The authors introduce a new approach to improving the therapeutic window of PDE4 (type 4 cAMP-specific phosphodiesterase) inhibitors, which is aimed at the identification of the specific targets for emesis and efficacy.To this end, they prepared an emetic, efficacious, and competitive PDE4 inhibitor (APIIMQ) (I) capable of covalently tagging its biol. targets upon photoactivation.This provides the possibility of identifying the emesis and efficacy targets of PDE4 inhibitors.To the authors knowledge, this is the first reported example of the preparation of a highly emetic and efficacious PDE4 photoaffinity probe.