Our bodies are under constant patrol by a vigilant team of defenders. These immune system superheroes set up barriers, decide whether an invader is dangerous, and, when necessary, go to battle.
White blood cells called T cells have the crucial task of protecting us from unwelcome pathogens and unhealthy cells. But like all superheroes, T cells have their weaknesses. Cancer cells have proven to be a pernicious foe by using a kind of invisibility shield to slip by T cells’ careful watch. Time after time, cancer wins.
Cancer researchers are starting to figure out how to boost the power of T cells. And it turns out that, once enhanced, they can be incredibly effective cancer killers.
How effective? The first inkling came in 2011, when a team led by Carl H. June at the University of Pennsylvania’s Abramson Cancer Center showed that enhanced T cells had obliterated tumors in three patients with advanced chronic lymphoid leukemia. These patients had been running out of options, and the treatment caused their cancer to essentially disappear.
Each patient had received a treatment, known as an immunotherapy, made from a superpowered version of his own T cells. The Penn researchers used inactivated HIV to insert into T cells’ DNA the instructions for a homing device that helps them find and fight cancer. After coaxing the reprogrammed cells—called CARTs, for chimeric antigen receptor T cells—into propagating, the researchers gave them back to the patient.
The results in that first CART trial were so dramatic that the researchers were incredulous. Bruce L. Levine, the director of Penn’s clinical cell and vaccine production facility, who has worked with June since 1992, recalls that when the first patients to be treated with CARTs came back for their checkup, “We’d get some lab data back, and it would be like, ‘The tumor cell counts were gone,’ or ‘The scans were clean,’ and Carl would say, ‘We’ve got to run that again.’ ”
The magnitude of the effect was a complete surprise. Depending on the patient, up to 8 lb of tumor melted away in a mere three weeks. And the engineered T cells kept up their battle. Six months after the infusion, patients came back to check whether the disease had resurfaced: In a test of a million cells, the scientists couldn’t detect a single tumor cell, Levine recalls.
The publication of the results kicked off a frenzy as drug firms and investors tried to get a foothold in a technology once considered a purely academic pursuit—cool science but way too challenging to commercialize. Since then, the major medical centers working on CART technology have spawned multiple start-ups, and venture capitalists have been more than happy to back their efforts.
Big pharma has also climbed on board. Novartis has a close relationship with Penn and an internal team of 320 researchers working on cell and gene therapies. This year Pfizer and GlaxoSmithKline also made significant commitments to modifying T cells through partnerships with biotech firms. Overall, since 2011, more than $800 million in public and private investment has flowed into a handful of companies developing the technology.
But commercializing CARTs in multiple cancer types is not going to be easy. Despite spending nearly a quarter-century on CARTs, scientists still need to make improvements throughout the process—from the point when T cells are isolated from a patient’s blood right up to when they are given back to the patient.
Key questions include: What is the best design for a CART? How many reengineered T cells are enough to be potent? What is the best way to manufacture CARTs to ensure they are safe, effective, and affordable? Perhaps the biggest question is whether this technology will be relevant in solid tumors, where the hurdles to designing a CART are much higher.
Industry veterans are encouraged that the mad rush into the field was accompanied by a large amount of cash. The hefty investment in CART technology “is an appreciation—maybe for the first time in biotech—that this is not a trivial process and we need a huge war chest,” says Mitchell H. Finer, chief scientific officer of Bluebird Bio, which is developing CART therapies in partnership with Celgene. Turning the technology into a product is going to be complicated, Finer adds.
You may have issues viewing the animation below when using Internet Explorer.
Please try another browser, view it as a PDF here, or view the static version at the bottom of this page.
Given that the immune system’s job is to find and destroy interlopers, it makes sense that scientists would want to direct it to find and destroy cancer cells. But researchers in industry and academia have spent decades searching for a good way to harness the immune system effectively.
The challenge is that cancer has sneaky ways of hiding from T cells. While scouting for invaders, a T cell stops regularly to assess whether a substance is out of the ordinary. If it is, the cell kills it. Checkpoint proteins mediate that process, alerting T cells to stop their attack before damage is done to healthy tissue. Cancer cells take refuge from immune attacks by hijacking the checkpoint protein’s signaling system.
The tide for cancer immunotherapy turned in 2011, when Bristol-Myers Squibb gained approval for the melanoma treatment Yervoy. The antibody inhibits the checkpoint protein CTLA-4, allowing T cells to get back in the battle. Although not a cure, Yervoy validated the idea of harnessing the immune system. Excitement has built as data have emerged for other checkpoint inhibitors, culminating last month in the approval of Merck & Co.’s Keytruda, a melanoma treatment.
Companies are encouraged by the potential for these immunotherapies, particularly as researchers learn how to maximize their efficacy by combining them with other drugs. But they have limitations. “There are also cancer patients who are really, really sick, and their immune systems are all beaten up,” says John C. Lin, Pfizer’s vice president of immunotherapy. “They may not have enough T cells left for these checkpoint inhibitors to work on.”
CART immunotherapy goes a step further. It overcomes the limitations of a weakened immune system by generating an army of reprogrammed T cells outside the patient’s body. Moreover, those reprogrammed T cells often stick around to keep up the battle if cancer should return.
The CARTs now undergoing clinical trials—from Penn-Novartis, Juno Therapeutics, Kite Pharma, and academic labs—all use the same basic construct. A vector embeds the DNA for the chimeric antigen receptor, or CAR, into the T cell; cells are multiplied and then returned to the patient.
The CAR includes all the components needed to give the T cell its enhanced powers: an antibody domain that sits on the outside of the T cell to bind to the tumor cell; a hinge and transmembrane domain that tethers the antibody to the cell; and costimulatory and essential activity domains, which sit inside the T cell and together signal it to divide.
So far, most of the CARTs in or nearing clinical studies seek out CD19, a protein found on B cells, a type of white blood cell that goes awry in many blood cancers. The differences in companies’ approaches boil down to nuances in design—the vector chosen or the choice of costimulatory domain or hinge. But these subtleties matter, Bluebird’s Finer explains. CARTs are normally expressed “in a very carefully coordinated fashion,” he says. “They’re not beads on a string, and so significant thought needs to be put into the details of their design.”
THE DOSAGE DILEMMA
Design is only one piece of making an immunotherapy work. Researchers also must decide how many reprogrammed cells are needed to elicit a safe and strong response against cancer.
Figuring out how much CART therapy to give a patient might require some mental jujitsu for those accustomed to the traditional ways of developing small-molecule or biologic drugs. Typically, drug developers run studies of different dosages of a medicine and make a tidy graph to determine how much can be given before side effects become worrisome.
But reprogrammed T cells, if working properly, should swiftly multiply inside the patient. Moreover, the therapeutic effect is dictated by the potency of the CART itself, which is a result of its design and how it was manufactured but can also depend on the state of the cancer patient, who might be very young or quite old and who often has been treated with many other drugs.
“I suppose the honest answer is that we really don’t know what the right dose is in a lot of situations,” says John Maher, an immunologist who leads the CAR mechanics lab at King’s College London. “These are living drugs. The cells can divide in the patient, so the dose you put in does not necessarily equate to the therapeutically active dose.”
Determining appropriate dosing for cellular therapy is “a question everybody in the field is trying to pinpoint,” says Margo R. Roberts, chief scientific officer of Kite Pharma. The current strategy is to infer from clinical data—in other words, to monitor safety and efficacy in individual cancer patients. But Roberts notes that the dose for the anti-CD19 CARTs has varied from as much as 30 million per kilogram of body weight to as low as 1 million per kilogram, the amount given in clinical trials of Kite’s lead CART.
Biotech executives all point to the results of those first three patients in the Penn study as evidence of the vagaries of dosage. Penn scientists had trouble manufacturing enough of the reprogrammed T cells for one patient. They ended up giving him a dose that was an order of magnitude lower than the other two received.
Remarkably, the man not only responded, but his cancer went into remission. “It tells you that numbers aren’t the only thing that matters,” Bluebird’s Finer says. “If you have the right cells and you treat them right, you can have an effective response.”
Getting the dosage right is as much about safety as it is about potency. CARTs quickly multiply, an effect that can obliterate cancer cells but also cause deadly side effects.
The concern was brought to the forefront this spring, when Juno, formed less than a year ago with a star-studded list of scientific founders, temporarily stopped adding new patients to its CART studies after several deaths. The halt—since lifted—was meant to give the company time to consider dosing and the overall health of patients it enrolled.
Paradoxically, the main safety problems arise when CARTs do too good a job of killing cancer cells. When T cells encounter a foe, they unleash cytokines, proteins that aid in and regulate the immune response. After CARTs rapidly multiply inside a patient’s body, oncologists have seen a cytokine “storm,” a dangerous—even deadly—release of the proteins. The other concern is tumor lysis syndrome, which occurs when a massive amount of dying tumor cells release metabolites that build up in the kidney.
Some progress is being made toward predicting the right number of cells to administer. “We continue to work this out diligently on a per-patient basis because, let’s face it, we’re dealing with personalized therapies where each individual is the control and each individual is unique,” says Usman Azam, head of Novartis’s cell and gene therapies unit.
Novartis and its collaborators at Penn have established a dose-ranging system that they expect to describe at the annual meeting of the American Society of Hematology in December. And because doctors now anticipate the side effects, they can try to mitigate serious damage to patients, Azam notes.
Two companies have responded to the dosing dilemma by embedding a gene in the CART that will be activated only in the event of a health crisis. The French biotech firm Cellectis has incorporated a cell-surface marker that, when turned on by the cancer drug Rituxan, causes the number of engineered T cells to plummet. Cellectis hopes to begin human tests of its first treatment a year from now.
Bellicum Pharmaceuticals has added a cell death switch to its CARTs that is activated by an otherwise inert small molecule. The company has shown in animal studies that it can eliminate the majority of the engineered T cells in as little as half an hour, according to Chief Executive Officer Thomas J. Farrell. Bellicum aims to put its first CART with the safety switch into clinical trials by the end of next year.
CARTs are proving in clinical studies to be effective killers of blood cancer, but it’s not clear yet if they can be turned into successful commercial products. Currently, most CARTs are made from scratch using a patient’s own T cells, putting the technology at the frontier of personalized medicine.
“This doesn’t fall into the kind of paradigm that big pharma uses as they try to develop drugs,” says Steven A. Rosenberg, chief of the National Cancer Institute’s surgery branch, who in 2010 published the first report of a patient responding to CART therapy. “They want vials on a shelf that are stable and can be widely distributed for treatment. And they don’t really care if it costs half a billion dollars to make the first vial, as long as they can make the second vial for a dollar.”
Companies are working diligently to understand how to translate what has primarily been an academic endeavor into an industrial process. Manufacturing CARTs for a wide audience will mean figuring out how to make individualized therapies quickly, consistently, and affordably.
“The thing that has kept many players away from this space is fear of manufacturing,” Novartis’s Azam says. But Novartis believes the challenge is surmountable. Soon after forging its pact with Penn, the company bought a manufacturing plant in Morris Plains, N.J., from Dendreon, a Seattle-based company that developed an approved, but commercially unsuccessful, personalized treatment for prostate cancer. Novartis has since been working diligently on how to ease the complexity and cost of CARTs.
Bringing down the cost of personalized immunotherapies will require improvements at every point in the process. Companies hope that properly designing CARTs will improve potency and cut down on the number of T cells needed to elicit a response. They are also honing the manufacturing process to ensure patients can be treated swiftly and safely.
Some firms are trying to work around the manufacturing challenge by developing “off the shelf” CARTs. Pfizer entered the field through a deal with Cellectis, which is developing CARTs that use donor T cells modified for use in a broad patient population. Like its competitors, Cellectis uses a viral vector to insert instructions for the CAR into T-cell DNA. But the company takes an additional measure: Using a gene-editing technology that precisely snips out segments of DNA, it strips the donor T cells of the proteins that would cause a patient to reject them as foreign.
The personalized approach “is very time-consuming, laborious, and also very expensive,” Pfizer’s Lin says. “We think there should be a better way.” He points out that some 300 patients are on Penn’s waiting list for CARTs for lack of capacity to make the personalized therapies. “Clearly it’s a problem that needs to be solved, so that’s why we’re drawn to the technology Cellectis is working on,” Lin adds.
Penn notes that last month it began construction of a 30,000-sq-ft facility—funded in part by Novartis—that, when completed, will enable more patients to be taken on.
But the manufacturing challenge isn’t just about logistics and economics. Research suggests that how CARTs are made could have an impact on their potency. As they multiply in the manufacturing process, T cells also become differentiated—that is, they change into other types of T cells, which can cause them to lose their ability to propagate once they are returned to the patient.
Moreover, T cells can undergo only so many divisions before they experience burnout. “Just like us, as we get older, we get exhausted more easily,” Bluebird’s Finer explains. Researchers are learning how to track so-called exhaustion markers on T cells to understand what they mean for the potency of a cell-based immunotherapy.
THE NEXT FIGHT
While companies sort out the nuances of bringing CARTs for blood cancer to market, they are also considering how to apply the technology to the wider world of solid tumors. It will be tricky. In CD19, researchers have as close to an ideal target as is possible in cancer. The receptor sits on both normal and cancerous B cells, but patients can live without healthy B cells as long as they are given immunoglobulin replacement therapy.
Similarly safe targets on solid tumors are less obvious. Moreover, solid tumors are harder to access. “When you’re dealing with a leukemia, you’ve got direct access to many of the cells circulating in the bloodstream,” says King’s College’s Maher, who is working on a CART for solid tumors. But solid tumors often have a complex microenvironment designed to deflect an immune attack, and many scientists question whether a CART for a solid tumor can elicit anywhere near the kind of response that it can in blood cancer.
Novartis’s Azam is sanguine about the prospects for moving CARTs into solid tumors, noting that the company even sees the technology being used one day in other disease areas, such as immunology. “There is clearly a very solid scientific rationale and basis for developing these in solid tumor types,” he says.
Novartis and Penn scientists are actively looking at diseases, such as pancreatic cancer, where few or no treatment options exist. Referring to solid tumors, Azam says Novartis is “definitely entering into clinical studies in a small-scale setting in the very near future.”
Beyond pursuit of other tumor types, researchers expect the coming years will bring new knowledge about how to pair CARTs with other treatments, particularly checkpoint inhibitors. “The cancer field really is built on combination therapy,” says David D. Chang, Kite’s chief medical officer. “There are a lot of discussions ongoing about combining CART therapy with other immunotherapy. I would say that the scientific rationale is strong and such a combination therapy is worth pursuing.”
Despite the many kinks to be worked out on how to optimize CARTs, researchers are happy to be making real progress in empowering T cells to fight cancer. They are betting that the technology will offer results of the type only dreamed about in the oncology world.
“A lot of people have talked a lot about personalized therapies and personalized medicines, but very few have achieved something successful in this space,” Azam says. “We feel we’re tantalizingly close to doing that.”