ABSTRACT. Two hallmarks of drug resistance in cancers are irregular metabolism and drug efflux. In multidrug- resistant cancers, both of these processes disarm the efficacy of chemotherapeutics, ultimately resulting in de- creased chemotherapeutic efficacy and increased mortality. Several strategies in development attempt to miti- gate the effects of drug resistance by modulating specific metabolic pathways or disrupting drug efflux. Specifi- cally, these strategies include inhibitors, interference RNAs, and nanomedicine approaches. However, a funda- mental challenge to these strategies is the off-target toxicity that arises from disrupting metabolism or drug efflux mediated by P-glycoprotein (P-gp), as these mechanisms are also critical to a number of healthy processes throughout the body. To address this, our long-term objective is to develop a therapeutic strategy that exploits both of these mechanisms of drug resistance in tandem to generate a therapeutic anti-cancer immune repsonse. Our central hypothesis is that rationally designed prodrugs can co-opt cancer cell metabolism and drug efflux to cause an anti-cancer immune response via a mechanism of action we have termed Bystander Assisted Immu- noTherapy (BAIT). In BAIT, an enzyme-directed prodrug is first metabolized to an immunotherapeutic metabolite by the irregular metabolism of multidrug-resistant cancer cells. Next, the immunotherapeutic is transported, via P-gp-mediated drug efflux, to the extracellular space. This results in the activation of bystander immune cells in local proximity, which initiate an anti-cancer immune response. Because BAIT requires tandem metabolism and drug efflux, we anticipate a uniquely enhanced specificity for multidrug-resistant phenotypes that exhibit both of these processes. To develop rationally designed BAIT prodrugs, we first identify small-molecule immunothera- peutics that are susceptible to drug efflux. In concurrent studies, we also develop synthetic enzyme-directing groups that modulate the activity of immunotherapeutics and are specifically removed by enzymes expressed in the irregular metabolism of multidrug-resistant cancer cells. Combining these two research areas, we generate enzyme-directed BAIT prodrugs that confer immunogenicity to multidrug-resistant cancers. In-vitro, this is con- firmed in co-cultures of immune cells and cancer cell lines that express these metabolic enzymes and P-gp. In- vivo, we use a murine model system for prostate cancer (TRAMP-C2 allograft) to demonstrate that BAIT pro- drugs result in lowered toxicity, decreased tumor volume, and increased progression-free survival, relative to conventional immunotherapeutics in immunocompetent mice. Taken together, we envision that this research will establish BAIT as a therapeutic strategy that is enhanced, rather than disarmed, by drug resistance. It is our long-term vision that this strategy could be widely applicable to multidrug-resistant cancers that evade the action of co...