IFNs represent an anti-neoplastic agent, effective for some malig- nancies resistant to conventional chemotherapy. Interleukin-2 (IL-2) IL-2, a lymphokine produced by activated T cells (Th1), plays a pivota l role in immune modulation, enhancing the growth of activated T cells, the proliferation o f lymphoid cells, and the migration of lym- phocytes from the peripheral blood. Anti-tumor activity of IL-2 includes the capacity to lyse fresh tumor cells, the regression of dis- ta nt metastases in murine models, and the in vivo release of other members of the cytokines f amily. IL-2 has been widely applied in the management of patients with advanced cancer. The system ic administration of high doses of IL-2, alone or in combination with lymphokine-activated killer (LAK) cells, activated ex vivo, may induce objective responses in a small OHO-11(189-204) 11/16/02 11:24 AM Page 196 IMMUNOTHERAPY OF CANCER 196 proportion of patients with renal cell carcinoma (5 \200\22315%) or metasta- tic melanoma ( <15%). Responses have also been demonstrated in patients with acute myeloid leukemia. These studies were per- formed in patients of poor prognosis, with advanced and refractory disea se. IL-2 can be given intravenously at doses from 72 000 to 720 000 IU/kg every 8 hours. Toxici ties associated with IL-2 administration include: (cid:2) Flu-like syndrome (cid:2) Capilla ry leak syndrome (cid:2) Severe hypotension (cid:2) Arrhythmia (cid:2) Angina (cid:2) Respi ratory distress (cid:2) Somnolence (cid:2) Anemia (cid:2) Thrombocytopenia (cid:2) Multi-or gan malfunction (cid:2) Toxic fatalities in up to 10% of patients In melanoma, administrati on of IL-2 alters the biodistribution of dacarbazine due to enhanced capillary permeability , resulting in syn- ergy. IFN-(cid:2) enhances IL-2 lymphocyte proliferation and IFN-(cid:2 ) /IL-2 combination therapy is undergoing clinical assessment in patients with renal cell c arcinoma and melanoma. Tumor necrosis factor (TNF) TNF is an important mediator of the in \202ammatory response, being involved in stress conditions, cachexia, and endotoxin shock. It is mainly produced b y monocytes, activated macrophages, and T cells. It induces the expression of MHC class I a nd II antigens, as well as adhesion molecules responsible for leukocyte migration and accum u- lation. TNF (mainly TNF-, has been used in various clinical trials, mostly in patients with advance d melanoma and sarcoma. Results have been disappointing, with <5% response rate. Loco-regio nal administration (intraperitoneally, intravesically, intralesionally) seems more promis- ing, but its clinical use is limited by the severe side-effects which include: (cid:2) Acut e fever (cid:2) Anemia (cid:2) Thrombocytopenia (cid:2) Hepatotoxicity OHO-11(189-204) 11/16/02 11:24 AM Page 197 ACTIVE IMMUNOTHERAPY 197 (cid:2) Cytochrome P450 depression (cid:2) Transient impairment of renal function (cid:2) C entral nervous system toxicity, especially in the elderly Local administration of rTNF and of TNF encapsulated liposomes could be used in the future to circumvent the problems of tox icity. Data show synergy between TNF and conventional chemothera- peutic agents such as cyclophosp hamide, doxorubicin, and cisplatin. TNF may reverse cisplatin resistance. On the other hand , cisplatin and cyclophosphamide may act as biological modulators by increasing TNF binding to the cell surface. Monoclonal antibodies Although the management of cancer by exploiting properties distin- gu ishing neoplastic and normal cells has always been an attractive concept, it was the develo pment of hybridoma technology and the resulting tumor-associated monoclonal antibodies (mAb s) that offered new prospects for this strategy. Some of the applications of mAbs in oncolo gy are now part of everyday diagnosis (i.e. immuno- histochemistry, radio-immunodetection). With regard to cancer therapeutic modalities, unconjugated or native mAbs can activate comp onents of the complement and of cytolytic cells and result in tumor lysis, with a mechanism known as Antibody-Dependent Cellular Cytotoxicity (ADCC). Several studies have been conduc ted, mainly in patients with hematological malig- nancies. The responses achieved were gene rally poor and of short duration: circulating malignant cells were reduced, but with little effect on lymph node or bone marrow disease. Since the cytotoxic effect of the mAbs was minimal, they have been used as carriers of more potent agents such as conventional cytotoxic drugs, radionuclides, liposomes, and toxins, to achieve speci \201c deliv- ery at the tumor sites and therefore reduced host toxicity. Another approach is to use mAbs-enzymes conjugates to catalyze various sub- strate conversi ons at the tumor. The pro-drug is administered after the conjugate has localized to the sur face of the malignant cells and cleared from the circulation. These approaches are currently undergoing clinical evaluation; the major problem seems to b e in the immune response elicited by murine antibodies, known as HAMA (Human Anti-Mouse Ant i- globulin response). Nevertheless, it is expected that the evolution of genetic engineeri ng and the subsequent production of single-chain, chimaeric and humanized antibodies and of fusion proteins will over- come this problem. Other limiting factors are the low antibody uptake by the tumor, the antigenic heterogeneity of the tumor, the poor tumor penetration, and the poor stability of the immuno-conjugates in vivo. OHO-11(189-204) 11/16/02 11:24 AM Page 198 IMMUNOTHERAPY OF CANCER 198 In conclusion, the use of mAbs as targeting devices for cytotoxic agents is a very attracti ve approach for tumor immunotherapy. However, there are still several complex issues to be resolved before they become part of the routine practice in oncology. OHO-11(189-204) 11/16/02 11:24 AM Page 199 ADOPTIVE IMMUNOTHERAPY 199 Adoptive immunotherapy The demonstration that the cell-mediated immune response is crucial in the rejection of all ogeneic and syngeneic tumors has prompted the use of cells with anti-tumor activity in pati ents with malignancies \200\224an approach known as adoptive immunotherapy of cancer. Several strategies have been applied to generate cells with reactivity to tumors, the most common being the production of lymphocyte- activated killer (LAK) cells that can lyse fresh tumor cells, by incu- bating human peripheral blood lymphocytes with IL-2, ex vivo. The ex act mechanism of recognition and destruction of tumors by LAK cells is not fully understood . Trials in renal cell carcinoma and melanoma patients did not reveal any therapeutic advan tage in the administration of LAK plus IL-2, compared to monotherapy with IL-2. In an alternative approach of adoptive immunotherapy, tumor- in \201ltrating lymphocytes ( TILs), that can recognize tumor-associated antigens, have been isolated from human tumors a nd administered to patients with advanced melanoma, with a response rate (PR + CR) of 25 \200\22335%. Recently, genetically manipulated TILs have been produced, with the transducti on of the gene encoding either the neomycin phosphotransferase or tumor necrosis factor. OHO-11(189-204) 11/16/02 11:24 AM Page 200 IMMUNOTHERAPY OF CANCER 200 Tumor vaccines Hepatitis B virus (HBV) vaccine is a widely used, very effective vaccine against hepatocell ular carcinoma, while studies are in progress to develop vaccines against Epstein \200\223B arr virus, which is closely linked to the development of: (cid:2) Burkitt \200\231s lymphom a (cid:2) Nasopharyngeal carcinoma (cid:2) Non-Hodgkin \200\231s lymphomas Other obvious ca ndidates for development of an anti-tumor vaccine are the human papilloma virus (HPV) and t he human retrovirus, HTLV, which are causative agents for several human malignancies. While the development of a vaccine is obvious in a virally-induced tumor, in non-virally-in duced tumors, the concept of a vaccine is more complicated. In this case, tumor cells or tu mor cell extracts are used as cancer vaccines intending to enhance a humoral or cell- media ted immune response to relevant tumor antigens, rather than to induce prophylactic immunity . The antibodies produced may kill the tumor cells by complement \201xation or antibody-d ependent cel- lular cytotoxicity, while the activation of cytotoxic T cells that recog- niz e antigens on the tumor cell surface may induce speci \201c cytolysis. This approach was e xpected to reduce tumor-induced immuno- suppression and selectively augment long-lasting hu moral and cel- lular anti-tumor immunity but, in general, it seems to have only minimal cli nical signi \201cance. More promising is the vaccination with anti-idiotype antibodies, directed against determina nts expressed on the variable regions of anti-tumor antibodies. However, this strategy is l imited by the fact that anti-idiotypic antibodies have to be produced individually and are thus expensive and labor-intensive. Several vaccines for melanoma, colorectal, breast, prostate, and lung cancers are currently under clinical evaluation. Preliminary data support the concept that active immunization w ill be effective to patients with high-risk recurrent disease, after surgical removal of th e tumor, when the tumor burden is small. Unfortunately, most of the clinical trials have be en in patients with advanced, extensive disease, refractory to conventional therapies and p robably already immunosuppressed. OHO-11(189-204) 11/16/02 11:24 AM Page 201 GENE THERAPY 201 Gene therapy Gene therapy is the therapeutic strategy in which a functioning gene is inserted into a cel l to provide a new function or to correct a genetic error. Since cancer is the result of \200\230misbehave \200\231 genes, it provides an ideal candidate for gene-therapy intervent ions. Two requirements exist for successful genetic manipulations: \201rst, a method to transfe r the gene into the correct cell and second, a way to adequately control its expression. Techniques to introduce DNA into cells (cid:2) Lipid complexed with DNA (cid:2) DNA in lipid vesicles (cid:2) DNA in red blood cel l ghosts (cid:2) Direct micro-injection of DNA (cid:2) Exposure of cells to high voltage (c id:2) Use of viruses The non-viral methods are convenient and have an obvious safety advant age, but they have low ef \201ciency and result in transient gene expression. On the contr ary, viral (retroviruses, adenoviruses, poxviruses) vectors represent the most ef \201cien t, stable manner of integrating DNA into large numbers of target cells. Based on the knowledge gained from other approaches of cancer immunotherapy, several strate gies have been proposed for using gene therapies in patients with malignancies. Genes encod ing for cytokines can be transferred in vivo to the tumor or to TILs. Alternatively, tumor cells or antigen-presenting cells (APC) can be genetically engineered ex vivo and re-inject ed in the patient to enhance tumor immunogeneity. Furthermore, genetic modi \201cation of immune effector cells may enhance their survival and increase their tumor recognition and a nti-tumor ef \201cacy, in vivo. Clinical trials are being conducted in patients with pancreatic, lung, breast, prostate, an d bladder cancer. These trials have demonstrated the feasibility and safety of genetic mani pulations in cancer patients, although the clinical bene \201t from gene therapy is not cl ear. Attempts are currently focused on the development of techniques that would warrant the ef \201cient gene delivery to selected tissues and control its expression, anticipating t he mobilization of an effective anti-tumor immune response. OHO-11(189-204) 11/16/02 11:24 AM Page 202 IMMUNOTHERAPY OF CANCER 202 Summary Molecular biology has opened new frontiers for the understanding of tumor immunology. Biolo gical therapies of cancer are based on sound scienti \201c rationale and show promising pr eliminary results. Immunotherapy in particular is based on the breaking of the host \200 \231s immunological tolerance to the tumor. This would allow the immune system of the patie nt to recognize the tumor as non-self and mediate an effective anti-tumor response directly or indirectly. To date, immunotherapy has failed to bring radical improvements in patients \200\231 care. Nevertheless, several complete and partial remissions have been induced in some cancer pati ents, mainly those with mini- mal tumor burden, indicating a potential role for immunothera py in the armentarium against cancer. The main challenge for tumor immunologists in the near future is to carry out properly cont rolled studies of the safety and ef \201cacy of the immunotherapy approaches, in order to identify the individuals most likely to bene \201t from them. OHO-11(189-204) 11/16/02 11:24 AM Page 203 FURTHER READING 203 Further reading Berd, D. (1998) Cancer vaccines: reborn or just recycled? Semin Oncol 24, 605 \200\22310. Efraim, B. (1999) One hundred years of cancer immunotherapy: a critical appraisal. Tumor Biol. 20, 1 \200\22324. Gore, M. and Riches, P. (ed.) (1996) Immunotherapy in cancer. John Wiley & Sons, London. Rosenberg, S.A. (1998) New opportunities for the development of cancer immunotherapies. Cancer J Sci Am 4, 1 \200\2234. Syrigos, K.N. and Epenetos, A.A. (1999) Antibody directed enzyme prodrug therapy (ADEPT): a review of the experimental and clinical considerations. Anticancer Res 1 9(1A), 605 \200\22314. Syrigos, K.N., Deonarian, D., and Epenetos, A.A. (1999) Use of monoclonal antibodies for th e diagnosis and treatment of bladder cancer. Hybridoma 18(3), 219 \200\22324. Syrigos, K.N., Karayiannakis, A.J. and Zbar, A. (2000) Mucins as immunogenic targets in cancer. Anticancer Research 20, 420 \200\2236. OHO-11(189-204) 11/16/02 11:24 AM Page 204 This page intentionally left blank OHO-12(205-220) 11/16/02 11:24 AM Page 205 Part 3 Principles of prevention and care 12 Cancer prevention and screening 207 13 Clinical trials 221 14 Principles of palliative c are 227 15 Psychological aspects of cancer 249 OHO-12(205-220) 11/16/02 11:24 AM Page 206 This page intentionally left blank OHO-12(205-220) 11/16/02 11:24 AM Page 207 Chapter 12 Cancer prevention and screening Prevention strategies 208 Cancer chemoprevention 213 Screening for cancer 217 Further readi ng 220 OHO-12(205-220) 11/16/02 11:24 AM Page 208 CANCER PREVENTION AND SCREENING 208 Prevention strategies Chemoprevention is the use of chemical agents or dietary compounds to reduce the incidence of cancer. The chemical compounds could be trace elements or hormones or other medicaments, the dietary com- pounds could be \201bre, nutrients, vitamins, etc. This \201eld of me dical oncology brings together the discipline of: (cid:2) Epidemiology (cid:2) Carcinogenesis (cid:2) Toxicology (cid:2) Pharmacology (cid:2) Molecular biology (cid:2) Genetics Burkitt observed that colorectal cancer was almost unkn own in numerous tribes in Africa, possibly due to their high- \201bre diet. Migration stud ies of these African tribesmen moving into townships showed that within a generation, bowel cancer increased in incidence, approaching that of the city dwellers. This had been accomp anied by a preceding change of diet to include less \201bre, more fat, and more sugar. Si milarly a number of studies associated breast cancer with obesity and numerous studies have subsequently attempted to explore the relationship of fat in the diet and the onset of bre ast cancer. There are numerous risk factors for breast cancer including: (cid:2) Delayed puberty (cid:2) History of nulliparity (cid:2) Lack of breast feeding (cid: 2) Contraceptive pill in early life (cid:2) Hormone replacement therapy around the menopaus e Interventions with hormones based on breast tumor biology are there- fore tempting but th e biological mechanisms need \201rst to be de \201ned. Biochemical alterations have been shown in population studies of cancer patients \200\231 b lood. Low levels of retinoids such as vitamin A and (cid:2) carotene and elements like sele nium have been associated with cancer. Whether these biochemical alterations are cause or e ffect is perplexing. It is clear that a number of carcinogens stimulate resting cells to pr oliferate and grow under the in \202uence of oncogenes such as ras, raf, Bcl2. This proces s should be inhibited by tumor suppressor genes such as Rb or p53 or substances such as TGF (cid:2). Another possible route for cells to become immortal is to avoid apoptosis. The apoptopic pa thway involves at least a dozen known OHO-12(205-220) 11/16/02 11:24 AM Page 209 PREVENTION STRATEGIES 209 mediators, including p53.Cancer cells classically will have either an increase of the proli feration genes or anti-apoptopic signals or defec- tive tumor suppressor genes. Having understood the cancer process and the predisposing factors it is easier to identify which patient group might be at highest risk and might bene \201t from an intervention eit her in change of diet or addi- tion of a medicine. Genetic risks Patients with certain genetic defects are more likely to get cancer \200\224 either they have overexpressed oncogenes such as Kras or a mutated tumor suppressor gene su ch as p53 or Rb. In the etiology of col- orectal cancer Kras is one of the \201rst of six or more genetic alterations known to accumulate in the malignant progression from normal m ucosa through adenoma or polyp to carcinoma. p53 mutations are associated with the last and critical stage. Injection of wild-type p53 into human cultured colorectal cells bearing se veral genetic alterations leads to reversion to a benign phenotype. Environmental risks A number of environmental factors are known to cause pre-malignant lesi ons e.g. chewing tobacco frequently causes leucoplakia, which pro- gresses to oral carcinom a. This is an ideal situation for testing interven- tions, including many retinoids. Metapl asia of the esophagus is known to herald frank carcinoma and high-risk populations, such as those found in China, are the subject of chemoprevention trials of drugs. Smoking and lung cancer is a clear-cut cause and effect, but trials of intervention require large numbers of smokers to be randomized. Two-tail analyzes are mandatory as evidence fro m two such trials indicate that (cid:2)carotene increases lung cancer incidence. Viruses ha ve been incriminated in the etiology of: (cid:2) Hepatoma (hepatitis viruses) (cid:2) Burkitt \200\231s lymphoma (cid:2) Nasopharyng eal carcinoma (Epstein \200\223Barr viruses, EBV) (cid:2) Cervical carcinoma (human papillo ma virus 16). Vaccines are available against each of these agents but only the \201rst ha s been tested for long enough to show ef \201cacy. Studies in Taiwan and West Africa have indicated a decreased incidence of hepatoma as well as a marked protective effect on hepati tis. Helicobacter pylori has been linked to gastric carcinoma and early claims of eradication of the organism by antibiotics and subsequent protection from cancer are being validated. The se examples could also be categorized as chemoprevention. The association of ultraviolet light with skin cancer is well- established, as is the incre ase of malignant melanoma in the UK OHO-12(205-220) 11/16/02 11:24 AM Page 210 CANCER PREVENTION AND SCREENING 210 (approximately 10% per annum). Primary intervention is the obvious solution. A recent rando mized trial of a retinoid showed adequate protection from new squamous carcinomas but no ef fect on basal carcinomas. Clinical trials The rules of intervention in prevention trials in normal people are very di fferent from the classical therapy trials known to oncologists. The disciplines come togeth er, for instance in breast cancer, with the observation made in the therapeutic trials of t amoxifen used as an adjuvant in early breast cancer that the incidence of second primary br east cancer in the contralateral breast was less with tamoxifen than with placebo. It led t o the notion that tamoxifen might be a preventive agent as well a cytotoxic drug. There fol lowed a series of trials leading to the landmark publication of the Breast Cancer Preventio n Trial (BCPT) from the United States in 1998, which showed that tamoxifen could halve the number of breast cancers observed in normal women at high risk by virtue of previous benign lesions in the breast or a pos- itive family history. However, in two further trials no pr oven bene \201t was observed in different patient populations. The tamoxifen illustration is important because it highlights a num- ber of problems. First is the duration of administration of tamox- ifen \200\224presumably those at risk from bre ast cancer, for instance due to mutated BRCA1 or BRCA2 genes, have a lifetime risk. Indeed it is measured to be around 80% for breast cancer and 50% for ovarian cancer in BRCA1 mutat ion carriers. How long should tamoxifen be given and when should it be started? The second issue is the incidence of side-effects, which may be tolerable in a cancer therapy trial bu t not in \200\230non-patients \200\231 in a prevention trial. It is known that the effectiveness of tamoxifen as a therapeutic agent plateaus after \201ve years, but the risk of uterine cancer does not. The risk of the latter is of the ord er of 1 in 500 women. The notion of giving a chemopreventive drug that is also a carcinogen , presents a dilemma, so second-generation tamoxifen agents have been developed to remove t he moiety of the drug which is associated with the carcinogenic effect on the uterus. Phase I/II trials The main objectives of early trials of chemopreventive agents that have b een proven to be effective in laboratory tests, either in cell lines or in animal models, i s to establish tolerance and side-effects. The major difference between the early trials of these agents, compared to cytotoxic agents, is that the duration of administration of the preven- tive agent will be much longer than a cytotoxic, so chronic side-effects are as imp ortant as acute side-effects. It is important to show proof of principle where possible, so if a particular drug is designed to show OHO-12(205-220) 11/16/02 11:24 AM Page 211 PREVENTION STRATEGIES 211 target-cell differentiation, then the endpoint would be a histological one. If the endpoint were to increase apoptosis where this had been defective, then measurement of apoptosis wo uld be important. A major side-effect would be either fatality or problems requiring intervention by a physic ian or long-term disability. Major side-effects would automatically rule out any further de velopment of an agent. Dose and duration of administration of the new agent are essential e ndpoints, but escalation of dose would not necessarily be carried out to the level at which toxicity is produced. Rather a dose might be de \201ned as ideal which achieves the biolo gical effect as described. An important part of Phase III trial evaluation is compliance and it is also important to g et some sort of measure of this in the early clini- cal trials. Clearly this is frequently related to appearance of toxicity, but can also be in \202uenced by the ease and route of administration. The target population will frequently be those at highest risk, for instanc e cancer patients who are cured but at high risk of a second malignancy. A Phase II trial will frequently be of longer duration with more emphasis on compliances; i t may well be randomized with a placebo control and may also, as in Phase I, evaluate furth er multiple-dose lev- els. Duration may be one to \201ve years and the sample size could be anything from one hundred to many thousands of \200\230patients \200\231 or poten- tial patients. Again the use of intermediate endpoints is extremely important for cost-ef \201cient studi es, though there is a paucity of good candidates for these biomarkers that are of proven va lue. Ease of recruitment is important because \200\230high risk \200\231 may be clear to a physician but not so clear to a normal individual. These parameters are important for calculating the Phase III trial size and the statistical power, by which is measured. Phase III trials Studies of chemopreventive agents in randomized Phase III design require t o be very large and very lengthy. For instance, a study of smokers randomized to retinoids needs to recruit thousands of smok- ers for a follow-up period of up to twenty years to det ect any impact on lung cancer rate or survival. As it is impossible to afford, in terms of time or money, to test each new agent with the classical Phase II design, two solutions are being tested. One is the concentration on high-risk groups of individuals and the other is the development of intermediate biomarkers. Individuals can be at high risk because of a g enetic predisposition or a previous treatment or by having had a pre- vious cancer. The Euroscan trial was designed to be cost-effective, as it tested patients who had been cu red of one smoking-related cancer either in the lung, head, or neck. The primary endpoint w as the appearance of OHO-12(205-220) 11/16/02 11:24 AM Page 212 CANCER PREVENTION AND SCREENING 212 a second smoking-related cancer, genotypically different from the \201rst, anywhere in th e aerodigestive tract. \200\230Ex-patients \200\231 were random- ized to receive retinol o r n-acetyl cysteine. Retinol induces differentiation and inhibits malignant trans- formation. It acts at the pro motion stage of carcinogenesis and there is evidence that it antagonizes a number of growth factors. It is an immune stimulant and it may actually be cytotoxic (transretinoic acid ha s been shown to be effective in the treatment of acute myelo- monocytic leukemia). N acetyl cysteine has been used widely in chronic bronchitis in Europe and works in a totally diffe rent way from retinol. It is a potent anti-oxidant and increases intracellular glutathione. It has been shown in laboratory animals to be an anti- carcinogen. In order to test the possible advantages in combining two chemo- preventive agents, which h ave different mechanisms of action, the third arm of the Euroscan trial includes both agent s and the fourth arm, neither. This allows two questions to be answered with half the numbe r of patients by analysis of the data at the end of the study by factorial methods. As seco nd primary tumors are seen within seven years in 15% of this cohort, the study only require s 2500 individuals to be randomized. Summary Chemoprevention is in its infancy. New methodologies are being evaluated and new su rrogate endpoints and novel candidate inter- ventions are emerging rapidly from the revolut ion in molecular bio- logy and genetics. It is an extremely promising and exciting branch o f oncology. OHO-12(205-220) 11/16/02 11:24 AM Page 213 CANCER CHEMOPREVENTION 213 Cancer chemoprevention Many human cancers are preventable, because their causes have been identi \201ed in the hu man environment. Wattenberg \201rst suggested that regular consumption of certain constit uents of fruits and vegetables might offer protection from cancer. He coined the term \200 \230cancer chemo- prevention \200\231, which can be de \201ned as \200\230the use of spec i \201c diets, or natur- al or synthetic chemicals, to reverse, suppress, or prevent carci nogenic progression to invasive cancer \200\231. Minimization of exposure towards carcinogens in the environment (primary prevention) is an effective strategy in cancer prevention. However, most environmental factors that initiate cancer remain to be identi \201ed and, once identi \201ed, the avoidance of such factors may necessitate dif \201cult lifestyle changes. Epidemiological data suggesting that cancer is preventable by inter- vention with chemicals are based on: (cid:2) Time trends in cancer incidence and mortality (cid:2) Geographic variations and effect of migration Identi \201cation of speci \201c ca usative factors (cid:2) (cid:2) Lack of simple patterns of genetic inheritance for the majority of human cancers Chemopreventive agents and their molecular targets Epithelial carcinogenesis proceeds via m ultiple, discernible steps of molecular and cellular alterations, culminating in invasive n eoplasms. These events can be separated into three distinct phases: Initiation (rapid; involves direct carcinogenic damage to DNA; and the resulting mutation i s irreversible). (cid:2) (cid:2) Promotion (follows initiation and is generally reversible; involves the clonal expa nsion of initiated cells induced by agents acting as mitogens for the initiated cell). (cid:2) Progression (results from promotion in the sense that cell pro- liferation caused b y promoters allows cellular damage in \202icted by initiation to be further propagated). During tumor progression, genotypically and phenotypically altered cells gradually emerge. Both promotion and progression phases are prolonged. Depending on which phase of carcinogen esis they affect, chemopreventive agents can be divided into tumor \200\230blocking \200 \231 agents, which counteract cancer by interfering with initiation, and tumor \200\230sup pressing \200\231 agents, which intercept promotion or progression. The OHO-12(205-220) 11/16/02 11:24 AM Page 214 CANCER PREVENTION AND SCREENING 214 synthetic dithiolethione compound, oltipraz, and the organic seleni- um compound, selenomet hionine, are examples of blocking agents. 13-cis retinoic acid, a member of the retinoid fa mily (which are natur- al and synthetic compounds related to vitamin A), is a tumor-sup- pr essing agent. Blocking agents such as oltipraz, which prevent metabolic activa- tion of carcinogens or th eir subsequent binding to DNA, probably play a signi \201cant role in reducing the accumul ation of initiating mutations. The fact that initiation can occur very early in life con- f ounds clinical chemoprevention strategies based only on anti-initia- tion. Suppression of t he development of the initiated cell to a malignant tumor is probably the strategy of choic e in human cancer chemoprevention. Altered states of cell and tissue differentiation are characteristic of pre-malignant lesio ns long before they become invasive and metastat- ic. This pathology of differentiation (dy splasia) offers a de \201ned target for pharmacological intervention because, in some circ umstances, it is possible to reverse abnormal differentiation with a hormone-like non-toxic agent. Two other approaches to the control of pre- neoplastic lesions are to block their e xpansion with non-toxic agents that suppress cell replication, or to induce an apoptotic st ate in cells which ordinarily would be programmed to die but may have under- gone carcinoge nic mutations providing an extended life span. Although in the past, cancer chemopreventive agents have been dis- covered serendipitously or developed empirically, recent advances in the molecular biology of carcinogenesis sugges t that it will be possible to develop new and better agents by a more mechanistic approach. A good example is colon cancer \200\224the recent discovery that over-expres- sion of the gene coding for inducible cyclo-oxygenase (COX-2), a key enzyme in the formation of prostag landins from arachidonic acid, is an early and central event in colon carcinogenesis now pr ovides an important target for new drug development. Recently, the novel speci \201c COX-2 inhibitors, celeoxib, was approved by the FDA for the reduction of polyps in familial aden omatous polyposis (FAP) patients. Tumor-suppressing compounds such as the retinoids, genistein, an iso \202avonoid constitue nt of soya, and curcumin (the pigment which gives curry its characteristic color) affect, i n a complex fashion, several biochemical and physiological cell parameters potentially asso - ciated with carcinogenesis. Some of these agents not only suppress tumor formation but al so possess a tumor-blocking component. Information about the pharmacokinetic behavior of chemo- preventive agents is scarce. It is therefore not yet possible to assess whether the concentrations at which these agents are known to elicit biochemical responses are achieved in vivo after eating foods that contain them. OHO-12(205-220) 11/16/02 11:24 AM Page 215 CANCER CHEMOPREVENTION 215 Clinical cancer chemoprevention The substance under test has to be innocuous from a toxicol ogical standpoint, as it is likely to be administered over a considerable period of time an d to healthy individuals. Foodstuffs are attractive sources of tumor-suppressive substances , as the effects of long-term exposure are well-documented. Currently, more than 60 randomized trials of potential chemo- preventive agents have been r eported. Only a few of these trials con- stituted \200\230de \201nitive trials \200\231. A primary chemopreventive trial with a signi \201cantly positive outcome is the retinol st udy that showed pro- tection against squamous cell skin carcinoma. Tamoxifen has been appro ved in the US to reduce the risk of breast cancer in high-risk women. Two de \201nitive trials of (cid:2)-carotene were signi \201cantly negative. In these tri als lung cancer incidence was studied in 50 000 individuals. The outcome of these trials ma de headlines, suggesting that in high- risk groups of smokers and/or workers occupationally exposed to asbestos. (cid:2)-carotene increases rather than decreases the risk of devel- o ping lung cancer. Subgroup analyzes of these two trials revealed that the risk of lung canc er was highest among those individuals who con- tinued to smoke at least 20 cigarettes per day and those in the highest quartile of alcohol consumption. It is conceivable that (cid:2 )-carotene suppresses tumors only in those individuals from whom the initiat- ing stimulus has been removed, but not in those who continue to be subjected to it. These results underline the importance of understanding how chemopreventive agents exert th eir effects and under which condi- tions they are bene \201cial or indeed detrimental, pri or to extensive clin- ical evaluation. Several trials were \200\230classically \200\231 negative \200\224the investigational agen ts (among them (cid:2)-carotene, 13-cis retinoic acid, retinol, selenium, and (cid:3)-tocop herol) failed to prevent a variety of cancers. Analyzes of sub- sets of populations in some of the trials yielded intriguing positive or negative results. For example, a nutritional supplement of selenium reduced total cancer mortality and incidence of prostate, lung, and colorectal cancer, and recently, (cid:3)-tocopherol was reported to protect against prostat e cancer. Whenever possible, trials of agents for \200\230primary prevention \200\231 should be prec eded by clinical evaluation of their ef \201cacy for \200\230secondary pre- vention \200 \231 of cancer in speci \201c epithelial target sites. These studies are aimed at the reve rsal or arrest of pre-malignant lesions, or the preven- tion of second primary tumors in pa tients cured of an initial cancer. Secondary prevention trials are more cost-effective than large and long-term primary prevention trials. The burden of carcinogenic stimuli on the p atient is high and a meaningful endpoint can be mea- sured in a reasonable time frame in a smaller number of patients. Such OHO-12(205-220) 11/16/02 11:24 AM Page 216 CANCER PREVENTION AND SCREENING 216 trials have furnished important leads concerning the bene \201t of retinoids to prevent pr e-malignant lesions and secondary primary malignancies of the head and neck, lung, skin, an d liver, and of the non-steroidal anti-in \202ammatory drug, sulindac, in the prevention o f the development of tumors in familial adenomatous polyposis. Summary Clinical and laboratory-based research in cancer chemoprevention is expanding in an attempt to prevent or postpone the disease. De \201nitive, clinical, Phase III trials inc lude the current studies of tamoxifen. But equally important are the mechanism of action st udies which will furnish better methods of designing and testing chemo- preventive agents, and the identi \201cation of speci \201c biomarkers of car- cinogenesis that can serve as surrogate endpoint markers. Table 12.1 Mechanisms of tumor suppression and examples of cancer chemopreventive agents Scavenging oxygen radicals Polyphenols (curcumin, genistein), selenium, tocopherol (vitamin E) Inhibition of arachidonic acid metabolism N-acetylcysteine, NSAIDs (sulindac, aspirin), polyphenols, tamoxifen Modulation of signal transduction NSAIDs, retinoids, tamoxifen, genistein, curcumin Modulation of hormonal/growth factor activity NSAIDs, retinoids, curcumin, tamoxifen Inhibition of oncogene activity Genistein, NSAIDs, monoterpenes (D-limonene, perillyl alcohol) Inhibition of polyamine metabolism 2-Di \202uoromethylornithine, retinoids, tamoxifen Induction of terminal differentiation Calcium, retinoids, vitamin D3 Genistein, curcumin, r etinoids, Induction of apoptosis tamoxifen NSAIDs = non-steroidal anti-in \202ammatory drugs OHO-12(205-220) 11/16/02 11:24 AM Page 217 SCREENING FOR CANCER 217 Screening for cancer Introduction The strongest evidence that early detection increases the chance of cure comes from randomized trials of screening, but there is also evi- dence that survival after diag nosis is related to duration of symptoms. This has led to public awareness campaigns to per suade individuals to seek advice regarding suspicious symptoms at an early stage. There is, however, no evidence as yet that this strategy can improve survival. Screening is the process whereby asymptomatic individuals are tested in order to detect a d isease that has yet to declare itself. For this to be effective in a population there are c ertain criteria that must be met by the disease in question, the screening test, and the sc reening program. The disease Its natural history is well understood It has a recognizable \200\230early \200\231 stage (cid:2) (cid:2) (cid:2) Treatment at an early stage is more effective than at a later stage It is suf \201ciently common in the target population to warrant screening (cid:2) The test (cid:2) Sensitive and speci \201c (cid:2) Acceptable (cid:2) Safe Inexpensive (cid:2) The program (cid:2) Adequate facilities for diagnosis in those with a positive test (cid:2) High qualit y of treatment for screen-detected disease (cid:2) Screening repeated at intervals if the d isease is of insidious onset (cid:2) Bene \201t must outweigh physical and psychological h arm (cid:2) Bene \201t must justify \201nancial cost It is crucial that treating the dis ease to be screened at an early stage is more effective than treating at a later stage. To justify a screening pro- gramme one cannot compare the outcome of screen-detected disease w ith that of symptomatic disease, because three biases operate in favor of screen-detected d isease: OHO-12(205-220) 11/16/02 11:24 AM Page 218 CANCER PREVENTION AND SCREENING 218 (cid:2) Lead-time bias arises from the fact that if early diagnosis advances the time of di agnosis of a disease, then the period from diagnosis to death will lengthen irrespective of whether or not treatment has altered the natural history of the disease. Screening will on ly be of value if it shifts the survival curve upwards. (cid:2) Length bias operates as slow-growing tumors are more likely to be detected by scree ning tests when compared to fast-growing tumors which are more likely to present with sympt oms before a screening test can be applied or between tests. Thus, screen-detect- ed tumors will tend to be less aggressive and associated with a relatively good prognosis.