Introduction 626 Malignant hypercalcaemia 627 Hyponatraemia 630 Hyperkalaemia 631 Renal tub ular dysfunction 632 Hyperglycaemia and hypoglycaemia 633 Summary 634 OHO-39(625-634) 11/18/02 9:33 AM Page 626 BIOCHEMICAL CRISES 626 Introduction Cancer and its treatment can cause a wide range of metabolic problems: (cid:2) Hypercalcaem ia (cid:2) Hyponatraemia (cid:2) Hyperkalaemia (cid:2) Renal tubular dysfunction (cid:2) Hy perglycaemia (cid:2) Hypoglycaemia The early symptoms of biochemical abnormalities may be s ubtle and easily confused with those of the underlying disease process. However, they undou btedly adversely affect quality of life and, when severe, may be life-threatening and prese nt as an oncological emergency. OHO-39(625-634) 11/18/02 9:33 AM Page 627 MALIGNANT HYPERCALCAEMIA 627 Malignant hypercalcaemia (cid:2) Complicates 5 \200\22310% of all cancers (cid:2) Especially associated with breast, myeloma, and squamous cell carcinoma of lung If \201ltered, calcium load increases \201ve-fold; kidney cannot excr ete and hypercalcaemia occurs (cid:2) (cid:2) Calcium above 3.0 mmol/L leads to dysfunctional GI tract, CNS, and kidneys (cid:2) Effective treatment improves quality of life Pathophysiology Mechanisms involved in the pathogenesis of malignant hyper- calcaemia inclu de increased bone resorption (osteolysis) and systemic release of humoral hypercalcaemic fa ctors, with or without evidence of metastatic bone disease. Local osteolytic hypercalcaemia is attrib- uted to tumor cell production of cytokines, particularly interleukins and tumor necrosis factors (TNF), prostaglandins, and growth factors, which stimulate prostaglandin function, while parathyroid hormone-related peptide (PTHrP) is the best characterized humor al mediator of hypercalcaemia. In some tumors, such as squamous cell cancers, humoral mech- anisms are dominant, while in others, for example multiple myeloma and lymphoma, osteolysis predominates. In breast cance r, both osteo- lysis and humoral mechanisms appear to be important. Dehydration is also an important contributory factor; calcium is a potent diuretic causing salt and water loss. In myeloma, renal impairment may also develop from deposition of Bence \200\223Jones protei ns. Some lymphomas produce active metabolites of vitamin D that increases the intestinal ab sorption of calcium. Clinical features (cid:2) Renal failure (cid:2) Cardiac arhythmias (cid:2) Non-malignant causes \200\224measu re PTH: inappropriately high in hyperparathyroidism, undetectable in malignancy Management The basic principles of management involve rehydration to restore glomerular fun ction and the use of drugs to inhibit osteoclastic bone OHO-39(625-634) 11/18/02 9:33 AM Page 628 BIOCHEMICAL CRISES 628 Table 39.1 Symptoms and signs of hypercalcaemia Symptoms Signs Gastrointestinal Nausea Vomiting Anorexia Constipation Renal Polyuria Dehydration Thirst Uraemia Hypercalciuria Nephrocalcinosis Neurological Lethargy Muscular weakness Drowsiness Stupor Weakness Confusion Disorientation Dysarthria Visual disturbance Diminished re \202exes Pain resorption. Where possible, speci \201c anti-tumor therapy should be instituted. Occasiona lly, the secretion of humoral factors by a primary tumor results in hypercalcaemia, and in such cases the serum level may be corrected by surgical removal of the tumor. Typically how - ever, the malignancy is advanced and treatment is palliative. Absorption of calcium from the gut is usually reduced in patients with malignant hypercalca emia and, except for the rare patient with lymphoma (usually T cell) associated with raised levels of vitamin D metabolites. They should be encouraged to eat what they like, when the y like, irrespective of the food \200\231s calcium content. Immobilization should be avoide d where possible as this may precipitate hyper- calcaemia, since the lack of weight-bearing induces increased osteo- clastic activity while reducing bone formation. Dehydration is an inevitable feature of symptomatic hypercalcaemia and it is essential to r ehydrate with 3 \200\2234 litres of 0.9% normal saline to restore glomerular function and i ncrease urinary excretion of calci- um. Rehydration will relieve many of the symptoms of hy percal- caemia but will rarely achieve total control. Prior to the 1990s, speci \201c treatment of hypercalcaemia relied main- ly on the use of calcitonin and mithramycin as inhibitors of osteoclast action, but these have largely been superceded by bisphosphonates. OHO-39(625-634) 11/18/02 9:33 AM Page 629 MALIGNANT HYPERCALCAEMIA 629 Corticosteroids have been widely used in the treatment of hyper- calcaemia but, except in s teroid-responsive tumors, add little to the response achieved by intravenous rehydration. ( cid:2) Selective inhibitors of osteoclast activity (cid:2) Agent of choice in hypercalcaemi a \200\224pamidronate, clodronate (cid:2) Highly effective (90% normocalcaemia) (cid:2) Usu ally normocalaemic in 3 \200\2237 days (cid:2) Side-effects \200\224transient fever, hypoca lcaemia IV route needed in acute situation (cid:2) (cid:2) Later, oral maintenance therapy. OHO-39(625-634) 11/18/02 9:33 AM Page 630 BIOCHEMICAL CRISES 630 Hyponatraemia A fall in the serum sodium to less than 130 mmol/l is associated with weakness, confusion, headache, drowsiness, and seizures. In cancer, the likely reason for hyponatraemia is the e ctopic tumor production of antidiuretic hormone (ADH). Small cell lung cancer is the most c ommonly associated malignancy, but carcinoid tumors, lymphoma, leukemia, and pancreatic can cers may also be responsible. Cytotoxic drugs used in the treatment of cancer, particularly ifosfamide, vin- cristine, and high-dose cyclophosphamide, may also stimulate ADH producti on. Other causes for hyponatraemia include pneumonia and raised intracranial pressure. Investigation will reveal continued renal excretion of sodium with an inappropriately high urinary sodium concentration, with the uri- nary osmolality exceeding that of the plasma. Treatments include a restricted \202uid intake of around 500 \200\223700 mls per day, dem eclocycline (which inhibits the action of ADH on the renal tubule), and, rarely, for severe life-threatening situations, the slow and closely monitored infusion of hypertonic (3%) sa line. OHO-39(625-634) 11/18/02 9:33 AM Page 631 HYPERKALAEMIA 631 Hyperkalaemia Renal failure is probably the most frequent cause of hyperkalaemia, but in cancer managemen t other causes should be considered. Most important, and usually preventable, is the tumor lysis syndrome that accompanies the rapid breakdown of malignant cells in response to effec tive chemotherapy. Tumor lysis is most likely in the manage- ment of high-grade lymphomas, leukemia, and trophoblastic and germ cell tumors, where the lysis of many millions of cells results in release of intracellular products, notably potassium and phosphate, into the ci rculation, along with urate generated by the breakdown of cellular proteins. Urate crystals may precipitate in the renal tubules causing acute urate nephropathy if appropriate precau tions are not taken. Prevention of tumor lysis is the key to management and patients who are predicted to respon d rapidly to chemotherapy should be vigorously hydrated intravenously, urinary pH should be maintained in the alkaline range by administration of sodium bicarbonate, and urate nephro pathy prevented by pre-treatment of the patient with allopurinol. Severe, and particularly acute, hyperkalaemia may cause cardiac dys- rhythmias and cardiac arrest. Emergency management is required and this should include intravenous rehydration wi th glucose, insulin, and sodium bicarbonate to correct acidosis and drive potassium into th e intracellular space, and intravenous administration of 10% calcium gluconate. In occasion al situations, haemodialysis may be necessary. Other causes of hyperkalaemia in cancer patients include septi- caemia, adrenal insuf \201 ciency (usually secondary to glucocorticoid withdrawal or adrenal destruction by a tumor), acute graft versus host disease following allogeneic bone marrow transplantation, and drugs (particularly diuretics such as spirinolactone). OHO-39(625-634) 11/18/02 9:33 AM Page 632 BIOCHEMICAL CRISES 632 Renal tubular dysfunction The kidney is normally very ef \201cient in ensuring that electrolyte con- centrations are maintained. However, this can be severely disrupted by cytotoxic drugs, particularly cispl atin and ifosfamide, which may cause tubular dysfunction and excessive loss of calcium, mag nesium, potas- sium, and sodium in the urine. Electrolyte loss results in lethargy, con- st ipation, confusion, and, in severe cases, seizures. Patients receiving these drugs require regular monitoring of electrolyte concentrations and replacement where necessary. Oral calcium and vitamin D may be required to maintain serum cal- cium levels and, for chro nic treatment of hypomagnasaemia, oral magnesium glycerophosphate is preferred to other ora l magnesium preparations as diarrhea is less of a problem with this formulation. OHO-39(625-634) 11/18/02 9:33 AM Page 633 HYPERGLYCAEMIA AND HYPOGLYCAEMIA 633 Hyperglycaemia and hypoglycaemia Disturbances in glucose metabolism may complicate cancer manage- ment. Administration of co rticosteroids to patients with diabetes mel- litus will increase glucose levels and increas e insulin requirements, while loss of appetite, nausea, and vomiting may predispose the dia - betic patient to hypoglycaemia. Inappropriate insulin production may occur from islet cell tumors and pancreatic APUDomas, while, rarely, large metastatic tumors, particularly in the liver, may produce insulin-like growth factors (IGF) which are released into the circulation, especially in response to tr eatment. OHO-39(625-634) 11/18/02 9:33 AM Page 634 BIOCHEMICAL CRISES 634 Summary Metabolic problems in cancer may present as a biochemical crisis. A high index of suspicion , understanding of the pathophysiological processes involved, and knowledge of the preventi on and treatment of these problems is an important component of good cancer medicine. OHO-40(635-654) 11/18/02 9:34 AM Page 635 Part 6 The way forward 40 Novel therapeutic strategies 637 41 Gene therapy for cancer 655 OHO-40(635-654) 11/18/02 9:34 AM Page 636 This page intentionally left blank OHO-40(635-654) 11/18/02 9:34 AM Page 637 Chapter 40 Novel therapeutic strategies New drug discovery 638 New drug development 643 Novel radiotherapeutic approaches 646 Targe ted radiotherapy 650 OHO-40(635-654) 11/18/02 9:34 AM Page 638 NOVEL THERAPEUTIC STRATEGIES 638 New drug discovery Introduction The process of drug discovery and development is undergoing a series of revolu tionary changes. This is due to three factors:- (cid:2) The view that conventional, mainly cytotoxic approaches have been successful in the past, but have reached a plateau of effect iveness and therapeutic index with respect to activity in human solid tumors. (cid:2) The opportunities for novel molecular targets opened up by our new understanding of the molecular biology of cancer. (cid:2) The range of new technologies that are available today to increase the quality of c andidate drugs and to accelerate their discovery and development. Earlier approaches to drug discovery The main approach used over the past 50 years has been to screen libraries of compounds (both chemically synthesized and natural products) either for cytotoxic activity against tumor cells in cell cul- ture or for anti-cancer activity i n tumor-bearing animals (e.g. inhibi- tion of tumor growth or increase in life span of the host). This has been successful in that it has led to the discovery and clinical use of man y of the major classes of established anti-cancer drugs e.g. alkylat- ing agents, anthracyc lines, topoisomerase inhibitors, and microtubule binders. Historical developments in screening are best illustrated by the evolution of methodologies used by the in \202uential National Cancer Institute in the US. In the early days, screen ing was commonly carried out against a mouse leukemia, either the P338 or L1210 models. But success in identifying drugs with activity in leukaemias and lympho- proliferative maligna ncies did not translate well to the major solid tumors. Hence, in the mid-1970s, transplant able mouse solid tumors, and then human tumor xenografts, were included as a secondary pane l. A decade later the need for an even greater focus on human cancer led to the introductio n of a 60-cell line in vivo tumor panel as the primary screen. Promising activities arising from this were followed up in animal studies. In some cases, the compound discovered in the screen was devel- oped through preclinical an d clinical testing. In others, a range of chemical analogs of the parent compound, already available or newly synthesized, were evaluated for improved performance. OHO-40(635-654) 11/18/02 9:34 AM Page 639 NEW DRUG DISCOVERY 639 These earlier approaches were very pragmatic. Studies of mode of action were commonly only conducted late in the development of a cytotoxic. Exceptions to this were the antimetabolit e and anti- hormonal approaches. In these cases, compounds were usually designed rationally to inhibit a target enzyme (e.g. dihydrofolate reductase, thymidylate synthetase, aromatas e) or to antagonize a receptor (e.g. for estrogen or androgen). Thus, the primary screen wa s frequently for the biochemical mode of action, and studies to check for compliance with t he desired mechanism could be included in subsequent in vitro and in vivo tests (e.g. thymi dine reversal for thymidylate synthetase, estrogen levels for aromatase). Contemporary approaches to drug discovery Over the last few years the increasing trend has been for a given drug discovery project to be aimed at a particular molecular target (e.g. a speci \201c oncogene product), with which pharmacological intervention might deliver a p articular desired biological or phenotypic effect (e.g. inhibition of proliferation, cell c ycle progression, motility, invasion, angiogenesis, and metastasis; or the induction of apo ptosis or differ- entiation) rather than a more general cytotoxic or cytostatic effect. This new molecular target orientated approach is now dominant in the pharmaceutical industr y and in biotechnology companies. Contemporary mechanism-based drug discovery can be divide d into the following several phases. Target identi \201cation and validation The objective is to identify genes and their cogna te proteins that are directly responsible for cancer causation and progression. Having iden ti \201ed a gene that is either mutated or shows deregulated expres- sion, a variety of ex periments can be carried out to validate the tar- get \200\224that is, to provide evidence that it is indeed involved in the disease process and to increase the level of con \201den ce that pharmaco- logical manipulations of the target would lead to an anti-tumor effect. The identi \201cation of new molecular targets in cancer has been revo- lutionized by huma n and other genome projects. All 30 000 human genes were published in Science and Nature in February 2001. Determining the speci \201c involvement of various genes in human tumors r equires molecular studies on familial and sporadic cancers to reveal the natural history of various malignant diseases at the genetic level. A leading example of this has been colore ctal cancer, where it is now clear that genes such as APC, ras, p53, and the mis- match rep air proteins play key roles. Various techniques can be used for target validation. These include transfection, gene knoc k-out and knock-in, micro-injection of inhibi- tory antibodies, dominant negative construct s, anti-sense constructs OHO-40(635-654) 11/18/02 9:34 AM Page 640 NOVEL THERAPEUTIC STRATEGIES 640 and oligonucleotides, inhibitory peptides, and pharmacological agents, through to clinical data linking gene mutation or deregulation to clinical outcome (e.g. overexpression of EGF and erbB2 receptors in ovarian and breast cancer). The choice of molecular target will relate to the strengths of the validation package, toge ther with the incidence of the abnormalities in human cancer and the technical feasibility of achieving pharmacological manipulation. For example, enzymes such as kinases and proteas es are more readily inhibited by drugs than are protein-protein interactions. Lead identi \201cation The objective of this phase is to identify a chemical structure tha t has some activity against the molecular target. This may be done in two ways. The \201r st way involves large, chemically diverse libraries being screened in automated, high-throu ghput assays. Here, both chemical diversity and speed can be increased by combinatorial che mistry, which uses robotic synthesis. The second way involves rational design based on a kn own substrate or ligand, or alternatively an X-ray crystallographic or NMR structure of the target itself. Lead optimization In this phase, the objective is to improve and re \201ne the desired pro p- erties of the lead (e.g. solubility, potency, and selectivity) and elimi- nate undesirab le features. This is done by making chemical derivatives or analogs of the lead, based on e merging biological results on the target structure. It involves an iterative process of \200\230making and testing \200\231 that leads to an understanding of structure \200\223act ivity relationships. Robotics may be used to make a series of analogs very quickly. Test cascade The key to successful lead identi \201cation and optimization is the test cas - cade. This is a series of hierarchically arranged tests or screens (usually biological) w hich allow the progressive optimization of chemical struc- ture, via the iterative \200 \230making and testing \200\231 approach, to give the desired performance. In industry, the desired performance will normally be de \201ned by a target pro \201le listing the essen tial and desirable features of the prospective drug. This will de \201ne the mechanism of action and bio- logical effect, as well as provide measures of physico-chemical proper- tie s, potency, selectivity, pharmacokinetics, and therapeutic index. A test cascade must be ro bust, rapid, and ef \201cient. The primary screen is usually a biochemical test using the recombi- nant protein target or a genetically engineered cell line. A selectivity screen will usually follow, together with an assay to con \201rm the bio- logical effect or a surrogate thereof in intact cells. Id eally, an assay should be available to con \201rm that this effect is achieved by the desi red molecular mode of action. Activity against tumor cells in culture is normally sought. OHO-40(635-654) 11/18/02 9:34 AM Page 641 NEW DRUG DISCOVERY 641 The transition from activity in cell culture to pharmacological effect in the intact animal is a challenging one. The latter requires the compound to display robust, \200\230drug-li ke \200\231 character, and inadequate pharmacokinetic/ADME (absorption, distribution, metab olism, and excretion) properties are often limiting. Hence a screen for blood levels, often using mixtures of compounds administered in cassette or cocktail dosing for higher through put, can be useful. Currently under evaluation by the NCI and others is the hollow \201bre assay that has ele ments of an in vitro \200\223in vivo hybrid test and gives an indication of the achievement of concentrations in vivo suf \201cient for at least minimal activity against human tumor cells. The \201nal stage of testing will involve seeking evidence of regression or growth arrest /delay in a human tumor xenograft. Depending on the biological effect sought, more complex tests such as orthoptic or Number of compounds tested Assay Biochemical screen (Usually high throughput test with recombinant reagents) Typically 200 000 \200\223500 000 Iterative lead development Rational design Structural biology Combinatorial chemistry 1000 \200\2235000 (Typically 0.5 \200\2231% hit rate from high throughput screen) Target cells assays (To measure cell activity, selectivity, mode of action) Pharmacokinetic endpoint (e.g. blood level as a measure of drug exposure) 50 \200\2231000 and/or Surrogate endpoint (e.g. normal tissue or animal tumor response as a measure of biol ogical effect) 20 \200\223250 Disease model (e.g. human tumor xenograft) 10 \200\22350 Specific organ toxicity Structure of a contemporary mechanism-based drug discovery test cascade. OHO-40(635-654) 11/18/02 9:34 AM Page 642 NOVEL THERAPEUTIC STRATEGIES 642 metastatic models may be useful. Transgenic mouse models can be valuable, as can surrogate non-tumor endpoints. Usually, activity will be sought in at least a small panel of tumors, including human xenog rafts. Ideally, these would be characterized for the molecular target and pathway involved, together with any other relevant features. Such a panel can be especially useful in select ing from a shortlist of potential clinical development candidates that may all dis- play th e target pro \201le. An alternative to this scheme is a modi \201cation to the NCI in vit ro cell panel approach, whereby compounds are selected based on correlation of sensitivity with molecular parameters. Preclinical drug development Following selection of a potential clinical candidate, a numbe r of pre- clinical development activities must be carried out. Formulation Choice of formulation is in \202uenced by solubility, stability, and dosage re quirements (e.g. acute or intermittent oral administration versus chronic oral dosing). App roaches involve the use of various mixed solvent systems as well as more novel delivery met hods such as liposomes. Preclinical pharmacology More detailed pharmacokinetic/ADME studies will be carried out. Th ese will de \201ne drug distribution in the body (usually rodents) and the rate and means of drug elimination, including identi \201cation of major metabolites. Preclinical toxicology Usually the \201nal step before Phase I trial in man, the objectiv es of preclinical toxicology are to de \201ne: (cid:2) Qualitative and quantitative organ toxicities (cid:2) The reversibility of the effects (cid:2) The initial safe starting dose Conditions for the preclinical toxicology should re \202ect as closely as possible the for mulation, schedule, route of administration, and dose/concentration features that will appl y in patients. Precise requirements vary between countries and regulatory authorities, but in general there is a two-step approach to de \201ne the acute maximum tolerated dose or L D10 (dose giving 10% lethality) in a rodent species usually the mouse and either the rat or dog, followed by a more extensive phase. Organ-speci \201c toxicities are often poorly pr edicted between species and the main objective is to de \201ne a safe starting dose in man . Conclusion Despite major advances in methodology, the drug discovery and devel- opment proc ess is still likely to take around seven years from new target to regulatory approval. Succ ess requires close co-operation of talented OHO-40(635-654) 11/18/02 9:34 AM Page 643 NEW DRUG DEVELOPMENT 643 multidisciplinary teams. With the trend away from empirical screening for anti-cancer activ ity to new mechanism-based approaches targeted to speci \201c molecular abnormalities resp onsible for cancer, we are now screening a range of exciting new agents emerging for clinic al evaluation. New drug development Once a novel anti-cancer agent has been discovered, its structure and pharmacological and p harmaceutical properties optimized, and a decision made to test it in humans, a number of w ell-de \201ned stages must be passed before the compound can enter clinical trials. The dr ug development process is very closely regulated to ensure the qual- ity of preclinical and clinical data and to protect the safety and the rights of the human subjects participating in the trials. Therefore, the requirements of the regulatory authority in the country wher e the trials are to be performed must be considered. The drug The ideal candidate will have potent activity against its target (e.g. VEGF recept or tyrosine kinase), minimal against other targets, be water soluble, and be physically and chemically stable both in powder form and in solution. The speci \201cation should ensure the material is of adequate quality and conforms to current requirements (i.e. ICH guideli nes). Methods should be developed with indicate stability and will detect and quantify, wit h suf \201cient sensitivity and accuracy, the drug, its degradation products, and the inte rmediates involved in its synthesis. Ideally, several batches should be made to con \201rm that the synthetic route is robust a nd that consistency of material with respect to purity of drug substance and levels of cont aminants (including solvents, synthetic intermediates, and degradation products) can be ach ieved. However, if several batches are not to be made, the batch produced initially should be large enough for all preclinical testing and early clinical trials. Formulation Formulation is the process by which the raw drug is converted into a form that can be administered to patients (e.g. an ampoule or vial for intravenous, intramuscular, or subcutaneous administration, or a capsule or tablet for oral administration). The route of administration OHO-40(635-654) 11/18/02 9:34 AM Page 644 NOVEL THERAPEUTIC STRATEGIES 644 selected must be appropriate for the biological and physico-chemical properties of the comp ound, and the agent must be shown to be active when given by that route. For example, for i ntravenous administra- tion, the compound must be soluble in a pharmaceutically acceptable solvent at biological pH, whereas for oral administration, the product must be stable in ac id. In some cases, it may be necessary to add excipients (other inert materials) to aid sol ubility, stability, etc. Once a putative formulation has been developed, the stability of t he product in the excipients must be con \201rmed for a minimum of three months (one month for a solution in saline). Batches of both the raw drug and the \201nal product should be assessed for stability und er a variety of environmental conditions for up to three years. These data will be used to attach a shelf life to the product when it is sent to the hospitals for clinical trials. Th e stability of the compound once reconstituted and compatibility with the giv- ing sets (e. g. stickiness to plastic) should also be investigated so that clear instructions can be pro vided for the appropriate administration of the drug (e.g. use within two hours of reconsti tution, or protect from light). Pharmacology Before undertaking a Phase I clinical trial, the pharmacological prop- erties of the drug need to be established. This is to ensure that ade- quate levels of drug can be delivered for it to have its desired effect and that the drug is working by the mechanism postulated. Preclinical toxicology Before clinical trials can begin, preclinical toxicology studies mus t be undertaken. This is to investigate the safety of the compound, to de \201ne its toxic ity pro \201le (side-effects), to investigate the reversibility of the toxic effects, and to determine a safe starting dose for the \201rst Phase I trial. These studies are genera lly undertaken in two species, often one rodent species (mouse or rat) and one non-rodent ( dog). The Cancer Research Campaign in the UK are, however, based on extensive experience, p ioneering the use of only rodent species for preclinical toxicology of anti-cancer agents. The exact design of the studies undertaken should be appropriate for the drug in question. The route and schedule of administration should mimic that proposed for the Phase I clinica l trial and the drug should be administered using the clinical formulation. The initial studies should determine the maximum tolerated dose (MTD) of the drug, or if th e compound is not very toxic, the maxi- mum administerable dose (MAD). This is used to calc ulate the start- ing dose of the clinical trial which is generally one tenth of the MTD (or MAD) in the most sensitive of the two species, when the dose is related to body surface ar ea (i.e. mg/m2). OHO-40(635-654) 11/18/02 9:34 AM Page 645 NEW DRUG DEVELOPMENT 645 The toxicity pro \201le of the drug is established by treating groups of animals at two or three doses (MTD and fractions of the MTD) or vehicle alone. Half of the animals in each g roup are then subject to hematology, clinical chemistry, and histopathology studies within 1 \200\2232 days of dosing. The other half are tested similarly after a 28-day recovery per iod to assess the reversibility of any toxicities seen. If there is any indication from its properties or its analogs that a particular drug may potentially have a particular toxicit y (e.g. nephrotoxicity), speci \201c tests should be undertaken during the toxicology stud ies to investigate this. Data from the toxicology studies should be used to design the Phas e I trial. Clinical trials The primary aim of the Phase I trials is to establish the safety and tolera bility of the compound being tested and to de \201ne an optimum dose and schedule for furt her (Phase II) studies. Other, secondary objectives may be to investigate the pharmacokinet ics of the drug in humans and study the ef \201cacy of the drug in the patients. Because of the toxicity of anti-cancer agents, Phase I trials of these compounds are genera lly performed in cancer patients and not healthy volunteers. Moreover, unless the drug is d esigned to treat a particular type of cancer, patients with advanced cancer of any type may be treated in Phase I trials. These patients will often be those who have already received , and their disease progressed through, the standard treatments for their particular condit ion. Patients entering all trials must be fully informed about the study and give their consent to participating. The details of the treatment, the patients to be entered, the doses, and the measurements to be made should all be detailed in the protocol that must be approved by the hospital ethics committee before the trial can begin. Phase I Phase I studies are generally dose escalation trials, where the initial dose level is calculated from the preclinical toxicology studies. This starting dose aims to be low en ough to ensure the safety of the patients, but high enough to minimize the number of patien ts treated at ineffective (too low) doses. Patients are generally treated in cohorts, with a number of patients all being treated at o ne dose level before the dose is escalated in the next cohort of patients. Traditionally, t hree patients are treated per dose level; however, increasingly trials are being designed w here only one patient is treated at a particular level if no toxicity is seen. The dose escalation scheme may vary (e.g. modi \201ed Fibonnaci scheme, pharmacokineticall y guided dose escalation), although the principle is the same \200\224to increase the dose rapidly at ineffective, non- toxic doses (thus minimizing the numbers of patients at these levels) OHO-40(635-654) 11/18/02 9:34 AM Page 646 NOVEL THERAPEUTIC STRATEGIES 646 and to reduce the rate of increase as effective and/or toxic doses are approached. The endpoint of the Phase I trial is normally toxicity (MTD and dose-limiting toxicity (DLT ) will be de \201ned in the protocol), except for non-cytotoxic agents, when the endpoint may be the optimum activity of the drug as de \201ned by its mechanism of action, unless u nacceptable toxicity is observed \201rst. For example, this may be inhi- bition of an enz yme or reduction of plasma levels of a hormone. On completion of the Phase I trial, the basic toxicity pro \201le of the agent in question should be known and an appropriate dose for further trials identi \201ed. Phase II and III Unless the drug has proved to be unacceptably toxic, it will then be subje ct to Phase II, and if successful, Phase III testing. The aim of Phase II trials is to asse ss the ef \201cacy of the drug. Each Phase II trial will be undertaken in patients with on e particular tumor type and they will all be treated with the same dose and schedule. Whist toxicity will continue to be monitored, the patients \200\231 disease will also be assesse d for response. The aim of Phase III trials is to compare the new agent with existing best treatment for the disease in question. If a new drug is proven to have an acceptable toxicity pro \201le, ef \201cacy in one or a number of disease types, and to compare favorably against existing standard treatments fo r the particular disease setting, then the compound may be submitted to regulatory authorit ies to register it as a product \200\224the culmination of a costly process which can take in the region of ten years to complete. Novel radiotherapeutic approaches Radiotherapy is an effective anti-cancer treatment modality. Increasing the dose delivered to the tumor while sparing the surrounding nor- mal tissue will commonly improve local tumo r control. Novel radio- therapeutic approaches aim to: Increase the dose delivered to tumors, by more effectively target- ing the tumor volume (bu t not at the expense of normal tissue damage). Increase the biological effectiveness of rad iation by affecting the radiosensitization of the tumor (but not of normal tissue). (cid:2) (cid:2) OHO-40(635-654) 11/18/02 9:34 AM Page 647 NOVEL RADIOTHERAPEUTIC APPROACHES 647 (cid:2) Use different forms of radiation which have improved biological effects. (cid:2) Use changed schedules of radiation delivery in order to increase the differential between tumor kill and normal tissue damage. Targeted radiotherapy The biological properties of the tumor itself provide the basis for s elective irradiation. In principle, this strategy should be capable of eradicating tumor ce lls anywhere in the body, but it is currently at an early stage of development for many sit es. Iodine (well differentiated thyroid carcinomas) (cid:2) (cid:2) Monoclonal antibodies to cancer cell surface antigens (B-cell lymphoma) (cid:2) Catecholamine precursor analog meta-iodo-benzl-guanine (MIBG) (neuroblastoma) (cid:2) Somatostatin (neuroendocrine tumors) Until recently, the nuclides used in targeted radiotherapy were selected because of their availability and low cost rather than on accoun t of their physical or radiobiological properties. Considerable experience has been gained with (cid:2) emitters such as iodine-131 and yttrium-90, but short lived (cid:3) emitters ( e.g. astatine-211 or bismuth-212) or Auger-emitting radionuclides (e.g. iodine-125) are mor e potent cell killers and may \201nd a clinical role. Targeting strategies to improve clinical results include: Improving the speci \201city of the existing targeting agents (cid:2) (cid:2) Regional targeting (e.g. intraperitoneal therapy) (cid:2) Multiple treatments (cid: 2) Decreasing side-effects (myelosuppression due to a radiation of bone marrow is the most important) by using peripheral blood stem cell support or haemopoietic growth factors (cid:2) Combination of targeted radiotherapy with other treatments The effectiveness of tar geted radiotherapy is strongly dependent on the extent to which a suf \201cient dose may b e delivered to critical clusters of tumor clonogens, and micodosimetric studies have shown that \202uc- tuations in dose uniformity can adversely affect treatment outcome. The opti mal strategy may be to use \200\230cocktails \200\231 of radionuclides, or to supplement t argeted radiotherapy with external-beam irradiation. Boron neutron capture therapy is a specialized approach to targeted therapy in which non-ra dioactive boron atoms are selectively incor- porated into tumor cells. When exposed to a be am of \200\230slow \200\231 neutrons the boron emits (cid:3) particles locally within the tumor, causing dense ionizations and DNA damage. So far, clinical experience in brain tumor s has produced controversial results. OHO-40(635-654) 11/18/02 9:34 AM Page 648 NOVEL THERAPEUTIC STRATEGIES 648 Improvements in external-beam radiotherapy delivery Improvements in focusing the external r adiotherapy beam on the tumor and avoiding normal tissue should allow a safe increase in th e dose of radiotherapy delivered. Conformal therapy This multi-step technique uses 3D image reconstruction and treatment plan ning, combined with sophisticated treatment veri \201cation, to con- form a high dose of r adiotherapy to the target volume (often irregu- larly shaped) but maintain a low dose to th e non-target tissues of the patients. The commercial development of multi-leaf collimators for linear accelerators and megavoltage imaging systems has made this treatment available i n a number of large radiotherapy treatment centers. Early clinical results suggest that nor mal tissue side-effects can be reduced and dose escalation is possible e.g. in the treatmen t of localized prostate cancer. Intensity-modulated radiation therapy (IMRT) Here the intensity of the radiotherapy beam is varied across the treat- ment \201eld to ensure that a uniform dose can be achieved in a regular or irregular shape target while avoiding delivery of a high dose to the surroundin g structures. Essentially, this may add to conformal therapy by further reducing the dose t o sensitive structures. Currently, 14 institutions worldwide specialize in this technology, prioritizing treatment of intracranial tumors and cancer of the head and neck and prostate. The beam can be produc ed by a modulatory accelerator or with the addition of a special collimator, but the tech- nique is computationally complex. Intra-operative radiotherapy Radiotherapy can be delivered to a tumor volume under direct v isual localization during an open operation within a designated radio- therapy suite. A lar ge single dose is delivered at the one procedure. The potential advantage of a targeted boo st (given in addition to external- beam fractionated therapy) is balanced by the theoretica l limitations of the biological effects of the single large dose. A few centers in the world currently use this technique and some results suggest a survival bene \201t. There remains controversy over the dif \201cult logistics of delivery and fu rther larger clinical studies are needed to de \201ne and quantify any bene \201t. Improvements in radiotherapy fractionation In general, tumors and critical normal tissues ( those which limit the dose which may safely be delivered in a course of radiotherapy) are a ssociated with different fractionation sensitivities. Thus, reducing the OHO-40(635-654) 11/18/02 9:34 AM Page 649 NOVEL RADIOTHERAPEUTIC APPROACHES 649 daily dose per fraction will allow a greater total dose to be delivered to the tumor withou t exceeding the normal tissue tolerance and the therapeutic index may be favorably altered. Studies have demonstrated fast rates of growth in certain tumors (e.g. squamous cell carcin omas of the head and neck region) such that clonogens have potential doubling times of less than \201ve days. This implies that the time taken to deliver a radical course of conven - tionally fractionated radiotherapy should not be extended. Indeed, clinical trials have d emonstrated bene \201ts with treatment acceleration where a radical course of treatment ma y be completed in two weeks (with thrice daily fractions), rather than the more usual 4 \200\2236 weeks.