Origin of CLL and the Molecular Mechanism of Ibrutinib
hematopoietic stem cells (HSC) give rise to healthy B lymphocytes.
Recent evidence suggests that genetic changes at the HSC level lead to
monoclonal B cells that then give rise to CLL cells. Enlargement: The
experimental oral agent ibrutinib blocks B-cell receptor (BCR)
signaling, thereby preventing B-cell activation and proliferation.
Ibrutinib binds irreversibly with Bruton’s tyrosine kinase (BTK), a
protein that is overexpressed in CLL and other B-cell malignancies. This
blocks signals from the B-cell receptor that are essential for CLL-cell
survival and proliferation and triggers cell death.
ABOUT-FACE FOR CHRONIC LEUKEMIA
Ibrutinib is a targeted drug that in early clinical testing has produced lasting remissions and few side effects in patients with chronic lymphocytic leukemia
By KENDALL POWELL
A few days before Christmas 2011, physician assistant Margaret Lucas walked into her chronic lymphocytic leukemia clinic to see a dozen patients and for a moment wondered if she'd entered a flu vaccination clinic by mistake. The patients were smiling, looking well, all had their hair and no one was fighting an infection or required a blood transfusion. "Nobody needed anything from me," she recalls. "It was really overwhelming."
They did need one thing: refills of their once-daily dose of the pill that was keeping these relapsed leukemia patients healthy.
That day was in stark contrast to the usual clinic day. Chronic lymphocytic leukemia (CLL) is a highly heterogeneous malignancy that typically requires a mix of treatment options depending on the individual. Many newly diagnosed patients have an asymptomatic, indolent form of the disease that requires close monitoring only, sometimes for many years. Often, a routine blood test reveals the abnormally high lymphocyte count and leads to diagnosis. Median survival can exceed 10 years.
Other patients have an aggressive form of CLL that requires immediate treatment. Survival for these patients is about one or two years when untreated and two to five years when treated.
"CLL is the most common and prevalent adult leukemia in the Western Hemisphere," says John C. Byrd, MD, a CLL specialist and professor of Medicine, of Medicinal Chemistry and of Veterinary Biosciences at The Ohio State University Comprehensive Cancer Center - Arthur G. James Cancer Hospital and Richard J. Solove Research Institute (OSUCCC - James).
"More than 100,000 people in the U.S. are walking around with it, and it is not curable with current therapy. The median age at diagnosis is 72, and nearly 4,600 patients were likely to die from CLL in 2012," says Byrd, who co-leads the CLL Experimental Therapeutics Research Laboratory.
CLL is believed to begin as a premalignant disorder of B lymphocytes called monoclonal B cell lymphocytosis (MBL). Additional gene mutations in B cells or signals from the bone-marrow microenvironment probably drive progression of MBL to CLL and result in the proliferation of ineffective B lymphocytes that lead to abnormal immunoglobulin production and autoimmune disorders. With progression, CLL cells crowd into lymph nodes and the spleen and liver, painfully enlarging those organs. The disease disrupts red-blood-cell and platelet production, leading to anemia and bleeding.
Treatment with traditional chemotherapy drugs could keep the disease in remission for a while but does not improve overall survival. In the late 1990s, drugs with greater activity against CLL were developed. They included rituximab, a monoclonal antibody designed to target leukemia cells, and fludarabine, a purine analogue that destroys both B and T cells. These agents produced more frequent and longer remissions, but the disease eventually recurs and usually resists further treatment.
Currently, there is no curative therapy for CLL, other than possibly a high-risk hematopoietic stem-cell transplant. Moreover, CLL is associated with a wide range of potentially fatal hematopoietic and immune-system complications. A common first-line treatment combines fludarabine, the alkylating agent cyclophosphamide, and rituximab. This regimen, called FCR, has a 70-percent complete-remission rate, but almost all patients eventually relapse, and FCR often worsens the hematopoietic and immune-system complications.
Unfortunately, patients over age 65, who represent about two-thirds of the CLL population, generally cannot tolerate FCR treatment because it puts patients at high risk of respiratory and other infections. Instead, older patients may be treated with drugs such as chlorambucil and rituximab, though these agents are less effective at producing lasting remissions.
"We need newer therapies for older patients - therapies that keep patients in remission longer and don't have the immunosuppressive side effects of chemotherapy," Byrd says. "Our quest is to identify targeted therapies that selectively kill leukemia cells."
Ibrutinib (or PCI-32765) has the potential to fulfill all of Byrd's requirements. Pharmacyclics, based in Sunnyvale, Calif., licensed the drug from Celera Genomics in 2006. Recognizing the OSUCCC - James group as a leader in preclinical research and early-phase trials of new drugs for CLL, Pharmacyclics approached Byrd and the OSUCCC - James CLL Experimental Therapeutics Laboratory about testing ibrutinib.
Ibrutinib belongs to a class of drugs called B-cell receptor (BCR) antagonists. The B-cell receptor is overexpressed in CLL, and BCR signaling is essential for B-cell maturation and growth. Having already tested another B-cell agonist, CAL101/GS1101, that was proving to be effective in early clinical testing, the team agreed to put ibrutinib through the same preclinical paces.
Ibrutinib binds to and inhibits a molecule called Bruton's tyrosine kinase (BTK), a protein that is essential for BCR signaling and is overexpressed by CLL cells. By inhibiting BTK, ibrutinib prevents BCR signaling and B-cell activation, says Amy Johnson, PhD, a research assistant professor in the Division of Hematology and a molecular pharmacologist.
"But when we expose CLL cells to ibrutinib in lab dishes, they don't instantly die. When we first saw this in the lab, we were kind of worried because ibrutinib is not highly cytotoxic," she says. "We learned there is more to it than that."
The team pushed ahead with experiments to test ibrutinib in CLL cells and mouse models. In a 2011 paper published in the journal Blood, the group showed that CLL cells overexpress BTK, that ibrutinib makes CLL cells more susceptible to apoptosis compared with normal B cells, and that the agent had no toxic effect on T cells.
Ibrutinib also quashed the proliferation of BCR-activated B cells and shut down their survival pathways. Finally, their laboratory studies showed that the drug blocked survival signals that CLL cells receive from the microenvironment, including IL-6, IL-4, TNF-α and stromal-cell contact.
In subsequent research, Johnson tested ibrutinib on a mouse strain that spontaneously develops a CLL-like leukemia at about nine months of age. (The mice carry B cells that overexpress a human oncogene called TCL1.) "If we provide continuous treatment with ibrutinib starting when they are really young, we can prevent the leukemia from developing," says Johnson.
In another experiment, the team transplanted the leukemic cells from the TCL1 mouse model into 100 mice without leukemia. Then, at the time of leukemia diagnosis (by flow cytometry), they gave the mice ibrutinib daily in their drinking water. The treated animals had a significantly longer overall survival than control animals at 46 days versus 24 days, respectively.
Using mouse genetic models, the team has also shown that BTK is indeed a critical target in the development of CLL. In both mice and humans, a mutation that knocks out the function of BTK causes the immune deficiency hypogammaglobulinemia. Johnson and her team crossed the mouse with the mutated BTK gene with the TCL1 mouse. The resulting offspring, which in essence are predisposed to develop CLL but have BTK permanently disabled, do not develop the leukemia at nine months as usually happens, and most were behaving normally and appeared free of side effects after more than 12 months.
"So we've shown that inhibiting BTK - both pharmacologically and genetically - prevents leukemia from developing in mice," says Johnson.
TRIALS WITHOUT TRIBULATIONS
In the meantime, clinical testing of ibrutinib was moving forward. In 2010, trials began in patients whose CLL had relapsed and was refractory to other treatments. In phase I and II trials, cohorts of CLL patients experienced a 90-percent response rate in terms of shrinking lymph nodes, spleens and white blood cell counts, and improving anemia. In 70 percent of patients with relapsed disease, ibrutinib removed detectable CLL cells from the blood, with a partial response or better, Byrd says.
"Ibrutinib has really opened up a door of hope for people," says Lucas. Turnarounds like those of her Christmastime-clinic patients were characteristic of most patients put on ibrutinib. "It's dramatic," she says.
One such patient is Brian Koffman, a family physician who lives in Newport Beach, Calif. The 61-year-old has been living with an aggressive variant of CLL for seven years. He underwent a stem-cell transplant and rituximab therapy, both of which failed. A complication called autoimmune thrombocytopenia made pursuing the harsher therapies, such as FCR, a high-risk choice.
At one point, he grew a "Santa-beard" to hide his massively swollen lymph nodes from his own patients. After participating in the phase II ibrutinib-plus-ofatumumab trial for eight months, his lymph nodes are no longer palpable and his blood work is "boringly normal," he says.
"What's most notable about ibrutinib is that, with more than two years of follow-up, the median time to relapse has not yet been reached. The drug - a pill taken daily - is holding people in remission," Byrd explains. And the side-effect profile of ibrutinib? "Compared to most new chemotherapy drugs, it's apples to oranges," Byrd says.
Koffman had some mild diarrhea, heartburn and a couple of rashes. All typical, he says.
"People are using the word revolutionary to describe ibrutinib – we have never had a medication that worked so well with so few side effects." -
"Overall, taking the medication is a non-event." Fatigue and bruising are also common. Even though the trial requires him to fly to Columbus frequently, Koffman calls his participation "one of the best decisions I've ever made."
Because the trial in relapsed patients has gone so well, and because the drug does not appear to increase infection risk further in CLL, Byrd wanted to move ibrutinib into a trial for first-line treatment of patients over 65, giving it as monotherapy.
The results in this previously untreated, over-65 patient group are "similarly spectacular," Lucas says. At 27 months, 96 percent of this group of 31 patients was progression-free. "With chemotherapy, we would expect this to be about 50 percent.
"People are using the word revolutionary to describe ibrutinib - we have never had a medication that worked so well with so few side effects."
Byrd, who typically speaks in measured, reserved tones, also cannot hide his enthusiasm: "It's beating the socks off other treatment options. This class of drug is going to transform CLL just as Gleevec did for CML," he says, referring to the molecularly targeted drug for chronic myelogenous leukemia.
Byrd doesn't use "transform" lightly - he and Lucas have seen patients' lymph nodes and spleens shrink within the first week of treatment; patients who thought they were headed for hospice care instead head out on cruises and trips to Europe. Simply put, says Byrd, "It's an amazing drug."
Ibrutinib could be game-changing for the treatment of end-stage CLL, which too often is a frustrating act of moving patients from one therapy to the next and hoping for the best. “With ibrutinib, we have some patients on their second and third years of treatment with no relapses," Lucas notes.
Byrd cautions that resistance could occur in the future. And even though the numbers of patients who have relapsed on ibrutinib or who cannot tolerate the drug are very small, doctors need a next-drug in the arsenal to offer those patients. This might be an improved, second-generation BTK inhibitor (akin to dasatinib and nilotinib for Gleevec), he explains.
Lab researchers will investigate the question of resistance and what happens when mice stop taking the drug. Clinicians want to know if ibrutinib, which also kills healthy B cells, will cause long-term immune suppression and whether the drug has benefit for treating early-stage CLL.
Currently, ibrutinib is entering phase III testing for both previously untreated CLL and for relapsed patients. In the RESONATE trial taking place in 44 centers around the world, 350 relapsed or refractory CLL patients will be randomized to receive either ibrutinib or the monoclonal antibody drug ofatumumab. In the RESONATE-2 trial, 272 patients 65 or older with previously untreated CLL will be randomized to receive either ibrutinib or chlorambucil.
Byrd has conducted translational studies of CLL for 15 years, moving from laboratory bench to patient bedside and back to the bench, working to improve CLL treatment. Lucas has worked with him that entire time and says ibrutinib is unlike anything they have seen before.
"I've seen Campath, rituximab and fludarabine as they were introduced. They can make patients incredibly sick and vulnerable to fatal infections," she says. "The more specifically targeted agents that we can find for malignancies, the better."