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MEDICAL BIOLOGY: ON TARGETED CANCER THERAPY

The following points are made by Charles Sawyers (Nature 2004 432:294):

1) Oncogene and tumor suppressor gene networks influence the decisions of cancer cells to proliferate or die. These decisions are further influenced by the tumor microenvironment and stress signals, such as DNA damage. Moreover, recent work suggests that a subpopulation of cancer cells with stem-cell-like properties may be critical for triggering tumor development. Together, recent studies have provided a conceptual framework within which practitioners of experimental cancer therapeutics can consider the design of targeted agents.

2) The term "targeted therapy" refers to a new generation of cancer drugs designed to interfere with a specific molecular target (typically a protein) that is believed to have a critical role in tumor growth or progression. The identification of appropriate targets is based on a detailed understanding of the molecular changes underlying cancer. This approach contrasts with the conventional, more empirical approach used to develop cytotoxic chemotherapeutics -- the mainstay of cancer drug development in past decades.

3) The clinical success of the small molecule kinase inhibitor imatinib mesylate (Gleevec) in chronic myeloid leukaemia (CML) and gastrointestinal stromal tumors (GIST) has established a paradigm for the treatment of tumors whose growth is acutely dependent on specific kinase targets. CML is driven by the mutant kinase fusion protein Bcr-Abl, which displays constitutive activation of the Abl kinase, whereas GIST is caused by activating point mutations in the c-Kit or platelet derived growth factor receptor (PDGFR)-alpha kinases. Imatinib effectively blocks the activity of all three kinases and produces dramatic clinical responses in all three situations in a manner that correlates precisely with the presence of these mutations in the tumor[1].

4) In lung cancer, clinical responses to epidermal growth factor receptor (EGFR) inhibitors are associated with point mutations in the EGFR kinase domain[2,3] (thereby explaining the rather modest 10% response rate in all patients). The clear prediction from this experience is that clinical responses to kinase inhibitors occur in tumors bearing activating mutations that drive tumor progression. Extending this paradigm to larger numbers of cancer patients would require establishing the frequency of kinase mutations in human cancer on a much broader scale -- presumably through global gene sequencing efforts analogous to the genome project. Indeed, initial efforts from groups at the Sanger Centre, the Eli Broad Institute, and Johns Hopkins have found previously unsuspected kinase mutations in human tumors[4,5].

References (abridged):

1. Sawyers, C. L. Opportunities and challenges in the development of kinase inhibitor therapy for cancer. Genes Dev. 17, 2998-3010 (2003)

2. Paez, J. G. et al. EGFR mutations in lung cancer: correlation with clinical response to gefitinib therapy. Science 304, 1497-1500 (2004)

3. Sordella, R., Bell, D. W., Haber, D. A. & Settleman, J. Gefitinib-sensitizing EGFR mutations in lung cancer activate anti-apoptotic pathways. Science 305, 1163-1167 (2004)

4. Stephens, P. et al. Lung cancer: intragenic ERBB2 kinase mutations in tumors. Nature 431, 525-526 (2004)

5. Davies, H. et al. Mutations of the BRAF gene in human cancer. Nature 417, 949-954 (2002)

Nature http://www.nature.com/nature

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P53 PROTEIN AS A TARGET IN CANCER THERAPY

The following points are made by B. Vogelstein and K.W. Kinzler (Nature 2001 412:865):

1) In 1989, it was discovered that the p53 protein is subtly mutated and thereby inactivated in almost all types of human cancer. These findings raised the possibility that drugs that restore the function of p53 would be broadly applicable cancer treatments. That hope has not yet been realized, but there have been some imaginative approaches to the problem.

2) For 10 years after its discovery in 1979, the p53 gene was thought to be an oncogene. Under normal circumstances, the protein products of oncogenes stimulate appropriate cell division or interfere with cell death. However, certain mutations in these genes result in their products being switched on all the time, or result in gaining a new function. This generally leads to inappropriate stimulation of cell growth, resulting in tumors.

3) Many commonly used drugs inhibit the ability of their target proteins to bind to other molecules or to catalyze reactions, and so mutated oncoproteins are, in theory, perfect drug targets. Theory has been put into practice with the development of STI-571 (Gleevec), a drug that inhibits the aberrantly activated enzyme found in chronic myeloid leukemia cells. But most genetic alterations in common human cancers affect tumor-suppressor genes, rather than oncogenes. The normal function of a tumor suppressor gene is to keep cell numbers down by stopping cells from multiplying or by promoting cell death. When mutated, tumor-suppressor proteins are switched off, and again the result is inappropriate net cell growth.

4) In some cancers, an entire tumor-suppressor gene may be deleted. In general, tumor suppressors are poor targets for conventional drugs, since one cannot inhibit an activity that is not present. Unfortunately, the same work that identified p53 as a potential target for anti-cancer drugs also demonstrated that it behaves genetically like a tumor-suppressor gene and not like an oncogene.

Nature http://www.nature.com/nature

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MEDICAL BIOLOGY: ON CHEMOKINES AND BREAST CANCER METASTASIS

Notes by ScienceWeek:

Although the term "metastasis" is usually used in connection with the spread of cancer, the term actually refers to the spread of any disease from an original location to other tissues. Concerning cancer, what is important is that malignant cells have the ability to invade healthy body tissues with very little of the restrictions that normally constrain non-malignant cells.

Following a nearby invasion, a number of malignant cells may detach from the initial ("primary") tumor and invade a body cavity (e.g., abdominal or thoracic) or enter the blood or lymph system. This latter condition can lead to widespread metastasis, with those malignant cells that survive in the blood or lymph invading adjacent body tissues and establishing secondary tumors.

In the final stage of metastasis, the secondary tumors become vascularized, with new networks of blood vessels providing nutrients for the further growth of these secondary tumors. (Any macroscopic new tissue, whether resulting from repairing a wound, normal growth, or tumors, requires a blood supply, with blood vessel growth in tumors initiated by chemical triggers called "tumor angiogenesis factors".)

In all stages of metastasis, malignant cells must resist the antitumor defenses of the body in order to survive.

A common popular misconception is that migration of tumor cells is a late development in the history of a tumor. On the contrary, almost from inception, a tumor may shed cells into the circulation. From animal models, it is estimated that a 1-centimeter tumor sheds more than 1 million cells a day into the venous circulation. In animals, circulating tumor cells usually die as a result of intravascular trauma: the longer a tumor cell spends in the circulation, the greater the chance of its death. The probability that a circulating tumor cell will become a metastatic tumor is estimated to be less than 10^(-6).

Experiments suggest that metastasis is not a random event, and that the primary tumor may regulate the growth of metastatic tumors, and removal of the primary tumor often results in accelerated growth of the metastases.

The term "cytokine" refers to any substance that promotes cell growth and cell division. As a promoter of cell growth and division, a cytokine acts as a messenger to cells, and the transmission of the message requires a binding of the cytokine molecule to a cytokine-specific receptor on the cell surface. "Chemokines" are a type of cytokine, the chemokines in general comprising several groups of polypeptides with molecular weights in the range 8 to 10 kilodaltons. The chemokines are chemokinetic and chemotactic, normally stimulating white blood cell (leukocyte) attraction and movement.

The G-proteins are a family of signal-coupling proteins that act as intermediaries between activated cell receptors and effectors, for example, the transduction of hormonal signals from the cell surface to the cell interior. The G-protein is apparently embedded in the cell membrane with parts exposed on the outside surface and inside surface. The outside moiety is activated by the first messenger, and the inside moiety activates the second messenger, the G-protein thus acting as a trans-membrane signal transducer.

Endothelial cells are flat cells forming a layer lining blood vessels, lymphatic vessels, the heart, etc., and when migrating tumor cells circulate in blood or lymph, they must adhere to endothelial cells before they can pass through the walls of blood or lymph vessels and into the adjacent tissues. The interaction of migrating tumor cells and endothelial cells is thus of considerable importance in metastasis. So-called "adhesion molecules" are molecules expressed on the surface of a cell that mediate the adhesion of the cell to other cells or to the extracellular matrix. Adhesion molecules bind to receptors that are classed collectively as "integrins".

In general, "hematopoietic cells" (hemopoietic cells) are any cells involved in the formation of blood cells.

In this context, a "growth factor" is any specific substance that must be present in a culture medium for multiplication of the cultured cells to occur. Certain growth factors have been identified as cytokine proteins (peptide hormones) that stimulate the growth and division of target cells by binding to cell membrane receptors. "Transforming growth factors" are cytokine growth factors that produce in target cells some of the growth characteristics of cancer cells.

The term "actin" refers to a family of ubiquitous structural proteins present in all cells with a nucleus (eukaryote cells), and the term "cytoskeleton" refers to the quasi-rigid matrix that among other things determines cell shape. Both actin and the cytoskeleton are of considerable importance in the production and movements of pseudopods, the protoplasmic extensions involved in the amoeba-like movements of migrating cells.

The following points are made by A. Mueller et al (Nature 2001 410:50):

1) Metastasis is the result of several sequential steps and represents a highly organized, non-random, and organ-selective process. Although various molecules have been implicated in the metastasis of breast cancer, the precise mechanisms determining the directional migration and invasion of tumor cells into specific organs remains to be established.

2) Through their interaction with G-protein coupled receptors, the secreted proteins called chemokines induce cytoskeletal rearrangement, firm adhesion to endothelial cells, and directional migration. Chemokines act in a coordinated fashion with cell surface proteins, including integrins, to direct the specific homing of various subsets of hematopoietic cells to specific anatomical sites. Breast cancer is characterized by a distinct metastatic pattern involving the regional lymph nodes, bone marrow, lung, and liver, with tumor cell migration and metastasis sharing many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors.

3) The chemokine receptors CXCR4 and CCR7 are highly expressed in human breast cancer cells, malignant breast tumors, and metastases, and that the known ligands of these receptors exhibit peak levels of expression in organs representing the first destination of breast cancer metastasis. In breast cancer cells, signaling via CXCR4 and CCR7 mediates actin polymerization and pseudopodia formation, and subsequently induces chemotactic and invasive responses. In vivo, neutralizing the interactions of the ligands of these chemokine receptors significantly impairs metastasis of breast cancer cells to regional lymph nodes and lung. Malignant melanoma, which has a similar metastatic pattern as breast cancer but also a high incidence of skin metastasis, shows high expression levels of the chemokine receptor CCR10 in addition to CXCR4 and CCR7. The authors suggest their findings indicate that chemokines and their receptors have a critical role in determining the metastatic destination of tumor cells.

4) The authors conclude: "Our findings are probably not unique to breast cancer. Other tumor entities of hematopoietic and non-hematopoietic origin... express functionally active chemokine receptors that mediate tumor cell migration in vitro. Our results in breast cancer and malignant melanoma suggest that malignant cells, in general, express distinct and non-random patterns of chemokine receptors... Currently, intense efforts are underway to identify small-molecule antagonists for many chemokine receptors. We propose that small molecule antagonists of chemokine receptors... may be useful to interfere with tumor progression and metastasis in tumor patients."

In a commentary on the above report, Lance A. Liotta (Nature 2001 410:24) makes the following points:

1) It was already recognized in the 19th century that secondary tumors are seeded by cells released from the original tumor, the seeding cells ferried around the body in the lymphatic and blood circulations. Autopsies have revealed that some organs have large numbers of secondary tumors, while other organs have relatively few. The basis for this bias has been a puzzle, because the pattern cannot be explained simply by differences in blood and lymph flow in different organs.

2) Three major theories have been proposed to explain the bias of metastases towards certain organs, but animals studies have failed to reveal which theory is correct. The three theories are as follows:

a) Tumor cells leave the blood and lymphatic systems to the same extent at all organs, but multiply only in those organs that have the appropriate growth factors.

b) The endothelial cells that line blood vessels in target organs express adhesion molecules that cause circulating tumor cells to stop circulating and invade those organs.

c) The chemokine theory holds that organ-specific attractant molecules enter the circulation, stimulating the migrating tumor cells to invade the walls of blood vessels and enter the organs.

3) The author (Liotta) points out that there is evidence to support all three theories, but the mediator molecules -- whether they be growth factors, adhesion molecules, or chemoattractants -- have remained unknown. "Now, however, Mueller et al have identified chemoattractants in target organs and chemoattractant receptors on tumor cells, providing molecular support for the chemoattraction theory. Moreover, [they] have shown that they can block metastasis in animals by blocking the receptors."

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