness to pathological changes that laid the foundation for scientific oncology. This observation in connection with discovery of anaesthesia in 1844 by Horace Wells (1815-1848) enabled development of precise diagnosis of cancer and modern radical cancer surgery. In 1838, Johannes Peter Muller (1801-1858) indicated cells as basic units of tumours and proposed the blastema theory that cancer cells developed from budding elements (blastema) between normal tissues. In 1860, Karl Thiersch (1822-1895), showed that cancers metastasize through the spread of malignant cells and described establishment of secondary cancer as a result of their spread by lymph. Rudolf Virchow (1821-1902), the founder of cellular pathology, recognized leukaemia cells. He showed that cancer cells can be differentiated from surrounding normal cells from which they originated and the stage of cancer can be deter‐ mined using microscopic images. Virchow also properly recognized chronic irritation as one of the factors favouring cancer development. Nowadays, we are aware that cancers arise from sites of infection, chronic irritation and inflammation. The next key step in understanding the mechanism of cancer development was the discovery of chromosome and mitosis credited to German botanist Wilhelm Hofmeister (1824-1877). In 1902 Theodor Boveri (1862-1915) reasoned that a cancerous tumour begins with a single cell, which divided uncontrollably, while David Paul von Hansemann (1858–1920), included multipolar mitoses among the factors responsible for the arise of abnormal chromosome numbers in cells leading to tumour formation. In fact Hansemann formulated chromosomal theory of cancer, while Boveri proposed the existence of cell-cycle checkpoints, tumour suppressor genes, and oncogenes and speculated that uncontrolled growth might be caused by physical (radiation), chemical (some chemicals) or biological (microscopic pathogens) factors. Thomas Hunt Morgan (1866-1945) made a key observations of chromosomal changes and demonstrated in 1915 the correctness of this theory. But still the carcinogens like chemical agents or irradiation could not explain the fact that sometimes cancer seemed to run in families. Already in the 17th c. Lusitani and

38 Drug Discovery

Tulp observed the appearance of breast cancer in whole families.

A rapid progress in understanding the cancer origins was possible thanks to the scientific progress and appearance of instruments required to solve complex interdisciplinary problems of chemistry and biology. The turning points in the research on cancer were the mapping of locations of the fruit fly (Drosophila melanogaster) genes by Alfred Sturtevant (1891-1970), the discovery that DNA is the genetic material by Oswald Avery (1877-1955), Colin Munro MacLeod (1909-1972) and Maclyn McCarty (1911-2005) and the resolution of the exact chemical structure of DNA, the basic material in genes, by James Watson and Francis Crick (1916-2004). Their results indicated that DNA was the cellular target for carcinogens and that mutations were the key to understanding the mechanisms of cancer. In 1970 the first oncogene (SRC, from sarcoma) a defective proto-oncogene i.e. gene which after mutation, predispose the cell to become a cancerous (stimulate cell proliferation) was discovered by G. Steve Martin in a chicken retrovirus. One year later, but long before the human genome was sequenced, Alfred George Knudson identified first tumour suppressor gene, the Rb gene, located on a region of In 1976 John Michael Bishop and Harold Elliot Varmus discovered the presence of oncogenes in many organisms including humans. Nowadays, after human genome sequencing in 2004, we know that human DNA contains approximately 20,500 genes [6]. About 50 of them are known to be proto-oncogenes, while 30 tumour suppressor genes. The proto-oncogenes (*conc*) initiate the process of cell division and code enzymes which control grows and division of cells. Proto-oncogenes can be activated to oncogenes by many factors including chromosome rearrangements, gene duplication, mutation or overexpression. For example, a chromosome rearrangement results in formation of BCR-ABL gene which leads to chronic myeloid leukae‐ mia [7], acquired mutation activate the KIT gene which results in gastrointestinal stromal tumour [8], while inheritance of BRCA1 or BRCA2 increase the risk of breast, ovarian, fallopian tube, and prostate cancers [9]. To cause cancer most oncogenes require an additional step, for example mutations in another gene or introduction of foreign DNA (e.g. by viral infection). Infection and inflammation significantly contribute to about 25% of cancer cases. During the inflammatory response to viral infection the free radicals - reactive oxygen and nitrogen species - are generated as a physiological protective response. During chronic inflammation the mechanism is different - free radicals induce genetic and epigenetic changes including somatic mutations in cancer-related genes and posttranslational modifications in proteins involved in DNA repair or apoptosis. However, irrespective of the origins, the tumour microenvironment created by inflammatory cells, is an essential factor in the whole neoplastic process. It facilitates proliferation and survival of malignant cells, promotes angiogenesis and metastasis, subverts adaptive immune responses, and alters responses to chemotherapeutic agents. If a cell accumulates critical mutations in a few of these proto-oncogenes (five or six), it will survive instead of undergoing apoptosis, will proliferate and become capable of forming a tumour. The protecting mechanism involves the tumour suppressor genes, "anti-oncogenes", which protect from developing or growing cancer by repairing DNA damages (mutations), inhibiting cell division and cell proliferation or prevent reproduction by stimulating apoptosis. Mutation of these genes may lead to cessation of the inhibition of cell division. As a result the cell will divide uncontrollably, and produce daughter cells with the same defect. For example, mutation in the TP53 gene (initially after discovery in 1979 by Arnold Levine, David Lane and William Old incorrectly believed to be an oncogene), one of the most commonly mutated tumour suppressor genes which encoding tumour protein - so called p53 protein, a key element in stress-induced apoptosis, is involved in the pathophysiology of leukaemias, lymphomas, sarcomas, and neurogenic tumours [10,11]. Homozygous loss of p53 is found in 70% of colon cancers, 50% of lung cancers and 30–50% of breast cancers. Other important tumour suppressor genes include p16, BRCA-1, BRCA-2, APC or PTEN [12]. Mutation of these genes may lead to melanoma (p16), breast and ovarian cancer in genetically related families (BRCA), colorectal cancer (APC) or glioblastoma, endometrial cancer, and prostate cancer (PTEN).

results in a decrease in the effectiveness of the therapy. The recent targeted therapies instead of interfering with rapidly dividing cells, interfere with selected targets in the cell and use small molecules to interfere with abnormal proteins (required for carcinogenesis and tumour growth) or cell receptors, or use monoclonal antibodies, which destroy malignant tumour cells and prevent tumour growth by blocking specific receptors. Targeted cancer therapy, which may be more effective and less harmful than classical chemotherapy but still are based on the use of chemical compounds is perceived as modern chemotherapy or chemotherapy of the

> **PROMOTION cumulations of mutations**

Currently we are aware that apart of inherited mutations, an important role in carcinogenesis play the factors connected with the expression to carcinogens [14]. This includes environmental factors (pollutions), lifestyle factors (tobacco smoking, diet, alcohol consumption, obesity, sedentary life), occupational factors (e.g. synthesis, dyes, fumes) and other factors (excessive exposure to sunlight, radiation, viruses, etc). Carcinogens (of chemical, physical or biological origin) include chemicals or non-chemical agents, which under certain conditions are able to induce cancer. Co-carcinogens, are not carcinogenic themselves but with other chemicals or non-chemical carcinogens, such as for example UV or ionizing radiation, promote the effects of a carcinogen in carcinogenesis. Carcinogens as well as co-carcinogens can be of natural or synthetic origins. In general, their carcinogenic action relay on direct or indirect action in the cellular DNA. Carcinogens acting directly can initiate the carcinogenesis by yielding highly reactive species that bind covalently to cellular DNA, while those acting indirectly can induce mutations to cellular DNA. Thus carcinogens are able to distort the conformation or function (replication/transcription) of DNA, which results not only in oncogene activation but also DNA amplification, gene transposition or chromosome translocation. Carcinogens may induce carcinogenesis directly by mutational activation of protooncogenes and/or inactivation of tumour suppression genes. Indirect action is realised through the mechanisms that generate chemical species (free radicals, reactive oxygen species, carcinogenic metabolites) which are

Over 80% of carcinogenic substances are of environmental origins [15]. Restriction of the exposition to carcinogens can substantially reduce the risk of cancers also those of occupational type, which make approximately 4-5% of all human cancers. Thus evaluation and classification of the carcinogens is required from the cancer prevention point of view. Although there are many international and national organizations that classify carcinogens, but only a few are

**PROGRESSION cancer in situ**

Anticancer Drug Discovery — From Serendipity to Rational Design

**MALIGNANCY metastasis**

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41

future.

**PREINITIATION exposure to cancerogenes**

**Figure 1.** Multi-step process of carcinogenesis.

capable of entering the nucleus of the cell.

**INITIATION mutations**

**3. Cancer risk — Carcinogens and co-carcinogens**

In general cancerogenesis is a multistep process thus it is usually a combination of protooncogene activation and tumour suppressor gene loss or inactivation is required. However, in a few cases (only 5-10% of cancer cases) this abnormal change in gene can be inherited, passed from generation to generation, in most cases it is a result of sporadic or somatic mutation acquired during a person's lifetime. Although cancer is generally believed to arise as a result of slow accumulation of multiple mutations, but in some cases (2-3%) massive multiple mutation can also arise in a single event. Thus cancer is described as a disease of abnormal gene function, genetically caused by the interaction of two factors: genetic suscept‐ ibility and environmental mutagens and carcinogens. Of key importance for the recognition of the molecular mechanism underlying cancer treatment - cell apoptosis - was the discovery of telomeres and telomerase. In the early 1970s, Alexei Olovnikov, on the basis of the Leonard Hayflick's concept of limited somatic cell divisions (Hayflick limit), suggested that chromo‐ somes cannot completely replicate their ends. In 1978 Elizabeth Blackburn discovered the unusual nature of stretches of DNA in the ends of the chromosomes of protozon *Tetrahyme‐ na* - the so-called telomeres. The sequence of human telomere was established 10 years later, in 1988, by Robin Allshire. Blackburn described telomere-shortening mechanism which limits cells to a fixed number of divisions and protect chromosomes from fusing each other or rearranging that can lead to cancer. Shortened telomeres have been found in many cancers, including pancreatic, bone, prostate, bladder, lung, kidney, and head and neck. In 1985, Carol Greider isolated the enzyme telomerase, controlling the elongation of telomeres. Four years later, in 1989, Gregg B. Morin reported the presence of telomerase in human tumour cells and linked its activity with the immortality of these cells (inability to apoptosis), while Greider discovered the lack of active telomerase in normal somatic cells apart from stem cells, kerati‐ nocytes, intestines, and hair follicle. It was discovered that deactivation of telomerase prompts the apoptosis of human breast and prostate cancer cells. These results indicated the important role of telomerase in the process of oncogenesis.

Carcinogenesis have been found a complex and multi-step (preinitiation, initiation, promotion and metastasis) biological process characterised by independence from growth factors, insensitivity to inhibitors of growth, unlimited potential for replication (reactivation of telomerase), invasiveness, the ability to metastasis and to sustain angiogenesis, and resistance of apoptosis [13]. DNA mutation inherited and caused by exposure to carcinogens (chemical: compounds including drugs, physical: radiation, or biological: the introduction of new DNA sequences by viruses) have been found to be the true origin of uncontrolled growth of cells coupled with malignant behaviour: invasion and metastasis (Fig. 1).

The knowledge of the molecular mechanisms involving the above mentioned factors, espe‐ cially apoptosis and cancer resistance to it, can improve cancer therapy through resensitization of tumour cells. Fundamental method of cancer treatment - classical chemotherapy (and radiotherapy) which is harmful also to normal cells, act primarily by inducing cell apoptosis either locally, in tumour, or globally, when cancer metastasize. Any disturbance in apoptosis results in a decrease in the effectiveness of the therapy. The recent targeted therapies instead of interfering with rapidly dividing cells, interfere with selected targets in the cell and use small molecules to interfere with abnormal proteins (required for carcinogenesis and tumour growth) or cell receptors, or use monoclonal antibodies, which destroy malignant tumour cells and prevent tumour growth by blocking specific receptors. Targeted cancer therapy, which may be more effective and less harmful than classical chemotherapy but still are based on the use of chemical compounds is perceived as modern chemotherapy or chemotherapy of the future.

**Figure 1.** Multi-step process of carcinogenesis.

genes include p16, BRCA-1, BRCA-2, APC or PTEN [12]. Mutation of these genes may lead to melanoma (p16), breast and ovarian cancer in genetically related families (BRCA), colorectal

In general cancerogenesis is a multistep process thus it is usually a combination of protooncogene activation and tumour suppressor gene loss or inactivation is required. However, in a few cases (only 5-10% of cancer cases) this abnormal change in gene can be inherited, passed from generation to generation, in most cases it is a result of sporadic or somatic mutation acquired during a person's lifetime. Although cancer is generally believed to arise as a result of slow accumulation of multiple mutations, but in some cases (2-3%) massive multiple mutation can also arise in a single event. Thus cancer is described as a disease of abnormal gene function, genetically caused by the interaction of two factors: genetic suscept‐ ibility and environmental mutagens and carcinogens. Of key importance for the recognition of the molecular mechanism underlying cancer treatment - cell apoptosis - was the discovery of telomeres and telomerase. In the early 1970s, Alexei Olovnikov, on the basis of the Leonard Hayflick's concept of limited somatic cell divisions (Hayflick limit), suggested that chromo‐ somes cannot completely replicate their ends. In 1978 Elizabeth Blackburn discovered the unusual nature of stretches of DNA in the ends of the chromosomes of protozon *Tetrahyme‐ na* - the so-called telomeres. The sequence of human telomere was established 10 years later, in 1988, by Robin Allshire. Blackburn described telomere-shortening mechanism which limits cells to a fixed number of divisions and protect chromosomes from fusing each other or rearranging that can lead to cancer. Shortened telomeres have been found in many cancers, including pancreatic, bone, prostate, bladder, lung, kidney, and head and neck. In 1985, Carol Greider isolated the enzyme telomerase, controlling the elongation of telomeres. Four years later, in 1989, Gregg B. Morin reported the presence of telomerase in human tumour cells and linked its activity with the immortality of these cells (inability to apoptosis), while Greider discovered the lack of active telomerase in normal somatic cells apart from stem cells, kerati‐ nocytes, intestines, and hair follicle. It was discovered that deactivation of telomerase prompts the apoptosis of human breast and prostate cancer cells. These results indicated the important

Carcinogenesis have been found a complex and multi-step (preinitiation, initiation, promotion and metastasis) biological process characterised by independence from growth factors, insensitivity to inhibitors of growth, unlimited potential for replication (reactivation of telomerase), invasiveness, the ability to metastasis and to sustain angiogenesis, and resistance of apoptosis [13]. DNA mutation inherited and caused by exposure to carcinogens (chemical: compounds including drugs, physical: radiation, or biological: the introduction of new DNA sequences by viruses) have been found to be the true origin of uncontrolled growth of cells

The knowledge of the molecular mechanisms involving the above mentioned factors, espe‐ cially apoptosis and cancer resistance to it, can improve cancer therapy through resensitization of tumour cells. Fundamental method of cancer treatment - classical chemotherapy (and radiotherapy) which is harmful also to normal cells, act primarily by inducing cell apoptosis either locally, in tumour, or globally, when cancer metastasize. Any disturbance in apoptosis

cancer (APC) or glioblastoma, endometrial cancer, and prostate cancer (PTEN).

40 Drug Discovery

role of telomerase in the process of oncogenesis.

coupled with malignant behaviour: invasion and metastasis (Fig. 1).
