An in vitro method for identifying an anti-cancer agent, the therapeutic efficacy of which is enhanced upon administration with a second agent that increases the expression of OPRK1 and/or BAD, comprising the steps of:



a. co-incubating a population of cells with a target anti-cancer agent and said second agent and
b. measuring the cytotoxicity and/or cytostasis in the population of cells;

wherein the therapeutic efficacy of the target anti-cancer agent is enhanced if the cytotoxicity and/or cytostasis of the target anti-cancer agent is increased compared to a control.