Cancer vaccines: a critical review on clinical impact.

Developing cancer vaccines to treat solid tumors is not an easy task. As solid tumors develop, the immune system places cancer cells under selective pressure; it detects those that are more 'antigenically different', selecting those cells that are not recognized by the immune system and are therefore resistant to immune surveillance. Resistant cells survive, and in turn their clones will spread to distant sites. This phenomenon, called 'immune editing', is not novel, but is quite similar in principle to the selective pressure induced by chemotherapy and hormone therapy, and is molecularly based on the clonal heterogeneity and molecular instability present in most solid tumors. This explains, at least in part, why many cancer vaccines work in animal models but not in a clinical setting. The aim of this article is to review the most commonly used cancer vaccine strategies and to evaluate the evidence supporting their efficacy. This is not as easy at it may sound, as each research group involved in cancer vaccine development uses different technologies, targets different antigens, combines different carriers and adjuvants to obtain an immune response, and immunizes patients with different administration schedules. The final picture is somewhat confusing, and comparison of different vaccine strategies is almost impossible. Most of the vaccines are still experimental, far from being approved by regulatory authorities, and their clinical utility is almost negligible.