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Targeted therapy is still trial-and-error treatment


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The latest thing in cancer treatment are so-called "targeted" therapies, the most prominent of which now are tyrosine kinase inhibitors (e.g. gefitinib, erlotinib, sunitinib, sorafenib). These drugs are very expensive and work most often modestly in a minority of cases.

We are now beginning to be able to use molecular profiling to provide personalized treatment that offers hope of improved survival and less toxicity from therapy. And the ability to provide this molecular diagnostic test on a worldwide basis with rapid turnaround will be critical for further clinical research and application in the clinic.

What were the data supporting the use of this testing?

Prospective, randomized trials showing improved treatment outcomes in patients so tested? Nope.

Prospective trials, showing survival advantages in patients with "positive" test results? Nope.

Prospective trials, showing response advantages in patients with "positive" test results? Nope.

Retrospective trials, showing both response and survival advantages in patients with "positive" test results, in thousands of patients, from multiple laboratories? Nope.

Two entirely retrospective studies, from two Harvard-affiliated hospitals, showing response, but not survival, advantages with a grand total of 26 assay/treatment correlations? Yes.

A subsequent study from another laboratory did not show correlations between gene mutations and patient survival.

So, the clinical application of DNA content assays have been shown to correlate only with response and not survival, and only in a handful of patients, and only in entirely retrospective studies.

What is going on here?

From a scientific perspective, the principal reason why the war on cancer has largely failed is due to an almost obsessive and myopic focus on targeting cancer cells and tumors at the expense of addressing the underlying factors that cause cancer in the first place.

Although the theory behind targeted therapy is appealing, the reality is more complex. For example, cancer cells often have many mutations in many different pathways, so even if one route is shut down by a targeted treatment, the cancer cell may be able to use other routes.

In other words, cancer cells have 'backup systems' that allow them to survive. The result is that the drug does not shrink the tumor as expected. One approach to this problem is to functionally target multiple pathways in a cancer cell.

Another challenge is to identify which of the targeted treatments will be effective (enzyme inhibitors, proteasome inhibitors, angiogenesis inhibitors, and monoclonal antibodies).

Targeted therapy is still trial-and-error treatment.

Medical research has focused a great deal on developing DNA (genomic) tests to identify gene expressions, amplifications and mutations relevant to cancer. The hope is that genetic information will enable researchers to better predict how you will respond to various treatment options.

However, when it comes to predicting the best treatment, unlocking the complexities of your DNA is simply not the answer. In fact, a March 2010 study in the Journal of the National Cancer Institute looked at the value of a number of gene tests and concluded none of the studies showed “clear usefulness.”


While genomic analysis can provide a veneer of information, unraveling the complexity of human tumor biology is beyond the scope of these analyses. Gene tests cannot capture the myriad of factors that ultimately determine how tumor cells will behave inside the body. Simply put, the human body is much more complex than the sum of its genes.

For example: a flower seed may have the genetic instructions to become a rose. But, its genes will not necessarily determine its size, number of blooms, etc. These features are heavily influenced by non-genetic and environmental factors, such as the soil, nutrients, water, sun exposure, pathogens and the climate in which the seed is nurtured.

No good gardener would attempt to tell you how your future bouquet will look by simply examining a packet of flower seeds. Similarly, no good doctor should attempt to choose drugs based solely on genomic analyses. Most physicians realize that genotype does not equal phenotype.

By testing your tumor in its native state, “functional profiling” takes not just your genomic make-up into consideration, but your cells’ entire biology. Treatment based on genetic testing is still a guessing game. Selecting drugs and combinations through the functional profiling of a tumor sample can predict response to treatment.

The functional profiling platform can explore multiple signaling pathways from the same test. It doesn't have to test for each and every signaling pathways there are.

There are many pathways to altered cellular function. Testing for these pathways, those which identify DNA, or RNA sequences or expression of individual genes or proteins often examine only one component of a much larger, interactive process. In testing for all "known" mutations, if you miss just one, it may be the one that gets through.

And it's not just only targeted drugs that may be effective as first-line treatment on your individual cancer cells. Cancers share pathways across tumor types. There really is no lung cancer chemos, or breast cancer chemos, or ovarian cancer chemos.

There are chemos that are sensitive (effective) or there are chemos that are resistant (ineffective) to each and every "individual" cancer patient, not populations. There are chemos that share across tumor types.

The functional profiling platform has the unique capacity to identify all of the operative mechanisms of response and resistance by gauging the result of drug exposure at its most important level: cell death.

Finding what targeted therapies would work for what cancers is very difficult. A lot of trial-and-error goes along trying to find out. However, finding the right targeted therapies for the right "individual" cancer cells can be improved by cell-based assays, using functional profiling.

Identifying DNA expression of individual proteins (that measure of RNA content, like Her2, EGFR, KRAS or ALK) often examine only one component of a much larger, interactive process. Gene (molecular) profiling measures the expression only in the "resting" state, prior to drug exposure. There is no single gene whose expression accurately predicts clinical outcome. Efforts to administer targeted therapies in randomly selected patients often will result in low response rates at significant toxicity and cost.

Functional profiling measures proteins before and after drug exposure. It measures what happens at the end (the effects on the forest), rather than the status of the individual trees. Molecular profiling is far too limited in scope to encompass the vagaries and complexities of human cancer biology when it comes to drug selection. The endpoints of molecular profiling are gene expression. The endpoints of functional profiling are expression of cell death (both tumor cell death and tumor associated endothelial [capillary] cell death).

In testing for all "known" mutations, if you miss just one, it may be the one that gets through. And it's not just only targeted drugs that may be effective as first-line treatment on your "individual" cancer cells. Cancers share pathways across tumor types.

Targeted treatments take advantage of the biologic differences between cancer cells and healthy cells by "targeting" faulty genes or proteins that contribute to the growth and development of cancer. Many times these drugs are combined with chemotherapy, biologic therapy (immunotherapy), or other targeted treatments.

Clinicians have learned that the same enzymes and pathways are involved in many types of cancer. However, understanding targeted treatments begins with understanding the cancer "cell." In order for cells to grow, divide, or die, they send and receive chemical messages. These messages are transmitted along specific pathways that involve various genes and proteins in the cell.

Cancer cells often have many mutations in many different pathways, so even if one route is shut down by a targeted treatment, the cancer cell may be able to use other routes.

Targeted therapies are typically not very effective when used singularly or even in combination with conventional chemotherapies. The targets of many of these drugs are so narrow that cancer cells are likely to eventually find ways to bypass them.

Physicians may have to combine several targeted treatments to try an achieve cures or durable responses for more complicated tumors like those that occur in the breast, colon and lung.

These targeted therapies produce limited results because they can help a relatively small subgroup of cancer patients. But when they work, they produce very good responses. With targeted therapy, the trick is figuring out which patients will respond. Tests to pinpoint those patients cannot be accomplished with genetic testing.

All the gene amplification studies, via genetic testing, tell us is whether or not the cancer cells are potentially susceptible to a mechanism/pathway of attack. They don't tell you if one drug is better or worse than another drug which may target a certain mechanism/pathway. Cell-based functional analysis can accomplish this.

The cell is a system, an integrated, interacting network of genes, proteins and other cellular constituents that produce functions. You need to analyze the systems' response to drug treatments, not just one target or pathway, or even a few targets/pathways.

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According to Harold DeMonaco of Massachusetts General Hospital, the promise of personalized medicine has been slow to come to fruition. The enthusiasm displayed a decade ago has been tempered by the difficulties in matching genetic mutations to specific diseases in individual patients.

Much of the difficulty has been in the inefficient methods in identifying candidate mutations. The search for or identification of specific mutations that are associated with a disease is both time consuming and expensive.

Traditional treatments for non-small cell lung cancer (NSCLC) have not been particularly effective with response to standard chemotherapy in the 20-30% range with a time to recurrence of 3-5 months. In addition to not being particularly effective, traditional chemotherapy carries with it a host of side effects.

The paradigm shifted with the development of targeted treatments that take advantage of the genetic differences seen between normal and cancer cell.

The problem was identifying which patients had the particular genetic mutations that were associated with their disease. Although the knowledge in this area moves very quickly, the identification process has been tedious and expensive. If more than one mutation was involved, multiple testing was required.

None of the targeted drugs are cures, they are life savers by being a life lengthener. Targeted therapy is what is used to stabilize the disease rather than cure it. Will patients have to take the drugs indefinitely?

Most targeted drugs do not directly kill cancer cells like nonspecific cytotoxic agents do (conventional chemotherapy), these drugs stop the growth of tumors. Therefore, patients must receive treatment for prolonged periods, and over time, they often develop secondary resistance. They eventually succumb to treatment resistance or disease recurrence.

To beat down your individual cancer mortality, an oncologist needs to target all the many cancers that make up your individual cancer, the dozens of different pathways that cells use to proliferate and spread. That is the leading edge of research and treatment, determining how an individual's tumor cells work and hitting those pathways with multiple drugs, simultaneously or sequentially, each chosen because it targets one of those growth, replication and angiogenesis pathways.

Trying to mate a notoriously heterogeneous disease into one-size-fits-all treatments is disingenuous to all who are inflicted with it. And the criticism remains: All of the clinical trials resources have gone toward driving a square peg (one-size-fits-all chemotherapy) into a round hole (notoriously heterogeneous disease).

More emphasis should be put on matching treatment to the patient, having more respect for minimal partial response or stable disease, when it can be achieved through use of the least toxic and mutagenic drug regimens, and reserve the use of higher dose therapy or aggressive combination chemotherapy to those patients with tumor biologies most amenable to attack and destroy by these treatments.

The hope is to match tumor type to drug. We need to make the next leap, getting the right drug to the right patient, during first-line treatment. That is why I am such an advocate of "individualized" treatment by using "individualized" pre-testing with valid biomarkers. I wish I could give a more positive outlook on cancer medicine, but targeted therapy is still "trial-and-error" treatment, what we had for the last thirty years.

I like to use the Coke vs Pepsi analogy. The Coke vs Pepsi scenario came by a French oncologist's comments during a presentation by Dr. Dave Alberts at an ASCO meeting some 15 years ago. The problem is not with using the prospective, randomized trial as a research instrument, the problem comes from applying this time and resource-consuming instrument to address hypotheses of trivial importance (do cancers prefer Coke or Pepsi?).

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In a periodical published by the American Society of Clinical Oncology (ASCO) in their September 1, 2011 issue of the ASCO Post, Margo J. Fromer, who participated in a conference sponsored by the Institute of Medicine, wrote about the obstacles that confront researchers in their efforts to develop effective combinations of targeted cancer agents.

One of the participants, Jane Perlmutter, PhD, of the Gemini Group, pointed out that advances in genomics have provided sophisticated target therapies, but noted, "cellular pathways contain redundancies that can be activated in response to inhibition of one or another pathway, thus promoting emergence of resistant cells and clinical relapse."

James Doroshow, MD, deputy director for clinical and translational research at the NCI, said, "the mechanism of actions for a growing number of targeted agents that are available for trials, are not completely understood."

He went on to say that the "lack of the right assays or imaging tools means inability to assess the target effect of many agents." He added that "we need to investigate the molecular effects . . . in surrogate tissues," and concluded "this is a huge undertaking."

Michael T. Barrett, PhD, of TGen, pointed out that "each patient's cancer could require it's own specific therapy." This was followed by Kurt Bachman of GlaxoSmithKline, who opined, "the challenge is to identify the tumor types most likely to respond, to find biomarkers that predict response, and to define the relationship of the predictors to biology of the inhibitors."

The complexities and redundancies of human tumor biology had finally dawned on these investigators. What they were describing was precisely the work that clinical oncologists involved with cell culture assays have been doing for the past two decades.

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By Robert Nagourney, M.D., PhD.

Rational Therapeutics, Inc.

The term “targeted therapy” has entered common parlance. Like personalized medicine, targeted therapy is a generic description of drugs and combinations that inhibit specific cancer-related pathways. I am impressed by how quickly esoteric phenomena like the downstream signal in the insulin factor pathway have entered the lexicon of medical oncologists. With the advent of temsirolimus and everolimus, both rapamycin derivatives that target mTOR, we now have at our disposal agents that are every bit a part of the therapy repertoire. Unlike erlotinib that targets a specific tyrosine kinase, mTOR is a complex and multifaceted target.

There are actually two separate forms of mTOR, TORC1 and TORC2, and they sit at a critical point in cellular determination. Stimulated by the insulin growth pathway, cells must decide whether they will grow in size or divide. The mTOR proteins participate in this process by regulating protein synthesis and glucose uptake among other functions. In turn, the mTOR pathway is regulated by numerous other factors like AMP kinase and AKT. The current crop of mTOR inhibitors all target TORC1.

New classes of compounds are being developed that inhibit both TORC1 and TORC2. More interesting are the compounds that influence upstream signaling, including phosphoinositol kinase (PI3K) and AKT. What we are coming to learn, however, is that these are not targets but collections of targets. Indeed, the PI3K inhibitors themselves have influence on one, two or all of the distinct classes of phosphoinositol kinases.

Most of the studies to date have used compounds that affect all the classes equally (pan-inhibitors). Pharmaceutical companies are now developing highly selective inhibitors of this fundamental pathway. In addition, duel inhibitors that target both PI3K and mTOR are in clinical trials. What we are coming to realize is the complexity of these pathways. What may prove more vexing still is their redundancy. One well-established by-product of successful inhibition of mTOR (principally TORC1) is the upstream activity of AKT via a feedback loop. This has the undesirable affect of redoubling mTOR stimulation through the very pharmacological manipulation that was designed to inhibit it. Again, an unintended consequence of a well laid plan.

To unravel the complexities and redundancies of these processes, we have utilized the primary culture platform. It enables us to examine the end result of signal inhibition and dissect disease specific profiles. Using this approach we can partner with collaborators to define the specific operative pathways in each disease entity.

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Clinical oncologists involved with "real time" functional profiling have what is called a "report out" (detailed account) of their analysis. And they have photomicrographs from those report outs. Based on FDA-apporved indications for a drug, they can mix and match 'targeted' drugs, as well as 'conventional' and 'targeted' drugs. What actually works against the cancer cells of the 'individual' patient (not populations). Even though a particular drug may be approved for a particular indication, 60% of all cancer drugs are used off-label. Manufacturers cannot promote their drug(s) for other than FDA-approved indications, but physicians can prescribe a drug for just about any indications they want.

Photomicrographs of the assay can show that some clones of tumor cells don't accumulate the drug. These cells won't get killed by it. The assay measures the net effect of everything which goes on (whole cell profiling methodology). Are the cells ultimately killed, or aren't they. In other words, photomicrographs of actual tumor cells sometime show that the exact same identical individual culture well, shows some clusters have taken up vast amounts of a drug, while right next door, clusters of the same size, same appearance, same everything haven't taken up any of the drug.

So it doesn't matter if there is a "target" molecule (protein or receptor) in the cell that the targeted drug is going after, if the drug either won't "get in" in the first place or if it gets pumped out/extruded or if it gets immediately metabolized inside the cell, drug resistance is multifactorial. The advantage of the functional profiling technique is that it can show this in the "population" of cells.

The functional profiling technique makes the statistically significant association between prospectively reported test results and patient survival. It can correlate test results which are obtained in the lab and reported to physicians prior to patient treatment, with significantly longer or shorter overall patient survival depending upon whether the drug was found to be effective or ineffective at killing the patient's tumor cells in the laboratory.

This could help solve the problem of knowing which patients can tolerate costly, new treatments and their harmful side-effects. These "smart" drugs are a really exciting element of cancer medicine, but do not work for everyone, and a test to determine the efficacy of these drugs in a patient could be the first crucial step in personalizing treatment to the individual. This kind of technique exists and is very valuable, especially when active chemoagents are limited in a particular disease, giving more credence to testing the tumor first.

But how does one get ASCO and others to understand this and allow its judicious use? They have single-handedly done more over the past 20 years to keep assay-testing (pre-testing) technology under a bushel basket and out of the public light. It has hurt literally hundreds of thousands of patients. We'd be much further along and technology would have improved, even more accurate. New treatments would have been discovered and targeted immediately to the people who could most benefit from them. This has been one great lost of opportunity in clinical cancer research.

Bibliography relevant to the functional profiling (microvascular viability) assay

1. Weisenthal, L. M. Patel,N., Rueff-Weisenthal, C. (2008). "Cell culture detection of microvascular cell death in clinical specimens of human neoplasms and peripheral blood." J Intern Med 264(3): 275-287.

2. Weisenthal, L., Lee,DJ, and Patel,N. (2008). Antivascular activity of lapatinib and bevacizumab in primary microcluster cultures of breast cancer and other human neoplasms. ASCO 2008 Breast Cancer Symposium. Washington, D.C.: Abstract # 166.

3. Weisenthal, L. M. (2010). Antitumor and anti-microvascular effects of sorafenib in fresh human tumor culture in comparison with other putative tyrosine kinase inhibitors. J Clin Oncol 28, 2010 (suppl; abstr e13617)

4. Weisenthal, L., H. Liu, Rueff-Weisenthal, C. (2010). "Death of human tumor endothelial cells in vitro through a probable calcium-associated mechanism induced by bevacizumab and detected via a novel method." Nature Precedings 28 May 2010.

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A conservative estimate of the number of targeted therapies tested in patients with cancer in the past decade is 700, yet no patients with solid tumors have been cured by targeted therapies over that time period, said Antonio Tito Fojo, PhD, head of the Experimental Therapeutics Section and senior investigator for Medical Oncology Branch Affiliates at the Center for Cancer Research at the National Cancer Institute in Bethesda, Maryland.

He was among speakers who looked at value in cancer care at a symposium on "Fighting a Smarter War Against Cancer." It was convened by the Ruesch Center for the Cure of Gastrointestinal Cancers at Georgetown Lombardi Comprehensive Cancer Center in Washington, DC.

"Zero [is] the number of targeted therapies that have prolonged survival by one year, when compared to a conventional treatment." Dr. Fojo noted in a talk that looked at the disconnect between the costs of cancer drugs and the magnitude of benefit they deliver.

In another example, Dr. Fojo noted that bevacizumab costs $90,816.00 per year to treat the average patient for breast cancer, yet it does not extend overall survival rate. Worse, he said, the chance that you'll experience a grade 3/4 toxicity if you give bevacizumab on top of paclitaxel more than doubles.

"This is not the innocuous drug that you were led to believe," he said. To tell a woman that you're going to have a 2-and-a-half-fold increase in toxicity and no benefit in terms of overall survival is unacceptable, he added.

The only "benefit," he said, was a prolonged progression-free survival of questionable value.

The disconnect between money spent and results is significant, said Thomas Smith, MD, FACP. He is director of palliative medicine at Johns Hopkins Medical Institutions and professor of oncology at the Sidney Kimmel Comprehensive Center in Bethesda, Maryland.

"The United States spends twice as much as any other country on cancer and medical care in general yet achieves the same survival, except for breast cancer and lymphoma, where you eke out maybe 1% to 2% better survival," he said.

With cancer care costs rising exponentially, Dr. Smith said it was time to change the efficacy and effectiveness curve by identifying therapies that deliver solid value.

Some therapies, such as imatinib for chronic myelogenous leukemia, "to use a baseball analogy, are truly home runs," said Dr. Smith. They are drugs that work so well, the question is: How are we going to pay for it? "Others are doubles or singles, where you get as [Dr. Fojo] said, 1.2 months, 2 months, 3 months — at the cost of substantial toxicity. "I would call those singles. Others are like sacrificed flies where you get 2 weeks."

Dr. Smith shared some of the ways to "bend the cost curve in cancer care."

"Target surveillance procedures to those most likely to benefit," he asserted. After a doctor has treated a patient for breast cancer, hoping to cure her, the only thing that helps the patient live the best life she can with the least chance of dying of recurrence is mammography — not blood tests or scans. Yet, in 1990, the United States wasted more than $1 billion doing tests on people who had been treated for breast cancer "with nothing to show for it," he said. The amount spent in recent years is probably even higher.

"Don't pay for these routine tests," he suggested. "Pay me to have a good survivorship care plan visit," where I can emphasize that you need to get your mammogram, and that I want you to eat lots of fruits and vegetables and avoid fat and meat, he said.

Another way to cut costs would be to limit chemotherapy to patients with good performance status. If they are fairly functional, they may benefit from chemotherapy.

If the person spends more than half the time in bed or a chair, you're just going to make that person sicker, rather than help them, Dr. Smith said. The risk for infection and toxicity are greater, yet the chance of benefit is near zero.

But it is not easy to call off chemotherapy. He told the story of a 67-year-old man who was suffering with severe abdominal pain from colorectal cancer. He had lost 20 pounds over the previous month and showed up in a wheelchair pushed by his wife. Dr. Smith called the oncologist and asked whether he had considered not treating the man with chemotherapy. The response he received was, "You want me to give up on him?"

Other pressures also may come into play. When Dr. Smith talked to a hospital administrator, he learned that if that doctor did not have so many of his patients hospitalized, the hospital would have one-third fewer hospitalizations and the institution would be way in the red. "It's a complicated web," said Dr. Smith.

One proven solution that reduces the chance chemotherapy will be pushed on patients who can't benefit from it is to document performance status, monitor oncologists' practices, and give feedback, Dr. Smith said.

At a University of Michigan faculty practice, too many people were getting chemotherapy within 2 weeks of their death. After giving feedback to oncologists, the number dropped from 50% to 20% in one quarter. "That's the beauty of feedback," he said.

After the day's many talks, including those by doctors, patients, insurers, big pharmaceutical companies, and government agencies, John Marshall, MD, director of the Ruesch Center for the Cure of Gastrointestinal Cancers, told Medscape Medical News that he felt the speakers had come closer to defining value in healthcare. He is also chief of hematology/oncology and associate director of Georgetown Lombardi Comprehensive Cancer Center.

"I actually felt a lot of clarity," he said. "The question is how to frame that in a way to have both government and industry, but most importantly, patient acceptance," he said, which would help barriers fall.

He noted that Dr. Fojo's and Dr. Smith's talks were well received by attendees. "They were just telling the truth," he said. "They recognize that to get to personalized medicine and get past the logjam we're in, we actually do need new regulations," he said. "We need an act in Congress…You have to start with why we need to change things. And they made a very clear demonstration of why we do."

Dr. Marshall said he dreams of a new law that would require drugs to be approved on the basis of value to the patient.

The Ruesch Center for the Cure of Gastrointestinal Cancers. Fighting a Smarter War Against Cancer: Linking Policy to the Patient. Presented December 2, 2011.

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Scientists are beginning to discover myriad strategies tumors use to avoid attacks by anti-cancer drugs.

By Tia Ghose | November 30, 2011 TheScientist

When cancer cells are first discovered, many drugs can blast them into oblivion. But over time, cancers begin to withstand those first line drugs and continue to grow and spread.

“If you already have 10 to the 10th tumor cells, the chances are you’re going to have some kind of resistance develop,” said William Pao, a physician scientist at Vanderbilt University, who first uncovered mechanisms of drug resistance in lung cancer. “Even if you kill 99.9 percent of cells you’re still left with a ton of cells which then can start to grow.”

A long-standing hurdle in cancer therapy, researchers are now making inroads into understanding how cancer cells acquire drug resistance, and they’re finding that genetic mutations are just one of many strategies cancers use to evade death. Cancer cells have been found to boost transcription of survival genes, drill out the cores of transport proteins, or even employ alternate protein configurations to avoid extinction. While researchers are learning to apply the findings to overcome specific types of drug resistance, it’s quickly becoming clear that there is no single pathway tumors use to avoid cancer drugs, and that the problem of cancer resistance is far from over.

Genetic changes

Each of the many cancer cells in a tumor can have slight genetic variations. While the vast majority of cells in a tumor may be susceptible to a drug, a few cells can harbor mutations that allow it to withstand the toxic assault. Over time, all the susceptible cells die off, while those which are resistant proliferate.

This selective pressure allows breast and other cancers to acquire resistance to the platinum-based chemotherapy drug Cisplatin. In 1996, researchers found that some cancer cells developed mutations that enable them to literally pump the drug out of the cytoplasm, thus allowing the cells to withstand higher and higher doses without dying, said David Solit, an oncologist at Memorial Sloan-Kettering Cancer Center in Boston.

Similarly, as pharmaceutical companies developed a class of drugs called kinase inhibitors that blocked key molecules in cell division pathways known to be hyperactive in a variety of cancers, tumors began to develop mutations that allowed them to divide in the absence of those molecules. Some cancers, including lung cancer, even evolved to no longer depend on the division pathway at all, making any type of drug that targeted that pathway ineffective, Pao said.

Epigenetic changes

In addition to genetic mutations, researchers are finding that epigenetic changes may also allow cancer cells to nimbly respond to drugs. Unlike mutations, epigenetic changes can occur more quickly in response to environmental changes, said Pamela Munster, an oncologist and hematologist at the University of California, San Francisco.

Last month, Munster and her colleagues published that epigenetic changes are responsible for the resistance many breast cancers acquire against the estrogen blocking drug Tamoxifen after around 18 months of treatment. When the researchers looked at the genetic profile of the breast cancer cells, they didn’t find any mutations that correlated with resistance, but found that resistant lines were transcribing a survival gene called Akt at much higher levels than susceptible cells. The cancer cells used histone tags, or chemical markers on the chromatin, to expose the Akt gene and increase its transcription. The higher Akt signal allowed cancer cells to stay alive even in the presence of the drug by stimulating growth and proliferation and preventing cell death.

“What we learned is the tumor cells have a way of tagging the genes with the resistance marker that they can then pass on to other generations,” Munster said, giving those cells’ progeny an advantage in the drug-treated environment.

Cancer cells may also take a page from viruses to switch between resistance and susceptibility, according to a study published November 9 in the American Journal of Pathology. D. Stave Kohiz, a molecular biologist at the Mount Sinai School of Medicine in New York, and his colleagues were studying why ovarian carcinomas sometimes become resistant to a drug, only to become susceptible again after the drug is no longer taken. Such a change is unlikely to be caused by genetic mutations, because those are usually not reversible so quickly, Kohtz said.

Prior studies had shown that nuclear pores, which help transport nuclear elements to the cytoplasm and vice versa, could influence gene expression by interacting with chromatin at the periphery of the nucleus. Specifically, nuclear pores can activate transcription by shielding DNA from repressors, or hinder transcription because repressor proteins lurk in the region near the nuclear boundary. Given their influence on gene expression, Kohtz and his colleagues wondered if nuclear pores might be altered in different cancer cell types.

Using electron microscopy to visualize the nuclei of cancer cells resistant to the drug Cisplatin, the researchers saw that the nuclear pores “didn’t look right,” Kohtz said. They appeared to be hollow, he said, with a key gatekeeper protein in the center disassembled—similar to pores affected by viruses that co-opt cellular transport machinery for its own purposes. When the team knocked out the cores of nuclear pores in other cancer cells, they found the cells were frozen in an early stage of cell division. These hibernating cells didn’t grow very much, but they were also resistant to Cisplatin. Kohtz believes these sleeper cells can lie in wait, resisting the toxic effects of drug therapy, until some other signal turns them on.

“In vivo, the idea is that they will sit at metastatic sites or even in the region of the original tumor until something wakes them up and makes them start growing again,” Kohtz said.

Alternative splicing

Yet another way cancers can acquire drug resistance has to do with protein processing. Between 40 to 80 percent of melanoma patients have a mutated BRAF gene, which turns on cellular growth and division signaling pathway, Solit said. Last year, drug maker Plexxicon showed that a compound targeting mutant forms of BRAF, called Vemurafenib, significantly lengthened lives in melanoma patients with the mutation.

Vemurafenib exploits the fact that BRAF proteins in healthy cells pair up with other BRAF proteins to form a multiprotein complex, while the mutated BRAF protein act as a lone compound. This solitary structure can be hundreds of times more effective in activating cell division than the normal paired BRAF complexes, said co-author Roger Lo, a dermatologist at the University of California, Los Angeles, who studies skin cancer. Vemurafenib targets tumor cells by only inhibiting the standalone mutant version, while allowing the twinned version in healthy cells to act unimpaired.

But within 18 months, many patients develop resistance to Vemurafenib, and their tumors progress. To understand why, Solit and his colleagues looked at resistant cancer cell lines and found “some of the resistant cells generate a variant form of BRAF” that is shorter, he said. But the shorter BRAF protein wasn’t made by a mutation in a protein coding region of DNA, according to a paper published November 23 in Nature. Instead, deletions of exon regions of the gene led to alternative splicing that generated the shorter version, which can bind to itself, rendering the protein undetectable by Vemurafenib.

Tackling resistant cancers

While cancer cells use an intimidating array of tactics to evade drug therapies, researchers are slowly developing ways to target resistant cells. In human trials, Munster’s team has found that adding a compound that removes histone tags to the Tamoxifen regimen can make resistant breast cancers sensitive to the drug once again. Similarly ongoing Phase II trials are testing the combination of Vemurafenib and another drug that inhibits a compound in the same the cellular division pathway in melanoma patients resistant to Vemurafenib. And for those cancers that have switched between quiescence and active growth, simply retreating with the same medicine later on can sometimes be effective, Kohtz said.

While there may be general principles that apply to cancer resistance, for now, treatment requires a tailored approach that uses frequent biopsies of tumors to see what genetic and epigenetic mutations they’ve acquired, Pao said. With more thorough genetic sequencing, it’s become clear that there’s no single answer, even for a single patient.

“We’re all looking for the common theme, so that we can find ways to overcome it,” Pao said. Unfortunately, “cancers are heterogeneous, not just across individuals, but within individuals.”

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By Michael Smith, MedPage Today

Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston

Studies from two separate groups found that KRAS mutations, both preexisting and acquired, explained resistance to EGFR that emerges during therapy for colorectal cancer with single-drug targeted treatment.

Note that the KRAS mutations were detectable in blood samples well before treatment failure was observed.

Resistance to targeted cancer therapies may be almost inevitable, at least if they are used alone, two groups of researchers reported online in Nature.

Mathematical modeling, based on genetic testing of colorectal cancer patients, suggests that resistance already exists even before targeted therapy begins, according to Luis Diaz, MD, of Johns Hopkins Kimmel Cancer Center, and colleagues.

One effect of single-agent targeted therapy, they noted, is to allow tumor cells containing resistance mutations to grow and prosper, leading to disease progression.

A second group, led by Alberto Bardelli, PhD, of the Institute for Cancer Research and Treatment in Turin, Italy, found some evidence of preexisting resistance, but added that resistance could also emerge as a result of single-agent targeted treatment.

The solution, both groups argued, may be to use combination therapies to delay or prevent progression.

Molecules that block the epidermal growth factor receptor (EGFR) often have a dramatic initial effect on cancers driven by the receptor, Diaz and colleagues noted.

But resistance almost always arises within a few months of starting therapy, leading to relapse, although the exact mechanisms of the resistance have been unclear.

To help clarify the situation, they studied 28 patients with metastatic colorectal cancer, a disease in which patients whose tumors have a wild-type KRAS gene are often sensitive to EGFR blockade.

Four of the patients already had KRAS mutations at the start of monotherapy with panitumumab (Vectibix), a monoclonal antibody aimed at EGFR. But nine of the remaining 24 with normal KRAS developed mutations about 5 or 6 months after starting treatment.

Mathematical modeling, Diaz and colleagues wrote, showed that the parent cells of those with KRAS mutations must have been present before the panitumumab treatment started.

“These resistance mutations develop by chance as cancer cells divide so that tumors always contain thousands of resistance cells,” Diaz said in a statement, adding that the findings likely apply to any targeted cancer therapy.

Co-author Bert Vogelstein, MD, also of Johns Hopkins, added that the finding means that “long-term remissions of advanced cancers will be nearly impossible with single targeted agents.”

The research team also noted that their method – testing tumor DNA found in the blood – is noninvasive and was able to detect changes in KRAS long before those changes translated into renewed tumor growth.

That should allow physicians the opportunity to alter the treatment, perhaps by adding agents to the regimen.

“The good news is that there is a limited number of pathways that go awry in cancer, so it should be possible to develop a small number of agents that can be used in a large number of patients,” Vogelstein said in a statement.

Bardelli and colleagues reached similar conclusions after studying colorectal tumor cell lines and a group of 10 patients with metastatic disease who were being treated with cetuximab (Erbitux), a chimeric antibody aimed at EGFR.

They found that preexisting KRAS mutations were amplified in one patient and emerged after treatment in six others.

The resistance mutations were detectable in blood samples as early as 10 months before radiological assessment confirmed that the disease had progressed, Bardelli and colleagues said.

“Our results suggest that blood-based noninvasive monitoring of patients undergoing treatment with anti-EGFR therapies … could allow for the early initiation of combination therapies that may delay or prevent disease progression,” they concluded.

The study by Diaz and colleagues was supported by The Virginia and D. K. Ludwig Fund for Cancer Research, the National Colorectal Cancer Research Alliance, the NIH, the National Cancer Institute, the European Research Council, the Austrian Science Fund, and the John Templeton Foundation.

The authors declared competing financial interests, including affiliations with Personal Genome Diagnostics and Inostics.

The study by Bardelli and colleagues had support from the European Union Seventh Framework Programme, the Associazione Italiana per la Ricerca sul Cancro, the Regione Piemonte, the Fondazione Piemontese per la Ricerca sul Cancro, Oncologia Ca’ Granda ONLUS, Mr William H. Goodwin and Mrs Alice Goodwin and the Commonwealth Foundation for Cancer Research, the Experimental Therapeutics Center of Memorial Sloan-Kettering Cancer Center, the Society of MSKCC, the NIH, the Beene Foundation, and the Regione Lombardia and Ministerio Salute.

The authors declared they had no competing financial interests

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