Conditionally replicating adenoviruses as anticancer agents and ways to improve their efficacy

Conditionally replicating adenoviruses (CRAds) were developed as new tools for cancer therapy. CRAds specifically replicate in and kill cancer cells. Within a solid tumor mass, release of newly formed infectious particles from infected cancer cells allows additional cell layers to be infected and destroyed. When used as monotherapeutic agents, CRAds showed only limited effects in clinical trials. Combination studies consisting of CRAd virotherapy with chemo-, radio-or enzyme prodrug therapy, however, showed additive or even synergistic effects and encouraging results in clinical studies. Furthermore, increased CRAd efficacy could be achieved in preclinical models by targeting CRAd infection specifically to tumor cells or via insertion of therapeutic genes in the CRAd genome. In this review, different mechanisms to achieve CRAd specificity, the efficacy of CRAds in clinical trials and ways to enhance their oncolytic potency are discussed.