A Q&A with Martin Brown D.Phil, FASTRO, Emeritus Professor (right), and Lawrence Recht, MD, Professor, at Stanford University’s Department of Neurology (left)
Q: The treatment of glioblastoma multiforme (GBM) is a serious challenge. Recurrence after initial surgery is common and subsequent treatment almost always unsuccessful. Just as “an army marches on its stomach,” GBM growth depends on blood supply. Successful use of the FDA-approved drug plerixafor (Mozobil) combined with irradiation for mouse model gliomas some years ago has lead to clinical trials in human GBM.
What is the current status of these investigations, and how could our readers help the effort?
A: A little over ten years ago, Dr. Brown began a series of preclinical studies to test the possibility that an important contributor to the recurrence of malignant brain tumors after radiation therapy was reconstitution of the tumor vasculature. Specifically, he hypothesized that this reconstitution stemmed at least in part from circulating pro-angiogenic cells not in the tumor at the time of radiation—a phenomenon known as “vasculogenesis.” In agreement with this concept, a finding common to all of the tumor models he tested was a major influx into the irradiated tumors of bone marrow-derived cells, most of which were macrophages, that correlated with when tumors began to grow two to three weeks after completion of radiation. Further, he demonstrated that the mechanism for this influx was a radiation-induced hypoxia that triggered a cascade that led to the secretion of stromal cell-derived factor-1 (SDF-1), which was instrumental in attracting these cells. The apparent importance of excluding these cells’ entry into tumors post-irradiation suggests a new treatment strategy, which we call macrophage exclusion radiation therapy (MERT).
In August of 2014, based on these strong preclinical data, we launched a phase I/II clinical trial of MERT. This study examined the effects of administering a four-week continuous infusion of plerixafor (Mozibil)—the only commercially available agent that blocked the SDF-1 binding receptor CXCR4—at the end of irradiation to newly diagnosed GBM patients (NCT01977677). We enrolled 29 patients and established in phase 1 that the treatment was well tolerated at a dose that resulted in plerixafor serum values being maintained above the threshold level for CXCR4 blockade.
Two findings in phase II of this trial were particularly noteworthy: (i) a persistently lower relative cerebral blood volume within the irradiated field, and (ii) a much-improved control of the cancer in the treated field.
The noted overall median survival of nearly 22 months compared favorably with the best results obtained in other studies of GBM. However, it fell short of the dramatic improvements in survival noted in our preclinical studies, which utilized whole-brain irradiation (WBRT). WBRT was abandoned by clinicians in the early 1990s as a treatment for GBM because the high rate of local recurrence did not seem to justify the associated potential treatment-related issues of irradiating the entire brain (i.e., cognitive decline). However, we have shown that MERT is actually radioprotective for cognitive decline in rats given WBRT, consistent with the fact that tissue inflammation after radiation is related in large part to macrophage entry. Therefore, we have opened a new trial (currently open to accrual)using the same basic strategy in which a modest dose of WBRT has been added. Our expectation is that the widened radiation fields will further patient survival without excessive toxicity.
It is also important to note that the MERT strategy can be applied to any solid tumor in which local control using radiation is challenging. Further study of this strategy can therefore be of benefit to a wide spectrum of cancer patients.
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