"The blood vessels that develop to supply nutrients to a tumor are not normal," says Rakesh Jain, PhD, director of the Steele Laboratory in the MGH Department of Radiation Therapy, the study's senior author. "The vessels are leaky, dilated, disfigured, and do not evenly inflitrate the tumor, which can interfere with standard cancer therapies. Chemotherapy drugs are not distributed throughout the tumor, and the oxygen level is low, making tumors resistant to radiation therapy. It now appears that antiangiogenic therapy transiently improves a tumor's blood supply and oxygenation, making it more vulnerable to radiation therapy."
Although some animal studies have suggested that combining antiangiogenesis and radiation therapy can slow tumor growth, in others the treatment appeared to spur tumor growth. The current investigation was designed to resolve these conflicting results and to improve understanding of the cellular and molecular underpinnings of antiangiogenesis treatment. The MGH researchers implanted human brain tumor tissue into mice that were then treated with various combinations of an angiogenesis inhibitor called DC101 and radiation therapy.
DC101 alone produced a minor delay in tumor growth, and radiation alone produced a more significant growth delay. But of five different schedules of combined treatment, only giving radiation from 4 to 6 days after initiation of DC101 therapy resulted in a synergistic effect that was greater than simply adding the effects of both treatments. Measurement of the oxygen levels within tumor tissue supported the theory that DC101 improves oxygen delivery to the tumor during the same time period, with hypoxia (oxygen starvation) almost eliminated on day 5 but returning by day 8.
To better understand the mechanism behind these changes, the researchers conducted several detailed analyses of the tumor tissue. They observed a shift toward more normal blood vessels that were smaller and less disfigured after DC101 treatment and also found that these vessels had been stabilized by the recruitment of pericytes – cells that normally help to support blood vessel walls. Mirroring the pattern of oxygen supply, pericyte coverage of blood vessels peaked around day 5 and then fell off by day 8.
The investigators also showed that greater pericyte coverage was the result of more pericytes being attracted to the tumor vessels, rather than the removal of pericyte-poor vessels as some researchers had assumed. Measurement of a factor known to be involved in pericyte recruitment found that its levels were temporarily increased after DC101 treatment. Examination of the effects of DC101 on vascular cells' basement membrane, which becomes abnormally thick in tumors, indicated that the membrane was thinner during the day-2-to-day-5 time period and also showed that this improvement resulted from the increased activity of specific enzymes.
One crucial result of these findings is alleviation of the concern that reducing a tumor's blood supply would actually worsen hypoxia and increase resistance to radiation therapy. "The success of this treatment approach depends on carefully scheduling when radiation is administered to take the greatest advantage of this window of vascular normalization," says Jain, who is Cook Professor of Tumor Biology at Harvard Medical School. His group plans further studies to determine how these results could be applied to treatment of cancer patients.
Additional authors of the current study are co-first authors Frank Winkler, MD, PhD, and Sergey Kozin, PhD, along with Ricky Tong, Sung-Suk Chae, PhD, Michael Booth, PhD, Igor Garkavtsev, MD, PhD, Lei Xu, MD, PhD, Dai Fukumura, MD, PhD, Emmanuelle di Tomaso, PhD, and Lance Munn, PhD, all of the Steele Laboratory at MGH; and Daniel Hicklin, PhD, of ImClone Systems Incorporated in New York.
Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $400 million and major research centers in AIDS, cardiovascular research, cancer, cutaneous biology, medical imaging, neurodegenerative disorders, transplantation biology and photomedicine. In 1994, MGH and Brigham and Women's Hospital joined to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups, and nonacute and home health services.
Journal
Cancer Cell