PROJECT SUMMARY/ABSTRACT Immune checkpoint inhibitors have shown remarkable oncologic efficacy, but their clinical benefit is limited by treatment-limiting adverse events. The efficacy of checkpoint inhibitors results from T-cell activation, which also triggers increased production of pro-inflammatory cytokines. Pro-inflammatory cytokines are known to inhibit hepatic drug-metabolizing enzymes, including members of the cytochrome P450 (CYP) family, and thereby increase the exposure and potential for adverse events with co-administered CYP substrates. Since check- point inhibitors are formulated as monoclonal antibodies that do not directly interact with CYP enzymes or drug transporters, clinical drug-drug interaction studies were not performed during development. However, the po- tential for clinically relevant drug-drug interactions with checkpoint inhibitors is supported by studies that have found increased adverse events during co-administration with CYP substrates, including adverse events spe- cific to co-administered CYP substrates. These findings suggest that adverse events during checkpoint inhibi- tor therapy may be, in part, caused by drug-drug interactions that increase the potential for adverse events with co-administered medications. Discovery of these novel drug-drug interactions will likely inform clinical strate- gies to reduce adverse events during checkpoint inhibitor therapy and thereby enhance their clinical benefit. The long-term goal of this research is to identify clinical strategies to manage adverse events during check- point inhibitor therapy. The research aims of this proposal are (1) to determine the impact of checkpoint inhibi- tor therapy on the metabolism of CYP probe drugs and the risk for adverse events with CYP substrate drugs commonly prescribed to cancer patients and (2) to identify associations between pro-inflammatory cytokine concentrations and CYP probe drug metabolism before and during checkpoint inhibitor therapy. To investigate these aims, we will conduct a two-phase crossover clinical drug interaction study in which a cocktail containing probe drugs for six CYP enzymes (CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6, and CYP3A) is adminis- tered to subjects before and after they initiate checkpoint inhibitor therapy. Metabolic ratios will be calculated from concentrations of probe drugs and their metabolites to assess potential changes in CYP metabolism. Changes in CYP metabolic ratios will be incorporated into physiologically-based pharmacokinetic models to estimate the potential for changes in exposure and adverse events with CYP substrate drugs that are com- monly prescribed to cancer patients with the potential for serious adverse events (e.g., citalopram, tramadol, fentanyl). Pro-inflammatory cytokine concentrations will be assayed in both study phases to determine poten- tial associations with CYP probe metabolic ratios. The expected outcomes of this proposal are to establish a mechanistic understanding o...