Human cancer is probably as old as the human race. It is obvious that cancer did not suddenly start appearing after modernization or industrial revolution. The oldest known description of human cancer is found in an Egyptian seven papyri or writing written between 3000-1500 BC.
Hippocrates, the great Greek physician (460-370 B.C), who is considered the father of medicine is though to be the first person to clearly recognize difference between benign and malignant tumours.
Hippocrates noticed that blood vessels around a malignant tumour looked like the claws of crab. He named the disease karkinos (the Greek name for crab) to describe tumours that may or may not progress to ulceration. In English this term translates to carcinos or carcinoma.
During the Renaissance, beginning in the 15th century, physicians acquired greater knowledge of the anatomy and physiology of the human body. This started an era, which has seen the advancement of surgery and development of rational therapies based on clinical observations.
Jean Astruc and chemist Bernard, two 18th century physicians conducted research to confirm or disprove then current theories related to the origin of cancer. These efforts were the very first steps of experimental oncology. The art and science of seeking better diagnosis, treatments and understanding of the causes of cancer evolved from many who followed their path.
The early 20th century saw great progress in our understanding of microscopic structure and functioning of the living cells. Researchers pursued different theories to the origin of cancer, subjecting their hypotheses to systematic research and experimentation. A virus causing cancer in chickens was identified in 1911.
Existence of many chemical and physical carcinogens was conclusively identified during later part of the 20th century. Later part of the 20th century showed tremendous improvement in our understanding of the cellular mechanisms related to cell growth and division. Many factors that suppress and activate the cell growth and division were identified. [1]
Cancer progresses from the uncontrolled growth of cells to the formation of a primary tumour mass, vascularisation and subsequent spread (metastasis) of cancer cells to other parts of the body where secondary tumours may form.
Cells on their way to becoming cancerous accumulate a number of genetic and chromosomal abnormalities, which push the cell towards unrestricted growth. At first, clusters of genetically identical cells are formed, each cell dividing more rapidly than its normal neighbours.
This dysplasia, or dysplastic changes include atypical changes in the nuclei of cells, the cytoplasm, or in the growth pattern of cells. They are considered to be pre-cancerous and divide into two categories, depending on how far the cells have progressed: low-grade dysplasia and high-grade dysplasia. This abnormal cell growth in humans is almost always monoclonal [2] [3]
In the context of cancer, monoclonal refers to the fact that tumours arise from a single damaged cell
Low-Grade Dysplasia – Atypical changes in less than 50% of nuclei. Growth patterns in tissue appear to be normal. Also called ‘Hyperplasia’.
High-Grade Dysplasia – Atypical changes (enlarged nuclei and frequent divisions) in more than 50% of cells, together with distorted/irregular growth patterns.
A benign tumour is a slow-growing, non-life threatening tumour that is often surrounded by a fibrous capsule, either formed from healthy cells or the tumour itself. Although usually easy to remove surgically, these tumours can put pressure on neighbouring tissues and organs, resulting in compression and atrophy, occasionally becoming lethal through impairment of critical function.
Tumours are usually named after their tissue of origin, e.g. Adenomas occur in connective tissues, myomas in muscles, lipomas in adipose tissue.
The most frequent types of Benign tumours:
Cysts: lumps filled with fluid. Common types include sebaceous cysts on the skin, filled with greasy sebum, and ovarian cysts.
Nodules: formed in inflammatory conditions such as arthritis, sarcoid and polyarteritis.
Fibromas and fibroademonas: lumps of fibrous or fibrous and glandular tissue.
Haematoma: lump formed by blood escaping into the tissues - simply a large bruise.
When the cell mass reaches a sufficient size, the cells release chemicals to recruit surrounding connective tissue and vascular cells to the tumour and induce them to grow into blood vessels, effectively growing their own blood supply from existing blood vessels, a process called angiogenesis (See Figure 1).
Figure 1 Angiogenesis, the Cascade of Events
The Angiogenesis Process:
• Tumour cells release angionec growth factors (e.g. Angiogenin, Angiopoietin-1, Del-1, Fibroblast growth factors: acidic (aFGF) and basic (bFGF) , Follistatin, Leptin, Placental growth factor, Tumour necrosis factor-alpha (TNF-alpha)) that diffuse into the nearby tissues.
• The angiogenic growth factors bind to specific receptors located on the endothelial cells (EC) of nearby pre-existing blood vessels.
• Once growth factors bind to their receptors, the endothelial cells are activated and signals are sent to the nucleus. Enzymes are released which create small pores in the basement membrane surrounding the existing blood vessels.
• The endothelial cells begin to proliferate, and migrate out through the pores of
the existing vessel towards the tumour tissue, whilst specialized adhesion molecules, or ‘integrins’ help to pull the new blood vessel sprout forward.
• Metalloproteinases (protein-degrading enzyme with a metal ion (Ca2+ or Zn2+) bound within its active site) are produced to disperse the tissue in front of the growing vessel, and as it extends, to remould it around the vessel.
• Endothelial cells proliferate to form the blood vessel tube, whilst individual vessels connect to form loops that can circulate blood.
• Finally, the newly formed blood vessels are provided structural support by specialized muscle cells. Blood flow then begins.
There is evidence that angiogenesis is initiated because cells in the interior of the tumour become starved of oxygen and the nutrients they need to continue growing. [4] [5]
Whilst most benign tumours will remain contained indefinitely, some undergo a series of changes to develop from a pre-malignant to a malignant state (See Figure 2). This process is known as tumour progression, and is characterised by a new, irreversible and inheritable change in the cells that leads to tumour heterogeneity (a tumour with two or more differing populations of cells).
Figure 2 Tumour Development to Malignant State
If some cells experience additional mutations that allow them to escape from the surrounding capsule and migrate, as only white blood cells are designed to do, to invade neighbouring tissues and release cells into the blood or lymph, the tumour is said to have become malignant.
The escaped cells may establish new tumours (metastases) at other locations in the body.[6] [7] [8]
Since the vast majority of cells are designed to remain fixed within their tissue of origin, metastasis requires the cancer cells to escape the tumour mass and migrate through the connective tissue capsule to reach a blood vessel large enough to carry them to a new organ. Once there, they must pass through the blood-vessel wall into the new organ and there form new connections. Here angiogenesis has an additional consequence. Connective tissue and vascular cells release factors that stimulate the growth and motility of cancer cells, meaning that the new tumour can only grow effectively if it’s supported by the surrounding healthy tissues.
This new, metastatic tumour is often more damaging than the original primary tumour. The metastatic cancer cells act to displace functional cells and deprive them of nutrients, resulting in lethality if the affected organ(s) can no longer perform vital functions. Secondary tumours can also occur in large numbers, and spread out into numerous organs and body systems. One of the major sites for these metastatic tumours is in the lung, due to cancerous cells travelling through the pulmonary circulation and lodging in alveolar capillaries.
Malignant tumours are graded in four degrees of severity – 1 to 4 – in terms of the differentiation of the tumour cells. If the cells appear well differentiated (i.e. more normal), then the less aggressive the tumour, and the lower the grade. Grade 3 and 4 tumours are (nearly) undifferentiated and consequently, more aggressive/invasive. [9] [10]
The initial neoplasm that forms is not malignant. Carcinogenesis occurs in a three-stage process (See Figure 3).
Figure 3 Process of Carcinogenesis
Initiation involves the formation of a mutant cell line through a change in the genetic information of a cell line. Promotion involves the division of the parent cell to produce many copies (NOTE: these are identical to the parent, and therefore termed clones). During this process, some of the clones pick up additional mutations and grow more rapidly than the others, a process termed progression. These cells therefore begin to outnumber the others. This process is repeated, the tumour cells becoming more aggressive after each cycle, in a process termed clonal selection. After 4-6 cycles the neoplasm becomes aggressive enough to become malignant. The loss of the usual characteristics of these cancerous cells is termed anaplasia.
Factors influencing cancer development
Agents which promote tumour initiation are known as mutagens
Agents which promote malignancy (promotion and progression) are known as carcinogens (See Figure 4).
Figure 4 Factors Influencing Cancer Development
The chemical agents can be Alkylating agents, polycyclic aromatic hydrocarbons (PAHs), or naturally some of the pesticides. [11]
In recent years, chemical pesticides have become the most important consciously-applied form of pest management. This is a generalization of course; for some crops in some areas, alternative forms of pest control are still used heavily.
In case of pesticides, the other largest field of dangerous substances beside the pharmaceuticals The Plant Protection Products Directive (91/414/EEC), 'The Authorisations Directive', was adopted by the Council of Ministers on 15 July 1991 and published on 19 August 1991 (OJ L230, ISSN 0378 6978). It came into force on 26 July 1993 and is implemented in Hungary by the Plant Protection Law, „2000. évi XXXV. Törvény”, and the regulation „89/2004. (V. 15.) FVM rendelet, a növényvédő szerek forgalomba hozatalának és felhasználásának engedélyezéséről, valamint a növényvédő szerek csomagolásáról, jelöléséről, tárolásáról és szállításáról”. [12] [13] [14]
The main elements of the Directive are as follows:
To harmonise the overall arrangements for authorisation of plant protection products within the European Union. This is achieved by harmonising the process for considering the safety of active substances at a European Community level by establishing agreed criteria for considering the safety of those products. Product authorisation remains the responsibility of individual Member States
The Directive provides for the establishment of a positive list of active substances (Annex I), that have been shown to be without unacceptable risk to people or the environment
Active substances are added to Annex I of the Directive as existing active substances are reviewed (under the European Commission (EC) Review Programme) and new ones authorised.
Member States can only authorise the marketing and use of plant protection products after an active substance is listed in Annex I, except where transitional arrangements apply.
Before an active substance can be considered for inclusion in Annex I of Directive 91/414/EEC, companies must submit a complete data package (dossier) on both the active substance and at least one plant protection product containing that active substance. The data required:
• identify an active substance and plant protection product;
• describe their physical and chemical properties;
• their effects on target pests, and;
• allow for a risk assessment to be made of any possible effects on workers, consumers, the environment and non-target plants and animals.
Comprehensive lists of the data required to be evaluated to satisfy inclusion in Annex I of the Directive, or the authorisation of a plant protection product are set out in the Directive, (Annexes II and III). Annex II data relate to the active substance and Annex III to the plant protection product. These data are submitted to one or more Member States for evaluation. A report of the evaluation is submitted to the European Food Safety Authority (EFSA). Following peer review of the report EFSA makes a recommendation to the European Commission on whether Annex I inclusion is acceptable. This recommendation is then discussed by all Member States in the framework of the Standing Committee on the Food Chain and Animal Health (SCFA) previously the Standing Committee on Plant Health (SCPH). Where necessary, the Scientific Panel is consulted before the SCFA can deliver an opinion on whether an active substance should be included in Annex I of 91/414/EEC.
The "Uniform Principles" (Annex VI of Directive 91/414/EEC) establishing common criteria for evaluating products at a national level were published on 27 September 1997 (OJ L265, p.87).
Application of the Uniform Principles ensures that authorisations issued in all Member States are assessed to the same standards. [15]
The objectives of carcinogenicity studies are to identify a tumourigenic potential in animals and to assess the relevant risk in humans. Any cause for concern derived from laboratory investigations, animal toxicology studies, and data in humans may lead to a need for carcinogenicity studies. The practice of requiring carcinogenicity studies in rodents was instituted for pharmaceuticals that were expected to be administered regularly over a substantial part of a patient's lifetime.
The design and interpretation of the results from these studies preceded much of the available current technology to test for genotoxic potential and the more recent advances in technologies to assess systemic exposure. These studies also preceded our current understanding of tumorigenesis with nongenotoxic agents. Results from genotoxicity studies, toxicokinetics, and mechanistic studies can now be routinely applied in preclinical safety assessment. These additional data are important not only in considering whether to perform carcinogenicity studies but for interpreting study outcomes with respect to relevance for human safety. Since carcinogenicity studies are time consuming and resource intensive they should only be performed when human exposure warrants the need for information from life-time studies in animals in order to assess carcinogenic potential. In Japan, according to the 1990 “Guidelines for Toxicity Studies of Drugs Manual”, carcinogenicity studies were needed if the clinical use was expected to be continuously for 6 months or longer. If there was cause for concern, pharmaceuticals generally used continuously for less than 6 months may have needed carcinogenicity studies. In the United States, most pharmaceuticals were tested in animals for their carcinogenic potential before widespread use in humans. According to the US Food and Drug Administration, pharmaceuticals generally used for 3 months or more required carcinogenicity studies. In Europe, the Rules Governing Medicinal Products in the European Community defined the circumstances when carcinogenicity studies were required. These circumstances included administration over a substantial period of life, i.e., continuously during a minimum period of 6 months or frequently in an intermittent manner so that the total exposure was similar. (ICH Harmonised Tripartite Guideline S1A, Guideline on the Need for Carcinogenicity Studies of Pharmaceuticals) [16] [17] [18]
In Europe according to the 91/414/EEC Directive the long-term studies should be conducted and reported, taken together with other relevant data and information on the active substance, must be sufficient to permit the identification of effects, following repeated exposure to the active substance, and in particular must be sufficient to:
— identify adverse effects resulting from exposure to the active substance,
— identify target organs, where relevant,
— establish the dose-response relationship,
— identify changes in toxic signs and manifestations observed, and
— establish the No Adverse Effect Level (NOAEL).
Similarly, the carcinogenicity studies taken together with other relevant data and information on the active substance, must be sufficient to permit the hazards for humans, following repeated exposure to the active substance, to be assessed, and in particular must be sufficient:
— to identify carcinogenic effects resulting from exposure to the active substance,
— to establish the species and organ specificity of tumours induced,
— to establish the dose-response relationship, and
— for non-genotoxic carcinogens, to identify the maximum dose eliciting no adverse effect (threshold dose).
The long-term toxicity and carcinogenicity of all active substances must be determined. If in exceptional circumstances, it is claimed that such testing is unnecessary, that claim must be fully justified, i.e. toxicokinetic data demonstrates that absorption of the active substance does not occur from the gut, through the skin or via the pulmonary system.
A long-term oral toxicity and carcinogenicity study (two years) of the active substance must be conducted using the rat as test species; these studies can be combined. [19] [20]
A carcinogenicity study of the active substance must be conducted using the mouse as test species.
Where a non-genotoxic mechanism for carcinogenicity is suggested, a well argued case, supported with relevant experimental data, including that necessary to elucidate the possible mechanism involved, must be provided.
While the standard reference points for treatment responses are concurrent control data, historical control data, may be helpful in the interpretation of particular carcinogenicity studies. Where submitted, historical control data should be from
the same species and strain, maintained under similar conditions and should be from contemporaneous studies. The information on historical control data provided must include:
— identification of species and strain, name of the supplier, and specific colony identification, if the supplier has more than one geographical location,
— name of the laboratory and the dates when the study was performed,
— description of the general conditions under which animals were maintained, including the type or brand of diet and, where possible, the amount consumed,
— approximate age, in days, of the control animals at the beginning of the study and at the time of killing or death,
— description of the control group mortality pattern observed during rat the end of the study, and other pertinent observations (e.g. diseases, infections),
— name of the laboratory and the examining scientists responsible for gathering and interpreting the pathological data from the study, and
— a statement of the nature of the tumours that may have been combined to produce any of the incidence data.
In the collection of data and compilation of reports, incidence of benign and malignant tumours must not be combined, unless there is clear evidence of benign tumours becoming malignant with time.
Similarly, dissimilar, un-associated tumours, whether benign or malignant, occurring in the same organ, must not be combined, for reporting purposes. In the interests of avoiding confusion, terminology such as that developed by American Society of Toxicologic Pathologists, or the Hannover Tumour Registry (RENI) should be used in the nomenclature and reporting of tumours. The system used must be identified.
As mentioned in the EEC Directive, the role of historical control data is very important during the evaluation of the results of the carcinogenicity studies.
Caloric intakes are now recognized to be important uncontrolled variables in bioassays because rodents chronically fed ad libitum become obese, reproductively senile and have increased incidences of age-related diseases, higher tumour burdens and decreased survival. The available literature suggests that ad libitum feeding neither optimizes the health and well-being of rodents nor
provides the best model for use in evaluation of pharmacological and toxicological profiles. Use of an optimized diet, restricted in terms of caloric intakes, has been proposed for chronic toxicity and carcinogenicity studies in rodents. [80, 81]
Caloric restriction and experimental tumorigenesis are evaluated in the article of Nutrition Today by David Kritchevsky as cited below:
Although dietary guidance directed to the reduction of cancer risk emphasizes reduction in fat intake, this article documents evidence from animal studies that caloric balance either through reduced intake or increased expenditure of energy without a reduction in fat is even more effective in controlling tumour growth.
Dietary fat has been correlated positively with incidence of human tumours,
Dietary fat has been correlated positively with incidence of human tumours,