Enhanced antitumor efficacy of a herpes simplex virus mutant isolated by genetic selection in cancer cells

Replication-competent, attenuated herpes simplex virus-1 (HSV-1) derivative s that contain engineered mutations into the viral gamma 34.5 virulence gen e have been used as oncolytic agents. However, as attenuated mutants often grow poorly, they may not completely destroy some tumors and surviving canc er cells simply regrow. Thus, although HSV-1 gamma 34.5 mutants can reduce the growth of human tumor xenografts in mice and have passed phase I safety studies, their efficacy is limited because they replicate poorly in many h uman tumor cells. Previously, we selected for a gamma 34.5 deletion mutant variant that regained the ability to replicate efficiently in tumor cells. Although this virus contains an extragenic suppressor mutation that confers enhanced growth in tumor cells, it remains attenuated. Here, we demonstrat e that the suppressor virus replicates to greater levels in prostate carcin oma cells and, importantly, is a more potent inhibitor of tumor growth in a n animal model of human prostate cancer than the gamma 34.5 parent virus. T hus, genetic selection in cancer cells can be used as a tool to enhance the antitumor activity of a replication-competent virus. The increased therape utic potency of this oncolytic virus may be useful in the treatment of a wi de variety of cancers.