A clinical trial targeting tumor immunosuppression by combining an IDO1 inhibitor with a therapeutic melanoma vaccine failed to increase tumor-fighting T-cell responses, according to results reported by researchers led by Craig L. Slingluff of the University of Virginia. The study was designed to amplify the vaccine’s effects by blocking IDO1, an immunosuppressive enzyme pathway active in the tumor microenvironment. Instead, the combination did not generate the intended boost in measurable T-cell activity within tumors. While the outcome is negative from an efficacy standpoint, the trial is likely to inform future biomarker selection and combination strategies—particularly which mechanisms can reliably translate from immunobiology to clinical immune readouts. For sponsors, the result reinforces how difficult it remains to “stack” immunotherapies without confirming the immunologic mechanism is engaged in the right compartment and at sufficient magnitude.