Therapeutic
cancer vaccines represent a new class of agents in the treatment of
cancer.
Sipuleucel-T is an antigen-presenting cell-based
vaccine that recently demonstrated a significant 4.8-month improvement in overall survival in advanced
prostate cancer patients and was well tolerated. The findings of that study have been met with skepticism, primarily because the agent did not change initial
disease progression and yet led to longer survival. Although the commonly accepted treatment paradigm suggests that treatments should initially decrease
tumor volume, perhaps
vaccines work differently.
Vaccines may induce delayed responses not seen in the first few months of
therapy or they may initiate a dynamic immune response that ultimately slows the
tumor growth rate, resulting in longer survival. Subsequent
therapies may also combine with the induced immune response, resulting in a combination that is more effective than conventional treatments alone. Also, other treatments may alter
tumor-associated
antigen expression, enhancing the immune response. Future trials are currently planned to investigate these hypotheses; however, the results of the
sipuleucel-T vaccine in
prostate cancer should not be dismissed. Results with another
vaccine in
prostate cancer are similar, perhaps suggesting a class effect. In a broader context, clinicians may need to reconsider how they measure success. Several agents have been approved that produce superior
disease progression results, but do not affect overall survival. Given the toxicity and costs of
cancer therapies, perhaps studies should put more weight on long-term survival endpoints than on short-term endpoints that may be less consequential.