The classic view of cancer is that it arises from the progressive accumulation of mutations. The cancer stem cell model, however, theorizes that tumors derive hierarchically from one or more progenitor cells.
Ravi Bhatia has seen more than his fair share of chronic myelogenous leukemia (CML) in his two decades as hematologist-oncologist.
When he started his hem-onc fellowship in 1989, the prognosis for CML was grim; therapeutic options included interferon and bone-marrow transplantation, yet neither really worked. “At that time bone-marrow transplantation was the treatment of choice, but was available to only a limited number of patients,” he says. “Patients who did not qualify for bone marrow transplant usually died from their leukemia.”
Today the situation is very different, thanks to Gleevec.
A small molecule inhibitor of the specific mutant enzyme that is the hallmark of CML, Gleevec (marketed by Novartis) is “remarkably effective for CML treatment” says Bhatia, director of the Division of Hematopoietic Stem Cell and Leukemia Research at the City of Hope Cancer Center in Los Angeles. Compare blood samples from patients before and after imatinib treatment and in most cases, the disease appears to vanish. In reality, though, the drug often drives the cancer into hiding.
“Patients who have been treated with Gleevec for many years, if we were to stop treatment in those patients, the disease will usually come back,” Bhatia explains. “There is a reservoir of disease that hangs around that allows the patient to relapse.
That reservoir, it turns out, is filled with cancer stem cells (CSCs). Like their “normal” counterparts, CSCs are tumor progenitors with the ability to self-renew (that is, make more stem cells) and differentiate into other components of the cancer (the “bulk tumor”). And like Gleevec, many anticancer therapies fail to target these cells efficiently, either because of their different growth or genetic characteristics. As a result, they are the target of intense drug development and research. Now, as the first drugs intentionally targeting these cells work their way into clinical trials, the question is, will they have an impact?
A controversial theory
The classic view of cancer is that it arises from the progressive accumulation of mutations over time—the so-called “stochastic model.” A normal cell acquires a mutation that gives it a slight growth advantage over its neighbors, say. Then one of that cell’s progeny acquires another mutation that provides yet another advantage, and so on. The implication is that all cells in a tumor have more or less an equivalent capacity to form another tumor—that is, to metastasize or cause relapse—and thus, the more doctors can do to shrink a tumor, the better the prognosis.
The CSC model posits instead that tumors derive hierarchically from one or more progenitor cells—whether a normal stem cell “gone bad” or a more differentiated cell that has somehow been reprogrammed to be “stem-like,” nobody knows. In this model, traditional therapies that target the bulk tumor are to some extent pointless, since only stem cells can propagate the tumor, and they often escape the treatment, for instance because they grow more slowly. The resulting shrinkage may look good on a CT scan, but the disease itself can still recur.
CSCs have been linked to some insidious properties, including metastatic and angiogenic potential, radioresistance, and chemotherapy tolerance. Markus Frank of Children’s Hospital Boston demonstrated recently that melanoma CSCs help that cancer evade the immune system by both downshifting immune activity and upregulating immunosuppressive cytokines.1
Yet not everyone buys the idea.
“All the things that really create problems for us as clinicians, you see as something being linked to cancer stem cells,” says Jeremy Rich, Chairman of the Department of Stem Cell Biology and Regenerative Medicine at the Cleveland Clinic. “The problem is this: People want cancer stem cells to be the answer, or they don’t believe it. And they want a single way to figure out what a cancer stem cell is, and we don’t have that.”
Indeed, “cancer” is not a homogeneous disease—even within a single tumor there is continuous clonal evolution at the genetic and biological levels—so it should come as no surprise that CSCs aren’t homogeneous either. No one cell surface marker can isolate all CSCs from all tumors, even those thought to have a common tissue and genetic basis; breast CSCs, for instance, are CD44high/CD24low, but melanoma stem cells bear the marker ABCB5, and glioblastoma CSCs, CD133.
Because individual tumors also evolve, so may their CSCs. “It can get to the point where every single cell behaves like a stem cell,” says Connie Eaves, vice president of research at the British Columbia Cancer Agency. “If you happen to be dealing with a tumor that has advanced to that stage, then the concept of a cancer stem cell becomes meaningless.” Indeed, in some tumors, CSCs seem to represent less than 1% of all the malignant cells—an observation that jibes with the notion that rare, sleeper stem cells hiding out from anticancer therapy are responsible for relapses even when initial reduction in tumor size is seen. But for others, CSCs may represent a substantial portion of the tumor mass—as much as 25% in one study.2
Plus, CSC is more a functional definition than a molecular one, says Rich. So it’s not enough to enrich for one fraction or another. Researchers must ask the “tough questions”: are the cells capable of self-renewal? Can they form tumors in animals that are comparable to the original tumor? And, because the cells are so plastic, do they retain those features over time?
Those questions are expensive to answer, Rich says, so researchers “try to cut corners all the time.” The result is uncertainty. “That’s one of the big reasons why people have skepticism about the field. Either that people are trying to claim too much about cancer stem cells, or they are not doing the hard studies.”
What makes them tick?
Controversy aside, researchers and drug developers are probing CSCs relentlessly for weaknesses. First up: figuring out what makes them tick.
Using microarrays, Scott Armstrong, codirector of the cancer program at the Harvard Stem Cell Institute, has addressed the gene expression differences that distinguish leukemia stem cells both from hematopoietic stem cells and more differentiated blood cells.
The results, he says, are “hopeful”: leukemia stem cells are effectively cellular chimeras, blending the expression characteristics of both stem and white blood cells. “We think that is probably happening in many different types of leukemias, if not other cancers, that what you have is aberrant activation of a stem cell program in the context of a cell where it shouldn’t be activated,” Armstrong says.
That observation, he adds, provides a “therapeutic opportunity,” a way to target CSCs while leaving their normal counterparts untouched, and without destroying the bone marrow—and it may be broadly generalizable.
“They are not going to be identical—some cancers will depend on one pathway, others will depend on a different one,” he explains. “But there will be a finite number of pathways that are involved in this process of cancer self renewal, and it’s just a matter of figuring out which diseases are dependent on which pathways.”
To the clinic
Now researchers and drug developers are beginning to capitalize on those opportunities. In September, Charles Rudin of the Johns Hopkins University Cancer Center, and colleagues described transient tumor regression in a patient with medulloblastoma, a brain tumor, following treatment directed at one key signaling pathway, called Hedgehog, which has been tied to CSCs from a variety of tissues, including brain.3
Other key pathways are also being targeted. Craig Jordan, Professor of Medicine at the University of Rochester School of Medicine, for instance, screened inhibitors that hit the NF-kappa-B pathway and identified the plant-derived molecule, parthenolide, as well as several, more “druggable” variants. Leuchemix, a company Jordan co-founded, has initiated phase 1 trials of one such compound, called LC-1, in AML patients. And Jennie Chang, medical director of the Lester & Sue Smith Breast Cancer Center at Baylor College of Medicine, presented data at the December San Antonio Breast Cancer Symposium on a phase 1 trial of a gamma-secretase inhibitor that targets Notch. Some breast cancer patients responded to the drug, Chang says, though not all. “It’s promising,” she says. “It’s probably not going to obliterate cancer stem cells, but it definitely hits the target.”
Other researchers hope to gain traction from another key difference between normal cells and CSCs, epigenetic variation. Bhatia presented data at the December American Society of Hematology meeting in New Orleans suggesting that supplementing tyrosine kinase inhibitors (like imatinib) with LBH589, a histone deacetylase inhibitor, can target CML stem cells in the test tube and in mice.
Meanwhile, the hunt is on for compounds to fill the pipeline.
As a postdoc with CSC expert John Dick, Sean McDermott, currently a research investigator with Max Wicha at the University of Michigan Medical School, screened a 4,000-compound library for potential leukemia stem cell inhibitors. Recognizing the overlap between the pathways driving cancer and normal stem cells, McDermott designed his screen to identify molecules that could hit acute myelogenous leukemia (AML) CSCs in culture, but not their normal hematopoietic counterparts.
“Basically, we didn’t want to kill off the normal bone marrow, which is common in chemotherapy and radiation therapy and is one of the big dose limiting toxicities,” McDermott explained. Only 10 of 80 compounds survived that test, which the team then tested in primary AML CSCs. Ultimately, McDermott settled on three compounds to move forward, including etopside and ciclopirox olamine, an FDA-approved topical antifungal that McDermott’s collaborator, Aaron Schimmer of the University of Toronto, is now trying to move into clinical trials.4
On a larger scale Eric Lander and Bob Weinberg of the Massachusetts Institute of Technology exploited the fact that stem cell populations apparently increase during epithelial-mesenchymal transitions to screen some 16,000 compounds for breast CSC inhibitors. The screen identified four compounds (including etopside), the most promising of which was salinomycin, a potassium ionophore.5
As these new drugs move toward the clinic, one thing is certain: they probably won’t be magic bullets. Instead, they likely will be added on to existing therapeutic regimens. “What we are trying to do is to target the root cause of the cancer and eliminate those cells,” says Bhatia, “and thus, to take the disease from remission to cure.”
1. T. Schatton et al., “Modulatin of T-cell activation by malignant melanoma initiating cells,” Cancer Res, 70:697–708, 2010.
2. E. Quintana et al., “Efficient tumor formation by single human melanoma cells,” Nature, 456:593-8, 2008.
3. C.M. Rudin et al., “Treatment of medulloblastoma with hedgehog pathway inhibitor GDC-0449,” N Engl J Med, 361:1173-8, 2009.
4. Y. Eberhard et al., “Chelation of intracellular iron with the antifungal agent ciclopirox olamine induces cell death in leukemia and myeloma cells,” Blood, 114:3064-73, 2009.
5. P.B. Gupta et al., “Identification of selective inhibitors of cancer stem cells by high-throughput screening,” Cell, 138:645-59, 2009