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Translating basic cancer research into new cancer therapeutics

2002; Elsevier BV; Volume: 8; Issue: 4 Linguagem: Inglês

10.1016/s1471-4914(02)02319-5

1471-499X

Paul Workman, Stanley B. Kaye,

Cancer-related Molecular Pathways

The medical need for better cancer treatment is clear. In the developed world, roughly one in three people contract cancer and around one in four of those die from the disease. The worldwide incidence of cancer is set to double from 10 to 20 million over the next two decades and the death rate will increase from six to 10 million [1.World Health Report (1998) Life in the 21st century. A vision for all. World Health Organisation, Geneva, SwitzerlandGoogle Scholar]. Advances in treatment, with surgery, radiotherapy and conventional cytotoxic chemotherapy, have made only a modest overall impact on mortality. Cures can be achieved in childhood cancers, testicular cancer and lymphoma, and improvements in survival rates have been made as a result of adjuvant drug treatment of breast and colorectal cancer. However, the majority of human cancers are difficult to treat, especially in their advanced, metastatic forms. There is thus a pressing need for novel and effective forms of systemic therapy. In this TRENDS Guide to Cancer Therapeutics, we have chosen to focus our attention on the translation of basic cancer research into new cancer medicines. In particular, we highlight the discovery and development of innovative cancer therapeutic agents that act on the new molecular targets that are emerging from our burgeoning understanding of the deregulated structure and function of the cancer genome; that is, the transition from oncogenomics to mechanism-based cancer medicines. Furthermore, we focus on those drugs that act upon novel molecular targets, and other new approaches that are now undergoing the transition from the laboratory into early clinical trials or entering clinical practice, as this is an area that is particularly vibrant at the moment. We must surely be living and working in the most exciting period in the history of cancer research. The sense of excitement applies across all aspects of the endeavour, from basic studies designed to understand at the fundamental level the molecular causation of cancer, through to the application of this knowledge for patient benefit. Both basic and translational cancer research, the latter being dependent on the former, are now developing at an unprecedented pace. As the Human Genome Project comes to completion [2.International Human Genome Sequencing ConsortiumInitial sequencing and analysis of the human genome.Nature. 2001; 409: 860-921Crossref PubMed Scopus (18245) Google Scholar, 3.Ventor J.C. et al.The sequence of the human genome.Science. 2001; 291: 1304-1351Crossref PubMed Scopus (10833) Google Scholar], the next challenge for biomedical research is to apply this wealth of information to understand biological function and to exploit our growing knowledge for the benefit of mankind. In cancer, the particular challenge is to discover in precise detail the series of molecular abnormalities that arise in the genomes of all types of cancer, to use that information to understand the process of malignancy, and then to develop more rational and effective strategies for diagnosis and treatment. A fundamental principle underlying contemporary approaches to developing new molecular therapeutics agents is that drugs that act upon or in some way exploit the precise molecular abnormalities that drive malignant progression should be more efficacious and selective against cancer cells than our current agents, which are largely cytotoxic in nature [4.Kaelin W.G. Choosing anti-cancer drug targets in the postgenomic era.J. Clin. Invest. 1999; 104: 1503-1506Crossref PubMed Scopus (64) Google Scholar, 5.Garrett M.D. Workman P. Millennium review. Discovering novel chemotherapeutic drugs for the third millennium.Eur. J. Cancer. 1999; 35: 2010-2030Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar, 6.Gibbs J.B. Mechanism-based target identification and drug discovery in cancer research.Science. 2000; 287: 1969-1973Crossref PubMed Scopus (458) Google Scholar, 7.Workman P. Scoring a bull's-eye against cancer genome targets.Curr. Opin. Pharmacol. 2001; 1: 342-352Crossref PubMed Scopus (37) Google Scholar, 8.Workman P. Changing times: developing cancer drugs in genomeland.Curr. Opin. Invest. Drugs. 2001; 2: 1128-1135PubMed Google Scholar]. Developing mechanism-based agents that pinpoint precisely the defects in cancer cells is rational and feels intuitively obvious in the postgenome era. Furthermore, we are beginning to see the first examples of this new approach showing genuine patient benefit in the cancer clinic. It is important to stress that cancer is not just one but many different diseases, each with distinct characteristics and therapeutic requirements. Genomic research provides even greater stratification of cancer types. Another important point is that cancer is also not simply a disease of unrestrained proliferation, although this is a significant part of the picture. The mutation and deregulation of cancer genes leads to a wide range of changes in cellular structure and function, all of which contribute to the malignant phenotype and pathological behaviour of human cancer. Hanahan and Weinberg [9.Hanahan D. Weinberg A. The hallmarks of cancer.Cell. 2000; 100: 57-70Abstract Full Text Full Text PDF PubMed Scopus (22974) Google Scholar] have usefully characterized cancer in terms of six hallmark traits or biological features, and these are shown in Box 1. All of the traits can be targeted by the new generation of molecular therapeutic agents, giving us multiple routes to attack malignant disease.Box 1. The six hallmark traits of cancer 1Information taken from [9.Hanahan D. Weinberg A. The hallmarks of cancer.Cell. 2000; 100: 57-70Abstract Full Text Full Text PDF PubMed Scopus (22974) Google Scholar]1Information taken from [9.Hanahan D. Weinberg A. The hallmarks of cancer.Cell. 2000; 100: 57-70Abstract Full Text Full Text PDF PubMed Scopus (22974) Google Scholar]•Self-sufficiency in proliferative growth signals•Insensitivity to growth inhibitory signals•Evasion of apoptosis•Acquisition of limitless replicative potential•Induction of angiogenesis•Induction of invasion and metastasis •Self-sufficiency in proliferative growth signals•Insensitivity to growth inhibitory signals•Evasion of apoptosis•Acquisition of limitless replicative potential•Induction of angiogenesis•Induction of invasion and metastasis The progress and challenges in the discovery and development of postgenomic cancer medicines are very well illustrated by the articles in this TRENDS Guide. Several underlying themes emerge. Features and potential advantages of contemporary postgenomic cancer drug development are summarized in Box 2. Of fundamental underlying importance, is that our increased understanding of the genomics and molecular pathology of cancer provides us with an intellectual framework for thinking about the disease and for devising innovative and imaginative strategies to combat it. The importance of continuing to discover new targets and pathways for therapeutic intervention in cancer is clear. The need to move forward extremely rapidly and exploit those targets for therapy is also obvious. The pace and efficiency of drug discovery is accelerated by a combination of modern technologies, particularly recombinant DNA methodology, genomics, high-throughput screening (HTS), combinatorial chemistry and structural biology. These technologies have contributed in a major way to the emerging success stories that are covered in this TRENDS Guide. The use of pharmacokinetic and pharmacodynamic endpoints to increase the rationality and hypothesis-testing power of early clinical trials is extremely important. These also provide a basis for making stop/go decisions on new agents, and reduce the risk of expensive, late-stage failure. The development of prognostic and pharmacogenomic biomarkers is essential to enable the targeting of individualized treatments to those patients most likely to benefit (Fig. 1).Box 2. Features and potential advantages of postgenomic cancer drug developmentFocus on new molecular targets driving the molecular pathology and progression of human cancers:•provides an intellectual framework for discovering new drugs with improved efficacy and selectivity.Application of new technologies such as high-throughput screening, combinatorial chemistry and microarrays: accelerates drug discovery and development.Use of pharmacokinetic and pharmacodynamic endpoints:•enhances the rationality and hypothesis-testing power of early clinical trials, provides the basis for early go/no-go decisions, and reduces the risk at late-stage failure.Development of diagnostic, prognostic and pharmacogenomic biomarkers:•enables the targeting of individualized treatments to patients most likely to benefit. Focus on new molecular targets driving the molecular pathology and progression of human cancers:•provides an intellectual framework for discovering new drugs with improved efficacy and selectivity. Application of new technologies such as high-throughput screening, combinatorial chemistry and microarrays: accelerates drug discovery and development. Use of pharmacokinetic and pharmacodynamic endpoints:•enhances the rationality and hypothesis-testing power of early clinical trials, provides the basis for early go/no-go decisions, and reduces the risk at late-stage failure. Development of diagnostic, prognostic and pharmacogenomic biomarkers:•enables the targeting of individualized treatments to patients most likely to benefit. As we review the progress in the areas described in the articles that follow, it is important to identify the learning points from our early therapeutic adventures in ‘genomeland’ and to prioritize the areas that we need to build on for future success. Many more exciting agents acting on cancer genomics targets will follow and it is important that we develop them not only quickly and imaginatively, but also critically. Do they work in the intended fashion against the desired target? If so, is this target going to be a useful one to modulate human malignancy, as distinct from preclinical models of cancer? The current successes are very clear. But how can we do the job even better and faster in the future? Some of the lessons and highlights from the topics covered in this TRENDS Guide are described below. It is entirely appropriate that the first article in this TRENDS Guide to Cancer Therapeutics describes the discovery and development of Gleevec™ (also known as Glivec, imatinib mesylate and STI571) as a paradigm for contemporary, post-genomic, mechanism-based drug development. Gleevec™ is the first small-molecule drug to receive regulatory approval that targets a particular cancer gene product. Brian Druker explains how Gleevec™ inhibits the activated tyrosine kinase activity of the Bcr-Abl protein. As a product of the so-called Philadelphia chromosome translocation, Bcr-Abl is responsible for the molecular pathogenesis of chronic myelogenous leukaemia (CML). The rapid approval of Gleevec™ in refractory CML was driven by the outstanding activity of this agent, its minimal side-effects and the limited alternative therapeutic options. There are several lessons to be learned from the story of Gleevec™, which of course is still incomplete. Although the elucidation of the role of Bcr-Abl in CML began as early as 1960, it has taken more than 40 years to develop a drug that exploits the specific molecular pathology that drives the disease. Translating oncogenomics into cancer medicines can now move much faster than this, but it must be recognized that elucidating the biological function of cancer genes, and developing drugs that antagonize these functions is not a trivial exercise. Chronic dosing is needed to maintain inhibition of the target – a common requirement with postgenomic drugs that will generally induce cytostasis or apoptosis, rather than the dramatic cell killing and regression that is sometimes seen with cytotoxic agents. A good tolerance profile is therefore essential. The emergence of resistance to Gleevec™ teaches us that our new postgenomic designer drugs will not escape this age-old problem of cancer therapy. Pinpointing the molecular mechanisms of resistance, involving, in this case, amplification of the Bcr-Abl gene or its mutation to drug-resistant forms, leads us to rational ways forward to overcome the resistance. The availability of a pharmacodynamic endpoint for Bcr-Abl inhibition proved extremely useful in understanding the development of Gleevec™ resistance – another valuable learning point. One of the most significant lessons to be learned from the Gleevec™ story is the importance of treating early- versus late-stage disease (that is, in this case, chronic as opposed to blast-phase patients). The greater sensitivity of early- versus late-stage CML might be caused by the accumulation of additional malignancy-driving mutations with advanced disease, a factor that could well apply to other cancers. If so, this suggests that the therapeutic targeting of multiple genetic abnormalities will be required. Also clear from the Gleevec™ story, is the continued role of serendipity in modern drug discovery. The drug emerged from a class of compounds known as the 2-phenylaminopyrimidines. These compounds were identified by HTS as showing promising activity against the platelet-derived growth factor receptor (PDGF-R) tyrosine kinase, and were subsequently optimized by medicinal chemistry for activity against that target. Further testing revealed even more potent activity against the Abl tyrosine kinase. Fortune played its hand again when Gleevec™ was also shown to exert equally potent inhibition against the c-kit tyrosine kinase enzyme. This led to its evaluation against gastrointestinal stromal tumours (GIST), which exhibit frequent activating mutations in c-kit, and the demonstration of remarkable therapeutic activity in this difficult to treat disease. The images on the front cover of this TRENDS Guide illustrate the type of dramatic response that has been achieved in GIST. The involvement of c-kit in other malignancies could broaden the use of Gleevec™ further still. Even greater therapeutic potential would be unleashed if therapeutic doses of Gleevec™ were sufficient to provide inhibition of the PDGF-R tyrosine kinase – this target is involved in various malignancies and is thought to play a key role in angiogenesis. Thus, we can still be surprised by the activity of designer drugs that are supposedly developed to be uniquely selective against a particular kinase. This experience also tells us that absolute selectivity is not necessarily essential in an effective and well-tolerated drug. Gleevec™ is of course the first kinase inhibitor drug to receive regulatory approval, finally laying to rest the view of the pessimists who predicted that ATP-competitive kinase inhibitors would never make selective medicines. Many more kinase inhibitor drugs are following closely behind Gleevec™. De Bono and Rowinsky describe progress and issues in the clinical development of the next group of kinase inhibitors challenging for approval – those inhibiting the ErbB receptor family of membrane receptor enzymes. Proof-of-concept for the drugability of these molecular targets is, of course, already established with the regulatory approval of the anti-ErbB2 antibody Herceptin® (trastuzumab). Leading the small molecule field is Iressa® (ZD1839). This is an optimized anilinoquinazoline, derived from a HTS lead, which exhibits selectivity for the epidermal growth factor receptor (EGF-R) tyrosine kinase. Initial clinical studies showed that chronic, oral administration resulted in objective responses at well-tolerated doses in non-small-cell lung cancer, head and neck cancer and hormone-refractory prostate cancer. Pivotal registration studies in non-small cell lung cancer involve randomization of Iressa® plus carboplatin/taxol, versus chemotherapy alone. Selection of dosage, schedule, disease-type and response endpoint are generic challenges to be faced with signal transduction inhibitors, especially as these are generally expected to be cytostatic rather that cytotoxic in nature. Because of this, it is a common view that agents like Iressa® will find their optimal usage in combination with cytotoxic agents, at least in the short to medium term. Selecting which combination to test clinically is a particular challenge as preclinical models are known to be poorly predictive in the combination setting. The historical, pragmatic approach has been to avoid combining drugs that have overlapping toxicities and antagonistic anticancer effects, but these criteria are insufficient for our needs in the postgenome era. Developing a mechanistic framework for selecting drug combinations would be both practically important and more intellectually satisfying. Molecular pharmacodynamic endpoints (e.g. receptor autophosphorylation) have been used to demonstrate that the EGF-R tyrosine kinase is indeed inhibited in tumour and/or normal tissues, and this was valuable in interpreting results from both animal models and patients. However, it is not yet clear whether responsive patients can be identified by target expression levels, as is the case with Herceptin®, which is approved for use in ErbB2 positive cancer and requires the use of an assay to measure expression of this receptor for patient selection. As successors to Iressa®, small-molecule inhibitors that target multiple members of the ErbB gene family are of particular interest. It is common, more for simplicity and because of genuine ignorance than anything else, to view signal transduction pathways as a linear sequence of events emanating from the cell membrane and reaching down through the cell into the nucleus. This simplistic textbook model is almost certainly incorrect as it fails to take into account even such well known factors as signalling redundancy and crosstalk between pathways. It is an ongoing debate as to where one should optimally intervene in oncogenic signalling pathways in order to maximize the anticancer effect and minimize the toxicity to normal tissues. Close to the membrane, for example, at the receptor tyrosine kinase (RTK)? Close to or inside the nucleus, for example, at the level of cyclin-dependent kinases (see later)? Or perhaps somewhere in between, such as targeting Ras in the RTK/Erk pathway, or hitting Akt in the RTK/PI3 kinase pathway? How about as close as possible to (but not above) the crucial molecular abnormality, for example, Raf, which is positioned immediately downstream of Ras? It could be argued that interfering with the central machinery of cell-cycle control, upon which multiple signalling cascades converge, would probably have a powerful anticancer effect, but also most likely have strong effects on normal cells. Based on this, it would follow that blocking a membrane receptor would be preferable; however, others would argue that too many downstream signals would then be affected, including those not required for oncogenicity. Hence, interaction at an intermediate level could be seen as optimal. In reality, we still don't know enough about the precise arrangement of the oncogenic wiring of individual cancers to make a rational selection. As we need to avoid analysis paralysis, it is important to support the development of multiple inhibitors and drugs, each of which is able to act at a key point in a given oncogenic signalling pathway. Ideally, we would have an inhibitor or drug available to evaluate the relevance and robustness of every potential target in all signalling cascades that induce or support malignancy. This is the long-term goal. Yet, given inevitably finite resources, some prioritization is clearly required. Focussing on what are regarded as key nodes in the most commonly deregulated, ‘mission critical’ pathways [10.Evan G.I. Vousden K.H. Proliferation, cell cycle and apoptosis in cancer.Nature. 2001; 411: 342-348Crossref PubMed Scopus (2794) Google Scholar] is one solution that has been adopted. Box 3 summarizes some of the criteria that can be used for target validation and selection.Box 3. Criteria for validation and selection of new drug targetsNote that not all criteria need to be met to embark on a drug discovery programme. However, if several of these criteria are met, it provides increasing confidence and reduces risk for the project.•Frequency of genetic or epigenetic deregulation of the target or pathway in human cancer.•Demonstration in a model system that the target contributes to the malignant phenotype.•Evidence of the reversal of the malignant phenotype; for example by gene knockout, dominant negative, antisense, RNAi, antibodies, peptides or drug leads.•Practical feasibility, tractability or drugability of the target; for example, enzymes are generally more tractable than are most protein–protein interactions.•Availability of a robust, efficient biological test cascade to support the drug discovery programme.•Ability to run a robust cost-effective high-throughput screen.•Availability of a structure-based drug design approach. Note that not all criteria need to be met to embark on a drug discovery programme. However, if several of these criteria are met, it provides increasing confidence and reduces risk for the project.•Frequency of genetic or epigenetic deregulation of the target or pathway in human cancer.•Demonstration in a model system that the target contributes to the malignant phenotype.•Evidence of the reversal of the malignant phenotype; for example by gene knockout, dominant negative, antisense, RNAi, antibodies, peptides or drug leads.•Practical feasibility, tractability or drugability of the target; for example, enzymes are generally more tractable than are most protein–protein interactions.•Availability of a robust, efficient biological test cascade to support the drug discovery programme.•Ability to run a robust cost-effective high-throughput screen.•Availability of a structure-based drug design approach. Herrera and Sebolt-Leopold enter this minefield by focussing on the sites for pharmacological intervention in the Ras/MEK/MAP kinase-signalling pathway. Mutant Ras proved to be intractable as a drug target, as exemplified by the failure to identify drug-like inhibitors of the GTP-bound ‘on-switch’ or of Ras-effector protein interactions. This experience reinforces our preference for the type of targets that have been proved by history to be tractable with respect to small-molecule intervention, and particularly for enzymes rather than large-domain size protein–protein interactions. Despite ongoing controversy about their precise molecular mechanism-of-action (which might not involve Ras directly but rather a prenylation switch on RhoB), several farnesyl transferase inhibitors are in preclinical and clinical development. Two such agents, the methylquinolone R115777 and the tricyclic SCH6636, have shown early evidence of clinical responses, including anti-leukaemic activity. It will not infrequently be the case that a potential cancer gene target proves to be intractable or ‘non-drugable.’ In such cases, it is necessary to ‘walk the pathway’ to find a downstream target. Next up to bat in the Ras/MEK/MAP kinase pathway are kinase targets – the so-called ‘many faceted’ Raf and the ‘master gatekeeper’ MEK. The leading c-Raf-1 inhibitor BAY 37-9751 and the MEK inhibitor CI-1040 (formerly PD184352) both showed promising activity in animal models and are now undergoing early clinical evaluation. Herrera and Sebolt-Leopold's brief discussion of the pros and cons of Raf and MEK as targets reveals how little we really understand about signal transduction in cancer cells, even for such a well-known pathway. Their proposal that the biochemical make-up of the cancer might guide the clinician's choice underlines a recurring theme in this TRENDS Guide – that is, the need for robust, predictive biomarkers of response and clinical outcome. Cancer pharmacologists should not be deterred by complex oncogenic pathways. The development of antimetabolite drugs has had to overcome similar obstacles such as salvage pathways, feedback loops and so on. The problem should be soluable through a combination of ongoing fundamental research to understand how oncogenic pathways actually work, coupled with pragmatic approaches to develop and evaluate inhibitors of currently prioritized targets. Another important oncogenic trunk route is the PI3 kinase (PI3K) pathway, which is often depicted as being positioned downstream of Ras, or alternatively operating in parallel with the Ras/Raf/MAP kinase cascade. PI3K is clearly important for several oncogenic effects, including proliferation, survival and angiogenesis. Many cancers exhibit loss of the PTEN tumour suppressor gene, which encodes the phosphatase that reverses the PI3K reaction. Others show increased expression of PI3K and the downstream kinase Akt/PKB. Although not discussed in this TRENDS Guide, selective PI3K inhibitors would be expected to have great therapeutic potential. In the validation and selection of molecular targets and pathways for therapeutic intervention in cancer, the frequency with which a particular target or pathway undergoes mutation or deregulation is a valuable indicator of its importance in the malignancy process and of the potential use of a drug that acts on that target or pathway. Mutation or functional impairment of the p53 tumour suppressor gene is the most common molecular defect in human cancer. p53 plays an important role in regulating the response of a cell to genomic damage, and has been described as the ‘guardian of the genome.’ Like Bcr-Abl and the RTK/Ras/Raf/MAP kinase pathway discussed earlier, basic research in this area goes back a long way, and yet therapeutic agents that exploit the fundamental knowledge have been slow to appear. But this situation is now changing. In their article, Lane and Lain review recent developments in our fundamental understanding of p53 function and highlight several therapeutic approaches based on p53 that are now undergoing preclinical research and development. Replacement gene therapy with p53 is already being evaluated in the clinic and a key factor in its success, like other gene therapy approaches, will be the efficiency of vector delivery. This issue is addressed in the article by Kirn et al. (see later), together with the engineering of oncolytic viruses to replicate selectively in cancers that lack p53 function. These include the Onyx 0-15 virus, which was designed to selectively replicate and lyse p53 deficient cells by virtue of EIB deletion. At least by intratumoural delivery, this virus has demonstrated interesting clinical activity, prompting the activation of large-scale randomized trials, particularly using the virus in combination with chemotherapy. Also discussed by Lane and Lain is the exploitation of p53 loss to drive selective expression of therapeutic genes. The holy grail of therapeutic research on p53, namely the discovery of agents that are able to convert mutant p53 into the active, wildtype conformation, has proved extremely difficult to achieve. It is well known that many companies have run p53 resurrection screens without success. A major challenge is the involvement of so many different mutations and the lack of any obvious pharmacologically tractable binding site. However, small-molecule compounds have now been identified that provide proof-of-principle for pharmacological rescue of p53 conformation and function. These act as a molecular brace to stabilize the active p53 structure [11.Foster B.A. et al.Pharmacological rescue of mutant p53 conformation and function.Science. 1999; 286: 2507-2510Crossref PubMed Scopus (699) Google Scholar]. In an alternative approach, a small-molecule compound, pithithrin α, was identified by HTS as an agent that induced transcription of p53-dependent genes. It is proposed that compounds of this type could be used to protect normal tissues from the damaging effects of chemotherapy and radiotherapy. However, this needs to be balanced against the potential for pro-mutagenic activity. Also discussed are strategies designed to treat tumours in which the wildtype p53 gene structure is retained but function is inhibited by the E6 protein in HPV-associated cancers or by Hdm2 in sarcomas and other malignancies. Additional therapeutic challenges include situations where the p53 response is impaired by mutation of upstream genes such as ATM and CHK2, and also by loss of p14ARF. Promising experimental approaches include blocking p53–Hdm2 binding, which involves a potentially tractable, small-domain size protein–protein interaction and also the inhibition of the nuclear export of p53, as seen with leptomycin B, which is a small-molecule inhibitor of the CRM-1 nuclear exportin. This latter agent has anti-tumour activity and previously underwent clinical trial, but was almost certainly tested in a sub-optimal way. Further work on the mechanism-of-action of leptomycin B or similarly acting agents could pave the way to a reinvestigation of its clinical potential. Efforts are also underway to mimic the function of p14ARF, which is an E3 ligase inhibitor, thereby preventing the ubiquitin-dependent degradation that involves Hdm2. Once again it is stressed that these various sophisticated approaches to interfere with the guardian of the genome will require diagnostic markers to be developed that enable the precise status of p53 within individual cancers to be determined. The cell cycle is commonly deregulated in cancer. This frequently occurs in a variety of ways at the level of cyclin-dependent kinases (CDKs) and their regulatory molecules. Several CDK inhibitors are now entering preclinical and clinical development. Such agents could potentially restore normal cell-cycle control or induce apoptosis in cancer cells. It is particularly appropriate to review