By Michael Eisenstein
Cancer operates like a well-disguised saboteur. Occasionally it slips up by displaying unusual proteins, tripping immunological surveillance systems that are checking for abnormal growth. For decades now, scientists have tried to exploit this vulnerability with therapeutic vaccines — injections of tumour-associated proteins that essentially hang a ‘Wanted’ poster, helping immune cells recognize and kill cancer cells.
After a string of expensive and dispiriting defeats, therapeutic cancer vaccines recently registered their first big win. In April 2010, the US Food and Drug Administration (FDA) approved Provenge (sipuleucel-T) — a mixture of a patient’s own cells incubated with a protein expressed by 95% of prostate tumours. This was not an unequivocal victory, however. “On average, patients live about four months longer,” says Martin Kast, a cancer vaccine specialist at the Norris Comprehensive Cancer Center at the University of Southern California (USC) in Los Angeles. “It certainly measures up to many chemotherapeutics, but there’s still a long way to go.”
The problem is that as cancer develops, it disrupts the immune system to prevent an effective counter-attack. Accordingly, Kast and other researchers have been refocusing their efforts towards developing cancer vaccines as preventative measures, before cancer sets its traps. “You don’t wait to get polio before taking the polio vaccine — you vaccinate prior to the engagement of the pathogen,” says Vincent Tuohy, an immunologist at the Cleveland Clinic in Ohio. With therapeutic vaccines “we were asking the immune system to go in and heroically eliminate this large and mature tumour load, and that’s a problem. If you want to get rid of a disease, you do it prophylactically”.