In an ideal world, the immune system would recognize cancerous cells and eliminate them before tumor formation could take place. Unfortunately, cancer cells appear to evade the immune system despite its best efforts, and past approaches taken to stimulate the immune system to recognize and attack cancer cells have not been successful. The issue for the numerous companies pursuing this approach is thus why have past efforts failed, and what can be done to increase their chances of success.

Cancer immunotherapies seek to use antigens present in or on cancer cells to alert the immune system to mount a full-scale attack - like vaccines used to prevent or treat infectious diseases. But despite the simple idea behind such approaches, no cancer vaccine has yet been approved.

Cancer vaccines in development can be grouped into three major classes. The first class is an older approach that involves immunization with whole cells, either intact or as lysates. The two other classes attempt to incorporate new technology to stimulate the immune response. One class focuses on generating an antibody (humoral) response, while the other focuses on a cytotoxic T cell (cellular) response.

Other approaches, such as nonspecific immunostimulation with bacterial polysaccharides or patient-specific immunization with autologous cells, do not fall neatly into these three classes (see chart for selected products in development). In each case, however, tumor immunology presents an obstacle to successful treatment.

Past problems

"The major obstacle we've had in the past is that we haven't known the precise nature of what we need to target," said Steven Rosenberg, chief of surgery at the National Cancer Institute.