Hitting cancer where it hurts: How have targeted therapies improved our approach. Karin Purshouse, Newcastle University ‘Will we ever cure cancer?’ Having completed a breast cancer research project, this is a question I am often asked. Honestly, I say? The more we know about cancer, the less convinced I am that we will ever find a cure. However, we are learning more about what makes cancer ‘tic’, and targeted therapies have paved the way for a more elegant approach compared the hit-andhope alternatives of surgery, chemotherapy and radiotherapy. Having spent four months collecting and analysing blood samples from breast cancer patients, looking particularly at bone metastasis, it is clear to me that targeted therapy could revolutionise care – however, there are significant biological and financial stumbling blocks that must first be overcome. The problem Cancer will affect around 1 in 3 people, and 1 in 4 will die from it. Breast cancer will affect 1 in 9 women, with rates increasing particularly in the developing world due to the oestrogen-promoting lifestyle choices seen for decades in the developed world such as obesity, later childbirth, early menarche, oral contraceptive use and a reduction in those breast feeding.(1,2) In addition, screening is becoming more widespread. Overall, incidence is increasing, and although mortality rates are decreasing, it remains that some 1.38 million women are diagnosed worldwide every year.(3) (Figure 1) QuickTime™ and a decompressor are needed to see this picture. Figure 1: Breast cancer incidence in the UK (Source – see figure references) Current options Conflict has given us many medical breakthroughs, and cancer is no different. The realisation that mustard gas was an alkylating agent triggered the development of chemotherapeutics broadly categorised as alkylating/pseudoalkylating agents, antimetabolites, anthracyclines, vinka alkaloids and taxanes, and topoisomerase inhibitors. Chemotherapy is used in breast cancer, with radiotherapy and surgery also mainstays. Targeted therapies have revolutionised breast cancer treatment already. Hormone therapies such as tamoxifen and aromatase inhibitors are utilised in hormone (estrogen (ER) and progesterone (PR)) receptor positive patients and increasingly, trastuzumab (aka Herceptin) and lapatanib are used for HER2 positive patients. Before treatment is given, breast cancer tissue is analysed. Traditionally, a core biopsy is stained by immunohistochemistry to identify the ER, PR and HER2 status (with additional analysis if necessary). However, Sorlie and colleagues identified alternative subtypes: basal, HER2, luminal A/B and normal, and genetic signatures for these subtypes indicate a completely different selection of potential targets.(4) Perhaps we are missing the mark with our existing diagnostic methods and the resulting treatment. The Hallmarks of Cancer It had been clear for a decade before Sorlie’s paper that the treatment methods were also failing to strike a fatal blow to cancer. The six Hallmarks of Cancer described by Hanahan and Weinberg illustrated cancer’s cure-evading qualities: self sufficiency, insensitivity to anti-growth signals, anti-apoptotic, invasive, pro-angiogenic and antisenescent.(5) (Figure 2) QuickTime™ and a decompressor are needed to see this picture. Figure 2: Hallmarks of Cancer Ultimately, genetic mutations give rise to these attributes, as well as epigenetic and RNA interference mechanisms contributing to the initiation, propagation and metastasis of cancer cells. Targeting the genome and related components may be an area for the future, but for now, limitations include the lack of a suitable vector, toxic side effects or immunoregulation. Further, our understanding of these mutations and alterations remains limited. For example, microRNAs involved in the initiation of breast cancer include microRNA 125b but antisense oligonucleotides, microRNA sponges and mimics remain very much at their therapeutic infancy.(6) To target the genome would only target that specific mutation – yes, at its origin, but with the limitation that many thousands of agents would need to be designed to target the genetic mutations involved in cancer. Consequently, targeted agents have sought to address the Hallmarks of Cancer, thereby tackling the qualities that make cancer so destructive. Targeted agents In understanding the significance of targeted therapies, their limitations and future directions, we should consider the hallmarks they attack and their mode of action, looking particularly at their role in breast cancer therapy.  Self Sufficiency and insensitivity to antigrowth signals o In contrast to normal cells, cancer cells enhance the manner in which growth factors induce their activation and proliferation. Growth Factor receptors on cells, such as fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF), are transmembrane tyrosine kinases. Their activation by extracellular growth factors leads to receptor dimerisation and downstream signalling by autophosphorylation. o Imatinib (Glivec) was the first drug to successfully inhibit this pathway by blocking phosphorylation via tyrosine kinase inhibition in patients with Chronic Myeloid Leukaemia (CML) demonstrating the bcr-abl (Philadelphia chromosome) mutation. o In breast cancer  • HER2 and VEGF receptors are often upregulated • VEGF expression may be increased Pro-angiogenic o An angiogenic ‘switch’ occurs, whereby proangiogenic factors such as VEGF outweigh anti-angiogenic factors like thrombospondin.  Antiapoptotic o Apoptosis occurs via intra- and extracellular activation of the common caspase-3 pathway, leading to cell death. Intracellular regulators of this are often mutated in various forms of cancer – p53, which acts as a gatekeeper of correct DNA replication and promotes apoptosis via mitochondrial signalling, is mutated in over 50% of cancers. Decoy FAS ligands prevent extracellular apoptosis activation. o Kinases, including MAPK and P13K/AKT, are key regulators of apoptosis.  Antisenescent o The Hayflick factors which regulate senescence (Telomere shortening, accumulation of DNA damage and activation of the ARF/INK4a/INK4b locus) also act as tumour suppressors. In cancer cells, telomerase is upregulated and gene mutations lead to a faulty senescence mechanism.  Invasive o Arguably the area where targeted therapies could have the most important role, metastasis is the commonest cause of cancer death. Cancer cells break free from their neighbouring cells and extracellular matrix and enter the blood and lymphatic systems. o In breast cancer, bone metastases affect 69% of those with advanced disease, with bone a particularly vulnerable site given its well-vascularised red marrow.(7) Existing therapies have, by design or by accident, addressed some of these qualities. For example, the antimetabolite chemotherapeutic methotrexate acts as a dihydrofolate reductase inhibitor, inhibiting synthesis of nucleotides and therefore proliferation and growth. Radiotherapy also prevents the survival and proliferation of cancer cells via ionising radiation. Surgery simply removes the tumour bulk, which may be curative in some cancers either alone or in combination with prior or subsequent treatments, or provide symptomatic relief. In some cancer types, these methods are becoming increasingly sophisticated. For example, in liver carcinoma, targeted radiofrequency ablation, direct radiotherapy via yttrium microspheres and arterial embolisation are revolutionising treatment of patients with moderate to palliative disease. However, cancer is still outwitting these approaches, and in my view, many research centres are focussing on new technology and crude methods of blocking the cancer waterfall ignoring the biological basis of cancer, which is in itself still being explored. As such, targeted therapies seem a far more logical approach to not just blocking the waterfall, but sealing it off for good. Breast cancer and targeted therapies The National Cancer Institute in the United States identifies hormonal therapies such as estrogen receptor modulators (e.g. tamoxifen, fulvestrant) and aromatase inhibitors (e.g. letrozole, exemestane) as the first targeted therapies.(8) These have revolutionised breast cancer therapy, although really they should be considered separately to novel targeted therapies. Targeted therapies can be broadly divided into monoclonal antibodies and small molecule inhibitors. Trastuzumab (or as it is better, known, Herceptin) targets the extracellular domain of HER-2 (Human epidermal growth factor receptor 2), thereby inhibiting dimerisation and activation of the receptor.’s downstream pathways HER- 2 is internalized and destroyed. HER-2 is overexpressed in 25% of breast cancers, which tend to be particularly aggressive. A classic example of monoclonab antibody therapy, Slamon and colleagues completed a revolutionary study demonstrating its superiority with chemotherapy over chemotherapy alone in metastatic breast cancer in terms of time to progression, response rate and survival.(9) Subsequent work by Piccard-Gebhard and colleagues in a large international, multicentre trial demonstrated a survival advantage at one year.(10) It is now recommended by NICE as adjuvant therapy for early breast cancer. Small molecule inhibitors, include lapatinib, an inhibitor of the intracellular domain of the receptor’s tyrosine kinase, thereby blocking autophosphorylation via the ATP binding domain. In 2006, Geyer published a landmark study exploring lapatinib with capecitabine versus capecitabine alone in advanced HER-2 positive breast cancer that had progressed despite treatment including anthracyclines, a taxane trastuzumab in terms of disease progression.(11) In the absence of recommendation by NICE, it is only used in exceptional cases or in clinical trials such as that recently completed by Coombes and colleagues at Imperial College London (Clinical Trial no: ISRCTN71165034). Given that advanced disease and its associated metastases are a more common cause of patient death than the primary tumour, this is a significant area of progress. Other small molecule inhibitors include intracellular kinase inhibitors and, notably in breast cancer, conserved site inhibitors such as Poly(ADP-Ribose) Polymerase (PARP) Inhibitors. These have a particular role in patients with a BRCA1/2 mutation by inhibiting normal DNA repair mechanisms and leading cancer cells with a BRCA mutation to expire. Fong et al demonstrated that Olaparib has few side effects and inhibits tumour activity.(13) Although this would only benefit a small proportion of patients, Tutt and colleagues identified in a recent ‘proof-of-concept’ study that showed a ‘favourable therapeutic index’ for olaparib.(14) Not all targeted agents have found success, and it is clear that we must take care in interpreting studies’ findings. Bevacizumab, a VEGF-binding agent, prevents the interaction between VEGF and various receptors, including VEGFR. Work by Miller and colleagues, and thereafter Hurwitz’s group (although this was in colorectal cancer), which explored bevacizumab in combination with various chemotherapeutic regimes, identified improved progression free survival but no overall increase in survival.(15,16) As such, NICE does not recommend bevacizumab,. Ongoing trials such as the Artemis trial in early breast cancer may yield enlightening results. There are other Hallmarks that these examples do not address – inhibitors of caspase activation, telomerase inhibitors and metallomatrix proteinase (MMP) inhibitors are other therapeutic agents under investigation. Bone metastasis An area of personal interest is bone metastasis, one of the major causes of morbidity and mortality among breast cancer patients. A significant cause of morbidity and ultimately mortality, the diagnostic methods for bone metastasis are predominantly image based, with no clinical use of potential biomarkers of bone turnover. Further, therapeutic options remain limited to bisphosphonates and radiotherapy. During my research in this area, I collected plasma samples from breast cancer patients attending oncology outpatient clinics. From the obvious difficulties, such as using a walking stick or a wheelchair, to the ones only understood through conversation, such as the pain, the psychological difficulty of accepting one’s diagnosis and the worry of how much worse it will get, it is clear that we need more answers and solutions. My research was concerned with Endo180, although many other targeted therapies, such as src kinase inhibitors, are also being developed to treat bone metastasis. Endo180 is a transmembrane recycling collagen receptor involved in both collagen remodeling and cell migration and metastasis. By exploring its potential role in bone metastasis in breast cancer, and observing the work of other research groups into different targets, I appreciated the difficulties of finding potential targets, characterizing the target and eventually designing agents to modify its activity. It is also clear that targeting the interactions between the cells and their microenvironment will be vital for future therapeutic agents. (Figure 3) QuickTime™ and a decompressor are needed to see this picture. Figure 3: Cell-cell and cell-microenvironment interactions. Limitations Biologically, the main target therapies, monoclonal antibodies and small molecule inhibitors, have several limitations. The former most be administered intravenously, are expensive, can only reach extracellular targets and are associated with allergic reactions. Small molecule inhibitors are short acting and can have significant off-target toxic effects. We must still identify potential targets, and need to modify existing agents to maximize their effects, such as radioimmunotherapeutic antibodies and immune response-potentiating agents. Other problems include communication. To most members of the general public, the information they receive about targeted therapies is via news reports of patients denied expensive treatments due to the postcode lottery. Although discussing the health economics of pharmaceutical agents would be another essay in itself, it cannot be ignored that these therapies are often expensive and offer two to three extra months of life at best. Drug companies increasingly determine success by ‘disease free survival’ rather than overall survival. A classic example of where this has led to misleadingly positive results is in the case of bevacizumab, where improved disease free survival did not translate into improved survival. The public is being misled by drug companies, interpreted by the media, who are keen for a quick fix to The Cancer Problem. The ethics of cancer drug trials is a further cause for concern. Patients invited to participate in current targeted cancer trials may participate with false hope of a wonder cure, despite the low rate of success of cancer trials compared with other pharmaceutical agents. Future Directions and Challenges The future for targeted therapies could be an exciting one. We may be on the brink of truly personalized cancer therapy. If we can identify all the possible targets, and, more importantly, a means of detecting them clinically, an individual’s cancer could be characterized and a detailed map generated. A personalized therapeutic strategy could then be developed. Such a strategy could even eliminate the cancer completely. But before we get ahead of ourselves, there are still significant challenges ahead. Research is facing the most difficult financial challenges in recent years, particularly with renewed government enthusiasm for teaching over research and charitable funding ever more challenging to obtain. Further, other targeted agents, such as sunitinib, which is used in renal cell carcinoma, recommended by NICE despite a cost per Quality Adjusted Life Years (QALY) of £79 000, are occasionally recommended because they fill a significant treatment gap. Such an argument is unlikely to be won for an expensive breast cancer targeted agent when so many other therapeutic options exist. The high quality of patient care, alongside improved understanding and resolving of the limitations of chemotherapy, radiotherapy and surgery makes it harder for targeted therapies to show a clear benefit over these widely accepted treatment modalities. It is far more appealing to improve the devil you know rather than constantly enter the unknown. The public has high expectations, and it remains a slow process, getting from the laboratory to the patient – perhaps we should review the processes for getting drugs through the system and create a more coordinated research effort. Although some cancer research partnerships exist, it is still a long way off the kind of research consortia that have been developed to solve other clinical problems, for example, the UK Cystic Fibrosis Gene Therapy Consortium. In conclusion, targeted agents have revolutionized how we look at cancer therapeutics and cancer biology. However we have a long way to go in fully understanding the mechanisms involved, and significant financial, communication and biological issues must be overcome before we can not only hit cancer where it hurts, but where it is fatal to its survival. References: 1. Cancer Research UK. CancerStats: Breast Cancer - UK. [Online] Available from: http://publications.cancerresearchuk.org/WebRoot/crukstoredb/CRUK_PDFs/CSBREA0 9.pdf. Accessed 25/6/2011 2. Jemal A, Center MM, DeSantis C, Ward EM. Global patterns of cancer incidence and mortality rates and trends. Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology 2010 Aug;19(8): pp. 1893-1907. 3. Ferlay J, Shin H, Bray F, Forman D, Mathers C, and Parkin D. GLOBOCAN 2008, Cancer Incidence and Mortality Worldwide: IARC CancerBase No. 10 [Internet]. Lyon, France: International Agency for Research on Cancer 2010. Accessed 24/4/2011 4. Sorlie T, Perou CM, Tibshirani R, Aas T, Geisler S, Johnsen H, et al. Gene expression patterns of breast carcinomas distinguish tumor subclasses with clinical implications. Proceedings of the National Academy of Sciences of the United States of America 2001 Sep 11;98(19): pp. 10869-10874. 5. Hanahan D, Weinberg RA. The hallmarks of cancer.Cell. 2000 Jan 7;100(1):57-70. 6. Iorio MV, Ferracin M, Liu CG, Veronese A, Spizzo R, Sabbioni S et al. MicroRNA gene expression deregulation in human breast cancer. Cancer Res. 2005 Aug 15;65(16):7065-70. 7. Coleman RE, Rubens RD. The clinical course of bone metastases from breast cancer. British journal of cancer 1987 Jan;55(1): pp. 61-66 8. National Cancer Institute, Targeted Therapies Fact Sheet, updated 28/6/11,[Online] Available from: http://www.cancer.gov/cancertopics/factsheet/Therapy/targeted. Accessed 25/7/11 9. Slamon DJ, Leyland-Jones B, Shak S, Fuchs H, Paton V et al. Use of chemotherapy plus a monoclonal antibody against HER2 for metastatic breast cancer that overexpresses HER2. N Engl J Med. 2001 Mar 15;344(11):783-92. 10. Piccart-Gebhart MJ, Procter M, Leyland-Jones B, Goldhirsch A, et al. Trastuzumab after adjuvant chemotherapy in HER2-positive breast cancer. N Engl J Med. 2005 Oct 20;353(16):1659-72. 11. Geyer CE, Forster J, Lindquist D, Chan S, Romieu CG, Pienkowski T, JagielloGruszfeld A et al. Lapatinib plus capecitabine for HER2-positive advanced breast cancer.N Engl J Med. 2006 Dec 28;355(26):2733-43. 12. Woodhouse EC, Chuaqui RF, Liotta LA. General mechanisms of metastasis. Cancer 1997 Oct 15;80(8 Suppl): pp. 1529-1537. 13. Fong PC, Boss DS, Yap TA, Tutt A, Wu P, Mergui-Roelvink M, Mortimer P et al. Inhibition of poly(ADP-ribose) polymerase in tumors from BRCA mutation carriers. N Engl J Med. 2009 Jul 9;361(2):123-34. Epub 2009 Jun 24. 14. Tutt A, Robson M, Garber JE, Domchek SM, Audeh MW, Weitzel JN, Friedlander M, et al. Oral poly(ADP-ribose) polymerase inhibitor olaparib in patients with BRCA1 or BRCA2 mutations and advanced breast cancer: a proof-of-concept trial. Lancet. 2010 Jul 24;376(9737):235-44. Epub 2010 Jul 6. 15. Miller KD. E2100: a phase III trial of paclitaxel versus paclitaxel/bevacizumab for metastatic breast cancer. Clin Breast Cancer. 2003 Feb;3(6):421-2. 16. Hurwitz H, Fehrenbacher L, Novotny W, Cartwright T, Hainsworth J, Heim W et al. Bevacizumab plus irinotecan, fluorouracil, and leucovorin for metastatic colorectal cancer. N Engl J Med. 2004 Jun 3;350(23):2335-42. Figures: Figure 1: Breast Cancer incidence in the UK - Office for National Statistics. Breast Cancer: Incidence rates rise, mortality rates fall. ; 2010. Accessed 29/7/11 Figure 2: Hallmarks of Cancer - Hanahan D, Weinberg RA. The hallmarks of cancer. Cell. 2000 Jan 7;100(1):57-70. Figure 3: Cell-cell and cell-microenvironment interactions - Hanahan D, Weinberg RA., Hallmarks of cancer: the next generation. Cell. 2011 Mar 4;144(5):646-74.